Study On Relation Between Branched Chain Structure And Properties Of Grafted Acrylamide-Based Copolymers

Three novel surfactant macromonomers: p–vinylbenzyl-terminated octylphenoxy poly(ethylene oxide) (VEn, polymerization number (n): 4, 18, 24) were synthesized. Three kinds of grafted copolymers were synthesized by an aqueous free radical copolymerization technique using acrylamide (AM), (VEn, n: 4, 18, 24), and sodium 2 -acrylamido-2-methyl propane sulphonate (NaAMPS), respectively. The influence of reaction conditions on viseosifieation performance of the polymer solutions was studied, and the suitable reaction parameters of PAAEn (n: 4, 18, 24) were obtained, respectively. The three macromonomers and their copolymers were characterized by Fourier transform infrared (FT-IR) spectroscopy and proton nuclear magnetic resonance (1H-NMR). Influence of branched chain length on solution performances, viscoelastic properties, and associating microstructure in solutions were investigated for the copolymers. Compared with partially hydrolyzed polyacrylamide (HPAM) with superhigh molecular weights, the molecular weights of the three kinds of copolymers were low. The copolymers exhibited the hydrophobic association behavior in water and brine solution, and good solution properties such as solubility, thickening properties, heat resistance, salt resistance, shear resistance, viscoelastic properties, and surfactant and interfacial activities. The critical associated concentrations were 0.10 g/dL for PAAEn. The PAAE4 polymers with the shortest branched chain length displayed the best solution performance. The benzyl-capped octylphenoxy poly(ethylene oxide) (PEO) side chain could not only increase the rigidity of polymer backbones, but also behave as the salt- and heat-resistant functional group.The three copolymers exhibited good thickening properties in pure water and brine solutions with Na+ or Ca2+. For the aqueous PAAE4, PAAE18 and PAAE24 solutions with a polymer concentration of 0.20 g/dL, the apparent viscosities were 1658 mPa·s, 1546 mPa·s and 1090 mPa·s, respectively, and the apparent viscosities of the 5000 mg/L NaCl solutions were 1233 mPa·s, 1167 mPa·s and 685 mPa·s at the same polymer concentration. In the range of 5000～150000 mg/L NaCl concentration, The strong intermolecular hydrophobic association could be formed via the hydrophobic interaction of the octylphenyl group; and the C-O bonds in PEO branched chains could be complex with the metallic uni- and bi-valent cations. Therefore, the PAAE4 and PAAE18 brine solutions exhibited the salt-thickening behavior twice. Their apparent viscosities were up to 350 mPa·s and 338 mPa·s at a high NaCl concentration of 90000 mg/L, respectively. For the PAAE24 polymer, the intermolecular hydrophobic associations were weakened because of excessive ethylene oxide groups in the PEO branched chains. Therefore, the salt-thickening performance of the polymer were not obvious, but the apparent viscosity of 90000 mg/L NaCl solution with 0.20 g/dL PAAE24 was still up to 172 mPa·s. In 5000 and 90000 mg/L NaCl solutions, the apparent viscosities of the three copolymers were higher than those of HPAM and modified acrylamide-based copolymers reported by literature, which were not higher than 500 mPa·s and 50 mPa·s. The solution viscosity of 0.20 g/dL PAAE24 was up to 55 mPa·s in 40000 mg/L CaCl2 brine solution.In the ranges of 20～85 oC, 0.20 g/dL PAAE4 and PAAE18 showed significant heat-thickening twice in 5000 mg/L NaCl solutions because of intermolecular association and hydrogen bonds of PEO with water. The apparent viscosities were the highest at 25 oC, and were 1343 mPa·s and 1304 mPa·s, respectively. The solution viscosities of the two copolymers were still up to 700 mPa·s and 685 mPa·s at 85 oC, respectively, which exhibited good heat resistance of the polymers. The brine solutions of the two copolymers exhibited good anti-aging properties. After aging for 45 days at 45 oC, the viscosity retention rates were 30% and 21%, respectively. The anti-aging properties were better than HPAM with the molecular weight of 25 million and the hydrolysis degree of 25% (viscosity retention rate: 14%). The results showed that the PAAE4 polymer exhibited the best heat resistance and anti-aging property. The molecular chains of PAAEn were extended by suitable shearing, resulting in the reinforced intermolecular association. As a result, the polymer solutions exhibited significant viscosification performance. The apparent viscosities decreased sharply with increasing shear rate at higher shear rate, and the apparent viscosities decreased more slowly with an increase in the branched chain length of the copolymers. The consecutive three shear cycles measurement indicated that the PAAEn copolymers exhibited thixotropy behaviour. The results also indicated that the degradation of polymer chains did not occur upon consecutive shear and that the PAAEn polymers displayed good anti-shearing properties.The dynamic viscoelasticity investigation showed that the elastic modulus (G') and viscous modulus (G") of 0.20 g/dL PAAEn increased with increasing the angular frequency in water and 5000 mg/L NaCl solution. The elastic character of aqueous and brine solutions were strengthened with increasing polymer concentration. The PAAE4 polymer exhibited the most significant viscoelastic properties in the solutions, and the solutions predominantly exhibited elastic character above 0.15 g/dL. The G' and G" increased with increasing the NaCl concentration from 50000 to 90000 mg/L for the PAAEn brine solutions, and the G' and G"of PAAE4 and PAAE18 brine solutions also increased with increasing the NaCl concentration from 20000 to 30000 mg/L. The PAAE4 polymer predominantly exhibited elastic character in the four brine solutions, and the viscoelastic character was the strongest among the three kinds of copolymers. The viscoelastic character of the brine solutions changed slightly with increasing temperature from 25 oC to 30 oC for the copolymers, especially PAAE4 and PAAE24. The G' value of PAAEn is higher than that of HPAM, which showed that the intermolecular hydrophobic association of extended grafted copolymer chains was more favorable for the elastic character than the entanglement of molecular chains.The fluorescent probe technique is used for investigating the formation of hydrophobic microenvironment for the PAAEn copolymers. The intermolecular association was reinforced with increasing the polymer concentrations in water and brine solutions, resulting in the enhancement of the non-polarity of hydrophobic microdomains. The I1/I3 values of three copolymers in 5000 mg/L NaCl solutions were significantly lower than those in water, and decreased more slowly than those in water. The I1/I3 values of PAAE18 were minimum in water and brine solutions among the PAAEn copolymers, and the non-polarity of hydrophobic microdomains of PAAE4 was between PAAE18 and PAAE24. This indicated that the associated microstructures with huge hydrodynamic volumes were formed via suitable intermolecular hydrophobic association of the PAAE4 chains with a shorter PEO branched chain. In water and 5000 mg/L NaCl solutions, the association of molecules was enhanced with increasing polymer concentration, and the number and size of hydrophobic microstructures were increased, resulting in an increase in the Ie/Im values. Moreover, the Ie/Im values of the brine solutions were remarkably higher than those in water. With the NaCl concentration increased, the variation of the Ie/Im values with NaCl concentration was similar for the PAAEn copolymers. The Ie/Im extreme value occurred at different NaCl concentrations because of different PEO chain length.Scanning electronic microscope (SEM) morphologies showed that the unique associated microstructures were formed via the intermolecular association of the extended branched chains of PAAE18 in water. The molecular chain bundles (diameter: 100~800 nm) were formed in an aqueous 0.15 g.dL-1 PAAE18 solution. These expanded polymer chain bundles aggregated with each other to form unique associated microstructures. The associating structures became larger and more compact with increasing the polymer concentration. The continuous associated microstructures were still formed in 5000 mg/L NaCl solution because of comparatively expanded molecular chains. It is because of the formation of supramolecular associated microstructures that the grafted polymer brine solutions displayed excellent the properties.