Unexpected permeability enhancement of thermally rearranged (TR) copolybenzoxazole membranes

Abstract

Gas transport properties of novel thermally rearranged copolybenzoxazole (TR-coPBO) membranes with various copolymerization ratios were investigated. The TR-coPBO membranes were obtained by thermal rearrangement of corresponding copoly(o-hydroxy imide)s with fluorene ring moiety via several rearranged temperatures. The TR-coPBO membranes had larger d-spacing values than corresponding neat TR-PBO membranes without copolymerization and larger specific volumes than those expected from an additive rule, suggesting excessively increased fractional free volumes. Permeability coefficients of the TR-coPBO membranes possessed positive deviations from the values expected by a semilogarithmic additive rule against the copolymerization ratio. The enhanced permeability might be brought by steric repulsion and suppressed molecular chain packing due to the partial introduction of fluorene ring moiety.