Membrane technology has been playing an increasingly important role in addressing the global challenges related to water, the environment, and energy. The majority of membranes used in industry are produced by using polymers. Membranes hosting nano-sized or even angstrom-sized pathways, that are fixed or dynamic, regulate the transport of fine particulates, molecules, or ions based on the mechanism of size discrimination, electrostatic interactions, or sorption and diffusion. Advances in polymer science have helped to develop better membranes, and in turn the ever-increasing demands for highly efficacious membrane separations drive further progress in fundamental polymer science. That is, polymer science is essential to the development of polymeric membranes with high performance (Figure 1). Macromolecules has long been an important platform for publishing highly relevant and impactful studies regarding polymeric membranes. This Virtual Issue highlights some membrane studies recently published in Macromolecules, mainly focusing on membranes for ultrafiltration (UF), gas separations, and fuel cell separators. Figure 1. Polymer science eventually determines the performances of polymeric membranes. UF is enabled by membranes with sizes in the range of ∼2–100 nm. UF membranes are typically prepared by phase inversion and have been widely used in the purification and concentration of, for example, colloids, polymers, and proteins. Unfortunately, typical UF membranes suffer from a wide pore size distribution which is intrinsic to the process of phase inversion, and consequently a trade-off between permeability and selectivity occurs. Self-assembly of block polymers is recognized to be one solution of this bottleneck of UF membranes by forming membranes with nearly monodispersed pores.(1) Such membranes are prepared either by etching away the dispersed cylindrical phases in the perpendicularly aligned block polymers or by performing nonsolvent-induced phase separation of block polymer micellar solutions. Alternatively, well-defined porosities can be generated in self-assembled block polymers by the strategy of selective swelling, which can be finished within 30 s with the assistance of microwave energy.(2) Interestingly, simulation reveals that vesicles of amphiphilic ABA triblock copolymers can be spontaneously perforated driven by the orientation entropy of the semirigid B-blocks, which may inspire efforts to experimentally realize such perforated BCP membranes with ON/OFF switchable pores.(3) Gas separation relies on much smaller pores, down to the angstrom scale if we consider the free volume in polymers as dynamic pores. Chemical functionalization has been performed to tune the free volume and consequently the gas-separation performance of engineering plastics, and fluorination of polyimide is a representative example.(4) Industrially used membranes for gas separation are usually prepared from glassy polymers with low free volumes and typically exhibit moderate selectivity and low permeability. To solve this issue, significant efforts have been devoted to explore highly permeable polymers, and polymers of intrinsic microscopy (PIMs) are identified as a very promising candidate in this regard. Because of their rigid and contorted backbone structures and consequently severely hindered segmental mobility, PIMs are characterized by remarkably high free volumes, thus greatly facilitating gas permeability. PIM-1, the most extensively studied PIM polymer, is generally synthesized from the step-growth polymerization of two tetrasubstituted monomers via double nucleophilic aromatic substitution. The microstructures of the synthesized PIM-1 and consequently its gas separation properties are sensitive to the applied polymerization conditions. It was revealed that cyclic or other nonlinear topologies identified in the polymerized products of PIM-1 contributed to the increase of the surface areas of PIM-1, and higher contents of branching and network topologies improved the gas pair selectivity of PIM-1.(5) The gas transport behaviors of PIMs can be tuned by postfunctionalization of PIMs. Of particular interest is to convert the nitrile groups in PIM-1 to carboxylic acids because thus-functionalized PIMs show enhanced CO₂ sorption and diffusion selectivity because of strong interaction between CO₂ and carboxylic acid groups. An optimized acid hydrolysis strategy was recently reported to functionalize PIM-1, and >89% conversion was obtained within 48 h while previous methods required 360 h. Thus-synthesized PIM-COOH exhibited smaller free-volume entities and showed significantly improved selectivities for CO₂-based gas pairs.(6) In another work, the nitrile group in PIM-1 was first converted to tetrazole, which was subsequently reacted with amines, thus incorporating ionic-liquid-like side groups to PIM-1. Thus-functionalized PIM-1 exhibited increased gas selectivities for both O₂/N₂ and CO₂/N₂ at the expense of reduced pure-gas permeabilities. This side-group modification strategy is expected to bring a variety of functional groups to PIMs because tetrazoles are versatile in composing a library of different ionic liquids.(7) Investigations on gas transport behaviors of PIMs are generally performed on PIM bulk films with thicknesses on the scale of 10 μm. However, thus-obtained transport properties cannot be directly used to predict the performances of PIM thin films with thickness down to the submicrometer scale required by industrial applications of gas separations. A recent work used in situ interference-enhanced spectroscopic ellipsometry to investigate dilation and sorption of pure and mixed gases in PIMs films.(8) It was found that film thinning promoted the collapse of the frozen-in free volume, and lower gas uptakes were observed in thinner films. Importantly, the mixture effect also occurred in thin PIM films as they dilated to a smaller degree in gas mixtures than in a pure gas. These observations are helpful in predicting the performances of PIM thin films and in designing of PIM membranes in the form of thin-film composites for real-world applications. Studies on anion exchange membranes (AEMs) are bursting nowadays mainly because AEM-enabled fuel cells have the potential to use cheap electrocatalysts under alkaline conditions, thus overcoming the cost barriers of the mainstream proton exchange membranes. Ion exchange membranes require to be suitably hydrated to maximize performance; therefore, it is essential to understand the hydration behavior and water mobility of these membranes.(9,10) Dekel et al. performed a systematic investigation on the equilibrium state and kinetics of water uptake (WU) in water vapor of different types of AEMs, demonstrating the potentials of the equilibrium WUs and WU kinetics in evaluating AEMs.(11) Kumar et al. investigated the effect of hydration on the transport properties of sulfonated polystyrene-based cationic exchange membranes (CEMs) and found that the water volume fraction underpinned an intrinsic trade-off between permselectivity and ion conductivity.(12) That is, an increase in water content will lead to enhanced ion conductivity but reduced permselectivity because of decreasing charge density. This is also applicable to AEMs. Freeman et al. developed a new and simple method to determine cation and anion diffusion coefficients in ion exchange membranes based on the Nernst–Planck equation and found that counterion diffusion coefficients were greater than co-ion diffusion coefficients after considering the contribution of differences in ion size.(13) Polymer chemistry is playing a crucial role in developing robust, high-performance AEMs. Poly(arylene ether)s (PAEs) are the most widely used polymers for AEMs. Unfortunately, the aryl ether bonds in PAEs are weak points and are prone to undergo chain scission in alkaline conditions, leading to progressive degradation of the AEMs. A few solutions have been developed to enhance the alkaline stability of PAEs. Bae et al. synthesized quaternized multiblock poly(phenylene-co-arylene ether)s from a “pre-aminated” monomer and a series of chloro-terminated oligo(arylene ether)s.(14) These copolymers exhibited enhanced fuel cell performances and alkaline durability because of the well-defined microphase separation of the hydrophilic and the hydrophilic domains and the presence of the phenylene-based structures in the hydrophilic domains. In contrast, He et al. converted the electron-withdrawing C═O groups in the conventional poly(aryl ether ketone) (PEAK) backbone into the electron-donating C–NH₂ groups through the Leuckart reaction, thus alleviating the nucleophilic attack of OH^– to the ether-connected phenyl–C in the elevated electron cloud density environment.(15) Alternatively, polymers containing no ether bonds in the backbones, for example, polystyrene, have been successfully explored to develop alkaline-robust AEMs.(16,17) Using Ziegler–Natta polymerization, Hickner et al. synthesized bromoalkyl-functionalized poly(olefin)s with 4-(4-methylphenyl)-1-butene and 11-bromo-1-undecene as the monomers.(18) The synthesized polyolefins were converted to anion-conductive copolymers by reacting the pendant bromoalkyl group with a custom-synthesized tertiary amine containing pendant quaternary ammonium moieties. Thus-prepared polyolefin-based AEMs carried three cations per side chain and showed good chemical and dimensional stability and considerably higher hydroxide conductivities likely due to phase separation in the triple-cation structure. It was also found that AEMs with longer pendant alkyl chains exhibited better alkaline stability because the steric and hydrophobic effects of the alkylene groups in the side chains mitigate the attack by the hydroxide ions to the ammonium groups.(19) Importantly, new approaches such as PIM-based polymers,(20) oppositely charged block copolymer mosaics,(21) and chemical vapor deposition methods(22) have found interesting applications in AEMs and other ion-conductive membranes. As the pores or “free volume” entities in membranes for desalination and ion exchanges are very small down to a few angstroms and polymeric structures of these membranes are hydrated, it is extremely important to use advanced characterization techniques to reveal the structures and dynamics of these membranes. Neutron scattering is emerging as a powerful tool to in situ probe the water and chain dynamics of these membranes because it can resolve motions of hydrogenous species on the length scale of several nanometers and the time scale on the order of nano- to picoseconds. Soles et al. employed neutron scattering to investigate a polyamide desalination membrane and an AEM.(23) It is revealed that on the nanometer length scale water diffuses through the desalination membrane at a rate similar to bulk water, while water diffuses through the AEM 2 times slower than bulk water. In another work, neutron reflectometry (NR) was used to distinguish the support and separation layers of membranes composed of polyelectrolyte multilayers.(24) NR reveals that the membranes have an asymmetric structure with distinct bottom and top multilayers in either the dried or hydrated state. It has also been demonstrated that reduced hydration leads to denser and consequently more selective separation layers. Digesting the works highlighted in this virtual issue, there is a strong impression that the development of a highly effective polymeric membrane relies on the adequate characterization and understanding of the transport behaviors inside the polymer as well as the rational design, synthesis, and processing of the polymer. Clearly, polymer science is playing a pivotal role in the research of polymeric membranes. As a leading journal in polymer science, Macromolecules will continue to serve the membrane community by publishing in-depth studies in this fast-growing field. This article references 24 other publications.

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虚拟特刊：用于高级分离的聚合物膜

膜技术在应对与水、环境和能源相关的全球挑战方面发挥着越来越重要的作用。工业中使用的大多数膜是通过使用聚合物生产的。承载纳米级甚至埃级通路的膜，这些通路是固定的或动态的，基于尺寸区分、静电相互作用或吸附和扩散的机制来调节细颗粒、分子或离子的传输。聚合物科学的进步有助于开发更好的膜，反过来，对高效膜分离的不断增长的需求推动了基础聚合物科学的进一步发展。也就是说，聚合物科学对于开发高性能聚合物膜至关重要（图 1）。大分子长期以来，它一直是发表有关聚合物膜的高度相关和有影响力的研究的重要平台。这个虚拟问题重点介绍了最近发表在Macromolecules 上的一些膜研究，主要专注于用于超滤（UF）、气体分离和燃料电池分离器的膜。图 1. 聚合物科学最终决定了聚合物膜的性能。超滤由尺寸在~2-100 nm 范围内的膜实现。超滤膜通常通过相转化制备，并已广泛用于胶体、聚合物和蛋白质的纯化和浓缩。不幸的是，典型的超滤膜具有较宽的孔径分布，这是相转化过程所固有的，因此在渗透性和选择性之间发生了折衷。通过形成具有几乎单分散孔的膜，嵌段聚合物的自组装被认为是解决超滤膜这一瓶颈的一种方法。(1) 通过蚀刻掉垂直排列的嵌段聚合物中分散的圆柱相或通过对嵌段聚合物胶束溶液进行非溶剂诱导相分离来制备此类膜。或者，可以通过选择性溶胀策略在自组装嵌段聚合物中产生明确定义的孔隙，这可以在微波能量的帮助下在 30 秒内完成。 (2) 有趣的是，模拟显示两亲性 ABA 三嵌段共聚物的囊泡可以由半刚性 B 块的取向熵驱动自发穿孔，这可能会激发人们努力通过实验实现这种具有开/关可切换孔的穿孔 BCP 膜。 (3) 气体分离依赖于小得多的孔，低至埃级如果我们将聚合物中的自由体积视为动态孔隙。已经进行化学官能化以调节工程塑料的自由体积，从而调节工程塑料的气体分离性能，聚酰亚胺的氟化是一个代表性的例子。 (4) 工业上用于气体分离的膜通常由低自由体积的玻璃状聚合物制备，并且通常表现出中等选择性和低渗透性。为了解决这个问题，已经付出了大量努力来探索高渗透性聚合物，并且本征显微镜 (PIM) 聚合物被认为是这方面非常有前途的候选者。由于其刚性和扭曲的骨架结构，因此严重阻碍了节段迁移，PIM 具有非常高的自由体积，从而极大地促进了气体渗透性。PIM-1，研究最广泛的 PIM 聚合物，通常由两个四取代单体通过双亲核芳族取代逐步聚合合成。合成的 PIM-1 的微观结构及其气体分离性能对所应用的聚合条件敏感。结果表明，在 PIM-1 聚合产物中确定的环状或其他非线性拓扑结构有助于 PIM-1 表面积的增加，并且更高的支化和网络拓扑含量提高了 PIM-1 的气体对选择性。 5) PIMs 的气体传输行为可以通过 PIMs 的后功能化来调节。特别令人感兴趣的是将 PIM-1 中的腈基转化为羧酸，因为如此官能化的 PIM 显示出增强的 CO₂吸附和扩散选择性是因为 CO ₂和羧酸基团之间的强相互作用。最近报道了一种优化的酸水解策略来功能化 PIM-1，并且在 48 小时内获得了 >89% 的转化率，而以前的方法需要 360 小时。由此合成的 PIM-COOH 表现出更小的自由体积实体，并显示出对基于CO ₂的气体对的选择性显着提高。 (6) 在另一项工作中，PIM-1 中的腈基首先转化为四唑，随后与胺，从而将类似离子液体的侧基结合到 PIM-1 中。如此官能化的 PIM-1 对 O ₂ /N ₂和 CO ₂ /N ₂均表现出更高的气体选择性以降低纯气体渗透率为代价。这种侧基修饰策略有望为 PIM 带来多种官能团，因为四唑可用于组成不同离子液体的库。 (7) PIM 气体传输行为的研究通常在厚度为10 μm 的尺度。然而，由此获得的传输特性不能直接用于预测气体分离工业应用所需的亚微米级厚度的 PIM 薄膜的性能。现场使用的近期作品(8) 发现薄膜变薄促进了冻结自由体积的坍塌，并且在较薄的薄膜中观察到较低的气体吸收。重要的是，混合效应也发生在 PIM 薄膜中，因为它们在气体混合物中的膨胀程度比在纯气体中要小。这些观察结果有助于预测 PIM 薄膜的性能和设计用于实际应用的薄膜复合材料形式的 PIM 膜。阴离子交换膜 (AEM) 的研究如今火爆，主要是因为支持 AEM 的燃料电池有可能在碱性条件下使用廉价的电催化剂，从而克服主流质子交换膜的成本障碍。离子交换膜需要适当水合以最大化性能；因此，了解这些膜的水合行为和水流动性至关重要。(9,10) Dekel 等人。对不同类型 AEM 的水蒸气中吸水 (WU) 的平衡状态和动力学进行了系统研究，证明了平衡 WU 和 WU 动力学在评估 AEM 中的潜力。(11) Kumar 等。研究了水合对磺化聚苯乙烯基阳离子交换膜 (CEM) 传输性能的影响，发现水的体积分数支持了渗透选择性和离子电导率之间的内在权衡。 (12) 即水含量的增加将导致离子电导率增强，但由于电荷密度降低而导致渗透选择性降低。这也适用于 AEM。弗里曼等人。基于 Nernst-Planck 方程开发了一种新的简单方法来确定离子交换膜中的阳离子和阴离子扩散系数，并发现在考虑离子大小差异的贡献后，反离子扩散系数大于共离子扩散系数。 （13 ) 聚合物化学在开发稳健、高性能的 AEM 方面发挥着至关重要的作用。聚亚芳基醚 (PAE) 是 AEM 中使用最广泛的聚合物。不幸的是，PAE 中的芳醚键是弱点，在碱性条件下容易发生断链，导致 AEM 逐渐降解。已经开发了一些解决方案来增强 PAE 的碱性稳定性。裴等人。合成季铵化多嵌段聚（亚苯基- 弗里曼等人。基于 Nernst-Planck 方程开发了一种新的简单方法来确定离子交换膜中的阳离子和阴离子扩散系数，并发现在考虑离子大小差异的贡献后，反离子扩散系数大于共离子扩散系数。 （13 ) 聚合物化学在开发稳健、高性能的 AEM 方面发挥着至关重要的作用。聚亚芳基醚 (PAE) 是 AEM 中使用最广泛的聚合物。不幸的是，PAE 中的芳醚键是弱点，在碱性条件下容易发生断链，导致 AEM 逐渐降解。已经开发了一些解决方案来增强 PAE 的碱性稳定性。裴等人。合成季铵化多嵌段聚（亚苯基- 弗里曼等人。基于 Nernst-Planck 方程开发了一种新的简单方法来确定离子交换膜中的阳离子和阴离子扩散系数，并发现在考虑离子大小差异的贡献后，反离子扩散系数大于共离子扩散系数。 （13 ) 聚合物化学在开发稳健、高性能的 AEM 方面发挥着至关重要的作用。聚亚芳基醚 (PAE) 是 AEM 中使用最广泛的聚合物。不幸的是，PAE 中的芳醚键是弱点，在碱性条件下容易发生断链，导致 AEM 逐渐降解。已经开发了一些解决方案来增强 PAE 的碱性稳定性。裴等人。合成季铵化多嵌段聚（亚苯基- 基于 Nernst-Planck 方程开发了一种新的简单方法来确定离子交换膜中的阳离子和阴离子扩散系数，并发现在考虑离子大小差异的贡献后，反离子扩散系数大于共离子扩散系数。 （13 ) 聚合物化学在开发稳健、高性能的 AEM 方面发挥着至关重要的作用。聚亚芳基醚 (PAE) 是 AEM 中使用最广泛的聚合物。不幸的是，PAE 中的芳醚键是弱点，在碱性条件下容易发生断链，导致 AEM 逐渐降解。已经开发了一些解决方案来增强 PAE 的碱性稳定性。裴等人。合成季铵化多嵌段聚（亚苯基- 基于 Nernst-Planck 方程开发了一种新的简单方法来确定离子交换膜中的阳离子和阴离子扩散系数，并发现在考虑离子大小差异的贡献后，反离子扩散系数大于共离子扩散系数。 （13 ) 聚合物化学在开发稳健、高性能的 AEM 方面发挥着至关重要的作用。聚亚芳基醚 (PAE) 是 AEM 中使用最广泛的聚合物。不幸的是，PAE 中的芳醚键是弱点，在碱性条件下容易发生断链，导致 AEM 逐渐降解。已经开发了一些解决方案来增强 PAE 的碱性稳定性。裴等人。合成季铵化多嵌段聚（亚苯基- (13) 聚合物化学在开发稳健、高性能的 AEM 方面发挥着至关重要的作用。聚亚芳基醚 (PAE) 是 AEM 中使用最广泛的聚合物。不幸的是，PAE 中的芳醚键是弱点，在碱性条件下容易发生断链，导致 AEM 逐渐降解。已经开发了一些解决方案来增强 PAE 的碱性稳定性。裴等人。合成季铵化多嵌段聚（亚苯基- (13) 聚合物化学在开发稳健、高性能的 AEM 方面发挥着至关重要的作用。聚亚芳基醚 (PAE) 是 AEM 中使用最广泛的聚合物。不幸的是，PAE 中的芳醚键是弱点，在碱性条件下容易发生断链，导致 AEM 逐渐降解。已经开发了一些解决方案来增强 PAE 的碱性稳定性。裴等人。合成季铵化多嵌段聚（亚苯基- 已经开发了一些解决方案来增强 PAE 的碱性稳定性。裴等人。合成季铵化多嵌段聚（亚苯基- 已经开发了一些解决方案来增强 PAE 的碱性稳定性。裴等人。合成季铵化多嵌段聚（亚苯基-共亚芳基醚）来自“预胺化”单体和一系列氯封端的低聚（亚芳基醚）。（14）这些共聚物表现出增强的燃料电池性能和碱性耐久性，因为它们具有明确的微相分离亲水域和亲水域以及亲水域中亚苯基结构的存在。相比之下，He 等人。通过 Leuckart 反应将传统聚（芳醚酮）（PEAK）骨架中的吸电子 C= O 基团转化为给电子 C-NH ₂基团，从而减轻了 OH 的亲核攻击^-到高电子云密度环境中与醚连接的苯基-C。(15) 或者，已经成功探索了主链中不含醚键的聚合物，例如聚苯乙烯，以开发耐碱性的 AEM。(16,17 ) 使用 Ziegler-Natta 聚合，Hickner 等人。以 4-(4-甲基苯基)-1-丁烯和 11-溴-1-十一烯为单体合成溴烷基官能化聚(烯烃)。 (18) 合成的聚烯烃通过侧链反应转化为阴离子导电共聚物溴烷基与定制合成的含有季铵侧链部分的叔胺。由此制备的基于聚烯烃的 AEM 的每个侧链带有三个阳离子，并表现出良好的化学和尺寸稳定性以及相当高的氢氧化物电导率，这可能是由于三重阳离子结构中的相分离。还发现具有更长烷基侧链的 AEM 表现出更好的碱稳定性，因为侧链中亚烷基的空间和疏水作用减轻了氢氧根离子对铵基团的攻击。 (19) 重要的是，新方法，如基于 PIM 的聚合物 (20) 带相反电荷的嵌段共聚物镶嵌 (21) 和化学气相沉积方法 (22) 已在 AEM 和其他离子导电膜中发现了有趣的应用。由于用于脱盐和离子交换的膜中的孔或“自由体积”实体非常小，低至几埃，并且这些膜的聚合物结构是水合的，因此使用先进的表征技术来揭示这些结构和动力学非常重要膜。中子散射正在成为一种强大的工具就地探测这些膜的水和链动力学，因为它可以在几个纳米的长度尺度和纳秒到皮秒的时间尺度上解析含氢物质的运动。鞋底等。使用中子散射来研究聚酰胺脱盐膜和 AEM。 (23) 结果表明，在纳米长度尺度上，水通过脱盐膜的扩散速度与本体水相似，而水通过 AEM 扩散的速度比本体慢 2 倍水。在另一项工作中，中子反射计 (NR) 用于区分由聚电解质多层膜组成的膜的支撑层和分离层。 (24) NR 表明膜具有不对称结构，在干燥或水合状态下具有不同的底部和顶部多层. 还已经证明，减少水合作用导致更致密且因此更具选择性的分离层。消化这个虚拟问题中突出的工作，有一个强烈的印象，即高效聚合物膜的开发依赖于对聚合物内部传输行为的充分表征和理解，以及对聚合物膜的合理设计、合成和加工。聚合物。显然，聚合物科学在聚合物膜的研究中起着举足轻重的作用。作为高分子科学领域的领先期刊，一种强烈的印象是，高效聚合物膜的开发依赖于对聚合物内部传输行为的充分表征和理解，以及聚合物的合理设计、合成和加工。显然，聚合物科学在聚合物膜的研究中起着举足轻重的作用。作为高分子科学领域的领先期刊，有一种强烈的印象，高效聚合物膜的开发依赖于对聚合物内部传输行为的充分表征和理解，以及聚合物的合理设计、合成和加工。显然，聚合物科学在聚合物膜的研究中起着举足轻重的作用。作为高分子科学领域的领先期刊，Macromolecules将通过在这个快速发展的领域发表深入的研究，继续为膜界服务。本文引用了 24 篇其他出版物。