EFFECT OF MOLECULAR-WEIGHT ON 2-STAGE SORPTION IN CONCENTRATED POLYSTYRENE-ETHYLBENZENE SOLUTIONS

We examine the effect of polymer molecular weight on two-stage weight uptake kinetics in concentrated polymer solutions just below the glass transition. New results are reported for ethylbenzene sorption in two monodisperse polystyrenes (M = 100 000 and 350 000). The two-stage so rption data were fitted with a linear model coupling diffusion and vis coelasticity. A data-fitting scheme is described, efficient enough for routine analysis of sorption data with small bench-top computers. Two dimensionless parameters, alpha and theta, and a collective diffusion coefficient, D, are determined from each two-stage sorption curve. al pha, proportional to the ratio of the shear and osmotic moduli of the mixture, is nearly constant over the entire concentration range examin ed, and is insensitive to molecular weight. The diffusion Deborah numb er, theta, decreases monotonically with ethylbenzene content, while D increases with ethylbenzene content, but no systematic changes in thet a or D were found with molecular weight. These results support the vie w that two-stage sorption results from a coupling of diffusion to high -frequency viscoelastic modes, which are insensitive to polymer molecu lar weight. (C) 1997 Elsevier Science Ltd.