Polymer Membranes with Configurational Free Volume for Gas Separations in Harsh Conditions

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.

本论文探究了先进高分子膜材料开发的创新策略，重点在于提升膜在高温、复杂进料组成等严苛工况下的稳定性与分离性能稳定性。核心目标是拓展高分子膜的应用边界，以推动高效节能、低排放的膜法气体分离技术发展，助力实现更可持续的未来。
膜辅助分离技术作为高效节能且环境友好的分离手段，已在研究领域受到日益广泛的关注。然而，现有高分子膜普遍面临渗透性-选择性权衡的难题，且在严苛工况下分离性能会出现劣化：例如，因CO₂等可凝性气体引发的塑化效应会导致筛分能力丧失；物理老化会使渗透性随时间显著下降；高温环境下选择性则会急剧降低。近数十年来，诸多研究致力于开发兼具优异稳定性与耐久性的高性能高分子膜材料。其中，通过交联使高分子链段显著刚性化的膜材料，在提升抗物理老化与抗塑化性能方面展现出应用潜力。但当前基于高分子链上随机分布的可交联基团制备的交联膜，往往因交联导致膜结构致密化，从而大幅降低气体渗透性。此外，随机交联膜形成的复杂网络结构几乎不具备结构可调性，这使得针对交联网络的结构-性能关系开展精准基础研究受到极大限制。高温严苛环境下的高分子气体分离膜应用以聚苯并咪唑（polybenzimidazoles, PBIs）为例，其可用于制氢过程中的高温H₂/CO₂分离。商业化PBI产品Celazole®（即间位聚苯并咪唑，m-PBI）是目前高温下H₂/CO₂选择性最优的聚合物材料，作为高温H₂/CO₂膜分离领域的基准参比材料。但m-PBI因强氢键作用导致高分子链段紧密堆积，气体渗透性极低。诸多研究尝试通过在PBI中引入大体积官能团来破坏其紧密链堆积，以此改善渗透性，但该策略往往会在高温下大幅损失筛分能力。
从分子层面来看，严苛工况下分离性能劣化的根源在于，高温或可凝性气体环境中高分子链段发生松弛、链运动性增强。这些问题主要源于气体传输通道的尺寸分布具有瞬态性与随机性，即所谓的非平衡构象自由体积。因此，开发具备精准可控、高度可定制且“永久”性自由体积结构的高分子膜材料，通过限制链段运动、抑制链松弛，以提升分离性能与性能稳定性，具有极强的研究必要性。本论文通过三项核心研究成果应对上述挑战，在高分子膜材料设计中引入构象自由体积，并系统开展结构-性能关系研究：
其一，采用可控端基交联策略，开发了具备高度可定制且结构明确的模型网络结构的刚性交联高分子膜（第2章与第3章）。通过将形状持久性五蝶烯（pentiptycene）单元引入热重排聚苯并恶唑（thermally rearranged polybenzoxazoles, TR-PBO）中，在高度刚性的交联膜中引入了不可坍塌的构象自由体积单元，从而大幅提升了抗塑化与抗物理老化性能。本研究系统考察了交联密度与交联不均匀性（即单峰或双峰网络结构）对这类创新性交联模型网络TR-PBO膜的基础气体传输性能的影响。其二，为探索模型网络结构在高温气体分离中的应用潜力，设计并开发了一类新型半互穿聚合物网络（semi-interpenetrating polymer network, s-IPN）结构，该结构将TR-PBO的单峰网络与商业化m-PBI相结合（第4章）。线性m-PBI穿透模型网络的刚性骨架并形成互锁结构，有效破坏了紧密链堆积，同时保持了较高的链段刚性，从而在维持高温H₂/CO₂分离选择性的同时，大幅提升了气体渗透性。其三，探究了将形状持久性蝶烯（iptycene）结构单元引入PBI的效果，以阐明构象自由体积对性能稳定性与高温耐受性的影响机制（第5章）。最后，第6章介绍了一种基于三苯甲烷类聚合物的通用高分子膜材料平台，该聚合物通过傅-克（Friedel-Crafts）聚合机制制备。该柔性聚合物平台具备多样的取代官能团，通过系统的结构-性能关系研究，可实现快速且选择性的气体传输路径的精准设计。

搜集汇总

背景与挑战

背景概述

该数据集聚焦于开发新型聚合物膜材料，旨在提升其在高温等严苛条件下的气体分离性能和稳定性。研究通过引入构型自由体积和设计特定网络结构，解决了传统聚合物膜在高温和复杂环境中的性能下降问题，为高效、低排放的膜气体分离技术提供了新思路。