ULTRAFILTRATION MEMBRANES FROM POLYSULFONE/CARBOXYLATED POLYSULFONE BLENDS

Abstract

Polysulfone (PSf) is a widely used thermoplastic for synthetic membranes, particularly in ultrafiltration (UF) processes and as porous backing supports for thin film composite membranes. However its somewhat hydrophobic nature precludes its use in some aqueous membrane applications that require hydrophilic characteristic. PSf can be hydrophilicised by adding carboxylic groups -COOH to the polymer backbone through a process of carboxylation to give carboxylated polysulfone (CPSf), or by blending PSf with CPSf. In this study hydrophilic UF membranes were successfully made from PSf/CPSf blends and their performance was evaluated. As immiscible PSf /CPSf blends would yield unstable membranes, the misciblility between PSf and CPSf was first investigated. It was found that miscibility between CPSf and PSf depends on its degree of carboxylation (DC), i.e., average number of -COOH groups attached to a repeated unit in the PSf backbone. CPSf of DC less than about 1.30 were found miscible with PSf. CPSf of DC higher than 1.30 is immiscible with PS£ due to strong self-association among CPSf molecules, probably through hydrogen bonding. There appeared to be higher intrinsic miscibility in CPS£ of DC 0.50 (CPSf0.50) with PSf. In this study CPSf 0.45 and PSf blended well to yield a blend membrane with high solute separation capability. PSf/CPSf blend membranes were cast from solution using the phase inversion technique. Performance of these membranes was measured in terms of solute separation factor (f) and membrane permeate flux using aqueous solution of NaCl, glucose, polyethylene glycols (PEG) of various molecular weights and bovine albumin as molecular probes. Based on the molecular weight cut-off method using PEG, it was estimated that the PSf/CPSf0.45 blends yielded membranes with average pore size in the range of 16 Å to 30 Å. PSf/CPSf0.87 blends and CPSf0.45 yielded membranes of larger pores in the range of 34 Å to 62 Å. Hence these membranes can be classified as UF membranes. Having smaller pores the PSf/CPSf0.45 membranes offered higher solute separation but lower permeate flux than the other membranes. Carboxyl contents in the membranes alone cannot offer an explanation for the difference in the membrane morphology. Carboxyl contents, considered in conjunction between PSf with and some possible synergistic interaction CPSf0.45 seems to offer a suitable explanation for these obsevations.