Based on NonEquilibrium Thermodynamics (NET-GP) approach, a model is here presented and discussed, aimed to the prediction of anomalous characteristics in kinetics for differential sorption of swelling agents in amorphous glassy polymer films. For mentioned conditions, the “two-stage sorption” kinetics is typically observed, in which a first stage of Fickian character is followed by a second stage with kinetic order larger than 0.5 and characteristic time independent of film thickness. NET-GP model, which interprets the out of equilibrium conditions of polymer mixtures below Tg through the use of polymer mass density as order parameter, was successfully used since its introduction [Doghieri and Sarti, Macromolecules 1996, 29, 7885] to describe several non-equilibrium features in solubility isotherms for a number of glassy polymeric systems. By coupling the description of thermodynamic properties from NET-GP with simplified model for the bulk rheology of the system and generalized Fickian law for mass transport, a modeling tool is derived in this work which allows to satisfactorily describe results from experimental analysis of two stage sorption kinetics.
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