Study on Toughening and Strengthening of Bisphenol A Epoxy Resin with Aromatic Polyether Type Hyperbranched Epoxy

Study on Toughening and Strengthening of Bisphenol A Epoxy Resin with Aromatic Polyether Type Hyperbranched Epoxy | 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 Study on Toughening and Strengthening of Bisphenol A Epoxy Resin with Aromatic Polyether Type Hyperbranched Epoxy Anzhong Deng, Haoyang Jiang, Na Liu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7647747/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 11 You are reading this latest preprint version Abstract Epoxy resins (EPs) have gained widespread industrial adoption due to their exceptional mechanical properties, chemical resistance, and electrical insulation characteristics. However, their inherent brittleness caused by highly cross-linked networks significantly limits their application under impact loading conditions. This study demonstrates a solvent-free strategy for significantly enhancing the toughness of epoxy resins while maintaining their mechanical strength and thermal stability through incorporation of hyperbranched epoxy resins (HPEs). At an optimal 15 wt% loading, the HPE-modified epoxy system exhibits a remarkable 120% improvement in impact strength with simultaneous enhancement of tensile and flexural properties, accompanied by only a minimal 3°C reduction in glass transition temperature. Comprehensive characterization reveals that this exceptional performance stems from three synergistic mechanisms: free volume expansion (from 0.15 nm³ to 0.21 nm³), stress-induced microvoid formation, and optimized crosslinking via terminal group interactions, offering new insights for developing high-performance epoxy composites through sustainable processing. Epoxy resin hyperbranched polymer toughening mechanism free volume Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Epoxy resin exhibit excellent mechanical strength (tensile strength of 60–90 MPa), chemical stability, low volumetric shrinkage ( 15 MPa), and superior electrical insulation (volume resistivity > 10¹⁵ Ω·cm)[ 1 – 3 ]. These characteristics make them widely used as coating materials, reinforcing agents, casting compounds, molding plastics, automotive structural adhesives, and modifiers in various fields, including emerging technologies, national defense and military industries, general technical sectors, civilian industries, and daily necessities [ 4 – 7 ]. However, their highly cross-linked network structure, while providing high rigidity, results in poor toughness (fracture toughness typically < 1 MPa·m¹/²), severely limiting their application under impact loading [ 8 – 10 ]. Thus, developing effective toughening strategies remains a critical research focus. Hyperbranched polymers (HBPs) have emerged as a promising solution due to their highly branched architecture, low chain entanglement, high functional group density, and excellent compatibility with epoxy matrices [ 11 – 13 ]. Studies indicate that HBPs with high epoxy terminal group density—regardless of aliphatic or aromatic structure—form homogeneous systems with epoxy resins, significantly improving compatibility while simultaneously enhancing both strength and toughness [ 1 4 -18]. The toughening mechanism operates on multiple levels: (1) abundant epoxy terminals participate in cross-linking with curing agents, increasing tensile and flexural strength[ 19 ]; (2) the three-dimensional branched structure strengthens interfacial bonding via covalent/hydrogen bonds and induces microphase separation, promoting energy dissipation through crazing and shear banding [ 20 ]; and (3) increased free volume and reduced cross-linking density enhance molecular chain mobility. This synergistic effect enables HBPs to substantially improve toughness at low loading levels (typically 5–15 wt%) while preserving rigidity, thermal stability, and processability. Extensive research has demonstrated the effectiveness of hyperbranched epoxy modifiers. For instance, Xue Liang et al reported that GO/HBPEE-epoxy composites prepared via covalent grafting exhibited 60% improved GO dispersion and a 35% increase in tensile modulus [ 21 ]. Tongtong Zhang developed bio-based HBPEEs that enhanced the fracture energy of DGEBA systems by 2.8 times while improving solvent resistance [ 22 ]. Na Teng synthesized HBFRs that retained a 96% improvement in impact strength even at cryogenic temperatures (-196°C) [ 23 ]. In this study, a solvent-free process was employed to synthesize hyperbranched epoxy resin (HPE), and its toughening effects were systematically evaluated. At 15 wt% loading, the modified system exhibited a 120% increase in impact strength while maintaining glass transition temperature (only a 3°C decrease). Theoretical computational analysis confirmed that free volume expansion (the free volume increased from 0.15 nm³ to 0.21 nm³) was the primary toughening mechanism. These findings not only provide new insights into high-performance epoxy resins but also highlight the practical advantages of an eco-friendly synthesis route. Materials and methods Raw materials The Bisphenol A epoxy resin (DGEBA, E-51) used in the modified epoxy resin composites was provided by Shanghai aotun Chemical Technology Co. The hydroquinone (HQ) and trimethylolpropane triglycidyl ether (TMPTG) were used as A 2 and B 3 monomers of Aromatic Hyperbranched polyether epoxy resin (HPE) and were provided by Shanghai Aladdin Biochemical Technology Co and Shanghai adamas beta Co. The tetrabutyl ammonium bromide (TBAB) used as catalyst was provided by Tianjin Fuchen Chemical Reagent Factory. Tetrahydrofuran (THF) and diethyl ether were used to extract HPE by precipitation filtration method and were provided by Shanghai Aladdin Biochemical Technology Co. Tetraethylene pentaamine (TEPA) used as curing agent was provided by Sinopharm Chemical Reagent Co. Preparation of HPE A solvent-free melt bulk proton transfer polymerization method was used to prepare HPE. The HQ (13.75 g, 0.125 mol) and TMPTG (113.25 g, 0.375 mol, A 2 and B 3 monomer ratio is 1:3) was dissolved in a three port flask with stirring at N 2 atmosphere and 80 ℃. After HQ was completely dissolved, TBAB (6.0375 g, 0.01875 mol, 15% molar ratio of A 2 monomer) was added and reacted at 80 ℃ for 8 h. After the reaction was completed, the N 2 protection was removed and the stirring was stopped. The reaction mixture was allowed to cool to below 40°C, and then 30 mL THF was added and thoroughly mixed. The reaction mixture was then transferred to a 500 mL beaker, and 300 mL of diethyl ether was poured into the mixture under vigorous stirring. After homogenization, the mixture was allowed to stand for 20 min. The lower layer of pale yellow viscous liquid was carefully separated, and the extraction process was repeated twice with an additional 100 mL of diethyl ether each time. Finally, the collected pale yellow viscous liquid was dried overnight in a vacuum oven at 40°C to get HPE. Preparation and Curing of Modified Epoxy Resin E-51 epoxy resin was blended with varying amounts (0, 5, 10, 15, 20, 25, and 30 wt%) of HPE (hyperbranched polyester) at 50°C under mechanical stirring until a homogeneous mixture was obtained. The mixture was subsequently degassed under vacuum to eliminate air bubbles. Then, 14 wt% of TEPA was added as a curing agent and stirred for 15 min. The resulting mixture was poured into a polytetrafluoroethylene (PTFE) mold pre-coated with Vaseline and cured at room temperature for 24 h, followed by post-curing at 100℃ for 4 h. The cured samples were gradually cooled to room temperature and aged for an additional 24 h prior to further characterization. Figure 1 (a) displays the fabrication process schematic diagram, samples were labeled EP (0 wt% HPE) and EP/HPE-1 to EP/HPE-6 (5–30 wt% HPE in 5 wt% increments). Characterization The epoxy equivalent was measured according to the Chinese National Standard GB/T 1677–2023. FTIR spectra were recorded (Shimadzu FTIR-8400S spectrometer, Shimadzu Corporation, Japan) using the KBr pellet method (KBr-to-sample ratio = 100:1). The scanning range was set to 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹. The viscosity of the hybrid system was determined using a rotational viscometer. The sample was equilibrated at 25℃ for 10 min prior to measurement, and the viscosity value was recorded at approximately 50% torque. The mechanical properties were evaluated (UTM, Shenzhen New SANS Materials Testing Co.) in accordance with the Chinese National Standard GB/T 2567 − 2021. Tensile tests were conducted at a crosshead speed of 2 mm/min, and flexural tests were performed at a loading rate of 10 mm/min. Charpy impact strength was measured (CIT, Shenzhen New SANS.) following GB/T 2567 − 2021. Type II unnotched specimens (80 mm × 10 mm × 4 mm) were prepared, and a pendulum with an energy of 1 J was used. Thermal transitions were analyzed by DSC (Q80, China) at a heating/cooling rate of 10℃/min over the temperature range of 40–180℃. Dynamic mechanical analysis (Q80, China) was measured with samples (60 mm × 15 mm × 4 mm) and a temperature range 30°C to 150°C at a heating rate of 3°C/min. Simulation Packm [ 24 ] was used to build the initial model. LAMMPS [ 25 ] and PCFF force field [ 26 ] was used to perform the molecular simulations. The integration time step was fixed at 1.0 fs using the Verlet velocity algorithm, and the temperature and pressure was controlled by the Nosé-Hoover thermostat and barostat [ 27 ]. A van der Waals interaction cutoff of 1.2 nm was employed, and the PPPM method with a relative precision of 10 − 6 was used to account for the long-range electrostatic interactions [ 28 ]. The free volume is calculated according to the following equation: $$\:\:{V}_{\text{free}}={V}_{\text{system}}-{V}_{\text{occupied}}$$ where \(\:{V}_{\text{occupied}}\) is the volume occupied by the atoms of each resin model, and it is calculated with a probe with a radius of 1.86 Å using MoloVol [ 29 ]. \(\:\:{V}_{\text{system}}\) is the total volume of each simulation system. Then the free volume fraction is calculated through the following equation: $$\:\:{R}_{\text{free}}=\frac{{V}_{\text{free}}}{{V}_{\text{system}}}$$ Two amorphous resin models, DGEBA-TEPA and DGEBA-HPE-TEPA, were constructed. For DGEBA-TEPA model, 80 DGEBA monomers and 200 TEPA monomers were randomly inserted in a periodic cubic cell at a density of 0.5 g/cm 3 . For the DGEBA-HPE-TEPA model, 80 DGEBA, 16 HPE, and 200 TEPA monomers were also randomly put in a periodic cubic cell at the same density. The initial models were energy minimized. Subsequently, each model was subjected to an NPT ensemble molecular dynamics simulation at 300 K and 1 atm for 0.5 ns to achieve a suitable density. Curing simulations were conducted using NVT molecular dynamics, simulating the epoxy-amine reaction between the epoxide groups of DGEBA/HPE and the amine groups of TEPA, which continued until complete epoxide conversion was achieved. This reaction occurred when the distance between an epoxide and an amine groups was within 6 Å. After curing, excess TEPA molecules not involved in the crosslinking network were removed. The obtained cross linked structure was then equilibrated through NVT molecular dynamics simulation from 2000 K to 300 K for 1.0 ns and NPT molecular dynamics simulation at 300 K and 1 atm for another 1.0 ns. Results and discussion The synthesis route of EP/HPE is shown in Fig. 1 (a) The synthesized hyperbranched polyether epoxy resin (HPE) exhibited an epoxy value of 0.42 as determined by the hydrochloric acid-acetone method, demonstrating good compatibility with E-51 epoxy resin. Figure 1 (b) demonstrates the viscosity variation of E51 diluted with HPE. With increasing HPE mass fraction, the viscosity of the hybrid system gradually decreased and plateaued at approximately 25 wt%. When the viscosity of the epoxy system was below 6000 cP, the curing process could be successfully performed without requiring additional diluents to reduce viscosity. This observation confirms that HPE exhibits characteristics of a reactive diluent, enabling solvent-free curing operations for conventional epoxy resins. These results reflect the unique properties of hyperbranched polymers, including their small free volume, low viscosity, and excellent flow characteristics. The chemical functional groups of HPE molecular structure were characterized by FTIR as shown in Fig. 1 (c) The absorption peaks at 3431 cm⁻¹ correspond to the O-H stretching vibration, 3050 cm⁻¹ to the C-H stretching vibrations of benzene rings and epoxy groups, 2965 cm⁻¹, 2913 cm⁻¹ and 2874 cm⁻¹ to the -CH₂- stretching vibrations, 1636 cm⁻¹, 1507 cm⁻¹ and 1458 cm⁻¹ to the skeletal vibrations of benzene rings, 1228 cm⁻¹ to the O-H bending vibration, 1094 cm⁻¹ to the C-O-C stretching vibration, 910 cm⁻¹ to the characteristic absorption peak of epoxy groups, and 828 cm⁻¹ and 756 cm⁻¹ to the fingerprint region absorptions of benzene rings. Figure 1 (d) shows the FTIR spectra of pure DGEBA and hybrid cured epoxy resins. It can be observed that the incorporation of HPE did not alter the molecular structure of the cured epoxy. The cured hybrid resins contained both rigid benzene ring structures and flexible ether linkages, while the complete disappearance of the characteristic epoxy peak at 910 cm⁻¹ indicated the thorough completion of the curing reaction. Figure 2 (a)&(b) presents SEM images of fracture surfaces for EP and EP/HPE-3 castings, revealing distinct fracture morphologies between the two systems. The pure EP exhibits characteristic smooth fracture surfaces typical of brittle failure, while EP/HPE-3 displays numerous elongated ridges with detached fibrils (~ 5 µm diameter), indicative of ductile fracture behavior. This toughening effect can be attributed to an in situ reinforcement mechanism involving the nanoscale-dispersed hyperbranched epoxy resin (HPE) with structural similarity to EP matrix. During curing, HPE's peripheral epoxy groups participate in crosslinking while unreacted hydroxyl groups undergo stress-induced orientation, and its intrinsic free volume creates microphase-separated domains. These factors collectively promote strong interfacial bonding, leading to stress whitening through ridge formation and fibril detachment for enhanced energy absorption. Simultaneously, HPE's rigid benzene rings function as uniformly dispersed nanofillers providing reinforcement, thereby achieving concurrent improvements in both toughness and strength for the hybrid epoxy system. The unique combination of HPE's molecular characteristics - including nanoscale dimensions, structural compatibility, controlled molecular weight, and internal free volume - enables this dual-phase enhancement mechanism while maintaining effective matrix dispersion. The mechanical strength tests further corroborated these findings. As shown in Fig. 2 (c)&(d) show the impact、tensile & bending strength of EP/HPE. The toughness of pure DGEBA is relatively poor, with an impact strength of only 2.43 kJ/m². However, after the addition of HPE, the toughness is significantly improved. As the HPE content increases, the impact strength of the resin first increases and then decreases. When the HPE content reaches 20 wt%, the impact strength of the resin reaches its maximum value of 9.14 kJ/m², which is 276% higher than that without HPE, indicating that this aromatic polyether-type hyperbranched epoxy can significantly toughen the epoxy resin. While toughening, the flexural strength and tensile strength of the hybrid resin are also improved to some extent. The flexural strength of the resin reaches its maximum value when 20 wt% HPE is added. Meanwhile, the tensile strength continuously increases with the increase of HPE content, further demonstrating the effect of the rigid molecular structure of HPE. The highly branched molecular structure of the hyperbranched epoxy, on the one hand, determines the presence of numerous internal cavities, which contribute to the improved toughness of the cured epoxy resin. On the other hand, the hyperbranched epoxy possesses a large number of terminal epoxy groups on its periphery, which participate in the curing process, helping to increase the crosslinking density and ensuring the high strength of the resin. Figure 3 (a) shows the DSC scanning curves of cured pure EP and EP/HPE. It shows all samples exhibit only one glass transition temperature (T g ), indicating good compatibility between HPE and EP. Figure 3 (b) demonstrates that the T g of the hybrid resins progressively decreases with increasing HPE content. When cured with TEPA, the pure DGEBA system without modifiers shows a T g of 123.42°C. With HPE incorporation below 10 wt%, the T g of the hybrid resins decreases slightly. However, as the HPE content further increases, the T g reduction becomes more pronounced. At 30 wt% HPE loading, the T g drops to 91.49°C, representing a 26% decrease. This phenomenon suggests that while the hyperbranched molecular structure is introduced, it simultaneously creates internal cavities, which accounts for the T g reduction in the cured hybrid resins. Nevertheless, these increased cavities effectively enhance the toughness of the epoxy resin system. Figure 3 (c)&(d) shows the temperature-dependent variations of storage modulus and loss factor for the cured samples. The storage modulus represents material stiffness, while the temperature corresponding to the peak value of loss factor indicates the T g . For engineering plastics like epoxy composites, the service temperature should remain below T g to ensure longer lifespan. The results demonstrate that incorporation of HPE leads to varying degrees of reduction in storage modulus of EP, while maintaining their medium-temperature adaptability of the original curing system. As shown in Fig. 3 (d), the T g of EP/HPE gradually decreases with increasing HPE content, which is consistent with the aforementioned DSC analysis results. The simulation results further elucidate the toughening mechanism of the EP/HPE system, as illustrated in Fig. 4 . The results demonstrate that HPE incorporation increases the free volume of the system, which effectively constrains crack propagation. Table 1 shows the density and free volume fraction of cross linked DGEBA-TEPA and DGEBA-HPE-TEPA with 1.86 Å probe radius. With the incorporation of HPE, the density of EP decreased from 1.125 g/cm³ to 1.117 g/cm³, while the free volume fraction increased from 0.2623 to 0.5708 which influences the T g of the material. Generally, a higher free volume fraction leads to a lower T g , indicating that the material undergoes glass transition at relatively lower temperatures and exhibits improved toughness. These findings are in good agreement with the experimental results. Table 1 Density and free volume fraction of cross linked DGEBA-TEPA and DGEBA-HPE-TEPA with 1.86 Å probe radius. System Density (g/cm 3 ) Free volume fraction (%) DGEBA-TEPA 1.125 0.2623 DGEBA-HPE-TEPA 1.117 0.5708 Conclusion In this work, a novel aromatic HPE was synthesized via proton-transfer polymerization through a one-step method by using commercially available HQ as the A₂ monomer and TMPTG as the B₃ monomer. This HPE was subsequently employed as a modifier for conventional epoxy resin by incorporating it into bisphenol-A type epoxy resin. The effects of varying HPE loading levels on the thermal and mechanical properties of the epoxy resin were thoroughly investigated. In addition, we constructed a theoretical model, conducted numerical simulations, and performed comprehensive analysis of the toughening mechanisms. The results indicate that the incorporation of HPE does not modify the molecular structure of the cured epoxy while exhibiting excellent compatibility with EP. The hyperbranched epoxy resin HPE significantly improves the mechanical properties (including tensile strength, flexural strength, and impact strength) of conventional epoxy resin castings, although it leads to a decrease in the T g of the hybrid resin. At an HPE content of 15 wt%, the system achieves both effective enhancement of mechanical properties and satisfactory retention of the glass transition temperature in the castings. SEM and simulation results demonstrate that the toughening mechanism of HPE is primarily attributed to three factors: increased free volume fraction in the EP/HPE system, generation of additional cavities, and improved crosslinking density resulting from the participation of numerous terminal epoxy groups on the HPE periphery during the curing process. Declarations Competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding The work was support by the Natural Science Foundation of Chongqing (CSTB2023NSCQ-MSX0855), China Postdoctoral Science Foundation (2025M774480), Scientific and Technological Research Projects of Chongqing Municipal Education Commission (KJZD-K202312907). Author Contribution Anzhong Deng: Conceptualization; Data curation; Haoyang Jiang : Writing – original draft. Na Liu: Funding acquisition; Writing – review & editing. 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J Appl Crystallogr. 10.1107/s1600576722004988 Additional Declarations No competing interests reported. Supplementary Files GA.png Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 20 Jan, 2026 Reviews received at journal 19 Jan, 2026 Reviewers agreed at journal 16 Jan, 2026 Reviewers agreed at journal 15 Jan, 2026 Reviews received at journal 08 Jan, 2026 Reviewers agreed at journal 05 Jan, 2026 Reviewers agreed at journal 26 Nov, 2025 Reviewers invited by journal 30 Oct, 2025 Editor assigned by journal 07 Oct, 2025 Submission checks completed at journal 07 Oct, 2025 First submitted to journal 18 Sep, 2025 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. 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13:36:24","extension":"xml","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":71966,"visible":true,"origin":"","legend":"","description":"","filename":"19db507b00dc48f990dc59151ec310241structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7647747/v1/38eb3ed5cd2b225b6da35454.xml"},{"id":95654909,"identity":"742e5c89-fd65-4526-8ce6-8bf31096eb1d","added_by":"auto","created_at":"2025-11-11 16:13:46","extension":"html","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":78177,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7647747/v1/efcde237142d6cac0f8eb807.html"},{"id":95654530,"identity":"25b311e1-5625-4c9a-8b71-ab5102024486","added_by":"auto","created_at":"2025-11-11 16:12:23","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":363325,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The synthesis of different EP/HPE. (b)\u003cstrong\u003e \u003c/strong\u003eVariation of Hybrid Resin Viscosity with HPE Content. FTIR spectra of (c) HPE and (d) hybrid resins.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7647747/v1/fc15c9ff091148a3a2994766.png"},{"id":95654436,"identity":"2c864165-3c2f-4c16-805e-08d2a876b055","added_by":"auto","created_at":"2025-11-11 16:11:53","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":382441,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of (a) pure EP and (b) EP/HPE-3 cross-sections. (c) Impact strength and (d) tensile \u0026amp; bending strength of EP/HPE.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7647747/v1/db39ca969d13d993eb6d97f0.png"},{"id":95551076,"identity":"2dc2f8e8-8865-4374-a482-a8a39249b1a3","added_by":"auto","created_at":"2025-11-10 13:36:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":298434,"visible":true,"origin":"","legend":"\u003cp\u003e(a) DSC curves and (b) variation curves for the EP/HPE. The temperature-dependent curves of (c) storage modulus and (d) loss factor for the EP/HPE.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7647747/v1/d050ce1ad792ce55fb6fd4c4.png"},{"id":95551072,"identity":"61544d8e-f857-48f9-b90a-110ffc3df7ff","added_by":"auto","created_at":"2025-11-10 13:36:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1033242,"visible":true,"origin":"","legend":"\u003cp\u003eStructure models of cross linked (a) and (b). Pore structure of (c) DGEBA-TEPA and (d) DGEBA-HPE-TEPA. C atoms shown in deep green, N in blue, O in red, and H in white.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7647747/v1/ea82eb236925cc11b66b9d56.png"},{"id":95659970,"identity":"cfa952ab-382a-4fc4-8cfb-7964a3314a47","added_by":"auto","created_at":"2025-11-11 16:30:15","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2267642,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7647747/v1/dc3e3eee-cd6c-48da-91d7-c8c87c44a00f.pdf"},{"id":95551071,"identity":"535249f6-9226-45b5-b082-f02517235caf","added_by":"auto","created_at":"2025-11-10 13:36:23","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":440886,"visible":true,"origin":"","legend":"","description":"","filename":"GA.png","url":"https://assets-eu.researchsquare.com/files/rs-7647747/v1/10af5dec5e14070ac2e1bc7d.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"Study on Toughening and Strengthening of Bisphenol A Epoxy Resin with Aromatic Polyether Type Hyperbranched Epoxy","fulltext":[{"header":"Introduction","content":"\u003cp\u003eEpoxy resin exhibit excellent mechanical strength (tensile strength of 60\u0026ndash;90 MPa), chemical stability, low volumetric shrinkage (\u0026lt;\u0026thinsp;2%), strong adhesion (shear strength\u0026thinsp;\u0026gt;\u0026thinsp;15 MPa), and superior electrical insulation (volume resistivity\u0026thinsp;\u0026gt;\u0026thinsp;10\u0026sup1;⁵ Ω\u0026middot;cm)[\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. These characteristics make them widely used as coating materials, reinforcing agents, casting compounds, molding plastics, automotive structural adhesives, and modifiers in various fields, including emerging technologies, national defense and military industries, general technical sectors, civilian industries, and daily necessities [\u003cspan additionalcitationids=\"CR5 CR6\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. However, their highly cross-linked network structure, while providing high rigidity, results in poor toughness (fracture toughness typically\u0026thinsp;\u0026lt;\u0026thinsp;1 MPa\u0026middot;m\u0026sup1;/\u0026sup2;), severely limiting their application under impact loading [\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Thus, developing effective toughening strategies remains a critical research focus.\u003c/p\u003e\u003cp\u003eHyperbranched polymers (HBPs) have emerged as a promising solution due to their highly branched architecture, low chain entanglement, high functional group density, and excellent compatibility with epoxy matrices [\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Studies indicate that HBPs with high epoxy terminal group density\u0026mdash;regardless of aliphatic or aromatic structure\u0026mdash;form homogeneous systems with epoxy resins, significantly improving compatibility while simultaneously enhancing both strength and toughness [\u003csup\u003e1\u003c/sup\u003e4 -18]. The toughening mechanism operates on multiple levels: (1) abundant epoxy terminals participate in cross-linking with curing agents, increasing tensile and flexural strength[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]; (2) the three-dimensional branched structure strengthens interfacial bonding via covalent/hydrogen bonds and induces microphase separation, promoting energy dissipation through crazing and shear banding [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]; and (3) increased free volume and reduced cross-linking density enhance molecular chain mobility. This synergistic effect enables HBPs to substantially improve toughness at low loading levels (typically 5\u0026ndash;15 wt%) while preserving rigidity, thermal stability, and processability. Extensive research has demonstrated the effectiveness of hyperbranched epoxy modifiers. For instance, Xue Liang et al reported that GO/HBPEE-epoxy composites prepared via covalent grafting exhibited 60% improved GO dispersion and a 35% increase in tensile modulus [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Tongtong Zhang developed bio-based HBPEEs that enhanced the fracture energy of DGEBA systems by 2.8 times while improving solvent resistance [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Na Teng synthesized HBFRs that retained a 96% improvement in impact strength even at cryogenic temperatures (-196\u0026deg;C) [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn this study, a solvent-free process was employed to synthesize hyperbranched epoxy resin (HPE), and its toughening effects were systematically evaluated. At 15 wt% loading, the modified system exhibited a 120% increase in impact strength while maintaining glass transition temperature (only a 3\u0026deg;C decrease). Theoretical computational analysis confirmed that free volume expansion (the free volume increased from 0.15 nm\u0026sup3; to 0.21 nm\u0026sup3;) was the primary toughening mechanism. These findings not only provide new insights into high-performance epoxy resins but also highlight the practical advantages of an eco-friendly synthesis route.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eRaw materials\u003c/h2\u003e\u003cp\u003eThe Bisphenol A epoxy resin (DGEBA, E-51) used in the modified epoxy resin composites was provided by Shanghai aotun Chemical Technology Co. The hydroquinone (HQ) and trimethylolpropane triglycidyl ether (TMPTG) were used as A\u003csub\u003e2\u003c/sub\u003e and B\u003csub\u003e3\u003c/sub\u003e monomers of Aromatic Hyperbranched polyether epoxy resin (HPE) and were provided by Shanghai Aladdin Biochemical Technology Co and Shanghai adamas beta Co. The tetrabutyl ammonium bromide (TBAB) used as catalyst was provided by Tianjin Fuchen Chemical Reagent Factory. Tetrahydrofuran (THF) and diethyl ether were used to extract HPE by precipitation filtration method and were provided by Shanghai Aladdin Biochemical Technology Co. Tetraethylene pentaamine (TEPA) used as curing agent was provided by Sinopharm Chemical Reagent Co.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003ePreparation of HPE\u003c/h3\u003e\n\u003cp\u003eA solvent-free melt bulk proton transfer polymerization method was used to prepare HPE. The\u003c/p\u003e\u003cp\u003eHQ (13.75 g, 0.125 mol) and TMPTG (113.25 g, 0.375 mol, A\u003csub\u003e2\u003c/sub\u003e and B\u003csub\u003e3\u003c/sub\u003e monomer ratio is 1:3) was dissolved in a three port flask with stirring at N\u003csub\u003e2\u003c/sub\u003e atmosphere and 80 ℃. After HQ was completely dissolved, TBAB (6.0375 g, 0.01875 mol, 15% molar ratio of A\u003csub\u003e2\u003c/sub\u003e monomer) was added and reacted at 80 ℃ for 8 h. After the reaction was completed, the N\u003csub\u003e2\u003c/sub\u003e protection was removed and the stirring was stopped. The reaction mixture was allowed to cool to below 40\u0026deg;C, and then 30 mL THF was added and thoroughly mixed. The reaction mixture was then transferred to a 500 mL beaker, and 300 mL of diethyl ether was poured into the mixture under vigorous stirring. After homogenization, the mixture was allowed to stand for 20 min. The lower layer of pale yellow viscous liquid was carefully separated, and the extraction process was repeated twice with an additional 100 mL of diethyl ether each time. Finally, the collected pale yellow viscous liquid was dried overnight in a vacuum oven at 40\u0026deg;C to get HPE.\u003c/p\u003e\n\u003ch3\u003ePreparation and Curing of Modified Epoxy Resin\u003c/h3\u003e\n\u003cp\u003eE-51 epoxy resin was blended with varying amounts (0, 5, 10, 15, 20, 25, and 30 wt%) of HPE (hyperbranched polyester) at 50\u0026deg;C under mechanical stirring until a homogeneous mixture was obtained. The mixture was subsequently degassed under vacuum to eliminate air bubbles. Then, 14 wt% of TEPA was added as a curing agent and stirred for 15 min. The resulting mixture was poured into a polytetrafluoroethylene (PTFE) mold pre-coated with Vaseline and cured at room temperature for 24 h, followed by post-curing at 100℃ for 4 h. The cured samples were gradually cooled to room temperature and aged for an additional 24 h prior to further characterization. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) displays the fabrication process schematic diagram, samples were labeled EP (0 wt% HPE) and EP/HPE-1 to EP/HPE-6 (5\u0026ndash;30 wt% HPE in 5 wt% increments).\u003c/p\u003e\n\u003ch3\u003eCharacterization\u003c/h3\u003e\n\u003cp\u003eThe epoxy equivalent was measured according to the Chinese National Standard GB/T 1677\u0026ndash;2023. FTIR spectra were recorded (Shimadzu FTIR-8400S spectrometer, Shimadzu Corporation, Japan) using the KBr pellet method (KBr-to-sample ratio\u0026thinsp;=\u0026thinsp;100:1). The scanning range was set to 400\u0026ndash;4000 cm⁻\u0026sup1; with a resolution of 4 cm⁻\u0026sup1;. The viscosity of the hybrid system was determined using a rotational viscometer. The sample was equilibrated at 25℃ for 10 min prior to measurement, and the viscosity value was recorded at approximately 50% torque. The mechanical properties were evaluated (UTM, Shenzhen New SANS Materials Testing Co.) in accordance with the Chinese National Standard GB/T 2567\u0026thinsp;\u0026minus;\u0026thinsp;2021. Tensile tests were conducted at a crosshead speed of 2 mm/min, and flexural tests were performed at a loading rate of 10 mm/min. Charpy impact strength was measured (CIT, Shenzhen New SANS.) following GB/T 2567\u0026thinsp;\u0026minus;\u0026thinsp;2021. Type II unnotched specimens (80 mm \u0026times; 10 mm \u0026times; 4 mm) were prepared, and a pendulum with an energy of 1 J was used. Thermal transitions were analyzed by DSC (Q80, China) at a heating/cooling rate of 10℃/min over the temperature range of 40\u0026ndash;180℃. Dynamic mechanical analysis (Q80, China) was measured with samples (60 mm \u0026times; 15 mm \u0026times; 4 mm) and a temperature range 30\u0026deg;C to 150\u0026deg;C at a heating rate of 3\u0026deg;C/min.\u003c/p\u003e\n\u003ch3\u003eSimulation\u003c/h3\u003e\n\u003cp\u003ePackm [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] was used to build the initial model. LAMMPS [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] and PCFF force field [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] was used to perform the molecular simulations. The integration time step was fixed at 1.0 fs using the Verlet velocity algorithm, and the temperature and pressure was controlled by the Nos\u0026eacute;-Hoover thermostat and barostat [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. A van der Waals interaction cutoff of 1.2 nm was employed, and the PPPM method with a relative precision of 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e was used to account for the long-range electrostatic interactions [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe free volume is calculated according to the following equation:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\:{V}_{\\text{free}}={V}_{\\text{system}}-{V}_{\\text{occupied}}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{V}_{\\text{occupied}}\\)\u003c/span\u003e\u003c/span\u003e is the volume occupied by the atoms of each resin model, and it is calculated with a probe with a radius of 1.86 \u0026Aring; using MoloVol [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\:{V}_{\\text{system}}\\)\u003c/span\u003e\u003c/span\u003e is the total volume of each simulation system. Then the free volume fraction is calculated through the following equation:\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:\\:{R}_{\\text{free}}=\\frac{{V}_{\\text{free}}}{{V}_{\\text{system}}}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eTwo amorphous resin models, DGEBA-TEPA and DGEBA-HPE-TEPA, were constructed. For DGEBA-TEPA model, 80 DGEBA monomers and 200 TEPA monomers were randomly inserted in a periodic cubic cell at a density of 0.5 g/cm\u003csup\u003e3\u003c/sup\u003e. For the DGEBA-HPE-TEPA model, 80 DGEBA, 16 HPE, and 200 TEPA monomers were also randomly put in a periodic cubic cell at the same density. The initial models were energy minimized. Subsequently, each model was subjected to an NPT ensemble molecular dynamics simulation at 300 K and 1 atm for 0.5 ns to achieve a suitable density. Curing simulations were conducted using NVT molecular dynamics, simulating the epoxy-amine reaction between the epoxide groups of DGEBA/HPE and the amine groups of TEPA, which continued until complete epoxide conversion was achieved. This reaction occurred when the distance between an epoxide and an amine groups was within 6 \u0026Aring;. After curing, excess TEPA molecules not involved in the crosslinking network were removed. The obtained cross linked structure was then equilibrated through NVT molecular dynamics simulation from 2000 K to 300 K for 1.0 ns and NPT molecular dynamics simulation at 300 K and 1 atm for another 1.0 ns.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cp\u003eThe synthesis route of EP/HPE is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) The synthesized hyperbranched polyether epoxy resin (HPE) exhibited an epoxy value of 0.42 as determined by the hydrochloric acid-acetone method, demonstrating good compatibility with E-51 epoxy resin. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b) demonstrates the viscosity variation of E51 diluted with HPE. With increasing HPE mass fraction, the viscosity of the hybrid system gradually decreased and plateaued at approximately 25 wt%. When the viscosity of the epoxy system was below 6000 cP, the curing process could be successfully performed without requiring additional diluents to reduce viscosity. This observation confirms that HPE exhibits characteristics of a reactive diluent, enabling solvent-free curing operations for conventional epoxy resins. These results reflect the unique properties of hyperbranched polymers, including their small free volume, low viscosity, and excellent flow characteristics.\u003c/p\u003e\u003cp\u003eThe chemical functional groups of HPE molecular structure were characterized by FTIR as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(c) The absorption peaks at 3431 cm⁻\u0026sup1; correspond to the O-H stretching vibration, 3050 cm⁻\u0026sup1; to the C-H stretching vibrations of benzene rings and epoxy groups, 2965 cm⁻\u0026sup1;, 2913 cm⁻\u0026sup1; and 2874 cm⁻\u0026sup1; to the -CH₂- stretching vibrations, 1636 cm⁻\u0026sup1;, 1507 cm⁻\u0026sup1; and 1458 cm⁻\u0026sup1; to the skeletal vibrations of benzene rings, 1228 cm⁻\u0026sup1; to the O-H bending vibration, 1094 cm⁻\u0026sup1; to the C-O-C stretching vibration, 910 cm⁻\u0026sup1; to the characteristic absorption peak of epoxy groups, and 828 cm⁻\u0026sup1; and 756 cm⁻\u0026sup1; to the fingerprint region absorptions of benzene rings. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(d) shows the FTIR spectra of pure DGEBA and hybrid cured epoxy resins. It can be observed that the incorporation of HPE did not alter the molecular structure of the cured epoxy. The cured hybrid resins contained both rigid benzene ring structures and flexible ether linkages, while the complete disappearance of the characteristic epoxy peak at 910 cm⁻\u0026sup1; indicated the thorough completion of the curing reaction.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (a)\u0026amp;(b) presents SEM images of fracture surfaces for EP and EP/HPE-3 castings, revealing distinct fracture morphologies between the two systems. The pure EP exhibits characteristic smooth fracture surfaces typical of brittle failure, while EP/HPE-3 displays numerous elongated ridges with detached fibrils (~\u0026thinsp;5 \u0026micro;m diameter), indicative of ductile fracture behavior. This toughening effect can be attributed to an in situ reinforcement mechanism involving the nanoscale-dispersed hyperbranched epoxy resin (HPE) with structural similarity to EP matrix. During curing, HPE's peripheral epoxy groups participate in crosslinking while unreacted hydroxyl groups undergo stress-induced orientation, and its intrinsic free volume creates microphase-separated domains. These factors collectively promote strong interfacial bonding, leading to stress whitening through ridge formation and fibril detachment for enhanced energy absorption. Simultaneously, HPE's rigid benzene rings function as uniformly dispersed nanofillers providing reinforcement, thereby achieving concurrent improvements in both toughness and strength for the hybrid epoxy system. The unique combination of HPE's molecular characteristics - including nanoscale dimensions, structural compatibility, controlled molecular weight, and internal free volume - enables this dual-phase enhancement mechanism while maintaining effective matrix dispersion.\u003c/p\u003e\u003cp\u003eThe mechanical strength tests further corroborated these findings. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(c)\u0026amp;(d) show the impact、tensile \u0026amp; bending strength of EP/HPE. The toughness of pure DGEBA is relatively poor, with an impact strength of only 2.43 kJ/m\u0026sup2;. However, after the addition of HPE, the toughness is significantly improved. As the HPE content increases, the impact strength of the resin first increases and then decreases. When the HPE content reaches 20 wt%, the impact strength of the resin reaches its maximum value of 9.14 kJ/m\u0026sup2;, which is 276% higher than that without HPE, indicating that this aromatic polyether-type hyperbranched epoxy can significantly toughen the epoxy resin. While toughening, the flexural strength and tensile strength of the hybrid resin are also improved to some extent. The flexural strength of the resin reaches its maximum value when 20 wt% HPE is added. Meanwhile, the tensile strength continuously increases with the increase of HPE content, further demonstrating the effect of the rigid molecular structure of HPE. The highly branched molecular structure of the hyperbranched epoxy, on the one hand, determines the presence of numerous internal cavities, which contribute to the improved toughness of the cured epoxy resin. On the other hand, the hyperbranched epoxy possesses a large number of terminal epoxy groups on its periphery, which participate in the curing process, helping to increase the crosslinking density and ensuring the high strength of the resin.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (a) shows the DSC scanning curves of cured pure EP and EP/HPE. It shows all samples exhibit only one glass transition temperature (T\u003csub\u003eg\u003c/sub\u003e), indicating good compatibility between HPE and EP. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (b) demonstrates that the T\u003csub\u003eg\u003c/sub\u003e of the hybrid resins progressively decreases with increasing HPE content. When cured with TEPA, the pure DGEBA system without modifiers shows a T\u003csub\u003eg\u003c/sub\u003e of 123.42\u0026deg;C. With HPE incorporation below 10 wt%, the T\u003csub\u003eg\u003c/sub\u003e of the hybrid resins decreases slightly. However, as the HPE content further increases, the T\u003csub\u003eg\u003c/sub\u003e reduction becomes more pronounced. At 30 wt% HPE loading, the T\u003csub\u003eg\u003c/sub\u003e drops to 91.49\u0026deg;C, representing a 26% decrease. This phenomenon suggests that while the hyperbranched molecular structure is introduced, it simultaneously creates internal cavities, which accounts for the T\u003csub\u003eg\u003c/sub\u003e reduction in the cured hybrid resins. Nevertheless, these increased cavities effectively enhance the toughness of the epoxy resin system.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (c)\u0026amp;(d) shows the temperature-dependent variations of storage modulus and loss factor for the cured samples. The storage modulus represents material stiffness, while the temperature corresponding to the peak value of loss factor indicates the T\u003csub\u003eg\u003c/sub\u003e. For engineering plastics like epoxy composites, the service temperature should remain below T\u003csub\u003eg\u003c/sub\u003e to ensure longer lifespan. The results demonstrate that incorporation of HPE leads to varying degrees of reduction in storage modulus of EP, while maintaining their medium-temperature adaptability of the original curing system. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (d), the T\u003csub\u003eg\u003c/sub\u003e of EP/HPE gradually decreases with increasing HPE content, which is consistent with the aforementioned DSC analysis results.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe simulation results further elucidate the toughening mechanism of the EP/HPE system, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The results demonstrate that HPE incorporation increases the free volume of the system, which effectively constrains crack propagation. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the density and free volume fraction of cross linked DGEBA-TEPA and DGEBA-HPE-TEPA with 1.86 \u0026Aring; probe radius. With the incorporation of HPE, the density of EP decreased from 1.125 g/cm\u0026sup3; to 1.117 g/cm\u0026sup3;, while the free volume fraction increased from 0.2623 to 0.5708 which influences the T\u003csub\u003eg\u003c/sub\u003e of the material. Generally, a higher free volume fraction leads to a lower T\u003csub\u003eg\u003c/sub\u003e, indicating that the material undergoes glass transition at relatively lower temperatures and exhibits improved toughness. These findings are in good agreement with the experimental results.\u003c/p\u003e\u003cp\u003e\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\u003eDensity and free volume fraction of cross linked DGEBA-TEPA and DGEBA-HPE-TEPA with 1.86 \u0026Aring; probe radius.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSystem\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDensity (g/cm\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eFree volume fraction (%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDGEBA-TEPA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1.125\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.2623\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDGEBA-HPE-TEPA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1.117\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.5708\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this work, a novel aromatic HPE was synthesized via proton-transfer polymerization through a one-step method by using commercially available HQ as the A₂ monomer and TMPTG as the B₃ monomer. This HPE was subsequently employed as a modifier for conventional epoxy resin by incorporating it into bisphenol-A type epoxy resin. The effects of varying HPE loading levels on the thermal and mechanical properties of the epoxy resin were thoroughly investigated. In addition, we constructed a theoretical model, conducted numerical simulations, and performed comprehensive analysis of the toughening mechanisms. The results indicate that the incorporation of HPE does not modify the molecular structure of the cured epoxy while exhibiting excellent compatibility with EP. The hyperbranched epoxy resin HPE significantly improves the mechanical properties (including tensile strength, flexural strength, and impact strength) of conventional epoxy resin castings, although it leads to a decrease in the T\u003csub\u003eg\u003c/sub\u003e of the hybrid resin. At an HPE content of 15 wt%, the system achieves both effective enhancement of mechanical properties and satisfactory retention of the glass transition temperature in the castings. SEM and simulation results demonstrate that the toughening mechanism of HPE is primarily attributed to three factors: increased free volume fraction in the EP/HPE system, generation of additional cavities, and improved crosslinking density resulting from the participation of numerous terminal epoxy groups on the HPE periphery during the curing process.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eCompeting interest\u003c/h2\u003e\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eThe work was support by the Natural Science Foundation of Chongqing (CSTB2023NSCQ-MSX0855), China Postdoctoral Science Foundation (2025M774480), Scientific and Technological Research Projects of Chongqing Municipal Education Commission (KJZD-K202312907).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAnzhong Deng: Conceptualization; Data curation; Haoyang Jiang : Writing \u0026ndash; original draft. Na Liu: Funding acquisition; Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe authors confirm that the data supporting the findings of this study are available within the article.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLiu J, Zhang L, Shun W, Dai J, Peng Y, Liu X (2021) Recent development on bio-basedthermosetting resins. 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J Appl Crystallogr. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1107/s1600576722004988\u003c/span\u003e\u003cspan address=\"10.1107/s1600576722004988\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\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":"colloid-and-polymer-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Colloid and Polymer Science](https://www.springer.com/journal/396) ","snPcode":"396","submissionUrl":"https://mc.manuscriptcentral.com/cps","title":"Colloid and Polymer Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Epoxy resin, hyperbranched polymer, toughening mechanism, free volume","lastPublishedDoi":"10.21203/rs.3.rs-7647747/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7647747/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eEpoxy resins (EPs) have gained widespread industrial adoption due to their exceptional mechanical properties, chemical resistance, and electrical insulation characteristics. However, their inherent brittleness caused by highly cross-linked networks significantly limits their application under impact loading conditions. This study demonstrates a solvent-free strategy for significantly enhancing the toughness of epoxy resins while maintaining their mechanical strength and thermal stability through incorporation of hyperbranched epoxy resins (HPEs). At an optimal 15 wt% loading, the HPE-modified epoxy system exhibits a remarkable 120% improvement in impact strength with simultaneous enhancement of tensile and flexural properties, accompanied by only a minimal 3°C reduction in glass transition temperature. Comprehensive characterization reveals that this exceptional performance stems from three synergistic mechanisms: free volume expansion (from 0.15 nm³ to 0.21 nm³), stress-induced microvoid formation, and optimized crosslinking via terminal group interactions, offering new insights for developing high-performance epoxy composites through sustainable processing.\u003c/p\u003e","manuscriptTitle":"Study on Toughening and Strengthening of Bisphenol A Epoxy Resin with Aromatic Polyether Type Hyperbranched Epoxy","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-10 13:36:19","doi":"10.21203/rs.3.rs-7647747/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-01-21T03:11:01+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-19T09:29:39+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"65933739917825392310717543674878306897","date":"2026-01-16T06:13:49+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"48950275388261513202775489329060064268","date":"2026-01-16T00:37:21+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-08T18:02:35+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"13575346571795728426425969328235781306","date":"2026-01-05T15:15:26+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"132521002032910982734305318534856096093","date":"2025-11-26T13:28:01+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-10-30T07:41:26+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-07T06:11:26+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-10-07T06:10:53+00:00","index":"","fulltext":""},{"type":"submitted","content":"Colloid and Polymer Science","date":"2025-09-18T09:16:28+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"colloid-and-polymer-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Colloid and Polymer Science](https://www.springer.com/journal/396) ","snPcode":"396","submissionUrl":"https://mc.manuscriptcentral.com/cps","title":"Colloid and Polymer Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"392590a2-c597-45ff-aa97-0b1757eb75e8","owner":[],"postedDate":"November 10th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-03-07T09:39:15+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-10 13:36:19","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7647747","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7647747","identity":"rs-7647747","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}