6FDA-DABA polyimide, polyphenylene, and polyimide-polyphenylene multiblock synthesis, characterization, and membrane gas separation performance and modeled transport

Abstract

A 6FDA-DABA (6FD) polyimide, polyphenylene (PP), and multiblock copolymer (6FD-PP) materials were synthesized using sequential polycondensation and Diels-Alder reactions. PP and 6FD solubility parameters were 19.4 (MPa)1/2 and 22.4 (MPa)1/2, as predicted using Group Contribution Theory. Due to their solubility parameter differences, PP and 6FD solutions in tetrahydrofuran became immiscible when mixed at all concentrations. However, 6FD-PP multiblock copolymer solutions were clear, and their solution-cast films were robust and creasable. Homopolymer and block copolymers estimated fractional free volume (FFV) ranged from 0.174 (6FD) to 0.196 (PP). Their densities and FFV largely obeyed the rule of mixtures, suggesting a "blend-like" morphology. The glass transition temperature (Tg) of 6FD and PP were 285 °C and 400 °C, which became two distinct Tgs at 340 °C and 420 °C for 6FD-PP-[14k:13k]. Larger PP block lengths appeared to suppress the first-order polyimide transition entirely. SAXS and AFM morphological analysis revealed two distinct domains with an average separation distance ranging from 9.0 to 47.7 nm, dependent on PP block length. Membrane gas separation performance was compared to Robeson's upper bound. 6FD-PP multiblock copolymer gas transport properties were found to be highly dependent upon copolymer composition and FFV. Increasing PP content within a copolymer significantly improved gas permeability at the expense of He/CH4 and CO2/CH4 selectivity. Membrane CO2 permeability ranged from 4.36 Barrers using 6FD to 155 Barrers with PP. Their CO2/CH4 selectivity varied between 80.6 (6FD) to 12.7 (PP). 6FD-PP-[14k:13k] membrane's O2 permeability was between these polymers. The membrane had a 30% improvement in its ideal O2/N2 selectivity. The N2/CH4 selectivity ranged from 3 to 1.70, suggesting that 6FD-PP multiblock systems could be efficiently tailored from N2 to CH4 selectivity. Five gas permeability models (rule-of-mixture, Maxwell, EBM, laminate, and blend) were used to study 6FD-PP block copolymer transport behavior. Among them, a blend model had the most reasonable gas permeability prediction. 6FD-PP multiblock membranes improved several ideal gas pairs leading to more permeable and selective materials. The synthetic approach using block copolymerization creates an opportunity for gas separation improvement and tuneability of specific gas pairs.