Effect of hydrophobic monomers with different carbon chains on the structure–activity relationship of associating polyacrylamides

Effect of hydrophobic monomers with different carbon chains on the structure–activity relationship of associating polyacrylamides | 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 Effect of hydrophobic monomers with different carbon chains on the structure–activity relationship of associating polyacrylamides Rong Yang, Xiaojuan Lai, Qiying Li, Xi Ding, Lei Wang, Xin Wen, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4393619/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 31 Jul, 2024 Read the published version in Journal of Polymer Research → Version 1 posted 5 You are reading this latest preprint version Abstract As temperature and salt-resistant materials, hydrophobically associating polymers can form a reversible spatial network structure through the interaction between their hydrophobic groups, effectively improve the viscosity of the polymer solution through association, and enhance the temperature and salt resistance of the polymer. Hydrophobically associating monomers have different effects on the properties of polymer solutions. Herein, acrylic acid, acrylamide, and 2-acrylamido-2-methylpropanesulfonic acid were used as hydrophilic monomers. The three hydrophobic monomers with different carbon chain lengths were prepared by the bromination reaction. Hydrophobic associating polymers DQM1-PAM, DQM2-PAM, and DQM3-PAM were prepared by aqueous solution free-radical polymerization. The structure–activity relationship of the hydrophobic monomers with different carbon chain lengths on polymers was studied. It was confirmed by Fourier-transform infrared spectroscopy and 1 H-NMR that the target product was successfully synthesized. Scanning electron microscopy revealed that with increasing hydrophobic carbon chain length, the hydrophobic microarea of molecular aggregation increased, forming a closer spatial network structure. Thermogravimetric and fluorescence tests revealed that with increasing hydrophobic carbon chain length of polymer molecules, the polymerization temperature resistance increased, intermolecular association degree increased, and critical association concentration decreased. Rheological property evaluation revealed that the viscosity of 0.5% polymer DQM1-PAM, DQM2-PAM, and DQM3-PAM was 71.32, 118.79, and 118.79 mPa·s after shearing at 120°C and 170 s − 1 for 1 h. With the increase in the carbon chain length, the retention rate of shear viscosity of polymer in a salt solution increased, showing good salt resistance. Concurrently, the molecular aggregation microarea of a solution with 0.5% polymer, degree of molecular chain action, viscoelasticity of the solution (G' > G''), and thixotropic area all increased. The performance of polymer solution can be improved by modifying hydrophobically associating polymers with long carbon chains, which has a broader application. hydrophobic association hydrophobic microarea viscoelasticity thixotropy structure–activity relationship Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Introduction Polymer flooding [1-2] can increase the swept area of a formation and is one of the effective means to enhance oil recovery. With increasing oilfield depth, the geological conditions become more complex and the burial depth of reservoirs, temperature, and salinity of formation water increase [3-5] . Conventional polyacrylamides cannot meet polymer flooding requirements. Under high-temperature and high-salt-concentration conditions, line polyacrylamide chains are prone to shear degradation [6- 8 ] and the solution performance is reduced, which cannot meet the water flooding in deep reservoirs [7-10] . Therefore, modified hydrophobically associating polyacrylamide has attracted the attention of many scholars. During acrylamide (AM) polymerization, aqueous solution specificity is achieved by grafting different side chain hydrophobic functional monomers [11-12] . When the polymer concentration is higher than the critical association concentration (CAC) [13-14] , the association between hydrophobic monomer molecules increases the viscosity and viscoelasticity. Moreover, the temperature and salt tolerance of polymers were considerably improved [15-17] . Yang et al [18] . synthesized long-chain hydrophobic initiator, 2,2'-azobisisobutyl dodecyl amidine hydrochloride (AIBL), from several raw materials such as sodium acrylate, AM, and a hydrophobic functional monomer, namely dodecyl dimethyl allyl ammonium chloride (C 12 DMAAC). The polymer exhibited good temperature resistance, shear stability, and viscoelasticity. Wan et al [19] . synthesized the terpolymer of acrylamide (AM/NaAA/DiC 8 AM), twin-tailed hydrophobic monomer (N, N-dioctylacrylamide), and sodium acrylate via micelle copolymerization . They determined the structure and rheological properties of the copolymer, which revealed its excellent temperature and salt resistances. Herein, hydrophobic functional monomers with different carbon chain lengths were synthesized to determine the effects of hydrophobic carbon chain length on the association behavior and solution properties of polymers. Acrylic acid (AA) and AM were used as the main molecular skeleton of polyacrylamide with different carbon chains, and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) was used as a salt-tolerant monomer to synthesize hydrophobic monomers with different carbon chains. These were then grafted into the main molecular skeleton. The intermolecular association strength varied because different hydrophobic carbon chain lengths have different hydrophobic effects on the polymer. Longer hydrophobic carbon chains lead to larger hydrophobic microareas in the polymer aqueous solution and enhanced intermolecular interactions; thus, the resistance of polymer to temperature and salt increases. Moreover, the structure–activity relationship of associating polymers with different carbon chain lengths was elucidated by analyzing the salt tolerance, rheological properties, and microstructure differences of macroscopic hydrophobically associating polymers in an aqueous solution. 1 Experimental 1.1 Reagents and instruments Acrylamide (AM) was provided by Henan Mingzhixin Chemical Co. Ltd. Acrylic acid (AA) was obtained from China Henan Mingzhixin Chemical Products Co. Ltd. 2-Acrylamido-2-methylpropanesulfonic acid (AMPS) was obtained Nanjing Bermuda Biotechnology Co. Ltd. Ammonium persulfate (APS) was purchased from Shandong Shenmao Chemical Co. Ltd. Ascorbic acid (Vc) and 2,2'-azodiisobutylamidine dihydrochloride (V50) were purchased from Hubei Shixing Chemical Co. Ltd. Sodium bisulfite, urea, and APS were purchased from China Chengdu Kelon Reagent Co. Ltd. Hydrophobic functional monomer (DQM) was prepared in the laboratory. Sodium formate was purchased from Jinan Mingguan Chemical Co. Ltd. Equipment used: Haake Mars 40 rotary rheometer was purchased from Haake, Germany. ADVANCE III 400MHz nuclear magnetic resonance spectrometer was purchased from Beijing Agilent Technology Co. Ltd. HELOS-OASIS dry and wet two-in-one laser particle size analyzer was purchased from New Patek Co. Ltd. Germany. ZEM desktop scanning electron microscope was purchased from Anhui Zeyou Technology Co. Ltd. FT-IR “Rocket” small Fourier-transform infrared spectrometer was purchased from Arcoptix, Switzerland. The BCQT98 nitrogen generator was purchased from Shijiazhuang Bochuang Air Separation Equipment Co. Ltd. RE-220 rotary evaporator was purchased from Zhengzhou Bohui Precision Technology Co. Ltd. Malvern Panaco X-ray diffractometer was purchased from Shanghai Sibaiji Instrument Co. Ltd. Thermogravimetric analyzer (TGAQ500) was purchased from Shanghai Lai Rui Scientific Instrument Co. Ltd. Lumina fluorescence spectrometer was purchased from Semir Fisher Technologies Inc. XGJ-S digital high-speed mixer was purchased from Qingdao Hong Yu Lin Petroleum Instrument Co. Ltd. 1.2 Hydrophobic monomer synthesis The molar ratio of bromododecane, bromohexadecane, bromodocosane, and N- (3-dimethylaminopropyl) methacrylamide (DMAPMA) was 1:1.1, and 50–60% of anhydrous acetone solution was placed in a three-neck flask. The mixture was held at 50°C for 30 h. The excess solvent was removed after the reaction in a rotary evaporator, which yielded a light-yellow liquid. The liquid was washed repeatedly with anhydrous ether, and three types of white precipitates were obtained, which were vacuum dried at room temperature to obtain DQM1, DQM2, and DQM3 (C12, C16, and C22, respectively). The synthesis reaction is shown in Figure.1. 1.3 Hydrophobically associating polymer synthesis AM, AA, AMPS, and DQM monomer solutions were prepared. The mass ratio of each component of the monomer solution was m(AM):m(AA):m(AMPS):(DQM):(H 2 O) = 20:11:3:1:65, and the mass ratio of the polymer monomer to the system was 35 wt%. The pH of the monomer solution was adjusted to 6.5–7.0 by mixing sodium hydroxide solution. The solution was deoxygenated with nitrogen for 30 min and cooled to 0°C–5°C. The mass ratio of the components of the initiator was m(APS):m(Vc):m(V50) = 1:5:2. The initiator accounted for 0.03% of the system, and sodium formate was used as a chain transfer agent to adjust the molecular weight of the polymer. The initiator was uniformly mixed with the solution by N 2 . N 2 was no longer introduced after the polymer solution turned sticky. The reactor was sealed and wrapped with an insulation sleeve to prevent heat dissipation. At the end of the reaction, the polymerization temperature reached its peak. The mixture was then cooled to obtain polymer blocks, which were cut, dried, crushed, and granulated sequentially. The powder hydrophobic association polymers DQM1-PAM, DQM2-PAM and DQM3-PAM with different carbon chain lengths were obtained. The reaction pathway of the final product is shown in the Figure. 2. 1.4 Structural characterization and performance test 1.4.1 Fourier-transform infrared spectroscopy The synthesized polymer was prepared using the KBr tablet method, and the characteristic functional groups and the structure of DQMn-PAM were characterized via Fourier-transform infrared spectroscopy (FTIR). 1.4.2 Proton nuclear magnetic resonance A certain amount of DQMn-PAM powder was collected and dissolved in deuterated water. 0.01–0.05 wt% of polymer solution was loaded into the nuclear magnetic resonance tube for testing. The molecular structure of DQMn-PAM was tested via proton nuclear magnetic resonance spectroscopy, and the 1 H-NMR spectrum was obtained. 1.4.3 Molecular weight characterization The viscosity average molecular weight of DQMn-PAM was measured using the viscosity method, and 1 mol/L NaCl aqueous solution was used as the standard sample solvent. A U-type viscometer was used to determine the viscosity of DQMn-PAM sample solution (concentration: 0.1–0.02%) at a constant temperature of 30.0°C ± 0.05°C. The relative viscosity ηr was calculated by recording the time t 0 and t of the nondiluted Ubbelohde viscometer. The intrinsic viscosity η was determined by the time ratio, and the molecular weight of DQMn-PAM was characterized using the Mark–Houwink equation. 1.4.4 Thermogravimetric analysis At 30°C–700°C, the system was introduced into N 2 , and DQMn-PAM was detected by thermogravimetric analyzer. 1.4.5 X-ray diffraction analysis DQMn-PAM was detected by X-ray diffractometer at a holding voltage of 40 kV and a current of 40 mA. 1.4.6 Determination of critical association concentration via fluorescence spectrophotometry DQMn-PAM was first conFigureured in a volumetric flask as a solution with a concentration of 0.5%. Then, 5 × 10 − 5 L of pyrene solution was dissolved in an ethanol solution in a 50-mL volumetric flask and mixed evenly. The ethanol was dried with N 2 . The DQMn-PAM aqueous solution was diluted to the scale line, ultrasonicated in a water bath for 30 min, and N 2 was continuously introduced for 30 min to remove O 2 from the aqueous solution. The fluorescence spectrum of pyrene was obtained using a fluorescence spectrophotometer at a wavelength of 335 nm, a detection temperature of 25°C, and a scanning range of 350–450 nm. 1.4.7 Scanning electron microscopy DQMn-PAM solution with a mass fraction of 0.15% was prepared by mixing the three polymer powders in distilled water and 500 mg/L CaCl 2 solution. The samples were frozen in liquid nitrogen and vacuum dried, and their microscopic aggregation morphology was observed via scanning electron microscopy. 1.4.8 Determination of thixotropic properties The thixotropic properties of 0.5% DQMn-PAM solution were tested using a rotary rheometer. The test program was set up according to the “up–down” shear process at up-shear and down-shear rates of 0–100 and 100–0 s − 1 , respectively [ 20 ] . 1.4.9 Thixotropy performance test The 0.5% polymer aqueous solution was prepared by mixing DQMn-PAM in clear water, 6% NaCl solution, and 1% CaCl 2 solution. The relation between elastic modulus (G') and viscous modulus (G") of polymers based on frequency and stress was tested using a rheometer at 30°C. 1.4.10 Determination of rheological properties The rheological properties of DQMn-PAM solution were tested using the Haake Mars 40 rotary rheometer. The 0.5% DQMn-PAM aqueous solution was prepared by mixing the polymer in clear water, 20000 mg/L NaCl, and 2000 mg/L CaCl 2 . Then, the temperature and shear resistances of these solutions were determined. 2 Results and discussion 2.1 Structural characterization and thermal performance analysis 2.1.1 Infrared spectroscopy analysis DQMn-PAM was characterized via FTIR; the results are shown in Figure. 3a.A peak corresponding to the stretching vibration of N–H was observed at 3474 cm − 1 . The peak at 1693 cm − 1 indicates the presence of olefinic double bonds (C = C) and that at 1671 cm − 1 corresponds to the C = O stretching vibration. The peak at 1425 cm − 1 reflects the presence of N + units in the bending vibration region of the saturated C–H plane. The peaks at 1110 cm − 1 correspond to the C-N and C-C stretching vibrations. The peak at 953 cm − 1 confirms the existence of disubstituted olefin units. The FTIR spectra showed that DQMn-PAM was successfully synthesized. 2.1.2 Proton nuclear magnetic resonance DQMn-PAM was tested using the ADVANCE III 400MHz nuclear magnetic resonance spectrometer; the spectra are shown in Figure. 3b.The peaks at δ5.75–5.80 and δ5.30–5.35 correspond to H-C = C-C = O and H-C = C-CH 3 , respectively. The peaks at δ4.70, δ3.23, and δ2.85 correspond to D 2 O, —CH 2 — in DQM, and —CH 2 — in AMPS and DQM, respectively. The peaks at δ1.57 and δ2.21 correspond to two methylene proton peaks in —CH 2 —CH 2 — on the molecular chain of DQMn-PAM. The peaks at δ1.49, δ1.22, δ0.86, δ0.98, and δ3.57 correspond to —CH 3 in AMPS, (—CH 2 —) n in DQM, —CH 3 in DQM, and ethanol solvent, respectively. The structure of the product was similar, which proved that the hydrophobic monomer DQM was successfully inserted into the main chain of DQMn-PAM. 2.1.3 Molecular weight characterization DQMn-PAM was synthesized via aqueous free-radical polymerization, and its molecular weight was measured using the one-point method (Table 1 ). As the carbon chain of DQM monomer lengthened, the steric hindrance effect increased during polymerization. Thus, the molecular weight of DQMn-PAM was > 6 million atomic units. Table 1 Determination of molecular weight of DQMn-PAM The flow time of polymer solution (s) The flowing time of NaCl solution (s) Strength of solutionc (g/mL) Relative viscosity [ηr] Characteristic viscosity [η] (mL/g) M DQM1-PAM 228 129 0.001226 1.23 1226 6967323 DQM2-PAM 220 129 0.001143 1.14 1144 6268456 DQM3-PAM 217 129 0.001121 1.11 1112 6008496 2.1.4 X-ray diffraction analysis Figure. 3c shows the X-ray diffraction pattern of DQMn-PAM. No sharp peaks were observed in the spectra of the three synthesized products, which confirms that polymer has an amorphous structure. The hydrophobic monomer carbon chain was grafted into the main chain of the polymer molecule. As the grafted carbon chain lengthened, the intermolecular association enhanced. Thus, the polymer crystallinity decreased and hence irregular molecular movement increased. The association between the molecular chains of DQMn-PAM gradually changed from intermolecular association to intramolecular association. 2.1.5 Thermogravimetric analysis of DQMn-PAM Figure. 3d shows the thermogravimetric curve of the weight loss of DQMn-PAM categorized into three stages. In the first stage, as the temperature increased, part of the free water in the polymer gradually volatilized when T < 223°C. When 223°C < T 450°C, the polymer began to decompose and the amide group was converted to ammonia. As DQM3 had a long carbon chain, the association effect in DQMn-PAM was stronger than those of DQM1 and DQM2. The degree of entanglement of DQMn-PAM macromolecules deepened, the thermodynamic decomposition temperature increased, and the internal structure of the molecular chain was more stable. 2.2 Critical association concentration of DQMn-PAM Figure. 4a shows the effect of DQMn-PAM concentration on its apparent viscosity. The apparent viscosity gradually increased at low polymer concentration and sharply as the concentration increased. This is because the self-aggregation behavior of DQMn-PAM polymer changed from intramolecular association to intermolecular association when a turning point appeared in the concentration–viscosity curve, i.e., the critical association concentration (C*) was achieved [ 21 ] . At high concentrations, the self-aggregation behavior was mainly dominated by intermolecular hydrophobic association. As the carbon chain of the hydrophobic monomer continued to lengthen, C* continued to decrease. In other words, the longer the hydrophobic monomer chain length in DQMn-PAM, the easier it is to form a larger volume of hydrophobic microdomains in aqueous solution and the lower the association concentration [ 22 ] . Pyrene was dissolved by the hydrophobic microdomain of hydrophobic association complex, and the I 1 /I 3 ratio responded to DQMn-PAM. The association behavior and critical association concentration of DQMn-PAM were studied. As shown in Figure. 4b, molecular polarity around the probe increased with increasing I 1 /I 3 ratio. Moreover, the hydrophobic microdomain became smaller and the hydrophobic association effect weakened. As the I 1 /I 3 ratio rapidly decreased, the polymer concentration reached the critical association concentration. Figure. 2c shows that when the DQMn-PAM concentration is 0.10–0.25 wt%, I 1 /I 3 is high and does not change considerably. During this time, the solution concentration is low, hydrophobic microdomain is small, intramolecular association is dominant, and association effect is poor. As the DQMn-PAM concentration increased, I 1 /I 3 began to decrease rapidly. Moreover, the effective concentration of hydrophobic monomer increased, polymerization between monomers gradually changed to between molecular chains, volume of associating hydrophobic microdomains increased, solubilization effect of pyrene molecules gradually enhanced, and the polarity of the environment around the pyrene probe. Figure. 5 shows the changes in the apparent viscosity of 0.5% DQMn-PAM in NaCl and CaCl 2 solutions with different mass fractions. The apparent viscosity of DQMn-PAM gradually decreased as the mass fraction of NaCl and CaCl 2 increased. Moreover, the apparent viscosity rapidly decreased when the polymer was mixed in the divalent salt ion solution, with good retention in both the salt ion solutions. The longer the carbon chain of DQMn-PAM, the stronger the interaction between the molecular chains, the closer the molecular space structure, and the stronger the association effect. This resulted in a higher viscosity retention rate and good salt resistance of DQMn-PAM. 2.3 Scanning electron microscopy of DQMn-PAM Figure. 6 shows the microscopic aggregation morphology of the DQMn-PAM molecules in fresh water and salt water. Figures. 6a–c show the microscopic aggregation morphology of DQMn-PAM molecules in clear water and salt water. In clear water, DQMn-PAM had a higher degree of molecular expansion and the association between hydrophobic monomers formed a spatial network structure. The association effect gradually increased with increasing carbon chain length, and the molecular aggregation state changed from a spatial network structure to a layered interpenetrating network structure. As shown in Figures. 6d–f, charged ions inhibited the expansion of polymer molecular chains in in salt water, and electrostatic shielding caused the molecular chains to curl. The molecular aggregation state changed from sheet to line [ 23 ] , thereby reducing the hydrodynamic volume of the solution. The apparent viscosity of DQMn-PAM in salt water was low, and the network density of DQM3-PAM in clear water and salt water was higher than that of DQM1-PAM with better solution characteristics. 2.4 Thixotropy analysis Figure. 7 shows the thixotropy test results of DQMn-PAM solution. As the shear rate increases, the externally applied energy is aggregated by the molecules into the network reservoir. When the shear rate decreases, the energy release is delayed and a thixotropic ring is formed. As the polymer carbon chain lengthens, the hysteresis area of the system considerably increases. This indicates that the strength of the DQMn-PAM network system is positively correlated with the growth of the polymer carbon chain. In other words, the introduction of hydrophobic monomers improves the network structure of the polymer system and therefore DQMn-PAM solution exhibits obvious thixotropy. As the hydrophobic carbon chain lengthens, the ability of the polymer to resist external mechanical action is gradually enhanced. The thixotropic ring increases, the association time increases, and the association effect is enhanced, resulting in enhanced thixotropy. 2.5 Viscoelastic energy analysis A 0.5% polymer aqueous solution was prepared by mixing DQMn-PAM in clear water, 6% NaCl solution, and 1% CaCl 2 solution. The variation curves of G' and G" with stress and frequency were tested, where G' is the storage modulus and G" is the energy dissipation modulus. The relationship between G' and G" with stress and frequency is shown in Figure. 8a-c. Figure. 8a-c shows that the hydrophobically associating polymer DQMn-PAM solution forms a supramolecular structural dynamic–physical cross-linking network, with high viscoelasticity. G' of DQM1-PAM solution is less than G" in the whole stress scanning range. At low frequencies, the solution has the highest viscosity. At a lower concentration, the orientation of the polymer molecular chain is affected by the shear force, the spatial structure is destroyed, and the polymer molecular chain freely expands. As the polymer concentration increases, the concentration of DQM2-PAM solution exceeds the critical association concentration and G' > G". At this time, the solution is mainly elastic and the hydrophobic microregion is positively correlated with DQMn-PAM concentration. When the hydrophobic carbon chain of the polymer solution lengthens, the degree of association between the polymer molecules increases and a closer spatial network structure is formed. Therefore, in the DQM3-PAM solution, G' > G" and the viscoelasticity of DQM3-PAM is considerably higher. Figure. 8d-f shows that the viscosity and elasticity of DQMn-PAM increased in the frequency scanning range of 0.1–10 Hz. This is because at low frequencies, the molecular chain gap of DQMn-PAM is relatively loose, intramolecular association is dominant, and most of the energy is lost by viscous flow. When the frequency gradually increases, the intramolecular association gradually changes to intermolecular association and the molecules are entangled with each other, thus improving the three-dimensional network structure of DQMn-PAM. Moreover, G' continues to increase. Under the same conditions, the G' and G" of clear water are smaller than those of the salt water. This is because the charge of the salt increases the hydrophobic microarea of DQMn-PAM, thereby increasing the degree of aggregation of micelles in the solution and improving its viscoelasticity. 2.6 Analysis of rheological properties DQMn-PAM solution with a mass fraction of 0.5% was prepared using fresh water, 20000 mg/L NaCl, and 2000 mg/L CaCl 2 . It was mixed until fully dissolved, and the temperature and shear resistances were tested; results are shown in Figure. 9. Figure. 9 shows that the viscosity of the three polymers decreases with increasing temperature. When the temperature tends to be balanced, the viscosity stabilizes. In the early stage of shearing, the viscosity of the polymer decreases because the shear force causes the disordered molecular chains to be arranged in an orderly manner. When the external conditions stabilize, the intermolecular interaction and hydrogen bonding are in dynamic equilibrium with the molecular thermal motion; thus, the viscosity remains unchanged. The initial viscosity of DQM1-PAM, DQM2-PAM, and DQM3-PAM before shearing was 122, 131, and 155 mPa·s, respectively. As the hydrophobic carbon chain of the polymer lengthened, the volume of the hydrophobic microdomain increased continuously and the association of the polymer gradually transformed into intermolecular association. Moreover, the association strength gradually increased, forming a more compact three-dimensional network structure. Thus, the viscosity retention rate was higher, which is more conducive to enhancing the salt resistance of the polymer. Figure. 10a shows the temperature and shear resistance curves of 0.5% polymer DQMn-PAM solution in 20000 mg/L NaCl aqueous solution. The apparent viscosity of the DQMn-PAM polymer solution first increases and then decreases with time and tends stabilize after long-term shearing. The viscosity retention rate after shearing is good, which is higher than that of the other two polymer solutions. As temperature increases, the molecular thermal motion of the hydrophobic chain of the polymer increases and the apparent viscosity of the polymer increases with the free expansion of the molecular chain. When the temperature further increases, the Brownian motion of the hydrophobic group of the polymer DQMn-PAM intensifies, association of the polymer system weakens, molecular chain shrinks, and viscosity of the polymer solution gradually reduces. Longer carbon chains of the polymer hydrophobic monomer lead to larger spatial three-dimensional networks formed by hydrophobic association and closer interconnections, resulting in increased viscosity. Figure. 10b shows the temperature and shear resistance curve of 0.5% polymer DQMn-PAM solution in 2000 mg/L CaCl 2 aqueous solution. As the temperature increases, the apparent viscosity of the DQMn-PAM polymer solution increases slowly. This is because the divalent salt ion (Ca 2+ ) in the solution interacts with surrounding hydrophobic monomer groups to increase the polarity and hydrodynamic volume of the DQMn-PAM solution, which further affects the self-aggregation behavior of the polymer solution. 2.7 Structure–activity relationship of hydrophobic monomers with different carbon chains on associating polymers Three linear alkyl hydrophobically associating polymers, DQMn-PAM, with different carbon chain lengths were obtained via aqueous free-radical polymerization. The effect of carbon chain lengths on the association properties of polymer solution was analyzed. As the carbon chain lengthened, the critical association concentration of the polymer decreased and the hydrophobicity of the hydrophobic carbon chain increased. The volume of hydrophobic association microdomain formed by the polymer in the aqueous solution increased, thereby increasing the strength of the polymer structure network. The thixotropic area of the polymer increased when the polymer was subjected to an external force, and its temperature resistance, shear resistance and viscoelasticity improved. The effect of carbon chain length on the association mechanism of DQMn-PAM is shown in Figure. 11. 3 Conclusions The hydrophobically associating polymer, DQMn-PAM, with three different carbon chains was prepared via aqueous free-radical polymerization and characterized via FTIR and 1 H-NMR spectroscopies. Results showed that the monomer was successfully incorporated into the molecular main chain and the target product was successfully synthesized. The molecular weight of DQMn-PAM was > 6 million atomic units. The structure–activity relationship of different carbon chain lengths on polymer solution was determined by testing. (1) X-ray, TG, and fluorescence performance tests showed that the association of the polymer increased as the hydrophobic carbon chain lengthened and the temperature resistance of the polymer improved. SEM images showed that the polymer molecules formed a more compact three-dimensional space network as the carbon chain lengthened and the association structure enhanced. (2) The association behavior and critical association concentration of DQMn-PAM were studied using a fluorescent pyrene probe. Results showed that the longer the chain length of the hydrophobic monomer in the polymer, the lower the critical association concentration (C*). Moreover, the more favorable the hydrophobically associating copolymer to form a spatial network structure, the larger the spatial size of the DQMn-PAM three-dimensional network. (3) The rheological property evaluation of DQMn-PAM revealed that with increasing hydrophobic carbon chain length, the association between polymer molecules increased, leading to improved temperature, salt, and shear resistances of the polymer. The strong intermolecular association improved the viscoelasticity of the polymer. The solution was mainly elastic, and the aggregation microarea of the polymer molecules increased, thereby improving the thixotropic area of the polymer. The modification of the polymer by long carbon chain hydrophobic monomers could effectively improve the performance of the polymer solution, broadening the application field of hydrophobically associating polyacrylamide. Declarations Conflict of Interest The authors declare no competing financial interest. Author Contributions Rong Yang : Writing–original draft (lead). Xiaojuan Lai : Supervision (lead). Qiying Li : Conceptualization (equal). Xi Ding : Project administration (equal); software (equal). Xin Wen : Supervision (equal). Yan Guo : Formal analysis (equal). Acknowledgments This research was supported by the Qin-Chuangyuan "Scientist + Engineer" Team Construction Project (2024QCY-KXJ−052), the Key R & D Program of Shaanxi Province (2024GX-YBXM−393), the Industrialization Project of Shaanxi Provincial Education Department (23JC008) in China. Data Availability Statement The data that support the findings of this study are available from corresponding author upon reasonable request. References J. H. Gao, G. H. Zhang, L. Wang, RSC Adv. 2019, 9 , 15246. J. M. Shi, Z. L. Wu, Q. C. Deng, L. Liu, X. F. Zhang, X. Y. Wu, Y. G. Wang, J Mol Liq. 2021, 79 , 4581. J. C. Mao, J. X. Xue, H. Zhang, X.J. Yang, C. Lin, Q. H. Wang, C.Li, Z. J. Liao, Colloid Polym. SciI. 2022, 300 , 569. J. Q. 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Supplementary Files GraphicalAbstract.png Cite Share Download PDF Status: Published Journal Publication published 31 Jul, 2024 Read the published version in Journal of Polymer Research → Version 1 posted Reviewers agreed at journal 22 May, 2024 Reviewers invited by journal 22 May, 2024 Editor invited by journal 12 May, 2024 Editor assigned by journal 10 May, 2024 First submitted to journal 09 May, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-4393619","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":305484408,"identity":"4048ef30-d8cc-4e4b-a5a2-8732e984cb39","order_by":0,"name":"Rong Yang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Rong","middleName":"","lastName":"Yang","suffix":""},{"id":305484409,"identity":"bebec805-2f10-4ec4-8ee4-66a14d53be7c","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":305484410,"identity":"81c26827-449f-4e9e-b802-c3b2f99edd7f","order_by":2,"name":"Qiying Li","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Qiying","middleName":"","lastName":"Li","suffix":""},{"id":305484411,"identity":"4668204c-e1a5-41e3-b031-683973b711c4","order_by":3,"name":"Xi Ding","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Xi","middleName":"","lastName":"Ding","suffix":""},{"id":305484412,"identity":"5c25ef6f-3a9d-4d94-857b-cdeb3665fbd2","order_by":4,"name":"Lei Wang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Lei","middleName":"","lastName":"Wang","suffix":""},{"id":305484413,"identity":"b4c196fa-907b-4f6f-b775-251ddf23f5c5","order_by":5,"name":"Xin Wen","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Xin","middleName":"","lastName":"Wen","suffix":""},{"id":305484414,"identity":"50bb86a7-0156-4d36-a83a-39994e4be5cb","order_by":6,"name":"Yan Guo","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Yan","middleName":"","lastName":"Guo","suffix":""}],"badges":[],"createdAt":"2024-05-09 08:11:16","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4393619/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4393619/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10965-024-04083-4","type":"published","date":"2024-07-31T15:57:12+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":57726817,"identity":"d0e9b344-24e4-49db-9768-13795f56aeed","added_by":"auto","created_at":"2024-06-04 21:05:57","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":7589,"visible":true,"origin":"","legend":"\u003cp\u003eSynthesis route of DQM\u003c/p\u003e","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/eae5fca30c711e8f2b6f12bb.png"},{"id":57726665,"identity":"fdbd5062-def7-4947-9236-721a7301cd41","added_by":"auto","created_at":"2024-06-04 20:57:57","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":13706,"visible":true,"origin":"","legend":"\u003cp\u003eSynthesis route of DQMn-PAM\u003c/p\u003e","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/8809049f02c74c29aeec2a8d.png"},{"id":57726675,"identity":"599ff93b-865b-48a2-aee7-a77be78a8b5b","added_by":"auto","created_at":"2024-06-04 20:57:58","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":58050,"visible":true,"origin":"","legend":"\u003cp\u003e(a) FTIR spectrum of DQMn-PAM. (b) Hydrogen spectra of DQMn-PAM. (c) XRD patterns of DQMn-PAM. (d) TG curve of the polymer DQMn-PAM.\u003c/p\u003e","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/e9203228b715660f035dc321.png"},{"id":57727031,"identity":"a0deb61c-edef-459f-a5ab-c8cbe5d4ad3e","added_by":"auto","created_at":"2024-06-04 21:13:57","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":49232,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Effect of the mass fraction of DQMn-PAM on its apparent viscosity. (b)Fluorescence spectra of DQMn-PAM of different concentrations. (c) I\u003csub\u003e1\u003c/sub\u003e/I\u003csub\u003e3\u003c/sub\u003e ratio for different DQMn-PAM concentrations.\u003c/p\u003e","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/7337ea96f5534be3dd16b339.png"},{"id":57726672,"identity":"ce62775c-3a35-4e04-905c-30b69519c800","added_by":"auto","created_at":"2024-06-04 20:57:57","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":28584,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in apparent viscosity of 0.5% polymer DQMn-PAM in solutions of different mass fractions of (a) NaCl. (b) CaCl\u003csub\u003e2.\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/a15f5e1868e2b787df6462f2.png"},{"id":57726671,"identity":"ca82f822-fce9-46b2-b663-a1417d868951","added_by":"auto","created_at":"2024-06-04 20:57:57","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":121892,"visible":true,"origin":"","legend":"\u003cp\u003eScanning electron micrograph of the DQMn-PAM. (a) 0.1% DQM1-PAM, fresh water, (b) 0.1% DQM2-PAM, fresh water, (c) 0.1% DQM3-PAM, fresh water, (d) 0.1% DQM1-PAM, 5% NaCl solution, (e) 0.1% DQM2-PAM, 5% NaCl solution, and (f) 0.1% DQM3-PAM, 5% NaCl solution\u003c/p\u003e","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/e130d357c1825e64f5f3310f.png"},{"id":57726667,"identity":"7ea57d01-57b9-4ad3-9950-ce4fbf9cdf5a","added_by":"auto","created_at":"2024-06-04 20:57:57","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":34318,"visible":true,"origin":"","legend":"\u003cp\u003eThixotropic diagram of DQMn-PAM: (a) DQM1-PAM, (b) DQM2-PAM, and (c) DQM3-PAM\u003c/p\u003e","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/15a2f202665b4525ee0b1733.png"},{"id":57726819,"identity":"3333e39a-8975-4705-9255-a8ed08d1acae","added_by":"auto","created_at":"2024-06-04 21:05:57","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":77683,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of stress scanning on the viscoelasticity of DQMn-PAM. (a) fresh water, (b) 6% NaCl, and (c) 1% CaCl\u003csub\u003e2. \u003c/sub\u003eEffect of frequency scanning on the viscoelasticity of DQMn-PAM. (d) fresh water, (e) 6% NaCl, and (f) 1% CaCl\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/76ca2876ad16933ddb3b1484.png"},{"id":57726676,"identity":"13751afb-35ac-4a47-abc4-46cb741ad9ac","added_by":"auto","created_at":"2024-06-04 20:57:58","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":46807,"visible":true,"origin":"","legend":"\u003cp\u003eTemperature and shear resistance curve of 0.5% DQMn-PAM. (a) DQM1-PAM, (b) DQM2-PAM, and (c) DQM3-PAM\u003c/p\u003e","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/fd023bedf2dda508e500114f.png"},{"id":57726674,"identity":"aafbda63-2d5e-4ed7-a7e7-32bbbbbc6948","added_by":"auto","created_at":"2024-06-04 20:57:58","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":45495,"visible":true,"origin":"","legend":"\u003cp\u003eViscosity variation of 170 s\u003csup\u003e−1\u003c/sup\u003e for 0.5% DQMn-PAM in salt solution with variations in thermal shear. (a) 120°C, 20000 mg/L NaCl aqueous solution and (b) 120°C, 2000 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"Onlinefloatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/e728725d53a643dca73ef324.png"},{"id":57726673,"identity":"551b157f-0754-4541-9b09-0b60bf55d77a","added_by":"auto","created_at":"2024-06-04 20:57:57","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":57156,"visible":true,"origin":"","legend":"\u003cp\u003eStructure–activity relationship diagram of DQMn-PAM\u003c/p\u003e","description":"","filename":"Onlinefloatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/f71f0a7d7db92fbcf19d06bd.png"},{"id":61793876,"identity":"d4126e88-29cc-4b6f-8cef-941195994f97","added_by":"auto","created_at":"2024-08-05 16:16:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1491407,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/2d103fcb-5e65-4849-9488-a17029405e52.pdf"},{"id":57726669,"identity":"4015e5e3-5d60-4725-bba8-b24027ae81b2","added_by":"auto","created_at":"2024-06-04 20:57:57","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":476261,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract.png","url":"https://assets-eu.researchsquare.com/files/rs-4393619/v1/9e0b16a188c2dcdc7bb95987.png"}],"financialInterests":"","formattedTitle":"Effect of hydrophobic monomers with different carbon chains on the structure–activity relationship of associating polyacrylamides","fulltext":[{"header":"Introduction","content":"\u003cp\u003ePolymer flooding \u003csup\u003e[1-2]\u003c/sup\u003e can increase the swept area of a formation and is one of the effective means to enhance oil recovery. With increasing oilfield depth, the geological conditions become more complex and the burial depth of reservoirs, temperature, and salinity of formation water increase \u003csup\u003e[3-5]\u003c/sup\u003e. Conventional polyacrylamides cannot meet polymer flooding requirements. Under\u0026nbsp;high-temperature\u0026nbsp;and high-salt-concentration conditions, line polyacrylamide chains are prone to shear degradation \u003csup\u003e[6-\u003c/sup\u003e\u003csup\u003e8\u003c/sup\u003e\u003csup\u003e]\u003c/sup\u003eand the solution performance is reduced, which\u0026nbsp;cannot\u0026nbsp;meet the water\u0026nbsp;flooding in deep reservoirs\u0026nbsp;\u003csup\u003e[7-10]\u003c/sup\u003e. Therefore, modified hydrophobically associating polyacrylamide has attracted the attention of many scholars. During\u0026nbsp;acrylamide\u0026nbsp;(AM)\u0026nbsp;polymerization,\u0026nbsp;aqueous solution specificity is achieved by grafting different side chain\u0026nbsp;hydrophobic functional monomers\u0026nbsp;\u003csup\u003e[11-12]\u003c/sup\u003e.\u0026nbsp;When the polymer concentration is higher than the critical association concentration (CAC) \u003csup\u003e[13-14]\u003c/sup\u003e, the association between hydrophobic monomer molecules increases the viscosity and viscoelasticity. Moreover, the temperature and salt tolerance of polymers were considerably improved \u003csup\u003e[15-17]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eYang et al\u003csup\u003e[18]\u003c/sup\u003e. synthesized long-chain hydrophobic initiator, 2,2\u0026apos;-azobisisobutyl dodecyl amidine hydrochloride (AIBL), from several raw materials such as sodium acrylate, AM, and a hydrophobic functional monomer, namely dodecyl dimethyl allyl ammonium chloride (C\u003csub\u003e12\u003c/sub\u003eDMAAC). The polymer exhibited good temperature resistance, shear stability, and viscoelasticity. Wan et al\u003csup\u003e[19]\u003c/sup\u003e. synthesized the terpolymer of acrylamide (AM/NaAA/DiC\u003csub\u003e8\u003c/sub\u003eAM), twin-tailed hydrophobic monomer (N, N-dioctylacrylamide), and sodium acrylate via micelle copolymerization . They determined the structure and rheological properties of the copolymer, which revealed its excellent temperature and salt resistances.\u003c/p\u003e\n\u003cp\u003eHerein, hydrophobic functional monomers with different carbon chain lengths were synthesized to determine the effects of hydrophobic carbon chain length on the association behavior and solution properties of polymers. Acrylic acid (AA) and AM were used as the main molecular skeleton of polyacrylamide with different carbon chains, and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) was used as a salt-tolerant monomer to synthesize hydrophobic monomers with different carbon chains. These were then grafted into the main molecular skeleton. The intermolecular association strength varied because different hydrophobic carbon chain lengths have different hydrophobic effects on the polymer. Longer hydrophobic carbon chains lead to larger hydrophobic microareas in the polymer aqueous solution and enhanced intermolecular interactions; thus, the resistance of polymer to temperature and salt increases. Moreover, the structure\u0026ndash;activity relationship of associating polymers with different carbon chain lengths was elucidated by analyzing the salt tolerance, rheological properties, and microstructure differences of macroscopic hydrophobically associating polymers in an aqueous solution.\u003c/p\u003e"},{"header":"1 Experimental","content":"\u003cdiv id=\"Sec2\" class=\"Section2\"\u003e \u003ch2\u003e1.1 Reagents and instruments\u003c/h2\u003e \u003cp\u003eAcrylamide (AM) was provided by Henan Mingzhixin Chemical Co. Ltd. Acrylic acid (AA) was obtained from China Henan Mingzhixin Chemical Products Co. Ltd. 2-Acrylamido-2-methylpropanesulfonic acid (AMPS) was obtained Nanjing Bermuda Biotechnology Co. Ltd. Ammonium persulfate (APS) was purchased from Shandong Shenmao Chemical Co. Ltd. Ascorbic acid (Vc) and 2,2'-azodiisobutylamidine dihydrochloride (V50) were purchased from Hubei Shixing Chemical Co. Ltd. Sodium bisulfite, urea, and APS were purchased from China Chengdu Kelon Reagent Co. Ltd. Hydrophobic functional monomer (DQM) was prepared in the laboratory. Sodium formate was purchased from Jinan Mingguan Chemical Co. Ltd.\u003c/p\u003e \u003cp\u003eEquipment used: Haake Mars 40 rotary rheometer was purchased from Haake, Germany. ADVANCE III 400MHz nuclear magnetic resonance spectrometer was purchased from Beijing Agilent Technology Co. Ltd. HELOS-OASIS dry and wet two-in-one laser particle size analyzer was purchased from New Patek Co. Ltd. Germany. ZEM desktop scanning electron microscope was purchased from Anhui Zeyou Technology Co. Ltd. FT-IR \u0026ldquo;Rocket\u0026rdquo; small Fourier-transform infrared spectrometer was purchased from Arcoptix, Switzerland. The BCQT98 nitrogen generator was purchased from Shijiazhuang Bochuang Air Separation Equipment Co. Ltd. RE-220 rotary evaporator was purchased from Zhengzhou Bohui Precision Technology Co. Ltd. Malvern Panaco X-ray diffractometer was purchased from Shanghai Sibaiji Instrument Co. Ltd. Thermogravimetric analyzer (TGAQ500) was purchased from Shanghai Lai Rui Scientific Instrument Co. Ltd. Lumina fluorescence spectrometer was purchased from Semir Fisher Technologies Inc. XGJ-S digital high-speed mixer was purchased from Qingdao Hong Yu Lin Petroleum Instrument Co. Ltd.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e1.2 Hydrophobic monomer synthesis\u003c/h2\u003e \u003cp\u003eThe molar ratio of bromododecane, bromohexadecane, bromodocosane, and N- (3-dimethylaminopropyl) methacrylamide (DMAPMA) was 1:1.1, and 50\u0026ndash;60% of anhydrous acetone solution was placed in a three-neck flask. The mixture was held at 50\u0026deg;C for 30 h. The excess solvent was removed after the reaction in a rotary evaporator, which yielded a light-yellow liquid. The liquid was washed repeatedly with anhydrous ether, and three types of white precipitates were obtained, which were vacuum dried at room temperature to obtain DQM1, DQM2, and DQM3 (C12, C16, and C22, respectively). The synthesis reaction is shown in Figure.1.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e1.3 \u003cb\u003eHydrophobically associating polymer synthesis\u003c/b\u003e\u003c/h2\u003e \u003cp\u003eAM, AA, AMPS, and DQM monomer solutions were prepared. The mass ratio of each component of the monomer solution was m(AM):m(AA):m(AMPS):(DQM):(H\u003csub\u003e2\u003c/sub\u003eO)\u0026thinsp;=\u0026thinsp;20:11:3:1:65, and the mass ratio of the polymer monomer to the system was 35 wt%. The pH of the monomer solution was adjusted to 6.5\u0026ndash;7.0 by mixing sodium hydroxide solution. The solution was deoxygenated with nitrogen for 30 min and cooled to 0\u0026deg;C\u0026ndash;5\u0026deg;C. The mass ratio of the components of the initiator was m(APS):m(Vc):m(V50)\u0026thinsp;=\u0026thinsp;1:5:2. The initiator accounted for 0.03% of the system, and sodium formate was used as a chain transfer agent to adjust the molecular weight of the polymer. The initiator was uniformly mixed with the solution by N\u003csub\u003e2\u003c/sub\u003e. N\u003csub\u003e2\u003c/sub\u003e was no longer introduced after the polymer solution turned sticky. The reactor was sealed and wrapped with an insulation sleeve to prevent heat dissipation. At the end of the reaction, the polymerization temperature reached its peak. The mixture was then cooled to obtain polymer blocks, which were cut, dried, crushed, and granulated sequentially. The powder hydrophobic association polymers DQM1-PAM, DQM2-PAM and DQM3-PAM with different carbon chain lengths were obtained. The reaction pathway of the final product is shown in the Figure. 2.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e1.4 Structural characterization and performance test\u003c/h2\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e1.4.1 Fourier-transform infrared spectroscopy\u003c/h2\u003e \u003cp\u003eThe synthesized polymer was prepared using the KBr tablet method, and the characteristic functional groups and the structure of DQMn-PAM were characterized via Fourier-transform infrared spectroscopy (FTIR).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e1.4.2 Proton nuclear magnetic resonance\u003c/h2\u003e \u003cp\u003eA certain amount of DQMn-PAM powder was collected and dissolved in deuterated water. 0.01\u0026ndash;0.05 wt% of polymer solution was loaded into the nuclear magnetic resonance tube for testing. The molecular structure of DQMn-PAM was tested via proton nuclear magnetic resonance spectroscopy, and the \u003csup\u003e1\u003c/sup\u003eH-NMR spectrum was obtained.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e1.4.3 Molecular weight characterization\u003c/h2\u003e \u003cp\u003eThe viscosity average molecular weight of DQMn-PAM was measured using the viscosity method, and 1 mol/L NaCl aqueous solution was used as the standard sample solvent. A U-type viscometer was used to determine the viscosity of DQMn-PAM sample solution (concentration: 0.1\u0026ndash;0.02%) at a constant temperature of 30.0\u0026deg;C\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u0026deg;C. The relative viscosity ηr was calculated by recording the time t\u003csub\u003e0\u003c/sub\u003e and t of the nondiluted Ubbelohde viscometer. The intrinsic viscosity η was determined by the time ratio, and the molecular weight of DQMn-PAM was characterized using the Mark\u0026ndash;Houwink equation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e1.4.4 Thermogravimetric analysis\u003c/h2\u003e \u003cp\u003eAt 30\u0026deg;C\u0026ndash;700\u0026deg;C, the system was introduced into N\u003csub\u003e2\u003c/sub\u003e, and DQMn-PAM was detected by thermogravimetric analyzer.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e1.4.5 X-ray diffraction analysis\u003c/h2\u003e \u003cp\u003eDQMn-PAM was detected by X-ray diffractometer at a holding voltage of 40 kV and a current of 40 mA.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e1.4.6 Determination of critical association concentration via fluorescence spectrophotometry\u003c/h2\u003e \u003cp\u003eDQMn-PAM was first conFigureured in a volumetric flask as a solution with a concentration of 0.5%. Then, 5 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e L of pyrene solution was dissolved in an ethanol solution in a 50-mL volumetric flask and mixed evenly. The ethanol was dried with N\u003csub\u003e2\u003c/sub\u003e. The DQMn-PAM aqueous solution was diluted to the scale line, ultrasonicated in a water bath for 30 min, and N\u003csub\u003e2\u003c/sub\u003e was continuously introduced for 30 min to remove O\u003csub\u003e2\u003c/sub\u003e from the aqueous solution. The fluorescence spectrum of pyrene was obtained using a fluorescence spectrophotometer at a wavelength of 335 nm, a detection temperature of 25\u0026deg;C, and a scanning range of 350\u0026ndash;450 nm.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e1.4.7 Scanning electron microscopy\u003c/h2\u003e \u003cp\u003eDQMn-PAM solution with a mass fraction of 0.15% was prepared by mixing the three polymer powders in distilled water and 500 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e solution. The samples were frozen in liquid nitrogen and vacuum dried, and their microscopic aggregation morphology was observed via scanning electron microscopy.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e1.4.8 Determination of thixotropic properties\u003c/h2\u003e \u003cp\u003eThe thixotropic properties of 0.5% DQMn-PAM solution were tested using a rotary rheometer. The test program was set up according to the \u0026ldquo;up\u0026ndash;down\u0026rdquo; shear process at up-shear and down-shear rates of 0\u0026ndash;100 and 100\u0026ndash;0 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively \u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e \u003ch2\u003e1.4.9 Thixotropy performance test\u003c/h2\u003e \u003cp\u003eThe 0.5% polymer aqueous solution was prepared by mixing DQMn-PAM in clear water, 6% NaCl solution, and 1% CaCl\u003csub\u003e2\u003c/sub\u003e solution. The relation between elastic modulus (G') and viscous modulus (G\") of polymers based on frequency and stress was tested using a rheometer at 30\u0026deg;C.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e \u003ch2\u003e1.4.10 Determination of rheological properties\u003c/h2\u003e \u003cp\u003eThe rheological properties of DQMn-PAM solution were tested using the Haake Mars 40 rotary rheometer. The 0.5% DQMn-PAM aqueous solution was prepared by mixing the polymer in clear water, 20000 mg/L NaCl, and 2000 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e. Then, the temperature and shear resistances of these solutions were determined.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"2 Results and discussion","content":"\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Structural characterization and thermal performance analysis\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec18\" class=\"Section3\"\u003e \u003ch2\u003e2.1.1 Infrared spectroscopy analysis\u003c/h2\u003e \u003cp\u003eDQMn-PAM was characterized via FTIR; the results are shown in Figure. 3a.A peak corresponding to the stretching vibration of N\u0026ndash;H was observed at 3474 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The peak at 1693 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicates the presence of olefinic double bonds (C\u0026thinsp;=\u0026thinsp;C) and that at 1671 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the C\u0026thinsp;=\u0026thinsp;O stretching vibration. The peak at 1425 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e reflects the presence of N\u003csup\u003e+\u003c/sup\u003e units in the bending vibration region of the saturated C\u0026ndash;H plane. The peaks at 1110 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e correspond to the C-N and C-C stretching vibrations. The peak at 953 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e confirms the existence of disubstituted olefin units. The FTIR spectra showed that DQMn-PAM was successfully synthesized.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section3\"\u003e \u003ch2\u003e2.1.2 Proton nuclear magnetic resonance\u003c/h2\u003e \u003cp\u003eDQMn-PAM was tested using the ADVANCE III 400MHz nuclear magnetic resonance spectrometer; the spectra are shown in Figure. 3b.The peaks at δ5.75\u0026ndash;5.80 and δ5.30\u0026ndash;5.35 correspond to H-C\u0026thinsp;=\u0026thinsp;C-C\u0026thinsp;=\u0026thinsp;O and H-C\u0026thinsp;=\u0026thinsp;C-CH\u003csub\u003e3\u003c/sub\u003e, respectively. The peaks at δ4.70, δ3.23, and δ2.85 correspond to D\u003csub\u003e2\u003c/sub\u003eO, \u0026mdash;CH\u003csub\u003e2\u003c/sub\u003e\u0026mdash; in DQM, and \u0026mdash;CH\u003csub\u003e2\u003c/sub\u003e\u0026mdash; in AMPS and DQM, respectively. The peaks at δ1.57 and δ2.21 correspond to two methylene proton peaks in \u0026mdash;CH\u003csub\u003e2\u003c/sub\u003e\u0026mdash;CH\u003csub\u003e2\u003c/sub\u003e\u0026mdash; on the molecular chain of DQMn-PAM. The peaks at δ1.49, δ1.22, δ0.86, δ0.98, and δ3.57 correspond to \u0026mdash;CH\u003csub\u003e3\u003c/sub\u003e in AMPS, (\u0026mdash;CH\u003csub\u003e2\u003c/sub\u003e\u0026mdash;)\u003csub\u003en\u003c/sub\u003e in DQM, \u0026mdash;CH\u003csub\u003e3\u003c/sub\u003e in DQM, and ethanol solvent, respectively. The structure of the product was similar, which proved that the hydrophobic monomer DQM was successfully inserted into the main chain of DQMn-PAM.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section3\"\u003e \u003ch2\u003e2.1.3 Molecular weight characterization\u003c/h2\u003e \u003cp\u003eDQMn-PAM was synthesized via aqueous free-radical polymerization, and its molecular weight was measured using the one-point method (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). As the carbon chain of DQM monomer lengthened, the steric hindrance effect increased during polymerization. Thus, the molecular weight of DQMn-PAM was \u0026gt;\u0026thinsp;6\u0026nbsp;million atomic units.\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 molecular weight of DQMn-PAM\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\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 \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eThe flow time of polymer solution (s)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eThe flowing time of NaCl solution (s)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eStrength of solutionc\u003c/p\u003e \u003cp\u003e(g/mL)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eRelative viscosity\u003c/p\u003e \u003cp\u003e[ηr]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCharacteristic viscosity\u003c/p\u003e \u003cp\u003e[η] (mL/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eM\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDQM1-PAM\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e228\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e129\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.001226\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.23\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1226\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e6967323\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDQM2-PAM\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e220\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e129\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.001143\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.14\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1144\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e6268456\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eDQM3-PAM\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e217\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e129\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.001121\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e1.11\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e1112\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003e6008496\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section3\"\u003e \u003ch2\u003e2.1.4 X-ray diffraction analysis\u003c/h2\u003e \u003cp\u003eFigure. 3c shows the X-ray diffraction pattern of DQMn-PAM. No sharp peaks were observed in the spectra of the three synthesized products, which confirms that polymer has an amorphous structure. The hydrophobic monomer carbon chain was grafted into the main chain of the polymer molecule. As the grafted carbon chain lengthened, the intermolecular association enhanced. Thus, the polymer crystallinity decreased and hence irregular molecular movement increased. The association between the molecular chains of DQMn-PAM gradually changed from intermolecular association to intramolecular association.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec22\" class=\"Section3\"\u003e \u003ch2\u003e2.1.5 Thermogravimetric analysis of DQMn-PAM\u003c/h2\u003e \u003cp\u003eFigure. 3d shows the thermogravimetric curve of the weight loss of DQMn-PAM categorized into three stages. In the first stage, as the temperature increased, part of the free water in the polymer gradually volatilized when T\u0026thinsp;\u0026lt;\u0026thinsp;223\u0026deg;C. When 223\u0026deg;C\u0026thinsp;\u0026lt;\u0026thinsp;T\u0026thinsp;\u0026lt;\u0026thinsp;312\u0026deg;C, the second stage of weight loss was reached, and the bound water and carboxyl group in the system began to decompose into water detachment system. When T\u0026thinsp;\u0026gt;\u0026thinsp;450\u0026deg;C, the polymer began to decompose and the amide group was converted to ammonia. As DQM3 had a long carbon chain, the association effect in DQMn-PAM was stronger than those of DQM1 and DQM2. The degree of entanglement of DQMn-PAM macromolecules deepened, the thermodynamic decomposition temperature increased, and the internal structure of the molecular chain was more stable.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec23\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Critical association concentration of DQMn-PAM\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure. 4a shows the effect of DQMn-PAM concentration on its apparent viscosity. The apparent viscosity gradually increased at low polymer concentration and sharply as the concentration increased. This is because the self-aggregation behavior of DQMn-PAM polymer changed from intramolecular association to intermolecular association when a turning point appeared in the concentration\u0026ndash;viscosity curve, i.e., the critical association concentration (C*) was achieved \u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e. At high concentrations, the self-aggregation behavior was mainly dominated by intermolecular hydrophobic association. As the carbon chain of the hydrophobic monomer continued to lengthen, C* continued to decrease. In other words, the longer the hydrophobic monomer chain length in DQMn-PAM, the easier it is to form a larger volume of hydrophobic microdomains in aqueous solution and the lower the association concentration \u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003ePyrene was dissolved by the hydrophobic microdomain of hydrophobic association complex, and the I\u003csub\u003e1\u003c/sub\u003e/I\u003csub\u003e3\u003c/sub\u003e ratio responded to DQMn-PAM. The association behavior and critical association concentration of DQMn-PAM were studied. As shown in Figure. 4b, molecular polarity around the probe increased with increasing I\u003csub\u003e1\u003c/sub\u003e/I\u003csub\u003e3\u003c/sub\u003e ratio. Moreover, the hydrophobic microdomain became smaller and the hydrophobic association effect weakened. As the I\u003csub\u003e1\u003c/sub\u003e/I\u003csub\u003e3\u003c/sub\u003e ratio rapidly decreased, the polymer concentration reached the critical association concentration. Figure. 2c shows that when the DQMn-PAM concentration is 0.10\u0026ndash;0.25 wt%, I\u003csub\u003e1\u003c/sub\u003e/I\u003csub\u003e3\u003c/sub\u003e is high and does not change considerably. During this time, the solution concentration is low, hydrophobic microdomain is small, intramolecular association is dominant, and association effect is poor. As the DQMn-PAM concentration increased, I\u003csub\u003e1\u003c/sub\u003e/I\u003csub\u003e3\u003c/sub\u003e began to decrease rapidly. Moreover, the effective concentration of hydrophobic monomer increased, polymerization between monomers gradually changed to between molecular chains, volume of associating hydrophobic microdomains increased, solubilization effect of pyrene molecules gradually enhanced, and the polarity of the environment around the pyrene probe.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure. 5 shows the changes in the apparent viscosity of 0.5% DQMn-PAM in NaCl and CaCl\u003csub\u003e2\u003c/sub\u003e solutions with different mass fractions. The apparent viscosity of DQMn-PAM gradually decreased as the mass fraction of NaCl and CaCl\u003csub\u003e2\u003c/sub\u003e increased. Moreover, the apparent viscosity rapidly decreased when the polymer was mixed in the divalent salt ion solution, with good retention in both the salt ion solutions. The longer the carbon chain of DQMn-PAM, the stronger the interaction between the molecular chains, the closer the molecular space structure, and the stronger the association effect. This resulted in a higher viscosity retention rate and good salt resistance of DQMn-PAM.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec24\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Scanning electron microscopy of DQMn-PAM\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure. 6 shows the microscopic aggregation morphology of the DQMn-PAM molecules in fresh water and salt water. Figures. 6a\u0026ndash;c show the microscopic aggregation morphology of DQMn-PAM molecules in clear water and salt water. In clear water, DQMn-PAM had a higher degree of molecular expansion and the association between hydrophobic monomers formed a spatial network structure. The association effect gradually increased with increasing carbon chain length, and the molecular aggregation state changed from a spatial network structure to a layered interpenetrating network structure. As shown in Figures. 6d\u0026ndash;f, charged ions inhibited the expansion of polymer molecular chains in in salt water, and electrostatic shielding caused the molecular chains to curl. The molecular aggregation state changed from sheet to line \u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e, thereby reducing the hydrodynamic volume of the solution. The apparent viscosity of DQMn-PAM in salt water was low, and the network density of DQM3-PAM in clear water and salt water was higher than that of DQM1-PAM with better solution characteristics.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec25\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Thixotropy analysis\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure. 7 shows the thixotropy test results of DQMn-PAM solution. As the shear rate increases, the externally applied energy is aggregated by the molecules into the network reservoir. When the shear rate decreases, the energy release is delayed and a thixotropic ring is formed. As the polymer carbon chain lengthens, the hysteresis area of the system considerably increases. This indicates that the strength of the DQMn-PAM network system is positively correlated with the growth of the polymer carbon chain. In other words, the introduction of hydrophobic monomers improves the network structure of the polymer system and therefore DQMn-PAM solution exhibits obvious thixotropy. As the hydrophobic carbon chain lengthens, the ability of the polymer to resist external mechanical action is gradually enhanced. The thixotropic ring increases, the association time increases, and the association effect is enhanced, resulting in enhanced thixotropy.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec26\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Viscoelastic energy analysis\u003c/h2\u003e \u003cp\u003eA 0.5% polymer aqueous solution was prepared by mixing DQMn-PAM in clear water, 6% NaCl solution, and 1% CaCl\u003csub\u003e2\u003c/sub\u003e solution. The variation curves of G' and G\" with stress and frequency were tested, where G' is the storage modulus and G\" is the energy dissipation modulus. The relationship between G' and G\" with stress and frequency is shown in Figure. 8a-c.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure. 8a-c shows that the hydrophobically associating polymer DQMn-PAM solution forms a supramolecular structural dynamic\u0026ndash;physical cross-linking network, with high viscoelasticity. G' of DQM1-PAM solution is less than G\" in the whole stress scanning range. At low frequencies, the solution has the highest viscosity. At a lower concentration, the orientation of the polymer molecular chain is affected by the shear force, the spatial structure is destroyed, and the polymer molecular chain freely expands. As the polymer concentration increases, the concentration of DQM2-PAM solution exceeds the critical association concentration and G' \u0026gt; G\". At this time, the solution is mainly elastic and the hydrophobic microregion is positively correlated with DQMn-PAM concentration. When the hydrophobic carbon chain of the polymer solution lengthens, the degree of association between the polymer molecules increases and a closer spatial network structure is formed. Therefore, in the DQM3-PAM solution, G' \u0026gt; G\" and the viscoelasticity of DQM3-PAM is considerably higher.\u003c/p\u003e \u003cp\u003eFigure. 8d-f shows that the viscosity and elasticity of DQMn-PAM increased in the frequency scanning range of 0.1\u0026ndash;10 Hz. This is because at low frequencies, the molecular chain gap of DQMn-PAM is relatively loose, intramolecular association is dominant, and most of the energy is lost by viscous flow. When the frequency gradually increases, the intramolecular association gradually changes to intermolecular association and the molecules are entangled with each other, thus improving the three-dimensional network structure of DQMn-PAM. Moreover, G' continues to increase. Under the same conditions, the G' and G\" of clear water are smaller than those of the salt water. This is because the charge of the salt increases the hydrophobic microarea of DQMn-PAM, thereby increasing the degree of aggregation of micelles in the solution and improving its viscoelasticity.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec27\" class=\"Section2\"\u003e \u003ch2\u003e2.6 Analysis of rheological properties\u003c/h2\u003e \u003cp\u003eDQMn-PAM solution with a mass fraction of 0.5% was prepared using fresh water, 20000 mg/L NaCl, and 2000 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e. It was mixed until fully dissolved, and the temperature and shear resistances were tested; results are shown in Figure. 9.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure. 9 shows that the viscosity of the three polymers decreases with increasing temperature. When the temperature tends to be balanced, the viscosity stabilizes. In the early stage of shearing, the viscosity of the polymer decreases because the shear force causes the disordered molecular chains to be arranged in an orderly manner. When the external conditions stabilize, the intermolecular interaction and hydrogen bonding are in dynamic equilibrium with the molecular thermal motion; thus, the viscosity remains unchanged. The initial viscosity of DQM1-PAM, DQM2-PAM, and DQM3-PAM before shearing was 122, 131, and 155 mPa\u0026middot;s, respectively. As the hydrophobic carbon chain of the polymer lengthened, the volume of the hydrophobic microdomain increased continuously and the association of the polymer gradually transformed into intermolecular association. Moreover, the association strength gradually increased, forming a more compact three-dimensional network structure. Thus, the viscosity retention rate was higher, which is more conducive to enhancing the salt resistance of the polymer.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure. 10a shows the temperature and shear resistance curves of 0.5% polymer DQMn-PAM solution in 20000 mg/L NaCl aqueous solution. The apparent viscosity of the DQMn-PAM polymer solution first increases and then decreases with time and tends stabilize after long-term shearing. The viscosity retention rate after shearing is good, which is higher than that of the other two polymer solutions. As temperature increases, the molecular thermal motion of the hydrophobic chain of the polymer increases and the apparent viscosity of the polymer increases with the free expansion of the molecular chain. When the temperature further increases, the Brownian motion of the hydrophobic group of the polymer DQMn-PAM intensifies, association of the polymer system weakens, molecular chain shrinks, and viscosity of the polymer solution gradually reduces. Longer carbon chains of the polymer hydrophobic monomer lead to larger spatial three-dimensional networks formed by hydrophobic association and closer interconnections, resulting in increased viscosity.\u003c/p\u003e \u003cp\u003eFigure. 10b shows the temperature and shear resistance curve of 0.5% polymer DQMn-PAM solution in 2000 mg/L CaCl\u003csub\u003e2\u003c/sub\u003e aqueous solution. As the temperature increases, the apparent viscosity of the DQMn-PAM polymer solution increases slowly. This is because the divalent salt ion (Ca\u003csup\u003e2+\u003c/sup\u003e) in the solution interacts with surrounding hydrophobic monomer groups to increase the polarity and hydrodynamic volume of the DQMn-PAM solution, which further affects the self-aggregation behavior of the polymer solution.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec28\" class=\"Section2\"\u003e \u003ch2\u003e2.7 Structure\u0026ndash;activity relationship of hydrophobic monomers with different carbon chains on associating polymers\u003c/h2\u003e \u003cp\u003eThree linear alkyl hydrophobically associating polymers, DQMn-PAM, with different carbon chain lengths were obtained via aqueous free-radical polymerization. The effect of carbon chain lengths on the association properties of polymer solution was analyzed. As the carbon chain lengthened, the critical association concentration of the polymer decreased and the hydrophobicity of the hydrophobic carbon chain increased. The volume of hydrophobic association microdomain formed by the polymer in the aqueous solution increased, thereby increasing the strength of the polymer structure network. The thixotropic area of the polymer increased when the polymer was subjected to an external force, and its temperature resistance, shear resistance and viscoelasticity improved. The effect of carbon chain length on the association mechanism of DQMn-PAM is shown in Figure. 11.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"3 Conclusions","content":"\u003cp\u003eThe hydrophobically associating polymer, DQMn-PAM, with three different carbon chains was prepared via aqueous free-radical polymerization and characterized via FTIR and \u003csup\u003e1\u003c/sup\u003eH-NMR spectroscopies. Results showed that the monomer was successfully incorporated into the molecular main chain and the target product was successfully synthesized. The molecular weight of DQMn-PAM was \u0026gt;\u0026thinsp;6\u0026nbsp;million atomic units. The structure\u0026ndash;activity relationship of different carbon chain lengths on polymer solution was determined by testing.\u003c/p\u003e \u003cp\u003e(1) X-ray, TG, and fluorescence performance tests showed that the association of the polymer increased as the hydrophobic carbon chain lengthened and the temperature resistance of the polymer improved. SEM images showed that the polymer molecules formed a more compact three-dimensional space network as the carbon chain lengthened and the association structure enhanced.\u003c/p\u003e \u003cp\u003e(2) The association behavior and critical association concentration of DQMn-PAM were studied using a fluorescent pyrene probe. Results showed that the longer the chain length of the hydrophobic monomer in the polymer, the lower the critical association concentration (C*). Moreover, the more favorable the hydrophobically associating copolymer to form a spatial network structure, the larger the spatial size of the DQMn-PAM three-dimensional network.\u003c/p\u003e \u003cp\u003e(3) The rheological property evaluation of DQMn-PAM revealed that with increasing hydrophobic carbon chain length, the association between polymer molecules increased, leading to improved temperature, salt, and shear resistances of the polymer. The strong intermolecular association improved the viscoelasticity of the polymer. The solution was mainly elastic, and the aggregation microarea of the polymer molecules increased, thereby improving the thixotropic area of the polymer. The modification of the polymer by long carbon chain hydrophobic monomers could effectively improve the performance of the polymer solution, broadening the application field of hydrophobically associating polyacrylamide.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflict of Interest\u003c/h2\u003e \u003cp\u003eThe authors declare no competing financial interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contributions\u003c/h2\u003e \u003cp\u003e \u003cb\u003eRong Yang\u003c/b\u003e: Writing\u0026ndash;original draft (lead). \u003cb\u003eXiaojuan Lai\u003c/b\u003e: Supervision (lead). \u003cb\u003eQiying Li\u003c/b\u003e: Conceptualization (equal). \u003cb\u003eXi Ding\u003c/b\u003e: Project administration (equal); software (equal). \u003cb\u003eXin Wen\u003c/b\u003e: Supervision (equal). \u003cb\u003eYan Guo\u003c/b\u003e: Formal analysis (equal).\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThis research was supported by the Qin-Chuangyuan \"Scientist\u0026thinsp;+\u0026thinsp;Engineer\" Team Construction Project (2024QCY-KXJ\u0026minus;052), the Key R \u0026amp; D Program of Shaanxi Province (2024GX-YBXM\u0026minus;393), the Industrialization Project of Shaanxi Provincial Education Department (23JC008) in China.\u003c/p\u003e\u003ch2\u003eData Availability Statement\u003c/h2\u003e \u003cp\u003eThe data that support the findings of this study are available from corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eJ. 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J. 2018, \u003cem\u003e354\u003c/em\u003e, 913.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH. Shi, W. Ge, Y. Wang, B. Fang, J. T. Huggins, T. A. Russell,Y. Talmon, D. J. Hart, J. L. Zakin,J. Colloid Interface Sci. 2014, \u003cem\u003e418\u003c/em\u003e,95.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-polymer-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jpol","sideBox":"Learn more about [Journal of Polymer Research](https://www.springer.com/journal/10965)","snPcode":"10965","submissionUrl":"https://www.editorialmanager.com/jpol/","title":"Journal of Polymer Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"hydrophobic association, hydrophobic microarea, viscoelasticity, thixotropy, structure–activity relationship","lastPublishedDoi":"10.21203/rs.3.rs-4393619/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4393619/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAs temperature and salt-resistant materials, hydrophobically associating polymers can form a reversible spatial network structure through the interaction between their hydrophobic groups, effectively improve the viscosity of the polymer solution through association, and enhance the temperature and salt resistance of the polymer. Hydrophobically associating monomers have different effects on the properties of polymer solutions. Herein, acrylic acid, acrylamide, and 2-acrylamido-2-methylpropanesulfonic acid were used as hydrophilic monomers. The three hydrophobic monomers with different carbon chain lengths were prepared by the bromination reaction. Hydrophobic associating polymers DQM1-PAM, DQM2-PAM, and DQM3-PAM were prepared by aqueous solution free-radical polymerization. The structure\u0026ndash;activity relationship of the hydrophobic monomers with different carbon chain lengths on polymers was studied. It was confirmed by Fourier-transform infrared spectroscopy and \u003csup\u003e1\u003c/sup\u003eH-NMR that the target product was successfully synthesized. Scanning electron microscopy revealed that with increasing hydrophobic carbon chain length, the hydrophobic microarea of molecular aggregation increased, forming a closer spatial network structure. Thermogravimetric and fluorescence tests revealed that with increasing hydrophobic carbon chain length of polymer molecules, the polymerization temperature resistance increased, intermolecular association degree increased, and critical association concentration decreased. Rheological property evaluation revealed that the viscosity of 0.5% polymer DQM1-PAM, DQM2-PAM, and DQM3-PAM was 71.32, 118.79, and 118.79 mPa\u0026middot;s after shearing at 120\u0026deg;C and 170 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 1 h. With the increase in the carbon chain length, the retention rate of shear viscosity of polymer in a salt solution increased, showing good salt resistance. Concurrently, the molecular aggregation microarea of a solution with 0.5% polymer, degree of molecular chain action, viscoelasticity of the solution (G' \u0026gt; G''), and thixotropic area all increased. The performance of polymer solution can be improved by modifying hydrophobically associating polymers with long carbon chains, which has a broader application.\u003c/p\u003e","manuscriptTitle":"Effect of hydrophobic monomers with different carbon chains on the structure–activity relationship of associating polyacrylamides","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-04 20:57:52","doi":"10.21203/rs.3.rs-4393619/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2024-05-22T13:48:55+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-05-22T12:15:44+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Journal of Polymer Research","date":"2024-05-12T18:16:57+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-05-10T06:08:24+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Polymer Research","date":"2024-05-09T04:10:51+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-polymer-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jpol","sideBox":"Learn more about [Journal of Polymer Research](https://www.springer.com/journal/10965)","snPcode":"10965","submissionUrl":"https://www.editorialmanager.com/jpol/","title":"Journal of Polymer Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"e1602bef-9056-4b9a-9241-328b0cc68bce","owner":[],"postedDate":"June 4th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-08-05T16:08:16+00:00","versionOfRecord":{"articleIdentity":"rs-4393619","link":"https://doi.org/10.1007/s10965-024-04083-4","journal":{"identity":"journal-of-polymer-research","isVorOnly":false,"title":"Journal of Polymer Research"},"publishedOn":"2024-07-31 15:57:12","publishedOnDateReadable":"July 31st, 2024"},"versionCreatedAt":"2024-06-04 20:57:52","video":"","vorDoi":"10.1007/s10965-024-04083-4","vorDoiUrl":"https://doi.org/10.1007/s10965-024-04083-4","workflowStages":[]},"version":"v1","identity":"rs-4393619","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4393619","identity":"rs-4393619","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}