Effect of molecular weight on two-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 sorption data were fitted with a linear model coupling diffusion and viscoelasticity. A data-fitting scheme is described, efficient enough for routine analysis of sorption data with small bench-top computers. Two dimensionless parameters, α and θ, and a collective diffusion coefficient, D, are determined from each two-stage sorption curve. α, proportional to the ratio of the shear and osmotic moduli of the mixture, is nearly constant over the entire concentration range examined, and is insensitive to molecular weight. The diffusion Deborah number, θ, decreases monotonically with ethylbenzene content, while D increases with ethylbenzene content, but no systematic changes in θ or D were found with molecular weight. These results support the view that two-stage sorption results from a coupling of diffusion to high-frequency viscoelastic modes, which are insensitive to polymer molecular weight.