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 semidilute aqueous solutions of poly(vinyl alcohol) (PVA) and on gelling PVA solutions in the presence of a constant amount of crosslinker agent. The time correlation data revealed, for all the solutions and the systems in the pregel domain, the existence of two relaxation modes. The ''fast'' mode (exhibits an almost exponential profile) was always diffusive. In the solutions, the ''slow'' mode (described by a stretched exponential) was diffusive at high concentrations and a somewhat stronger wave vector (q) dependence was observed at lower concentrations. The concentration dependence of the slow relaxation time (tau(s)) could be described by a power law (tau(s) similar to c(4)). In the pregel domain, the slow mode was diffusive for systems of high polymer concentration, while for systems of low concentration, a much stronger q dependence was detected when the gelation threshold was approached. The cooperative diffusion coefficient (D-c), evaluated from the ''fast'' mode, decreased for systems of low polymer concentration as the gelation process proceeded, while for systems of higher concentration, D-c was practically constant in the whole pregel zone. These results suggest that incipient gels formed from systems of low polymer concentration are more heterogeneous than their corresponding solutions, while in systems of higher polymer concentration, the nonuniformities of the network are practically the same for the solution and the corresponding incipient gel. Close to the gelation threshold, the slow mode could be described 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 single exponential. Incipient gels and gels in the postgel zone exhibited nonergodic features.