# Preparation of Ester-Crosslinked PI Membranes with Enhanced Gas Selectivity and Plasticization Resistance

## TL;DR

This paper introduces a new method to create polyimide membranes that separate gases more efficiently and resist plasticization, using ester crosslinking.

## Contribution

The study introduces novel diamine monomers and a transesterification crosslinking strategy to enhance gas separation performance and plasticization resistance in polyimide membranes.

## Key findings

- Ester-crosslinked PI membranes showed improved CO2/CH4 selectivity and resistance to plasticization.
- The PI-PGBO-300 membrane achieved a CO2 permeability of 371.05 Barrer and a CO2/CH4 selectivity of 28.11.
- Thermal treatment enhanced solvent resistance while reducing d-spacing and BET surface area.

## Abstract

Fabricating polyimide (PI) membranes with outstanding anti-plasticization ability and gas separation performance remains a challenge. In this study, two novel diamine monomers, DAMBO (methyl 3,5-diamino-4-methylbenzoate) and DAPGBO (3-hydroxypropyl 3,5-diamino-4-methylbenzoate), were synthesized through esterification reactions. Then, we copolymerized each of these two new monomers with 4,4′-diaminodiphenylmethane (DAM) and 4,4′-(Hexafluoroisopropylidene) diphthalic anhydride (6FDA) separately to yield two monoesterified PIs. Following this, we further prepared the ester-crosslinked PIs by inducing a transesterification crosslinking reaction within the PI-PGBO membrane via thermal treatment. As expected, we found that the formation of cross-linked structures can effectively regulate the microporous structure, enhance its sieving performance, and thus improve the membrane’s gas selectivity. Furthermore, the resulting network structure endowed the thermally treated PI membrane with excellent anti-plasticization ability. Physical characterization results show that after heat treatment, both the d-spacing and BET surface area of the PI membrane decreased, but the solvent resistance of the thermally treated PIs was significantly improved. Gas separation experiments revealed that the representative membrane (PI-PGBO-300) exhibited the optimal CO2/CH4 separation performance, with a CO2 permeability of 371.05 Barrer, a CO2/CH4 selectivity of 28.11, and a CO2 plasticization pressure exceeding 30 bar. This study provides valuable insights into the design of cross-linked polyimides (PIs) via transesterification reactions, which are capable of enhancing the performance of membrane-based gas separation processes.