Modeling Sorption and Swelling Behavior in Non-Equilibrium Polymer Phases

The solubility of low molecular weight species in glassy polymers can be described by using suitable adaptations to the non-equilibrium state of well established thermodynamic equations for the Helmholtz free energy, which were developed and tested to describe equilibrium phases, such as Lattice Fluid models, PHSC models and SAFT. The procedure is already recognized as effective to model the solubility isotherm of a series of gases in various glassy polymers, and is indeed predictive of different behaviors observed. The non-equilibrium properties of the glassy phase is lumped into the value of the polymer density, which is thus an effective measure, for isotropic systems, of the departure from equilibrium. In the present work we review several different aspects of the models, i.e. NE-LF. NE-PHSC and NE-SAFT, which can be obtained from different equilibrium models, and show how the model can calculate mixed-gases effects, the behavior of polymer blends, the trend observed for the solubility coefficients vs. the condensability of penetrants. As it was also observed by Morbidelli, the model can also been used for the case of semi-crystalline polymers, for which peculiar departures from equilibrium can be observed.