Anomalous diffusion in poly(vinyl alcohol)-poly(acrylic acid) thin films

Dynamic in situ spectroscopic ellipsometry studies of poly(vinyl alcohol)-poly(acrylic acid) (PVA-PAA) composite films are presented. Thin films immobilized on glass substrates were conditioned over time in aqueous media. Dynamic measurements consisted of thickness (swelling ratio) and optical constant changes in time. The films displayed anomalous diffusion that was characterized by fast initial swelling followed by a much slower thickness reduction. A simple mechanism based on polymer network relaxation was proposed as the likely explanation for thickness reduction. A diffusion/relaxation formalism was used to quantitatively model measured data. From this modeling the diffusion coefficient of water and the network relaxation rate constants were estimated for three different types of composite films that differed in the degree of cross-linking. The diffusion Deborah number was estimated to be a high value ((DEB)(D) approximate to 5 x 10(3)) which suggested elastic diffusion and non-Fickian behavior. Slow polymer network relaxation was the likely cause of a considerable amount of water expelled from the films. Alternative explanations for such anomalous diffusion were tested, and it was concluded that they did not play a significant role in the dynamic properties of the composites. It was proposed that the existence of the two relaxation rate constants (k(r2) approximate to 8 x 10(-3) min(-1), k(r1) = variable) could be related to mutually independent PAA and cross-linked PVA backbone relaxation, respectively.