Microporous Polymer Membranes Research Articles

Carboxylated PIM‐1 incorporating sulfonated graphene oxide effectively improves the ion transport properties of the membrane

AbstractThis study explores the ion transport properties of self‐microporous polymers by introducing a novel combination of carboxylated PIM‐1 with sulfonated graphene oxide (SGO) to fabricate membranes. The resulting membranes exhibit enhanced structural stability, hydrophilicity, and ion exchange capacity (IEC) compared with the original carboxylated PIM‐1 (CPIM‐1), while preserving the subnanoporous structure. However, it was observed that excessive SGO loading leads to a detrimental “blocking effect” that compromises various membrane properties. Through electrically driven ion transport tests in a 0.01 M NaCl solution, it is demonstrated that a moderate amount of SGO effectively enhances membrane conductivity from 46.96 μS m−1 (for carboxylated PIM‐1 membranes without SGO) to 56.55 μS m−1. Additionally, the membranes exhibit selective sieving of cations and anions. The presence of small‐sized ion channels and the electrostatic repulsion generated by the abundant carboxyl and sulfonic acid groups significantly hinder Cl− transport. Consequently, the Na+/Cl− migration ratio (t+/t−) reaches 98 at a concentration ratio of 10:1 on both sides of the membrane, surpassing the value of 3.74 observed for the pure CPIM‐1 membrane. This investigation provides valuable insights for the practical application of easily prepared, processable, and cost‐effective hydrophilic self‐contained microporous polymer membranes in ion transport applications.

Efficient acid/alkali recovery enabled by only one negatively charged microporous polymer membrane

AbstractThe recovery of acid/base from industrial wastewater via membrane technology is energy efficient and has a low carbon footprint, while demanding high‐performing ion exchange membranes. It remains a grand challenge for ion exchange membranes, enabling concomitantly fast H+/OH− transport and high ion selectivity. This article presents a negatively charged microporous polymer membrane, a sulfonated polyxanthene membrane, which has intrinsic micropore channels for H+/OH− permeation, and we find that highly selective H+ and OH− transport can be acquired with the same membrane. This membrane exhibits a high H+/Fe2+ selectivity of 617 (6.3 times that of the commercial CIMS membrane) in acid recovery and a very high OH−/WO42− selectivity of 649 (the highest value ever reported) in alkali recovery while maintaining comparable H+/OH− transport rate. The results highlight the importance of hydrophilic micropores as ion‐selective channels in constructing high‐performing ion exchange membranes for resource recovery.

Penetrant-induced plasticization in microporous polymer membranes.

Penetrant-induced plasticization has prevented the industrial deployment of many polymers for membrane-based gas separations. With the advent of microporous polymers, new structural design features and unprecedented property sets are now accessible under controlled laboratory conditions, but property sets can often deteriorate due to plasticization. Therefore, a critical understanding of the origins of plasticization in microporous polymers and the development of strategies to mitigate this effect are needed to advance this area of research. Herein, an integrative discussion is provided on seminal plasticization theory and gas transport models, and these theories and models are compared to an exhaustive database of plasticization characteristics of microporous polymers. Correlations between specific polymer properties and plasticization behavior are presented, including analyses of plasticization pressures from pure-gas permeation tests and mixed-gas permeation tests for pure polymers and composite films. Finally, an evaluation of common and current state-of-the-art strategies to mitigate plasticization is provided along with suggestions for future directions of fundamental and applied research on the topic.

Projection Micro Stereolithography of Skin Layered Microporous Polymer Membranes As a Separator for Li-Ion Batteries

Commercial liquid electrolyte lithium-ion batteries (LIBs) traditionally incorporate microporous polymer membranes, or separators, to isolate electrode contact and enable ionic transport. Although they do not actively participate in cell chemistry, well-designed separators can influence cell performance and safety, and thus, are essential components of a cell. Current separator manufacturing processes include "wet" or "dry" mechanical stretching , phase inversion, and evaporation-induced phase separation. However, these processes have limited morphological control of geometry and layered structures. Additive manufacturing techniques have been used to rapidly prototype materials that have both high customizability and fine microstructural control. We investigate hexanediol diacrylate (HDDA) as a polymer system using projection micro-stereolithography, a layer-by-layer photopolymerization technique, to develop separators with controlled microstructures for Li-ion batteries. We successfully fabricate a unique polymer architecture of alternating nanometer thin skin layers and microporous network. The fine control of printing parameters (i.e., printing platform, skin layers, and exposure time), results in tunable porosity and morphology. This unique approach to manufacturing LIB separators provides hierarchical homogeneity for more uniform Li-ion transport. It’s potential as a LIB separator is explored by characterizing its electrolyte wettability, thermal tolerance, electrochemical properties, and mechanical stability. Finally, lithium symmetric cells using the HDDA microporous membranes are evaluated against Celgard 2325 separators. This work provides a promising new route for developing separators using additive manufacturing techniques to enable high performance Li-ion batteries.

Fine-tuning gas separation performance of intrinsic microporous polyimide by the regulation of atomic-level halogen substitution

The molecular structure of polymers is essential to membrane separation performance. However, the inherent relationship between molecular structure and gas separation performance of polymeric membranes is unclear. Herein, the halogen atoms (F, Cl, and Br) were introduced into intrinsic micropore polyimide (PIM-PI) to precisely regulate its interchain cavity and elucidate the structure-property relationship. Based on the pristine BAPI, the halogen atoms were precisely designed at the ortho position of the amino group and synthesized three PIM-PIs, namely BFPI, BCPI, and BBPI, respectively. Then, it can be found that the packing structure of polymer chains and fine-tuning of fractional free volume (FFV) exhibits a regular trend in halogen-containing PIM-PIs. In addition, as the van der Waals volume of halogen atoms increases, the gas permeability of halogen-containing PIM-PIs exhibits an increasing trend. Specifically, compared with BFPI, the CO2 permeability of BCPI and BBPI is increased by 48 % and 100 %, respectively. Our findings may elucidate the structure-property relationship of microporous polymer membranes and provide a brand-new insight to design an advanced high-performance membrane.

Experimental Investigation of a Plate-Frame Water Gap Membrane Distillation System for Seawater Desalination.

This study presented a detailed investigation into the performance of a plate-frame water gap membrane distillation (WGMD) system for the desalination of untreated real seawater. One approach to improving the performance of WGMD is through the proper selection of cooling plate material, which plays a vital role in enhancing the gap vapor condensation process. Hence, the influence of different cooling plate materials was examined and discussed. Furthermore, two different hydrophobic micro-porous polymeric membranes of similar mean pore sizes were utilized in the study. The influence of key operating parameters, including the feed water temperature and flow rate, was examined against the system vapor flux and gained output ratio (GOR). In addition, the used membranes were characterized by means of different techniques in terms of surface morphology, liquid entry pressure, water contact angle, pore size distribution, and porosity. Findings revealed that, at all conditions, the PTFE membrane exhibits superior vapor flux and energy efficiency (GOR), with 9.36% to 14.36% higher flux at a 0.6 to 1.2 L/min feed flow rate when compared to the PVDF membrane. The copper plate, which has the highest thermal conductivity, attained the highest vapor flux, while the acrylic plate, which has an extra-low thermal conductivity, recorded the lowest vapor flux. The increasing order of GOR values for different cooling plates is acrylic < HDPE < copper < aluminum < brass < stainless steel. Results also indicated that increasing the feed temperature increases the vapor flux almost exponentially to a maximum flux value of 30.36 kg/m2hr. The system GOR also improves in a decreasing pattern to a maximum value of 0.4049. Moreover, a long-term test showed that the PTFE membrane, which exhibits superior hydrophobicity, registered better salt rejection stability. The use of copper as a cooling plate material for better system performance is recommended, while cooling plate materials with very low thermal conductivities, such as a low thermally conducting polymer, are discouraged.

Textile dye removal in wastewater by peroxymonosulfate (PMS) activation on a zero-valent iron nanoparticle-modified ultrafiltration catalytic membrane (nZVI@PES).

The use of the nano zero-valent iron (nZVI) nanoparticle-based advanced oxidation systems in conjunction with an activator such as peroxymonosulfate (PMS) to generate hydroxyl and sulfate radicals for the degradation of organic pollutants has been extensively used in recent studies. In this study, a nZVI-modified polyethersulfone (PES) membrane (nZVI@PES) was produced successfully by attaching the nZVI catalytic nanoparticles on the surface of a commercial microporous polymeric membrane material using a simple and easy filter press coating method. The presence of nZVI nanoparticles on the nZVI@PES membrane was confirmed by XRD, SEM, and EDS analyses. The nZVI@PES membrane was applied in the dead-end filtration system in the presence of the PMS activator to treat the reactive black 5 (RB5) dye solution. The effect of catalyst loading, RB5 dye concentration, PMS dosage, and pH level on the nZVI@PES membrane/PMS system was investigated. Quenching experiments were carried out to identify the reactive species responsible, and reusability tests were conducted on the membrane. The highest decolorization efficiency (96.8%) was obtained at 20 mg/L RB5 dye solution, initial pH of 3, the nZVI loading of 5 mg/cm2, and the PMS dosage of 300 mg/L at the end of a reaction time of 30 min. The formation of HO•, [Formula: see text], [Formula: see text] and, 1O2 was confirmed by quenching experiments. The results indicate that the nZVI@PES membrane/PMS system could successfully treat wastewater contaminated with an organic dye.

Ultra-stable fully-aromatic microporous polyamide membrane for molecular sieving of nitrogen over volatile organic compound

Microporous polymer membranes are promising candidates for industrial membrane-based gas separation because of their high separation performance. However, their relatively low stability due to the local rearrangement of polymer chains during usage remains a problem. Hence, we propose the construction of a fully aromatic polymer structure in a microporous polymer membrane to enhance membrane stability. Four triptycene-based microporous polyamides were synthesized via the polymerization of 2,6,14-triaminotriptycene with aromatic acyl chloride and/or aliphatic acyl chlorides. Their properties were characterized and compared by using nuclear magnetic resonance (NMR) and Brunauer–Emmett–Teller analyses. The synthesized polyamides were fabricated into composite membranes by employing a solution process; their stability was evaluated for the molecular sieving of nitrogen over volatile organic compounds such as cyclohexane. Low-field NMR and X-ray photoelectron spectroscopy were used to investigate the differences in the properties of membranes with different structures at different times. The results showed that the fully aromatic polyamide membrane made from 2,6,14-triaminotriptycene and aromatic acyl chloride displayed constant rejection (99 %) and nitrogen permeability (approximately 50 Barrer) for the molecular sieving of nitrogen over cyclohexane during 100-d experiments, indicating good stability. This approach paves the way for the industrialization of microporous polymer membranes from a theoretical perspective.

Ion-Selective Microporous Polymer Membranes with Hydrogen-Bond and Salt-Bridge Networks for Aqueous Organic Redox Flow Batteries.

Redox flow batteries (RFBs) have great potential for long-duration grid-scale energy storage. Ion-conducting membranes are a crucial component in RFBs, allowing charge-carrying ions to transport while preventing the cross-mixing of redox couples. Commercial Nafion membranes are widely used in RFBs, but their unsatisfactory ionic and molecular selectivity, as well as high costs, limit the performance and the widespread deployment of this technology. To extend the longevity and reduce the cost of RFB systems, inexpensive ion-selective membranes that concurrently deliver low ionic resistance and high selectivity toward redox-active species are highly desired. Here, high-performance RFB membranes are fabricated from blends of carboxylate- and amidoxime-functionalized polymers of intrinsic microporosity, which exploit the beneficial properties of both polymers. The enthalpy-driven formation of cohesive interchain interactions, including hydrogen bonds and salt bridges, facilitates the microscopic miscibility of the blends, while ionizable functional groups within the sub-nanometer pores allow optimization of membrane ion-transport functions. The resulting microporous membranes demonstrate fast cation conduction with low crossover of redox-active molecular species, enabling improved power ratings and reduced capacity fade in aqueous RFBs using anthraquinone and ferrocyanide as redox couples.

Mechanically robust conjugated microporous polymer membranes prepared using polyvinylpyrrolidone (PVP) electrospun nanofibers as a template for efficient PM capture

Air pollution has become a challenging environmental problem worldwide due to rapid industrial development and excessive emissions of vehicle exhaust. Herein, we report a preparation of conjugated microporous polymer membranes (CMPM) with a hierarchical porous structure by electrospun polyvinylpyrrolidone (PVP) nanofibers as a template for effective removal of PM from airborne and vehicle exhaust. CMP membranes have hierarchical holes, where the macropores are from electrospun nanofiber membranes and the mesopores are from polymer synthesis. Taking advantage of its inherent physicochemical and thermal stability and hierarchical hole characteristics, the CMPM-based filter can work continuously for up to 36 h and still maintains a high removal efficiency (>99.56%), and also has a high filtration efficiency in the treatment of vehicle exhausts, with 95.18% for PM0.3, 98% for PM0.5 and >99% for PM2.5-10.0. The superior mechanical properties of CMPM allow the filter to be cleaned and reused. After three cycles, the filtration effectiveness of CMPM is still 94.83% for respirable particulate matter. Under high humidity (RH ≥ 95%) conditions, the CMPM-based filter showed higher than 95.37% filtration of PM0.3-10, and the oil adsorption rate could be maintained at 284% at high speed, proving the great potential of CMPM to clean air in complex situations.

Ultrafast organic solvent nanofiltration in C[sbnd]C bonded conjugated microporous membrane-based nanochannels by space-confined polymerization

Organic solvent nanofiltration is a new membrane technology for separation of organic solvents with low energy consumption and environmental compatibility. However, commercial ultrafiltration membranes are unsuitable for organic solvents separation owing to their low retention and poor solvent resistance. CC bonded conjugated microporous polymer (CCMP) membranes consisting of all-rigid conjugated systems feature well-developed microporosity and excellent organic resistance. Herein, we report a strategy to prepare solvent-resistant CCMP@PTFE composite membrane by portioning channels in polytetrafluoroethylene (PTFE) ultrafiltration membrane by space-confined polymerization. Owing to intrinsic micropores from crosslinking of multifunctional rigid building units and the robustness of PTFE substrate, three CCMP@PTFE membranes exhibit outstanding separation performance, solvent resistance and high flexibility, which rival against the best of those reported membranes. This strategy inspires the design of OSN membranes with high performance.

Ultra-microporous anion conductive membranes for crossover-free pH-neutral aqueous organic flow batteries

Aqueous organic redox flow battery (AORFB) operating in pH-neutral solutions is an attractive electrochemical energy storage device because of its decoupled energy and power, high safety, low corrosivity, and potentially low cost. However, it remains largely unclear how to develop high-performing AORFB membranes that enable fast ion transport while impeding electrolyte cross-contamination. Here we report a series of anion conductive membranes derived from Tröger's Base polymers with intrinsic microporosity as promising contender membranes for pH-neutral AORFBs. The quantity of sub-nanometer-sized pores in these membranes is regulated by tuning chain rigidity, while a similar pore size distribution is retained. We discover that membrane conductivity is determined by the quantity of pores, whereas the energy barrier for ion transport is related to the pore size distribution. Contrasting conventional knowledge, crossover-free operation of pH-neutral AORFB can be accomplished by implementing microporous polymer membranes that enable fast ion conduction but are moderately selective to redox-active electrolytes. The pH-neutral AORFB exhibits a low capacity fade rate of 0.002% per cycle (0.02% per hour) during 500 consecutive charge/discharge cycles at 40 mA cm−2 while maintaining an energy efficiency of up to 80% and a coulombic efficiency of ∼100%. These results demonstrate that ultra-microporous membranes possess great potential for AORFB applications.

2D conjugated microporous polymer membranes for organic solvent nanofiltration

Two-dimensional conjugated microporous polymer (CMP) membranes have gained increasing attention in the nanofiltration field owing to their tunable pore structure and solvent stability. Nevertheless, fabricating freestanding thin films with accessible porosity is challenging. In this study, we fabricated four freestanding CMP membranes based on the benzobisoxazole linkage and investigated their performance for organic solvent nanofiltration (OSN). The effect of pore size on the OSN performance was studied by investigating three CMP structures (B-BOP, TPPy-BOP, and TPB-BOP)-based membranes with various theoretical pore sizes (1.4, 1.6, and 2.8 nm, respectively). The effect of pore functionalization on the molecular sieving properties was evaluated by comparing two otherwise equivalent CMP structures with and without fluorine functionalization (TPB-BOP and TPB-F-BOP, respectively). The inherent control on the membranes’ molecular sieving properties is evidenced by the proportionality between the pore sizes of the CMP structures and their filtration performances. The molecular weight cutoff (MWCO) values for B-BOP, TPPy-BOP, TPB-BOP, and TPB-F-BOP were 676, 760, 1759, and 1402 g mol−1, respectively, which were consistent with the expected trend based on theoretical pore sizes. The experimental results are consistent with pore flow model predictions, indicating that the pore structure remains intact despite the materials lack long-range order. All the CMP membranes demonstrated long-term stability, exhibiting a constant acetone flux (212 L m−2 h−1 for B-BOP and 879 L m−2 h−1 for TPB-BOP) and Rose Bengal rejection (100 % for B-BOP and 67 % for TPB-BOP) over 120 h of continuous operation at 30 bar.

Effects of Co-diluent on the pore structure, patterning fidelity, and properties of membranes fabricated by lithographically templated thermally induced phase separation

Surface patterning as a physical surface modification method is promising for improving performance of liquid-based membrane processes. Lithographically templated, thermally induced phase separation (lt-TIPS) process is recently demonstrated for fabricating surface patterned microporous polymer membranes. However, the mechanical properties of the poly(vinylidene fluoride) (PVDF) membranes casted from tributyl O-acetylcitrate (ATBC) were poor, due to the predominant spherulitic morphology. In this study, we investigate the influences of diphenyl carbonate (DPC) as co-diluent to ATBC on the pore structure and patterning fidelity of lt-TIPS fabricated PVDF membranes. With the increase of DPC content in the mixed diluents, the phase separation path of the PVDF solution transitions from solid-liquid (S-L) to liquid-liquid (L-L), and correspondingly, the membrane pore structures evolve from dispersed spherulitic to bi-continuous cellular. The interconnected cellular morphology resulted from ATBC/DPC ratio of 20/80 shows superior mechanical properties of membranes compared with all other membranes, improving the toughness by nine times compared with pure ATBC system. The patterning fidelity improves from 59% to 79% as DPC content increases. The presence of the surface patterns on the PVDF membranes improves both the water contact angle WCA (by 12–18%) and the liquid entry pressure LEP (by 5–20%) which are desirable for membrane distillation (MD) applications. Concomitantly, patterned membranes also showed improved pure water permeance and water vapor flux in MD measurement.

Conjugated microporous polymer membranes for light-gated ion transport.

Inspired by the light-gated ion channels in cell membranes that play important roles in many biological activities, herein, we developed an artificial light-gated ion channel membrane out of conjugated microporous polymers. Through bottom-up design of the monomer molecular structure and by the electropolymerization method, the membrane pore size and thickness were precisely controlled on the molecular level. The obtained membrane exhibited uniform pore size and highly sensitive light-switchable response. The photoisomerization of the polymer chain resulted in a reversible “on and off” light control over the pore size and subsequently led to light-gated ion transport across the membrane for a series of ions including hydrogen, potassium, sodium, lithium, calcium, magnesium, and aluminum ions.

High-performance gas separation membranes derived from thermal-oxidative block poly(benzoxazole-co-imide)

Thermally rearranged(TR) polymers are usually prepared at an insert atmosphere and display high gas permeability, but their low gas selectivity results in high energy consumption. Thermal oxidation is an effective method to enhance the gas selectivity of microporous polymers. In this work, the effect of oxygen in the purge environment on imide-to-benzoxazole conversion and membrane structures was investigated. 1% oxygen content of the purge environment could restrain imide-to-benzoxazole conversion of 6FDA-DETDA/bisAPAF hydroxyl block polyimide(bHPI) membranes effectively, and oxygen induced the oxygen bridge formation, improving the gas selectivities of the resulting thermal-oxidative membranes with the increased oxygen content. Thermal-oxidative PBOCI membranes (TO-PBOCI) based on block poly(benzoxazole-co-imide)(PBOCI) precursor membranes were prepared to enhance gas separation performance of microporous polymers. The stable polybenzoxazole structures and microphase separation of polybenzoxazole blocks maintained membrane structures and the gas transport channels in the thermal-oxidative process at high temperature. Significantly, the remaining hydroxyl groups in PBOCI precursor membranes were beneficial for oxygen bridge formations of TO-PBOCI membranes, enhancing H2/CH4 selectivity. As a result, the TO-PBOCI membranes treated at a high temperature kept high H2 permeability and H2/CH4 selectivity, surpassing the 2008 Robeson upper bound. The study about TO-PBOCI membranes promotes a better understanding of the polymeric materials and process development of microporous polymer membranes for hydrogen separation applications.

Conjugated microporous polymer membranes for chemical separations

• Conjugated microporous polymer (CMP) membranes for chemical separations are reviewed. • The historical development of CMPs is discussed. • Fabrication routes of CMP membranes are critically compared and reviewed. • Applications of CMP membranes are comprehensively summarized. • The inference on the future prospects of CMPs as membranes is provided. Conjugated microporous polymers (CMPs) are a unique class of porous organic materials, which are constructed with π-conjugation structures leading to intrinsic micropores. The CMPs properties such as high surface area, intrinsic and rich micropores, interlocking and rigid structure, extensive π-conjugation and tunable band-gap, chemical and thermal stability, together with tailored functionalities, contribute to its abundant potential for application in fields such as photocatalysis, optoelectronics, energy storage, and chemical sensors. Recently, CMPs have gained importance in the field of membranes for chemical separation. In this review, we briefly discuss the historical development of CMPs, followed by a detailed description of the progress in state-of-the-art design, preparation, and application of CMPs in membranes. Additionally, we provide inference on the future prospects of CMPs as membranes.

Two-stage polymerization towards C–C bonded Conjugated microporous polymer membranes with excellent nanofiltration performance

C–C bonded conjugated microporous polymer (CMP) membranes are promising candidates for organic solvent nanofiltration (OSN), in which molecular-sieving selectivity, high permeability, and chemical/structural stability can be integrated. However, it is a big challenge to fabricate CMP membranes by simple coating due to the insolubility and poor processability of CMPs. In this work, three C–C bonded CMP membranes were fabricated by a two-stage polymerization strategy via cyclotrimerization of acetyls. Firstly, the viscous prepolymer solution was coated on a glass plate. After thermal curing based on cyclotrimerization, three C–C bonded CMP membranes were synthesized. Thanks to the aromatic nature of the backbone, high surface area, and narrowly distributed pore sizes, these membranes exhibited high permeability and sharp selectivity for OSN. Especially, even though they have a thickness up to μm-scale, their permeability is still fast, which breaks the stereotype that the ultra-thin property was necessary for fast permeance. The extraordinary stability integrated with excellent permeability and selectivity renders these C–C bonded CMP membranes promising candidates for real-world OSN applications. Besides, this work may be extended to prepare CMP membranes for other applications. • A simple two-stage polymerization[1] was developed to create CCMP membranes with prepolymer as the coating material. • The membrane was fabricated by cyclotrimerization of acetyl groups under mild and atmosphere conditions. • The CCMP membrane exhibited fast solvent permeance, high Young's modulus and large solute retention. • This work solved the paradox between the membrane thickness and the membrane flux.

Electropolymerized thin films with a microporous architecture enabling molecular sieving in harsh organic solvents under high temperature

Electropolymerized conjugated microporous polymer (CMP) membranes enable molecular sieving in organic solvents under extreme conditions.

Pattern Recognition and Modeling of Asymmetric Microporous Polymer Membrane Structure for the Bio-Separation of Viruses from Blood Cells

Pattern Recognition and Modeling of Asymmetric Microporous Polymer Membrane Structure for the Bio-Separation of Viruses from Blood Cells