Effect of block versus random copolyimide structure on hollow fiber membrane spinnability

Abstract

Soluble homopolyimides can be made into spinnable dopes; but copolyimides often present challenges. A 6FDA (4,4′-(Hexafluoroisopropylidene)diphthalic anhydride)-based copolyimide was used to illustrate these challenges when forming both defect-free and “nodular” skinned fibers. This copolyimide is soluble in strong solvents; but when combined in proportions with non-solvents to allow forming an asymmetric structure, a gel, rather than a flowable dope solution can occur in some cases, making fiber spinning impossible. Analysis of problems and solution are presented to show how varied copolymer chemical structures affect dope formation. Studies were extended to other 6FDA-based copolyimides to show the generality of the problem and solution to the problem. We found a random copolyimide that forms a soluble spinnable dope becomes problematic when a block copolyimide with sufficient insoluble chain segments is present. Branching in copolyimide, impurity, and excessive molecular weight also affect dope formation, but to a less drastic degree.