Emulsion pathways to composite polymeric membranes for separation processes

Abstract

A novel method for the preparation of selective composite membranes from emulsions is suggested. The dispersed phase is chosen to yield a polymer soluble in those components for which the membrane should be selective; the continuous phase, on the other hand, is selected to yield a polymer that is insoluble in any of the components of the mixture. Conventional emulsions (which have a maximum dispersed phase volume fraction of 0.74) or microemulsions can be employed to generate composites. However, concentrated emulsions which allow volume fractions as large as 0.99 are most suitable as precursors to selective and efficient membranes. These concentrated emulsions have the appearance of gels with a structure similar to that of foams. The relatively high permeabilities obtained with the resultant membranes are due to the small thickness of the films of the continuous phase. A concentrated emulsion of a hydrophobic (hydrophilic) monomer dispersed in a hydrophilic (hydrophobic) continuous phase is first prepared at room temperature, with suitable initiators in each phase for later polymerization, and with an appropriate dispersant in the continuous phase. To ensure the stability of the emulsion, the hydrophilic monomer is, in general, replaced by monomer plus water. On heating the gel at 50 °C, polymerization occurs in both phases and the emulsion transforms into a composite polymer membrane. As examples, composite membranes containing polystyrene as the dispersed phase and polyacrylamide as the continuous phase are used to separate toluene from cyclohexane, while other composite membranes containing acrylamide as the dispersed phase and a crosslinked polystyrene as the continuous phase are used to separate water from ethanol.