3.1 Analysis of FT-IR
Figure 1a displays the various absorption peaks: the peak at 3605 cm− 1 corresponds to the stretching vibration of the O–H bond in the carboxyl group; and the peak at 3165 cm− 1 corresponds to the stretching vibration of the N–H bond in the amide group; the peak at 2931 cm− 1 corresponds to the stretching vibration of the N–H bond in the methyl and methylene groups; the peak at 1691 cm− 1 corresponds to the stretching vibration of the C = O bond; the peak at 1552 cm− 1 corresponds to the stretching vibration of the C–N bond and the bending vibration of the N–H bond in the amide group; the peak at 1415 cm− 1 is the characteristic peak of the sulfonic acid group; the peak at 1315 cm− 1 corresponds to the stretching vibration of the C–N bond and the bending vibration of the N–H bond in the secondary amine in the amide group; the peak at 640 cm− 1 is the characteristic peak of –(CH2)9–; GTE-10 and p(AM/AMC12S/GTE-10) both exhibit corresponding stretching vibration peaks of the C = C bond at 1415, 1450, and 1552 cm− 1, a bending vibration peak of the C–H bond on the benzene ring at 813 cm− 1, and a corresponding stretching vibration peak of the C–O–C bond at 1315 cm− 1. The results indicate that the hydrophobic monomer GTE-10 has been successfully incorporated into the polymer, and the structure of the polymer molecule obtained is consistent with expectations.
3.2 Analysis of 1H NMR spectroscopy
Figure 1b displays peaks at δ 4.71 corresponding to the solvent peak of deuterated oxide; at δ 0.63 (a) corresponding to the proton peak of the methyl group –CH3 on the long alkyl chain –(CH2)7CH3 in the monomer GTE-10; at δ 1.16 (b), δ 1.72 (c), and δ 3.11 (d) corresponding to the proton peaks of –CH2– within the repeating unit –(CH2)7CH3; at δ 6.68 (e) and δ 6.95 (f) corresponding to the two sets of proton peaks on the benzene ring; at δ 4.12 (g), δ 3.88 (h), and δ 3.50 (i) corresponding to the proton peaks of –CH2– within the repeating unit –(CH2CH2O)10–; at δ 1.79 (j) corresponding to the proton peak of the –CH3 on the double bond carbon at the end of GTE-10; and at δ 5.31 (k) and δ 6.09 (l) corresponding to the two sets of proton peaks on the double-bonded carbon.
Figure 1c displays a peak at δ 13.9 (a), which corresponds to the absorption of the methyl group –CH3 on the long alkyl chain –(CH2)7CH3 in the monomer GTE-10. The peaks at δ 43.2 (b) and δ 46.6 (c) represent the absorption peaks of the repeating unit –(CH2)7–. Additionally, the peaks at δ 135.8 (d), δ 129.6 (e), δ 113.7 (f), and δ 156.3 (g) correspond to the absorption peaks of the four carbon groups on the benzene ring. The peak at δ 69.8 (h) corresponds to the absorption peak of the –CH2– within the repeating unit –(CH2CH2O)10. Moreover, the peak at δ 167.7 (i) represents the absorption peak of the carbonyl carbon, while the peak at δ 17.8 (k) corresponds to the absorption peak of the –CH3 group on the double-bonded carbon at the end of GTE-10. Lastly, the peaks at δ 135.2 (j) and δ 126.6 (l) correspond to the absorption peaks of the two double-bonded carbons.
Figure 1d displays the peak at δ 4.71 corresponds to the solvent peak of deuterated oxide. The proton peaks at δ 1.71 (a), δ 1.43 (b), and δ 0.76 (c) are attributed to the main-chain protons of –CH2–, –CH–, and –CH3, respectively. Additionally, the proton peaks at δ 3.20 (d) and δ 3.28 (g) correspond to the side chain protons of –CH– and –CH2– in the monomer AMC12S. The peaks at δ 0.91 (e) and δ 1.26 (f) represent the proton peaks of -CH2- and -CH3 in the repeating unit of –(CH2)9CH3. Furthermore, the peak at δ 3.36 (h) corresponds to the proton peak of –CH2– in the repeating unit –(CH2CH2O)10– in the side chain of the functional monomer GTE-10. Lastly, the peak at δ 6.78 (i) represents the proton peak of the benzene ring. These results indicate that the monomers AMC12S and GTE-10 both participated in the polymerization reaction, resulting in polymer molecules with the expected structure.
3.3 Particle Size Analysis
Figure 2 displays the particle size distribution and microstructure of the emulsion before and after polymerization. As depicted in Fig. 2a and c, it can be observed that the oil-in-water emulsion formed after pre-emulsification has a larger average particle size and uneven distribution, with a D50 of 10.45 µm. In contrast, in Fig.s 2b and d, it is evident that the oil-in-water structure becomes more uniform after the completion of the polymerization reaction, with a narrower particle size distribution range and a D50 of 5.837 µm. This results in a polymer with improved solubility.
3.4 Mechanism of Inverse Emulsion Polymerization
The schematic diagram of the reaction mechanism for the synthesis of polymer p(AM/AMC12S/GTE-10) via reverse-phase free radical polymerization is shown in Fig. 3. The polymerization process can be divided into three stages: initiation, propagation, and termination. During the initiation stage, the initiator undergoes redox reactions, and the pre-polymerization radicals are continuously transferred to the growing micelles, driven by the equilibrium swelling of latex particles, to initiate monomer polymerization, leading to an increase in the polymerization rate. As the free monomers in the system are completely encapsulated within the monomer droplets, the polymerization rate becomes relatively constant, and the system enters the propagation stage. In this stage, only latex particles and monomer droplets coexist, representing the golden period for acrylamide polymerization, which is characterized by high exothermicity. The addition of sodium formate as a chain-transfer agent promotes a more regular polymer structure. This stage is characterized by a long duration. Finally, the reaction enters the termination stage, during which the remaining residual monomers continue to react, resulting in a significant decrease in the polymerization rate until all the active monomers are converted into polymer molecules.
3.5 Analysis of Critical Aggregation Concentration by Fluorescent Probes
In Fig. 4a, the hydrophobically associating polymer spontaneously forms micelles and small hydrophobic microdomains capable of dissolving pyrene, serving as a fluorescence probe reflecting the strength of the hydrophobic association effect. In the fluorescence spectrum, the ratio of the absorbance at 373 nm (I1) to the absorbance at 391 nm (I3), denoted as I1/I3, depends on the polarity of the surrounding environment. When the surrounding microenvironment is more polar, the I1/I3 value is higher; conversely, once the polymer molecules form a distinct dynamic physical cross-linking network, the I1/I3 value significantly decreases. Figure 4b illustrates the variation in I1/I3 at different concentrations of the p(AM/AMC12S/GTE-10). When the concentration is 0.30 wt.%, the value of I1/I3 suddenly decreases. This indicates that when the concentration of the p(AM/AMC12S/GTE-10) solution exceeds 0.30 wt.%, the polymer molecules form a large number of hydrophobic regions, weakening the polarity of the surrounding microenvironment of the pyrene probe. Therefore, the apparent viscosity and fluorescence spectrum measurements indicate that the critical association concentration (C*) of p(AM/AMC12S/GTE-10) is approximately 0.30–0.35 wt.%.
3.6 Analysis of Salt Tolerance Performance
At room temperature, the apparent viscosity changes of the polymers with mass fractions of 0.3%, 0.5%, and 0.7% in different mass concentrations of NaCl and CaCl2 aqueous solutions are shown in Fig. 5. The viscosity of the polymer solutions exhibits a trend of first increasing and then decreasing with an increase in the salt solution mass concentration. The initial viscosity of the 0.7% mass fraction polymer solution is 123 mPa·s. In the 80000 mg/L NaCl and CaCl2 solutions, the apparent viscosities of the 0.7% mass fraction polymer solution were 39 and 24 mPa·s, with viscosity retention rates of 31.71% and 19.51%, respectively. The ability of the phenylethylene group contained in monomer GTE-10 to chelate metal ions reduces the negative effects of electrostatic shielding, increases the hydrodynamic volume, and promotes the formation of spatial network structures, thereby increasing the apparent viscosity of the polymer with increasing polymer mass fraction [18–19]. The introduction of hydrophobic long chains and benzene rings enhances the rigidity of the molecular chains, preventing them from easily coiling and enhancing the salt resistance capabilities of the system. At high salt mass concentrations, the electrostatic shielding of metal ions on the molecular chains exceeds the intermolecular binding effect, compressing the hydrodynamic volume of the polymer molecules and resulting in a decrease in the viscosity of the polymer solution.
3.7 Analysis of SEM
SEM images of the polymer p(AM/AMC12S/GTE-10) with a mass fraction of 0.3% in pure water and sodium chloride solution with a mass concentration of 10000 mg/L are shown in Fig. 6. In pure water, the polymer is fully extended, and the functional monomer GTE-10 in the polymer spontaneously aggregates, forming hydrophobic microdomains with certain strength. This leads to the formation of a three-dimensional network structure with intermolecular cohesive interactions, resulting in the viscoelastic behavior of the polymer solution on a macroscopic scale. In saline solution, the phenylethylene group in the functional monomer GTE-10 acts as a chelating agent for salt ions. The filling of the vacant orbital of the metal ions by the lone pair electrons of the ethylene group enhances the intermolecular forces, reduces the impact of electrostatic shielding on the polymer skeleton, and enhances the resistance of the polymer to metal cations. Therefore, the polymer exhibits a stronger spatial network structure in saline solution [20–21].
3.8 Analysis of temperature resistance performance
A mass fraction of 0.7% p(AM/AMC12S/GTE-10) solution was prepared in a water solution of NaCl and CaCl2 with equal mass concentrations of 20000 mg/L at 140 ℃, and its temperature resistance performance is shown in Fig. 7. The viscosity of the polymer solution continues to decrease with increasing temperature. Hydrophobic modification enhances the cohesive force of the polymer chains, increasing the viscous flow activation energy of the fluid; thus, the polymer exhibits a certain dependency on temperature. As the temperature was increased, the viscosity of the polymer solution decreased. At 140 ℃, the viscosity of the polymer solution in both pure water (Fig. 7a) and NaCl (Fig. 7b) solutions was greater than 50 mPa·s (Fig. 7b). In the CaCl2 solution (Fig. 7c), the viscosity of the polymer solution decreased below 50 mPa·s at temperatures higher than 115.3 ℃. Overall, the polymer solution exhibited good temperature resistance.
3.9 Analysis of shear resistance performance
A 0.7% (w/w) p(AM/AMC12S/GTE-10) solution was prepared in pure water and aqueous solutions of NaCl and CaCl2 at mass concentrations of 20000 mg/L. The shear-thinning performance of the polymer solution was measured at 90 and 120°C at a constant shear rate of 170 s–1, as shown in Fig. 8. With increasing temperature, the viscosity of the polymer solution initially decreased and then plateaued. The intensified thermal motion of the functional groups weakens intermolecular aggregation, leading to a reduction in the viscosity of the polymer solution. Once the temperature stabilized, the temperature dependency of the polymer solution viscosity was eliminated, and a balance was reached between the intermolecular association and molecular thermal motion. Figure 8a, b, after shearing at 90 and 120°C for 1 h, the viscosity of the polymer solution remained essentially unchanged at 88.7 and 73.2 mPa·s, respectively. Figure 8c, d the viscosity of the polymer solution after shearing at 90 and 120°C for 1 h in an aqueous NaCl solution with a mass concentration of 20000 mg/L was 79.3 and 64.7 mPa·s, respectively. Meanwhile, in the aqueous CaCl2 solution with the same mass concentration, the viscosities of the polymer solutions at 90 and 120°C after shearing for 1 h were 63.9 and 54.2 mPa·s, respectively. The presence of hydrophobic long chains and benzene rings in the monomer GTE-10 enhanced the rigidity of the molecular chains and hindered conformational transitions, resulting in a more stable spatial network structure. Additionally, metal ions in the salt solutions form complexes with ethylene oxide groups, reducing the negative impact of the salt ions and enhancing the shear resistance of the polymer solution.
3.10 Analysis of shear resistance performance testing
At 30 ℃, the effect of the shear rate at 170, 510, and 1022 s–1 on the viscosity of the polymer solution was alternately investigated, and the results are shown in Fig. 9a. When the shear rate increased from 170 s–1 to 510 s–1 and then further to 1022 s–1, the viscosity continued to decrease. When the shear rate was constant, viscosity remained essentially unchanged. After shearing at 1022 s–1 for a certain period of time, the viscosity returned to its initial value upon reverting to 170 s–1. The results indicate that the polymer viscosity exhibits good shear recovery performance, possibly due to the fact that under high shear rates, the dynamically physical cross-linked network structure formed by molecular aggregation is not completely disrupted. When the shear rate decreases, the aggregation forces lead to the reformation of separated molecular chains into hydrophobic microdomains, which macroscopically manifests as either an increase or return to the initial value of apparent viscosity [22].
The rheological behavior of polymer solutions can be described by the power law equation as follows:
\(\lg \tau =\lg \kappa +{\text{n}}\lg \gamma\)
Where τ is the shear stress, κ is the consistency coefficient, γ is the shear rate and n is the power law index. When 0 < n < 1, it represents a pseudoplastic flow, where the apparent viscosity decreases with increasing shear stress or shear rate. When n = 1, it represents an ideal Newtonian fluid.
Figure 9b, A fitting line of the shear rate and shear stress was obtained with lgγ as the abscissa and lgτ as the ordinate, and the fitting equation was y = 0.28839x + 0.90339 (R2 = 0.99805). Where the power law index is n = 0.28839 < 1, and the consistency coefficient is κ = 8.01. Therefore, the polymer solution is a pseudoplastic fluid and exhibits shear-thinning behavior.
3.11 Analysis of thixotropic performance
The thixotropic properties of the polymer aqueous solutions with quality fractions of 0.3, 0.5, and 0.7% were tested, as shown in Fig. 10a, b, c. As the mass fraction of the polymer solution increased, both the hysteresis loop area and the shear stress increased. This indicates that the increase in intermolecular bonding of the polymer leads to the strengthening of the spatial network structure, resulting in greater resistance to deformation. This, in turn, increases the energy required to break the network structure, demonstrating the excellent shear resistance capabilities of the polymer.
3.12 Analysis of viscoelastic performance
Polymer solutions are viscoelastic fluids that exhibit non-Newtonian viscosity and normal stress differences under steady shear flow. The presence of normal stress differences leads to various special flow phenomena such as the Weissenberg effect, suspended siphons, and jet expansion [23]. Therefore, to analyze the variation rules of viscoelasticity in polymer solutions, parameters such as G″, G′, complex modulus (G*), and the first normal stress difference (N1) were determined.
3.12.1 Trends in changes of G' and G''
The curves of G' and G" versus the stress and frequency for polymer solutions with different mass fractions in NaCl at a mass concentration of 20,000 mg/L, CaCl2 at a mass concentration of 10,000 mg/L, and deionized water are presented in Figs. 11 and 12.
As shown in Fig. 11a, for polymer solutions with mass fractions of 0.3% and 0.5%, G′ and G″ remain stable in the low-stress range, showing a linear viscoelastic region that reflects the structural strength of the polymer solution. As the stress continues to increase, G′ and G″ show a decreasing trend, with G′ < G″, transitioning to a non-linear viscoelastic region, indicating the disruption of the spatial network structure of the solution under shear stress, displaying shear thinning behavior. For the polymer solution with a mass fraction of 0.7%, there is a clear linear viscoelastic region in the stress scan range of 0 ~ 10 Pa, with G′ > G″, exhibiting elastic fluid behavior. As the mass fraction of the polymer increases, the intermolecular binding becomes stronger, resulting in a more stable and difficult-to-break spatial structure. Additionally, the Fig. 11b, c appropriately concentrated salt solutions can enhance the binding effect, contributing to an increase in the viscoelasticity of polymer molecules.
As shown in Fig. 12, as the shear frequency increases, the G′ and G″ values of the polymer solution increase. As shown in Fig. 12a, the 0.3% mass fraction polymer solution in pure water exhibits predominantly intramolecular association at lower frequencies, where the viscoelasticity is mainly contributed by the higher molecular weight, leading to G′ < G″, showing viscous flow. As the frequency increases from low to high, the intermolecular association strengthens, and the complex network structure provides better elasticity, with G′ dominating, indicating elastic flow. As shown in Fig. 12b and c, the viscoelasticity of the polymer in a salt solution is greater than that in pure water because of the chelation reaction between metal ions and styrene groups, which enhances intermolecular forces and results in a dense network structure, accompanied by an increase in structural viscosity. Therefore, increasing the structural viscosity of the polymer can increase the viscoelasticity of the solution.
3.12.2 Trends in Changes of G*
G* studies the linear viscoelastic behavior of polymer solutions, reflecting both the viscosity and elastic characteristics of the fluid. It can be obtained based on G′ and G″ [24, 25], and the equation is as follows:
$${G^{\text{*}}}=G^{\prime}+{\text{i}}G^{\prime\prime}=\sqrt {{{G^{\prime}}^2}+{{G^{\prime\prime}}^2}}$$
Figure 13a depicts the increase in G* as the shear frequency increases, approaching a linear trend. This phenomenon is attributed to the formation of a stable spatial network structure between polymer molecules. This results in increased resistance to deformation in polymer solutions as shear frequency rises, as evidenced by the increase in G*. Under the same mass fraction, the value of G* in salt solutions is greater than that in pure water. The dynamically physical crosslinked network formed by hydrophobically modified polymers is less affected by inorganic salts. This results in enhanced resistance to salt-induced viscosity enhancement and an increase in viscoelasticity. To compare the viscoelastic properties of polymer solutions with varying mass fractions, the complex modulus values at a frequency of 1 Hz were chosen for comparison. With the increasing mass fractions of the polymer, the values of G* in both pure water and saline solutions showed an increased trend. Among them, the trend of Fig. 13b increase in a 20000 mg/L NaCl solution is the steepest, and the Fig. 13c followed by the 10000 mg/L CaCl2 solution. This indicates that appropriately concentrated salt solutions are conducive to enhancing the viscoelasticity of polymer molecules. When the mass fraction of the polymer is in the range of 0.3–0.5%, the increase in G* is slow. However, when the mass fraction is in the range of 0.5–0.7%, the slope of the curve is steeper, leading to a faster increase in G*. At lower mass fractions, viscoelasticity is primarily determined by the intramolecular binding of polymer molecules. With the increase in mass fractions, intermolecular binding strengthens, leading to the formation of a complex network structure, resulting in a significant increase in G*.