Research on the crosslinking mechanism of polyacrylamide/resol using molecular simulation and X-ray photoelectron spectroscopy

Abstract

Resol crosslinked polyacrylamide (PAM) hydrogel can be used as the chemical flooding agent in enhanced oil recovery because of its excellent temperature- and salt-resistant properties. It is known that crosslinking reactions, including ortho–ortho, ortho–para and para–para polycondensation of resols and ortho and para polycondensation between PAM and resol, are involved in the researched system, but the extent and activity of the reactions are still not clear. In this study, these crosslinking reactions were all investigated by a combination of molecular simulation and X-ray photoelectron spectroscopy (XPS) techniques. When propionamide was investigated as the model compound of PAM, the Gibbs-free energy and energy barrier of the reactions were calculated by density-functional theory. The atomistic model of the crosslinked PAM network was constructed by molecular mechanics and molecular dynamics, and the density of crosslinked PAM and the amount of active crosslinking sites were obtained. The results show that these reactions occurred spontaneously at 353 K because of thermodynamics. The ortho condensation between PAM and resol is the main kinetic reaction, and the reacted amidocyanogen reached 60% of the total amount, which is consistent with the XPS analysis. The weak gel was prepared by polycondensation of polyacrylamide and resol. The reaction mechanism was clarified by molecular simulation and experiment characterization. The reaction thermodynamics was analyzed to judge the reaction possibility by DMol3. Reaction dynamics were applied to determine the predominant reaction by transition state research, and the results of X-ray photoelectron spectrum confirmed the simulation results.