Molecular Insight into 6fd Polyimide-Branched Poly(Phenylene) Copolymers: Synthesis, Block Compatibility, Ffv, Morphology, Physical Properties, Gas Separation, and Modeled Gas Transport Study

Abstract

A series of 6FDA-DABA (6FD) polyimide and branched poly(phenylene) (PP) block copolymers and homopolymers were successfully synthesized using a Diels-Alder and polycondensation reaction. PP and 6FD homopolymer blends in tetrahydrofuran were immiscible. The result coincides with their large chemical dissimilarity and theoretical solubility parameter differences of 25.47 and 33.17 (MJ/m 3 ) 1/2 . However, 6FD-PP block copolymer solutions were clear and thin films were robust and creasable. Densities and fractional free volumes (0.162~0.346) largely obeyed the rule of mixing, suggesting a "blend-like" morphology. At moderate PP block lengths, two distinct glass transition temperatures (340°C and 420°C) were evident, while large PP block lengths suppressed first-order polyimide transitions entirely. A SAXS and AFM morphological analysis revealed two distinct domains, with separation lengths increasing with PP block length. Their gas permeation, diffusion, sorption and separation properties were thoroughly investigated and exhibited a strong correlation with polymer chemistry, block length and FFV. A block copolymer had an O 2 permeability roughly between 6FD and PP, resulting in a 30% increase in O 2 /N 2 selectivity. The N 2 /CH 4 selectivities ranged from 4.2 to 0.58, suggesting that this 6FD-PP system could be efficiently tuned from highly N 2 selective to CH 4 selective performance. Five structural models, rule-of-mixture, Maxwell, EBM, laminate, and blend, were used to predict gas transport properties. Compared with experimental data, the miscible blend model provided the best results for the 6FD-PP block copolymer system. Block copolymerization by combining highly selective polyimide and highly permeable branched poly(phenylene) provides an opportunity for gas separation tuneability and improvement in selected gas pairs.

Keywords: Polyimide and branched polyphenylene block copolymers, Block compatibility, Fractional free volume (FFV), Gas separation, Gas Transport Models