EFFECT OF INHOMOGENEOUS STRESS ON THE SELECTIVITY OF MEMBRANES

A theory is presented in which stress undergoing spatial fluctuations on a mesoscopic length scale is shown to be capable of increasing the separation factors of membrane-penetrant systems, provided it is of su fficient magnitude, i.e., if the ratio of the stored elastic energy to the thermal energy exceeds unity. The theory is developed in schemati c form for a one-dimensional case and is used to demonstrate that memb ranes consisting of glassy polymers can meet the stated criterion. Ext rapolation to the three-dimensional case indicates that such fluctuati ng stresses produce ''blurred supersieves'' in which the widths of the channels depend on the partial molar volumes of the penetrants in the membrane phase. This effect account for the increased selectivity. Th e theory also predicts the frequently observed behavior in which incre ased selectivity is accompanied by decreased membrane permeability. Th e theory is not in conflict with most of the conventional theories of membrane selectivity where mechanisms are considered on the molecular scale of length so that these mechanisms could still be operative whil e the new mechanism simply enhances the separation factor based on the conventional ones. It is suggested that treatments of membranes that lead to morphological heterogeneity on the mesoscopic scale can produc e the desired fluctuating stress fields.