Polyethersulfone and polyphenylsulfone hollow fiber trilayer membranes spun from Lewis acid: base complexes — Structure determination by SEM, DSC, and oxygen plasma ablation

Abstract

Hollow fiber membranes of polyethersulfone and polyphenylsulfone were prepared from propionic acid: N-methylpyrrolidone Lewis acid:base complex solvent system and from noncomplexing N-methylpyrrolidone/formamide solvent/nonsolvent mixture. Differential scanning calorimetry showed that both membranes prepared from the complexing system exhibit “first heat” glass transition temperatures higher than those of bulk polymers from which the membranes were prepared. The elevated T g's result from increased fractional free volume incorporated into the membranes by accelerated sol-to-gel transition in the phase inversion process. Structures of the membranes were further examined by scanning electron microscopy, oxygen plasma ablation, and oxygen and nitrogen gas permeation. Measurements on silicone coated membranes show much higher permeation rates for the complex spun membranes than for those spun from the noncomplexing system with only slight differences in selectivity. Differences in gas flux rates across untreated and oxygen plasma etched specimens indicate profound differences in skin and substructure morphologies. These are discussed in terms of combined effects of enhanced fractional free volume, reduced separating layer thickness and greater porosity of both skin and substructure for membranes prepared from the complexing system.