Tolerance of polymer-zeolite composite membranes to mechanical strain

Abstract

Numerous studies have been reported for zeolite membranes grown on inflexible supports, such as alumina and metals, and examined for gas transport measurements. Polymer membranes are also used for gas separations, and have a far greater practical impact, primarily because of lower cost. Polymer membranes are made by rapid roll to roll methods, and large quantities can be generated, whereas zeolite membranes are typically made in a batch fashion at much smaller sizes. Because the mechanism of gas transport between zeolites and polymers is different, zeolite membranes have certain performance advantages. Combining the attributes of both zeolites and polymers in a membrane is of significant interest. In this paper, we examine two types of zeolite polymer membranes based on faujasitic zeolites, one with zeolite grown on top of porous polyethersulfone (Type I), and the second where the zeolite is grown within the pores of the porous polymer (Type II). We have previously reported that both Type I and II membranes perform satisfactorily for CO2/N2 separation. However, Type I membranes’ performance is less reproducible and more sensitive to handling. In this study, we explore the elastic properties of these two types of membranes using Peak Force Quantitative Nanomechanical Measurement (PFQNM) technique by Atomic Force Microscopy. It is found that Type II membranes have lower elastic modulus even though the zeolite thickness in these membranes exceeds those of Type I membranes by an order of magnitude. Mechanical stress leads to cracks within the zeolite layer for both types of membranes, though Type II membranes tolerate higher stress. We propose that the higher stress-resistance of the Type II membranes arises because the polymer surrounding the zeolite acts to arrest the crack propagation. The genesis of cracking and elastic property measurements by AFM will be relevant for other polymer-inorganic composites, where an inorganic film exists on/within a polymer.