Stress effects on diffusion in polymers: a model for volume relaxation

A thermodynamic model is discussed for the representation of volume relaxation phenomenon induced by sorption of gases or vapours in glassy polymer matrixes. The model is based on results from NELF approach to the description of thermodynamic properties of glassy polymeric mixtures and it assumes a general linear viscoelastic law holds for the bulk rheology of the glassy polymer. Representation of volume relaxation properties of the mixture allows for the prediction of sorption kinetics in glassy polymer samples for those conditions in which diffusive resistance is negligible. Results from the use of the model are compared with experimental data for n-hexane sorption kinetics in glassy polystyrene microspheres, for relatively low vapour activity, at different temperatures. Satisfactory representation of experimental data is obtained by means of temperature dependent bulk viscosity which is consistent with results obtained form purely mechanical volume relaxation experiments.