Structural engineering of matrimid using an amino-terminated polydimethylsiloxane crosslinker to enhance CO2 permeability

Abstract

Polyimides have been utilized for gas separation because of their high gas diffusivity selectivity. Matrimid is one of glassy polyimides that show high selectivity for CO₂/light gases. However, its applications for gas separation have been limited due to its low gas permeability. We report here a novel and facile method to enhance its gas permeability by crosslinking it with high molecular weight poly(dimethylsiloxane) bis(3-aminopropyl) terminated (PDMS-NH₂). PDMS-NH₂ was chosen because of its known high gas permeability due to its flexible siloxane linkages. The structural rearrangement and chemical interactions between Matrimid and PDMS-NH₂ were investigated using various characterization methods. Both XRD and position annihilation lifetime spectroscopy (PALS) results confirmed that the d-spacing and fractional free volume (FFV) significantly increased after incorporating PDMS-NH₂ into the Matrimid polymer matrix. The EDX elemental mapping of both top and cross-sectional surfaces show uniform distributions of elements. The resultant membranes display synergistic separation performance, not only exhibiting high gas permeability but also maintaining high CO₂/N₂ selectivity close to Matrimid. Specifically, the additions of 10, 25, and 50 wt% of PDMS-NH₂ into the Matrimid matrix enhance the CO₂ permeability from the initial 12.01 to 58.22, 82.60, and 260.51 Barrer, respectively (i.e., increasing CO₂ permeability by about 385 %, 588 %, and 2069 %, respectively) without much compromise in CO₂/N₂ selectivity. Interestingly, the mixed gas tests have higher CO₂/N₂ selectivity than those pure gas tests, possibly due to strong interactions between CO₂ and the amine groups of PDMS-NH₂. In addition, the plasticization phenomenon of the Matrimid/PDMS-NH₂ membranes reduces due to the less CO₂ adsorption. Dual-mode sorption analyses indicate the shift of separation mechanism from a diffusivity-selectivity controlled one in the Matrimid membrane to a solubility-selectivity controlled one in the 50/50 Matrimid/PDMS-NH₂ membrane. Comparing with other studies, the newly developed membranes show a relatively higher CO₂ permeability, while exhibiting a comparable CO₂/N₂ selectivity. This study may provide new insights to design advanced polymeric membrane materials for CO₂ capture.