Simultaneous Occurrence of Chemical Grafting, Cross-Linking, and Etching on the Surface of Polyimide Membranes and Their Impact on H2/CO2 Separation

Surface modifications of copoly(4,4'-diphenylene oxide/1,5-naphthalene-2,2'-bis(3,4-dicarboxylphenyl)hexafluoropropanediimide (6FDA−ODA/NDA) dense membranes were performed using ethylenediamine (EDA), 1,3-diaminopropane (PDA), and 1,4-diaminobutane (BuDA). Chemical grafting, cross-linking, and etching on the film surface have been verified by ATR−FTIR, UV−vis, AFM, and gel content analyses. The varying extent of the above-mentioned types of surface modification is dependent on the nucleophilicity and molecular dimensions of the diamines, which were computed using molecular simulation. Highly severe chemical etching occurred with EDA modification due to its small molecular dimensions and high nucelophilicity. The greatest degree of cross-linking was provided by PDA modification due to its favorable kinetic property and appropriate nucleophilicity. The ideal H2/CO2 permselectivity escalated from a polymer intrinsic permselectivity of 2.3 to a remarkable value of 64 after PDA modification for 90 min. This promising result was reconfirmed by the binary gas tests showing a H2/CO2 permselectivity of 45 at 35 °C. To the best of our knowledge, this mixed gas permselectivity emerged as the highest ever reported for glassy polymers in the literature. However, because of the inherent nature of the diamines modified films, direct comparison of the gas separation performances reported in this work with the Robeson upper bound cannot be established. In short, appropriate selections of diamino reagent and modification duration are required to cross-link the polymeric chains substantially while maintaining the main-chain rigidity, thereby giving the desired gas separation performance of the membrane.