Polymeric membranes: chemistry, physics, and applications
Haiqing Lin, Yifu Ding
Abstract
Membranes have emerged as a critical component in solving vital energy and environmental problems and are intensively explored for gas separations, water purifications, and fuel cell and battery applications. These applications demand the membranes with the capability of controlling the transport of small molecules (such as gas and water) or ions. Polymeric materials play a leading role in membrane development because of their excellent processability, low cost, and abundance, and they will remain at the core of membrane technology, as manifested through the chemistry-processing-structure-performance paradigm. New polymer chemistry provides opportunities to tune polymer-penetrant interactions to improve the separation of molecules and ions. A better understanding of control over the structure formation during self-assembly and phase separation can lead to membranes with desired free volumes or pores. Innovation in polymer processing is critical for realizing the potentials of promising polymer chemistry and structures. This special issue highlights the role of polymer science in membrane technology applications. Polymeric membranes are attractive for industrial gas separations due to their inherently high energy-efficiency. However, there exists a trade-off, i.e., polymers with high gas permeability often exhibit low gas selectivity.1, 2 A variety of strategies have been developed to molecularly engineer polymers to enhance gas separation properties to cross the upper bound. Young Moo Lee and colleagues review two approaches to effectively manipulate polymer microporosity (resulting in polymers re-defining the permeability/selectivity trade-off), i.e., incorporating intrinsically microporous units to form polymers with intrinsic microporosity (PIMs) and increasing chain rigidity by thermal rearrangement (TR) to enhance microporosity. Jianyong Jin and colleagues demonstrate that PIMs can be post-modified with metalation (such as Na+) to improve CO2/CH4 and CO2/N2 selectivity due to the pore blocking by Na+ ions. Jason Bara and colleagues introduce cations in the backbones of conventional 6FDA-based polyimides and show that the doping with ionic liquids (ILs) improves CO2/CH4 and CO2/N2 separation properties. Liyuang Deng and colleagues synthesize cross-linked poly(ethylene glycol) (PEG) via thiol-ene/epoxy reaction and show that the doping with ILs can improve CO2 solubility and diffusivity and CO2/N2 separation properties. Chulsung Bae and colleagues graft a block copolymer of polystyrene-b-polybutadiene-b-polystyrene (SBS) using CO2-philic triethylene oxide (TEO) and demonstrate that increasing the TEO content improves the CO2/gas separation properties while retaining excellent mechanical properties from the SBS. Polymer-based mixed matrix materials (MMMs) have been widely studied for gas separations, as they combine the unique molecular sieving ability of the fillers and excellent processability of the polymers. Bin Mu and colleagues provide an exhaustive overview of the MMMs based on metal-organic frameworks (MOFs) for gas separations, including modeling, challenges (such as interfacial incompatibility), strategies to address them, and future outlook. Ruilan Guo and colleagues report surface modification of ZIF-90 nanoparticles with triptycene to effectively improve their interfacial compatibility with a triptycene-based polyimide and thus gas separation properties. The evolvement of the polymeric gas separation membranes benefits tremendously from the urgent need of CO2 capture, utilization, and sequestration (CCUS) from fossil fuel-derived power plants to mitigate the CO2 emissions to the atmosphere. With the inherently high energy-efficiency, membrane technology is essential for CO2/N2 and CO2/H2 separations to enable economically viable CCUS.3 Yang Han and Winston Ho comprehensively review facilitated transport membranes based on amine-containing polymers, which exhibit extremely high CO2/N2 and CO2/H2 selectivity in the presence of water vapor. Brian Long and colleagues conduct systematic studies of the effect of CO2-philic functional groups (such as amidoxime and ethereal side chains) on polynorbornene backbones on CO2/N2 separation properties. The aforementioned cross-linked PEG doped with the ILs by Deng's group and SEBS grafted with TEO by Bae's group also shed some light on designing CO2-philic polymers for CO2/N2 separation. Haiqing Lin and colleagues examine the state-of-the-art of molecularly engineered polymers for high-temperature H2/CO2 separation, including chemical functionalization, cross-linking, polymer blending, thermal treatment, and mixing with porous fillers and H2-sorptive nanoparticles. Polymeric membranes have been widely used for water purification (such as desalination and wastewater treatment), but the trade-off issue between water permeance and selectivity still remains. Janina Gaalken and Mathias Ulbricht synthesize amphiphilic poly(ethylene oxide)-b-poly(isopropyl methacrylate) diblock copolymers (BCPs) to produce isoporous ultrafiltration (UF) membranes using a self-assembling nonsolvent induced phase separation (SNIPS) process. The obtained UF membranes display promising performance in overcoming permeance/selectivity trade-off. Putting positive charges onto UF membranes can improve their performance in treating wastewaters containing cationic dyes and heavy metal ions from the textile and printing industries. However, a direct coating of polyelectrolyte on the membrane surface can form a dense layer that reduces surface porosity. Jianxin Li and colleagues report a novel way of incorporating positively charged polyelectrolyte into UF membranes through blending charge-functionalized polysulfone with polyethersulfone that is used for the phase inversion process. The membranes display superior separation performance in terms of rejecting cationic dyes and robustness against fouling. Polymeric membranes have also been explored for hydrocarbon liquid separations, which are currently performed using energy-intensive distillation processes. Chen Zhang and colleagues provide a comprehensive review of hydrocarbon separations using glassy polymers. The review compares different separation processes (including vapor separation, pervaporation, and the emerging organic solvent reverse osmosis process), surveys performances of different glassy polymers for several hydrocarbon mixtures, and highlights the significant plasticization challenges and aging for the membranes as well as the fabrication of defect-free membranes. Finally, the review provides an outlook into the future commercialization of the membrane technology for hydrocarbon separations and formulates several specific areas of research needs. MMMs are also evaluated for hydrocarbon liquid separations. Gongping Liu and colleagues incorporate polyhedral oligomeric silsesquioxanes (POSS) particles into polydimethylsiloxane (PDMS) membranes via cross-linking between POSS-containing monomer and PDMS precursors. At low loadings, the POSS can be molecularly dispersed in the PDMS. The incorporation of POSS shifts the free volume distribution of the MMMs and leads to superior pervaporation performance surpassing the upper bound for butanol/water separation. The incorporation of molecular fillers into MMMs often leads to complex adsorption behaviors that need to be understood. Kazukiyo Nagai and colleagues develop POSS-containing methacrylate polymers with varying POSS substituents and spacer lengths. The sorption of methanol and ethanol is dictated by the POSS moieties with a solid adsorption mechanism, unlike the conventional dissolution diffusion mechanism. As a result, the chemical nature of the substituents on the POSS has a profound impact on the sorption. For example, the polymer containing isobutyl-substituted POSS displays endothermic mixing, which is unusual for glassy polymers. Plasticization of polymers by penetrants is a significant challenge for organic solvent nanofiltration (OSN). Michele Galizia and colleagues report the use of in situ FTIR measurements to monitor the sorption of methanol in polybenzimidazole (PBI). Methanol forms hydrogen bonding with PBI, disrupting the hydrogen bonding network of PBI and enhancing its chain mobility. Both findings and new methodology can be valuable for the design of solvent-resistant OSN membranes. Last but not least, polymer-based proton exchange membranes (PEMs) with a combination of high proton conductivity and excellent stability are critical to fuel cell applications. Jun Lin and colleagues design novel MMMs using amino-modified halloysite nanotubes to stabilize phosphotungstic acid within the sulfonated poly(ether ether ketone). The functionalized halloysite nanotubes form strong acid-based pairs with both the acid and the polymer matrix and improve both proton conductivity and long-term stability. We would like to conclude by thanking all the authors, reviewers, and editorial staff at the Journal of Polymer Science for contributing to this special issue. We hope this issue can further stimulate the interests and efforts of polymer scientists to address significant challenges in the field of membrane technology.