Non-ideal effects in organic-inorganic materials for gas separation membranes

Organic-inorganic hybrids, so-called 'mixed matrix', materials comprising highly selective rigid phases, such as zeolites, dispersed in a continuous polymeric matrix are leading candidates for challenging membrane applications. In addition to ideal additive effects modeled in terms of the intrinsic properties of the continuous and dispersed phases, the dispersed phase may affect the surrounding polymer matrix at the interface between the two phases. For instance, the dispersed phase may cause an undesirable void at the interface or create varying degrees of rigidification in the surrounding polymer. The observed performance as a gas separation material depends strongly upon the specific preparation conditions. The properties of the interface may, therefore, be understood better by considering gas permeation and sorption experiments, coupled with appropriate modeling. For a given polymer and dispersed phase, the stress at the interface during membrane preparation is believed to determine whether a void or a rigidified region of polymer forms at the interface and to what extent. This stress depends primarily on the amount of solvent left to be removed when the nascent polymer matrix vitrifies. The response to preparation-generated stresses can also be affected by chemical 'priming', for example using silane or other coupling agents. Here, we discuss how solvent evaporation, thermal effects, and the resulting stresses at the polymer-dispersed phase interface cause a complex, but at least partially understood array of effects ranging from void formation to stress-dilated regions or even zones of compression in the polymer layers nearest the polymer-dispersed phase interface. Other non-ideal effects can be due to partial or apparent clogging of the dispersed phase. In fact, depending on particle size, shape and preparation protocols, two or more of these effects may superimpose to create a rich array of properties beyond those described by simple models based on only pure material properties. Using this framework, preparation conditions to tailor the interfacial morphology and therefore, membrane transport properties are discussed.