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 CO2 permeability of 817% than that of its pre-crosslinked counterpart without sacrificing CO2/N-2 or CO2/CH4 selectivity due to a combination of decarboxylation and amidation-induced cross-linking. Molecular dynamics simulations revealed that such a drastic increase in CO2 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 CO2 feed pressure of 22 bar.