DOI: 10.7274/25562946
title: Polymer Membranes with Configurational Free Volume for Gas Separations in Harsh Conditions

description: This dissertation explores innovative strategies for the development of advanced polymeric membrane materials, with an emphasis to enhance membrane stability and separation performance stability under challenging conditions such as elevated temperatures and complex feed compositions. The overarching goal is to expand the applicability of polymeric membranes to facilitate the advancement of energy-efficient, low-emission membrane gas separation technologies for a more sustainable future. 
Recognized as an energy-efficient and environmentally friendly separation technology, membrane-assisted separation has gained increasing attention in research. However, existing polymer membranes are commonly challenged by the permeability-selectivity trade-off as well as compromised separation performance in harsh conditions, e.g., loss of size-sieving capability due to plasticization from condensable gases like CO2, significantly declined permeability over time due to physical aging), and drastically decreased selectivity at elevated temperatures. In the past few decades, many research endeavors have focused on developing high-performance polymer membrane materials with outstanding stability and durability. Crosslinked polymer membranes with significantly stiffened polymer segments have shown promise in improving resistance to physical aging and plasticization. However, existing crosslinked membranes derived from randomly distributed crosslinkable groups along the polymer chains often show significantly reduced permeability due to crosslinking-induced densification. Moreover, intricate network structures in the randomly crosslinked membranes have little structure tunability, which prevents unambiguous fundamental studies on structure-property relationships on crosslinked networks. The use of polymeric gas separation membranes in thermally challenging environments is exemplified by polybenzimidazoles (PBIs) for high-temperature H2/CO2 separations in hydrogen production processes. A commercial PBI, known as Celazole? (i.e., m-PBI), which exhibits the highest H2/CO2 selectivity at high temperatures among polymers serves as a benchmark for high-temperature H2/CO2 membrane separation. However, m-PBI suffers from extremely low gas permeability due to densely packed polymer segments resulting from strong hydrogen bonding interactions. Numerous research efforts have attempted to address this issue by incorporating bulky functional groups in PBI to disrupt its tight chain packing. However, the enhanced permeability achieved through this strategy often results in a substantial loss of size-sieving capability at elevated temperatures. 
At the molecular level, the compromised separation performance under harsh conditions stems from chain relaxation and increased chain mobility at elevated temperatures or in the environment of condensing gases. These issues largely originate from the transient and random size distribution of gas transport pathways, referred to as non-equilibrium conformational free volume. Therefore, there is a compelling motivation to develop polymer membrane materials with precisely controlled, highly tailorable, and “permanent” free volume architecture to enhance separation performance and performance stability by restricting segmental mobility and suppressing chain relaxation. This dissertation addresses these challenges through three major accomplishments to instill configurational free volume in the polymer membrane materials design while conducting a comprehensive examination of structure-property relationships. 
Firstly, following a controlled end-linking approach, rigid crosslinked polymer membranes with highly tailorable yet well-defined model network structures were developed (Chapter 2 and Chapter 3), wherein non-collapsible configurational free volume elements were instilled in the highly rigid crosslinked membranes by incorporating the shape-persistent pentiptycene units into thermally rearranged polybenzoxazoles (TR-PBO), leading to much improved resistance to plasticization and physical aging. Systematic studies were performed to thoroughly investigate the effects of crosslink density and crosslink inhomogeneity (i.e., unimodal or bimodal networks) on fundamental gas transport properties of these innovative TR-PBO membranes with crosslinked model network structures. Secondly, to explore the potential of model network structures for high-temperature gas separations a new class of semi-interpenetrating polymer network (s-IPN) structures that integrate unimodal network of TR-PBOs with commercial m-PBI was designed and developed (Chapter 4). The penetration of linear m-PBI through the rigid scaffolds of model networks and the interlocked architecture effectively disrupted tight chain packing while maintaining high segmental rigidity, resulting in greatly enhanced gas permeability with well-maintained selectivity for high-temperature H2/CO2 separation. Thirdly, the incorporation of shape-persistent iptycene structure units into PBIs was explored to understand how configurational free volume influences performance stability and high-temperature tolerance (Chapter 5). Lastly, Chapter 6 introduces a new versatile platform of polymer membrane materials based on triphenylmethane-based polymers prepared via Friedel-Crafts polymerization mechanism. This flexible polymer platform with versatile substituent functional groups enables the engineering of fast and selective gas transport based on a rigorous investigation of the structure-property relationship.