The engaged species induced clustering (ENSIC) model: A unified mechanistic approach of sorption phenomena in polymers

The description of sorption equilibria of solvents in polymers is a key problem which has to be solved in order to achieve a correct understanding of transport processes through dense membranes. Up to now, this purpose is limited essentially to solvents in elastomers, which are described by Flory-Huggins thermodynamics theory, while sorption of gases in glassy polymers is usually based on a mechanistic one, the so-called dual mode model. Given the difficulties encountered as soon as polymer solvent peculiarities (non-regular mixing enthalpy, network elastic contribution) are taken into account, other types of mixtures can hardly be described by purely thermodynamic models unless complicated expressions are used. A simple mechanistic approach has been developed in order to circumvent these limitations; it is based on the assumption that insertion of a solvent molecule into the polymer solvent matrix will be governed by the intrinsic affinity of the solvent for either a polymer segment or an already sorbed solvent molecule. The model enabling cluster formation description has been named engaged species induced clustering (ENSIC). A series of data related to the most frequently used polymer dense membrane materials have been used in order to check the fitting efficiency of the newly developed expression; it is shown to give a very good description of sorption isotherms of many binary polymer solvent systems, including polar and apolar c:ompounds, in either elastomeric, glassy or thermoplastic membrane materials. The physico-chemical interpretation of the two probabilistic insertion parameters used in the ENSIC model, as well as prospects concerning the extension to multicomponent systems or transport mechanisms simulations are discussed.