Synthesis and salt thickening mechanism of salt-tolerant copolymers based on functional monomer synergy

Synthesis and salt thickening mechanism of salt-tolerant copolymers based on functional monomer synergy | 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 Synthesis and salt thickening mechanism of salt-tolerant copolymers based on functional monomer synergy Wenfei Wang, Xiaojuan Lai, Xianyun Shi, Wenwen Yang, Lei Wang, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6302949/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract To improve the utilization rate of thickening agents used for oil recovery in high-temperature and high-salinity reservoirs, a medium-molecular-weight water-soluble hydrophobic association copolymer (APDM) was prepared via the copolymerization of acrylamide (AM) with acrylic acid (AA), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), octadecyldimethylallyl ammonium chloride (DMAAC-18), and N -vinylpyrrolidone (NVP) using the free-radical polymerization method. The APDM copolymer was characterized and its salt resistance, viscoelastic properties, temperature resistance, and shear resistance were determined. Research has shown that the molecular weight of APDM was around 5 million. The apparent viscosity of a 0.5 wt% APDM solution was higher in 2 × 10 4 mg/L NaCl solution (187 mPa·s) than in clean water but decreased to 87 mPa·s when the NaCl concentration increased to 20 × 10 4 mg/L. In a high-salinity environment, the hydrophobic groups associated owing to the hydrophobic effect, causing the molecular chains to form a physical crosslinking network that increased the solution viscosity. Moreover, the steric hindrance stemming from the aggregation of hydrophobic groups prevented salt ions from approaching the hydrophobic groups, reducing the electrostatic interaction between salt ions and the hydrophobic long chains, endowing APDM with excellent salt thickening ability and salt resistance in NaCl solutions. At 140 °C and a shear rate of 170 s −1 , the apparent viscosity of a 0.5 wt% APDM solution with deionized water and 20,000 mg/L NaCl aqueous solution as a solvent was >50 mPa·s after shearing for 1 h. Owing to its good temperature resistance and shear resistance, the APDM copolymer can find application as a thickener in high-salinity reservoirs. hydrophobic association effect salt resistance synergy viscoelasticity temperature resistance Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction With the continuous increase of the global energy demand and the depletion of oil and gas resources, the development and utilization of conventional oil and gas reservoirs have recently reached a bottleneck, and ultradeep reservoirs have attracted increasing attention [ 1 – 4 ]. Since the temperature and complexity of the environment increase with depth, the development of an enhanced oil recovery technology has become a key research target in the petroleum industry [ 5 – 7 ]. As an important branch of chemical flooding, copolymer flooding can significantly improve oil recovery by injecting a polymer thickener solution into the formation to increase the viscosity of the displacement phase, improve the fluidity ratio, and expand the sweep volume [ 8 ]. In particular, partially hydrolyzed polyacrylamide (HPAM) is the most widely used thickener for oil displacement because of its excellent viscosity, low cost, and mature process [ 9 , 10 ]. However, in high-temperature (> 90°C) and high-salinity (> 10 4 mg/L) reservoirs, the molecular chains of traditional HPAM undergo accelerated thermal degradation as well as deformation and crosslink failure induced by metal ions, resulting in a sharp decrease in the solution viscosity, seriously restricting its application in deep high-temperature and high-salinity reservoirs [ 11 – 15 ]. In a recent study on acrylamide (AM) copolymers, Shi [ 16 ] synthesized a thickener (ASDM) consisting of AM, acrylic acid (AA), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), Non-ionic polymeric surfactant (NPS), and Double tailed hydrophobic monomer (DHM) under the synergistic effect of hydrophobic association and linear entanglement. He found that ASDM could be quickly dissolved within 5 min and exhibited an apparent viscosity of 175.9 mPa·s in 10 × 10 4 mg/L brine at a concentration of 0.5 wt% and a viscosity of 85.9 mPa·s after shearing at 120°C for 2 h. Li [ 17 ] mixed β-cyclodextrin with a hydrophobic association polymer (HAP), improving the thickening ability of the HAP solution compared with that of pure HAP. Peng [ 18 ] prepared two acid thickeners (ADMC and ADOM) and investigated the double-layer thickening mechanism of ADOM, finding that the self-thickening of an ADOM acid solution in the initial stage was mainly affected by the Ca 2+ concentration and temperature and the self-thickening in the middle to late stage was mainly affected by temperature. The viscosities of a 0.8 wt% ADOM solution were 250, 201.5, and 61.3 mPa·s after shearing at 90°C, 120°C, and 150°C for 1 h, respectively. Gou [ 19 ] synthesized hydrophobic association copolymers modified with α-aminophosphonic acid. The apparent viscosity of a 2000 mg/L copolymer solution was maintained at 40.20 mPa·s after shearing at 120°C. Moreover, the apparent viscosities were 55.41, 59.95, and 52.97 mPa·s in 10,000 mg/L NaCl, 1200 mg/L MgCl 2 , and 1200 mg/L CaCl 2 solutions, respectively. According to these studies, water-soluble hydrophobic association polyacrylamide copolymers [ 20 ] exhibit special association behavior in water due to the presence of a small amount of hydrophobic monomers, which in turn endows the aqueous solution with good viscosity properties, resulting in good shear resistance, temperature resistance, and salt resistance. The molecular chains of hydrophobic association copolymers contain a small number of hydrophobic groups that associate when the copolymer is dissolved in water, forming supramolecular aggregates with a reversible network structure that considerably increase the viscosity of the solution. As a result, hydrophobic association copolymers exhibit a special rheology that differs from that of general water-soluble copolymers [ 21 – 23 ]. Therefore, hydrophobic association copolymers can overcome to a certain extent the poor temperature resistance and salt resistance and easy shear degradation of polyacrylamides commonly used in oil and gas exploration, which render them promising water-soluble copolymer materials for oil and gas exploration with good application prospects [ 24 ]. On the basis of this background, we designed and synthesized a medium-molecular-weight water-soluble hydrophobic association copolymer with strong temperature and salt tolerances (hereinafter referred to as APDM) by copolymerizing five functional monomers, i.e., AM, AA, the anionic monomer AMPS, the cationic hydrophobic monomer octadecyldimethylallyl ammonium chloride (DMAAC-18), and the amphiphilic monomer N -vinylpyrrolidone (NVP), via free-radical polymerization. Moreover, due to the moderate molecular weight of the copolymer, it has a good thickening effect at low concentrations, can quickly dissolve and disperse in water, and has good adaptability in both aqueous and salt solutions. DMAAC-18 and NVP were introduced to further enhance the salt resistance, temperature resistance, and shear resistance of the resulting APDM thickener. The water solubility of the monomers made the synthesis of APDM simpler compared with that of general hydrophobic association copolymers. Furthermore, their long hydrophobic alkyl chains produced hydrophobic association effects, further improving the temperature resistance and salt resistance of the copolymer. 2. Materials and Methods 2.1 Materials and instruments Analytically pure AM, AMPS, and AA were purchased from Shanghai Maclean’s Biochemical Technology Co., Ltd. Analytically pure DMAAC-18 was obtained from Guangzhou Shang he Chemical Technology Co., Ltd. Analytically pure NVP was provided by Shanghai Maclean’s Biochemical Technology Co., Ltd. Analytically pure sodium hydroxide (NaOH) was purchased from Tianjin Obokai Chemical Co., Ltd. Analytically pure NaCl, CaCl 2 , and MgCl 2 were purchased from Tianjin Ding sheng Chemical Co., Ltd. Analytically pure 2,2'-azo(2-methylpropionamidine) dihydrochloride (V50) was provided by Shanghai Maclean’s Biochemical Science & Technology Co., Ltd. Analytically pure ammonium persulfate (APS) was obtained from Tian li Chemical Reagent Co., Ltd. Analytically pure ammonium ferrous sulfate was provided by Comio Chemical Reagents Co., Ltd. The ACDM and HPAD copolymers were prepared for comparative purposes according to the APDM synthesis method. Self-made deionized water (DI) and high-purity nitrogen were used throughout the study. The following instruments were used for material characterization: a Haake Mars 40 rotational rheometer, Thermopower Ltd.; an Avater370 Fourier transform infrared (FTIR) spectrometer, Nicole Corporation, USA; an Advance III 400 MHz nuclear magnetic resonance (NMR) spectrometer, Bruker, Switzerland; an S4800 field emission scanning electron microscope, Hitachi, Japan; an LGJ-12 vacuum freeze dryer, Beijing Song yuan Hua xing Technology Development Co., Ltd.; a Model 8510 thermometer, Shanghai Medical Instrument Factory; a TST101A-1B electric constant temperature blower drying oven, Chengdu Tester Instrument Co., Ltd.; a JJ-1 precision booster electric stirrer, Changzhou Surui Instrument Co., Ltd.; and a ZNN-D6 II electric six-speed viscometer, Qingdao Hongyulin Petroleum Instrument Co., Ltd. 2.2 Preparation of APDM AM (0.23 mol), AA (0.08 mol), AMPS (0.015 mol), DMAAC-18 (0.003 mol), and NVP (0.009 mol) were fully dissolved in deionized (DI) water (total monomer concentration: 27 wt%), and the pH of the resulting solution was adjusted to 6.4–6.6 using 32 wt% NaOH. The solution was cooled to about 4°C, poured into a thermos flask equipped with a thermometer and a nitrogen blower, and V50, the APS oxidant, and the ammonium ferrous sulfate reducing agent were added while flowing nitrogen gas for 60 min. When the solution became sticky, the nitrogen flow was removed and the flask was sealed. After 3–4 h, the obtained product was crushed into colloidal particles with a mixer to obtain the copolymer colloid. After the colloidal aging was complete, the rubber block was taken out from the thermos flask, cut into granules with scissors, and finally the copolymer colloid was crushed, dried at 60°C, and then beaten into powder using a high-speed grinder, yielding the APDM copolymer. The synthesis route is shown in Fig. 1 (a). Then, DMAAC-18 and NVP were replaced with dimethyldiallylammonium chloride to ensure that the other monomers remained unchanged. The hydrophobic copolymer HPAD and the ACDM copolymer without DMAAC-18 and NVP were synthesized under the same conditions. The synthesis route is shown in Fig. 1 (b). 2.3 APDM characterization and performance testing The FTIR spectroscopy characterization was performed using the KBr tableting method, for which the copolymer was dried and ground into powder. The 1 H NMR spectra were recorded using deuterated water (D 2 O) as the solvent. A relative-molecular-weight determination test was performed according to GB 12005.1–1989 “Method for Determination of Polyacrylamide Intrinsic Viscosity” and GB/T 12005.10–1992 “Polyacrylamide Molecular Weight Determination Viscosity Method.” To observe the microscopic morphology and structural characteristics of the copolymers via scanning electron microscopy (SEM), the copolymer powder was prepared into a 0.1 wt% aqueous copolymer solution and then freeze–dried and placed in a scanning electron microscope. The temperature resistance and shear resistance tests were performed on a rotational rheometer using a 0.5 wt% APDM copolymer solution in deionized (DI) water. Shearing was conducted at 120°C for 1 h at a shear rate of 170 s − 1 . The conventional HPAD copolymer and the ACDM copolymer without hydrophobic monomers were also tested for comparison. For the salt tolerance test, aqueous solutions with different mass fractions of NaCl, CaCl 2 , and MgCl 2 were used as solvents to prepare 0.5 wt% APDM, HPAD, and ACDM copolymer solutions. The apparent viscosity of the APDM solution at 30°C and 170 s − 1 was determined using a six-speed viscometer. The viscoelastic performance test was performed using a 0.5 wt% APDM solution in deionized (DI) water and NaCl, CaCl 2 , and MgCl 2 aqueous solutions with a mass concentration of 20,000 mg/L, and the relationship of the elastic modulus (G′) and the viscous modulus (G′′) with frequency and stress was determined using a rotational rheometer at 30°C. The temperature resistance test was conducted on a 0.5 wt% aqueous copolymer solution in deionized (DI) water and brine using a rotational rheometer with a temperature range of 30°C–120°C, a shear rate of 170 s − 1 , and a heating rate of 0.05°C/s. The temperature and shear resistance of the APDM copolymer was tested using a rheometer and 0.5 wt% APDM solutions in deionized (DI) water and 20,000 mg/L NaCl, CaCl 2 , and MgCl 2 aqueous solutions. The effect of shear on the rheological properties was determined using the rheometer at 140°C. 3. Results and Discussion 3.1 FTIR spectroscopy analysis Figure 2 (a) shows the FTIR spectrum of the five-membered APDM copolymer. The peaks at 3480 and 1674 cm − 1 are the bending vibration peaks of the N–H bonds of the primary amines in the amide groups and that at 1550 cm − 1 is the bending vibration peak of the N–H bonds of the secondary amines. The expansion vibration peak of the methylene C–H group and the absorption peak of the C–N expansion vibration appear at 2931 and 1188 cm − 1 , respectively. The peak at 3388 cm − 1 is due to the expansion and contraction vibration of the O–H bonds in the carboxyl group. The characteristic peak of the symmetrical stretching vibration of the sulfonic acid group and the absorption peak of the C–S bending vibration appear at 1042 and 628 cm − 1 , respectively. The peaks at 3200 and 1320 cm − 1 are the C–H telescopic vibration and C–C telescopic vibration peaks on the pyrrole ring, respectively. The peak at 1450 cm − 1 is the characteristic vibrational peak of the quaternary ammonium ions. These results confirm that the AM, AMPS, AA, DMAAC-18, and NVP monomers underwent copolymerization reactions to afford the five-membered APDM copolymer. 3.2 1 H NMR spectroscopy analysis The 1 H NMR spectrum of the APDM copolymer is shown in Fig. 2 (b). The presence of the following peaks attributable to the AM, AA, AMPS, NVP, and DMAAC-18 monomers indicates that the five functional monomers were successfully copolymerized: δ 1.42 (a) and δ 2.14 (b) signals for –CH 2 –CH– on the APDM backbone, δ 3.16 (c) for the –CH 2 – groups in AMPS, δ 1.11 (d) for –CH 3 in AMPS, δ 3.57 (e) for methylene linked to nitrogen atoms in DMAAC-18, δ 3.05 (f) for –CH 3 in DMAAC-18, δ 1.23 (j) for –(CH 2 ) 17 – in DMAAC-18, δ 0.82 (h) for the –CH 3 group on the long-chain alkyl group on DMAAC-18, and δ 2.02 (i) for the methylene group linked to nitrogen on NVP. Note that the signal at δ 4.70 corresponds to the D 2 O solvent. 3.3 Relative-molecular-weight determination and temperature rise curve analysis The relative molecular weight test results of the copolymer (Table. 1) show that its relative molecular weight is around 5 million, indicating that we have synthesized a copolymer with a medium molecular weight. Table 1 Determination of the relative molecular weight of the APDM copolymer Polymer solution flow time (s) NaCl solution flow time (s) Solution concentration (g/mL) Relatively viscosity η r Characteristic viscosity number [ η ] (mL/g) M 166 100 0.0005 1.66 1089.28 5018794 The instantaneous temperature was recorded during the synthesis of the APDM copolymer, and the plots shown in Fig. 2 (c) were constructed. The polymerization process was induced by the redox system, and the initiation induction period was due to the high concentration of monomer molecules in the solution. Specifically, the addition of the oxidant and reducing agent caused a redox reaction with the release of active free radicals, which reduced the activation energy of the double bonds. Hence, the double bonds underwent a rapid prepolymerized free-radical polymerization reaction, releasing a large amount of heat that produced a drastic increase in the temperature. However, with the progress of the redox reaction, the oxidant and reducing agent were gradually consumed and the temperature rise trend gradually slowed down. When the temperature reached 40°C, the decomposition of the water-soluble azo initiator V50 formed active free radicals, which activated monomer molecules, leading to polymer chain growth. This caused another period of rapid temperature increase, but as the reaction progressed, the number of monomer molecules in the system decreased, the reaction entered a deceleration period, and the coupling termination and disproportionation termination began to occur. 3.4 SEM analysis As shown in Fig. 3 (a), the copolymer molecular chains of APDM in deionized (DI) water are intertwined with each other to form an irregular spatial structure. This physical crosslinking network structure can maintain a certain structural strength at high temperatures. The excellent rheological properties of APDM can be attributed to the hydrogen bonding between the molecular chains. As shown in Fig. 3 (b, c), ACDM and HPAD are relatively dispersed in deionized (DI) water, their molecular chains cannot be stretched at high temperature, and the viscosity loss is large. The images shown in Fig. 3 (d, e, f,) suggest that the hydroxyl groups in the copolymer molecules can chelate with the metal cations of the salts, which can not only weaken the negative influence of the metal cations on the viscosity of the copolymers but also enhance the intermolecular forces, promoting the formation of a denser and regular structure that prevents the molecular chains from deforming and improves the solubility of the copolymer and the viscoelasticity of the system. The strength of the intermolecular structure increases, which is macroscopically manifested as the enhanced salt tolerance of the copolymer. 3.5 Temperature resistance and shear resistance of the copolymers The temperature resistance and shear resistance were tested at a shear rate of 170 s − 1 at 120°C for 1 h using 0.5 wt% solutions of APDM, HPAD, and ACDM in deionized (DI) water. As shown in Fig. 4 (a), the viscosity of the APDM solution was higher than those of the HPAM and ACDM solutions. The apparent viscosity of the 0.5 wt% APDM solution was 76.50 mPa·s after shearing for 1 h, and the viscosity retention rate was 52.76%. These results meet the industry standard for water-based fracturing fluids, which indicates that APDM has good temperature resistance as a thickener for fracturing fluids. However, the viscosity of self-made HPAM decreased to approximately 65.69 mPa·s after shearing at 120°C for 1 h. Meanwhile, the viscosity of ACDM decreased to 51.43 mPa·s after shearing at 120°C for 1 h and kept decreasing with time. Because ACDM does not contain hydrophobic monomers, the copolymer molecular chain winding is simple and cannot resist the continuous external shear, resulting in a continuous decrease in viscosity. In contrast, APDM and HPAD contain hydrophobic monomers, and the hydrophobic long chains form a dynamic physical crosslinking network through hydrophobic association in aqueous solutions, which can still maintain a certain structural strength at high temperatures. Moreover, the hydrophobic monomer side chain in APDM is longer compared to that in HPAD, and its stereo effect and hydrophobic accumulation increase the rigidity of the molecular chains and further improve the stability of the copolymer. The cyclic rigid structure of NVP can limit the excessive curling of the copolymer chain at high temperatures, maintaining the stretched state of the molecular chains. This stretched state helps maintain the viscosity of the solution, and the carbonyl group of NVP can also form intermolecular hydrogen bonds with the amide group, which protects the adjacent amide side groups at high temperatures, slows down the breaking rate of the main chain, reduces the thermal movement of the molecular chain segment at high temperature, and reduces the viscosity loss. As a result, the APDM copolymer exhibits better temperature resistance and shear resistance than HPAD. 3.6 Salt tolerance analysis Next, the effects of different concentrations of NaCl, CaCl 2 , and MgCl 2 on the apparent viscosities of the three copolymer solutions were investigated. Figure 4 (b) shows the effect of different NaCl concentrations on the apparent viscosities of the copolymers. The apparent viscosity of the copolymer solution increased with increasing NaCl concentration up to 2×10 4 mg/L. Due to the low positive charge density of the monovalent ions, the electrostatic attraction between Na + and the negative groups in the copolymers was weaker than the molecular forces and hydrogen bonding between the copolymer molecular chains at a low salt concentration. As a result, the copolymer thread clusters dissolved in the monovalent salt solution, and the apparent viscosity of the solution increased slightly. When the NaCl concentration was high, the relative density of the positive charge of the monovalent ions increased, the molecular chains curled up, and the apparent viscosity of the solution decreased. When the NaCl concentration increased from 0 to 20 × 10 4 mg/L, the apparent viscosities of 0.5 wt% APDM, 0.5 wt% HPAD, and 0.5 wt% ACDM solutions decreased from 154 to 87 mPa·s, from 120 to 57 mPa·s, and from 114 to 42 mPa·s, respectively, and the corresponding viscosity retention rates were 56.5%, 47.5%, and 36.8%, respectively. The higher apparent viscosity and viscosity retention rate of the APDM copolymer solution indicate that the APDM copolymer exhibits better salt tolerance than the HPAD and ACDM copolymers. As shown in Fig. 4 (c, d), the viscosity of APDM copolymer solutions in CaCl 2 and MgCl 2 was higher than that of the HPAD and ACDM solutions, indicating that the ACDM copolymer solution had a good anti-divalent-salt effect. When the concentrations of CaCl 2 and MgCl 2 reached 15 × 10 4 mg/L, the viscosity of the 0.5 wt% APDM solution decreased from 154 to 96 and 78 mPa·s, respectively, and the viscosity retention rates were 62.3% and 50.6%, respectively. Meanwhile, when the concentrations of CaCl 2 and MgCl 2 reached 15 × 10 4 mg/L, the viscosity of the 0.5 wt% HPAD solution decreased from 120 to 54 and 51 mPa·s, respectively, and that of the 0.5 wt% ACDM solution decreased from 114 to 33 and 36 mPa·s, respectively. The viscosity retention rates of the HPAD and ACDM copolymer solutions were lower than 50%, confirming that the APDM copolymer has better salt tolerance. Due to the unique structure of DMAAC-18 and NVP functional monomers introduced, as well as the synergistic effects of hydrogen bonding, hydrophobic association, and electrostatic forces in aqueous solution, the salt resistance of copolymer APDM is superior to that of HPAD and ACDM. The ionization of sulfonic acid groups on the APDM molecular chains of copolymers generates charge repulsion, causing the molecular chains to stretch and exhibit good viscosity increasing properties in saltwater. The introduction of DMAAC-18 and NVP rigid groups further enhances the interaction between the copolymer molecular chains, making the molecular structure more stable, which enables the copolymer to maintain higher viscosity in high concentration inorganic salt solutions. 3.7 Viscoelastic analysis of the APDM copolymer Figure 5 shows the viscoelastic properties of the copolymers in deionized (DI) water and brine as a function of frequency and stress. The mass fraction of APDM was 0.5% and the concentration of NaCl, CaCl 2 , and MgCl 2 was 20,000 mg/L. The copolymer exhibits better viscoelasticity in saltwater medium, with better viscoelasticity in NaCl solution with a mass concentration of 20,000 mg/L than in deionized (DI) water. As can be seen from Fig. 5 (a), it can be seen that in the frequency range of 0.1–10 Hz, G′ is always greater than G′′ in the pure water solution of the copolymer, indicating that the elastic modulus of the copolymer solution dominates. In the aqueous NaCl solution, the G′ and G′′ values increased with increasing NaCl concentration, indicating that the copolymer had good viscoelasticity. In 2 × 10 4 mg/L NaCl solution, G′ was always greater than G′′, which suggests that the addition of NaCl enhanced the network structure of the copolymer molecules, resulting in good viscoelasticity and a dominant elastic modulus. In the 10 × 10 4 mg/L NaCl solution, G′ was smaller than G′′ in the low-frequency region (< 1 Hz), indicating that the viscous modulus plays a more significant role. The trend displayed in Fig. 5 (a) further illustrates the thickening effect of the NaCl solution on the copolymer at low salinity, which is consistent with the effect of the NaCl concentration on the viscosity. As shown in Fig. 5 (b), in 20,000 mg/L CaCl 2 solution, G′ was smaller than G′′ in the low-frequency region ( 1 Hz), suggesting that the copolymer solution is dominated by viscosity and elasticity in each case, respectively. In 20,000 mg/L MgCl 2 solution, G′ was always less than G′′ in the frequency range of 0.1–10 Hz and the copolymer solution was mainly viscous. In 20,000 mg/L CaCl 2 and MgCl 2 solutions, the G′ and G′′ of the copolymer solution were smaller than those in deionized (DI) water, indicating that the copolymer lost more in the divalent salt solution, which is consistent with the effect of the divalent salt concentration on the viscosity. The stress scanning results shown in Fig. 5 (c, d) reveal that the copolymer molecular chains in water mainly interact intramolecularly and the solution changes from elastomer to viscous during the stress scanning process. In brine, G′ > G"; therefore, the solution shows good elasticity and changes from viscous to elastomeric. 3.8 Salt tolerance mechanism of the APDM copolymer As shown in Fig. 6 , APDM molecular chain is longer, can form a “bridge” between different molecules, connecting them to form a larger aggregate, so that the internal structure of the solution is more complex, the resistance to flow increases, the viscosity rises. The hydrophobic groups in the copolymer reduce the polarity of water under high-salinity conditions but also associate with each other through hydrophobic effects to form physical crosslinking points. As a result, the copolymer molecular chains form a physical crosslinking network that accommodates a large number of solvent molecules, thereby increasing the solution viscosity. The aggregation of the hydrophobic groups also produces a steric hindrance effect, which reduces the electrostatic interaction between the salt ions and the hydrophobic long chains and the influence of the salts on the molecular chains, improving the salt tolerance. Meanwhile, the hydrophilic group of NVP can increase the number of hydrogen bonds between the copolymer molecular chains and water molecules, forming a thicker hydration film that reduces the charge shielding effect of the salt ions on the copolymer chains in a high-salinity environment and maintains the molecular chains stretched. The sulfonic acid group can interact with the cations in brine, hinder the compression of the copolymer molecular chains due to the salt ions to a certain extent, and maintain the stretched state of the molecular chains; therefore, the copolymer can still maintain good viscosity in brine. In addition, NVP can also balance the electrostatic attraction effect of the hydrophobic long chains and avoid the reduction of viscosity caused by the excessive curling of molecular chains. Owing to the synergistic effect of the hydrophobic long chains and NVP, as well as the role of the sulfonic acid group, the chain rigidity, thermal stability, and hydration ability of the APDM copolymer are improved, resulting in a stable performance in high-temperature and high-salinity environments. 3.9 Temperature resistance analysis of the APDM copolymer Changes in the viscosities of a 0.5 wt% APDM solution in DI water and NaCl, CaCl 2 , and MgCl 2 aqueous solutions with a mass concentration of 20,000 and 40,000 mg/L were measured in a temperature range of 30°C–120°C and a shear rate of 170 s − 1 . As shown in Fig. 7 , with increasing temperature, the thermal movement of the molecular chains intensifies, the system gradually moves toward chaos, and the molecular chains begin to stretch in water and are wound during the shearing process, resulting in a decrease in the viscosity. When the temperature exceeds 100 ℃, some molecular chains undergo toughness fracture and viscosity loss accelerates. The final viscosity of APDM in deionized (DI) water is 80.28 mPa·s. The final apparent viscosities of APDM in 20,000 and 40,000 mg/L NaCl solutions were maintained at 74.81 and 69.07 mPa·s, respectively, owing to the chelating structure of the metal ions and hydroxyl groups in the molecular chains as well as the ability of the winding structure to resist temperature. The final apparent viscosities were maintained at 69.51 and 68.41 mPa·s in 20,000 and 40,000 mg/L CaCl 2 solutions, respectively, and at 54.51 and 52.85 mPa·s in 20,000 and 40,000 mg/L MgCl 2 solutions, respectively. These results show that at 120°C, the viscosity of the copolymer solution in water and brine was > 50 mPa·s and the overall temperature resistance of the copolymer solution was better. Thus, it can be concluded that the copolymer has good temperature resistance. 3.10 Analysis of the temperature and shear resistance of the APDM copolymer The temperature and shear resistance of a 0.5 wt% APDM solution was measured at 140°C in DI water and 20,000 mg/L NaCl, CaCl 2 , and MgCl 2 solutions, respectively, and the results are shown in Fig. 8 . The shear viscosity gradually decreased with increasing temperature in the heating stage, due to an increase in the thermal movement of molecules and the hydrophobic association being in a chaotic state. When the temperature and shear rate reach stability, the intermolecular association opens and reaches a dynamic equilibrium, and the viscosity of the system is also stable. When the system temperature increased to 140°C, the viscosity of the APDM solution in deionized (DI) water remained basically unchanged after shearing, with the final apparent viscosity being 57.49 mPa·s at 140°C and 170 s − 1 (Fig. 8 (a)), demonstrating the excellent temperature and shear resistance of APDM. This is because without the influence of salt ions in deionized (DI) water, the molecular chains can form a relatively stable network structure via van der Waals force interactions and hydrogen bonds owing to the long-chain macromolecular structure of the copolymer and the large number of active groups such as amide groups. When subjected to temperature and shear force, this network structure can absorb and dissipate part of the energy, preventing the molecular chains from breaking. As shown in Fig. 8 (b), the final apparent viscosity of APDM in 20,000 mg/L NaCl aqueous solution was maintained at 62.16 mPa·s. When the shear time of APDM in a 20,000 mg/L NaCl aqueous solution is less than 500 s, the apparent viscosity increases with the increase of temperature, indicating excellent salt thickening ability. This is because the monovalent salt ions shield part of the charge on the chains and weaken the electrostatic repulsion between the anionic groups; therefore, the copolymer molecular chains can be more fully extended and their hydrodynamic volume in the solution and the viscosity increase. This affects the self-aggregation behavior of the copolymer solution, endowing it with excellent temperature and shear resistance. As shown in Fig. 8 (c, d), the final apparent viscosity of APDM in 20,000 mg/L CaCl 2 and MgCl 2 aqueous solutions remained at 42.43 and 36.54 mPa·s, respectively, at 140°C and 170 s − 1 . The temperature at which the viscosity is below 50 mPa·s in CaCl 2 solution is 130.9 ℃, and the temperature at which the viscosity is below 50 mPa·s in MgCl 2 solution is 128.5 ℃. This is because the cations of the divalent salt form complexes with the carboxyl groups and other groups on the molecular chains, resulting in a crosslinked structure and producing flocculation precipitation, which decreases the stability of the solution and the original thickening performance at high temperatures. However, compared to conventional copolymers, APDM has stronger shear resistance due to the combination of special cationic hydrophobic monomers and NVP, resulting in better stability of the copolymer molecular chain. Therefore, modified APDM has higher viscosity retention and temperature and shear resistance. 4. Conclusion A hydrophobic association copolymer with a medium molecular weight (APDM) was synthesized by copolymerizing AM as the backbone with AA, AMPS, DMAAC-18, and NVP via free-radical polymerization. The optimal synthesis conditions were as follows: a mass ratio of the AM, AA, AMPS, DMAAC-18, and NVP monomers of 16:6:3:1:1; a total monomer concentration of 27 wt%, and an initiator dosage of 0.2 wt% relative to the total monomer mass. Due to the moderate molecular weight of the copolymer, it has a good thickening effect at low concentrations, can quickly dissolve and disperse in water, has good adaptability in both aqueous and salt solutions, and has good thermal stability. A 0.5 wt% solution of the resulting APDM copolymer exhibited apparent viscosities of 87, 96, and 78 mPa·s in 20 × 10 4 mg/L NaCl, 15 × 10 4 mg/L CaCl 2 , and 15 × 10 4 mg/L MgCl 2 solutions, respectively, indicating that the APDM copolymer maintained high viscosity in a high-salinity environment. The viscoelastic test showed that the G′ of APDM was greater than G′′ in 2 × 10 4 mg/L NaCl solution and G′ and G′′ were higher in the salt solution than in deionized water. The salt tolerance test further showed that the apparent viscosity of 0.5 wt% APDM in 2 × 10 4 mg/L NaCl solution was 187 mPa·s, which was higher than that in deionized water. This can be attributed to the low salinity charge stimulating the hydrophobic micelle microregion of APDM, which facilitates the solubilization of the hydrophobic monomers and the aggregation of micelles. The hydrophobic association between molecules was also more strong and the viscoelastic ratio was higher than those in deionized water, which further verified that APDM showed excellent salt thickening ability in NaCl solutions. The rheological property test showed that the final apparent viscosities of APDM were maintained at 57.49 and 62.16 mPa·s after shearing a 0.5 wt% APDM solution in deionized water and a 20,000 mg/L NaCl solution, respectively, for 1 h at 140°C and 170 s − 1 . Under the same conditions, 0.5 wt% APDM in 20,000 mg/L CaCl 2 and MgCl 2 solutions exhibited an apparent viscosity of > 50 mPa·s at temperatures below 130°C and showed excellent temperature resistance and shear resistance. These results demonstrate that APDM is suitable for use as a thickener in high-salinity oil reservoirs. Declarations Funding This research was financially supported by the Qin-Chuangyuan "Scientist and Engineer" Team Construction Project (2024QCY-KXJ-052), Key R&D Program of Shaanxi Province (2024GX-YBXM-393), Industrialization Project of Shaanxi Provincial Education Department (23JC008) and the Youth Innovation Team Project of Shaanxi Universities(24JP022). Author Contributions Wenfei Wang: Conceptualization, data curation and writing-original draft Xiaojuan Lai: Funding acquisition and methodology Xianyun Shi: Investigation and project administration Wenwen Yang: Software Lei Wang: Supervision Rui Wang: Validation and visualization Haibin Li: Data curation Xiaojiang Song: Review and editing Data Availability Statement All of the material is owned by the authors and no permissions are required. Conflicts of Interest There are no conflicts of interest to declare. Institutional Review Board Statement Not applicable. References Yang L, Yang D, Liu C H, et al. A review of research on imbibition mechanism of unconventional oil and gas reservoirs, 2023, 26(4):119–142. Lei Q, Yun X, Cai B, et al. Progress and prospects of horizontal well fracturing technology for shale oil and gas reservoirs[J]. Petroleum Exploration and Development, 2022, 49(1):166–172. He D F, Jia C Z, Zhao W Z, et al. Research progress and key issues of ultra-deep oil and gas exploration in China[J]. Petroleum Exploration and Development, 2023, 50(6):1333–1344. Duan Y, Chen B, Li Y. Experimental Evaluation of Authigenic Acid Suitable for Acidification of Deep Oil and Gas Reservoirs at High Temperatures[J]. Processes, 2023, 11(10). Gen J, Fan H, Zhao Y, et al. A correlation between interfacial tension, emulsifying ability and oil displacement efficiency of ASP system for Daqing crude oil[J]. Petroleum Science & Technology, 2018, 36(24):2151–2156. Du H, Chen F, Guo L, et al. Laboratory study on high-efficiency viscosity reducer in high temperature and salinity of heavy oil reservoir[J]. IOP Conference Series Earth and Environmental Science, 2020, 585:012179. Kang H S, Zhang W L, Chen L M, et al. Experimental study on a fine emulsion flooding system to enhance oil recovery for low permeability reservoirs[J]. Journal of Petroleum Science and Engineering, 2018, 171:974–981. Xiao N, Zhang Y, Zhao C. Study on increasing oil and gas recovery in oilfield by active water - driven surfactant[J]. Chemical engineering transactions, 2017, 62:445–450. Wu Y, Wang Z, Yuan L. Deep and Ultra-deep oil/gas-source correlation methods: A review[J]. IOP Conference Series: Earth and Environmental Science, 2020, 600(1):012038. Jiang T, Teng X, Yang X. Integrated techniques for rapid and highly-efficient development and production of ultra-deep tight sand gas reservoirs of Keshen 8 Block in the Tarim Basin[J]. Natural Gas Industry B, 2017, 4(1):30–38. Pu W, Shen C, Yang Y, et al. Application potential of in situ emulsion flooding in the high-temperature and high-salinity reservoir[J]. Journal of Dispersion Science and Technology, 2018, 40(7):1–9. Jiang F, Wang H, Ye Z, et al. Thickening mechanism of water-soluble polymer in the presence of Ca 2+ and Na + [J]. Polymer Bulletin, 2022. Wang J, Kang W, Yang H, et al. Study on salt tolerance mechanism of hydrophobic polymer microspheres for high salinity reservoir[J]. Journal of Molecular Liquids, 2022. Fang J, Wang J, Wen Q, et al. Research of phenolic crosslinker gel for profile control and oil displacement in high temperature and high salinity reservoirs[J]. John Wiley & Sons, Ltd, 2018. Lv Y, Zhang S, Zhang Y, et al. Hydrophobically Associating Polyacrylamide "Water-in-Water" Emulsion Prepared by Aqueous Dispersion Polymerization: Synthesis, Characterization and Rheological Behavior[J]. Molecules, 2023, 28(6). Shi S l, Sun J S, Lv K H, et al. Fracturing Fluid Polymer Thickener with Superior Temperature, Salt and Shear Resistance Properties from the Synergistic Effect of Double-Tail Hydrophobic Monomer and Nonionic Polymerizable Surfactant. [J]. Molecules (Basel, Switzerland), 2023, 28 (13): 20–23. Li X, Ye Z, Luo P. Performance Evaluation of Enhanced Oil Recovery by Host–Guest Interaction of β-Cyclodextrin Polymer/Hydrophobically Associative Polymer [J]. Molecules, 2024, 30(1): 109–109. Li P, Wang L, Lai X J, et al. Two-Level Self-Thickening Mechanism of a Novel Acid Thickener with a Hydrophobic-Associated Structure during High-Temperature Acidification Processes [J]. Polymers, 2024, 16(5):52–54. Gou S H, Zhang Q, Yang C, et al. A novel a-aminophosphonic acid-modified acrylamide-based hydrophobic associating copolymer with superb water solubility for enhanced oil recovery[J]. RSC ADVANCES, 2016. Viralkumar P, Japan T, Tushar S. Influence of hydrophobic association in the aqueous media on the rheology and polymer conformation of associative polymers[J]. Polymer bulletin, 2023. Shi J, Wu Z, Deng Q, et al. Synthesis of hydrophobically associating polymer: temperature resistance and salt tolerance properties[J]. Polymer bulletin, 2022, 20(7):79. Zhu R, Feng Y, Luo P. Net Contribution of Hydrophobic Association to the Thickening Power of Hydrophobically Modified Polyelectrolytes Prepared by Micellar Polymerization[J]. Macromolecules, 2020, 53(4). Ma X, Mu H, Hu Y, et al. Synthesis and properties of hydrophobically associating polymer fracturing fluid[J]. Colloids and Surfaces A Physicochemical and Engineering Aspects, 2021, 626:127013. Ma X, Huang Q, Zhou Z, et al. Synthesis and evaluation of water-soluble fracturing fluid thickener based on hydrophobic association[J]. Materials Letters, 2022. Cite Share Download PDF Status: Posted Version 1 posted 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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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-6302949","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":444104625,"identity":"fbd66cdc-aea6-41d4-b628-22962d9b10c7","order_by":0,"name":"Wenfei Wang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Wenfei","middleName":"","lastName":"Wang","suffix":""},{"id":444104626,"identity":"ececf9b3-37ed-44cc-ad83-519a5002a4a4","order_by":1,"name":"Xiaojuan Lai","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAw0lEQVRIiWNgGAWjYBACefnzHx9IVNjIsbG3HyBOi+EMBmMDizNpxnw8ZxKItOYGg5lEZcuhxHkSDgbE6WCc3ZBscLPhQHqbBEMCw4+KbYS1sMscOPhw5o47uW3SjQcYe87cJsKWhsRmY8kzz3LbZA4kMDO2EaGF4UAym/TftsPpbBIJBkRquZHGJiHZdjiBeC2GPWeYDSTOpBm2AQP5IFF+kWfvYQRFpbx8e/vBBz8qiHEYMjhAovpRMApGwSgYBbgAAO+wQIzOUzJ1AAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0003-3824-6703","institution":"Shaanxi University of Science \u0026Technology","correspondingAuthor":true,"prefix":"","firstName":"Xiaojuan","middleName":"","lastName":"Lai","suffix":""},{"id":444104627,"identity":"8e7a86a0-65a3-4259-bb15-8b45364793a2","order_by":2,"name":"Xianyun Shi","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Xianyun","middleName":"","lastName":"Shi","suffix":""},{"id":444104628,"identity":"de7b6681-57db-464a-835f-ddd3197b692a","order_by":3,"name":"Wenwen Yang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Wenwen","middleName":"","lastName":"Yang","suffix":""},{"id":444104629,"identity":"5323f666-307a-4903-a35a-b233cfb476cd","order_by":4,"name":"Lei Wang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Lei","middleName":"","lastName":"Wang","suffix":""},{"id":444104630,"identity":"ff6f6c6a-2c5c-4caf-b5c8-e1c3d79efd32","order_by":5,"name":"Rui Wang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Rui","middleName":"","lastName":"Wang","suffix":""},{"id":444104631,"identity":"ed68cda7-267f-4d67-a6a8-37767f7161e3","order_by":6,"name":"Haibin Li","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Haibin","middleName":"","lastName":"Li","suffix":""},{"id":444104632,"identity":"bdbae1ed-8b54-46f9-b859-f277ad5105b6","order_by":7,"name":"Xiaojiang Song","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Xiaojiang","middleName":"","lastName":"Song","suffix":""}],"badges":[],"createdAt":"2025-03-25 10:44:51","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6302949/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6302949/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":81391088,"identity":"c670c05a-d380-4ec1-9c1a-97a4efc47bcc","added_by":"auto","created_at":"2025-04-25 14:43:38","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":51438,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Synthesis of APDM; (b) molecular structures of HPAD and ACDM\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6302949/v1/248a8ed8e2bdebdd429183f8.jpg"},{"id":81391417,"identity":"78bdb537-f50e-4e9f-98ce-c52f9d88a7cc","added_by":"auto","created_at":"2025-04-25 14:51:38","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":99570,"visible":true,"origin":"","legend":"\u003cp\u003e(a) FTIR spectrum of APDM; (b) \u003csup\u003e1\u003c/sup\u003eH NMR spectrum of APDM; (c) temperature curve of the APDM polymerization process over time\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6302949/v1/8fa38d0227814629fef39037.jpg"},{"id":81391423,"identity":"3d1ee9e1-dbd5-432a-8f5e-0396a8b31378","added_by":"auto","created_at":"2025-04-25 14:51:39","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":142756,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of (a) APDM in deionized (DI) water, (b) HPAD in DI water, (c) ACDM in DI water, (d) APDM in 20,000 mg/L NaCl, (e) APDM in 20,000 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e, and (f) APDM in 20,000 mg/L MgCl\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6302949/v1/bbf491529bec6290feb415e0.jpg"},{"id":81391426,"identity":"47797095-60c1-4d0a-bceb-67b55bdf91c1","added_by":"auto","created_at":"2025-04-25 14:51:39","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":138270,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Temperature resistance and shear resistance of different copolymers; apparent viscosities of different copolymer solutions as a function of (b) NaCl concentration, (c) CaCl\u003csub\u003e2\u003c/sub\u003e concentration, and (d) MgCl\u003csub\u003e2\u003c/sub\u003e concentration\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6302949/v1/37dc2708b26fe26362546286.jpg"},{"id":81391427,"identity":"4137892a-3e19-4497-9a47-b7168f7a1905","added_by":"auto","created_at":"2025-04-25 14:51:39","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":160484,"visible":true,"origin":"","legend":"\u003cp\u003eElastic modulus (G′) and viscous modulus (G′′) of APDM copolymer solutions as a function of frequency and stress\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6302949/v1/a953b3a88c4a64230d62b93c.jpg"},{"id":81391093,"identity":"1bc5734e-fbc3-454e-b434-62502bfb0fb0","added_by":"auto","created_at":"2025-04-25 14:43:39","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":76325,"visible":true,"origin":"","legend":"\u003cp\u003eSalt thickening mechanism of the APDM solution\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6302949/v1/ad3ff791208e6585d3959b10.jpg"},{"id":81391095,"identity":"f324f94c-c927-4699-b7db-55dedf2e3fdf","added_by":"auto","created_at":"2025-04-25 14:43:39","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":120095,"visible":true,"origin":"","legend":"\u003cp\u003eTest results of the temperature resistance of the APDM copolymer in (a) deionized (DI) water, (b) NaCl, (c) CaCl\u003csub\u003e2\u003c/sub\u003e, and (d) MgCl\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6302949/v1/5fb2ee49fd828f337d140c2e.jpg"},{"id":81391099,"identity":"326d3528-378f-4c06-bc46-1e09e7939922","added_by":"auto","created_at":"2025-04-25 14:43:39","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":110380,"visible":true,"origin":"","legend":"\u003cp\u003eTest results of temperature and shear resistance of the APDM copolymer in (a) deionized (DI) water, (b) 20,000 mg/L NaCl, (c) 20,000 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e, and (d) 20,000 mg/L MgCl\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6302949/v1/deafca74de665129ec31d0ec.jpg"},{"id":90972612,"identity":"bcac3fb8-b84a-4f1b-928d-a99c64281e18","added_by":"auto","created_at":"2025-09-10 08:03:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1760515,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6302949/v1/048396c4-84fd-4be7-a55d-76458033a9c6.pdf"}],"financialInterests":"","formattedTitle":"Synthesis and salt thickening mechanism of salt-tolerant copolymers based on functional monomer synergy","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eWith the continuous increase of the global energy demand and the depletion of oil and gas resources, the development and utilization of conventional oil and gas reservoirs have recently reached a bottleneck, and ultradeep reservoirs have attracted increasing attention [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Since the temperature and complexity of the environment increase with depth, the development of an enhanced oil recovery technology has become a key research target in the petroleum industry [\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. As an important branch of chemical flooding, copolymer flooding can significantly improve oil recovery by injecting a polymer thickener solution into the formation to increase the viscosity of the displacement phase, improve the fluidity ratio, and expand the sweep volume [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. In particular, partially hydrolyzed polyacrylamide (HPAM) is the most widely used thickener for oil displacement because of its excellent viscosity, low cost, and mature process [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. However, in high-temperature (\u0026gt;\u0026thinsp;90\u0026deg;C) and high-salinity (\u0026gt;\u0026thinsp;10\u003csup\u003e4\u003c/sup\u003e mg/L) reservoirs, the molecular chains of traditional HPAM undergo accelerated thermal degradation as well as deformation and crosslink failure induced by metal ions, resulting in a sharp decrease in the solution viscosity, seriously restricting its application in deep high-temperature and high-salinity reservoirs [\u003cspan additionalcitationids=\"CR12 CR13 CR14\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn a recent study on acrylamide (AM) copolymers, Shi [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] synthesized a thickener (ASDM) consisting of AM, acrylic acid (AA), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), Non-ionic polymeric surfactant (NPS), and Double tailed hydrophobic monomer (DHM) under the synergistic effect of hydrophobic association and linear entanglement. He found that ASDM could be quickly dissolved within 5 min and exhibited an apparent viscosity of 175.9 mPa\u0026middot;s in 10 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e mg/L brine at a concentration of 0.5 wt% and a viscosity of 85.9 mPa\u0026middot;s after shearing at 120\u0026deg;C for 2 h. Li [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] mixed β-cyclodextrin with a hydrophobic association polymer (HAP), improving the thickening ability of the HAP solution compared with that of pure HAP. Peng [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] prepared two acid thickeners (ADMC and ADOM) and investigated the double-layer thickening mechanism of ADOM, finding that the self-thickening of an ADOM acid solution in the initial stage was mainly affected by the Ca\u003csup\u003e2+\u003c/sup\u003e concentration and temperature and the self-thickening in the middle to late stage was mainly affected by temperature. The viscosities of a 0.8 wt% ADOM solution were 250, 201.5, and 61.3 mPa\u0026middot;s after shearing at 90\u0026deg;C, 120\u0026deg;C, and 150\u0026deg;C for 1 h, respectively. Gou [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] synthesized hydrophobic association copolymers modified with α-aminophosphonic acid. The apparent viscosity of a 2000 mg/L copolymer solution was maintained at 40.20 mPa\u0026middot;s after shearing at 120\u0026deg;C. Moreover, the apparent viscosities were 55.41, 59.95, and 52.97 mPa\u0026middot;s in 10,000 mg/L NaCl, 1200 mg/L MgCl\u003csub\u003e2\u003c/sub\u003e, and 1200 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e solutions, respectively.\u003c/p\u003e \u003cp\u003eAccording to these studies, water-soluble hydrophobic association polyacrylamide copolymers [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] exhibit special association behavior in water due to the presence of a small amount of hydrophobic monomers, which in turn endows the aqueous solution with good viscosity properties, resulting in good shear resistance, temperature resistance, and salt resistance. The molecular chains of hydrophobic association copolymers contain a small number of hydrophobic groups that associate when the copolymer is dissolved in water, forming supramolecular aggregates with a reversible network structure that considerably increase the viscosity of the solution. As a result, hydrophobic association copolymers exhibit a special rheology that differs from that of general water-soluble copolymers [\u003cspan additionalcitationids=\"CR22\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Therefore, hydrophobic association copolymers can overcome to a certain extent the poor temperature resistance and salt resistance and easy shear degradation of polyacrylamides commonly used in oil and gas exploration, which render them promising water-soluble copolymer materials for oil and gas exploration with good application prospects [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOn the basis of this background, we designed and synthesized a medium-molecular-weight water-soluble hydrophobic association copolymer with strong temperature and salt tolerances (hereinafter referred to as APDM) by copolymerizing five functional monomers, i.e., AM, AA, the anionic monomer AMPS, the cationic hydrophobic monomer octadecyldimethylallyl ammonium chloride (DMAAC-18), and the amphiphilic monomer \u003cem\u003eN\u003c/em\u003e-vinylpyrrolidone (NVP), via free-radical polymerization. Moreover, due to the moderate molecular weight of the copolymer, it has a good thickening effect at low concentrations, can quickly dissolve and disperse in water, and has good adaptability in both aqueous and salt solutions. DMAAC-18 and NVP were introduced to further enhance the salt resistance, temperature resistance, and shear resistance of the resulting APDM thickener. The water solubility of the monomers made the synthesis of APDM simpler compared with that of general hydrophobic association copolymers. Furthermore, their long hydrophobic alkyl chains produced hydrophobic association effects, further improving the temperature resistance and salt resistance of the copolymer.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials and instruments\u003c/h2\u003e \u003cp\u003eAnalytically pure AM, AMPS, and AA were purchased from Shanghai Maclean\u0026rsquo;s Biochemical Technology Co., Ltd. Analytically pure DMAAC-18 was obtained from Guangzhou Shang he Chemical Technology Co., Ltd. Analytically pure NVP was provided by Shanghai Maclean\u0026rsquo;s Biochemical Technology Co., Ltd. Analytically pure sodium hydroxide (NaOH) was purchased from Tianjin Obokai Chemical Co., Ltd. Analytically pure NaCl, CaCl\u003csub\u003e2\u003c/sub\u003e, and MgCl\u003csub\u003e2\u003c/sub\u003e were purchased from Tianjin Ding sheng Chemical Co., Ltd. Analytically pure 2,2'-azo(2-methylpropionamidine) dihydrochloride (V50) was provided by Shanghai Maclean\u0026rsquo;s Biochemical Science \u0026amp; Technology Co., Ltd. Analytically pure ammonium persulfate (APS) was obtained from Tian li Chemical Reagent Co., Ltd. Analytically pure ammonium ferrous sulfate was provided by Comio Chemical Reagents Co., Ltd. The ACDM and HPAD copolymers were prepared for comparative purposes according to the APDM synthesis method. Self-made deionized water (DI) and high-purity nitrogen were used throughout the study.\u003c/p\u003e \u003cp\u003eThe following instruments were used for material characterization: a Haake Mars 40 rotational rheometer, Thermopower Ltd.; an Avater370 Fourier transform infrared (FTIR) spectrometer, Nicole Corporation, USA; an Advance III 400 MHz nuclear magnetic resonance (NMR) spectrometer, Bruker, Switzerland; an S4800 field emission scanning electron microscope, Hitachi, Japan; an LGJ-12 vacuum freeze dryer, Beijing Song yuan Hua xing Technology Development Co., Ltd.; a Model 8510 thermometer, Shanghai Medical Instrument Factory; a TST101A-1B electric constant temperature blower drying oven, Chengdu Tester Instrument Co., Ltd.; a JJ-1 precision booster electric stirrer, Changzhou Surui Instrument Co., Ltd.; and a ZNN-D6 II electric six-speed viscometer, Qingdao Hongyulin Petroleum Instrument Co., Ltd.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Preparation of APDM\u003c/h2\u003e \u003cp\u003eAM (0.23 mol), AA (0.08 mol), AMPS (0.015 mol), DMAAC-18 (0.003 mol), and NVP (0.009 mol) were fully dissolved in deionized (DI) water (total monomer concentration: 27 wt%), and the pH of the resulting solution was adjusted to 6.4\u0026ndash;6.6 using 32 wt% NaOH. The solution was cooled to about 4\u0026deg;C, poured into a thermos flask equipped with a thermometer and a nitrogen blower, and V50, the APS oxidant, and the ammonium ferrous sulfate reducing agent were added while flowing nitrogen gas for 60 min. When the solution became sticky, the nitrogen flow was removed and the flask was sealed. After 3\u0026ndash;4 h, the obtained product was crushed into colloidal particles with a mixer to obtain the copolymer colloid. After the colloidal aging was complete, the rubber block was taken out from the thermos flask, cut into granules with scissors, and finally the copolymer colloid was crushed, dried at 60\u0026deg;C, and then beaten into powder using a high-speed grinder, yielding the APDM copolymer. The synthesis route is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a). Then, DMAAC-18 and NVP were replaced with dimethyldiallylammonium chloride to ensure that the other monomers remained unchanged. The hydrophobic copolymer HPAD and the ACDM copolymer without DMAAC-18 and NVP were synthesized under the same conditions. The synthesis route is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 APDM characterization and performance testing\u003c/h2\u003e \u003cp\u003eThe FTIR spectroscopy characterization was performed using the KBr tableting method, for which the copolymer was dried and ground into powder.\u003c/p\u003e \u003cp\u003eThe \u003csup\u003e1\u003c/sup\u003eH NMR spectra were recorded using deuterated water (D\u003csub\u003e2\u003c/sub\u003eO) as the solvent.\u003c/p\u003e \u003cp\u003eA relative-molecular-weight determination test was performed according to GB 12005.1\u0026ndash;1989 \u0026ldquo;Method for Determination of Polyacrylamide Intrinsic Viscosity\u0026rdquo; and GB/T 12005.10\u0026ndash;1992 \u0026ldquo;Polyacrylamide Molecular Weight Determination Viscosity Method.\u0026rdquo;\u003c/p\u003e \u003cp\u003eTo observe the microscopic morphology and structural characteristics of the copolymers via scanning electron microscopy (SEM), the copolymer powder was prepared into a 0.1 wt% aqueous copolymer solution and then freeze\u0026ndash;dried and placed in a scanning electron microscope.\u003c/p\u003e \u003cp\u003eThe temperature resistance and shear resistance tests were performed on a rotational rheometer using a 0.5 wt% APDM copolymer solution in deionized (DI) water. Shearing was conducted at 120\u0026deg;C for 1 h at a shear rate of 170 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The conventional HPAD copolymer and the ACDM copolymer without hydrophobic monomers were also tested for comparison.\u003c/p\u003e \u003cp\u003eFor the salt tolerance test, aqueous solutions with different mass fractions of NaCl, CaCl\u003csub\u003e2\u003c/sub\u003e, and MgCl\u003csub\u003e2\u003c/sub\u003e were used as solvents to prepare 0.5 wt% APDM, HPAD, and ACDM copolymer solutions. The apparent viscosity of the APDM solution at 30\u0026deg;C and 170 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was determined using a six-speed viscometer.\u003c/p\u003e \u003cp\u003eThe viscoelastic performance test was performed using a 0.5 wt% APDM solution in deionized (DI) water and NaCl, CaCl\u003csub\u003e2\u003c/sub\u003e, and MgCl\u003csub\u003e2\u003c/sub\u003e aqueous solutions with a mass concentration of 20,000 mg/L, and the relationship of the elastic modulus (G\u0026prime;) and the viscous modulus (G\u0026prime;\u0026prime;) with frequency and stress was determined using a rotational rheometer at 30\u0026deg;C.\u003c/p\u003e \u003cp\u003eThe temperature resistance test was conducted on a 0.5 wt% aqueous copolymer solution in deionized (DI) water and brine using a rotational rheometer with a temperature range of 30\u0026deg;C\u0026ndash;120\u0026deg;C, a shear rate of 170 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and a heating rate of 0.05\u0026deg;C/s.\u003c/p\u003e \u003cp\u003eThe temperature and shear resistance of the APDM copolymer was tested using a rheometer and 0.5 wt% APDM solutions in deionized (DI) water and 20,000 mg/L NaCl, CaCl\u003csub\u003e2\u003c/sub\u003e, and MgCl\u003csub\u003e2\u003c/sub\u003e aqueous solutions. The effect of shear on the rheological properties was determined using the rheometer at 140\u0026deg;C.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1 FTIR spectroscopy analysis\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a) shows the FTIR spectrum of the five-membered APDM copolymer. The peaks at 3480 and 1674 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are the bending vibration peaks of the N\u0026ndash;H bonds of the primary amines in the amide groups and that at 1550 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is the bending vibration peak of the N\u0026ndash;H bonds of the secondary amines. The expansion vibration peak of the methylene C\u0026ndash;H group and the absorption peak of the C\u0026ndash;N expansion vibration appear at 2931 and 1188 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. The peak at 3388 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is due to the expansion and contraction vibration of the O\u0026ndash;H bonds in the carboxyl group. The characteristic peak of the symmetrical stretching vibration of the sulfonic acid group and the absorption peak of the C\u0026ndash;S bending vibration appear at 1042 and 628 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. The peaks at 3200 and 1320 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are the C\u0026ndash;H telescopic vibration and C\u0026ndash;C telescopic vibration peaks on the pyrrole ring, respectively. The peak at 1450 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is the characteristic vibrational peak of the quaternary ammonium ions. These results confirm that the AM, AMPS, AA, DMAAC-18, and NVP monomers underwent copolymerization reactions to afford the five-membered APDM copolymer.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2 \u003csup\u003e1\u003c/sup\u003eH NMR spectroscopy analysis\u003c/h2\u003e \u003cp\u003eThe \u003csup\u003e1\u003c/sup\u003eH NMR spectrum of the APDM copolymer is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b). The presence of the following peaks attributable to the AM, AA, AMPS, NVP, and DMAAC-18 monomers indicates that the five functional monomers were successfully copolymerized: δ 1.42 (a) and δ 2.14 (b) signals for \u0026ndash;CH\u003csub\u003e2\u003c/sub\u003e\u0026ndash;CH\u0026ndash; on the APDM backbone, δ 3.16 (c) for the \u0026ndash;CH\u003csub\u003e2\u003c/sub\u003e\u0026ndash; groups in AMPS, δ 1.11 (d) for \u0026ndash;CH\u003csub\u003e3\u003c/sub\u003e in AMPS, δ 3.57 (e) for methylene linked to nitrogen atoms in DMAAC-18, δ 3.05 (f) for \u0026ndash;CH\u003csub\u003e3\u003c/sub\u003e in DMAAC-18, δ 1.23 (j) for \u0026ndash;(CH\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e17\u003c/sub\u003e\u0026ndash; in DMAAC-18, δ 0.82 (h) for the \u0026ndash;CH\u003csub\u003e3\u003c/sub\u003e group on the long-chain alkyl group on DMAAC-18, and δ 2.02 (i) for the methylene group linked to nitrogen on NVP. Note that the signal at δ 4.70 corresponds to the D\u003csub\u003e2\u003c/sub\u003eO solvent.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Relative-molecular-weight determination and temperature rise curve analysis\u003c/h2\u003e \u003cp\u003eThe relative molecular weight test results of the copolymer (Table. 1) show that its relative molecular weight is around 5\u0026nbsp;million, indicating that we have synthesized a copolymer with a medium molecular weight.\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\u003eDetermination of the relative molecular weight of the APDM copolymer\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\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 \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePolymer solution flow time (s)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNaCl solution flow time (s)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSolution concentration\u003c/p\u003e \u003cp\u003e(g/mL)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRelatively\u003c/p\u003e \u003cp\u003eviscosity \u003cem\u003eη\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCharacteristic viscosity number [\u003cem\u003eη\u003c/em\u003e] (mL/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eM\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e166\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.0005\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1089.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e5018794\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\u003eThe instantaneous temperature was recorded during the synthesis of the APDM copolymer, and the plots shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(c) were constructed. The polymerization process was induced by the redox system, and the initiation induction period was due to the high concentration of monomer molecules in the solution. Specifically, the addition of the oxidant and reducing agent caused a redox reaction with the release of active free radicals, which reduced the activation energy of the double bonds. Hence, the double bonds underwent a rapid prepolymerized free-radical polymerization reaction, releasing a large amount of heat that produced a drastic increase in the temperature. However, with the progress of the redox reaction, the oxidant and reducing agent were gradually consumed and the temperature rise trend gradually slowed down. When the temperature reached 40\u0026deg;C, the decomposition of the water-soluble azo initiator V50 formed active free radicals, which activated monomer molecules, leading to polymer chain growth. This caused another period of rapid temperature increase, but as the reaction progressed, the number of monomer molecules in the system decreased, the reaction entered a deceleration period, and the coupling termination and disproportionation termination began to occur.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.4 SEM analysis\u003c/h2\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a), the copolymer molecular chains of APDM in deionized (DI) water are intertwined with each other to form an irregular spatial structure. This physical crosslinking network structure can maintain a certain structural strength at high temperatures. The excellent rheological properties of APDM can be attributed to the hydrogen bonding between the molecular chains. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b, c), ACDM and HPAD are relatively dispersed in deionized (DI) water, their molecular chains cannot be stretched at high temperature, and the viscosity loss is large. The images shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(d, e, f,) suggest that the hydroxyl groups in the copolymer molecules can chelate with the metal cations of the salts, which can not only weaken the negative influence of the metal cations on the viscosity of the copolymers but also enhance the intermolecular forces, promoting the formation of a denser and regular structure that prevents the molecular chains from deforming and improves the solubility of the copolymer and the viscoelasticity of the system. The strength of the intermolecular structure increases, which is macroscopically manifested as the enhanced salt tolerance of the copolymer.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Temperature resistance and shear resistance of the copolymers\u003c/h2\u003e \u003cp\u003eThe temperature resistance and shear resistance were tested at a shear rate of 170 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 120\u0026deg;C for 1 h using 0.5 wt% solutions of APDM, HPAD, and ACDM in deionized (DI) water. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a), the viscosity of the APDM solution was higher than those of the HPAM and ACDM solutions. The apparent viscosity of the 0.5 wt% APDM solution was 76.50 mPa\u0026middot;s after shearing for 1 h, and the viscosity retention rate was 52.76%. These results meet the industry standard for water-based fracturing fluids, which indicates that APDM has good temperature resistance as a thickener for fracturing fluids. However, the viscosity of self-made HPAM decreased to approximately 65.69 mPa\u0026middot;s after shearing at 120\u0026deg;C for 1 h. Meanwhile, the viscosity of ACDM decreased to 51.43 mPa\u0026middot;s after shearing at 120\u0026deg;C for 1 h and kept decreasing with time. Because ACDM does not contain hydrophobic monomers, the copolymer molecular chain winding is simple and cannot resist the continuous external shear, resulting in a continuous decrease in viscosity. In contrast, APDM and HPAD contain hydrophobic monomers, and the hydrophobic long chains form a dynamic physical crosslinking network through hydrophobic association in aqueous solutions, which can still maintain a certain structural strength at high temperatures. Moreover, the hydrophobic monomer side chain in APDM is longer compared to that in HPAD, and its stereo effect and hydrophobic accumulation increase the rigidity of the molecular chains and further improve the stability of the copolymer. The cyclic rigid structure of NVP can limit the excessive curling of the copolymer chain at high temperatures, maintaining the stretched state of the molecular chains. This stretched state helps maintain the viscosity of the solution, and the carbonyl group of NVP can also form intermolecular hydrogen bonds with the amide group, which protects the adjacent amide side groups at high temperatures, slows down the breaking rate of the main chain, reduces the thermal movement of the molecular chain segment at high temperature, and reduces the viscosity loss. As a result, the APDM copolymer exhibits better temperature resistance and shear resistance than HPAD.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Salt tolerance analysis\u003c/h2\u003e \u003cp\u003eNext, the effects of different concentrations of NaCl, CaCl\u003csub\u003e2\u003c/sub\u003e, and MgCl\u003csub\u003e2\u003c/sub\u003e on the apparent viscosities of the three copolymer solutions were investigated. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b) shows the effect of different NaCl concentrations on the apparent viscosities of the copolymers. The apparent viscosity of the copolymer solution increased with increasing NaCl concentration up to 2\u0026times;10\u003csup\u003e4\u003c/sup\u003e mg/L. Due to the low positive charge density of the monovalent ions, the electrostatic attraction between Na\u003csup\u003e+\u003c/sup\u003e and the negative groups in the copolymers was weaker than the molecular forces and hydrogen bonding between the copolymer molecular chains at a low salt concentration. As a result, the copolymer thread clusters dissolved in the monovalent salt solution, and the apparent viscosity of the solution increased slightly. When the NaCl concentration was high, the relative density of the positive charge of the monovalent ions increased, the molecular chains curled up, and the apparent viscosity of the solution decreased.\u003c/p\u003e \u003cp\u003eWhen the NaCl concentration increased from 0 to 20 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e mg/L, the apparent viscosities of 0.5 wt% APDM, 0.5 wt% HPAD, and 0.5 wt% ACDM solutions decreased from 154 to 87 mPa\u0026middot;s, from 120 to 57 mPa\u0026middot;s, and from 114 to 42 mPa\u0026middot;s, respectively, and the corresponding viscosity retention rates were 56.5%, 47.5%, and 36.8%, respectively. The higher apparent viscosity and viscosity retention rate of the APDM copolymer solution indicate that the APDM copolymer exhibits better salt tolerance than the HPAD and ACDM copolymers.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c, d), the viscosity of APDM copolymer solutions in CaCl\u003csub\u003e2\u003c/sub\u003e and MgCl\u003csub\u003e2\u003c/sub\u003e was higher than that of the HPAD and ACDM solutions, indicating that the ACDM copolymer solution had a good anti-divalent-salt effect. When the concentrations of CaCl\u003csub\u003e2\u003c/sub\u003e and MgCl\u003csub\u003e2\u003c/sub\u003e reached 15 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e mg/L, the viscosity of the 0.5 wt% APDM solution decreased from 154 to 96 and 78 mPa\u0026middot;s, respectively, and the viscosity retention rates were 62.3% and 50.6%, respectively. Meanwhile, when the concentrations of CaCl\u003csub\u003e2\u003c/sub\u003e and MgCl\u003csub\u003e2\u003c/sub\u003e reached 15 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e mg/L, the viscosity of the 0.5 wt% HPAD solution decreased from 120 to 54 and 51 mPa\u0026middot;s, respectively, and that of the 0.5 wt% ACDM solution decreased from 114 to 33 and 36 mPa\u0026middot;s, respectively. The viscosity retention rates of the HPAD and ACDM copolymer solutions were lower than 50%, confirming that the APDM copolymer has better salt tolerance. Due to the unique structure of DMAAC-18 and NVP functional monomers introduced, as well as the synergistic effects of hydrogen bonding, hydrophobic association, and electrostatic forces in aqueous solution, the salt resistance of copolymer APDM is superior to that of HPAD and ACDM. The ionization of sulfonic acid groups on the APDM molecular chains of copolymers generates charge repulsion, causing the molecular chains to stretch and exhibit good viscosity increasing properties in saltwater. The introduction of DMAAC-18 and NVP rigid groups further enhances the interaction between the copolymer molecular chains, making the molecular structure more stable, which enables the copolymer to maintain higher viscosity in high concentration inorganic salt solutions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.7 Viscoelastic analysis of the APDM copolymer\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the viscoelastic properties of the copolymers in deionized (DI) water and brine as a function of frequency and stress. The mass fraction of APDM was 0.5% and the concentration of NaCl, CaCl\u003csub\u003e2\u003c/sub\u003e, and MgCl\u003csub\u003e2\u003c/sub\u003e was 20,000 mg/L. The copolymer exhibits better viscoelasticity in saltwater medium, with better viscoelasticity in NaCl solution with a mass concentration of 20,000 mg/L than in deionized (DI) water. As can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a), it can be seen that in the frequency range of 0.1\u0026ndash;10 Hz, G\u0026prime; is always greater than G\u0026prime;\u0026prime; in the pure water solution of the copolymer, indicating that the elastic modulus of the copolymer solution dominates. In the aqueous NaCl solution, the G\u0026prime; and G\u0026prime;\u0026prime; values increased with increasing NaCl concentration, indicating that the copolymer had good viscoelasticity. In 2 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e mg/L NaCl solution, G\u0026prime; was always greater than G\u0026prime;\u0026prime;, which suggests that the addition of NaCl enhanced the network structure of the copolymer molecules, resulting in good viscoelasticity and a dominant elastic modulus. In the 10 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e mg/L NaCl solution, G\u0026prime; was smaller than G\u0026prime;\u0026prime; in the low-frequency region (\u0026lt;\u0026thinsp;1 Hz), indicating that the viscous modulus plays a more significant role. The trend displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a) further illustrates the thickening effect of the NaCl solution on the copolymer at low salinity, which is consistent with the effect of the NaCl concentration on the viscosity. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b), in 20,000 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e solution, G\u0026prime; was smaller than G\u0026prime;\u0026prime; in the low-frequency region (\u0026lt;\u0026thinsp;1 Hz) but G\u0026prime; was greater than G\u0026prime;\u0026prime; in the high-frequency region (\u0026gt;\u0026thinsp;1 Hz), suggesting that the copolymer solution is dominated by viscosity and elasticity in each case, respectively. In 20,000 mg/L MgCl\u003csub\u003e2\u003c/sub\u003e solution, G\u0026prime; was always less than G\u0026prime;\u0026prime; in the frequency range of 0.1\u0026ndash;10 Hz and the copolymer solution was mainly viscous. In 20,000 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e and MgCl\u003csub\u003e2\u003c/sub\u003e solutions, the G\u0026prime; and G\u0026prime;\u0026prime; of the copolymer solution were smaller than those in deionized (DI) water, indicating that the copolymer lost more in the divalent salt solution, which is consistent with the effect of the divalent salt concentration on the viscosity.\u003c/p\u003e \u003cp\u003eThe stress scanning results shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c, d) reveal that the copolymer molecular chains in water mainly interact intramolecularly and the solution changes from elastomer to viscous during the stress scanning process. In brine, G\u0026prime; \u0026gt; G\"; therefore, the solution shows good elasticity and changes from viscous to elastomeric.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.8 Salt tolerance mechanism of the APDM copolymer\u003c/h2\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, APDM molecular chain is longer, can form a \u0026ldquo;bridge\u0026rdquo; between different molecules, connecting them to form a larger aggregate, so that the internal structure of the solution is more complex, the resistance to flow increases, the viscosity rises. The hydrophobic groups in the copolymer reduce the polarity of water under high-salinity conditions but also associate with each other through hydrophobic effects to form physical crosslinking points. As a result, the copolymer molecular chains form a physical crosslinking network that accommodates a large number of solvent molecules, thereby increasing the solution viscosity. The aggregation of the hydrophobic groups also produces a steric hindrance effect, which reduces the electrostatic interaction between the salt ions and the hydrophobic long chains and the influence of the salts on the molecular chains, improving the salt tolerance. Meanwhile, the hydrophilic group of NVP can increase the number of hydrogen bonds between the copolymer molecular chains and water molecules, forming a thicker hydration film that reduces the charge shielding effect of the salt ions on the copolymer chains in a high-salinity environment and maintains the molecular chains stretched. The sulfonic acid group can interact with the cations in brine, hinder the compression of the copolymer molecular chains due to the salt ions to a certain extent, and maintain the stretched state of the molecular chains; therefore, the copolymer can still maintain good viscosity in brine. In addition, NVP can also balance the electrostatic attraction effect of the hydrophobic long chains and avoid the reduction of viscosity caused by the excessive curling of molecular chains. Owing to the synergistic effect of the hydrophobic long chains and NVP, as well as the role of the sulfonic acid group, the chain rigidity, thermal stability, and hydration ability of the APDM copolymer are improved, resulting in a stable performance in high-temperature and high-salinity environments.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e3.9 Temperature resistance analysis of the APDM copolymer\u003c/h2\u003e \u003cp\u003eChanges in the viscosities of a 0.5 wt% APDM solution in DI water and NaCl, CaCl\u003csub\u003e2\u003c/sub\u003e, and MgCl\u003csub\u003e2\u003c/sub\u003e aqueous solutions with a mass concentration of 20,000 and 40,000 mg/L were measured in a temperature range of 30\u0026deg;C\u0026ndash;120\u0026deg;C and a shear rate of 170 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, with increasing temperature, the thermal movement of the molecular chains intensifies, the system gradually moves toward chaos, and the molecular chains begin to stretch in water and are wound during the shearing process, resulting in a decrease in the viscosity. When the temperature exceeds 100 ℃, some molecular chains undergo toughness fracture and viscosity loss accelerates. The final viscosity of APDM in deionized (DI) water is 80.28 mPa\u0026middot;s. The final apparent viscosities of APDM in 20,000 and 40,000 mg/L NaCl solutions were maintained at 74.81 and 69.07 mPa\u0026middot;s, respectively, owing to the chelating structure of the metal ions and hydroxyl groups in the molecular chains as well as the ability of the winding structure to resist temperature. The final apparent viscosities were maintained at 69.51 and 68.41 mPa\u0026middot;s in 20,000 and 40,000 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e solutions, respectively, and at 54.51 and 52.85 mPa\u0026middot;s in 20,000 and 40,000 mg/L MgCl\u003csub\u003e2\u003c/sub\u003e solutions, respectively. These results show that at 120\u0026deg;C, the viscosity of the copolymer solution in water and brine was \u0026gt;\u0026thinsp;50 mPa\u0026middot;s and the overall temperature resistance of the copolymer solution was better. Thus, it can be concluded that the copolymer has good temperature resistance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e3.10 Analysis of the temperature and shear resistance of the APDM copolymer\u003c/h2\u003e \u003cp\u003eThe temperature and shear resistance of a 0.5 wt% APDM solution was measured at 140\u0026deg;C in DI water and 20,000 mg/L NaCl, CaCl\u003csub\u003e2\u003c/sub\u003e, and MgCl\u003csub\u003e2\u003c/sub\u003e solutions, respectively, and the results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. The shear viscosity gradually decreased with increasing temperature in the heating stage, due to an increase in the thermal movement of molecules and the hydrophobic association being in a chaotic state. When the temperature and shear rate reach stability, the intermolecular association opens and reaches a dynamic equilibrium, and the viscosity of the system is also stable. When the system temperature increased to 140\u0026deg;C, the viscosity of the APDM solution in deionized (DI) water remained basically unchanged after shearing, with the final apparent viscosity being 57.49 mPa\u0026middot;s at 140\u0026deg;C and 170 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a)), demonstrating the excellent temperature and shear resistance of APDM. This is because without the influence of salt ions in deionized (DI) water, the molecular chains can form a relatively stable network structure via van der Waals force interactions and hydrogen bonds owing to the long-chain macromolecular structure of the copolymer and the large number of active groups such as amide groups. When subjected to temperature and shear force, this network structure can absorb and dissipate part of the energy, preventing the molecular chains from breaking.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(b), the final apparent viscosity of APDM in 20,000 mg/L NaCl aqueous solution was maintained at 62.16 mPa\u0026middot;s. When the shear time of APDM in a 20,000 mg/L NaCl aqueous solution is less than 500 s, the apparent viscosity increases with the increase of temperature, indicating excellent salt thickening ability. This is because the monovalent salt ions shield part of the charge on the chains and weaken the electrostatic repulsion between the anionic groups; therefore, the copolymer molecular chains can be more fully extended and their hydrodynamic volume in the solution and the viscosity increase. This affects the self-aggregation behavior of the copolymer solution, endowing it with excellent temperature and shear resistance. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(c, d), the final apparent viscosity of APDM in 20,000 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e and MgCl\u003csub\u003e2\u003c/sub\u003e aqueous solutions remained at 42.43 and 36.54 mPa\u0026middot;s, respectively, at 140\u0026deg;C and 170 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The temperature at which the viscosity is below 50 mPa\u0026middot;s in CaCl\u003csub\u003e2\u003c/sub\u003e solution is 130.9 ℃, and the temperature at which the viscosity is below 50 mPa\u0026middot;s in MgCl\u003csub\u003e2\u003c/sub\u003e solution is 128.5 ℃. This is because the cations of the divalent salt form complexes with the carboxyl groups and other groups on the molecular chains, resulting in a crosslinked structure and producing flocculation precipitation, which decreases the stability of the solution and the original thickening performance at high temperatures. However, compared to conventional copolymers, APDM has stronger shear resistance due to the combination of special cationic hydrophobic monomers and NVP, resulting in better stability of the copolymer molecular chain. Therefore, modified APDM has higher viscosity retention and temperature and shear resistance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eA hydrophobic association copolymer with a medium molecular weight (APDM) was synthesized by copolymerizing AM as the backbone with AA, AMPS, DMAAC-18, and NVP via free-radical polymerization. The optimal synthesis conditions were as follows: a mass ratio of the AM, AA, AMPS, DMAAC-18, and NVP monomers of 16:6:3:1:1; a total monomer concentration of 27 wt%, and an initiator dosage of 0.2 wt% relative to the total monomer mass. Due to the moderate molecular weight of the copolymer, it has a good thickening effect at low concentrations, can quickly dissolve and disperse in water, has good adaptability in both aqueous and salt solutions, and has good thermal stability. A 0.5 wt% solution of the resulting APDM copolymer exhibited apparent viscosities of 87, 96, and 78 mPa\u0026middot;s in 20 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e mg/L NaCl, 15 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e mg/L CaCl\u003csub\u003e2\u003c/sub\u003e, and 15 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e mg/L MgCl\u003csub\u003e2\u003c/sub\u003e solutions, respectively, indicating that the APDM copolymer maintained high viscosity in a high-salinity environment.\u003c/p\u003e \u003cp\u003eThe viscoelastic test showed that the G\u0026prime; of APDM was greater than G\u0026prime;\u0026prime; in 2 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e mg/L NaCl solution and G\u0026prime; and G\u0026prime;\u0026prime; were higher in the salt solution than in deionized water. The salt tolerance test further showed that the apparent viscosity of 0.5 wt% APDM in 2 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e mg/L NaCl solution was 187 mPa\u0026middot;s, which was higher than that in deionized water. This can be attributed to the low salinity charge stimulating the hydrophobic micelle microregion of APDM, which facilitates the solubilization of the hydrophobic monomers and the aggregation of micelles. The hydrophobic association between molecules was also more strong and the viscoelastic ratio was higher than those in deionized water, which further verified that APDM showed excellent salt thickening ability in NaCl solutions. The rheological property test showed that the final apparent viscosities of APDM were maintained at 57.49 and 62.16 mPa\u0026middot;s after shearing a 0.5 wt% APDM solution in deionized water and a 20,000 mg/L NaCl solution, respectively, for 1 h at 140\u0026deg;C and 170 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Under the same conditions, 0.5 wt% APDM in 20,000 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e and MgCl\u003csub\u003e2\u003c/sub\u003e solutions exhibited an apparent viscosity of \u0026gt;\u0026thinsp;50 mPa\u0026middot;s at temperatures below 130\u0026deg;C and showed excellent temperature resistance and shear resistance. These results demonstrate that APDM is suitable for use as a thickener in high-salinity oil reservoirs.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was financially supported by the Qin-Chuangyuan \"Scientist and Engineer\" Team Construction Project (2024QCY-KXJ-052), Key R\u0026amp;D Program of Shaanxi Province (2024GX-YBXM-393), Industrialization Project of Shaanxi Provincial Education Department (23JC008) and the Youth Innovation Team Project of Shaanxi Universities(24JP022).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWenfei Wang: Conceptualization, data curation and writing-original draft\u003c/p\u003e\n\u003cp\u003eXiaojuan Lai: Funding acquisition and methodology\u003c/p\u003e\n\u003cp\u003eXianyun Shi: Investigation and project administration\u003c/p\u003e\n\u003cp\u003eWenwen Yang: Software\u003c/p\u003e\n\u003cp\u003eLei Wang: Supervision\u003c/p\u003e\n\u003cp\u003eRui Wang: Validation and visualization\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHaibin Li: Data curation\u003c/p\u003e\n\u003cp\u003eXiaojiang Song: Review and editing\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll of the material is owned by the authors and no permissions are required.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere are no conflicts of interest to declare.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInstitutional Review Board Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eYang L, Yang D, Liu C H, et al. 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Materials Letters, 2022.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"hydrophobic association effect, salt resistance, synergy, viscoelasticity, temperature resistance","lastPublishedDoi":"10.21203/rs.3.rs-6302949/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6302949/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTo improve the utilization rate of thickening agents used for oil recovery in high-temperature and high-salinity reservoirs, a medium-molecular-weight water-soluble hydrophobic association copolymer (APDM) was prepared via the copolymerization of acrylamide (AM) with acrylic acid (AA), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), octadecyldimethylallyl ammonium chloride (DMAAC-18), and \u003cem\u003eN\u003c/em\u003e-vinylpyrrolidone (NVP) using the free-radical polymerization method. The APDM copolymer was characterized and its salt resistance, viscoelastic properties, temperature resistance, and shear resistance were determined. Research has shown that the molecular weight of APDM was around 5 million. The apparent viscosity of a 0.5 wt% APDM solution was higher in 2 × 10\u003csup\u003e4\u003c/sup\u003e mg/L NaCl solution (187 mPa·s) than in clean water but decreased to 87 mPa·s when the NaCl concentration increased to 20 × 10\u003csup\u003e4\u003c/sup\u003e mg/L. In a high-salinity environment, the hydrophobic groups associated owing to the hydrophobic effect, causing the molecular chains to form a physical crosslinking network that increased the solution viscosity. Moreover, the steric hindrance stemming from the aggregation of hydrophobic groups prevented salt ions from approaching the hydrophobic groups, reducing the electrostatic interaction between salt ions and the hydrophobic long chains, endowing APDM with excellent salt thickening ability and salt resistance in NaCl solutions. At 140 °C and a shear rate of 170 s\u003csup\u003e−1\u003c/sup\u003e, the apparent viscosity of a 0.5 wt% APDM solution with deionized water and 20,000 mg/L NaCl aqueous solution as a solvent was \u0026gt;50 mPa·s after shearing for 1 h. Owing to its good temperature resistance and shear resistance, the APDM copolymer can find application as a thickener in high-salinity reservoirs.\u003c/p\u003e","manuscriptTitle":"Synthesis and salt thickening mechanism of salt-tolerant copolymers based on functional monomer synergy","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-25 14:43:34","doi":"10.21203/rs.3.rs-6302949/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"e1602bef-9056-4b9a-9241-328b0cc68bce","owner":[],"postedDate":"April 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-09-10T07:55:48+00:00","versionOfRecord":[],"versionCreatedAt":"2025-04-25 14:43:34","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6302949","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6302949","identity":"rs-6302949","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}