Theoretical studies of hydrolysis and stability of polyacrylamide polymers

Abstract

Thermal stability of water-soluble polyacrylamide (PAM) and partially hydrolyzed polyacrylamide (HPAM) polymers under various solution conditions have been studied utilizing Quantum Mechanics Density Functional Theory (DFT) computational modeling method. The hydrolysis reaction of the amide group (AM) to form acrylic acid (AA) is significantly affected by the pH of solution and the presence of cationic species such as Ca2+. Without catalyzed, PAM is thermally stable with transition barriers as high as 50 kcal/mol. Both acid and base can catalyze the hydrolysis reaction by lowering activation energies with more than 10 kcal/mol. Formation of the 6-membered ring transition state (TS) structure arising from the assist of a second water or an ammonia molecule yields a lower enthalpic energy, which is sufficient to overcome the entropic penalty for bring three molecular species together for reactions. Ammonia/ammonium as products of hydrolysis also catalyze the reaction, rendering an auto-accelerated amide hydrolysis kinetics. The divalent Ca+2 cation not only interacts with carboxylic groups on HPAM to alter their rheological and phase behaviors, but also catalyzes the hydrolysis reaction to increase the degree of hydrolysis of HPAM. These results provide theoretical insights for molecular modifications of PAM/HAPM for their high-temperature (HT) and high-salinity (HS) applications.