Effects of Repeat Unit Sequences on Physical and Gas transport Properties of Soluble Copolyimides

Polyimides are widely utilized in functional applications such as gas separation membranes, owing to their outstanding properties. However, the influence of repeat unit sequences on physical and gas transport properties remains insufficiently explored. In this work, three imide-containing diamines, Diamine N, Diamine B, and Diamine 1, were synthesized by reacting 2,5-dimethyl-1,4-phenylenediamine (DPD) with 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA), and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), respectively. Subsequently, alternating copolymers NT6F-A and BP6F-A were prepared from Diamine N or Diamine B with 6FDA, while BPBT-A and 6FBT-A were obtained from Diamine B or Diamine 1 with 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA). For comparative evaluation, four corresponding random copolymers (NT6F-R, BP6F-R, BPBT-R, and 6FBT-R) were also synthesized under identical conditions. The effects of repeat unit sequencing on the copolyimides’ physical and gas transport properties were systematically investigated. Compared to their random analogs, alternating copolymers exhibited more ordered and dense chain packing that imparts higher diffusion selectivity, thereby substantially enhancing membrane gas separation selectivity. And the alternating copolymer 6FBT-A in particular demonstrated superior solubility, mechanical properties and thermal conductivity relative to its random counterpart 6FBT-R. Specifically, 6FBT-A exhibited tensile strength and modulus of 98.9 MPa and 2.80 GPa, representing increases of 23.9% and 12.0%, respectively. Its thermal conductivity reached 0.1640 W·m -1 ·K -1 , a 138% improvement over 6FBT-R. Furthermore, the O 2 /N 2 selectivity increased by 26%, from 5.8 to 7.3. This study provides valuable insights into the rational design of advanced polyimides with optimal physical and gas separation performance.