Case-II diffusion and solvent-polymer films drying: A meso-scale model

We propose a model for calculating a constitutive relation for the dynamics of solvent-polymer systems close to the glass transition. We do it by considering a layer of thickness $\xi $ of a few nanometers which is submitted to an arbitrary time varying solvent activity $a(t)$. We show then how these constitutive relations allow to explain case-II diffusion in glassy polymers and provide a physical interpretation for the parameters of the Thomas-Windle model. Regarding the process of film drying, we show that films up to $1\,\mu {\rm m}$ thick can be almost completely dried in an accessible experimental time, even at temperatures well below the polymer glass transition temperature. When drying a thicker film, we show that a glassy crust may appear on the free surface, as has been shown experimentally