DYNAMIC LIGHT-SCATTERING OF POLY(VINYL ALCOHOL) SOLUTIONS AND THEIR DYNAMICAL BEHAVIOR DURING THE CHEMICAL GELATION PROCESS

Dynamic light scattering experiments have been carried out on semidilu te aqueous solutions of poly(vinyl alcohol) (PVA) and on gelling PVA s olutions in the presence of a constant amount of crosslinker agent. Th e time correlation data revealed, for all the solutions and the system s in the pregel domain, the existence of two relaxation modes. The ''f ast'' mode (exhibits an almost exponential profile) was always diffusi ve. In the solutions, the ''slow'' mode (described by a stretched expo nential) was diffusive at high concentrations and a somewhat stronger wave vector (q) dependence was observed at lower concentrations. The c oncentration dependence of the slow relaxation time (tau(s)) could be described by a power law (tau(s) similar to c(4)). In the pregel domai n, the slow mode was diffusive for systems of high polymer concentrati on, while for systems of low concentration, a much stronger q dependen ce was detected when the gelation threshold was approached. The cooper ative diffusion coefficient (D-c), evaluated from the ''fast'' mode, d ecreased for systems of low polymer concentration as the gelation proc ess proceeded, while for systems of higher concentration, D-c was prac tically constant in the whole pregel zone. These results suggest that incipient gels formed from systems of low polymer concentration are mo re heterogeneous than their corresponding solutions, while in systems of higher polymer concentration, the nonuniformities of the network ar e practically the same for the solution and the corresponding incipien t gel. Close to the gelation threshold, the slow mode could be describ ed by a power law. In the postgel regime, the slow mode was absent and the decay of the correlation function could be approximated by a sing le exponential. Incipient gels and gels in the postgel zone exhibited nonergodic features.