Gel formation of polyacrylic acid in dilute aqueous solutions via radiation crosslinking

Linear polyacrylic acid was crosslinked in dilute aqueous solution via gamma radiation, and gel formation was investigated as a function of radiation dose, pH, salt concentration and polymer concentration. High gel fractions were found at low pH values whereas above pH similar to 4 no gel was formed. However, the gel content was increased at higher pH by adding salts. The gelation results were explained in terms of ionic interactions. The formation of crosslinks by the combination of chain-bound radicals was inhibited by interchain electrostatic repulsion when the polyacrylate chains were charged and the ionic strength was low. The experimental results showed that gel formation in dilute solution did not occur when kappa lambda < 1, where lambda is the average distance between neighboring dissociated carboxyl groups on the polyacrylate chains and 1/kappa is the Debye-Huckel interaction distance. Thus, we propose kappa lambda > 1 is a necessary, but not sufficient, criterion for the onset of gelation of charged polymers in dilute solution. (C) 1998 Elsevier Science Ltd. All rights reserved.