The impact of cyclic freezing-thawing on the physicochemical properties of superabsorbent polyacrylic acid (PAA) hydrogel used as a soil conditioner

The impact of cyclic freezing-thawing on the physicochemical properties of superabsorbent polyacrylic acid (PAA) hydrogel used as a soil conditioner | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article The impact of cyclic freezing-thawing on the physicochemical properties of superabsorbent polyacrylic acid (PAA) hydrogel used as a soil conditioner Christian Buchmann, Lars Neumann, Janina Neff This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8848966/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 10 You are reading this latest preprint version Abstract Superabsorbent polymers (SAPs) such as polyacrylic acid (PAA) are increasingly used as soil conditioners to enhance water retention under drought, yet their long-term stability and transformation under environmental stress remain poorly understood. In particular, the effects of cyclic freezing-thawing on their physicochemical properties and behavior in soils have not been systematically assessed. Here, we examined the structural and physicochemical evolution of PAA hydrogel, both freely incubated and embedded in contrasting soil matrices (sand, loam), under static and cyclic freezing-thawing conditions. A multi-method approach combining swelling index (SI), 1 H-NMR relaxometry, rheometry, FTIR spectroscopy, and ESEM imaging was applied to assess hydration dynamics, network rigidity, and polymer-mineral interactions. Freely incubated PAA remained highly hydrated, flexible, and chemically stable, confirming that freezing-thawing alone does not alter its physicochemical structure. In sand, moderate confinement and cation-mediated crosslinking produced a thin, reversible organomineral shell that increased stiffness while preserving an elastic core. In loam, finer texture, higher cation exchange capacity, and organic ligands promoted deeper polymer-mineral integration and compaction, yielding dense, solid-like composites. Across all treatments, soil matrix effects clearly dominated over freezing-thawing. Overall, PAA followed a mechanistic transformation pattern from a hydrated flexible hydrogel to a confined organomineral composite, overall driven by soil-induced mechanical and ionic interactions. These findings highlight how SAPs may gradually persist as plastic-like residues in soil, emphasizing the need to evaluate soil-specific polymer-mineral interactions in future SAP applications. Superabsorbent polymer soil conditioner polyacrylic acid freezing-thawing cycles hydrogel aging Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1 Introduction The availability and retention of soil moisture is one of the most important and simultaneously limiting factors for plant productivity in many agricultural systems, especially in regions where irregular rainfall patterns, increasing evaporation and climatic extremes exacerbate water stress [ 1 , 2 ]. Improving soil water retention is therefore central to sustainable agricultural production and climate-adaptation strategies under increasingly variable environmental conditions - core priorities in the transition toward more resilient and resource-efficient agri-food systems [ 3 ]. In this context, superabsorbent polymers (SAPs) have received significant attention as soil amendments capable of improving water retention, protecting plants against short-term drought, and reducing irrigation demand [ 4 – 7 ]. Their proposed benefits align with broader sustainability goals by potentially lowering water use, supporting crop productivity under climatic stress, and enabling more efficient input management [ 8 ]. Besides polyacrylamide (PAM), polyacrylic acid (PAA) is one of the most studied synthetic SAPs and is widely used in modern agriculture, particularly in arid and semi-arid regions, as its high water-absorption capacity can improve water-use efficiency and support crop yields under drought [ 5 , 9 ]. When incorporated into soil or used as seed coatings and nutrient carriers in fertilizers, SAPs not only enhance soil moisture retention but also reduce nutrient leaching, increase fertilizer efficiency, and promote robust seedling establishment, leading to yield increases of up to 15% in cereals and even higher in legumes and other crops [ 10 , 11 ]. These multifunctional application possibilities make SAPs a promising tool for sustainable agricultural transitions, yet their long-term behavior, persistence, and potential environmental trade-offs remain insufficiently understood [ 5 ]. Beyond agriculture, SAPs are increasingly utilized in construction materials, e.g., to improve concrete workability and durability, in geoengineering for soil stabilization and erosion control, and in environmental remediation for wastewater treatment and pollution mitigation [ 12 ]. Their widespread use therefore highlights the need for further mechanistic understanding of SAP behavior and transformation as striking challenges seem to restrict their sustainable use in agricultural systems, including limited biodegradability, cost-effectiveness, and inconsistent field performance [ 5 , 13 ]. When SAPs are incorporated into soil, they can progressively transform in response to soil physiochemistry and environmental stressors, altering their structure, water-holding capacity, and functional integrity, with significant implications for agronomic performance and the formation of persistent, plastic-like residues [ 4 , 13 – 15 ]. Recent studies indicate that soil matrix properties and external stress can gradually convert interparticulate hydrogel structures into increasingly condensed, rigid, particle-like residues, yet the underlying mechanistic processes remain poorly understood [ 5 ]. On the one hand, soil texture, ionic strength, pH, SOM and clay content, and soil solution composition strongly influence SAP swelling, network organization, and mechanical stability [ 16 – 18 ]. For example, high cation concentrations crosslink and thus compact the polymer chains, while SOM and mineral particles can bind to the hydrogel, further reducing swelling and accessibility [ 20 ]. On the other hand, SAPs continuously experience environmental dynamics in parallel, including moisture fluctuations, mechanical stress from roots or compaction, freezing-thawing, and UV exposure, which can further modify their physicochemical properties [ 20 – 23 ]. In this regard, repeated drying-rewetting has been shown to induce surface cracking, densification, and swelling hysteresis due to cyclic shrinking and expansion of the hydrogel network [ 24 – 26 ]. Although some environmental factors have been shown to alter the physicochemical properties of SAPs in soil, freezing-thawing cycles remain one of the least explored [ 14 , 21 , 27 ]. Although frost has been a minor issue in the regions where SAPs are typically applied, its importance grows as their use expands into temperate regions facing climate change-induced drought and erratic rainfall [ 5 , 21 ]. In those regions, freezing-thawing dynamics are widespread and expected to become increasingly irregular as winter temperatures fluctuate and snow cover declines, further emphasizing the need to understand SAP behavior under such conditions. [ 21 , 28 , 29 ]. Here, residual SAPs in soil could be inevitably exposed to recurring freezing-thawing dynamics during the winter months, which might play a crucial role in shaping their transformation pathways and physicochemical properties in terms of (re)swelling performance, water binding, structural morphology, as well as their contribution to plastic pollution. Thus, understanding how SAPs respond to freezing-thawing cycling is essential for evaluating both their performance as soil amendments and their potential to form persistent, solid-like residues in agricultural soils. To address these knowledge gaps, this study examines the physicochemical and structural evolution of PAA hydrogel incorporated into contrasting soil matrices (sand and loam) under static and cyclic freezing-thawing conditions. For this, a multi-method approach was applied to quantify changes in swelling capacity (swelling index - SI), water entrapment ( 1 H proton nuclear magnetic resonance (NMR) relaxometry), mechanical stability (rheometry), chemical structure (attenuated total reflectance fourier-transform infrared (ATR-FTIR) spectroscopy), and morphology (environmental scanning electron microscopy - ESEM). Together, these complementary techniques allow a comprehensive assessment of PAA transformation processes in realistic soil environments. When freely swollen in demineralized water (dH 2 O), PAA should show the highest SI and water mobility because no mineral surfaces, ions, or structural constraints restrict network expansion. Under static incubation, only minor physical ageing is expected, reflected in largely stable SI, relaxation times, rheological responses, FTIR spectra, and regular morphology. Even under cyclic freezing-thawing, only slight ageing should occur, as the low crosslinking density and high flexibility allow the hydrogel to accommodate freezing-induced water expansion through chain reorganization, preventing strong internal stresses. Freezing should therefore cause only limited compaction with modest narrowing of polymer segments and small pore-like cavities. Accordingly, SI, relaxation times, and deformability should decline only marginally, and FTIR spectra should only slightly shift in the carbonyl/carboxyl region due to minimal deprotonation, while no new bands and no substantial intensity changes in other band regions are expected. Furthermore, no mineral-associated SiO and Al-O-Si bands should be present, reflecting the lack of mineral interactions. When incubated in sand, swelling and water mobility should be reduced by mechanical confinement and moderate ion-induced crosslinking. SI should therefore be lower than in dH 2 O and decrease further under cyclic freezing-thawing, as repeated ice formation promotes network densification with compacted lamellae, fracture edges, and localized collapse. Consequently, NMR relaxation times should be shortened, with higher τ max , τ YP , and reduced γ YP indicating more solid-like behavior. FTIR ratios should remain comparatively stable, with only moderate intensity increases and band broadening in the carbonyl/carboxyl region due to partial cation coordination at the PAA hydrogel-sand interface. Further, due to the interaction of PAA with fine quartz particles at the interface, we assume an increasing SiO absorption, while Al-O-Si signals should remain negligible. Thus, intensity shifts should primarily reflect moderate interface-related coordination rather than structural modification of the overall hydrogel network. In loam, the strongest PAA densification and crosslinking are expected due to higher ionic strength, OM, and clay content. SI should thus be lowest and remain greatly reduced, reflecting permanently bound and confined water. This should yield the highest τ max and τ YP and the lowest γ YP , indicating pronounced mechanical reinforcement. FTIR spectra should reflect the strongest matrix-induced changes, with pronounced intensity increases and broadening of the carbonyl/carboxyl region and systematic shifts in the carbonyl/carboxyl region caused by extensive cation coordination, partial deprotonation, and OM-related complexation. The CH 2 region should intensify through sorbed SOM, while strong SiO and Al-O-Si signals are expected from incorporated clay and silt particles, reflecting the formation of a dense, multi-layered organomineral shell and deep integration of mineral fines into the hydrogel network. ESEM should reveal dense composite aggregates, collapsed hydrogel structures, and thick polymer-mineral coatings. However, freezing-thawing cycles should have a comparatively minor effect on loam-incubated PAA because strong clay-OM-polymer interactions and cation-mediated crosslinking impose high confining pressure and strongly suppress ice formation. SI should therefore decrease only moderately, and changes in NMR relaxation times and rheological parameters should remain less pronounced than in dH 2 O or sand. Also, FTIR should retain the broadened carbonyl/carboxyl region, and strong SiO/Al-O-Si signals imposed by the loam matrix, with only minor freezing-thawing-related intensity shifts. The other band regions should remain unchanged, confirming that freezing-thawing effects are negligible compared to matrix-driven organomineral integration. ESEM should display compact, particle-rich structures with fewer freezing-thawing-related cracks and irregularities than in the corresponding dH 2 O and sand variants. 2 Materials and methods In this study, two well-characterized reference soils from the Agricultural Investigation and Research Institute (Lufa Speyer, Germany) were used. Both soils significantly differed in their texture and overall physicochemical properties (Table 1 ). For the PAA hydrogels, hydrogel-forming PAA powder (Viscosity average molar mass M v = 4,000,000 g/mol) (Sigma-Aldrich, Germany; CAS 9003-01-4) was used and prepared according to [ 26 ] by allowing 200 mg dry powder to completely swell for 72 h on a pre-wetted dialysis membrane (flat width 44 mm, MWCO 14000, Carl Roth GmbH & Co. KG), which was in direct contact with a demineralized water (dH 2 O) reservoir. After 72 h, the swollen PAA hydrogel (~ 15 g) was gently removed from the membrane and further used for the incubation experiment. For the incubation experiment, the PAA hydrogel was applied to the two reference soils at a medium depth using a point application method to ensure reproducible geometry and minimize edge effects. Each incubated sample contained 40 g soil with an initial bulk density of 1.47 g cm − 3 for the sand and 1.18 g cm − 3 for the loam. Both soils were preset to 40% of their WHC max prior to incubation, with moisture checked weekly and readjusted with dH₂O as needed. During the checks, the incubation vessels were briefly opened to allow for ventilation. In addition, pure PAA hydrogel was also investigated as soil-free control. In total, 81 samples were prepared with three replicates for each treatment. After PAA incorporation, the soil samples were incubated under two thermal regimes: a static temperature regime at 10°C (S) and a cyclic temperature regime at 10°C with 10 additional weekly 24-hour freezing events at -10°C (C). Three days after PAA application (timepoint 0) and after the 1st, 3rd, 5th, and 10th incubation week (analogous to the freezing-thawing cycle), the respective PAA hydrogels were gently removed from the soil and examined for various physicochemical properties as described below. To improve clarity in the illustrations, we use the general term ‘measuring point’ (MP) for both treatments: for the static incubation, MP n corresponds to incubation week n at constant temperature, whereas in the cyclic one, MP n corresponds to the n -th freezing-thawing cycle (e.g., MP3 denotes incubation week 3 for static (S3) and freezing-thawing cycle 3 (C3) for cyclic samples). Table 1 Selected physicochemical properties of the investigated soils used for the experiment and the respective 1:5 soil extracts Soil name (acc. to Lufa) 2.1 2.4 Soil type sand loam Bulk soil Organic carbon [% C] 0.55 ± 0.10 1.83 ± 0.17 Nitrogen [% N] 0.06 ± 0.01 0.23 ± 0.02 pH 4.60 ± 0.10 7.50 ± 0.10 CEC [meq 100g − 1 ] 2.90 ± 0.20 17.40 ± 0.80 Density [g cm − 3 ] 1.47 ± 0.06 1.18 ± 0.04 WHC max [g 100g − 1 ] 32.50 ± 1.50 44.60 ± 2.20 PSD (mm) [%] < 0.002 3.10 ± 0.90 26.60 ± 0.70 0.002–0.05 10.70 ± 1.20 41.20 ± 1.30 0.05-2.0 86.20 ± 0.70 32.30 ± 1.40 Soil extracts EC [µS cm − 1 ] 138 ± 1 424 ± 1 pH 7.0 ± 0.1 7.4 ± 0.1 Al 3+ [mg L − 1 ] 0.40 ± 0.01 0.04 ± 0.00 Fe 3+ [mg L − 1 ] 0.29 ± 0.04 0.23 ± 0.02 Mn 2+ [mg L − 1 ] 1.17 ± 0.15 0.018 ± 0.001 Zn 2+ [mg L − 1 ] 0.06 ± 0.01 0.007 ± 0.00 Ca 2+ [mg L − 1 ] 31.15 ± 4.25 90.62 ± 5.10 Mg 2+ [mg L − 1 ] 5.34 ± 0.13 5.87 ± 0.10 K + [mg L − 1 ] 10.29 ± 0.90 2.92 ± 0.17 Na + [mg L − 1 ] 2.62 ± 0.06 3.67 ± 0.05 Swelling index After the first free swelling (MP0), the different freezing-thawing cycles/incubation weeks, the swelling index (SI) of each PAA hydrogel was estimated according to [ 26 ] as the volume of absorbed water by the swollen PAA hydrogel (in ml) divided by its dry weight (in g), representing the overall ability of the hydrogel network to absorb water from its surroundings. 1 H-NMR relaxometry Water entrapment was investigated by 1 H-NMR relaxometry using a Bruker Minispec MQ (Bruker, Germany) at a magnetic field strength of 0.176T (corresponding proton Larmor frequency of 7.5 MHz) [ 30 ]. The transverse relaxation (T 2 ) decay curves were acquired at an echo time (T E ) of 0.3 ms and further processed using MATLAB R2014a (MathWorks, United States), performing an inverse Laplace transformation (ILT) [ 31 ] and applying the Butler, Reeds, and Dawson (BRD) algorithm [ 32 ]. The respectively obtained T 2 distributions (RTDs) were further evaluated according to [ 18 ] by calculating the longest relaxation time in the respective RTDs (T 2WL ) in terms of the 95th percentile of the sum of all amplitudes. Additionally, the peak positions (T 2peak ) within the RTDs were calculated as proxy for the predominant water fraction in the respective hydrogel sample. Thus, both longer T 2WL and T 2peak indicate less-restricted water populations, typically associated with larger pore domains and a more expanded hydrogel network, whereas shorter times indicate stronger confinement, e.g., due to condensed network structures, stronger water-polymer interactions, or smaller pore domains [ 30 , 33 ]. Rheometry Rheological measurements were conducted according to [ 26 ] using a MCR 102 rheometer (Anton Paar, Germany) using a cone-plate measuring geometry. For the measurements, swollen SAP hydrogel was placed on the rheometer plate, rested for 60 s to ensure undisturbed measurements, and subjected to an amplitude sweep test (AST) at a constant frequency of 0.681 s⁻¹ for a total of 37 measurement points to assess the respective viscoelasticity in terms of shear stress (τ) and shear rate (γ) at the yield point (YP) as well as the maximum shear stress (τ max ). In this regard, τ max represents the maximum resistance of the sample before structural failure, with higher values indicating a stronger, more rigid structure [ 34 ]. Further, τ YP indicates the onset of irreversible structural deformation and the shift from elastic to viscous solid-like behavior. Finally, γ YP reflects sample deformability, where higher values indicate a more flexible structure and lower values a stiffer structure with limited deformation capacity. ATR-FTIR ATR-FTIR measurements were performed on the freeze-dried hydrogels using a Cary 630 ATR-FTIR spectrometer (Agilent Technologies, United States). The resulting spectra were normalized and further analyzed using the open Specy R package [ 35 ]. Striking PAA absorbance bands typically include CH (3,000–2,800 cm − 1 ), C = O - carbonyl (1,870-1,550 cm − 1 ), COOR (1,800-1,715 cm − 1 ), COOH (1,730-1,710 cm − 1 ), COH (1,750-1,650 cm − 1 ), COO⁻ (1,620-1,550 cm − 1 ), and CH 2 (1,490-1,150 cm − 1 ) [ 35 – 39 ]. Because several carbonyl-related functional groups exhibited strong spectral overlaps and could not be reliably resolved in the solid-state ATR configuration, the individual C = O, COOR, COOH, COH, and COO⁻ bands were evaluated as one consolidated ‘carbonyl/carboxyl region’. Concerning sand- and loam-matrix-related absorbance bands, additional mineral-associated regions were evaluated in terms of Si-O/Al-O-Si (1030 − 960 cm − 1 ) band, which arise from silicate and aluminosilicate structures [ 40 , 41 ]. In addition, shifts in band intensities (I) were quantitatively evaluated according to [ 38 , 42 ] as the absorbance at the band-specific peak maximum above the baseline. All bands, together with their intensity changes, allow assessing cation coordination, partial deprotonation, and organomineral interactions at the hydrogel-soil interface. In particular, shifts, broadening, and intensity changes in the carbonyl/carboxyl region reflect cation-induced crosslinking and associated changes in protonation state, while increases or decreases in the CH 2 region indicate varying degrees of SOM sorption and local ordering or loosening of polymer side chains. The emergence and intensity shift of the SiO/Al-O-Si band are used to assess the physical incorporation of fine mineral particles and the formation of organomineral structures [ 43 ]. ESEM Morphological and structural features were exemplarily examined at MP0, MP1, and MP10 for all SAP hydrogels, both freely swollen in dH 2 O and incubated in the two soils using a Quanta 250 ESEM (FEI Company, United States) equipped with a backscattered electron detector (BSED). Prior to the measurements, all hydrogels were freeze-dried using an ALPHA 1–2 LDplus freeze-dryer (Martin Christ, Germany) and coated with a 30 nm thick gold layer using a Q150R S sputter coater (Quorom Technologies Ltd, United Kingdom). All measurements were performed under high vacuum (< 10 − 4 Pa) at an acceleration voltage of 30kV and an average spot size of 3.5. Statistical analysis Statistical analyses were performed with R (v. 4.3.1) using the packages dplyr, car , and DescTools. Variations within the three replicates were presented as standard errors (SE) of the arithmetic means. Prior to analysis, data were tested for normality and homogeneity of variances using Shapiro-Wilk and Levene’s tests. As all assumptions were met, no data transformation was required. ANOVA was conducted using type-III sums of squares via the Anova() function in the car package. When significant effects were detected (p < 0.05), Tukey’s HSD test was applied for post-hoc comparisons. All analyses were based on three independent replicates per treatment. Fixed factors included treatment (static vs. cyclic), matrix (dH₂O, sand, loam), and measurement point (0–10). A full-factorial three-way ANOVA (Treatment × Matrix × MP) was used to evaluate factor effects and interactions. To complement significance testing, effect sizes (η²) were calculated as the proportion of variance explained by each factor. Given the replicate variability and the sample size, η 2 can help to identify the magnitude of treatment effects and interpreting incubation- and matrix-related differences in the investigated PAA properties, even where statistical significance was not given. To support interpretation of multivariate patterns, principal component analysis (PCA) was performed to reduce data complexity and visualize treatment-related clustering. PCA was conducted in R using the FactoMineR package, with variables scaled to unit variance. Matrix and measurement point were included as supplementary variables. Visualizations were generated with factoextra and ggplot2 , using 95% confidence ellipses. R version 4.3.1 (RStudio 2024.04.2) and Excel Office 16 were used to carry out all calculations and figures. All detailed statistical parameters, including degree of freedom, Sum of squares, R 2 , F-value and p-values are presented in the supplementary information (Tables A1-A10). 3 Results Swelling index At the beginning of the experiment (MP0), PAA showed the highest swelling in dH₂O with an SI dH₂O of 77.07 ± 0.12 ml g − 1 , followed by PAA incubated in the sand with SI sand = 74.32 ± 0.81 ml g − 1 and the loam matrix with SI loam = 67.04 ± 0.73 ml g − 1 , corresponding to an initial SI reduction of -4% in sand and − 13% in loam relative to dH 2 O (Fig. 1 ). Under static incubation, SI dynamics of PAA diverged between dH 2 O and the two soils. From MP0 to MP1, SI dH₂O decreased only slightly by -5.4% to 72.91 ± 2.13 ml g − 1 , whereas SI sand and SI loam decreased more strongly to 42.23 ± 5.64 ml g − 1 (-43.2%) and 53.17 ± 4.68 ml g − 1 (-21.7%), respectively. After MP1, SI dH₂O remained comparatively stable at 70.72 ± 1.63 ml g − 1 at MP10 (-8.2% relative to MP0). However, SI sand showed a pronounced re-swelling, from 42.23 ± 5.64 ml g − 1 at MP1 to 61.88 ± 4.49 ml g − 1 at MP10 (+ 46.5% relative to MP1, -16.7% relative to MP0). Although SI loam increased to 58.23 ± 2.18 ml g − 1 at MP5, it approached 51.64 ± 3.62 ml g − 1 at MP10 (-2.9% relative to MP1 and − 24.0% relative to MP0). Cyclic freezing-thawing produced a similar initial response but slightly different long-term developments: from MP0 to MP1, SI dH₂O again decreased modestly to 72.91 ± 2.13 ml g − 1 (-5.4%), whereas SI sand and SI loam dropped to 54.94 ± 5.91 ml g − 1 (-26.1%) and 51.57 ± 5.90 ml g − 1 (-24.1%), respectively. Over the subsequent freezing-thawing cycles, SI dH₂O remained high and ended at 72.17 ± 1.41 ml g − 1 at MP10 (-6.4% vs. MP0). SI sand fluctuated between 55–63 ml g − 1 and finally stabilized at 56.80 ± 2.21 ml g − 1 at MP10 (-23.6% vs. MP0). In contrast, SI loam progressively increased after MP1 and reached 59.24 ± 8.52 ml g − 1 at MP10 (-12.8% vs. MP0). According to the three-way ANOVA, matrix had a highly significant effect on SI (p < 0.001), whereas treatment (static vs. cyclic) and MP alone were not significant (p = 1.000). The interaction between treatment and measurement point was significant (p = 0.001), while the three-way Treatment × Matrix × MP interaction was not (p = 0.182). Effect size analysis indicated that the matrix factor explained a large proportion of the variance (η 2 = 0.45), with additional moderate contributions from the Treatment × MP interaction (η 2 = 0.16) and the three-way interaction (η 2 = 0.11). 1 H-NMR Relaxometry At MP0, PAA swollen in dH₂O exhibited a single, narrow unimodal relaxation time distribution (RTD) with T 2WL = 2494.51 ± 0.00 ms and T 2peak = 2171.12 ± 0.00 ms (Figs. 2 and 3 ). In contrast, PAA swelling in both soils markedly broadened the RTDs with shifts towards shorter relaxation times. In sand, T 2WL was 117.77 ± 4.72 ms and T 2peak 79.39 ± 1.86 ms, corresponding to reductions of -95% and − 96% relative to PAA swollen dH 2 O. PAA incubated in loam showed a T 2WL of 291.38 ± 20.88 ms and a T 2peak = 219.99 ± 8.81 ms (-88% and − 90% compared to dH 2 O), with broad and multi-modal RTDs already at MP0. During static incubation, PAA showed a pronounced RTD shift between MP0 and MP1 for all matrices: For dH 2 O, both T 2WL and T 2peak decreased about − 7% to 2327.20 ± 0.00 ms and 2025.50 ± 0.00 ms, respectively, accompanied by an overall broadening of the RTD. Even stronger reductions were observed for PAA incubated in sand (T 2WL = 70.71 ± 1.62 ms, -40%; T 2peak = 52.34 ± 1.23 ms, -34%) and in loam (T 2WL = 29.38 ± 1.18 ms, -90%; T 2peak = 20.27 ± 0.46 ms, -91%). Also, the RTD shapes became broader but still dominated by long T₂ for dH 2 O, whereas the RTDs of PAA in sand and loam were dominated by shorter T 2 . After MP1 and until MP5, both T 2 indices re-increased for PAA swollen in dH 2 O to the initial values (T 2WL = 2494.51 ± 0.00 ms; T 2peak = 2171.12 ± 0.00 ms), stabilizing at slightly lower values at MP10 (T 2WL = 2382.97 ± 55.77 ms; T 2peak = 2122.58 ± 48.54 ms). A partial recovery was also observed for PAA incubated in sand: after the strong initial drop, T 2WL increased again to 36.99 ± 0.87 ms at MP5 (-69% compared to MP0) and 31.44 ± 0.00 ms at MP10 (-73%), while T 2peak re-increased slightly from MP1 to MP5 to finally 22.22 ± 0.00 ms at MP10 (-72%). PAA incubated in loam showed also showed a re-increase of T 2WL to 47.96 ± 17.58 ms at MP5 (+ 63%) but decreased again to 37.11 ± 2.31 ms at MP10 (-87% compared to MP0). In contrast, T 2peak remained consistently low at 20–21 ms throughout the remaining incubation time. Regarding cyclic freezing-thawing of PAA swollen in dH 2 O, both T 2WL and T 2peak remained at 2494.51 ± 0.00 ms and 2171.12 ± 0.00 ms at MP1, respectively, decreased at MP5 (T 2WL = 2099.69 ± 227.51 ms; T 2peak = 1827.49 ± 198.01 ms;, -16%), before re-increasing at MP10 to T 2WL = 3145.73 ± 73.62 ms and T 2peak = 2223.15 ± 52.03 ms (+ 26% compared to MP0). For PAA in sand, relaxation times decreased strongly from directly after the first freezing-thawing cycle with T 2WL = 78.43 ± 8.64 (-33%) and T 2peak = 51.11 ± 0.00 (-36%) at MP1. At MP5, T 2WL and T 2peak further decreased by -65% (T 2WL = 41.57 ± 1.67 and T 2peak = 27.41 ± 1.10 ms) and slightly re-increased at MP10 (T 2WL = 61.54 ± 1.41 ms; T 2peak = 33.70 ± 0.00 ms). PAA incubated in loam exhibited the strongest and most persistent reduction of both T 2WL and T 2peak after the first freezing-thawing cycle: T 2WL decreased to 78.22 ± 13.73 ms at MP1 and further to 30.54 ± 4.13 ms at MP5 but partially recovered to 72.13 ± 22.75 ms at MP10. T 2peak also decreased from 219.99 ± 8.81 ms at MP0 to 24.39 ± 0.57 ms at MP1 (-89%) and remained nearly constant until MP10 (24.30 ± 3.57 ms). Accordingly, the RTDs under cyclic conditions showed a loss of relaxation signals at longer T₂ times. For PAA swollen in dH₂O, the primary peak shifted towards shorter relaxation times, whereas PAA incubated in sand and loam showed broad, predominantly left-shifted distributions dominated by short T 2 . Three-way ANOVA revealed a highly significant effect of the matrix on both T 2WL and T 2peak (p < 0.001), confirming the strong and persistent differences between dH₂O, sand, and loam. Neither treatment nor MP showed statistically significant effects, and no significant interactions were detected (all p > 0.19). However, effect size analysis indicated moderate contributions of the Treatment × MP interaction (η 2 = 0.13–0.15) and the Treatment × Matrix × MP interaction (η 2 = 0.28–0.30). Rheometry After initial swelling in dH 2 O (MP0), PAA exhibited the lowest maximum shear stress (τ max ) with 258.85 ± 27.15 Pa, whereas the incubation in soil caused a marked increase in mechanical resistance (Fig. 4 ): In sand, τ max increased to 406.05 ± 24.59 Pa (+ 57%), and in loam to 833.32 ± 112.90 Pa (+ 222% compared to dH 2 O at MP0). A similar pattern at MP0 was observed for the shear stress at the yield point (τ YP ), which was 185.73 ± 20.43 Pa for PAA swollen in dH₂O, 321.29 ± 20.28 Pa for PAA incubated in sand (+ 73%) and 724.98 ± 55.35 Pa (+ 290%) in loam. In parallel, the strain at the yield point (γ YP ) decreased in both soils at MP0 compared to dH 2 O, from 7.20 ± 0.26% in dH₂O to 2.71 ± 0.08% in sand (-62%) and to 1.34 ± 0.38% in loam (-81%). Under static incubation, the rheological behavior of PAA differed between the matrices but showed a general moderate strengthening: In dH 2 O, τ max increased by totally + 77% to 457.97 ± 67.58 Pa at MP10, whereas τ YP decreased to 138.86 ± 7.05 Pa (-25%). Also, γ YP decreased markedly from 7.20 ± 0.26% at MP0 to 0.77 ± 0.10% at MP10 (-89%), indicating a progressive loss of deformability with time. For PAA incubated in sand, τ max increased to finally 1258.13 ± 162.76 Pa at MP10 (+ 210%), whereas τ YP increased to 1214.53 ± 138.45 Pa (+ 278%). In contrast, γ YP decreased finally to 2.97 ± 0.20 at MP10. The loam matrix induced the strongest mechanical reinforcement under static conditions, with an increase of τ max to 2691.20 ± 521.31 Pa and τ YP to 2344.38 ± 669.61 Pa at MP10 (both + 223% compared to MP0). However, γ YP remained consistently low between 0.17–0.26% over the incubation time. During subsequent cyclic freezing-thawing, the microstructural stability of PAA increased steadily across all matrices: For PAA swollen in dH 2 O, both τ max and τ YP increased by + 57%, from 258.85 ± 27.15 Pa at MP0 to 405.89 ± 49.78 Pa at MP10, and from 185.73 ± 20.43 to 291.61 ± 43.67 Pa, respectively. In contrast, γ YP varied between 6.57 ± 0.18 at MP1, 8.30 ± 0.09 at MP3, and 6.78 ± 0.21 at MP5, before reaching 7.25 ± 0.39 at MP10. For PAA incubated in sand, τ max and τ YP successively increased with freezing-thawing cycles to finally 941.99 ± 96.59 Pa (+ 132% compared to MP0) and 767.90 ± 149.06 Pa (+ 139% compared to MP0) at MP10, respectively. PAA incubated in loam revealed the most pronounced increases in microstructural stability: here, τ max increased from 833.32 112.90 Pa at MP0 to 2753.87 ± 184.13 Pa at MP10 (+ 230%). Also, τ YP increased from 724.98 55.35 Pa at MP0 to 1532.93 ± 457.94 Pa (+ 112%). Again, γ YP remained consistently low between 0.17–0.26% throughout the freezing-thawing cycles. The three-way ANOVA revealed that only the matrix had a statistically significant effect on all rheological parameters (p 0.27), and no two- or three-way interactions were statistically significant (all p > 0.20). Effect size analysis indicated that the matrix accounted for 94–96% of the total variance (η 2 = 0.94–0.96) across all rheological parameters. All remaining main effects and interaction terms showed small effect sizes below 3% (η 2 < 0.03). ATR-FTIR At the beginning of the experiment (MP0), freely incubated PAA showed the highest absorbance (A) in both the carbonyl/carboxyl and CH 2 regions, while PAA incubated in sand and loam exhibited distinctly lower values (Figs. 5 and 6 ). Specifically, in the carbonyl/carboxyl region, A of freely incubated PAA reached 0.217 ± 0.013, compared to 0.181 ± 0.007 in sand (-16.6%) and 0.072 ± 0.006 in loam (-66.8%). For the CH 2 region, a similar trend was observed, with freely incubated PAA showing A = 0.191 ± 0.005, followed by sand (A = 0.179 ± 0.007; -6.3%) and loam (A = 0.126 ± 0.006; -34.0%). For the SiO/Al-O-Si region, A was lowest in freely incubated PAA (0.083 ± 0.002), increased in sand (0.100 ± 0.007; +20.5%), and was highest in loam (0.330 ± 0.037; +297.5%). In contrast, the COC band was only evaluable for freely incubated PAA (A = 0.159 ± 0.007). Under static incubation, A for freely incubated PAA remained relatively stable over time with only a slightly decrease for carbonyl/carboxyl region from MP0 (0.217 ± 0.013) to MP1 (0.213 ± 0.009) to finally 0.205 ± 0.010 at MP10 (-5.5%). CH 2 absorbance also remained constant, from A = 0.191 ± 0.005 at MP0 to A = 0.183 ± 0.005 at MP10 (-4.2%). The SiO/Al-O-Si band was not detectable in freely incubated PAA, but the COC signal remained stable (A = 0.160 ± 0.004 at MP10 vs. 0.159 ± 0.007 at MP0). PAA incubated in sand, A decreased more strongly over time. For the carbonyl/carboxyl band, A dropped from 0.181 ± 0.007 (MP0) to 0.151 ± 0.002 (MP1) and further to 0.129 ± 0.007 at MP10 (-28.7%). CH 2 absorbance remained constant, from A = 0.179 ± 0.007 at MP0 to 0.180 ± 0.002 at MP10 (± 0.0%). Also, the SiO/Al-O-Si band remained stable at A = 0.131 ± 0.006 at MP10 (+ 31.0% compared to MP0). For PAA incubated in loam, A for the carbonyl/carboxyl region decreased to 0.068 ± 0.002 from MP0 to MP1 and to finally 0.063 ± 0.003 at MP10 (-12.5%). The CH 2 region followed a similar pattern, dropping from A = 0.126 ± 0.006 (MP0) to A = 0.111 ± 0.002 (MP1) and A = 0.108 ± 0.002 at MP10 (-14.3%). Notably, the SiO/Al-O-Si signal remained high overall, with A = 0.349 ± 0.009 at MP10 (+ 5.8% compared to MP0).Under cyclic freezing-thawing, A of freely incubated PAA remained nearly unchanged across most bands. The carbonyl/carboxyl region decreased marginally from 0.217 ± 0.013 (MP0) to 0.199 ± 0.005 at MP10 (-8.3%), while CH 2 increased slightly to finally 0.193 ± 0.004 at MP10 (+ 1.0%). Also, the SiO/Al-O-Si band remained not visible, and the COC band increased only slightly to 0.162 ± 0.004 at MP10 (+ 1.9%). When PAA was incubated in sand, A of the carbonyl/carboxyl region decreased from 0.181 ± 0.007 at MP0 to 0.172 ± 0.002 at MP10 (-5.0%), whereas CH 2 remained nearly constant (from 0.179 ± 0.007 at MP0 to 0.178 ± 0.003 at MP10; -0.6%). The SiO/Al-O-Si band slightly increased to 0.102 ± 0.003 at MP10 (+ 2.0%). For PAA incubated in loam, the carbonyl/carboxyl region decreased from A = 0.072 ± 0.006 at MP0 to 0.065 ± 0.002 (-9.7%) at MP10, whereas CH 2 decreased slightly to finally 0.123 ± 0.002 (-2.4%), and SiO/Al-O-Si to 0.315 ± 0.012 (-4.5%). Overall, freely incubated PAA exhibited all major absorbance regions, including pronounced carbonyl/carboxyl, CH 2 , and COC bands, with no detectable signals in the SiO/Al-O-Si region. In contrast, PAA incubated in sand showed a clear reduction in carbonyl/carboxyl band intensity, stable CH 2 absorbance, disappearance of the COC band, and moderately increased signals in the SiO/Al-O-Si region. For PAA incubated in loam, carbonyl/carboxyl and CH 2 bands markedly decreased, the COC band was no longer detectable, and high SiO/Al-O-Si absorbance dominated the spectrum. These matrix-specific patterns were further supported by the results of the three-way ANOVA, which confirmed that the matrix (dH 2 O, sand, loam) had a statistically significant effect on absorbance across all evaluated bands. For the carbonyl/carboxyl region, the matrix effect was highly significant (p < 0.001) and explained a large share of the variance (η 2 = 0.52). Similar strong effects were found for the CH 2 band (p < 0.001; η 2 = 0.45) and SiO/Al-O-Si (p < 0.001; η 2 = 0.61). The treatment (static vs. cyclic) showed smaller but relevant contributions, particularly for SiO/Al-O-Si (p = 0.012; η 2 = 0.12), followed by the carbonyl/carboxyl band (p = 0.042; η 2 = 0.08) and CH 2 (p = 0.063; η 2 = 0.09). No significant main effect was found for MP (p > 0.1 for all bands). Finally, a significant Matrix × Treatment interaction was observed for the carbonyl/carboxyl band (p = 0.038; η 2 = 0.14) and for SiO/Al-O-Si (p = 0.021; η 2 = 0.11), suggesting matrix-specific responses of PAA hydrogels to the applied freezing-thawing cycles. ESEM At MP0, pronounced structural differences were evident between PAA swollen in dH 2 O and incubated in sand and loam (Fig. 7 with striking features indicated by arrows). In dH 2 O, PAA exhibited a uniform sheet-like morphology with thin, tightly layered lamellae without noticeable cavities or embedded particles. In contrast, PAA incubated in sand showed thicker, flake-like lamellae with localized compaction zones, angular fracture edges, and several small voids. In loam, PAA displayed the most heterogeneous network structure, consisting of coarse polymer-particle aggregates, extensive porous regions, and numerous mineral- and organic-rich inclusions. Under static incubation, the matrix-specific differences observed at MP0 remained and further evolved: at MP1, PAA swollen in dH 2 O largely maintained its initial lamellar arrangement, showing only minor local tightening. When incubated in sand, the PAA network formed more compacted, interconnected flake assemblies with collapsed segments and distinct fracture lines. PAA in loam also developed bulky composite clusters with rounded pores, particle-filled cavities, and polymer sheets visibly wrapped around mineral soil constituents. At the end of the static incubation time (MP10), the lamellar structure of PAA swollen in dH 2 O still remained intact but exhibited greater variability in network spacing and densification areas. PAA incubated in sand showed substantially thickened, interlocked lamellae with angular voids and particle-stabilized cavities, whereby PAA incubated loam formed large, cohesive aggregates with multiple cavity openings, and distinct layered shells around embedded soil particles. Under cyclic freezing-thawing conditions, structural properties differed from those observed under static conditions: after the first freezing-thawing cycle (MP1), PAA swollen in dH 2 O showed localized disruptions within the lamellae and newly formed small cavities and sheet-like structures. In sand, the PAA network irregularly folded with strongly collapsed flake structures and several pronounced fracture edges. PAA incubated in loam showed rough, highly particle-permeated, compacted structures with numerous large cavities and layered edges surrounding soil mineral particles and aggregates. After ten freezing-thawing cycles (MP10), PAA swollen in dH 2 O exhibited strongly contracted lamellar regions forming dense clusters with thin, folded edges. PAA incubated in sand revealed fragmented, thick lamellae arranged in irregular stacks with enclosed pores and angular breaks. Especially in loam, the PAA network was highly condensed, with large composite aggregates, crater-like cavities, and multiple discontinuities. Relationships between investigated parameters PCA analysis of the static incubation showed that the first two principal components explained 79.1% of the total variance (Dim1 = 67.4%, Dim2 = 11.7%) (Fig. 8 A, C, E). In detail, T 2peak , T 2WL , and SI showed the highest contributions to Dim1, projecting strongly in the positive direction. In contrast, τ max , CH 2 , and the carbonyl/carboxyl region showed weaker positive loadings, while τ YP projected distinctly in the negative Dim1 direction. The opposite projection of τ YP relative to T 2peak , T 2WL , and SI indicates that samples with high water mobility and swelling index are associated with lower microstructural stability. Dim2 was primarily shaped by the SiO/Al-O-Si band, which loaded positively, whereas the remaining bands showed only minor contributions. The near-orthogonal orientation of the SiO/Al-O-Si vector to the T 2 variables further reflects their limited correlation, highlighting that mineral-related signals vary independently from hydration-related parameters. (C-D) Individual factor maps grouped by matrix (dH 2 O, sand, loam) with 95% confidence ellipses. (E-F) Data grouped by measurement points MP (80 − 10), shaded from white to black. Sample clustering showed clear matrix-dependent separation (Fig. 8 C). Freely incubated PAA clustered in the strongly positive Dim1 region, driven by high SI, T 2peak , and T 2WL , whereas PAA incubated in sand grouped close to the origin, reflecting intermediate parameter values. However, PAA incubated in loam shifted towards the negative Dim1 and positive Dim2 region, corresponding to reduced relaxation times and increased SiO/Al-O-Si absorbance. MP did not form distinct temporal trajectories (Fig. 8 E), indicating that incubation time had comparatively minor effects relative to the matrix. Under cyclic freezing-thawing conditions, the PCA structure remained similar but showed a stronger dominance of Dim1. The first two components explained 84.3% of the total variance (Dim1 = 73.5%, Dim2 = 10.8%) (Fig. 8 B, D, F). Again, T 2peak , T 2WL , and SI contributed most strongly to Dim1. However, compared to the static treatment, τ max showed a more pronounced positive loading on Dim1, while τ YP remained strongly negative. Again, the opposition of τ YP to T 2peak and SI again reflects their inverse relationship, while the strengthened positive contribution of τ max indicates a closer association between increased structural stress resistance and overall matrix variance. Dim2 was again dominated by SiO/Al-O-Si, which clearly separated the loam-incubated samples from the other matrices. The near-perpendicular vector orientation between SiO/Al-O-Si, SI, and T 2 also remained, confirming that the mineral variables contribute independently to the secondary axis. Matrix-related clustering was maintained under cyclic freezing-thawing conditions, with freely incubated PAA samples still projecting to the positive Dim1 side, PAA incubated in sand remaining near the origin, and PAA incubated in loam again shifting to negative Dim1 and positive Dim2 (Fig. 8 D). MP showed no systematic evolution along either dimension (Fig. 8 F), confirming limited freezing-thawing-induced changes in the multivariate parameter space. All in all, across both treatments, the PCA consistently showed the dominant contribution of the matrix to the hydrogel properties, with dH 2 O, sand, and loam forming robustly separated clusters along Dim1 and Dim2. 4 Discussion When freely incubated, the PAA hydrogel exhibited the highest SI and longest T 2 values, reflecting a highly hydrated and mobile polymer network that remained stable under both constant and cyclic incubation. FTIR spectra stayed stable, showing only minor intensity changes in the carbonyl/carboxyl (-5.5%) and CH 2 (-4.2%) regions and no detectable SiO/Al-O-Si signals. Similarly, rheological data consistently indicated a soft, deformable network, confirming the flexible character of the PAA hydrogel. This overall pattern aligns well with various studies that reported negligible or even beneficial effects of repeated freezing-thawing on physically crosslinked PVA/PAA hydrogels [ 44 , 45 ]. In this regard, freezing-thawing resistance has been attributed to polymer chain rearrangement and the gradual formation of crystallite junction zones that serve as stress buffers, dissipating mechanical energy during ice formation and thawing and preventing network collapse [ 46 , 47 ]. Similarly, [ 48 ] showed that cyclic freezing-thawing improved structural order in chitosan-based hydrogels through hydrogen-bond reorganization, suggesting that the PAA hydrogel in our study underwent comparable, subtle structural stabilization, consistent with its unchanged rheological behavior. This observation is also in line with [ 49 ], who found that double-network hydrogels initially lost but later recovered mechanical stability due to hydrogen-bond realignment. On the one hand, the negligible overall changes in the physicochemical properties of freely incubated PAA hydrogel underline the absence of ionic coordination or chemical modification, consistent with the chemically inert environment. [ 47 ] demonstrated that PVA/PAA hydrogels maintained mechanical and thermal stability through several freezing-thawing cycles in the absence of crosslinking cations, confirming the intrinsic resilience of physically crosslinked networks. In this regard, the NMR relaxation data also confirmed that most water in the PAA hydrogel remained relatively free, as both T 2peak and T 2WL remained comparable to free water (~ 2000–3000 ms) [ 17 ] and stable throughout the experiment. The predominance of free, mobile water domains corresponds to weakly bound hydration shells typical of flexible, coiled PAA chains [ 18 , 26 , 50 ]. In such systems, dynamic hydrogen-bond exchange allows continuous water reorganization during freezing-thawing, maintaining flexibility and preventing structural collapse [ 49 , 51 ]. On the other hand, the molecular structure and pH-dependent ionization of PAA resulted in only partial deprotonation, since the pH of dH 2 O (~ 7) is slightly above the pK a (~ 5.0) [ 52 ]. This limited deprotonation under near-neutral conditions restricts electrostatic repulsion and sustains a moderately coiled configuration, as reported for pH-sensitive PAA networks [ 50 ]. Thus, the PAA polymer chains retained flexibility and maintained high water mobility and limited network stiffening, supporting the dominance of hydrogen-bonded rather than electrostatic interactions. As already shown for chitosan/PVA and chitosan/PAA hydrogels, hydrogen bonding between water molecules and carbonyl/carboxyl groups produces bound-water domains with depressed freezing points that resist ice crystal formation and preserve mechanical integrity [ 48 , 53 ]. This is further supported by molecular simulations showing that hydration shells around carboxylated polymers prevent ice nucleation [ 54 ]. Similarly, [ 55 ] visualized polymer networks restricting ice crystal growth, leading to homogeneous solidification without microcracking, a mechanism likely contributing to the stable NMR and rheological data observed here. Also, [ 18 ] showed that alginate hydrogel-treated soil revealed higher proportions of non-freezable water due to strong water entrapment within the interparticulate hydrogel network. Thus, only a small portion of free water likely crystallized during freezing, and hydrogen bonds dynamically broke and reformed, enabling local polymer relaxation. These reversible processes prevented any permanent structural compaction, further explaining the statistically unchanged SI, rheological parameters, and FTIR spectra across the freezing-thawing cycles. Comparable reversibility was also reported for natural polysaccharide hydrogels and synthetic root-exudate mimics, which display similar cyclic flexibility through protonation-deprotonation equilibria [ 56 ], applying analogously to the carboxylic groups of PAA, sustaining reversible hydrogen-bond dynamics and preventing fracture formation. Here, [ 55 ] also confirmed that macromolecular hydrogel networks can constrain ice crystallization and prevent microcrack formation using cryo magnetic resonance imaging (cryo-MRI). In line with these findings, the PCA positioned all freely incubated PAA samples distinctly on the positive Dim1 side, confirming their stable physicochemical and rheological characteristics. The tight clustering across MP and treatment supports the negligible effect of freezing-thawing, consistent with the ANOVA results showing no significant treatment or MP effect (p > 0.25; η 2 < 0.03). Similar stability under repeated freezing-thawing has been reported for physically crosslinked PVA/PAA hydrogels with the crystalline domains acting as reversible junctions that dissipated mechanical stress without altering overall network integrity [ 44 , 45 ]. Altogether, these findings confirm that freely incubated PAA hydrogels maintain a flexible, hydrated, and reversible network stabilized mainly by hydrogen bonding. When incubated in sand, however, the structural dynamics changed markedly due to confinement and ionic effects. When incubated in sand, the behavior of PAA differed substantially compared to the freely swollen one, mainly due to a combination of soil matrix-induced mechanical confinement and cation-mediated interactions. In such confined environments, osmotic gradients and partial dehydration promote more rigid, mechanically stressed interparticulate hydrogel structures, as observed in SOM-, PVA-, and PAA-based systems [ 18 , 57 , 58 ]. On the one hand, the mineral soil matrix restricted the swelling of the interparticle hydrogel and induced a steady release of water into the environment, while, on the other hand, multivalent cations in the soil solution (especially Ca²⁺, Mn²⁺, and Al³⁺) promoted additional crosslinking between carboxylate groups [ 18 , 26 , 49 ]. Together, these two well-known soil-related mechanisms already explain the reduced SI, the shorter T 2peak and T 2WL values, and the slight reduction in carbonyl/carboxyl absorbance with a moderate increase in SiO/Al-O-Si intensity, corroborated by the ANOVA showing a dominant matrix effect (p 0.25; η 2 < 0.03). Reduced T 2 values and increased stiffness as observed in our study are consistent with findings by [ 20 , 26 , 59 , 60 ], who showed that polymer-soil systems progressively compact through crosslinking via cation bridges between carboxyl groups and mineral hydroxyls. For instance, [ 46 ] and [ 26 ] have shown that increased cationic strength increases viscoelasticity and yield stress in crosslinked PAA systems, suggesting that similar electrostatic bridging mechanisms underpin the observed increases in τ YP and τ max . Further, shortened relaxation times ≈ 100 ms has already been attributed to hydrated mineral-associated organic hydrogel structures formed by hydrated SOM and PAA in silty sand soil [ 18 , 58 ] and hydrated cross-linked lignite humic acids [ 61 ]. In this regard, the studies conducted by [ 18 , 49 , 58 ] demonstrated that organomineral complexation contributes to the formation of non-freezing water fractions that are strongly bound even under severe thermal stress. Consequently, the formation of such water fractions in the sand-incubated PAA likely reflects enhanced hydrogen bonding and reduced mobility at the polymer-mineral interface. The PCA overall supported these findings, as sand-incubated PAA samples clustered centrally between the dH 2 O and loam samples, reflecting an intermediate physicochemical state. Dim1, explaining 63.4% of the total variance, was dominated by SI, T 2peak , and τ YP , whereas Dim2 (21.7%) was mainly influenced by FTIR carbonyl/carboxyl intensity and τ max . This distribution clearly separated sand-incubated hydrogels from dH 2 O by lower swelling and increased mechanical resistance, while maintaining limited internal modification. FTIR spectra corroborated this interpretation, showing moderate broadening in the carbonyl/carboxyl region and a weak yet distinct SiO/Al-O-Si signal, indicating localized mineral contact and transient surface crosslinking without deeper structural integration as shown in the ESEM images. All in all, this suggests that mechanical confinement and moderate cation crosslinking govern PAA behavior in sand rather than freezing-thawing cycles and its overall chemical transformation as it has already been observed by [ 44 ] for physically crosslinked hydrogels under extended cyclic temperature stress. At this point, the application type of PAA to the two soils becomes critical for understanding the differing degrees of confinement, ion exchange, and interface-driven structural modifications. In this study, PAA hydrogel was applied as a single pre-swollen unit, so soil contact was initially limited to its outer interface, creating a narrow reaction zone where minerals, SOM, and multivalent cations interacted with the polymer. The pressure exerted by mineral grains restricted local expansion, while cations transiently bridged adjacent carboxylate groups through coordination interactions [ 17 – 19 , 62 ]. The combined effect of mechanical confinement and cation-mediated crosslinking gradually tightened the outer polymer network, forming a thin organomineral shell composed of densified chain segments, adsorbed mineral and organic residues, and locally crosslinked domains that stabilized the interface. ESEM images clearly visualized this structure as a compact, particle-rich layer surrounding a still porous, sponge-like inner hydrogel region. These structural differences from freely incubated PAA were reflected in an increased solid-like rheological behavior, indicating that the hydrogel could still deform substantially before yielding and evidencing a preserved core elasticity. However, the missing effect of cyclic freezing-thawing on PAA in sand likely resulted from (i) a large fraction of bound, nonfreezing water immobilized through ionic coordination, limiting ice formation and freeze-induced stresses, and (ii) the already reduced amount of mobile water under soil confinement. This agrees with studies showing that confinement and ion coordination shift water populations from free to non-freezable states tightly bound to polymer or mineral surfaces [ 18 , 49 , 58 , 63 ], thereby reducing crack formation and enhancing cryostability [ 55 ]. The enrichment of such water likely suppressed ice-induced microcracking and maintained network elasticity under repeated cycles. Meanwhile, the flexible hydrogel core still acted as a viscoelastic buffer absorbing volumetric changes without structural damage [ 50 , 64 ]. Additionally, the organomineral shell likely absorbed and redistributed freezing-induced stresses over the hydrogel surface, preventing local compaction or fracture. During thawing, weak ionic bridges relaxed rapidly, allowing the interface region to recover the shell to function as a stress-dissipating layer protecting the inner hydrogel from cyclic freezing-thawing damage. In loam, the mechanisms governing PAA hydrogel behavior extended beyond those observed for freely- and sand-incubated PAA due to its even finer texture, higher CEC, and SOM content. Fine-textured soils exert higher osmotic and mechanical confinement and provide more reactive mineral surfaces [ 18 , 20 , 65 , 66 ]. Consequently, carboxylate-cation bridging and polymer-SOM complexation further reduced swelling and resulted in an even denser organomineral composite. Similar mechanisms have already been shown for synthetic exudate analogs and other synthetic polymers that formed hybrid networks with mineral particles [ 56 , 67 , 68 ]. While the PAA hydrogel remained fully hydrated and chemically stable when freely incubated and interactions in sand were largely limited to surface compaction and transient crosslinking, the incubation in loam promoted the formation of an even more consolidated and complex structures. This difference was directly reflected in the SI, NMR, rheometry, and FTIR, which all indicated a stronger confinement and a more pronounced interfacial structuring of the polymer network. The low and constant SI revealed a restricted swelling and water release directly from the beginning of the incubation, attributed to both the fine-grained mineral particles closely enveloped the hydrogel, limiting its physical expansion, and a high concentration of Ca 2+ and organic ligands that diffused into the interfacial zone [ 20 , 26 , 38 , 42 , 69 ]. Additionally, fine material such as silt and clay particles can be absorbed and transported into the inner regions of the hydrogel, where they interacted directly with polymer chains through ionic crosslinking and physical entanglement [ 70 ]. For instance, [ 71 ] and [ 70 ] reported that mineral particle migration and their penetration into hydrogel matrices enhanced mechanical heterogeneity and reduced overall elasticity, reflected by our ESEM images and rheometry measurements in terms of a dense and particle-coated interface with compacted and solid core structures and overall low γ YP , respectively. Thus, the highly condensed, crosslinked hydrogel structures that extended from the interface into the interior produced a markedly rigid, solid-like architecture, with the loam environment—more than sand—driving this deep compaction and polymer reorganization. In this regard, the FTIR spectra displayed pronounced decreases in carbonyl/carboxyl (-12.5%) and CH 2 (-14.3%) absorbance, together with strong and persistent SiO/Al-O-Si bands, reflecting a progressive substitution of polymer-water by polymer-mineral interactions, further strengthening its structure and immobilizing interfacial water molecules as it has already been shown for chitosan-silica and PAA-bentonite nanocomposite [ 43 , 72 ]. This also aligns with recent conceptual models suggesting that hydrogel-soil systems evolve into semi-permanent hybrid domains through progressive polymer-mineral coupling and water immobilization [ 21 , 56 ]. In line with previous findings for PAA incubated in sand, where freezing-thawing cycles showed only marginal effects due to surface compaction and partial water immobilization, the influence of the loam matrix was even more pronounced. Its higher mineral and Ca²⁺ content, together with the finer texture and stronger polymer-mineral interactions, further reduced the fraction of mobile water. As a result, freezing-thawing cycles caused no measurable changes in SI, τ YP , or FTIR spectra, indicating that the loam-incubated hydrogel had already attained a structurally confined state with relatively low amounts of freezable water. Also, PCA visualization confirmed the pronounced matrix effect in loam, with PAA samples clustering at the negative Dim1 and positive Dim2 side, far from the dH 2 O and sand samples. This separation reflected the lowest SI, shortest T 2 values, and highest τ YP and τ max , all indicating strong confinement and interfacial compaction (ANOVA: matrix p 0.75). The combination of these chemical and mechanical indicators revealed that loam not only induced external densification but also promoted internal reorganization of the polymer network toward consolidated, solid-like organomineral structures. Although the present study provides valuable insights into PAA behavior under repeated freezing-thawing conditions, several limitations should be acknowledged. First, The ten freezing-thawing cycles applied represent only a short-term scenario and may not reflect long-term field dynamics. Therefore, future studies should extend both the number and intensity of cycles to simulate realistic seasonal variability. Second, the performed bulk-scale analyses of the PAA hydrogel limit insights into microscale dynamics, which might be overcome by advanced techniques such as cryo-scanning electron microscopy, differential scanning calorimetry (DSC), and neutron scattering to resolve ice formation and hydration mechanisms in more detail [ 73 – 75 ]. Third, other field-relevant factors such as application types and field cultivation techniques (e.g., tillage) were not investigated but may significantly affect the fate and behavior of PAA when utilized as soil conditioner [ 5 ]. Finally, field-scale validation under natural soil temperature and moisture conditions is essential to confirm the laboratory findings. However, from an ecological perspective, [ 21 ] and [ 76 ] reported that SAP-treated soils show improved frost resistance, particularly under repeated freezing-thawing conditions, underlining the relevance of targeted in-situ studies further investigating the soil matrix and the soil-polymer system. Concerning the recently discussed potential of synthetic SAPs to form plastic-like solid residues in soil, the findings of this study provide mechanistic evidence for how such residues may originate: the gradual densification, surface crosslinking, and water loss observed for PAA in soil indicate the early stages of solid residue formation as already discussed in literature [ 5 , 13 ]. Although freezing-thawing may locally enhance dehydration and bonding, its overall contribution appears minor. Dominant processes seem rather soil-polymer interactions, particularly crosslinking and mechanical confinement, rather than temperature-induced effects. Once the hydrogel loses flexibility and becomes incorporated into the organomineral structure, it may persist as a non-biodegradable fragment, potentially contributing to plastic-like accumulation and altering soil structure and hydraulic behavior. Consequently, understanding how soil chemistry, confinement, and climate interact to shape the long-term fate of SAPs will be critical for predicting their persistence and ecological impact. 5 Conclusion This study provides first mechanistic insights on how polyacrylic acid (PAA) hydrogels respond to repeated freezing-thawing cycles under varying environmental and soil conditions. Across all treatments, the findings consistently show that soil-specific physical and chemical interactions, rather than temperature fluctuations, govern the evolution of structural and physicochemical properties of PAA when incorporated in soil. When freely incubated in demineralized water, the hydrogel remained fully hydrated, flexible, and chemically stable independent of temperature changes. The absence of ionic coordination and external confinement of the soil matrix preserved the polymer network, allowing reversible hydrogen-bond rearrangements that prevented compaction and maintained high water mobility. In sand, however, mechanical confinement and weak cation-mediated crosslinking introduced clear interfacial structuring as PAA hydrogel developed a thin organomineral shell at its outer surface via transient bridges between carboxylate groups and multivalent cations such as Ca 2+ and Al 3+ . This shell increased local stiffness and limited swelling, whereas the inner core stayed largely hydrated and elastic, allowing the system to buffer freezing-thawing stresses without structural collapse. In loamy soil, finer particles and higher concentrations of reactive cations and organic ligands enhanced these processes, driving deeper ion penetration and more extensive crosslinking within the hydrogel network. As a result, the organomineral shell thickened and gradually extended inward, transforming the hydrogel into a compact, solid-like composite with immobilized water fractions and cryostable mechanical properties. Taken together, these results reveal a progressive transformation pathway, from freely swollen and fully hydrated networks to increasingly confined and mineral-integrated composites, governed primarily by soil matrix-induced mechanical compression and ionic coordination. The observed shell formation, densification, and water immobilization provide a mechanistic basis for understanding how synthetic hydrogels may gradually evolve into persistent, plastic-like residues once embedded in soil matrices. These insights underline the need to consider soil-polymer interactions as a dominant driver of both hydrogel performance and long-term persistence in terrestrial environments. Declarations Competing interests The authors declare no competing interests. Clinical trial number Not applicable. Ethics, Consent to Participate, and Consent to Publish declarations Not applicable Acknowledgements We kindly thank Gabriele E. Schaumann for her feedback on the results. Data Availability: The data that supports the findings of this study are available from the corresponding author upon reasonable request. Author contribution Conceptualization: J.N. and C.B.; Methodology and experimental setup: J.N. and C.B.; Material preparation and data collection: L.N., J.N., and C.B.; Data evaluation and interpretation: C.B., L.N., and J.N.; Writing-review and editing: C.B., J.N.; Funding acquisition: C.B.; Project management: C.B.; Supervision: C.B. Funding This research was financially supported by the Deutsche Forschungsgemeinschaft (Grant No. BU 3763/1-1) References Xing Y, Wang X. Precision Agriculture and Water Conservation Strategies for Sustainable Crop Production in Arid Regions. Plants. 2024;13:3184. Naorem A, et al. Soil Constraints in an Arid Environment—Challenges, Prospects, and Implications. Agronomy. 2023;13:220. Fragomeli R, Annunziata A, Punzo G. Promoting the Transition towards Agriculture 4.0: A Systematic Literature Review on Drivers and Barriers. Sustainability. 2024;16:2425. Takahashi M, Kosaka I, Ohta S. 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Supplementary Files SITables.pdf Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 20 Apr, 2026 Reviews received at journal 10 Apr, 2026 Reviewers agreed at journal 26 Mar, 2026 Reviews received at journal 06 Mar, 2026 Reviewers agreed at journal 26 Feb, 2026 Reviewers invited by journal 24 Feb, 2026 Editor invited by journal 24 Feb, 2026 Editor assigned by journal 17 Feb, 2026 Submission checks completed at journal 17 Feb, 2026 First submitted to journal 11 Feb, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8848966","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":596356244,"identity":"0219b075-4f45-4849-b85e-6a268a1f4252","order_by":0,"name":"Christian Buchmann","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAx0lEQVRIiWNgGAWjYFACHgYGxgYQg/kAROAA8VrYEhgYEkjTwmNAnBb+Bt5jEj93bJM3l+759rnwx2EGvuMN+LVIHOBLk+w9c9tw55yzm2fPSDjMIHmGgDUGDDxm0oxttxk33MjdzMwD1GJwI4E4LfYbbuQ8hmi5/4A4LYlALcxQW/DrYJA4zJds2dt2O3nDjTRj5hlp6TySZwg4jL+99+CNn223bTfcSH7MXGBjLcd3/AABa5jR2DwE1OPRPgpGwSgYBaMADgAOmURLrXPKTQAAAABJRU5ErkJggg==","orcid":"","institution":"IES Landau, RPTU University Kaiserslautern-Landau","correspondingAuthor":true,"prefix":"","firstName":"Christian","middleName":"","lastName":"Buchmann","suffix":""},{"id":596356246,"identity":"eea27ac8-9e9c-4522-8146-50d63c2d2c88","order_by":1,"name":"Lars Neumann","email":"","orcid":"","institution":"IES Landau, RPTU University Kaiserslautern-Landau","correspondingAuthor":false,"prefix":"","firstName":"Lars","middleName":"","lastName":"Neumann","suffix":""},{"id":596356247,"identity":"786c8cb3-174b-4748-8948-20f192453776","order_by":2,"name":"Janina Neff","email":"","orcid":"","institution":"IES Landau, RPTU University Kaiserslautern-Landau","correspondingAuthor":false,"prefix":"","firstName":"Janina","middleName":"","lastName":"Neff","suffix":""}],"badges":[],"createdAt":"2026-02-11 08:23:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8848966/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8848966/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103590647,"identity":"06b38c5e-ad97-4280-bd24-dd484a0f462d","added_by":"auto","created_at":"2026-02-27 12:06:49","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":34288,"visible":true,"origin":"","legend":"\u003cp\u003eSwelling index (SI) for polyacrylic acid (PAA) swollen in either dH\u003csub\u003e2\u003c/sub\u003eO, sand, and loam at different measurement points (0, 1, 3, 5, and 10) for the static (triangle) and cyclic freezing-thawing (circle) samples, respectively. Error bars represent standard error of the arithmetic means (n = 3) and are not shown when smaller than the symbol size. Three-way ANOVA showed a significant matrix effect on SI (p \u0026lt; 0.001), treatment and MP were not significant (p \u0026gt; 0.5). A significant Treatment × MP interaction (p = 0.001) and a moderate three-way effect (η\u003csup\u003e2\u003c/sup\u003e = 0.11) indicate measurable temporal differences between regimes. Matrix accounted for the largest share of explained variance (η\u003csup\u003e2\u003c/sup\u003e = 0.45), with additional moderate contribution from the Treatment × MP interaction (η\u003csup\u003e2\u003c/sup\u003e = 0.16).\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8848966/v1/366668643bfcb79f93d7152b.png"},{"id":104398705,"identity":"db0ed92a-6435-44b8-b8f8-363f618575d2","added_by":"auto","created_at":"2026-03-11 12:03:20","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":134784,"visible":true,"origin":"","legend":"\u003cp\u003eMean transverse relaxation time distribution (RTD) of polyacrylic acid (PAA) swollen in either dH\u003csub\u003e2\u003c/sub\u003eO, sand, and loam at different measurement points MP (0, 1, 3, 5, and 10) for the static (A-C) and cyclic freezing-thawing samples (D-F), respectively.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8848966/v1/6742392eef5012f6d0d2ed5a.png"},{"id":104398339,"identity":"95acf465-267e-4320-bd9e-cff417bc811f","added_by":"auto","created_at":"2026-03-11 12:01:49","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":49889,"visible":true,"origin":"","legend":"\u003cp\u003eMean transverse relaxation times\u003cstrong\u003e \u003c/strong\u003eT\u003csub\u003e2WL\u003c/sub\u003e (A) and T\u003csub\u003e2peak\u003c/sub\u003e (B) for polyacrylic acid (PAA) swollen in either dH\u003csub\u003e2\u003c/sub\u003eO, sand, and loam at different measurement points MP (0, 1, 3, 5, and 10) for the static (triangle) and cyclic freezing-thawing (circle) samples, respectively. Error bars represent standard error of the arithmetic means (n = 3) and are not shown when smaller than the symbol size. Three-way ANOVA showed a significant matrix effect (p \u0026lt; 0.001), whereas treatment, MP, and all interaction terms were not significant (p \u0026gt; 0.19). Post-hoc test showed no significant pairwise differences; therefore, no statistical group indicators are shown. Moderate effect sizes for Treatment × MP and Treatment × Matrix × MP (η\u003csup\u003e2\u003c/sup\u003e\u003csup\u003e\u003cem\u003e \u003c/em\u003e\u003c/sup\u003e= 0.13-0.30) indicate non-significant but measurable interaction trends.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8848966/v1/5c5e6694fa5de159be4e2e54.png"},{"id":103590651,"identity":"37d8dc29-5dba-48e5-85d7-3db2def77e29","added_by":"auto","created_at":"2026-02-27 12:06:49","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":91455,"visible":true,"origin":"","legend":"\u003cp\u003e(A)\u003cstrong\u003e \u003c/strong\u003eMaximum shear stress (τ\u003csub\u003emax\u003c/sub\u003e)\u003csub\u003e,\u003c/sub\u003e (B)\u003csub\u003e \u003c/sub\u003eshear stress at the yield point\u003cstrong\u003e \u003c/strong\u003e(τ\u003csub\u003eYP\u003c/sub\u003e), and (C) strain at the yield point (γ\u003csub\u003eYP\u003c/sub\u003e) for polyacrylic acid (PAA) swollen in either dH\u003csub\u003e2\u003c/sub\u003eO, sand, and loam at different measurement points MP (0, 1, 3, 5, and 10) for the static (triangle) and cyclic freezing-thawing (circle) samples, respectively. Error bars represent standard error of the arithmetic means (n = 3) and are not shown when smaller than the symbol size. Three-way ANOVA revealed the matrix as the only significant factor (p \u0026lt; 0.001), while regime, MP, and all interaction terms were non-significant (p \u0026gt; 0.20). Post-hoc test showed no significant pairwise differences; therefore, no statistical group indicators are shown. Effect size analysis showed that matrix explained nearly all variance (η\u003csup\u003e2\u003c/sup\u003e = 0.96), whereas all other effects were minimal (η\u003csup\u003e2 \u003c/sup\u003e\u0026lt; 0.03).\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8848966/v1/7bb35b3777de05c2d9a238ac.png"},{"id":103590654,"identity":"b985ef12-7b52-46cf-b13f-f30aec2e567a","added_by":"auto","created_at":"2026-02-27 12:06:49","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":148251,"visible":true,"origin":"","legend":"\u003cp\u003eNormalized ATR‑FTIR absorbance spectra of polyacrylic acid (PAA) swollen in dH\u003csub\u003e2\u003c/sub\u003eO and incubated freely (A), in sand (B), and in loam (C) at different measurement points MP (0, 1, 3, 5, and 10) under static (black/gray) and cyclic freezing-thawing (blue) conditions. Dashed vertical lines indicate the evaluated regions. Three‑way ANOVA revealed significant matrix effects for all bands (p \u0026lt; 0.001), while treatment, MP, and interaction terms showed weaker or non‑significant effects (p \u0026gt; 0.05). Effect sizes indicated matrix as the dominant factor (η\u003csup\u003e2\u003c/sup\u003e = 0.45-0.61).\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8848966/v1/b092ab4b6ba28e58cc62b099.png"},{"id":105033531,"identity":"7008bea9-773c-4bd3-b940-3b104d8248b9","added_by":"auto","created_at":"2026-03-20 07:19:23","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":96801,"visible":true,"origin":"","legend":"\u003cp\u003eNormalized ATR‑FTIR absorbance of polyacrylic acid (PAA) swollen in dH\u003csub\u003e2\u003c/sub\u003eO and incubated freely, in sand, and in loam for the CH (A), CH\u003csub\u003e2\u003c/sub\u003e (B), carbonyl/carboxyl (C), and SiO/Al-O-Si (D) regions at different measurement points MP (0, 1, 3, 5, and 10) under static (triangles) and cyclic freezing-thawing (circles) conditions. Error bars represent the standard error of the arithmetic means (n = 3) are not shown when smaller than the symbol size. Three‑way ANOVA indicated a significant matrix effect for all evaluated regions (p \u0026lt; 0.001), while treatment, MP, and interaction terms showed weaker or non‑significant effects (p \u0026gt; 0.05). Effect size analysis confirmed matrix as the dominant factor (η\u003csup\u003e2\u003c/sup\u003e = 0.45-0.61).\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8848966/v1/2309b410626760e793c4d80d.png"},{"id":104399003,"identity":"04d1aa13-b631-449f-adc8-941cd2cd5fde","added_by":"auto","created_at":"2026-03-11 12:04:28","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":810704,"visible":true,"origin":"","legend":"\u003cp\u003eESEM pictures of swollen and freeze-dried hydrogel networks of polyacrylic acid (PAA) swollen in either dH\u003csub\u003e2\u003c/sub\u003eO, sand, and loam at selected measurement points MP (0, 1, and 10) for the static and cyclic freezing-thawing samples, respectively. Arrows indicate key microstructural features such as condensed gel structures, fracture edges, pore-like cavities, and organo-mineral interface zones where a compacted shell has formed. Insets in the top-left corners display representative macroscopic views of selected PAA hydrogels (shown exemplarily for MP0 and MP10 under cyclic conditions) after removal from the respective soils.\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8848966/v1/550b878ccd31c116eb50b98a.jpeg"},{"id":103590655,"identity":"e5d7f9d2-f821-444a-9329-4440619b8869","added_by":"auto","created_at":"2026-02-27 12:06:49","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":407598,"visible":true,"origin":"","legend":"\u003cp\u003ePrincipal component analysis (PCA) of PAA hydrogel properties under static (left, A/C/E) and cyclic (right, B/D/F) conditions. (A-B) Variable contributions (colored vectors) to the first two principal components.\u003cbr\u003e\n(C-D) Individual factor maps grouped by matrix (dH\u003csub\u003e2\u003c/sub\u003eO, sand, loam) with 95% confidence ellipses.\u003cbr\u003e\n(E-F) Data grouped by measurement points MP (80-10), shaded from white to black.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8848966/v1/da1b45eb46d77bfa4b067454.png"},{"id":105036449,"identity":"81a46472-e3bd-4986-9692-0b93d71d98c5","added_by":"auto","created_at":"2026-03-20 07:33:06","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2447888,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8848966/v1/2da76931-36a5-431d-8ad6-df7780b09751.pdf"},{"id":103590649,"identity":"d2a7b2a5-d849-4f73-9234-3f5fe424db65","added_by":"auto","created_at":"2026-02-27 12:06:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":218376,"visible":true,"origin":"","legend":"","description":"","filename":"SITables.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8848966/v1/dab09b37be22168e6aa7fca1.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"The impact of cyclic freezing-thawing on the physicochemical properties of superabsorbent polyacrylic acid (PAA) hydrogel used as a soil conditioner","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eThe availability and retention of soil moisture is one of the most important and simultaneously limiting factors for plant productivity in many agricultural systems, especially in regions where irregular rainfall patterns, increasing evaporation and climatic extremes exacerbate water stress [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Improving soil water retention is therefore central to sustainable agricultural production and climate-adaptation strategies under increasingly variable environmental conditions - core priorities in the transition toward more resilient and resource-efficient agri-food systems [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. In this context, superabsorbent polymers (SAPs) have received significant attention as soil amendments capable of improving water retention, protecting plants against short-term drought, and reducing irrigation demand [\u003cspan additionalcitationids=\"CR5 CR6\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Their proposed benefits align with broader sustainability goals by potentially lowering water use, supporting crop productivity under climatic stress, and enabling more efficient input management [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eBesides polyacrylamide (PAM), polyacrylic acid (PAA) is one of the most studied synthetic SAPs and is widely used in modern agriculture, particularly in arid and semi-arid regions, as its high water-absorption capacity can improve water-use efficiency and support crop yields under drought [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. When incorporated into soil or used as seed coatings and nutrient carriers in fertilizers, SAPs not only enhance soil moisture retention but also reduce nutrient leaching, increase fertilizer efficiency, and promote robust seedling establishment, leading to yield increases of up to 15% in cereals and even higher in legumes and other crops [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. These multifunctional application possibilities make SAPs a promising tool for sustainable agricultural transitions, yet their long-term behavior, persistence, and potential environmental trade-offs remain insufficiently understood [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Beyond agriculture, SAPs are increasingly utilized in construction materials, e.g., to improve concrete workability and durability, in geoengineering for soil stabilization and erosion control, and in environmental remediation for wastewater treatment and pollution mitigation [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Their widespread use therefore highlights the need for further mechanistic understanding of SAP behavior and transformation as striking challenges seem to restrict their sustainable use in agricultural systems, including limited biodegradability, cost-effectiveness, and inconsistent field performance [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWhen SAPs are incorporated into soil, they can progressively transform in response to soil physiochemistry and environmental stressors, altering their structure, water-holding capacity, and functional integrity, with significant implications for agronomic performance and the formation of persistent, plastic-like residues [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Recent studies indicate that soil matrix properties and external stress can gradually convert interparticulate hydrogel structures into increasingly condensed, rigid, particle-like residues, yet the underlying mechanistic processes remain poorly understood [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. On the one hand, soil texture, ionic strength, pH, SOM and clay content, and soil solution composition strongly influence SAP swelling, network organization, and mechanical stability [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. For example, high cation concentrations crosslink and thus compact the polymer chains, while SOM and mineral particles can bind to the hydrogel, further reducing swelling and accessibility [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. On the other hand, SAPs continuously experience environmental dynamics in parallel, including moisture fluctuations, mechanical stress from roots or compaction, freezing-thawing, and UV exposure, which can further modify their physicochemical properties [\u003cspan additionalcitationids=\"CR21 CR22\" citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. In this regard, repeated drying-rewetting has been shown to induce surface cracking, densification, and swelling hysteresis due to cyclic shrinking and expansion of the hydrogel network [\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAlthough some environmental factors have been shown to alter the physicochemical properties of SAPs in soil, freezing-thawing cycles remain one of the least explored [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Although frost has been a minor issue in the regions where SAPs are typically applied, its importance grows as their use expands into temperate regions facing climate change-induced drought and erratic rainfall [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. In those regions, freezing-thawing dynamics are widespread and expected to become increasingly irregular as winter temperatures fluctuate and snow cover declines, further emphasizing the need to understand SAP behavior under such conditions. [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Here, residual SAPs in soil could be inevitably exposed to recurring freezing-thawing dynamics during the winter months, which might play a crucial role in shaping their transformation pathways and physicochemical properties in terms of (re)swelling performance, water binding, structural morphology, as well as their contribution to plastic pollution. Thus, understanding how SAPs respond to freezing-thawing cycling is essential for evaluating both their performance as soil amendments and their potential to form persistent, solid-like residues in agricultural soils.\u003c/p\u003e \u003cp\u003eTo address these knowledge gaps, this study examines the physicochemical and structural evolution of PAA hydrogel incorporated into contrasting soil matrices (sand and loam) under static and cyclic freezing-thawing conditions. For this, a multi-method approach was applied to quantify changes in swelling capacity (swelling index - SI), water entrapment (\u003csup\u003e1\u003c/sup\u003eH proton nuclear magnetic resonance (NMR) relaxometry), mechanical stability (rheometry), chemical structure (attenuated total reflectance fourier-transform infrared (ATR-FTIR) spectroscopy), and morphology (environmental scanning electron microscopy - ESEM). Together, these complementary techniques allow a comprehensive assessment of PAA transformation processes in realistic soil environments.\u003c/p\u003e \u003cp\u003eWhen freely swollen in demineralized water (dH\u003csub\u003e2\u003c/sub\u003eO), PAA should show the highest SI and water mobility because no mineral surfaces, ions, or structural constraints restrict network expansion. Under static incubation, only minor physical ageing is expected, reflected in largely stable SI, relaxation times, rheological responses, FTIR spectra, and regular morphology. Even under cyclic freezing-thawing, only slight ageing should occur, as the low crosslinking density and high flexibility allow the hydrogel to accommodate freezing-induced water expansion through chain reorganization, preventing strong internal stresses. Freezing should therefore cause only limited compaction with modest narrowing of polymer segments and small pore-like cavities. Accordingly, SI, relaxation times, and deformability should decline only marginally, and FTIR spectra should only slightly shift in the carbonyl/carboxyl region due to minimal deprotonation, while no new bands and no substantial intensity changes in other band regions are expected. Furthermore, no mineral-associated SiO and Al-O-Si bands should be present, reflecting the lack of mineral interactions.\u003c/p\u003e \u003cp\u003eWhen incubated in sand, swelling and water mobility should be reduced by mechanical confinement and moderate ion-induced crosslinking. SI should therefore be lower than in dH\u003csub\u003e2\u003c/sub\u003eO and decrease further under cyclic freezing-thawing, as repeated ice formation promotes network densification with compacted lamellae, fracture edges, and localized collapse. Consequently, NMR relaxation times should be shortened, with higher τ\u003csub\u003emax\u003c/sub\u003e, τ\u003csub\u003eYP\u003c/sub\u003e, and reduced γ\u003csub\u003eYP\u003c/sub\u003e indicating more solid-like behavior. FTIR ratios should remain comparatively stable, with only moderate intensity increases and band broadening in the carbonyl/carboxyl region due to partial cation coordination at the PAA hydrogel-sand interface. Further, due to the interaction of PAA with fine quartz particles at the interface, we assume an increasing SiO absorption, while Al-O-Si signals should remain negligible. Thus, intensity shifts should primarily reflect moderate interface-related coordination rather than structural modification of the overall hydrogel network.\u003c/p\u003e \u003cp\u003eIn loam, the strongest PAA densification and crosslinking are expected due to higher ionic strength, OM, and clay content. SI should thus be lowest and remain greatly reduced, reflecting permanently bound and confined water. This should yield the highest τ\u003csub\u003emax\u003c/sub\u003e and τ\u003csub\u003eYP\u003c/sub\u003e and the lowest γ\u003csub\u003eYP\u003c/sub\u003e, indicating pronounced mechanical reinforcement. FTIR spectra should reflect the strongest matrix-induced changes, with pronounced intensity increases and broadening of the carbonyl/carboxyl region and systematic shifts in the carbonyl/carboxyl region caused by extensive cation coordination, partial deprotonation, and OM-related complexation. The CH\u003csub\u003e2\u003c/sub\u003e region should intensify through sorbed SOM, while strong SiO and Al-O-Si signals are expected from incorporated clay and silt particles, reflecting the formation of a dense, multi-layered organomineral shell and deep integration of mineral fines into the hydrogel network. ESEM should reveal dense composite aggregates, collapsed hydrogel structures, and thick polymer-mineral coatings.\u003c/p\u003e \u003cp\u003eHowever, freezing-thawing cycles should have a comparatively minor effect on loam-incubated PAA because strong clay-OM-polymer interactions and cation-mediated crosslinking impose high confining pressure and strongly suppress ice formation. SI should therefore decrease only moderately, and changes in NMR relaxation times and rheological parameters should remain less pronounced than in dH\u003csub\u003e2\u003c/sub\u003eO or sand. Also, FTIR should retain the broadened carbonyl/carboxyl region, and strong SiO/Al-O-Si signals imposed by the loam matrix, with only minor freezing-thawing-related intensity shifts. The other band regions should remain unchanged, confirming that freezing-thawing effects are negligible compared to matrix-driven organomineral integration. ESEM should display compact, particle-rich structures with fewer freezing-thawing-related cracks and irregularities than in the corresponding dH\u003csub\u003e2\u003c/sub\u003eO and sand variants.\u003c/p\u003e"},{"header":"2 Materials and methods","content":"\u003cp\u003eIn this study, two well-characterized reference soils from the Agricultural Investigation and Research Institute (Lufa Speyer, Germany) were used. Both soils significantly differed in their texture and overall physicochemical properties (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFor the PAA hydrogels, hydrogel-forming PAA powder (Viscosity average molar mass M\u003csub\u003ev\u003c/sub\u003e = 4,000,000 g/mol) (Sigma-Aldrich, Germany; CAS 9003-01-4) was used and prepared according to [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] by allowing 200 mg dry powder to completely swell for 72 h on a pre-wetted dialysis membrane (flat width 44 mm, MWCO 14000, Carl Roth GmbH \u0026amp; Co. KG), which was in direct contact with a demineralized water (dH\u003csub\u003e2\u003c/sub\u003eO) reservoir. After 72 h, the swollen PAA hydrogel (~\u0026thinsp;15 g) was gently removed from the membrane and further used for the incubation experiment.\u003c/p\u003e \u003cp\u003eFor the incubation experiment, the PAA hydrogel was applied to the two reference soils at a medium depth using a point application method to ensure reproducible geometry and minimize edge effects. Each incubated sample contained 40 g soil with an initial bulk density of 1.47 g cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e for the sand and 1.18 g cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e for the loam. Both soils were preset to 40% of their WHC\u003csub\u003emax\u003c/sub\u003e prior to incubation, with moisture checked weekly and readjusted with dH₂O as needed. During the checks, the incubation vessels were briefly opened to allow for ventilation. In addition, pure PAA hydrogel was also investigated as soil-free control. In total, 81 samples were prepared with three replicates for each treatment.\u003c/p\u003e \u003cp\u003eAfter PAA incorporation, the soil samples were incubated under two thermal regimes: a static temperature regime at 10\u0026deg;C (S) and a cyclic temperature regime at 10\u0026deg;C with 10 additional weekly 24-hour freezing events at -10\u0026deg;C (C). Three days after PAA application (timepoint 0) and after the 1st, 3rd, 5th, and 10th incubation week (analogous to the freezing-thawing cycle), the respective PAA hydrogels were gently removed from the soil and examined for various physicochemical properties as described below. To improve clarity in the illustrations, we use the general term \u0026lsquo;measuring point\u0026rsquo; (MP) for both treatments: for the static incubation, MP\u003cem\u003en\u003c/em\u003e corresponds to incubation week \u003cem\u003en\u003c/em\u003e at constant temperature, whereas in the cyclic one, MP\u003cem\u003en\u003c/em\u003e corresponds to the \u003cem\u003en\u003c/em\u003e-th freezing-thawing cycle (e.g., MP3 denotes incubation week 3 for static (S3) and freezing-thawing cycle 3 (C3) for cyclic samples).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSelected physicochemical properties of the investigated soils used for the experiment and the respective 1:5 soil extracts\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eSoil name (acc. to Lufa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.4\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSoil type\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003esand\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eloam\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003eBulk soil\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eOrganic carbon [% C]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.55\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.83\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNitrogen [% N]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.06\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003epH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.50\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCEC [meq 100g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.90\u0026thinsp;\u0026plusmn;\u0026thinsp;0.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e17.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.80\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDensity [g cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.47\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.18\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eWHC\u003csub\u003emax\u003c/sub\u003e [g 100g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e32.50\u0026thinsp;\u0026plusmn;\u0026thinsp;1.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e44.60\u0026thinsp;\u0026plusmn;\u0026thinsp;2.20\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePSD (mm) [%]\u003c/p\u003e \u003cp\u003e\u003cem\u003e\u0026lt;\u0026thinsp;0.002\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e26.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.70\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003e0.002\u0026ndash;0.05\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.70\u0026thinsp;\u0026plusmn;\u0026thinsp;1.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e41.20\u0026thinsp;\u0026plusmn;\u0026thinsp;1.30\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003e0.05-2.0\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e86.20\u0026thinsp;\u0026plusmn;\u0026thinsp;0.70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e32.30\u0026thinsp;\u0026plusmn;\u0026thinsp;1.40\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"9\" rowspan=\"10\"\u003e \u003cp\u003eSoil extracts\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEC [\u0026micro;S cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e138\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e424\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003epH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e7.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAl\u003csup\u003e3+\u003c/sup\u003e [mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.04\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e [mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.29\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMn\u003csup\u003e2+\u003c/sup\u003e [mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.018\u0026thinsp;\u0026plusmn;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eZn\u003csup\u003e2+\u003c/sup\u003e [mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.06\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.007\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCa\u003csup\u003e2+\u003c/sup\u003e [mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e31.15\u0026thinsp;\u0026plusmn;\u0026thinsp;4.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e90.62\u0026thinsp;\u0026plusmn;\u0026thinsp;5.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMg\u003csup\u003e2+\u003c/sup\u003e [mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eK\u003csup\u003e+\u003c/sup\u003e [mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.29\u0026thinsp;\u0026plusmn;\u0026thinsp;0.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.92\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNa\u003csup\u003e+\u003c/sup\u003e [mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.62\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3.67\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eSwelling index\u003c/b\u003e \u003c/p\u003e \u003cp\u003eAfter the first free swelling (MP0), the different freezing-thawing cycles/incubation weeks, the swelling index (SI) of each PAA hydrogel was estimated according to [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] as the volume of absorbed water by the swollen PAA hydrogel (in ml) divided by its dry weight (in g), representing the overall ability of the hydrogel network to absorb water from its surroundings.\u003c/p\u003e \u003cp\u003e \u003csup\u003e \u003cb\u003e1\u003c/b\u003e \u003c/sup\u003e \u003cb\u003eH-NMR relaxometry\u003c/b\u003e \u003c/p\u003e \u003cp\u003eWater entrapment was investigated by \u003csup\u003e1\u003c/sup\u003eH-NMR relaxometry using a Bruker Minispec MQ (Bruker, Germany) at a magnetic field strength of 0.176T (corresponding proton Larmor frequency of 7.5 MHz) [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. The transverse relaxation (T\u003csub\u003e2\u003c/sub\u003e) decay curves were acquired at an echo time (T\u003csub\u003eE\u003c/sub\u003e) of 0.3 ms and further processed using MATLAB R2014a (MathWorks, United States), performing an inverse Laplace transformation (ILT) [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] and applying the Butler, Reeds, and Dawson (BRD) algorithm [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. The respectively obtained T\u003csub\u003e2\u003c/sub\u003e distributions (RTDs) were further evaluated according to [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] by calculating the longest relaxation time in the respective RTDs (T\u003csub\u003e2WL\u003c/sub\u003e) in terms of the 95th percentile of the sum of all amplitudes. Additionally, the peak positions (T\u003csub\u003e2peak\u003c/sub\u003e) within the RTDs were calculated as proxy for the predominant water fraction in the respective hydrogel sample. Thus, both longer T\u003csub\u003e2WL\u003c/sub\u003e and T\u003csub\u003e2peak\u003c/sub\u003e indicate less-restricted water populations, typically associated with larger pore domains and a more expanded hydrogel network, whereas shorter times indicate stronger confinement, e.g., due to condensed network structures, stronger water-polymer interactions, or smaller pore domains [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cb\u003eRheometry\u003c/b\u003e \u003c/p\u003e \u003cp\u003eRheological measurements were conducted according to [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] using a MCR 102 rheometer (Anton Paar, Germany) using a cone-plate measuring geometry. For the measurements, swollen SAP hydrogel was placed on the rheometer plate, rested for 60 s to ensure undisturbed measurements, and subjected to an amplitude sweep test (AST) at a constant frequency of 0.681 s⁻\u0026sup1; for a total of 37 measurement points to assess the respective viscoelasticity in terms of shear stress (τ) and shear rate (γ) at the yield point (YP) as well as the maximum shear stress (τ\u003csub\u003emax\u003c/sub\u003e). In this regard, τ\u003csub\u003emax\u003c/sub\u003e represents the maximum resistance of the sample before structural failure, with higher values indicating a stronger, more rigid structure [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Further, τ\u003csub\u003eYP\u003c/sub\u003e indicates the onset of irreversible structural deformation and the shift from elastic to viscous solid-like behavior. Finally, γ\u003csub\u003eYP\u003c/sub\u003e reflects sample deformability, where higher values indicate a more flexible structure and lower values a stiffer structure with limited deformation capacity.\u003c/p\u003e \u003cp\u003e \u003cb\u003eATR-FTIR\u003c/b\u003e \u003c/p\u003e \u003cp\u003eATR-FTIR measurements were performed on the freeze-dried hydrogels using a Cary 630 ATR-FTIR spectrometer (Agilent Technologies, United States). The resulting spectra were normalized and further analyzed using the open Specy R package [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eStriking PAA absorbance bands typically include CH (3,000\u0026ndash;2,800 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), C\u0026thinsp;=\u0026thinsp;O - carbonyl (1,870-1,550 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), COOR (1,800-1,715 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), COOH (1,730-1,710 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), COH (1,750-1,650 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), COO⁻ (1,620-1,550 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), and CH\u003csub\u003e2\u003c/sub\u003e (1,490-1,150 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) [\u003cspan additionalcitationids=\"CR36 CR37 CR38\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Because several carbonyl-related functional groups exhibited strong spectral overlaps and could not be reliably resolved in the solid-state ATR configuration, the individual C\u0026thinsp;=\u0026thinsp;O, COOR, COOH, COH, and COO⁻ bands were evaluated as one consolidated \u0026lsquo;carbonyl/carboxyl region\u0026rsquo;. Concerning sand- and loam-matrix-related absorbance bands, additional mineral-associated regions were evaluated in terms of Si-O/Al-O-Si (1030\u0026thinsp;\u0026minus;\u0026thinsp;960 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) band, which arise from silicate and aluminosilicate structures [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. In addition, shifts in band intensities (I) were quantitatively evaluated according to [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e] as the absorbance at the band-specific peak maximum above the baseline.\u003c/p\u003e \u003cp\u003eAll bands, together with their intensity changes, allow assessing cation coordination, partial deprotonation, and organomineral interactions at the hydrogel-soil interface. In particular, shifts, broadening, and intensity changes in the carbonyl/carboxyl region reflect cation-induced crosslinking and associated changes in protonation state, while increases or decreases in the CH\u003csub\u003e2\u003c/sub\u003e region indicate varying degrees of SOM sorption and local ordering or loosening of polymer side chains. The emergence and intensity shift of the SiO/Al-O-Si band are used to assess the physical incorporation of fine mineral particles and the formation of organomineral structures [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cb\u003eESEM\u003c/b\u003e \u003c/p\u003e \u003cp\u003eMorphological and structural features were exemplarily examined at MP0, MP1, and MP10 for all SAP hydrogels, both freely swollen in dH\u003csub\u003e2\u003c/sub\u003eO and incubated in the two soils using a Quanta 250 ESEM (FEI Company, United States) equipped with a backscattered electron detector (BSED). Prior to the measurements, all hydrogels were freeze-dried using an ALPHA 1\u0026ndash;2 LDplus freeze-dryer (Martin Christ, Germany) and coated with a 30 nm thick gold layer using a Q150R S sputter coater (Quorom Technologies Ltd, United Kingdom). All measurements were performed under high vacuum (\u0026lt;\u0026thinsp;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e Pa) at an acceleration voltage of 30kV and an average spot size of 3.5.\u003c/p\u003e \u003cp\u003e \u003cb\u003eStatistical analysis\u003c/b\u003e \u003c/p\u003e \u003cp\u003eStatistical analyses were performed with R (v. 4.3.1) using the \u003cem\u003epackages dplyr, car\u003c/em\u003e, and \u003cem\u003eDescTools.\u003c/em\u003e Variations within the three replicates were presented as standard errors (SE) of the arithmetic means. Prior to analysis, data were tested for normality and homogeneity of variances using Shapiro-Wilk and Levene\u0026rsquo;s tests. As all assumptions were met, no data transformation was required. ANOVA was conducted using type-III sums of squares via the Anova() function in the \u003cem\u003ecar\u003c/em\u003e package. When significant effects were detected (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), Tukey\u0026rsquo;s HSD test was applied for post-hoc comparisons. All analyses were based on three independent replicates per treatment. Fixed factors included treatment (static vs. cyclic), matrix (dH₂O, sand, loam), and measurement point (0\u0026ndash;10). A full-factorial three-way ANOVA (Treatment \u0026times; Matrix \u0026times; MP) was used to evaluate factor effects and interactions. To complement significance testing, effect sizes (η\u0026sup2;) were calculated as the proportion of variance explained by each factor. Given the replicate variability and the sample size, η\u003csup\u003e2\u003c/sup\u003e can help to identify the magnitude of treatment effects and interpreting incubation- and matrix-related differences in the investigated PAA properties, even where statistical significance was not given.\u003c/p\u003e \u003cp\u003eTo support interpretation of multivariate patterns, principal component analysis (PCA) was performed to reduce data complexity and visualize treatment-related clustering. PCA was conducted in R using the \u003cem\u003eFactoMineR\u003c/em\u003e package, with variables scaled to unit variance. Matrix and measurement point were included as supplementary variables. Visualizations were generated with \u003cem\u003efactoextra\u003c/em\u003e and \u003cem\u003eggplot2\u003c/em\u003e, using 95% confidence ellipses.\u003c/p\u003e \u003cp\u003eR version 4.3.1 (RStudio 2024.04.2) and Excel Office 16 were used to carry out all calculations and figures. All detailed statistical parameters, including degree of freedom, Sum of squares, R\u003csup\u003e2\u003c/sup\u003e, F-value and p-values are presented in the supplementary information (Tables A1-A10).\u003c/p\u003e"},{"header":"3 Results","content":"\u003cp\u003e \u003cb\u003eSwelling index\u003c/b\u003e \u003c/p\u003e \u003cp\u003eAt the beginning of the experiment (MP0), PAA showed the highest swelling in dH₂O with an SI\u003csub\u003edH₂O\u003c/sub\u003e of 77.07\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, followed by PAA incubated in the sand with SI\u003csub\u003esand\u003c/sub\u003e = 74.32\u0026thinsp;\u0026plusmn;\u0026thinsp;0.81 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and the loam matrix with SI\u003csub\u003eloam\u003c/sub\u003e = 67.04\u0026thinsp;\u0026plusmn;\u0026thinsp;0.73 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to an initial SI reduction of -4% in sand and \u0026minus;\u0026thinsp;13% in loam relative to dH\u003csub\u003e2\u003c/sub\u003eO (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eUnder static incubation, SI dynamics of PAA diverged between dH\u003csub\u003e2\u003c/sub\u003eO and the two soils. From MP0 to MP1, SI\u003csub\u003edH₂O\u003c/sub\u003e decreased only slightly by -5.4% to 72.91\u0026thinsp;\u0026plusmn;\u0026thinsp;2.13 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, whereas SI\u003csub\u003esand\u003c/sub\u003e and SI\u003csub\u003eloam\u003c/sub\u003e decreased more strongly to 42.23\u0026thinsp;\u0026plusmn;\u0026thinsp;5.64 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (-43.2%) and 53.17\u0026thinsp;\u0026plusmn;\u0026thinsp;4.68 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (-21.7%), respectively. After MP1, SI\u003csub\u003edH₂O\u003c/sub\u003e remained comparatively stable at 70.72\u0026thinsp;\u0026plusmn;\u0026thinsp;1.63 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at MP10 (-8.2% relative to MP0). However, SI\u003csub\u003esand\u003c/sub\u003e showed a pronounced re-swelling, from 42.23\u0026thinsp;\u0026plusmn;\u0026thinsp;5.64 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at MP1 to 61.88\u0026thinsp;\u0026plusmn;\u0026thinsp;4.49 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at MP10 (+\u0026thinsp;46.5% relative to MP1, -16.7% relative to MP0). Although SI\u003csub\u003eloam\u003c/sub\u003e increased to 58.23\u0026thinsp;\u0026plusmn;\u0026thinsp;2.18 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at MP5, it approached 51.64\u0026thinsp;\u0026plusmn;\u0026thinsp;3.62 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at MP10 (-2.9% relative to MP1 and \u0026minus;\u0026thinsp;24.0% relative to MP0).\u003c/p\u003e \u003cp\u003eCyclic freezing-thawing produced a similar initial response but slightly different long-term developments: from MP0 to MP1, SI\u003csub\u003edH₂O\u003c/sub\u003e again decreased modestly to 72.91\u0026thinsp;\u0026plusmn;\u0026thinsp;2.13 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (-5.4%), whereas SI\u003csub\u003esand\u003c/sub\u003e and SI\u003csub\u003eloam\u003c/sub\u003e dropped to 54.94\u0026thinsp;\u0026plusmn;\u0026thinsp;5.91 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (-26.1%) and 51.57\u0026thinsp;\u0026plusmn;\u0026thinsp;5.90 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (-24.1%), respectively. Over the subsequent freezing-thawing cycles, SI \u003csub\u003edH₂O\u003c/sub\u003e remained high and ended at 72.17\u0026thinsp;\u0026plusmn;\u0026thinsp;1.41 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at MP10 (-6.4% vs. MP0). SI\u003csub\u003esand\u003c/sub\u003e fluctuated between 55\u0026ndash;63 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and finally stabilized at 56.80\u0026thinsp;\u0026plusmn;\u0026thinsp;2.21 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at MP10 (-23.6% vs. MP0). In contrast, SI\u003csub\u003eloam\u003c/sub\u003e progressively increased after MP1 and reached 59.24\u0026thinsp;\u0026plusmn;\u0026thinsp;8.52 ml g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at MP10 (-12.8% vs. MP0).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAccording to the three-way ANOVA, matrix had a highly significant effect on SI (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), whereas treatment (static vs. cyclic) and MP alone were not significant (p\u0026thinsp;=\u0026thinsp;1.000). The interaction between treatment and measurement point was significant (p\u0026thinsp;=\u0026thinsp;0.001), while the three-way Treatment \u0026times; Matrix \u0026times; MP interaction was not (p\u0026thinsp;=\u0026thinsp;0.182). Effect size analysis indicated that the matrix factor explained a large proportion of the variance (η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.45), with additional moderate contributions from the Treatment \u0026times; MP interaction (η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.16) and the three-way interaction (η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.11).\u003c/p\u003e \u003cp\u003e \u003csup\u003e \u003cb\u003e1\u003c/b\u003e \u003c/sup\u003e \u003cb\u003eH-NMR Relaxometry\u003c/b\u003e \u003c/p\u003e \u003cp\u003eAt MP0, PAA swollen in dH₂O exhibited a single, narrow unimodal relaxation time distribution (RTD) with T\u003csub\u003e2WL\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2494.51\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms and T\u003csub\u003e2peak\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2171.12\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn contrast, PAA swelling in both soils markedly broadened the RTDs with shifts towards shorter relaxation times. In sand, T\u003csub\u003e2WL\u003c/sub\u003e was 117.77\u0026thinsp;\u0026plusmn;\u0026thinsp;4.72 ms and T\u003csub\u003e2peak\u003c/sub\u003e 79.39\u0026thinsp;\u0026plusmn;\u0026thinsp;1.86 ms, corresponding to reductions of -95% and \u0026minus;\u0026thinsp;96% relative to PAA swollen dH\u003csub\u003e2\u003c/sub\u003eO. PAA incubated in loam showed a T\u003csub\u003e2WL\u003c/sub\u003e of 291.38\u0026thinsp;\u0026plusmn;\u0026thinsp;20.88 ms and a T\u003csub\u003e2peak\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;219.99\u0026thinsp;\u0026plusmn;\u0026thinsp;8.81 ms (-88% and \u0026minus;\u0026thinsp;90% compared to dH\u003csub\u003e2\u003c/sub\u003eO), with broad and multi-modal RTDs already at MP0.\u003c/p\u003e \u003cp\u003eDuring static incubation, PAA showed a pronounced RTD shift between MP0 and MP1 for all matrices: For dH\u003csub\u003e2\u003c/sub\u003eO, both T\u003csub\u003e2WL\u003c/sub\u003e and T\u003csub\u003e2peak\u003c/sub\u003e decreased about\u0026thinsp;\u0026minus;\u0026thinsp;7% to 2327.20\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms and 2025.50\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms, respectively, accompanied by an overall broadening of the RTD. Even stronger reductions were observed for PAA incubated in sand (T\u003csub\u003e2WL\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;70.71\u0026thinsp;\u0026plusmn;\u0026thinsp;1.62 ms, -40%; T\u003csub\u003e2peak\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;52.34\u0026thinsp;\u0026plusmn;\u0026thinsp;1.23 ms, -34%) and in loam (T\u003csub\u003e2WL\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;29.38\u0026thinsp;\u0026plusmn;\u0026thinsp;1.18 ms, -90%; T\u003csub\u003e2peak\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;20.27\u0026thinsp;\u0026plusmn;\u0026thinsp;0.46 ms, -91%). Also, the RTD shapes became broader but still dominated by long T₂ for dH\u003csub\u003e2\u003c/sub\u003eO, whereas the RTDs of PAA in sand and loam were dominated by shorter T\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAfter MP1 and until MP5, both T\u003csub\u003e2\u003c/sub\u003e indices re-increased for PAA swollen in dH\u003csub\u003e2\u003c/sub\u003eO to the initial values (T\u003csub\u003e2WL\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2494.51\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms; T\u003csub\u003e2peak\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2171.12\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms), stabilizing at slightly lower values at MP10 (T\u003csub\u003e2WL\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2382.97\u0026thinsp;\u0026plusmn;\u0026thinsp;55.77 ms; T\u003csub\u003e2peak\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2122.58\u0026thinsp;\u0026plusmn;\u0026thinsp;48.54 ms). A partial recovery was also observed for PAA incubated in sand: after the strong initial drop, T\u003csub\u003e2WL\u003c/sub\u003e increased again to 36.99\u0026thinsp;\u0026plusmn;\u0026thinsp;0.87 ms at MP5 (-69% compared to MP0) and 31.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms at MP10 (-73%), while T\u003csub\u003e2peak\u003c/sub\u003e re-increased slightly from MP1 to MP5 to finally 22.22\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms at MP10 (-72%). PAA incubated in loam showed also showed a re-increase of T\u003csub\u003e2WL\u003c/sub\u003e to 47.96\u0026thinsp;\u0026plusmn;\u0026thinsp;17.58 ms at MP5 (+\u0026thinsp;63%) but decreased again to 37.11\u0026thinsp;\u0026plusmn;\u0026thinsp;2.31 ms at MP10 (-87% compared to MP0). In contrast, T\u003csub\u003e2peak\u003c/sub\u003e remained consistently low at 20\u0026ndash;21 ms throughout the remaining incubation time.\u003c/p\u003e \u003cp\u003eRegarding cyclic freezing-thawing of PAA swollen in dH\u003csub\u003e2\u003c/sub\u003eO, both T\u003csub\u003e2WL\u003c/sub\u003e and T\u003csub\u003e2peak\u003c/sub\u003e remained at 2494.51\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms and 2171.12\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms at MP1, respectively, decreased at MP5 (T\u003csub\u003e2WL\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2099.69\u0026thinsp;\u0026plusmn;\u0026thinsp;227.51 ms; T\u003csub\u003e2peak\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1827.49\u0026thinsp;\u0026plusmn;\u0026thinsp;198.01 ms;, -16%), before re-increasing at MP10 to T\u003csub\u003e2WL\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;3145.73\u0026thinsp;\u0026plusmn;\u0026thinsp;73.62 ms and T\u003csub\u003e2peak\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2223.15\u0026thinsp;\u0026plusmn;\u0026thinsp;52.03 ms (+\u0026thinsp;26% compared to MP0).\u003c/p\u003e \u003cp\u003eFor PAA in sand, relaxation times decreased strongly from directly after the first freezing-thawing cycle with T\u003csub\u003e2WL\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;78.43\u0026thinsp;\u0026plusmn;\u0026thinsp;8.64 (-33%) and T\u003csub\u003e2peak\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;51.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 (-36%) at MP1. At MP5, T\u003csub\u003e2WL\u003c/sub\u003e and T\u003csub\u003e2peak\u003c/sub\u003e further decreased by -65% (T\u003csub\u003e2WL\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;41.57\u0026thinsp;\u0026plusmn;\u0026thinsp;1.67 and T\u003csub\u003e2peak\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;27.41\u0026thinsp;\u0026plusmn;\u0026thinsp;1.10 ms) and slightly re-increased at MP10 (T\u003csub\u003e2WL\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;61.54\u0026thinsp;\u0026plusmn;\u0026thinsp;1.41 ms; T\u003csub\u003e2peak\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;33.70\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms).\u003c/p\u003e \u003cp\u003ePAA incubated in loam exhibited the strongest and most persistent reduction of both T\u003csub\u003e2WL\u003c/sub\u003e and T\u003csub\u003e2peak\u003c/sub\u003e after the first freezing-thawing cycle: T\u003csub\u003e2WL\u003c/sub\u003e decreased to 78.22\u0026thinsp;\u0026plusmn;\u0026thinsp;13.73 ms at MP1 and further to 30.54\u0026thinsp;\u0026plusmn;\u0026thinsp;4.13 ms at MP5 but partially recovered to 72.13\u0026thinsp;\u0026plusmn;\u0026thinsp;22.75 ms at MP10. T\u003csub\u003e2peak\u003c/sub\u003e also decreased from 219.99\u0026thinsp;\u0026plusmn;\u0026thinsp;8.81 ms at MP0 to 24.39\u0026thinsp;\u0026plusmn;\u0026thinsp;0.57 ms at MP1 (-89%) and remained nearly constant until MP10 (24.30\u0026thinsp;\u0026plusmn;\u0026thinsp;3.57 ms). Accordingly, the RTDs under cyclic conditions showed a loss of relaxation signals at longer T₂ times. For PAA swollen in dH₂O, the primary peak shifted towards shorter relaxation times, whereas PAA incubated in sand and loam showed broad, predominantly left-shifted distributions dominated by short T\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eThree-way ANOVA revealed a highly significant effect of the matrix on both T\u003csub\u003e2WL\u003c/sub\u003e and T\u003csub\u003e2peak\u003c/sub\u003e (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), confirming the strong and persistent differences between dH₂O, sand, and loam. Neither treatment nor MP showed statistically significant effects, and no significant interactions were detected (all p\u0026thinsp;\u0026gt;\u0026thinsp;0.19). However, effect size analysis indicated moderate contributions of the Treatment \u0026times; MP interaction (η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.13\u0026ndash;0.15) and the Treatment \u0026times; Matrix \u0026times; MP interaction (η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.28\u0026ndash;0.30).\u003c/p\u003e \u003cp\u003e \u003cb\u003eRheometry\u003c/b\u003e \u003c/p\u003e \u003cp\u003eAfter initial swelling in dH\u003csub\u003e2\u003c/sub\u003eO (MP0), PAA exhibited the lowest maximum shear stress (τ\u003csub\u003emax\u003c/sub\u003e) with 258.85\u0026thinsp;\u0026plusmn;\u0026thinsp;27.15 Pa, whereas the incubation in soil caused a marked increase in mechanical resistance (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e): In sand, τ\u003csub\u003emax\u003c/sub\u003e increased to 406.05\u0026thinsp;\u0026plusmn;\u0026thinsp;24.59 Pa (+\u0026thinsp;57%), and in loam to 833.32\u0026thinsp;\u0026plusmn;\u0026thinsp;112.90 Pa (+\u0026thinsp;222% compared to dH\u003csub\u003e2\u003c/sub\u003eO at MP0). A similar pattern at MP0 was observed for the shear stress at the yield point (τ\u003csub\u003eYP\u003c/sub\u003e), which was 185.73\u0026thinsp;\u0026plusmn;\u0026thinsp;20.43 Pa for PAA swollen in dH₂O, 321.29\u0026thinsp;\u0026plusmn;\u0026thinsp;20.28 Pa for PAA incubated in sand (+\u0026thinsp;73%) and 724.98\u0026thinsp;\u0026plusmn;\u0026thinsp;55.35 Pa (+\u0026thinsp;290%) in loam. In parallel, the strain at the yield point (γ\u003csub\u003eYP\u003c/sub\u003e) decreased in both soils at MP0 compared to dH\u003csub\u003e2\u003c/sub\u003eO, from 7.20\u0026thinsp;\u0026plusmn;\u0026thinsp;0.26% in dH₂O to 2.71\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08% in sand (-62%) and to 1.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.38% in loam (-81%).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUnder static incubation, the rheological behavior of PAA differed between the matrices but showed a general moderate strengthening: In dH\u003csub\u003e2\u003c/sub\u003eO, τ\u003csub\u003emax\u003c/sub\u003e increased by totally\u0026thinsp;+\u0026thinsp;77% to 457.97\u0026thinsp;\u0026plusmn;\u0026thinsp;67.58 Pa at MP10, whereas τ\u003csub\u003eYP\u003c/sub\u003e decreased to 138.86\u0026thinsp;\u0026plusmn;\u0026thinsp;7.05 Pa (-25%). Also, γ\u003csub\u003eYP\u003c/sub\u003e decreased markedly from 7.20\u0026thinsp;\u0026plusmn;\u0026thinsp;0.26% at MP0 to 0.77\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10% at MP10 (-89%), indicating a progressive loss of deformability with time. For PAA incubated in sand, τ\u003csub\u003emax\u003c/sub\u003e increased to finally 1258.13\u0026thinsp;\u0026plusmn;\u0026thinsp;162.76 Pa at MP10 (+\u0026thinsp;210%), whereas τ\u003csub\u003eYP\u003c/sub\u003e increased to 1214.53\u0026thinsp;\u0026plusmn;\u0026thinsp;138.45 Pa (+\u0026thinsp;278%). In contrast, γ\u003csub\u003eYP\u003c/sub\u003e decreased finally to 2.97\u0026thinsp;\u0026plusmn;\u0026thinsp;0.20 at MP10. The loam matrix induced the strongest mechanical reinforcement under static conditions, with an increase of τ\u003csub\u003emax\u003c/sub\u003e to 2691.20\u0026thinsp;\u0026plusmn;\u0026thinsp;521.31 Pa and τ\u003csub\u003eYP\u003c/sub\u003e to 2344.38\u0026thinsp;\u0026plusmn;\u0026thinsp;669.61 Pa at MP10 (both +\u0026thinsp;223% compared to MP0). However, γ\u003csub\u003eYP\u003c/sub\u003e remained consistently low between 0.17\u0026ndash;0.26% over the incubation time.\u003c/p\u003e \u003cp\u003eDuring subsequent cyclic freezing-thawing, the microstructural stability of PAA increased steadily across all matrices: For PAA swollen in dH\u003csub\u003e2\u003c/sub\u003eO, both τ\u003csub\u003emax\u003c/sub\u003e and τ\u003csub\u003eYP\u003c/sub\u003e increased by +\u0026thinsp;57%, from 258.85\u0026thinsp;\u0026plusmn;\u0026thinsp;27.15 Pa at MP0 to 405.89\u0026thinsp;\u0026plusmn;\u0026thinsp;49.78 Pa at MP10, and from 185.73\u0026thinsp;\u0026plusmn;\u0026thinsp;20.43 to 291.61\u0026thinsp;\u0026plusmn;\u0026thinsp;43.67 Pa, respectively. In contrast, γ\u003csub\u003eYP\u003c/sub\u003e varied between 6.57\u0026thinsp;\u0026plusmn;\u0026thinsp;0.18 at MP1, 8.30\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09 at MP3, and 6.78\u0026thinsp;\u0026plusmn;\u0026thinsp;0.21 at MP5, before reaching 7.25\u0026thinsp;\u0026plusmn;\u0026thinsp;0.39 at MP10. For PAA incubated in sand, τ\u003csub\u003emax\u003c/sub\u003e and τ\u003csub\u003eYP\u003c/sub\u003e successively increased with freezing-thawing cycles to finally 941.99\u0026thinsp;\u0026plusmn;\u0026thinsp;96.59 Pa (+\u0026thinsp;132% compared to MP0) and 767.90\u0026thinsp;\u0026plusmn;\u0026thinsp;149.06 Pa (+\u0026thinsp;139% compared to MP0) at MP10, respectively. PAA incubated in loam revealed the most pronounced increases in microstructural stability: here, τ\u003csub\u003emax\u003c/sub\u003e increased from 833.32 112.90 Pa at MP0 to 2753.87\u0026thinsp;\u0026plusmn;\u0026thinsp;184.13 Pa at MP10 (+\u0026thinsp;230%). Also, τ\u003csub\u003eYP\u003c/sub\u003e increased from 724.98 55.35 Pa at MP0 to 1532.93\u0026thinsp;\u0026plusmn;\u0026thinsp;457.94 Pa (+\u0026thinsp;112%). Again, γ\u003csub\u003eYP\u003c/sub\u003e remained consistently low between 0.17\u0026ndash;0.26% throughout the freezing-thawing cycles.\u003c/p\u003e \u003cp\u003eThe three-way ANOVA revealed that only the matrix had a statistically significant effect on all rheological parameters (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001 for each variable). Neither treatment (static vs. cyclic) nor MP (0, 1, 3, 5, 10) showed significant main effects (all p\u0026thinsp;\u0026gt;\u0026thinsp;0.27), and no two- or three-way interactions were statistically significant (all p\u0026thinsp;\u0026gt;\u0026thinsp;0.20). Effect size analysis indicated that the matrix accounted for 94\u0026ndash;96% of the total variance (η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.94\u0026ndash;0.96) across all rheological parameters. All remaining main effects and interaction terms showed small effect sizes below 3% (η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.03).\u003c/p\u003e \u003cp\u003e \u003cb\u003eATR-FTIR\u003c/b\u003e \u003c/p\u003e \u003cp\u003eAt the beginning of the experiment (MP0), freely incubated PAA showed the highest absorbance (A) in both the carbonyl/carboxyl and CH\u003csub\u003e2\u003c/sub\u003e regions, while PAA incubated in sand and loam exhibited distinctly lower values (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Specifically, in the carbonyl/carboxyl region, A of freely incubated PAA reached 0.217\u0026thinsp;\u0026plusmn;\u0026thinsp;0.013, compared to 0.181\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007 in sand (-16.6%) and 0.072\u0026thinsp;\u0026plusmn;\u0026thinsp;0.006 in loam (-66.8%). For the CH\u003csub\u003e2\u003c/sub\u003e region, a similar trend was observed, with freely incubated PAA showing A\u0026thinsp;=\u0026thinsp;0.191\u0026thinsp;\u0026plusmn;\u0026thinsp;0.005, followed by sand (A\u0026thinsp;=\u0026thinsp;0.179\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007; -6.3%) and loam (A\u0026thinsp;=\u0026thinsp;0.126\u0026thinsp;\u0026plusmn;\u0026thinsp;0.006; -34.0%).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor the SiO/Al-O-Si region, A was lowest in freely incubated PAA (0.083\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002), increased in sand (0.100\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007; +20.5%), and was highest in loam (0.330\u0026thinsp;\u0026plusmn;\u0026thinsp;0.037; +297.5%). In contrast, the COC band was only evaluable for freely incubated PAA (A\u0026thinsp;=\u0026thinsp;0.159\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007).\u003c/p\u003e \u003cp\u003eUnder static incubation, A for freely incubated PAA remained relatively stable over time with only a slightly decrease for carbonyl/carboxyl region from MP0 (0.217\u0026thinsp;\u0026plusmn;\u0026thinsp;0.013) to MP1 (0.213\u0026thinsp;\u0026plusmn;\u0026thinsp;0.009) to finally 0.205\u0026thinsp;\u0026plusmn;\u0026thinsp;0.010 at MP10 (-5.5%). CH\u003csub\u003e2\u003c/sub\u003e absorbance also remained constant, from A\u0026thinsp;=\u0026thinsp;0.191\u0026thinsp;\u0026plusmn;\u0026thinsp;0.005 at MP0 to A\u0026thinsp;=\u0026thinsp;0.183\u0026thinsp;\u0026plusmn;\u0026thinsp;0.005 at MP10 (-4.2%). The SiO/Al-O-Si band was not detectable in freely incubated PAA, but the COC signal remained stable (A\u0026thinsp;=\u0026thinsp;0.160\u0026thinsp;\u0026plusmn;\u0026thinsp;0.004 at MP10 vs. 0.159\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007 at MP0).\u003c/p\u003e \u003cp\u003ePAA incubated in sand, A decreased more strongly over time. For the carbonyl/carboxyl band, A dropped from 0.181\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007 (MP0) to 0.151\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002 (MP1) and further to 0.129\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007 at MP10 (-28.7%). CH\u003csub\u003e2\u003c/sub\u003e absorbance remained constant, from A\u0026thinsp;=\u0026thinsp;0.179\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007 at MP0 to 0.180\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002 at MP10 (\u0026plusmn;\u0026thinsp;0.0%). Also, the SiO/Al-O-Si band remained stable at A\u0026thinsp;=\u0026thinsp;0.131\u0026thinsp;\u0026plusmn;\u0026thinsp;0.006 at MP10 (+\u0026thinsp;31.0% compared to MP0). For PAA incubated in loam, A for the carbonyl/carboxyl region decreased to 0.068\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002 from MP0 to MP1 and to finally 0.063\u0026thinsp;\u0026plusmn;\u0026thinsp;0.003 at MP10 (-12.5%).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe CH\u003csub\u003e2\u003c/sub\u003e region followed a similar pattern, dropping from A\u0026thinsp;=\u0026thinsp;0.126\u0026thinsp;\u0026plusmn;\u0026thinsp;0.006 (MP0) to A\u0026thinsp;=\u0026thinsp;0.111\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002 (MP1) and A\u0026thinsp;=\u0026thinsp;0.108\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002 at MP10 (-14.3%). Notably, the SiO/Al-O-Si signal remained high overall, with A\u0026thinsp;=\u0026thinsp;0.349\u0026thinsp;\u0026plusmn;\u0026thinsp;0.009 at MP10 (+\u0026thinsp;5.8% compared to MP0).Under cyclic freezing-thawing, A of freely incubated PAA remained nearly unchanged across most bands. The carbonyl/carboxyl region decreased marginally from 0.217\u0026thinsp;\u0026plusmn;\u0026thinsp;0.013 (MP0) to 0.199\u0026thinsp;\u0026plusmn;\u0026thinsp;0.005 at MP10 (-8.3%), while CH\u003csub\u003e2\u003c/sub\u003e increased slightly to finally 0.193\u0026thinsp;\u0026plusmn;\u0026thinsp;0.004 at MP10 (+\u0026thinsp;1.0%). Also, the SiO/Al-O-Si band remained not visible, and the COC band increased only slightly to 0.162\u0026thinsp;\u0026plusmn;\u0026thinsp;0.004 at MP10 (+\u0026thinsp;1.9%).\u003c/p\u003e \u003cp\u003eWhen PAA was incubated in sand, A of the carbonyl/carboxyl region decreased from 0.181\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007 at MP0 to 0.172\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002 at MP10 (-5.0%), whereas CH\u003csub\u003e2\u003c/sub\u003e remained nearly constant (from 0.179\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007 at MP0 to 0.178\u0026thinsp;\u0026plusmn;\u0026thinsp;0.003 at MP10; -0.6%). The SiO/Al-O-Si band slightly increased to 0.102\u0026thinsp;\u0026plusmn;\u0026thinsp;0.003 at MP10 (+\u0026thinsp;2.0%). For PAA incubated in loam, the carbonyl/carboxyl region decreased from A\u0026thinsp;=\u0026thinsp;0.072\u0026thinsp;\u0026plusmn;\u0026thinsp;0.006 at MP0 to 0.065\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002 (-9.7%) at MP10, whereas CH\u003csub\u003e2\u003c/sub\u003e decreased slightly to finally 0.123\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002 (-2.4%), and SiO/Al-O-Si to 0.315\u0026thinsp;\u0026plusmn;\u0026thinsp;0.012 (-4.5%).\u003c/p\u003e \u003cp\u003eOverall, freely incubated PAA exhibited all major absorbance regions, including pronounced carbonyl/carboxyl, CH\u003csub\u003e2\u003c/sub\u003e, and COC bands, with no detectable signals in the SiO/Al-O-Si region. In contrast, PAA incubated in sand showed a clear reduction in carbonyl/carboxyl band intensity, stable CH\u003csub\u003e2\u003c/sub\u003e absorbance, disappearance of the COC band, and moderately increased signals in the SiO/Al-O-Si region. For PAA incubated in loam, carbonyl/carboxyl and CH\u003csub\u003e2\u003c/sub\u003e bands markedly decreased, the COC band was no longer detectable, and high SiO/Al-O-Si absorbance dominated the spectrum. These matrix-specific patterns were further supported by the results of the three-way ANOVA, which confirmed that the matrix (dH\u003csub\u003e2\u003c/sub\u003eO, sand, loam) had a statistically significant effect on absorbance across all evaluated bands. For the carbonyl/carboxyl region, the matrix effect was highly significant (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) and explained a large share of the variance (η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.52). Similar strong effects were found for the CH\u003csub\u003e2\u003c/sub\u003e band (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001; η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.45) and SiO/Al-O-Si (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001; η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.61).\u003c/p\u003e \u003cp\u003eThe treatment (static vs. cyclic) showed smaller but relevant contributions, particularly for SiO/Al-O-Si (p\u0026thinsp;=\u0026thinsp;0.012; η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.12), followed by the carbonyl/carboxyl band (p\u0026thinsp;=\u0026thinsp;0.042; η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.08) and CH\u003csub\u003e2\u003c/sub\u003e (p\u0026thinsp;=\u0026thinsp;0.063; η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.09). No significant main effect was found for MP (p\u0026thinsp;\u0026gt;\u0026thinsp;0.1 for all bands).\u003c/p\u003e \u003cp\u003eFinally, a significant Matrix \u0026times; Treatment interaction was observed for the carbonyl/carboxyl band (p\u0026thinsp;=\u0026thinsp;0.038; η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.14) and for SiO/Al-O-Si (p\u0026thinsp;=\u0026thinsp;0.021; η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.11), suggesting matrix-specific responses of PAA hydrogels to the applied freezing-thawing cycles.\u003c/p\u003e \u003cp\u003e \u003cb\u003eESEM\u003c/b\u003e \u003c/p\u003e \u003cp\u003eAt MP0, pronounced structural differences were evident between PAA swollen in dH\u003csub\u003e2\u003c/sub\u003eO and incubated in sand and loam (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e with striking features indicated by arrows). In dH\u003csub\u003e2\u003c/sub\u003eO, PAA exhibited a uniform sheet-like morphology with thin, tightly layered lamellae without noticeable cavities or embedded particles. In contrast, PAA incubated in sand showed thicker, flake-like lamellae with localized compaction zones, angular fracture edges, and several small voids. In loam, PAA displayed the most heterogeneous network structure, consisting of coarse polymer-particle aggregates, extensive porous regions, and numerous mineral- and organic-rich inclusions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUnder static incubation, the matrix-specific differences observed at MP0 remained and further evolved: at MP1, PAA swollen in dH\u003csub\u003e2\u003c/sub\u003eO largely maintained its initial lamellar arrangement, showing only minor local tightening. When incubated in sand, the PAA network formed more compacted, interconnected flake assemblies with collapsed segments and distinct fracture lines. PAA in loam also developed bulky composite clusters with rounded pores, particle-filled cavities, and polymer sheets visibly wrapped around mineral soil constituents.\u003c/p\u003e \u003cp\u003eAt the end of the static incubation time (MP10), the lamellar structure of PAA swollen in dH\u003csub\u003e2\u003c/sub\u003eO still remained intact but exhibited greater variability in network spacing and densification areas. PAA incubated in sand showed substantially thickened, interlocked lamellae with angular voids and particle-stabilized cavities, whereby PAA incubated loam formed large, cohesive aggregates with multiple cavity openings, and distinct layered shells around embedded soil particles.\u003c/p\u003e \u003cp\u003eUnder cyclic freezing-thawing conditions, structural properties differed from those observed under static conditions: after the first freezing-thawing cycle (MP1), PAA swollen in dH\u003csub\u003e2\u003c/sub\u003eO showed localized disruptions within the lamellae and newly formed small cavities and sheet-like structures. In sand, the PAA network irregularly folded with strongly collapsed flake structures and several pronounced fracture edges. PAA incubated in loam showed rough, highly\u003c/p\u003e \u003cp\u003eparticle-permeated, compacted structures with numerous large cavities and layered edges surrounding soil mineral particles and aggregates.\u003c/p\u003e \u003cp\u003eAfter ten freezing-thawing cycles (MP10), PAA swollen in dH\u003csub\u003e2\u003c/sub\u003eO exhibited strongly contracted lamellar regions forming dense clusters with thin, folded edges. PAA incubated in sand revealed fragmented, thick lamellae arranged in irregular stacks with enclosed pores and angular breaks. Especially in loam, the PAA network was highly condensed, with large composite aggregates, crater-like cavities, and multiple discontinuities.\u003c/p\u003e \u003cp\u003e \u003cb\u003eRelationships between investigated parameters\u003c/b\u003e \u003c/p\u003e \u003cp\u003ePCA analysis of the static incubation showed that the first two principal components explained 79.1% of the total variance (Dim1\u0026thinsp;=\u0026thinsp;67.4%, Dim2\u0026thinsp;=\u0026thinsp;11.7%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eA, C, E). In detail, T\u003csub\u003e2peak\u003c/sub\u003e, T\u003csub\u003e2WL\u003c/sub\u003e, and SI showed the highest contributions to Dim1, projecting strongly in the positive direction. In contrast, τ\u003csub\u003emax\u003c/sub\u003e, CH\u003csub\u003e2\u003c/sub\u003e, and the carbonyl/carboxyl region showed weaker positive loadings, while τ\u003csub\u003eYP\u003c/sub\u003e projected distinctly in the negative Dim1 direction. The opposite projection of τ\u003csub\u003eYP\u003c/sub\u003e relative to T\u003csub\u003e2peak\u003c/sub\u003e, T\u003csub\u003e2WL\u003c/sub\u003e, and SI indicates that samples with high water mobility and swelling index are associated with lower microstructural stability. Dim2 was primarily shaped by the SiO/Al-O-Si band, which loaded positively, whereas the remaining bands showed only minor contributions. The near-orthogonal orientation of the SiO/Al-O-Si vector to the T\u003csub\u003e2\u003c/sub\u003e variables further reflects their limited correlation, highlighting that mineral-related signals vary independently from hydration-related parameters.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e(C-D) Individual factor maps grouped by matrix (dH\u003csub\u003e2\u003c/sub\u003eO, sand, loam) with 95% confidence ellipses.\u003c/p\u003e \u003cp\u003e(E-F) Data grouped by measurement points MP (80\u0026thinsp;\u0026minus;\u0026thinsp;10), shaded from white to black.\u003c/p\u003e \u003cp\u003eSample clustering showed clear matrix-dependent separation (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eC). Freely incubated PAA clustered in the strongly positive Dim1 region, driven by high SI, T\u003csub\u003e2peak\u003c/sub\u003e, and T\u003csub\u003e2WL\u003c/sub\u003e, whereas PAA incubated in sand grouped close to the origin, reflecting intermediate parameter values. However, PAA incubated in loam shifted towards the negative Dim1 and positive Dim2 region, corresponding to reduced relaxation times and increased SiO/Al-O-Si absorbance. MP did not form distinct temporal trajectories (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eE), indicating that incubation time had comparatively minor effects relative to the matrix.\u003c/p\u003e \u003cp\u003eUnder cyclic freezing-thawing conditions, the PCA structure remained similar but showed a stronger dominance of Dim1. The first two components explained 84.3% of the total variance (Dim1\u0026thinsp;=\u0026thinsp;73.5%, Dim2\u0026thinsp;=\u0026thinsp;10.8%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eB, D, F). Again, T\u003csub\u003e2peak\u003c/sub\u003e, T\u003csub\u003e2WL\u003c/sub\u003e, and SI contributed most strongly to Dim1.\u003c/p\u003e \u003cp\u003eHowever, compared to the static treatment, τ\u003csub\u003emax\u003c/sub\u003e showed a more pronounced positive loading on Dim1, while τ\u003csub\u003eYP\u003c/sub\u003e remained strongly negative. Again, the opposition of τ\u003csub\u003eYP\u003c/sub\u003e to T\u003csub\u003e2peak\u003c/sub\u003e and SI again reflects their inverse relationship, while the strengthened positive contribution of τ\u003csub\u003emax\u003c/sub\u003e indicates a closer association between increased structural stress resistance and overall matrix variance. Dim2 was again dominated by SiO/Al-O-Si, which clearly separated the loam-incubated samples from the other matrices. The near-perpendicular vector orientation between SiO/Al-O-Si, SI, and T\u003csub\u003e2\u003c/sub\u003e also remained, confirming that the mineral variables contribute independently to the secondary axis.\u003c/p\u003e \u003cp\u003eMatrix-related clustering was maintained under cyclic freezing-thawing conditions, with freely incubated PAA samples still projecting to the positive Dim1 side, PAA incubated in sand remaining near the origin, and PAA incubated in loam again shifting to negative Dim1 and positive Dim2 (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eD). MP showed no systematic evolution along either dimension (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eF), confirming limited freezing-thawing-induced changes in the multivariate parameter space.\u003c/p\u003e \u003cp\u003eAll in all, across both treatments, the PCA consistently showed the dominant contribution of the matrix to the hydrogel properties, with dH\u003csub\u003e2\u003c/sub\u003eO, sand, and loam forming robustly separated clusters along Dim1 and Dim2.\u003c/p\u003e"},{"header":"4 Discussion","content":"\u003cp\u003eWhen freely incubated, the PAA hydrogel exhibited the highest SI and longest T\u003csub\u003e2\u003c/sub\u003e values, reflecting a highly hydrated and mobile polymer network that remained stable under both constant and cyclic incubation. FTIR spectra stayed stable, showing only minor intensity changes in the carbonyl/carboxyl (-5.5%) and CH\u003csub\u003e2\u003c/sub\u003e (-4.2%) regions and no detectable SiO/Al-O-Si signals. Similarly, rheological data consistently indicated a soft, deformable network, confirming the flexible character of the PAA hydrogel. This overall pattern aligns well with various studies that reported negligible or even beneficial effects of repeated freezing-thawing on physically crosslinked PVA/PAA hydrogels [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. In this regard, freezing-thawing resistance has been attributed to polymer chain rearrangement and the gradual formation of crystallite junction zones that serve as stress buffers, dissipating mechanical energy during ice formation and thawing and preventing network collapse [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. Similarly, [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e] showed that cyclic freezing-thawing improved structural order in chitosan-based hydrogels through hydrogen-bond reorganization, suggesting that the PAA hydrogel in our study underwent comparable, subtle structural stabilization, consistent with its unchanged rheological behavior. This observation is also in line with [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e], who found that double-network hydrogels initially lost but later recovered mechanical stability due to hydrogen-bond realignment.\u003c/p\u003e \u003cp\u003eOn the one hand, the negligible overall changes in the physicochemical properties of freely incubated PAA hydrogel underline the absence of ionic coordination or chemical modification, consistent with the chemically inert environment. [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e] demonstrated that PVA/PAA hydrogels maintained mechanical and thermal stability through several freezing-thawing cycles in the absence of crosslinking cations, confirming the intrinsic resilience of physically crosslinked networks. In this regard, the NMR relaxation data also confirmed that most water in the PAA hydrogel remained relatively free, as both T\u003csub\u003e2peak\u003c/sub\u003e and T\u003csub\u003e2WL\u003c/sub\u003e remained comparable to free water (~\u0026thinsp;2000\u0026ndash;3000 ms) [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] and stable throughout the experiment. The predominance of free, mobile water domains corresponds to weakly bound hydration shells typical of flexible, coiled PAA chains [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]. In such systems, dynamic hydrogen-bond exchange allows continuous water reorganization during freezing-thawing, maintaining flexibility and preventing structural collapse [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOn the other hand, the molecular structure and pH-dependent ionization of PAA resulted in only partial deprotonation, since the pH of dH\u003csub\u003e2\u003c/sub\u003eO (~\u0026thinsp;7) is slightly above the pK\u003csub\u003ea\u003c/sub\u003e (~\u0026thinsp;5.0) [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. This limited deprotonation under near-neutral conditions restricts electrostatic repulsion and sustains a moderately coiled configuration, as reported for pH-sensitive PAA networks [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]. Thus, the PAA polymer chains retained flexibility and maintained high water mobility and limited network stiffening, supporting the dominance of hydrogen-bonded rather than electrostatic interactions. As already shown for chitosan/PVA and chitosan/PAA hydrogels, hydrogen bonding between water molecules and carbonyl/carboxyl groups produces bound-water domains with depressed freezing points that resist ice crystal formation and preserve mechanical integrity [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. This is further supported by molecular simulations showing that hydration shells around carboxylated polymers prevent ice nucleation [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. Similarly, [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e] visualized polymer networks restricting ice crystal growth, leading to homogeneous solidification without microcracking, a mechanism likely contributing to the stable NMR and rheological data observed here. Also, [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] showed that alginate hydrogel-treated soil revealed higher proportions of non-freezable water due to strong water entrapment within the interparticulate hydrogel network. Thus, only a small portion of free water likely crystallized during freezing, and hydrogen bonds dynamically broke and reformed, enabling local polymer relaxation. These reversible processes prevented any permanent structural compaction, further explaining the statistically unchanged SI, rheological parameters, and FTIR spectra across the freezing-thawing cycles. Comparable reversibility was also reported for natural polysaccharide hydrogels and synthetic root-exudate mimics, which display similar cyclic flexibility through protonation-deprotonation equilibria [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e], applying analogously to the carboxylic groups of PAA, sustaining reversible hydrogen-bond dynamics and preventing fracture formation. Here, [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e] also confirmed that macromolecular hydrogel networks can constrain ice crystallization and prevent microcrack formation using cryo magnetic resonance imaging (cryo-MRI). In line with these findings, the PCA positioned all freely incubated PAA samples distinctly on the positive Dim1 side, confirming their stable physicochemical and rheological characteristics. The tight clustering across MP and treatment supports the negligible effect of freezing-thawing, consistent with the ANOVA results showing no significant treatment or MP effect (p\u0026thinsp;\u0026gt;\u0026thinsp;0.25; η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.03). Similar stability under repeated freezing-thawing has been reported for physically crosslinked PVA/PAA hydrogels with the crystalline domains acting as reversible junctions that dissipated mechanical stress without altering overall network integrity [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAltogether, these findings confirm that freely incubated PAA hydrogels maintain a flexible, hydrated, and reversible network stabilized mainly by hydrogen bonding. When incubated in sand, however, the structural dynamics changed markedly due to confinement and ionic effects.\u003c/p\u003e \u003cp\u003eWhen incubated in sand, the behavior of PAA differed substantially compared to the freely swollen one, mainly due to a combination of soil matrix-induced mechanical confinement and cation-mediated interactions. In such confined environments, osmotic gradients and partial dehydration promote more rigid, mechanically stressed interparticulate hydrogel structures, as observed in SOM-, PVA-, and PAA-based systems [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]. On the one hand, the mineral soil matrix restricted the swelling of the interparticle hydrogel and induced a steady release of water into the environment, while, on the other hand, multivalent cations in the soil solution (especially Ca\u0026sup2;⁺, Mn\u0026sup2;⁺, and Al\u0026sup3;⁺) promoted additional crosslinking between carboxylate groups [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. Together, these two well-known soil-related mechanisms already explain the reduced SI, the shorter T\u003csub\u003e2peak\u003c/sub\u003e and T\u003csub\u003e2WL\u003c/sub\u003e values, and the slight reduction in carbonyl/carboxyl absorbance with a moderate increase in SiO/Al-O-Si intensity, corroborated by the ANOVA showing a dominant matrix effect (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001; large η\u003csup\u003e2\u003c/sup\u003e) and non-significant contributions of treatment and MP (p\u0026thinsp;\u0026gt;\u0026thinsp;0.25; η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.03). Reduced T\u003csub\u003e2\u003c/sub\u003e values and increased stiffness as observed in our study are consistent with findings by [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e], who showed that polymer-soil systems progressively compact through crosslinking via cation bridges between carboxyl groups and mineral hydroxyls. For instance, [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e] and [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] have shown that increased cationic strength increases viscoelasticity and yield stress in crosslinked PAA systems, suggesting that similar electrostatic bridging mechanisms underpin the observed increases in τ\u003csub\u003eYP\u003c/sub\u003e and τ\u003csub\u003emax\u003c/sub\u003e. Further, shortened relaxation times\u0026thinsp;\u0026asymp;\u0026thinsp;100 ms has already been attributed to hydrated mineral-associated organic hydrogel structures formed by hydrated SOM and PAA in silty sand soil [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e] and hydrated cross-linked lignite humic acids [\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e]. In this regard, the studies conducted by [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e] demonstrated that organomineral complexation contributes to the formation of non-freezing water fractions that are strongly bound even under severe thermal stress. Consequently, the formation of such water fractions in the sand-incubated PAA likely reflects enhanced hydrogen bonding and reduced mobility at the polymer-mineral interface.\u003c/p\u003e \u003cp\u003eThe PCA overall supported these findings, as sand-incubated PAA samples clustered centrally between the dH\u003csub\u003e2\u003c/sub\u003eO and loam samples, reflecting an intermediate physicochemical state. Dim1, explaining 63.4% of the total variance, was dominated by SI, T\u003csub\u003e2peak\u003c/sub\u003e, and τ\u003csub\u003eYP\u003c/sub\u003e, whereas Dim2 (21.7%) was mainly influenced by FTIR carbonyl/carboxyl intensity and τ\u003csub\u003emax\u003c/sub\u003e. This distribution clearly separated sand-incubated hydrogels from dH\u003csub\u003e2\u003c/sub\u003eO by lower swelling and increased mechanical resistance, while maintaining limited internal modification. FTIR spectra corroborated this interpretation, showing moderate broadening in the carbonyl/carboxyl region and a weak yet distinct SiO/Al-O-Si signal, indicating localized mineral contact and transient surface crosslinking without deeper structural integration as shown in the ESEM images. All in all, this suggests that mechanical confinement and moderate cation crosslinking govern PAA behavior in sand rather than freezing-thawing cycles and its overall chemical transformation as it has already been observed by [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e] for physically crosslinked hydrogels under extended cyclic temperature stress.\u003c/p\u003e \u003cp\u003eAt this point, the application type of PAA to the two soils becomes critical for understanding the differing degrees of confinement, ion exchange, and interface-driven structural modifications. In this study, PAA hydrogel was applied as a single pre-swollen unit, so soil contact was initially limited to its outer interface, creating a narrow reaction zone where minerals, SOM, and multivalent cations interacted with the polymer. The pressure exerted by mineral grains restricted local expansion, while cations transiently bridged adjacent carboxylate groups through coordination interactions [\u003cspan additionalcitationids=\"CR18\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e]. The combined effect of mechanical confinement and cation-mediated crosslinking gradually tightened the outer polymer network, forming a thin organomineral shell composed of densified chain segments, adsorbed mineral and organic residues, and locally crosslinked domains that stabilized the interface. ESEM images clearly visualized this structure as a compact, particle-rich layer surrounding a still porous, sponge-like inner hydrogel region. These structural differences from freely incubated PAA were reflected in an increased solid-like rheological behavior, indicating that the hydrogel could still deform substantially before yielding and evidencing a preserved core elasticity.\u003c/p\u003e \u003cp\u003eHowever, the missing effect of cyclic freezing-thawing on PAA in sand likely resulted from (i) a large fraction of bound, nonfreezing water immobilized through ionic coordination, limiting ice formation and freeze-induced stresses, and (ii) the already reduced amount of mobile water under soil confinement. This agrees with studies showing that confinement and ion coordination shift water populations from free to non-freezable states tightly bound to polymer or mineral surfaces [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e, \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e], thereby reducing crack formation and enhancing cryostability [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. The enrichment of such water likely suppressed ice-induced microcracking and maintained network elasticity under repeated cycles. Meanwhile, the flexible hydrogel core still acted as a viscoelastic buffer absorbing volumetric changes without structural damage [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e, \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e]. Additionally, the organomineral shell likely absorbed and redistributed freezing-induced stresses over the hydrogel surface, preventing local compaction or fracture. During thawing, weak ionic bridges relaxed rapidly, allowing the interface region to recover the shell to function as a stress-dissipating layer protecting the inner hydrogel from cyclic freezing-thawing damage.\u003c/p\u003e \u003cp\u003eIn loam, the mechanisms governing PAA hydrogel behavior extended beyond those observed for freely- and sand-incubated PAA due to its even finer texture, higher CEC, and SOM content.\u003c/p\u003e \u003cp\u003eFine-textured soils exert higher osmotic and mechanical confinement and provide more reactive mineral surfaces [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e, \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e]. Consequently, carboxylate-cation bridging and polymer-SOM complexation further reduced swelling and resulted in an even denser organomineral composite. Similar mechanisms have already been shown for synthetic exudate analogs and other synthetic polymers that formed hybrid networks with mineral particles [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e, \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e, \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWhile the PAA hydrogel remained fully hydrated and chemically stable when freely incubated and interactions in sand were largely limited to surface compaction and transient crosslinking, the incubation in loam promoted the formation of an even more consolidated and complex structures. This difference was directly reflected in the SI, NMR, rheometry, and FTIR, which all indicated a stronger confinement and a more pronounced interfacial structuring of the polymer network. The low and constant SI revealed a restricted swelling and water release directly from the beginning of the incubation, attributed to both the fine-grained mineral particles closely enveloped the hydrogel, limiting its physical expansion, and a high concentration of Ca\u003csup\u003e2+\u003c/sup\u003e and organic ligands that diffused into the interfacial zone [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e, \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e]. Additionally, fine material such as silt and clay particles can be absorbed and transported into the inner regions of the hydrogel, where they interacted directly with polymer chains through ionic crosslinking and physical entanglement [\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e]. For instance, [\u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e] and [\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e] reported that mineral particle migration and their penetration into hydrogel matrices enhanced mechanical heterogeneity and reduced overall elasticity, reflected by our ESEM images and rheometry measurements in terms of a dense and particle-coated interface with compacted and solid core structures and overall low γ\u003csub\u003eYP\u003c/sub\u003e, respectively. Thus, the highly condensed, crosslinked hydrogel structures that extended from the interface into the interior produced a markedly rigid, solid-like architecture, with the loam environment\u0026mdash;more than sand\u0026mdash;driving this deep compaction and polymer reorganization.\u003c/p\u003e \u003cp\u003eIn this regard, the FTIR spectra displayed pronounced decreases in carbonyl/carboxyl (-12.5%) and CH\u003csub\u003e2\u003c/sub\u003e (-14.3%) absorbance, together with strong and persistent SiO/Al-O-Si bands, reflecting a progressive substitution of polymer-water by polymer-mineral interactions, further strengthening its structure and immobilizing interfacial water molecules as it has already been shown for chitosan-silica and PAA-bentonite nanocomposite [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e, \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e72\u003c/span\u003e]. This also aligns with recent conceptual models suggesting that hydrogel-soil systems evolve into semi-permanent hybrid domains through progressive polymer-mineral coupling and water immobilization [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. In line with previous findings for PAA incubated in sand, where freezing-thawing cycles showed only marginal effects due to surface compaction and partial water immobilization, the influence of the loam matrix was even more pronounced. Its higher mineral and Ca\u0026sup2;⁺ content, together with the finer texture and stronger polymer-mineral interactions, further reduced the fraction of mobile water. As a result, freezing-thawing cycles caused no measurable changes in SI, τ\u003csub\u003eYP\u003c/sub\u003e, or FTIR spectra, indicating that the loam-incubated hydrogel had already attained a structurally confined state with relatively low amounts of freezable water.\u003c/p\u003e \u003cp\u003eAlso, PCA visualization confirmed the pronounced matrix effect in loam, with PAA samples clustering at the negative Dim1 and positive Dim2 side, far from the dH\u003csub\u003e2\u003c/sub\u003eO and sand samples. This separation reflected the lowest SI, shortest T\u003csub\u003e2\u003c/sub\u003e values, and highest τ\u003csub\u003eYP\u003c/sub\u003e and τ\u003csub\u003emax\u003c/sub\u003e, all indicating strong confinement and interfacial compaction (ANOVA: matrix p\u0026thinsp;\u0026lt;\u0026thinsp;0.001; η\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.75). The combination of these chemical and mechanical indicators revealed that loam not only induced external densification but also promoted internal reorganization of the polymer network toward consolidated, solid-like organomineral structures.\u003c/p\u003e \u003cp\u003eAlthough the present study provides valuable insights into PAA behavior under repeated freezing-thawing conditions, several limitations should be acknowledged. First, The ten freezing-thawing cycles applied represent only a short-term scenario and may not reflect long-term field dynamics. Therefore, future studies should extend both the number and intensity of cycles to simulate realistic seasonal variability. Second, the performed bulk-scale analyses of the PAA hydrogel limit insights into microscale dynamics, which might be overcome by advanced techniques such as cryo-scanning electron microscopy, differential scanning calorimetry (DSC), and neutron scattering to resolve ice formation and hydration mechanisms in more detail [\u003cspan additionalcitationids=\"CR74\" citationid=\"CR73\" class=\"CitationRef\"\u003e73\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e75\u003c/span\u003e]. Third, other field-relevant factors such as application types and field cultivation techniques (e.g., tillage) were not investigated but may significantly affect the fate and behavior of PAA when utilized as soil conditioner [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Finally, field-scale validation under natural soil temperature and moisture conditions is essential to confirm the laboratory findings. However, from an ecological perspective, [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] and [\u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e76\u003c/span\u003e] reported that SAP-treated soils show improved frost resistance, particularly under repeated freezing-thawing conditions, underlining the relevance of targeted in-situ studies further investigating the soil matrix and the soil-polymer system.\u003c/p\u003e \u003cp\u003eConcerning the recently discussed potential of synthetic SAPs to form plastic-like solid residues in soil, the findings of this study provide mechanistic evidence for how such residues may originate: the gradual densification, surface crosslinking, and water loss observed for PAA in soil indicate the early stages of solid residue formation as already discussed in literature [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Although freezing-thawing may locally enhance dehydration and bonding, its overall contribution appears minor. Dominant processes seem rather soil-polymer interactions, particularly crosslinking and mechanical confinement, rather than temperature-induced effects. Once the hydrogel loses flexibility and becomes incorporated into the organomineral structure, it may persist as a non-biodegradable fragment, potentially contributing to plastic-like accumulation and altering soil structure and hydraulic behavior. Consequently, understanding how soil chemistry, confinement, and climate interact to shape the long-term fate of SAPs will be critical for predicting their persistence and ecological impact.\u003c/p\u003e"},{"header":"5 Conclusion","content":"\u003cp\u003eThis study provides first mechanistic insights on how polyacrylic acid (PAA) hydrogels respond to repeated freezing-thawing cycles under varying environmental and soil conditions. Across all treatments, the findings consistently show that soil-specific physical and chemical interactions, rather than temperature fluctuations, govern the evolution of structural and physicochemical properties of PAA when incorporated in soil.\u003c/p\u003e \u003cp\u003eWhen freely incubated in demineralized water, the hydrogel remained fully hydrated, flexible, and chemically stable independent of temperature changes. The absence of ionic coordination and external confinement of the soil matrix preserved the polymer network, allowing reversible hydrogen-bond rearrangements that prevented compaction and maintained high water mobility.\u003c/p\u003e \u003cp\u003eIn sand, however, mechanical confinement and weak cation-mediated crosslinking introduced clear interfacial structuring as PAA hydrogel developed a thin organomineral shell at its outer surface via transient bridges between carboxylate groups and multivalent cations such as Ca\u003csup\u003e2+\u003c/sup\u003e and Al\u003csup\u003e3+\u003c/sup\u003e. This shell increased local stiffness and limited swelling, whereas the inner core stayed largely hydrated and elastic, allowing the system to buffer freezing-thawing stresses without structural collapse.\u003c/p\u003e \u003cp\u003eIn loamy soil, finer particles and higher concentrations of reactive cations and organic ligands enhanced these processes, driving deeper ion penetration and more extensive crosslinking within the hydrogel network. As a result, the organomineral shell thickened and gradually extended inward, transforming the hydrogel into a compact, solid-like composite with immobilized water fractions and cryostable mechanical properties.\u003c/p\u003e \u003cp\u003eTaken together, these results reveal a progressive transformation pathway, from freely swollen and fully hydrated networks to increasingly confined and mineral-integrated composites, governed primarily by soil matrix-induced mechanical compression and ionic coordination. The observed shell formation, densification, and water immobilization provide a mechanistic basis for understanding how synthetic hydrogels may gradually evolve into persistent, plastic-like residues once embedded in soil matrices. These insights underline the need to consider soil-polymer interactions as a dominant driver of both hydrogel performance and long-term persistence in terrestrial environments.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eClinical trial number\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics, Consent to Participate, and Consent to Publish declarations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe kindly thank Gabriele E. Schaumann for her feedback on the results.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability:\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe data that supports the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contribution\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConceptualization: J.N. and C.B.; Methodology and experimental setup: J.N. and C.B.; Material preparation and data collection: L.N., J.N., and C.B.; Data evaluation and interpretation: C.B., L.N., and J.N.; Writing-review and editing: C.B., J.N.; Funding acquisition: C.B.; Project management: C.B.; Supervision: C.B.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was financially supported by the Deutsche Forschungsgemeinschaft (Grant No. BU 3763/1-1)\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eXing Y, Wang X. 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Dev Built Environ. 2025;22:100666.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"discover-soil","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Discover Soil](https://link.springer.com/journal/44378)","snPcode":"44378","submissionUrl":"https://submission.nature.com/new-submission/44378/3","title":"Discover Soil","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Superabsorbent polymer, soil conditioner, polyacrylic acid, freezing-thawing cycles, hydrogel aging","lastPublishedDoi":"10.21203/rs.3.rs-8848966/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8848966/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSuperabsorbent polymers (SAPs) such as polyacrylic acid (PAA) are increasingly used as soil conditioners to enhance water retention under drought, yet their long-term stability and transformation under environmental stress remain poorly understood. In particular, the effects of cyclic freezing-thawing on their physicochemical properties and behavior in soils have not been systematically assessed.\u003c/p\u003e \u003cp\u003eHere, we examined the structural and physicochemical evolution of PAA hydrogel, both freely incubated and embedded in contrasting soil matrices (sand, loam), under static and cyclic freezing-thawing conditions. A multi-method approach combining swelling index (SI), \u003csup\u003e1\u003c/sup\u003eH-NMR relaxometry, rheometry, FTIR spectroscopy, and ESEM imaging was applied to assess hydration dynamics, network rigidity, and polymer-mineral interactions.\u003c/p\u003e \u003cp\u003eFreely incubated PAA remained highly hydrated, flexible, and chemically stable, confirming that freezing-thawing alone does not alter its physicochemical structure. In sand, moderate confinement and cation-mediated crosslinking produced a thin, reversible organomineral shell that increased stiffness while preserving an elastic core. In loam, finer texture, higher cation exchange capacity, and organic ligands promoted deeper polymer-mineral integration and compaction, yielding dense, solid-like composites. Across all treatments, soil matrix effects clearly dominated over freezing-thawing.\u003c/p\u003e \u003cp\u003eOverall, PAA followed a mechanistic transformation pattern from a hydrated flexible hydrogel to a confined organomineral composite, overall driven by soil-induced mechanical and ionic interactions. These findings highlight how SAPs may gradually persist as plastic-like residues in soil, emphasizing the need to evaluate soil-specific polymer-mineral interactions in future SAP applications.\u003c/p\u003e","manuscriptTitle":"The impact of cyclic freezing-thawing on the physicochemical properties of superabsorbent polyacrylic acid (PAA) hydrogel used as a soil conditioner","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-27 12:06:39","doi":"10.21203/rs.3.rs-8848966/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-04-20T10:59:19+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-10T13:46:08+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"25489481246901044453746732724213981078","date":"2026-03-26T09:46:25+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-06T21:02:27+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"134680691885692040551518000859355320057","date":"2026-02-26T16:08:16+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-24T13:54:21+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-02-24T12:43:09+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-17T08:44:14+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-17T08:41:31+00:00","index":"","fulltext":""},{"type":"submitted","content":"Discover Soil","date":"2026-02-11T07:55:02+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"discover-soil","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Discover Soil](https://link.springer.com/journal/44378)","snPcode":"44378","submissionUrl":"https://submission.nature.com/new-submission/44378/3","title":"Discover Soil","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"1d034a34-3993-4f1b-b4e3-657f970d8410","owner":[],"postedDate":"February 27th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-18T12:24:18+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-27 12:06:39","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8848966","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8848966","identity":"rs-8848966","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}