Nonwoven Membrane Research Articles

Hierarchical Self‐Assembly of Poly(Urethane)/Poly(Vinylidene Fluoride‐co‐Hexafluoropropylene) Blends into Highly Hydrophobic Electrospun Fibers with Reduced Protein Adsorption Profiles

Electrospinning of blend systems, combining two or more polymers, has gained increasing interest for the fabrication of fibers that combine properties of the individual polymers. Here, a versatile method to produce hydrophobic fibers composed of poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDFhfp) and polyurethane (PUR) is presented. PVDFhfp containing fibers are expected to reduce protein adsorption. In a one‐step process, blend solutions are electrospun into homogeneous nonwoven membranes with fiber diameters in the range of 0.6 ± 0.2 to 1.4 ± 0.7 µm. Surface fluorine concentrations measured by X‐ray photospectroscopy show an asymptotic dependency in function of the PVDFhfp to PUR ratio, reaching values close to pure PVDFhfp at a weight per weight ratio of 10% PVDFhfp to 90% PUR. This fluorine enrichment on the surface suggests a gradient structure along the fiber cross‐section. At increased surface fluorine concentration, the contact angle changes from 121 ± 3° (PUR) to 141 ± 4° (PUR/PVDFhfp). Furthermore, these highly hydrophobic fibers present significantly reduced fibrinogen or albumin adsorption compared to PUR membranes.

A low-cost, environment-friendly lignin-polyvinyl alcohol nanofiber separator using a water-based method for safer and faster lithium-ion batteries

A lignin/polyvinyl alcohol (lignin/PVA) nanofiber membrane has been prepared as a separator for lithium-ion batteries (LIBs) using a simple, low-cost, environment-friendly, water-based method. Lignin is a widely abundant, naturally occurring biopolymer and polyvinyl alcohol (PVA) is a biocompatible polymer with good chemical and thermal stability. In this work, lignin and PVA were dissolved in water, and fibers were drawn from the solution to form a non-woven membrane. A highly porous interpenetrating network structure resulted that showed high electrolyte uptake, excellent compatibility, and superior wettability. The membrane also showed high thermal stability and flame retardant properties. Electrochemical performance revealed high rate capability with the lignin/PVA membrane that is attributed to its high ionic transport property. The as prepared lignin/PVA membrane, thus, can be considered as a separator for safer and faster LIBs with various different anodes.

Nanofiber Single‐Ion Conducting Electrolytes: An Approach for High‐Performance Lithium Batteries at Ambient Temperature

AbstractExploring the benefits of a nanofibrous morphology in electrolyte materials for Li‐battery applications, an approach of fabricating single‐ion conducting electrolyte (SICE) membranes is reported. A nonwoven nanofabric SICE membrane, delivering an outstanding performance, surpassing the conventional liquid electrolyte system at ambient conditions, was fabricated by using an electrospinning technique. When soaked in a carbonate solvent system, the membrane shows high ionic conductivity, electrochemical stability above 5.2 V (vs. Li/Li+) and a lithium transference number ( ) close to unity (0.93). Moreover, a LiFePO4|Li cell assembled with this membrane performs charge/discharge up to the 5 C rate under ambient conditions with a coulombic efficiency close to 100%. The discharge capacities of this battery are found to be higher than those of a battery assembled with a commercial separator/dual‐ion salt electrolyte system up to 2 C, which offers significant progress for SICEs. Unlike most of the earlier reported SICEs that performed like a bulk‐material entity, the innovative approach might be valuable for future developments.

Elastic Evaluation of Poly(Lactic Acid) Electrospun Membranes Using the Pulsed Photoacoustic Technique

Fibrous membranes manufactured by electrospinning possess unique features such as a high porosity and large specific surface area, making them suitable for applications in tissue engineering. However, the determination of their mechanical behavior under different loading conditions remains one of the most difficult technical problems for researchers to overcome. While the tensile properties of this kind of membrane are commonly reported in the literature, few explorations of their properties in other directions have been reported. In this paper, the pulsed photoacoustic technique is employed to obtain the elastic constants of electrospun non-woven membranes, specifically in two directions (L, T). The electrospun samples are hybrid fiber membranes of poly(lactic acid) and hydroxyapatite (HA) nanoparticles at different concentrations. It is found that the concentration of HA nanoparticles determines the mechanical response of the membrane, where the nanoparticles act either as a reinforcement or as a mesh defect. The elastic constants ( $$E_{L}, E_{T}, G_{L}, G_{T}, v_{L}$$ , $$\nu _{T}$$ ) are obtained through velocity waves related to the stress–strain equations, using samples with two different geometries and considering the electrospinning mats as a transversely isotropic material. These values are compared to those acquired using macro-tensile testing equipment according to the ASTM D1708 standard.

Exploration of an innovative draw solution for a forward osmosis-membrane distillation desalination process.

Forward osmosis (FO) has emerged as a viable technology to alleviate the global water crisis. The greatest challenge facing the application of FO technology is the lack of an ideal draw solution with high water flux and low reverse salt flux. Hence, the objective of this study was to enhance FO by lowering reverse salt flux and maintaining high water flux; the method involved adding small concentrations of Al2(SO4)3 to a MgCl2 draw solution. Results showed that 0.5M MgCl2 mixed with 0.05M of Al2(SO4)3 at pH 6.5 achieved a lower reverse salt flux (0.53gMH) than that of pure MgCl2 (1.55gMH) using an FO cellulose triacetate nonwoven (CTA-NW) membrane. This was due possibly to the flocculation of aluminum hydroxide in the mixed draw solution that constricted membrane pores, resulting in reduced salt diffusion. Moreover, average water fluxes of 4.09 and 1.74L/m2-h (LMH) were achieved over 180min, respectively, when brackish water (5g/L) and sea water (35g/L) were used as feed solutions. Furthermore, three types of membrane distillation (MD) membranes were selected for draw solution recovery; of these, a polytetrafluoroethylene membrane with a pore size of 0.45μm proved to be the most effective in achieving a high salt rejection (99.90%) and high water flux (5.41LMH) in a diluted draw solution.

Sustainable dewatering and drying of self-flocculating microalgae and study of cake properties

Efficient dewatering and drying at lower cost have been a requirement in reducing the cost of microalgal biomass production. The self-flocculation capacity of Scenedesmus obliquus and the hydrophobic nature of polypropylene non-woven fabric membrane (PNM) are explored in this study. Formation of ‘algal bio-filter layer’ due to natural forces showed the efficiency of the process. Compared to the conventional methods, the cost of harvesting reduced substantially. The physico-chemical and mechanical properties of the harvested cake were studied. To the best of our knowledge, this is the first report on PNM usage for dewatering and drying as well as algal cake properties. The membrane showed high biomass recovery efficiency (94%) for Scenedesmus sp. but very low (58%) for Chlorella sorokiniana (non-self-flocculating). Dewatering on sand-bed followed by drying on wire-gauge (10.5% m.c) found to be the best arrangement. Solar drying of 1–2 cm thick slurry was achieved within 34 ± 2 h. Based on drying period, true density, bulk density, inter-cake porosity, breaking strength, grinding and selective physical properties, obtaining a 2-mm-thick cake was comparatively more energy efficient for post-harvest operations. The estimated cost of pilot-scale dewatering-drying was $ 0.048 per kg of dry algal biomass. Dehydration using PMN followed by natural drying was found to be energy efficient and comparatively very cost effective for pilot scale harvesting.

Engineering mechanical gradients in next generation biomaterials - Lessons learned from medical textile design.

External and internal biomaterial membranes prescribe boundary conditions for treatment of medical disorders, from oedema to tissue defects. Studies are needed to guide the design of next generation biomaterials and devices that incorporate gradient engineering approaches, which offer great potential to enhance function in a dynamic and physiological context. Mechanical gradients intrinsic to currently implemented biomaterials such as medical textiles and surgical interface membranes are poorly understood. Here we characterise quantitatively the mechanics of textile and nonwoven biomaterial membranes for external and internal use. The lack of seamless gradients in compression medical textiles contrasts with the graded mechanical effects achieved by elastomeric exercise bands, which are designed to deliver controlled, incremental increases in loading to facilitate healing as injured tissues return to normal structure and function. Engineering textiles with a prescient choice of fibre composition/size, type of knit/weave and inlay fibres, and weave density/anisotropy will enable creation of fabrics that can deliver spatially and temporally controlled mechanical gradients to maintain force balances at tissue boundaries, e.g. to treat oedema or tissue defects.

Theoretical modeling of thermal stress imposed by selective permeation membranes reinforced with graphene oxide

ABSTRACTThe main purpose of the present work is to study the thermal stress imposed by selective permeation hydrogel‐filled nonwoven membranes (SPHM) in various environmental conditions, including cold, moderate and hot, in view of high and low wearer activity levels. In addition, graphene oxide (GO) has been used in the matrix structure of SPHM to reduce thermal stress. Hence, a mathematical model is proposed to study one‐dimensional heat transfer through SPHM reinforced with GO. Heat transfer equation was solved using the differential quadrature method and the resulting model was verified by experiments using a dynamic heat transfer simulation apparatus. It was observed that SPHM causes a significant thermal stress, especially in hot environments, and high activity level due to the low thermal conductivity of hydrogels. The results also showed that an increase in the GO content from 0.1% up to 0.5% leads to an increase in thermal conductivity up to 85% of blank SPHM without GO. Therefore, SPHM reinforced with GO is a promising candidate for protective clothing, especially in hot environments. Also, the mathematical model can be useful in predicting thermal stress for designing SPHM‐based PCs in various environmental conditions and activity levels. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44752.

Super-stable non-woven fabric-based membrane as a high-efficiency oil/water separator in full pH range

A polystyrene coated non-woven fabric hybrid membrane with superoleophilicity has been synthesized by a facile polymerization reaction for high-efficiency separation of oils in harsh environments.

Poly(amidoamine) dendrimers grafted on electrospun poly(acrylic acid)/poly(vinyl alcohol) membranes for host-guest encapsulation of antioxidant thymol.

Amino-terminated fifth generation poly(amidoamine) (PAMAM G5-NH2) dendrimers were grafted onto the surface of poly(acrylic acid)/poly(vinyl alcohol) (PAA/PVA) electrospun fibres with the purpose of creating a host-guest architecture for the controlled delivery of a natural antioxidant, thymol. The nanofibers were stabilized by esterification crosslinking to produce a water insoluble non-woven membrane. The functionalization with PAMAM G5-NH2 led to dendrimer loadings in the 7.4 × 10-7-2.25 × 10-6 mol dendrimer per g membrane range. The resulting materials were characterized using SEM, ATR-FTIR and surface ζ-potential measurements. The loading capacity for thymol reached 2.5 × 10-4 mol thymol per g membrane. The membranes were tested for thymol release in different aqueous and non-aqueous food simulants. Computational modelling was used to get a further insight into the host-guest association of thymol and PAMAM G5-NH2 molecules through docking studies. For this purpose, we examined the molecular level details of the dendrimer-guest complex, calculated the number of included or attached molecules, the exact location of thymol in host-guest complexes and the local environment around the thymol molecules. Docking studies showed that PAMAM-G5-NH2 dendrimers can encapsulate thymol molecules through hydrophobic interactions and hydrogen bonding. The maximum amount of thymol molecules theoretically encapsulated was 16, while another 25 could be hosted at the dendrimer surface through interaction with the outer part or the dendritic branches. The experimental value was 37 ± 5, in agreement with theoretical predictions.

Nanoporous nanocomposite membranes via hybrid twin-screw extrusion—multijet electrospinning

Non-woven nanoporous membranes of poly(caprolactone), PCL, incorporated with multi-walled carbon nanotubes, CNTs, could be fabricated via an industrially-scalable hybrid twin screw extrusion and electrospinning process. The utilization of a spinneret with multiple nozzles allowed the increase of the flow rate beyond what is possible with conventional electrospinning using a single nozzle, albeit at the expense of difficulties in the control of the thickness distributions of the nanofibrous membranes. The thickness and orientation distributions and the resulting mechanical properties of the membranes could be modified via changes in voltage, angular velocity of the collector mandrel and separation distance of the collector from the spinneret. The increases in crystallinity due to the presence of the CNTs and the preferential alignment of the nanofibers via rotation of the collecting mandrel led to increases in the tensile properties of the nanoporous membranes. The use of poly(ethylene oxide), PEO, together with PCL, followed by the dissolution of the PEO, rendered the nanofibers themselves nanoporous with typical surface porosity values of around 50% and pore sizes of about 220 nm. The demonstrated versatility of the hybrid twin screw extrusion and electrospinning process and the manipulation of mesh dimensions and properties are indicative of the applicability of the hybrid process for fabrication of nanoporous membranes for myriad diverse industrial applications ranging from water treatment to tissue engineering applications.

Effect of drying temperature on a thin PVDF-HFP/PET composite nonwoven separator for lithium-ion batteries

A thin composite separator with polyethylene terephthalate nonwoven membrane as the structural support and polyvinylidene fluoride-hexafluoropropylene as the coating layer for lithium-ion batteries was prepared by a simple dip-coating process. The effect of different drying temperatures on the performance of the composite separator was investigated. The results indicate that 80 °C is the optimal drying temperature, preventing leakage current problems and providing a well-developed porous structure. The drying of the composite separator at 80 °C provides a superior thermal stability, better wettability with electrolyte, higher electrolyte uptake, and ionic conductivity compared to commercially available polypropylene (PP) separator. Furthermore, the electrochemical performance consisting of electrochemical stability, self-discharge, cycle performance, rate performance of the composite separator, and PP were determined. The drying of the composite separator at 80 °C shows almost the same oxidation stability and self-discharge performance, but a better cycling and rate performance than the PP separator.

Hydrophilic and antimicrobial properties of acrylic acid and chitosan bigrafted polypropylene melt-blown nonwoven membrane immobilized with silver nanoparticles

The polypropylene melt-blown nonwoven membrane (PPM) is widely used in healthcare; however, the highly hydrophobic nature of the PPM readily adsorbs proteins and polysaccharides, which are conducive to bacteria being retained in the network, resulting in biofouling. Therefore, to improve the hydrophilic and antimicrobial properties of PPM, acrylic acid (AA) was first graft-polymerized on PPM (PPM- g-AA) by ultraviolet (UV)-induced photo-grafting polymerization. Chitosan (CS) was then covalently grafted onto PPM- g-AA to obtain the bigrafted PPM (PPM- g-AA- g-CS). Finally, silver (Ag) nanoparticles were immobilized onto PPM- g-AA- g-CS to create the hydrophilic and antibacterial PPM. The surface chemical composition and morphology of the samples were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The hydrophilic and antimicrobial properties of the modified PPM were assessed using static water contact angle measurements, wetting time, and bacteria colony-counting assays. The results show that PPM- g-AA- g-CS with immobilized Ag nanoparticles has excellent antibacterial and hydrophilic properties.

Experimental wound dressings of degradable PHA for skin defect repair.

The present study reports construction of wound dressing materials from degradable natural polymers such as hydroxy derivatives of carboxylic acids (PHAs) and 3-hydroxybutyrate/4-hydroxybutyrate [P(3HB/4HB)] as copolymer. The developed polymer films and electrospun membranes were evaluated for its wound healing properties with Grafts-elastic nonwoven membranes carrying fibroblast cells derived from adipose tissue multipotent mesenchymal stem cells. The efficacy of nonwoven membranes of P(3HB/4HB) carrying the culture of allogenic fibroblasts was assessed against model skin defects in Wistar rats. The morphological, histological and molecular studies revealed the presence of fibroblasts on dressing materials which facilitated wound healing, vascularization and regeneration. Further it was also observed that cells secreted extracellular matrix proteins which formed a layer on the surface of membranes and promoted the migration of epidermal cells from the neighboring tissues surrounding the wound. The wounds under the P(3HB/4HB) membrane carrying cells healed 1.4 times faster than the wounds under the cell-free membrane and 3.5 times faster than the wounds healing under the eschar (control).The complete wound healing process was achieved at Day 14. Thus the study highlights the importance of nonwoven membranes developed from degradable P(3HB/4HB) polymers in reducing inflammation, enhancing angiogenic properties of skin and facilitating better wound healing process.

Activity of enzymes immobilized on plasma treated polyester

The aim of this study was to investigate the effect of 3 different surface plasma treatments on the immobilization of β-galactosidase on a fibrous PET nonwoven membrane. Two methods 1. Entrapment in a thin calcium alginate coating and 2. Direct sorption, were used to immobilize the enzyme. The three different plasma treatments for surface activation of PET nonwovens were: 1. Air atmospheric DBD plasma, 2. Cold remote plasma-CRP with 100% N2 and 3.- CRP with a mixture of N2/O2 gases. Plasma treatment of the PET fiber surface increased the quantity of immobilized enzyme using the entrapment method, and the degree of alginate film cross-linking highly influenced the enzyme activity. Highest enzyme activity was reached for the PET treated with air atmospheric plasma and cross-linked with 0.25g/l of CaCl2. With the direct sorption method, greater amounts of enzyme were immobilized as compared to the entrapment method, but a considerable proportion of enzyme lost their catalytic activity. Only with the CRP N2/O2 plasma treatment, up to 90% of sorbed enzyme maintained their activity. Reusability study showed that for the optimized entrapment method, a progressive decrease in activity was observed after each use cycle. With the optimized sorption method using N2/O2 CRP plasma, no decrease in enzyme activity was detected, and the immobilized enzyme could be used over more than 15 cycles.

Electrospun melamine resin-based multifunctional nonwoven membrane for lithium ion batteries at the elevated temperatures

Abstract A flame retardant and thermally dimensional stable membrane with high permeability and electrolyte wettability can overcome the safety issues of lithium ion batteries (LIBs) at elevated temperatures. In this work, a multifunctional thermoset nonwoven membrane composed of melamine formaldehyde resin (MFR) nano-fibers was prepared by a electro-spinning method. The resultant porous nonwoven membrane possesses superior permeability, electrolyte wettability and thermally dimensional stability. Using the electrospun MFR membrane, the LiFePO 4 /Li battery exhibits high safety and stable cycling performance at the elevated temperature of 120 °C. Most importantly, the MFR membrane contains lone pair electron in the nitrogen element, which can chelate with Mn 2+ ions and suppress their transfer across the separator. Therefore, the LiMn 2 O 4 /graphite cells with the electrospun MFR multifunctional membranes reveal an improved cycle performance even at high temperature. This work demonstrated that electrospun MFR is a promising candidate material for high-safety separator of LIBs with stable cycling performance at elevated temperatures.

Synthesis and Characterization of Polymer Electrolyte Using Deep Eutectic Solvents and Electrospun Poly(vinyl alcohol) Membrane

Organic polymer electrolytes attracted a great deal of attention and research in the field of energy conversion and energy storage due to their benefits such as appreciable ionic conductivity and transference number, improved mechanical strength, thermal stability, and better interaction with the electrodes. In this study, a polymer electrolyte was synthesized from poly(vinyl alcohol) (PVA) by electrospinning technique followed by soaking in a series of deep eutectic solvents (DESs) to enhance the electrical conductivity of the product. The result was a very thin, nonwoven membrane. The produced membranes showed appreciable improvement in electrical conductivity from 2.78 × 10–6 to 2.27 × 10–2 S/cm after soaking in DES2. The observed improvement in electrical properties opens the possibility for the PVA membranes to find their applications in batteries, storage cells, flexible thermoelectric devices, sensors, and solar cells.

A simple method to prepare a polydopamine modified core-shell structure composite separator for application in high-safety lithium-ion batteries

In this work, we report a simple dip coating method to grow a thin polydopamine (PDA) layer on the surface of polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) nano-fibers as the composite separator for application in high-safety lithium-ion batteries. A thin adherent PDA film uniformly distributed on the surface of PVDF-HFP nano-fibers, formed a unique core-shell structure. Compared with pure PVDF-HFP nonwoven membrane, the PDA coating PVDF-HFP composite membrane (PVDF-HFP-PDA) not only exhibits much higher thermal stability, but also shows enhanced mechanical strength and tensile strength, which are very important for the assemble and long-term performance of the batteries. The ionic conductivity of the PVDF-HFP-PDA composite membrane is 1.40mScm−1 which is much higher than 0.80mScm−1 of the polypropylene (PP) separator. The lithium-ion batteries with PVDF-HFP-PDA composite membrane show excellent cyclic stability and good rate performance. The PVDF-HFP-PDA composite membrane is suitable for applications in secondary lithium-ion batteries due to its advantages mentioned above.

Safety-Reinforced and High-Voltage Poly(propylene carbonate)-Based All-Solid-State Polymer Electrolyte for Ambient-Temperature Solid Polymer Lithium Batteries

1. Introduction There were huge potential safety hazards when commercialized lithium ion battery employing organic carbonate solvent and polyolefin-based separators. These potential risks (combustion and explosion) would retard the commercialization of electric vehicles or hybrid electric vehicles. Thus, the safety issue of lithium ion battery merits further study. Solid polymer electrolyte is attracting strong interest and regarded as a perfect way to handle the potential safety risk of lithium batteries. 2. Experimental poly(propylene carbonate) (PPC) based solid polymer electrolytes were fabricated by a facile solution casting method. PPC (10 g) and lithium bis (trifluoromethanesulphonyl) imide (LiTFSI) (3 g) were added into anhydrous acetonitrile (100 mL) in sequence under intense stirring to form a homogeneous solution. Subsequently, flame retardant cellulose nonwoven membrane was prepared according to the method of our previous literature. Followingly, homogeneous solution poured into cellulose nonwovens followed by the evaporation of the solvent in a vacuum oven at 100 oC for 24 hrs. The thickness of the resultant cellulose-supported PPC solid polymer electrolyte (CPPC-SPE) was 75±5 μm. Pristine PPC solid polymer electrolyte (PPC-SPE) and pristine polyethylene oxide (PEO) solid polymer electrolyte (PEO-SPE) were fabricated via similar method without cellulose nonwoven supporting matrix. 3. Results and Discussion In order to demonstrate the feasibility of the novel CPPC-SPE, all-solid-state polymer LiFePO4/Li battery was assembled and typical charge-discharge curves at various rates and long-term cycling stability were also examined (Fig. 1). It is obvious that rate capability and cycle performance of CPPC-SPE was much better than those of PEO-SPE at room temperature. This better rate performance is an indicator of a low resistance, which can be contributed to better ionic transport capability, demonstrating that CPPC-SPE has great potential in ambient temperature all-solid-state lithium batteries. It was noted that the cell with CPPC-SPE displayed superior cycle durability after 1000 cycles at 20 oC, 95 % of the initial capacity. This outstanding cycle performance would be attributed to better electrochemically interfacial stability. It can be seen from Figure 4c that the value of charge-transfer resistance after first cycle was 73 Ω. After the 1000 cycles, the value became 83 Ω, indicative of a minor augment of charge-transfer resistance. More importantly, solid state soft-package lithium batteries using CPPC-SPE can present excellent safety characteristic even in extremely harsh condition. A corollary of this is that, this class of CPPC-SPE is very promising to develop next-generation all-solid-state lithium batteries that require high energy density, superior safety and excellent reliability. Most significantly, the main innovative of our work will highlight a reference for subsequent scientific research of room temperature solid polymer electrolyte and also boost the development of high performance all solid state polymer lithium batteries. References 1) G. L. Cui, et. al Advanced Energy Materials DOI: 10.1002/aenm.201501082. 2) G. L. Cui, et. al Sci entific Rep orts 2014, 4, 6272. Figure 1

Enhanced Water Vapor Transmission through Porous Membranes Based on Melt Blending of Polystyrene Sulfonate with Polyethylene Copolymers and Their CNT Nanocomposites.

A novel concept for the use of an immiscible and non-meltable polymer, such as sodium polystyrene sulfonate (PSSNa), in order to prepare polyethylene non-woven breathable membranes is described. Membranes were fabricated by melt compounding of properly functionalized PE (P(E-co-AA)) and PSSNa (P(SSNa-co-GMA)) copolymers in the presence of water soluble polyethylene glycol (PEG). The inability of PSSNa derivatives to be melted was overcome by using PEG, which was easily meltable thus inducing PSSNa processability improvement. PEG was removed after membrane fabrication and therefore also acted as a porogen. Carbon nanotubes, functionalized with PSSNa moieties or alkyl groups, were also incorporated in the membranes with the aim of improving the porous connectivity and increasing the water vapor transmission rate. The morphology of the membranes was investigated through Scanning Electron Microscopy (SEM). Water vapor transmission rate (permeation) (WVTR) measurements for the porous membranes showed increased values in comparison with the neat PE ones. A further increase of WVTR was observed with the addition of CNTs to the polymer membranes.