Effects of gamma radiation on aqueous polymer solutions-A comparative study

The effects of 60Co γ radiation on aqueous solutions of poly(vinyl pyr-rolidone), poly(vinyl alcohol), poly(ethylene oxide), and partially hydrolyzed polyacrylamide were investigated. Concentrations below and above the critical concentration for macrogel formation were studied at radiation doses below and above the dose for incipient gelation. The changes in viscosity of solutions were followed to characterize the effect of radiation below the critical concentration. It was observed that, at a given fraction of the respective critical concentrations, the polymers behaved similarly upon irradiation despite differences in atomic composition and molecular architecture. The dependence of reduced viscosity on absorbed dose showed three distinct regions which were associated with intramolecular cross-linking and chain scission (decrease in viscosity), intermolecular cross-linking (increase in viscosity), and intramolecular cross-linking via ring formation (a second decrease in viscosity). The postgelation properties were characterized by dynamic mechanical measurements in a torsion pendulum using cone-plate geometry. For a given polymer system, storage modulus showed a linear increase with absorbed dose (after a small induction dose), indicating the formation of additional crosslinks. The efficiency of cross-link formation could be correlated in terms of the critical concentration. The logarithmic decrement showed a sudden drop at low doses (the induction dose) and then leveled off with further irradiation indi-eating a rapid elimination of loose chain ends. It was observed that the gels showed syneresis above a limiting dose. This phenomenon was associated with the creation of high internal stresses which appear to develop more readily in polymers with bulky side groups.