Membranes with non-homogeneous sorption and transport properties

Abstract

The present review deals with selected developments in a variety of fields of membrane and polymer research organized around the central theme indicated by the title. The emphasis throughout is on fundamental principles and examples which best illustrate these principles, rather than on an encyclopedic review. The membrane systems considered are of topical interest, on one hand, and exemplify various degrees of inhomogeneity of sorption and transport properties, on the other hand. Inhomogeneities at the microscopic level can affect sorption and transport properties markedly by creating new sorption modes. This is well exemplified by dual mode gas sorption and diffusion in glassy polymers; which is being actively studied also from a dual point of view, namely the fundamental understanding of the glassy state, on one hand, and the prediction of gas separation properties, on the other hand. Polyamide-acid dye systems have been quoted as another physically very different example, which is important in textile dyeing, but could prove important also from the biomimetic point of view. Non-homogeneity at the microscopic level is exemplified by the gas permeability of composite membranes and the ionic sorption and transport properties of “homogeneous” ion exchange and other membranes. In the former case, the problem of interest is the relation between the properties of the composite and those of the component phases. In the latter case, there are no recognizable component phases and the effects of non-homogeneity have often been interpreted in other ways. Finally, membranes exhibiting macroscopically nonhomogeneous sorption and diffusion properties are considered. The major development here concerns the characterization of such properties by means of transient-state measurements.