Modeling of vapor sorption in glassy polymers using a new dual mode sorption model based on multilayer sorption theory

Conventional dual mode sorption (CDMS) model is one of the most effective models in describing vapor sorption isotherms with a concave towards the activity axis in glassy polymers, while engaged species induced clustering (ENSIC) model has been approved to be highly successful in modeling vapor sorption isotherms in polymers with a convex to the activity axis (BET type III) over a wide range. However, neither of them is effective to describe other types of vapor sorption isotherms, especially sigmoidal isotherms. The Guggenheim-Anderson-de Boer (GAB) model fits extremely well with sigmoidal isotherms such as some vapor especially water vapor sorption data in food and related natural materials. However, one assumption of the GAB model for vapor sorption in glassy polymers is inconsistent with the fact that there are two species of sorption sites as the CDMS model assumes. Based on multilayer sorption theory on which the Guggenheim-Anderson-de Boer (GAB) model is based, a new dual mode sorption (DMS) model for vapor sorption in the glassy polymers is deduced. The mathematical meanings and the physicochemical significances of the parameters in the new model are analyzed. The new model has been verified experimentally by some special cases. Comparisons of the new DMS model with the CDMS and the ENSIC models prove that only the new model fits extremely well with all types of vapor sorption isotherms in the glassy polymers.