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.

本论文探究了先进聚合物膜材料（advanced polymeric membrane materials）开发的创新策略，重点旨在提升膜在高温、复杂进料组成等严苛条件下的稳定性与分离性能稳定性。其总体目标是拓展聚合物膜的应用范围，以推动节能低排放的膜法气体分离技术发展，助力构建更可持续的未来。

膜辅助分离（membrane-assisted separation）作为一种节能且环保的分离技术，已在研究领域受到日益广泛的关注。然而，现有聚合物膜普遍面临渗透率-选择性权衡（permeability-selectivity trade-off）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（condensable gases）引发的塑化（plasticization）效应会导致筛分能力丧失；物理老化（physical aging）会使渗透率随时间显著下降；高温下选择性会大幅降低。近几十年来，众多研究致力于开发兼具优异稳定性与耐久性的高性能聚合物膜材料。其中，链段刚性显著提升的交联聚合物膜（crosslinked polymer membranes）在改善抗物理老化与抗塑化性能方面展现出应用潜力。但现有基于聚合物链上随机分布可交联基团（crosslinkable groups）制备的交联膜，往往因交联诱导的膜致密化（crosslinking-induced densification）而导致渗透率大幅降低。此外，随机交联膜的复杂网络结构几乎不具备结构可调性，这阻碍了对交联网络结构-性能关系（structure-property relationships）开展明确的基础研究。

聚合物气体分离膜在高温严苛环境中的应用，以聚苯并咪唑（polybenzimidazoles, PBIs）用于制氢过程（hydrogen production processes）中的高温H₂/CO₂分离为例。商业化的间位聚苯并咪唑（meta-polybenzimidazole, m-PBI，商品名Celazole®）是目前高温下H₂/CO₂选择性最高的聚合物材料，常作为高温H₂/CO₂膜分离的基准材料。但由于强氢键相互作用（hydrogen bonding interactions）导致聚合物链段紧密堆积（tight chain packing），m-PBI的气体渗透率极低。诸多研究尝试通过在PBI中引入大体积官能团（bulky functional groups）来破坏其紧密链堆积，以此改善渗透率，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，链松弛（chain relaxation）现象加剧且链段运动性（segmental mobility）提升。这些问题主要源于气体传输通道的尺寸分布呈瞬态且随机的状态，即所谓的非平衡构象自由体积（non-equilibrium conformational free volume）。因此，开发具备精确可控、高度可定制且“永久”构型自由体积（configurational free volume）结构的聚合物膜材料，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。

本论文针对上述挑战，通过三项核心研究成果，在聚合物膜材料设计中引入构型自由体积，并系统开展了结构-性能关系的全面研究：
其一，采用可控端基交联法（end-linking approach），开发了具备高度可定制且结构明确的模型网络结构（model network structures）的刚性交联聚合物膜（第2章与第3章）。通过将形状持久性五ptycene单元（pentiptycene units）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型自由体积，显著提升了抗塑化与抗物理老化性能。本研究系统探究了交联密度与交联不均一性（即单峰（unimodal）或双峰（bimodal）网络）对这类创新型交联模型网络结构TR-PBO膜的基础气体传输性能（gas transport properties）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，本研究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）。线型m-PBI（linear m-PBI）渗透进入模型网络的刚性骨架（rigid scaffolds）中，形成的互锁结构（interlocked architecture）有效破坏了紧密链堆积，同时维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率。
其三，本研究探究了将形状持久性ptycene结构单元引入PBI的策略，以阐明构型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章介绍了一种基于傅克聚合（Friedel-Crafts polymerization）机制制备的三苯甲烷基聚合物（triphenylmethane-based polymers）的通用型聚合物膜材料新平台。该聚合物平台具备灵活的取代基官能团修饰空间，通过系统研究结构-性能关系，可实现对快速且选择性气体传输过程的精准调控。

搜集汇总

背景与挑战

背景概述

该数据集总结了博士论文中关于聚合物膜材料的研究，重点在于开发在高温和复杂气体组成条件下具有优异稳定性和分离性能的新型膜材料。研究通过引入构型自由体积元素和设计模型网络结构，显著提高了膜的耐塑化和抗物理老化能力，同时保持了高选择性和渗透性。