A series of 6FDA-DABA (6FD) polyimide
and branched poly(phenylene)
(PP) block copolymers and homopolymers were successfully synthesized
using Diels–Alder and polycondensation reactions. 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/m3)1/2. However, 6FD-PP block copolymer solutions were clear,
and thin films were robust and creasable. Densities and fractional
free volumes (FFV) (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
and 420 °C) were evident, while large PP block lengths suppressed
first-order polyimide transitions entirely. A small-angle X-ray scattering
and atomic force microscopy morphological analysis revealed two distinct
domains, with separation lengths increasing with the 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
O2 permeability roughly between 6FD and PP, resulting in
a 30% increase in O2/N2 selectivity. The N2/CH4 selectivities ranged from 4.2 to 0.58, suggesting
that this 6FD-PP system could be efficiently tuned from highly N2-selective to CH4-selective performance. Five structural
models, rule-of-mixture, Maxwell, equivalent box model, 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 tunability
and improvement in selected gas pairs.