Thermally cross-linked ultra-robust membranes for plasticization resistance and permeation enhancement – A combined theoretical and experimental study

Abstract

This study reports enhanced flux in plasticization-resistive ultra-robust membranes by thermally cross-linking a blend of carboxylated polyimide (PI) and ladder-like amino-polysilsesquioxane (LAPSQ). A series of BTDA-Durene:DABA PIs (BTDA: 3,3′,4,4′-benzophenonetetracarboxylic dianhydride) with three different 2,3,5,6-tetramethyl-1,4-phenylenediamine (Durene):3,5-diaminobenzoic acid (DABA) molar ratios (3:2, 2:1, and 4:1) exhibited an increase in gas permeability with an increasing Durene:DABA molar ratio before and after dehydration-induced cross-linking; this indicated that the bulky Durene moiety was more critical for flux enhancement than the number of carboxylated sites post-cross-linking. More importantly, the thermally cross-linked PI/LAPSQ (80/20) membrane exhibited a significantly enhanced CO 2 permeability of 817% than that of its pre-crosslinked counterpart without sacrificing CO 2 /N 2 or CO 2 /CH 4 selectivity due to a combination of decarboxylation and amidation-induced cross-linking. Molecular dynamics simulations revealed that such a drastic increase in CO 2 permeability was due to larger and/or more interconnected cavities formed in the thermally cross-linked PI/LAPSQ (80/20) membrane. In addition, it showed a substantial increase in hardness and reduced modulus owing to the rigid double-stranded siloxane backbone of LAPSQ and plasticization resistance up to a CO 2 feed pressure of 22 bar. ● Membranes were cross-linked by decarboxylation/amidation of a hybrid polymer blend. ● Siloxane backbone in cross-linked membranes increased hardness and reduced modulus. ● CO 2 permeability of hybrid membranes increased 817% after thermal cross-linking. ● MD simulations showed cross-linking forms larger and more interconnected cavities. ● Cross-linked hybrid membranes were plasticization resistant up to 22 bar of CO 2 .