The effect of free volume on enhanced transport rates of polysulfone hollow fiber membranes spun from lewis acid:base solvent complexes

Abstract

Asymmetric hollow fiber membranes can be prepared by phase inversion from Lewis acid:base complex solvents. Formation of a complex from the nonsolvent (Lewis acid) and the solvent (Lewis base) permits higher concentrations of nonsolvent to be included in the spinning dope than can be added with traditional solvent/nonsolvent mixtures. These membranes exhibit gas permeation rates which are a multiple of those obtained with membranes fabricated from traditional solvent/nonsolvent mixtures as well as maintenance of selectivity. This enhancement in permeation performance results primarily from the creation of a skin structure with a much thinner effective separating layer than can be obtained from spinning processes utilizing solvent-nonsolvent mixtures. However, it is also believed that these membranes possess enhanced free volume which is derived from the kinetics of the sol-to-gel transition. The rapid dissociation of the Lewis acid:base complex by contact with water accelerates the coagulation process, which limits conformational rearrangement. The dissociation of the Lewis acid:base complex into smaller moieties facilitates their removal from the nascent hollow fiber membranes. The acceleration of the coagulation process increases the free volume in the resultant membrane, which is reflected by an increase in the glass transition temperature. This increase is readily seen during the first heat in a DSC scan, and it is believed that this increase is not a result of superheating, which yields the “ T g overshoot” commonly observed in glassy polymers annealed below their glass transition temperatures.