Morphological Effects of Microphase Separation on the Permselectivity for Aqueous Ethanol Solutions of Block and Graft Copolymer Membranes Containing Poly(dimethylsiloxane)

Abstract

The permselectivity of block copolymer membranes consisting of ethanol-permselective poly(dimethylsiloxane) (PDMS) plus water-permselective poly(methyl methacrylate) (PMMA) was compared to the permselectivity of graft copolymer membranes for the separation of an aqueous ethanol solution. This paper focuses on the difference in molecular architecture between the block and graft copolymers and relates microphase separation in these membranes to their permeability and permselectivity for an aqueous ethanol solution in pervaporation. With increasing DMS content, the block copolymer membranes changed from water- to ethanol-permselective at a DMS content of 55 mol %. As reported in a previous paper, however, the graft copolymer membranes showed a dramatic change in the permselectivity at a DMS content of 35 mol %. Transmission electron micrography demonstrated that both membranes had distinct microphase separation consisting of PDMS and PMMA phases and that the morphology was quite different between the block and graft copolymer membranes. The morphological changes in these membranes were investigated by image processing of the micrographs and analysis using a combined model consisting of both parallel and series models. These investigations revealed that the percolation transition of the PDMS phase in the block and graft copolymer membranes takes place at a DMS content of about 55 and 35 mol %, respectively. This suggests that the continuity of the PDMS phase in the microphase separation strongly influences the ethanol permselectivity of these membranes for an aqueous ethanol solution. This report concludes that the design of the molecular architecture in multicomponent polymer membranes is very important in controlling membrane characteristics which are governed by microphase separation.