(285a) Polybenzimidazole-Based Membranes with Semi-Interpenetrating Network (s-IPN) Structures for High-Temperature Pre-Combustion Carbon Capture

H₂/CO₂ separation that relates to pre-combustion CO₂ capture from syngas after the water-gas shift reactions has attracted increasing attention in recent years. Membrane-assisted separation technology is more promising to save energy usage and capital costs compared to current technologies such as physical absorptions. Separating H₂ from CO₂ at high temperatures (e.g., 100-300 ^oC) demands polymer membranes with high thermal stability and strong size-sieving capability, which is challenging for most conventional polymer materials. Polybenzimidazoles (PBIs), e.g., commercial m-PBI, are the leading polymer membrane materials for high-temperature H₂/CO₂ separation due to outstanding thermal stability and good size-sieving capability imparted by strong hydrogen bonding and pi-pistacking interactions of imidazole rings. However, m-PBI has extremely low H₂ permeability (i.e., <3 Barrer at 35 ^oC) limiting its applications in industrial processes. To improve H₂ permeability while maintaining good size-sieving capability at high temperatures, we developed a novel macromolecular design of semi-interpenetrating polymer network (s-IPN) structures, where linear m-PBI chains penetrate through and get interlocked with a rigid unimodal crosslinked networks with precisely controlled and tailorable crosslink density. In s-IPN design, penetration of m-PBI chains through a rigid network scaffold disrupts the tight chain packing of m-PBI by breaking up pi-pi stacking and hydrogen bonds, enabling enhanced H₂ permeability. Simultaneously, the interlocking of m-PBI chains with a rigid network helps retain size-sieving capability at high temperatures due to suppressed chain relaxation. More importantly, the separation performance of s-IPN films can be finely tailored by adjusting crosslinked structure and network content, allowing for realizing superior separation performance. We have demonstrated that m-PBI-based s-IPN films containing pentiptycene-based polybenzoxazole (PPBO) networks and linear m-PBI exhibited significantly improved H₂ permeability (e.g., by ~9 times at 35 ^oC) and well-maintained H₂/CO₂ selectivity spanning a wide temperature range up to 180 ^oC. Through synergistical efforts in adjusting the crosslink density and network content, these s-IPN films outperformed the predicted H₂/CO₂ upper bound at 180 ^oC.

In our recent work, we furthered the structure-property relationship study of the m-PBI-based s-IPN by examining the effect of the nature of unimodal network components via architecting s-IPN structures using a variety of glassy polymer networks of different types, i.e., crosslinked Matrimid^®-like polyimide and crosslinked PBIs. By comparing s-IPN films of the same crosslink density and network content but different network structures (i.e., PBO vs. polyimide vs. PBI), we demonstrated that the segmental rigidity and microporosity of the crosslinked networks play an important role in regulating the gas transport in the resulting s-IPN films. For example, the Matrimid^®-like polyimide networks with more well-defined crosslinked structures and narrower interchain distance favor more efficient penetration and interlocking of m-PBI chains compared to PPBO-based networks, leading to simultaneous enhancement of H₂ permeability and H₂/CO₂ selectivity at high temperatures and exceeding the H₂/CO₂ upper bound at 180 ^oC. The greatly enhanced H₂ permeability, superior size-sieving capability, and highly tailorable free volume architecture in PBI-based s-IPN design exemplify their promising prospects of energy-efficient pre-combustion CO₂ capture. This talk will emphasize the preparation of new PBI-based s-IPN membranes and the fundamental structure-property relationships of these new PBI membranes for high-temperature H₂/CO₂ separation.