Microporous Polymer Membranes Research Articles

Membrane technology in air pollution control: prospect and challenge

Rapid growth in environmental awareness raised the strict regulations on air pollution control and led to great developments in air filtration or cleaning technologies. The membrane is one of the promising technologies for air filtration due to its high efficiency, low cost, and easy to scale-up. Two types of air filtration membranes have been used to provide high efficiencies in contaminant removal, namely electro-spun fibrous and microporous polymeric membranes. The fibrous membranes composed of randomly oriented fibers, either micro-or nano-fiber, and have been widely used to remove fine particles, such as aerosols. Meanwhile, the microporous membranes have a pore size that can remove both particulate and molecular, such as toxic gases. This paper reviews the role of membrane technologies in air pollution control. The performances of the membranes are presented comprehensively. Besides, the developments of the membranes to improve the filtration efficiency and reduce air-flow resistance are also discussed.

FINE PARTICLE REMOVAL USING HYDROPHOBIC MICROPOROUS POLYMERIC MEMBRANES

The air quality in the world has been worsening in the last decades due to industrial, vehicle, cigarettes smoke, forest fire, and fuel usage. In this case, fine particles are the world’s greatest concern due to its aerodynamic properties which enable it to travel throughout the world. The current conventional technologies seem to have lost their reliability due to complexity, low removal efficiency, and high equipment cost. Membrane air filter brings new hope to answer this challenge. It gives high removal efficiency with an acceptable pressure drop to fulfill the need for clean air at a lower price. Recently, the introduction of nanofibre membrane as a low-cost membrane may broaden membrane application in air filtration. Compared to conventional membrane, nanofibre membrane offers some interesting features such as higher porosity, interconnected pore structure, and narrow pore size distribution that provide remarkable permeability. In this paper, the microporous polymeric membrane for air filtration especially for fine particles removal is reviewed including mechanism of fine particle removal, membrane preparation, and factor affecting filtration performance.

Dissolved Methane Harvesting Using Omniphobic Membranes for Anaerobically Treated Wastewaters

Dissolved biomethane in anaerobic effluents has long been a hurdle for energy harvesting through anaerobic wastewater treatment processes. Here, we present a novel membrane process for dissolved methane recovery through the normal range of domestic wastewater temperatures by utilizing an omniphobic (nonwetting) microporous membrane. In a process driven by a solubility gradient, dissolved methane is extracted from a methane-rich aqueous solution (feed), transported across the omniphobic membrane, and absorbed into a nonpolar organic solvent (draw) that has a high solubility for methane. We fabricated the omniphobic membrane by coating a microporous polymeric membrane with silica nanoparticles, followed by surface fluorination. Using the omniphobic membrane, we demonstrate that nearly ≥90% of dissolved methane is recovered from methane-saturated feedwater at 15, 25, and 35 °C, simulating anaerobic effluents produced in the psychrophilic to mesophilic temperature range, while negligible transport of water is...

Molecular Design of Microporous Polymer Membranes for the Upgrading of Natural Gas

Microporous polymer membranes (MPMs) are attractive for gas separation due to their high porosity and tuneable functionalization. In this study, two new MPMs (namely PILP-1 and PILP-3) are designed...

Membranes with Intrinsic Micro-Porosity: Structure, Solubility, and Applications.

Microporous polymer membranes have been widely studied because of their excellent separation performance. Among them, polymers of intrinsic micro-porosity (PIMs) have been regarded as a potential next-generation membrane material for their ultra-permeable characteristics and their solution-processing ability. Therefore, many reviews have been reported on gas separation and monomers for the preparation of PIMs. This review aims to provide an overview of the structure-solubility property. Different structures such as non-network and network macromolecular structure made of different monomers have been reviewed. Then their solubility with different structures and different separation applications such as nanofiltration, pervaporation, and gas/vapor separation are summarized. Lastly, we also provide our perspectives on the challenges and future directions of the microporous polymer membrane for the structure-property relationship, anti-physical aging, and more.

High-performance microporous polymer membranes prepared by interfacial polymerization for gas separation

Microporous polymers possessing good processability and low cost show promising applications for large-scale industrial membrane separation. The relatively low gas selectivity and physical aging of microporous polymer membranes are current challenges which become more serious for ultrathin membranes. Crosslinking is an effective way to improve gas selectivity and physical aging resistance of glassy polymers, and the cross-linked ultrathin membranes can be directly produced by interfacial polymerization (IP). However, there are only a few reports of IP formed microporous polymer membranes, and the reported gas separation performance is not satisfying. In this work, ultrathin microporous polyarylate (PAR) membranes were prepared by IP of 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethylspirobisindane (TTSBI) and trimesoyl chloride (TMC). The crosslinking structure, molar ratio of TTSBI to TMC (n(TTSBI)/n(TMC)) in PAR, and membrane thickness characterized by zeta potential measurements, X-ray photoelectron spectroscopy and scanning electronic microscope were optimized by adjusting IP parameters such as pH value of aqueous phase that decides the hydrolysis of both TTSBI and TMC in IP, and the concentrations of TTSBI and TMC. The microporous structures of PAR characterized by CO2 adsorption are highly relevant to the n(TTSBI)/n(TMC) in PAR. The resulted membranes exhibit CO2 permeance in a wide range of 100–2115 GPU with CO2/N2 selectivity of 45–21 that is a remarkable improvement in gas separation performance of IP formed microporous polymer membranes and also shows significant advantages over other reported ultrathin microporous polymer membranes. In addition, the membranes also exhibit excellent stability, and only 29% decrease in CO2 permeance was observed after 180 h aging. This work provides an effective strategy to fabricate high-performance and stable microporous polymer membranes by IP, which is expected to push the particle applications of microporous polymer membranes in gas separation.

The evolution of free volume and gas transport properties for the thermal rearrangement of poly(hydroxyamide-co-amide)s membranes

Within a microporous polymer membrane, its high gas separation performance is much dependent on the free volume element architecture. In this study, thermally rearranged poly(benzoxazole-co-amide) (TR-PBOA) copolymer membranes were prepared by in-situ thermal treating poly(o-hydroxyamide-co-amide) (PHAA) precursors, basing on commercially available TR-able and non TR-able diamines with different molar ratio. Free-volume topologies were tailored by controlling the degree of thermal rearrangement and the flexibility of the original chains. Upon thermal conversion, small cavities coalesced into bigger ones, representing hourglass-shaped cavities with larger cavities and small bottlenecks, resulting in the significant increase in permeability. It was found that thermal rearrangement mainly occurred near or above glass transition temperature (Tg) where chain segments obtained enough motion ability, and TR-PBOA membrane prepared at this temperature possessed the maximal selectivity due to effective packing of rigid chains. When thermally treated at temperature much higher than Tg, there was a compromise between thermal conversion and chain annealing. Compared to thermal treatment temperature, the effect of dwelling time on thermal conversion ratio was minor, as the formed rigid structure limited chain motion until enough energy was received at higher temperature. Furthermore, TR-PBOA membranes with appropriate ratio of PBO and PA contents displayed superior mechanical properties and gas transport performance, especially for CO2/CH4 separation (CO2 permeability was about 237 Barrer, CO2/CH4 ideal selectivity was 36.6, plasticization pressure of CO2 was 2.9 MPa) (1 Barrer=10−10 cm3 (STP) cm cm−2 s−1 cmHg−1)

Scanning Probe Microscopy Used to Estimate the Structural Asymmetry of Filters with Micro- and Nanopores

The features of the surface structure of nanoporous and microporous polymer-membrane films promising for use in cleanrooms during the manufacturing of microelectronics products are examined. The degrees of asymmetry of selective and supporting surfaces are determined via scanning probe microscopy. The hydrodynamic characteristics of the membranes under study are analyzed.

Conjugated Microporous Polymers with Rigid Backbones for Organic Solvent Nanofiltration

It is very challenging to develop highly permeable and selective polymer membranes for organic solvent nanofiltration. Recently in Nature Chemistry, Liang et al. reported conjugated microporous polymer membranes with all-rigid conjugated systems through a surface-initiated polymerization strategy; the membranes exhibited high resistance to organic solvents and high permeance for hexane and methanol.

Microporous membranes comprising conjugated polymers with rigid backbones enable ultrafast organic-solvent nanofiltration

Conventional technology for the purification of organic solvents requires massive energy consumption, and to reduce such expending calls for efficient filtration membranes capable of high retention of large molecular solutes and high permeance for solvents. Herein, we report a surface-initiated polymerization strategy through C-C coupling reactions for preparing conjugated microporous polymer (CMP) membranes. The backbone of the membranes consists of all-rigid conjugated systems and shows high resistance to organic solvents. We show that 42-nm-thick CMP membranes supported on polyacrylonitrile substrates provide excellent retention of solutes and broad-spectrum nanofiltration in both non-polar hexane and polar methanol, the permeance for which reaches 32 and 22 l m-2 h-1 bar-1, respectively. Both experiments and simulations suggest that the performance of CMP membranes originates from substantially open and interconnected voids formed in the highly rigid networks.

Highly Selective and Permeable Microporous Polymer Membranes for Hydrogen Purification and CO2 Removal from Natural Gas

This paper reports a new macromolecular design that incorporates hierarchical triptycene unit into thermally rearranged polybenzoxazole (TR-PBO) structures for highly selective and permeable gas se...

Additive-manufactured microporous polymer membranes for biomedical in vitro applications.

Microscale porous membranes are used in a wide range of technical and medical applications such as water treatment, dialysis and in vitro test systems. A promising approach to control membrane properties and overcome limitations of conventional fabrication techniques is given by additive manufacturing (AM). In this study, we designed and printed a microporous membrane via digital light processing and validated its use for biomedical in vitro applications based on the example of a cell culture insert. A multi-layer technique was developed, resulting in an eight-layer membrane with an average pore diameter of 25 µm. Image analyses proved the printing accuracy to be high with small deviations for an increasing number of layers. Permeability tests with brilliant blue FCF (E133, triarylmethane dye) and growth factors comparing the printed to track-etched membranes showed similar transfer dynamics and confirmed sufficient separation properties. Overall, the results showed that printing microporous polymer membranes is possible and highlight the potential of AM for biomedical in vitro applications such as cell culture inserts, scaffolds for tissue engineering or bioreactors.

Nanofiller-incorporated porous polymer electrolyte for electrochemical energy storage devices

We report the poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP)-based microporous polymer membranes, prepared by phase inversion technique, incorporated with different amounts of nanosized zirconium dioxide (ZrO2) filler. Scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and thermal studies confirm the role of ZrO2 nanofiller to modify the polymer structure, pore geometry and crystallinity. The nanofillers interact with the PVdF-HFP chains via surface groups and electrostatic interactions, and their incorporation led to an increase in crystalline content of the membrane and ionic conductivity (when activated with a liquid electrolyte (LE)). A possible mechanism for the increase in crystallinity in the polymer due to interaction with nanofiller particles has also been presented. The optimized membrane has been saturated with an LE sodium perchlorate-ethylene carbonate:propylene carbonate for use as a separator/electrolyte in electrical double-layer capacitor (EDLC). The cells fabricated with the nanofiller-incorporated membrane show better performance in terms of specific electrode capacitance, specific energy and specific power (approximately 76 F g−1, approximately 20.9 Wh kg−1 and 2.62 kW kg−1) than the cells using the membrane devoid of nanofillers (approximately 61 F g−1, approximately 17.3 Wh kg−1 and approximately 3.16 kW kg−1), respectively. The EDLC shows approximately 85% retention in specific capacitance for 10,000 charge–discharge cycles.

Coarse-grained molecular dynamics study of membrane distillation through meso-size graphene channels

Molecular dynamics simulations have now been broadly applied to investigate membrane transport and optimize membrane design, but mostly for nanostructures without the involvement of phase change due to the enormous computational time and spatial scale requirements. In the present study, a coarse-grained molecular dynamics model is developed to overcome the limitations in simulating membrane distillation through meso-size (2–5 nm) channels formed by graphene bilayer at the direct contact mode. The new coarse-grained approach enables the comprehensive evaluation of the influences of channel opening, hydrostatic pressure, and temperature. In addition, the evaporation processes at the membrane surface as well as the water vapour transport can now be analysed in a statistically reliable manner. The coarse-grained results show that the permeate flux through the graphene bilayer channels is almost three-order-of-magnitude higher than the commonly used microporous polymer membranes nowadays, which demonstrate the significant application potential. Increasing the hydrostatic pressure is found to be uneconomical due to its limited effects. The enhanced evaporation at small channel opening is due to the elevated collisions among the interface water molecules. Most importantly, the permeate flux shows a non-monotonic dependence on the channel opening. The flux is highest when the channel opening is 2 nm, after which the dominant transport of water molecules inside the channel transit from surface diffusion to activated Knudsen transport. This finding has an important implication towards the design of graphene bilayer membranes for membrane distillation in the future.

Microporous polymeric composite membranes with advanced film properties: pore intercalation yields excellent CO2 separation performance

PIM-1 based blend membranes with advanced gas transport and mechanical properties.

A rapidly-reversible absorptive and emissive vapochromic Pt(II) pincer-based chemical sensor

Selective, robust and cost-effective chemical sensors for detecting small volatile-organic compounds (VOCs) have widespread applications in industry, healthcare and environmental monitoring. Here we design a Pt(II) pincer-type material with selective absorptive and emissive responses to methanol and water. The yellow anhydrous form converts reversibly on a subsecond timescale to a red hydrate in the presence of parts-per-thousand levels of atmospheric water vapour. Exposure to methanol induces a similarly-rapid and reversible colour change to a blue methanol solvate. Stable smart coatings on glass demonstrate robust switching over 104 cycles, and flexible microporous polymer membranes incorporating microcrystals of the complex show identical vapochromic behaviour. The rapid vapochromic response can be rationalised from the crystal structure, and in combination with quantum-chemical modelling, we provide a complete microscopic picture of the switching mechanism. We discuss how this multiscale design approach can be used to obtain new compounds with tailored VOC selectivity and spectral responses.

Selective Blocking Property of Microporous Polymer Membranes Fabricated by Chemical Vapor Deposition

Poly-p-xylylene films have been utilized as protective and barrier layers for gases and solvents on electronic and implantable devices. Here we report a new approach to create highly permeable and selective nanofiltration membranes coated with microporous poly-p-xylylene nanofilms fabricated through a dry chemical vapor deposition process by using [2.2]paracyclophanes derivatives on ultrafiltration membranes. The introduction of crosslinking points into rigid poly-p-xylylenes enhanced microporosity and mechanical strength due to insufficient packing and depression of structural relaxation among polymer chains in three-dimensional networks. Crosslinked nanofilms with thicknesses down to 50 nm showed outstanding permeability for water and alcohols at a pressure difference of 0.5 MPa and exhibited higher rejection ratios for water-soluble organic dyes than non-crosslinked nanofilms. Poly-p-xylylene nanofilms also showed an excellent blocking property for non-polar organic solvent permeation through specific interaction of hydrophilic pores with organic solvents.

Plasma modification of microporous polymer membranes for application in biomimetic dissolution studies

Biorelevant dissolution is an indispensable tool utilized during formulation development and optimization for the prediction of in vivo bioavailability of pharmaceutical agents. Within that framework, membrane-permeation dissolution methodologies are widely used to model drug absorption. The current work evaluates polymer membrane surface modifications for production of biomimetic membranes to be employed in biorelevant dissolution studies. Biomimetic membranes exhibit hydrophilic and hydrophobic properties to simulate the intestinal membrane environment. Low temperature plasma treatment of microporous polyethersulfone (PES), nylon and polypropylene (PP) polymer membranes was applied to produce low energy surface layers with permanent hydrophobic and hydrophilic functionalities. Surface modifications on microporous polymer membranes were achieved by plasma treatments using tetrafluoromethane (CF4), perfluorohexane (C6F14), dichloromethane (DCM) and water (H2O). Surface characterization of treated membranes was evaluated using scanning electron microscopy energy dispersive x-ray spectroscopy (SEM-EDS), water contact angle (CA) and x-ray photoelectron spectroscopy (XPS) techniques. SEM-EDS analysis of polymer membranes treated with fluorinated and chlorinated solvents/gases depicts altered surface morphologies with enriched porosity. SEM-EDS and XPS analyses demonstrate the chemical modification at the surface of treated membranes is strongly influenced by the type of treatment gas or solvent. Results show fluorination as a more effective and less destructive treatment technique. XPS confirmed the presence of elemental fluorine functional groups at the surface of the PES and nylon membranes. Evaluating elemental changes (F/C ratio) from multiple techniques confirms fluorinated plasma treatments are localized to the surface of the membrane and do not significantly affect the bulk properties. In a supplemental study, a detailed comparison of the plasma treated polymer membranes and porcine intestines revealed the biomimetic nature of the modified membranes.

Diamine-Appended Mg2(dobpdc) Nanorods as Phase-Change Fillers in Mixed-Matrix Membranes for Efficient CO2/N2 Separations.

Despite the availability of chemistries to tailor the pore architectures of microporous polymer membranes for chemical separations, trade-offs in permeability and selectivity with functional group manipulations nevertheless persist, which ultimately places an upper bound on membrane performance. Here we introduce a new design strategy to uncouple these attributes of the membrane. Key to our success is the incorporation of phase-change metal-organic frameworks (MOFs) into the polymer matrix, which can be used to increase the solubility of a specific gas in the membrane, and thereby its permeability. We further show that it is necessary to scale the size of the phase-change MOF to nanoscopic dimensions, in order to take advantage of this effect in a gas separation. Our observation of an increase in solubility and permeability of only one of the gases during steady-state permeability measurements suggests fast exchange between free and chemisorbed gas molecules within the MOF pores. While the kinetics of this exchange in phase-change MOFs are not yet fully understood, their role in enhancing the efficacy and efficiency of the separation is clearly a compelling new direction for membrane technology.

Fabrication of a conjugated microporous polymer membrane and its application for membrane catalysis

A flexible and free standing conjugated microporous polymer (CMP) membrane was prepared using a polyvinylpyrrolidone (PVP) electrospun membrane as a template. The PVP nanofibers of the template membrane were coated with a thin layer of the CMP through the in situ Sonogashira-Hagihara coupling reaction of 1,3,5-triethynylbenzene and 1,4-diiodobenzene. The PVP nanofibers were removed by the solvent extraction to produce the CMP membrane, which retained the entangled fibrous structure of the template membrane. Each fiber showed a hollow tubular structure having a CMP wall with a thickness of tens of nanometers. The microporous polymer membrane exhibited a high BET surface area with hierarchical porosity and good permeability. As a catalytic CMP membrane, the Ag nanoparticle-immobilized microporous polymer membrane was fabricated using an electrospun PVP@Ag membrane as a template. After being coated with the CMP, the PVP nanofibers were removed by the solvent extraction, but the Ag nanoparticles were trapped in the microporous polymer shell. The catalytic CMP membrane was successfully used for the catalytic reduction reaction of 4-nitrophenol. The hollow tubular structure and hierarchical porosity of the membrane allowed for the reactants to easily penetrate into the CMP wall and to contact the Ag nanoparticles, resulting in the high catalytic activity.