NONLINEAR VISCOELASTIC DIFFUSION IN CONCENTRATED POLYSTYRENE ETHYLBENZENE SOLUTIONS/

Nonlinear gradient-driven diffusion was studied in concentrated polyst yrene (PS)/ethylbenzene (EB) solutions using vapor sorptions with a fi nite driving force. The nonlinear sorptions were carried out on thin f ilms (congruent to 2.05, 3.50 mu m thick) at conditions where non-Fick ian, ''viscoelastic'' effects appear. These data were modeled with the nonlinear diffusion equation studied by Tang. Four dimensionless mate rial parameters in the model were determined from a limited amount of linear-response, differential sorption data on PS/EB mixtures measured in the same range of experimental conditions as for the nonlinear sor ptions. The nonlinear model successfully predicts the observed nonline ar response either above or below the glass transition (T-g). In order to simultaneously capture the nonlinear response both above and below T-g, the abrupt change in the concentration dependence of physical pr operties at T-g must be accounted for. (C) 1997 John Wiley & Sons, Inc .