Enhancing the Properties of Water-Soluble Copolymer Nanocomposites by Controlling the Layer Silicate Load and Exfoliated Nanolayers Adsorbed on Polymer Chains

Novel polymer nanocomposites of methacryloyloxy ethyl dimethyl hexadecyl ammonium bromide-modified montmorillonite (O-MMt) with acrylamide/sodium p-styrene sulfonate/methacryloyloxy ethyl dimethyl hexadecyl ammonium bromide (ASD/O-MMt) were synthesized via in situ polymerization. The molecular structures of the synthesized materials were confirmed using Fourier-transform infrared and 1H-nuclear magnetic resonance spectroscopy. X-ray diffractometry and transmission electron microscopy revealed well-exfoliated and dispersed nanolayers in the polymer matrix, and scanning electron microscopy images revealed that the well-exfoliated nanolayers were strongly adsorbed on the polymer chains. The O-MMt intermediate load was optimized to 1.0%, and the exfoliated nanolayers with strongly adsorbed chains were controlled. The properties of the ASD/O-MMt copolymer nanocomposite, such as its resistance to high temperature, salt, and shear, were significantly enhanced compared with those obtained under other silicate loads. ASD/1.0 wt% O-MMt enhanced oil recovery by 10.5% because the presence of well-exfoliated and dispersed nanolayers improved the comprehensive properties of the nanocomposite. The large surface area, high aspect ratio, abundant active hydroxyl groups, and charge of the exfoliated O-MMt nanolayer also provided high reactivity and facilitated strong adsorption onto the polymer chains, thereby endowing the resulting nanocomposites with outstanding properties. Thus, the as-prepared polymer nanocomposites demonstrate significant potential for oil-recovery applications.


Introduction
Petroleum is an important energy source and crucial raw material for chemicals; thus, its worldwide demand is expected to continually increase. Because of the low efficiency of the traditional primary and secondary oil recovery methods, the need for enhanced oil recovery (EOR) alternatives has become a major priority [1][2][3]. In recent years, water-soluble polymer nanocomposites are increasingly being used in EOR [4][5][6][7]. Montmorillonite (MMt) is a common inorganic phase in the synthesis of water-soluble polymer nanocomposites, and its nano-effects have received considerable research attention [8][9][10][11][12][13][14][15][16][17]. MMt is a natural two-dimensional layered silicate material whose crystal structure is composed of two sheets of silicon-oxygen tetrahedra sandwiched between sheets of aluminum-oxygen octahedra [18][19][20]. The surface of a MMt layer has ample hydroxyl groups and negative charges, and abundant exchangeable ions exist between layers [21,22]. Therefore, MMt can be easily modified via layer intercalation to enlarge the layer-layer distance and reduce the interlayer interaction force. This enables reactive monomers to fill into the nano-layers and polymerize in the interlayer. The heat generated in the polymerization process causes the continuous expansion of the nano-slice spacing until an exfoliate state is reached. Exfoliated MMt layers have been prepared in this manner [23][24][25]. MMts with this structure Polymers 2023, 15, 1413 3 of 20 In this study, we designed a tunable MMt NI using intercalation nanochemistry principles [54]. The selection of an appropriate modifier led to a MMt NI with a controllable interlayer distance; this intermediate underwent in situ copolymerization to produce well-exfoliated O-MMt active nanolayers that were uniformly dispersed in the polymer matrix. Thereafter, water-soluble copolymer nanocomposites (ASD/O-MMt) exhibiting the unique nano-effects of the exfoliated O-MMt nanolayers were synthesized. Depending on the O-MMt content and degree of exfoliation, these nanolayers strongly adsorbed onto polymer chains to yield polymer nanocomposites with excellent properties. Systematic analyses of the structure, properties, and solution flooding performance of these polymer nanocomposites were performed to optimize the nanomaterial composition. ASD/1.0 wt% O-MMt exhibited excellent properties because the exfoliated layers of the inorganic clay simultaneously exerted a positive effect on enhancing the final nanocomposite performance and a negative effect on increasing the molecular mass of the barrier polymer. The asprepared optimized sample demonstrated potential applicability to the EOR process under high-temperature and high-salt reservoir conditions.

Materials
Unmodified Na-MMt with a cationic exchange capacity of 100 mmol/100 g was supplied by Huai An Saibei Technology Co., Ltd., Zhangjiakou, China. Sodium chloride (NaCl, 98%), sodium hydroxide (NaOH, 96%), ammonium persulfate ((NH 4 ) 2 S 2 O 8 , 98%), sodium hydrogen sulfite (NaHSO 3 , 98%), and absolute ethanol (CH 3 CH 2 OH, 99.7%) were purchased from the Tianjin Fuchen Fine Chemical Research Institute, Tianjin, China. AM (98%), acetone (CH 3 COCH 3 , 99.5%), and N,N-dimethylformamide (DMF, 99.5%) were purchased from the Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China. SSS (98%) was obtained from Shanghai Macklin Biochemical Co., Ltd., Shanghai, China. Methacrylic acid 2-(dimethylamino)ethyl ester (stabilized with MEHQ, DMA, >98.5%) was supplied by TCI Industry Co., Ltd., Shanghai, China. 1-Bromohexadecane (97%), β-CD (98%), AOI (stabilized with BHT, >98.0%), and stannous octoate (95%) were purchased from Aladdin Biochemical Co., Ltd., Shanghai, China. All reagents were of analytical grade, and deionized water was used in all experiments. 2 1-Bromohexadecane (20.15 g, 0.066 mol) was added to a stirred solution of DMA (10.38 g, 0.066 mol) in 100 mL of anhydrous ethanol. Thereafter, the mixture was heated to 50 • C by continuously stirring in a water bath for 24 h. After the reaction, the solvent was removed using a rotary evaporator. The residue was first recrystallized in acetone and then washed several times with acetone to obtain DMB as a white powder. A scheme depicting the preparation of DMB is shown in Figure 1a. β-CD (10 g, 7.6 mmol) was dissolved in 50 mL of anhydrous DMF under nitrogen. Thereafter, AOI (2.14 g, 15.2 mmol) was added to this mixture and stirred vigorously, after which 80 μL of tin(II) 2-ethylhexanoate (a catalyst) was added to the solution. The mixture β-CD (10 g, 7.6 mmol) was dissolved in 50 mL of anhydrous DMF under nitrogen. Thereafter, AOI (2.14 g, 15.2 mmol) was added to this mixture and stirred vigorously, after which 80 µL of tin(II) 2-ethylhexanoate (a catalyst) was added to the solution. The mixture was stirred for 90 min at 25 • C under nitrogen. The temperature was increased to 50 • C, and the reaction was allowed to proceed for 8 h under continuous stirring. At the end of the reaction, most of the solvent was removed using a rotary evaporator. The residue was recrystallized via the addition of cold acetone and then washed several times with deionized water and acetone to remove the DMF and unreacted AOI. The product was dried at 45 • C for 48 h in a vacuum oven. β-CD-AOI 2 was obtained as a white powder and characterized using Fourier-transform infrared (FTIR) spectroscopy. A schematic depicting the synthesis of β-CD-AOI 2 is shown in Figure 1b.

Preparation of the O-MMt Intermediate
Pristine Na-MMt (3.33 g) was ultrasonically dispersed in 50 mL of distilled water to prepare a homogeneous mixture; this mixture was then heated to 50 • C. Subsequently, 50 mL of the DMB solution (3.33 g of DMB) were slowly added dropwise to the aforementioned mixture under continuous stirring. The cation-exchange reaction was allowed to proceed for 18 h under nitrogen. Finally, the sample was collected via filtration and washed several times with distilled water until the AgNO 3 titration test confirmed the absence of bromide ions in the solution. The collected sample was dried in a vacuum oven at 70 • C for 24 h, ground into a powder, and carefully stored under dry conditions for further use. A schematic of the synthesis of O-MMt is shown in Figure 2. β-CD (10 g, 7.6 mmol) was dissolved in 50 mL of anhydrous DMF under nitro Thereafter, AOI (2.14 g, 15.2 mmol) was added to this mixture and stirred vigorously, which 80 μL of tin(II) 2-ethylhexanoate (a catalyst) was added to the solution. The mi was stirred for 90 min at 25 °C under nitrogen. The temperature was increased to 5 and the reaction was allowed to proceed for 8 h under continuous stirring. At the e the reaction, most of the solvent was removed using a rotary evaporator. The residue recrystallized via the addition of cold acetone and then washed several times with d ized water and acetone to remove the DMF and unreacted AOI. The product was dri 45 °C for 48 h in a vacuum oven. β-CD-AOI2 was obtained as a white powder and ch terized using Fourier-transform infrared (FTIR) spectroscopy. A schematic depictin synthesis of β-CD-AOI2 is shown in Figure 1b.

Preparation of the O-MMt Intermediate
Pristine Na-MMt (3.33 g) was ultrasonically dispersed in 50 mL of distilled wa prepare a homogeneous mixture; this mixture was then heated to 50 °C. Subsequent mL of the DMB solution (3.33 g of DMB) were slowly added dropwise to the afore tioned mixture under continuous stirring. The cation-exchange reaction was allow proceed for 18 h under nitrogen. Finally, the sample was collected via filtration washed several times with distilled water until the AgNO3 titration test confirme absence of bromide ions in the solution. The collected sample was dried in a vacuum at 70 °C for 24 h, ground into a powder, and carefully stored under dry condition further use. A schematic of the synthesis of O-MMt is shown in Figure 2.

Preparation of the ASD/O-MMt Nanocomposite
The ASD/O-MMt nanocomposite was synthesized via in situ polymerization. First, a mixture of 0-0.67 g of O-MMt, 23.00 g of AM, 6.67 g of SSS, 3.33 g of DMB, 0.33 g of β-CD-AOI 2 , and 100 g of deionized water was added to a 250 mL three-necked flask and continuously stirred for 1 h under nitrogen. Thereafter, the pH of this mixture was adjusted to 8-9 using 1.0 mol/L NaOH. Second, a certain amount of (NH 4 ) 2 S 2 O 8 and NaHSO 3 (molar ratio of 1:1; 0.3 wt%) was added as an initiator to the reaction mixture with stirring under nitrogen. The mixture was allowed to react at 50 • C for 12 h, after which the obtained product was washed several times with absolute ethanol and water (volume ratio of 9:1) to remove residual monomers and initiator. Finally, the gel sample was cut into pieces and dried in vacuo at 75 • C for 24 h. The final product was ground, sieved through a 200-mesh screen (74-µm eye size), and referred to as "ASD/x wt% O-MMt" (x = 0.5, 1.0, or 2.0). The pure polymer was prepared using the aforementioned process in the absence of O-MMt and referred to as "ASD." The synthetic scheme of the ASD/O-MMt nanocomposites is shown in Figure 3. to remove residual monomers and initiator. Finally, the gel sample was cut into pie and dried in vacuo at 75 °C for 24 h. The final product was ground, sieved through a 2 mesh screen (74-μm eye size), and referred to as "ASD/x wt% O-MMt" (x = 0.5, 1.0, or 2 The pure polymer was prepared using the aforementioned process in the absence of MMt and referred to as "ASD." The synthetic scheme of the ASD/O-MMt nanocomposi is shown in Figure 3.

Characterization
FTIR spectra were obtained at 25 °C on an FTS-3000 spectrophotometer (Americ Digilab) in the wavenumber range of 400-4000 cm −1 . 1 H-nuclear magnetic resonan (NMR) experiments were performed on a Bruker ASCEND-400 NMR spectrometer usi D2O as the solvent. Thermogravimetric analysis (TGA) was performed on a NETZSC analyzer (STA409PC, Selbu, Germany); the samples were heated from 25 to 700 °C a rate of 10 °C/min. The morphologies of the nanocomposites were examined via scanni electron microscopy (SEM; SU8010, Hitachi, Japan), and the energy dispersive X-ray sp troscopy (EDS) was used to characterize the element in nanocomposites. O-M nanolayer exfoliation and dispersion in the polymer matrix were observed via transm sion electron microscopy (TEM; JEM-2100, Tokyo, Japan) at an accelerating voltage of 2 kV.
X-ray diffractometry (XRD) was performed on a Bruker D8 Advance X-ray diffr tometer (Karlsruhe, Germany) with Ni-filtered Cu-Kα radiation (λ = 1.54 Å). The diffr tion angle (2θ) was varied from 1.0° to 10° at a rate of 1°/min and a scanning interval 0.02°. The distance between silicate layers was calculated from the XRD data using Brag law:

Characterization
FTIR spectra were obtained at 25 • C on an FTS-3000 spectrophotometer (American Digilab) in the wavenumber range of 400-4000 cm −1 . 1 H-nuclear magnetic resonance (NMR) experiments were performed on a Bruker ASCEND-400 NMR spectrometer using D 2 O as the solvent. Thermogravimetric analysis (TGA) was performed on a NETZSCH analyzer (STA409PC, Selbu, Germany); the samples were heated from 25 to 700 • C at a rate of 10 • C/min. The morphologies of the nanocomposites were examined via scanning electron microscopy (SEM; SU8010, Hitachi, Japan), and the energy dispersive X-ray spectroscopy (EDS) was used to characterize the element in nanocomposites. O-MMt nanolayer exfoliation and dispersion in the polymer matrix were observed via transmission electron microscopy (TEM; JEM-2100, Tokyo, Japan) at an accelerating voltage of 200 kV.

Intrinsic Nanocomposite Viscosity
The intrinsic viscosity [η] of each nanocomposite was measured using a Ubbelohde capillary viscometer at 25 • C. Prior to the measurements, the nanocomposite was dissolved in a 1.0 mol/L NaCl solution and diluted with 1.0 mol/L NaCl solution. The flow time for each nanocomposite solution was measured in triplicate; the difference between flow times did not exceed 0.2 s. The viscosity-average molecular weight (M η , g/mol) of each sample was calculated using the Mark-Houwink equation:  (5) where t and t 0 are the flow times of the nanocomposite solution and 1.0 mol/L NaCl solution (s), respectively; η sp and η r are the specific and relative viscosities, respectively; H is the y-intercept of the two corresponding fitted straight lines; and c is the concentration of the nanocomposite solution (g/mL).

Viscosity
The viscosities of the nanocomposite solutions at different concentrations, temperatures, and salinities were measured using a Brookfield DV-II + Pro viscometer at a rotor speed of 60 rpm. Each sample's shear resistance at 0-200 s −1 and dynamic rheological behavior at 0-10 Hz were evaluated using a HAAKE RS600 controlled rheometer.

EOR Tests
EOR tests were performed to investigate the performance of the ASD/1.0 wt% O-MMt polymer nanocomposite for EOR. The temperature and back pressure were maintained at 50 • C and 1500 psi, respectively. The detailed experimental procedures were as follows: (1) The core was saturated with synthetic brine (5217 mg/L), after which crude oil was injected into the core until water ceased to flow out from the outlet. (2) Water flooding was conducted at a flow rate of 0.5 mL/min until the water cut reached 98%. (3) A polymer slug having an injection volume (PV) of 0.3 was injected into the core, and water flooding was continued until the water cut reached 98%.
An artificial sandstone core was employed for the experiments, and the basic experimental parameters are listed in Table 1. Table 2 lists the composition of the synthetic brine used in the experiments. The oil recovery ratio was calculated using the following equation [55]: where EOR is the enhanced oil recovery obtained using the polymer solution (%); E T is the total oil recovery from the total flooding process (%); and E W is the oil recovery from the water flooding process (%).

FTIR Spectroscopy
The chemical structure of β-CD-AOI 2 was identified from the FTIR spectra of β-CD and β-CD-AOI 2 . As shown in Figure 4I, the new characteristic IR bands at 1705 and 1595 cm −1 , which are absent in the spectrum of β-CD, correspond to the stretching vibrations of secondary amides, thereby confirming the successful grafting of AOI onto β-CD [8].

FTIR Spectroscopy
The chemical structure of β-CD-AOI2 was identified from the FTIR spectra of β-C and β-CD-AOI2. As shown in Figure 4I, the new characteristic IR bands at 1705 and 15 cm −1 , which are absent in the spectrum of β-CD, correspond to the stretching vibrations secondary amides, thereby confirming the successful grafting of AOI onto β-CD [8].

1 H-NMR Spectroscopy
To further confirm that the ASD copolymer had been successfully synthesized, we obtained the 1 H-NMR spectrum of ASD using D 2 O as the solvent ( Figure 5). The strong peak at 4.68 ppm corresponds to residual H 2 O in D 2 O. The peaks at 1.75-1.35 (1, 8) and 2.07-1.95 ppm (2) correspond to the -CH 2 -and -CH-protons of the polymer backbone, respectively. The singlet at 2.22 ppm (9) corresponds to the -CH 3 protons in DMB, and the peaks at 6.25-5.53 ppm (4, 5) are characteristic -CH 2 -and -CH proton peaks of the polymer backbone of SSS. The peaks at 7.68 (7) and 7.20 ppm (6) are attributable to benzene ring protons. The small peak at 6.89 ppm (3) corresponds to the -NH 2 proton in AM, and the multiplets at 3.34 (12) and 3.72 ppm (11) correspond to the -CH 2 -and -CH 3 protons in the long alkyl chain of DMB. The peak at 2.37 ppm (10) and multiplet at 3.56-3.50 ppm  (13) are attributable to the -OCH 2 -and -CH 2 -protons that are bonded to DMB, while the triplet at 1.05 ppm (14) corresponds to the terminal -CH 3 protons in the hydrophobic chain of DMB. Therefore, 1 H-NMR analysis confirmed that the target product had been successfully prepared.
backbone of SSS. The peaks at 7.68 (7) and 7.20 ppm (6) are attributable to benzene rin protons. The small peak at 6.89 ppm (3) corresponds to the -NH2 proton in AM, and t multiplets at 3.34 (12) and 3.72 ppm (11) correspond to the -CH2and -CH3 protons in t long alkyl chain of DMB. The peak at 2.37 ppm (10) and multiplet at 3.56-3.50 ppm (1 are attributable to the -OCH2and -CH2-protons that are bonded to DMB, while the trip at 1.05 ppm (14) corresponds to the terminal -CH3 protons in the hydrophobic chain DMB. Therefore, 1 H-NMR analysis confirmed that the target product had been succes fully prepared.

XRD
The layer spacing is usually determined using XRD. The diffraction peak position the d001 surface of MMt can reflect the interlayer spacing (d) of MMt [58]. The XRD patter of Na-MMt, O-MMt, ASD, ASD/0.5 wt% O-MMt, ASD/1.0 wt% O-MMt, and ASD/2.0 wt O-MMt are shown in Figure 6. According to Figure 6a,b, Na-MMt exhibits a characteris diffraction peak at 7.30°, while the diffraction peaks of O-MMt are observed at 4.50° an

XRD
The layer spacing is usually determined using XRD. The diffraction peak position on the d 001 surface of MMt can reflect the interlayer spacing (d) of MMt [58].  (Figure 6c-f). This shows that the polymer chains entered into the interlayer of O-MMt during the exothermic polymerization process. This heat generation accelerates the exfoliation of O-MMt nanolayers dispersed in the polymer matrix. In addition, the dispersion and exfoliation degree of O-MMT sheets and the microstructure of the polymer matrix were confirmed via TEM analysis.  a considerably more compact network structure, with a stronger skeleton and fewer but smaller cavities. These features are ascribable to the surface of the exfoliated O-MMt layers having abundant and highly reactive groups, enabling the strong adsorption of polymer molecular chains onto the material surface; these layers act as a nanobarrier that protects the polymer. Therefore, a coarse polymer skeleton, less porous structure, and strong network structure are formed. Furthermore, Figure 7d shows that the exfoliated O-MMt nanolayers are strongly adsorbed on the polymer chain surface because the abundant hydroxyl (Al-OH and Si-OH) and amino groups on the exfoliated O-MMt layers participate in intramolecular and intermolecular hydrogen bonding, resulting in the formation of a thicker and stronger network structure. These results indicate that ASD/1.0  has a considerably more compact network structure, with a stronger skeleton and fewer but smaller cavities. These features are ascribable to the surface of the exfoliated O-MMt layers having abundant and highly reactive groups, enabling the strong adsorption of polymer molecular chains onto the material surface; these layers act as a nanobarrier that protects the polymer. Therefore, a coarse polymer skeleton, less porous structure, and strong network structure are formed. Furthermore, Figure 7d shows that the exfoliated O-MMt nanolayers are strongly adsorbed on the polymer chain surface because the abundant hydroxyl (Al-OH and Si-OH) and amino groups on the exfoliated O-MMt layers participate in intramolecular and intermolecular hydrogen bonding, resulting in the formation of a thicker and stronger network structure. These results indicate that ASD/1.0 wt% O-MMt exhibits better mechanical properties than ASD and has good comprehensive properties that are important for EOR from high-temperature and high-salinity reservoirs.

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wt% O-MMt exhibits better mechanical properties than ASD and has good comprehensive properties that are important for EOR from high-temperature and high-salinity reservoirs.

TEM
Well-exfoliated and well-dispersed O-MMt nanolayers in the polymer matrix can significantly improve the properties of a polymer nanocomposite, such as its viscosity, temperature resistance, salt resistance, shear resistance, and viscoelasticity. To obtain clearer and more reliable information on the exfoliated O-MMt nanolayers dispersed in the nanocomposite, we captured TEM images of ASD/1.0 wt% O-MMt (Figure 8). During TEM, the electron beam penetrates the sample, and the black regions in the image provide estimates of the degrees of electron beam penetration. wt% O-MMt exhibits better mechanical properties than ASD and has good comprehensive properties that are important for EOR from high-temperature and high-salinity reservoirs.

TEM
Well-exfoliated and well-dispersed O-MMt nanolayers in the polymer matrix can significantly improve the properties of a polymer nanocomposite, such as its viscosity, temperature resistance, salt resistance, shear resistance, and viscoelasticity. To obtain clearer and more reliable information on the exfoliated O-MMt nanolayers dispersed in the nanocomposite, we captured TEM images of ASD/1.0 wt% O-MMt ( Figure 8). During TEM, the electron beam penetrates the sample, and the black regions in the image provide estimates of the degrees of electron beam penetration.  Figure S2.
In addition to strong adsorption ascribable to the reactive groups on the surfaces of the O-MMt layers, ion-dipole interactions between sulfonic groups and exchangeable cations, as well as those between quaternary ammonium groups and negative charges on the O-MMt sheets, accelerate the exfoliation of O-MMt. In particular, polymer reorganization and the presence of functional groups (e.g., hydroxyl, amido, and quaternary ammonium groups) facilitate the exfoliation of the O-MMt layers. These well-exfoliated and welldissipated O-MMt nanolayers can act as nano barriers and strongly adsorb onto the polymer chains, thereby slowing down the thermal decomposition rate of the nanocomposite. Consequently, improved thermal stability can be expected from the O-MMt-loaded sample. Thus, ASD/1.0 wt% O-MMt shows high temperature stability toward water-soluble fluids, which is beneficial for EOR.

Intrinsic Viscosity and Viscosity-Average Molecular Weight
The viscosity of a polymer depends on its concentration and size at a particular tem perature in a specific solvent. The property [η] is a physical quantity that reflects the na ture of the molecule. The [η] and Mη of a polymer can be obtained via the dilution method

Intrinsic Viscosity and Viscosity-Average Molecular Weight
The viscosity of a polymer depends on its concentration and size at a particular temperature in a specific solvent. The property [η] is a physical quantity that reflects the nature of the molecule. The [η] and M η of a polymer can be obtained via the dilution method. A plot of η sp /c or lnη r /c against concentration yields a straight line, and [η] can be determined by extrapolating this line to the y-intercept. The M η of each sample was calculated using the Mark-Houwink equation. The plot of η sp /c or (lnη r /c) versus ASD concentration is shown in Figure 10, and the details of the ASD/O-MMt nanocomposites are listed in Table 3

Intrinsic Viscosity and Viscosity-Average Molecular Weight
The viscosity of a polymer depends on its concentration and size at a particular temperature in a specific solvent. The property [η] is a physical quantity that reflects the nature of the molecule. The

Viscosity
Polymer nanocomposite samples with various concentrations were prepared by dissolving the appropriate amount of the polymer in 100 mL of distilled water, after which the apparent viscosity of each sample solution was measured at 25 • C. Figure 11 shows that the apparent viscosity increases with an increasing solution concentration. The apparent viscosity of ASD/1.0 wt% O-MMt changed from 18.8 to 208.9 mPa·s as the concentration was increased from 1 to 11 g/L. The apparent viscosities of ASD, ASD/0.5 wt% O-MMt, and ASD/2.0 wt% O-MMt were 145.9, 112.3, and 132.9, respectively, at 11 g/L. Therefore, ASD/1.0 wt% O-MMt exhibited the highest apparent viscosity among the analyzed samples under the same conditions. Strong interactions between the well-exfoliated and well-dispersed O-MMt nanolayers and the polymer, as explained previously, significantly increased the viscosity of the WSP. This tackifying property increased the sweep area of a WSP, which is beneficial for EOR. that the apparent viscosity increases with an increasing solution concentration. The apparent viscosity of ASD/1.0 wt% O-MMt changed from 18.8 to 208.9 mPa·s as the concentration was increased from 1 to 11 g/L. The apparent viscosities of ASD, ASD/0.5 wt% O-MMt, and ASD/2.0 wt% O-MMt were 145.9, 112.3, and 132.9, respectively, at 11 g/L. Therefore, ASD/1.0 wt% O-MMt exhibited the highest apparent viscosity among the analyzed samples under the same conditions. Strong interactions between the well-exfoliated and well-dispersed O-MMt nanolayers and the polymer, as explained previously, significantly increased the viscosity of the WSP. This tackifying property increased the sweep area of a WSP, which is beneficial for EOR.

Temperature Resistance
High temperatures can significantly damage polymer chains by destroying intermolecular physical interactions and accelerating polymer decomposition. Therefore, the apparent viscosity decreases with increasing temperature. Sample solutions were prepared at a concentration of 5 g/L, and the effects of temperature on the viscosities of the ASD and ASD/O-MMt solutions were investigated ( Figure 12). The viscosities of the ASD and ASD/O-MMt solutions were found to depend on the temperature, with the viscosity decreasing as the temperature increased from 25 to 95 • C. Apparent viscosities of 11.1, 6.0, 23.5, and 7.9 mP·s were determined for the ASD, ASD/0.5 wt% O-MMt, ASD/1.0 wt% O-MMt, and ASD/2.0 wt% O-MMt solutions, respectively, at 95 • C. The apparent viscosity of ASD/1.0 wt% O-MMt was consistently higher than those of the other samples at a given temperature, which is indicative of its superior temperature resistance. Polymer nanocomposite degradation was retarded because the well-exfoliated and well-dispersed O-MMt nanolayers strongly adsorbed onto the surfaces of the polymer chains and acted as nano barriers. Moreover, strong interactions between the O-MMt nanolayers and the polymer restricted polymer chain movement at high temperatures and improved the temperature resistance of the nanocomposite, thereby contributing to EOR from high-temperature reservoirs.

Temperature Resistance
High temperatures can significantly damage polymer chains by destroying intermolecular physical interactions and accelerating polymer decomposition. Therefore, the apparent viscosity decreases with increasing temperature. Sample solutions were prepared at a concentration of 5 g/L, and the effects of temperature on the viscosities of the ASD and ASD/O-MMt solutions were investigated ( Figure 12). The viscosities of the ASD and ASD/O-MMt solutions were found to depend on the temperature, with the viscosity decreasing as the temperature increased from 25 to 95 °C. Apparent viscosities of 11.1, 6.0, 23.5, and 7.9 mP·s were determined for the ASD, ASD/0.5 wt% O-MMt, ASD/1.0 wt% O-MMt, and ASD/2.0 wt% O-MMt solutions, respectively, at 95 °C. The apparent viscosity of ASD/1.0 wt% O-MMt was consistently higher than those of the other samples at a given temperature, which is indicative of its superior temperature resistance. Polymer nanocomposite degradation was retarded because the well-exfoliated and well-dispersed O-MMt nanolayers strongly adsorbed onto the surfaces of the polymer chains and acted as nano barriers. Moreover, strong interactions between the O-MMt nanolayers and the polymer restricted polymer chain movement at high temperatures and improved the temperature resistance of the nanocomposite, thereby contributing to EOR from high-temperature reservoirs.

Salt Resistance
Salt resistance is an important index for the use of an ASD/O-MMt nanocomposite in EOR applications. Therefore, we investigated the effect of different NaCl concentrations on the apparent viscosities of ASD and ASD/O-MMt nanocomposite solutions (5 g/L) at 25 °C (Figure 13). The apparent viscosity of the ASD solution decreased from 48.2 to 5.8 mPa·s as the NaCl concentration increased from 0 to 10 g/L, with apparent viscosities de-

Salt Resistance
Salt resistance is an important index for the use of an ASD/O-MMt nanocomposite in EOR applications. Therefore, we investigated the effect of different NaCl concentrations on the apparent viscosities of ASD and ASD/O-MMt nanocomposite solutions (5 g/L) at 25 • C ( Figure 13). The apparent viscosity of the ASD solution decreased from 48.2 to 5.8 mPa·s as the NaCl concentration increased from 0 to 10 g/L, with apparent viscosities decreasing from 13.5, 87.4, and 21.7 mPa·s to 2.0, 9.2, and 3.8 mPa·s for ASD/0.5 wt% O-MMt, ASD/1.0 wt% O-MMt, and ASD/2.0 wt% O-MMt, respectively. The apparent viscosity of ASD/1.0 wt% O-MMt was consistently higher than those of the other composites. The strongly adsorbed O-MMt nanolayers on the polymer surface prevented inorganic salt ions from affecting the polymer chains; hence, the polymer chains remained fully extended in aqueous solution, thereby maintaining its viscosity. The barrier effect of the exfoliated O-MMt nanolayers improved the salt resistance of the solution; however, adsorption on the polymer surface was poor in the absence of well-exfoliated and well-dispersed O-MMt nanolayers; under these conditions, no nanolayers were available to act as nano barriers, resulting in polymer degradation and a decrease in apparent viscosity. Electrostatic repulsion in the polymer chain decreased when the Na + and -SO 3 2− concentration changes were neutralized, leading to the curling of the macromolecular chain and an associated decrease in viscosity. ASD/1.0 wt% O-MMt exhibited a higher viscosity retention rate than ASD in the same solvent. These results indicate that ASD/1.0 wt% O-MMt can be used for EOR in high-salinity reservoirs.

Shear Resistance
Shear forces from the pump, pipeline, wellbore, bullet hole, and porous media can significantly reduce the viscosity of the polymer solution and affect oil recovery during the injection and displacement processes. Figure 14

Shear Resistance
Shear forces from the pump, pipeline, wellbore, bullet hole, and porous media can significantly reduce the viscosity of the polymer solution and affect oil recovery during the injection and displacement processes. Figure 14

Viscoelasticity
Viscoelastic fluids can enhance oil recovery; hence, we studied the dynamic rheological behavior of the polymer nanocomposites. Relationships between the G′ and G″ moduli and frequency are shown in Figure 15. ASD/1.0 wt% O-MMt displayed G′ and G″ values that are higher than those of ASD, ASD/0.5 wt% O-MMt, and ASD/2.0 wt% O-MMt, indicating better viscoelasticity. G′ and G″ crossover at Gc, and the frequency corresponding to this point is the characteristic frequency (fc) of the material. G″ is higher than G′ for ASD/1.0 wt% O-MMt ( Figure 15) at frequencies less than fc, where the viscidity of the polymer nanocomposite dominates, and the fluid exhibits liquid-like properties. By contrast, elasticity dominates at frequencies above fc, where G' is higher than G″; the G′ value of the fluid rapidly increases with increasing frequency, and the fluid tends to exhibit elasticfluid-like behavior, which increases the sweep area and promotes adsorption to oil surfaces on rocks, thereby enhancing oil recovery. The O-MMt nanolayers have ample hydroxyl groups, hydrogen bonds and ion-dipole interactions among the nanolayers, polymer chains, and oil surface, thus facilitating the removal of oil droplets from rocks, thereby yielding more oil.

Viscoelasticity
Viscoelastic fluids can enhance oil recovery; hence, we studied the dynamic rheological behavior of the polymer nanocomposites. Relationships between the G and G moduli and frequency are shown in Figure 15. ASD/1.0 wt% O-MMt displayed G and G values that are higher than those of ASD, ASD/0.5 wt% O-MMt, and ASD/2.0 wt% O-MMt, indicating better viscoelasticity. G and G crossover at G c , and the frequency corresponding to this point is the characteristic frequency (f c ) of the material. G is higher than G for ASD/1.0 wt% O-MMt ( Figure 15) at frequencies less than f c , where the viscidity of the polymer nanocomposite dominates, and the fluid exhibits liquid-like properties. By contrast, elasticity dominates at frequencies above f c , where G' is higher than G ; the G value of the fluid rapidly increases with increasing frequency, and the fluid tends to exhibit elastic-fluidlike behavior, which increases the sweep area and promotes adsorption to oil surfaces on rocks, thereby enhancing oil recovery. The O-MMt nanolayers have ample hydroxyl groups, hydrogen bonds and ion-dipole interactions among the nanolayers, polymer chains, and oil surface, thus facilitating the removal of oil droplets from rocks, thereby yielding more oil.

EOR Experiments under Laboratory Conditions
Experiments were designed and conducted to evaluate the potential applications of ASD/1.0 wt% O-MMt for EOR. The results are shown in Figure 16. First, during water flooding, the injection pressure and oil recovery increased with the injection of water. The oil recovery of this first water flooding process was EW. Second, the injection pressure and oil recovery increased rapidly with polymer flooding. Third, in the subsequent water flooding process, the oil recovery reached a stable value (ET). However, the injection pres-

EOR Experiments under Laboratory Conditions
Experiments were designed and conducted to evaluate the potential applications of ASD/1.0 wt% O-MMt for EOR. The results are shown in Figure 16. First, during water flooding, the injection pressure and oil recovery increased with the injection of water. The oil recovery of this first water flooding process was E W . Second, the injection pressure and oil recovery increased rapidly with polymer flooding. Third, in the subsequent water flooding process, the oil recovery reached a stable value (E T ). However, the injection pressure first increased and then decreased with an increasing PV. Compared with the water flooding processes, the use of ASD/1.0 wt% O-MMt led to 10.5% higher oil recovery. The experimental parameters and results are listed in Table 4. The EOR test of ASD, ASD/0.5 wt% O-MMt, and ASD/2.0 wt% O-MMt are presented in Figures S6-S8. The results indicate that ASD/1.0 wt% O-MMt has significant potential for application to EOR.

Proposed Adsorption Mechanism of the Exfoliated O-MMt Nanolayers
The well-exfoliated and well-dissipated O-MMt nanolayers strongly adsorb onto the surfaces of the polymer chains, thereby significantly improving the properties of the polymer nanocomposite. The mechanism for the adsorption of exfoliated O-MMt nanolayers is shown in Figure 17. The amino groups of AM form hydrogen bonds with the hydroxyl groups on the exfoliated O-MMt nanolayers. Meanwhile, hydronium ions (H3O + ) act as bridging molecules by forming hydrogen bonds with the hydroxyl and N-H groups of AM. Because of its sulfonic groups, SSS strongly adsorbs onto the surfaces of the polymer chains through H3O + . Moreover, ionic bonds are formed between the ammonium centers (N + ) of DMB molecules and the exfoliated O-MMt nanolayers. The terminal double bond of DMB polymerizes to form covalent bonds. Overall, polymer chains are strongly adsorbed onto exfoliated O-MMt nanolayers as a result of covalent, ionic, and hydrogen bonding; these strongly adsorbed nanolayers protect the polymer nanocomposite from damage. Our results reveal that ASD/1.0 wt% O-MMt, with well-exfoliated and well-dispersed O-MMt nanolayers, exhibits excellent temperature resistance, salt resistance, shear resistance, and viscoelasticity. Therefore, this polymer nanocomposite is potentially useful for EOR applications [17,60].

Proposed Adsorption Mechanism of the Exfoliated O-MMt Nanolayers
The well-exfoliated and well-dissipated O-MMt nanolayers strongly adsorb onto the surfaces of the polymer chains, thereby significantly improving the properties of the polymer nanocomposite. The mechanism for the adsorption of exfoliated O-MMt nanolayers is shown in Figure 17. The amino groups of AM form hydrogen bonds with the hydroxyl groups on the exfoliated O-MMt nanolayers. Meanwhile, hydronium ions (H 3 O + ) act as bridging molecules by forming hydrogen bonds with the hydroxyl and N-H groups of AM. Because of its sulfonic groups, SSS strongly adsorbs onto the surfaces of the polymer chains through H 3 O + . Moreover, ionic bonds are formed between the ammonium centers (N + ) of DMB molecules and the exfoliated O-MMt nanolayers. The terminal double bond of DMB polymerizes to form covalent bonds. Overall, polymer chains are strongly adsorbed onto exfoliated O-MMt nanolayers as a result of covalent, ionic, and hydrogen bonding; these strongly adsorbed nanolayers protect the polymer nanocomposite from damage. Our results reveal that ASD/1.0 wt% O-MMt, with well-exfoliated and welldispersed O-MMt nanolayers, exhibits excellent temperature resistance, salt resistance, shear resistance, and viscoelasticity. Therefore, this polymer nanocomposite is potentially useful for EOR applications [17,60].     Institutional Review Board Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.