Effect of Casting Solution on Morphology and Performance of PVDF Microfiltration Membranes

The effects of preparation‐influencing parameters such as polymer concentration, thickness of casting solution, and type of solvent on morphology and performance of poly(vinylidene difluoride) (PVDF) microfiltration membranes for the treatment of emulsified oily wastewater were investigated. Flat‐sheet membranes were prepared from a casting solution of polymer and additive in various solvents by immersing the prepared films in nonsolvent‐containing mixtures of water and 2‐propanol. The membranes were characterized using scanning electron microscopy. Increasing the polymer concentration and membrane thickness significantly affected the pore size, leading to permeate flux decrease. An attempt was made to correlate the effect of the solvent on membrane morphology and performance employing solubility parameters between solvent and nonsolvent).     

聚丙烯腈—聚氨酯共混超滤膜的研究

用聚丙烯腈和聚氨酯为膜材料,N,N-二甲基乙酰胺（DMAC）为溶剂,PEG 4000为添加剂制备超滤膜。研究了聚合物浓度、比例对膜结构和性能的影响。结果表明：随着聚合物浓度的增大,膜的水通量下降,截留率先升高后降低;当聚合物的比例为8∶2、质量分数为14%时,性能最佳。     

Preparation of defect-free asymmetric membranes for gas separations

A technique was developed to prepare defect-free, asymmetric, polymer membranes for gas separation. The preparation method eliminates the need for coatings, which are usually required to render asymmetric, polymer based, membranes gas selective. In this method, a casting solution containing a polymer, solvent, and salt additive is given a desired shape and immersed in a coagulation bath containing a nonsolvent. The nonsolvent is selected to have a low affinity for both the solvent and salt additive. After the complete coagulation of the membrane, the additive salt is leached out in a second bath. This leads to the formation of an asymmetric membrane that has a well-interconnected porous network. The fine membrane structure is preserved by solvent exchange before it is finally dried. Polyetherimide (PEI) (Ultem® 1000) membranes were prepared from casting solutions containing 23, 25, and 26.5% (wt) PEI, various amounts of lithium nitrate and N-methyl-2-pyrrolidinone (NMP). Membrane performance was determined for the separation of oxygen from air. The effects of polymer concentration, additive salt concentration and the drying process on oxygen permeance, and the actual separation factor of the membrane are discussed. The addition of a small amount of solvent to the coagulation bath improved the leaching of the salt additive and produced membranes with a more open structure. A polymer concentration of 23% produced membranes with the highest performance. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1471–1482, 1999     

Characterization of poly(vinylidene fluoride) flat sheet membranes prepared in various ratios of water/ethanol for separator of li‐ion batteries: Morphology and other properties

PVDF, poly(vinylidene fluoride), membranes were prepared and investigated by a scanning electron microscope, a universal testing machine, and capillary porometer for its potential use as a separator in lithium ion batteries. The membranes were prepared by phase inversion with different polymer types, concentrations of solution, amounts of additive, and nonsolvent ratios of water/ethanol. The morphology of membranes is affected by the ratio of both the coagulation bath (water/ethanol) and a low molecular weight additive (polymer/solvent/additive). The results showed that significant variations in the membrane were detected when adding an additive to the casting solution or ethanol to the coagulation bath. With an increased concentration of ethanol, the upper structure was found to be transformed into a sponge‐like arrangement. In the case of Solef®1015 of the same polymer concentration, despite the higher molecular weight of 1015, a relatively small sized nucleus is formed, resulting in a denser network and relatively uniform membrane structure being formed. Mechanical testing showed that the tensile strength of the PVDF membranes increased when added to a 25 wt % ethanol coagulation bath, whereas it is decreased when added to higher concentrations of ethanol in the bath or additives in the casting solution. In a bath condition of water/ethanol = 75/25 wt % (Bath no. 2), the value of tensile strength is 7.11 and 7.52 MPa, for Solef®6010 20 wt % and Solef®1015 17 wt %, respectively. The thickness of the prepared membrane is 21–34 μm and the porosity is up to 50%. The electrolyte absorption changes of the fabricated membranes at different conditions are measured from 151 to 223 ± 15%. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Studies on ultrafiltration membranes. I. Development of cellulose nitrate membranes

Ultrafiltration membranes were prepared from cellulose nitrate under different conditions and were characterized using a solution of 1.0% dextran-150 having a salinity of 1500 ppm. The role of various parameters such as polymer concentration, ratio of ether to alcohol, methanol as the solvent, and other additives is systematically studied and presented. Similarly, the effect of operational and hydraulic parameters on the membrane performance is evaluated and the minimum activation energy for the solvent permeability is experimentally deduced.     

Nanofiltration membranes based on PA6/EVOH with variable composition and morphology

Nanofiltration PA6/EVOH membranes were prepared through a nonsolvent induced phase separation technique. The effects of polymer concentration in the solution and solvent evaporation time on the performance and morphology of the resulting membranes were investigated by cloud point titration, permeation, and scanning electron microscopy (SEM). Experimental cloud point data for various prepared membranes suggested that polymer solutions with higher concentrations of PA6/EVOH need a less content of nonsolvent. SEM observations show that an increase in polymer concentration leads to formation of a thin dense layer on the surface of the membrane thanks to pore size reduction. However, dense top layer of membrane becomes thicker as polymer concentration increases from 15 wt% to 20 wt%. The performance of membranes reveals a decrease with polymer concentration in casting solution. By contrast, Polyamide/Poly(ethylene‐co‐vinyl alcohol) membranes show an optimal performance with various formic acid evaporation times. J. VINYL ADDIT. TECHNOL., 25:E28–E34, 2019. © 2018 Society of Plastics Engineers     

Studies on solvent evaporation and polymer precipitation pertinent to the formation of asymmetric polyetherimide membranes

The characteristics of solvent evaporation and polymer precipitation during the formation of asymmetric aromatic polyetherimide (PEI) membranes via the dry/wet phase inversion method are studied and the results are discussed with reference to membrane preparation. It is shown that the solvent evaporation from the surface of freshly cast films in early evaporation stages can be quantified by an empirical equation with two parameters. Analysis of the evaporation parameters partially explains the interaction effect of membrane preparation variables on membrane performance. The phase separation data for systems PEI/DMAc/H₂O and PEI/NMP/H₂O with and without LiNO₃ additive are determined using the turbidimetric titration method. The kinetic data on solvent–nonsolvent exchange and additive leaching during polymer precipitation in nonsolvent water are measured. The results presented here offer a qualitative basis for the development of asymmetric PEI membranes. © 1995 John Wiley & Sons, Inc.     

Optimization of CO2–CH4 separation performance of integrally skinned asymmetric membranes prepared from poly(2,6‐dimethyl‐1,4‐phenylene oxide) by factorial design

Integrally skinned asymmetric flat sheet membranes were prepared from poly(2,6‐dimethyl 1,4‐phenylene oxide)(PPO) for CO₂–CH₄ separation. Various experiments were carried out to identify PPO membranes, which have good mechanical strength and gas separation abilities. Membrane strength and selectivity depend on the interplay of the rate of precipitation and the rate of crystallization of the PPO. The effects of major variables involved in the membrane formation and performance, including the concentration of the polymer, solvent, and additive, the casting thickness, the evaporation time before gelation, and the temperature of the polymer solution, were investigated. Factorial design experiments were carried out to identify the factor effects. The membrane performance was modelled and optimized to approach preset values for high CO₂ permeance and a high CO₂ : CH₄ permeance ratio. Membranes were prepared based on the optimum conditions identified by the model. Essentially, defect‐free membranes were prepared at these conditions, which resulted in a pure gas permeance of 9.2 × 10⁻⁹ mol/m² s Pa for CO₂ and a permeance ratio of 19.2 for CO₂ : CH₄. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1601–1610, 1999     

Synthesis and characterization of high molecular weight perfluorocyclobutyl-containing polybenzimidazoles (PFCB-PBI) for high temperature polymer electrolyte membrane fuel cells

High molecular weight perfluorocyclobutyl-containing polybenzimidazoles (PFCB-PBI) were synthesized from 4,4′-((1,2,3,3,4,4-hexafluorocyclobutane-1,2-diyl)bis(oxy))dibenzoic acid (PFCB diacid) and 3,3′,4,4′-tetraaminobiphenyl (TAB) in phosphorous pentoxide/methanesulfonic acid (PPMA). PPMA was used as a reaction medium to replace PPA due to the higher monomer solubility. High molecular weight polymer was achieved via optimization of the monomer/solvent ratio, polymerization temperature, and polymerization time. The resulting polymer showed good thermal and chemical stability. Several different phosphoric acid doping membrane preparation processes were investigated. Conventional DMAc solvent casting and direct-casting of the PBI/PPMA solution did not produce sufficiently strong membranes to fabricate into MEAs. A modified PPA process was developed that produced improved membranes. The mechanical properties of these membranes were low compared to other PBI membranes; however, they were sufficiently strong to fabricate into membrane electrode assemblies and tested in single cell fuel cells under various conditions. The polymer dissolved in phosphoric acid at temperatures above 140 °C, which limited the operation of fuel cells below 140 °C. The maximum power densities of fuel cells operated with these membranes were similar to meta-PBI membranes prepared by the conventional casting process and lower than para-PBI membranes prepared by the PPA process.     

Morphology and performance of poly(vinylidene fluoride) flat sheet membranes: Thermodynamic and kinetic aspects

In this study, poly(vinylidene fluoride) (PVDF) membranes were prepared using two different solvents with various polymer concentrations to investigate the predominant kinetic or thermodynamic aspects of membrane preparation in a phase separation process. For this purpose, dimethyl sulfoxide (DMSO) as a weak solvent and N‐2‐methylpyrrolidone (NMP) as a strong solvent were used with polymer concentrations between 8 and 15 wt %. Scanning electron microscopy and water content, contact angle, and pore size measurements were used to assess the factors affecting the physicochemical properties of the prepared membranes. The results showed that in the case of NMP, the membrane structure is mainly controlled by thermodynamic parameters, while when using DMSO, kinetic parameters are predominant. According to the results, the prepared PVDF‐based membranes with DMSO exhibited a relatively denser top layer and less permeation compared to the NMP/PVDF membranes. The difference between the viscosities of the casting solutions with equal polymer concentrations in DMSO and NMP was considered to be the main effective factor in solvent/nonsolvent exchange, resulting in denser top layers in the DMSO/PVDF membranes. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46419.     

Cellulose acetate and sulfonated polysulfone blend ultrafiltration membranes. I. Preparation and characterization

Modification of polymeric membrane materials by incorporation of hydrophilicity results in membranes with low fouling behavior and high flux. Hence, Polysulfone was functionalized by sulfonation and ultrafiltration membranes were prepared based on sulfonated polysulfone and cellulose acetate in various blend compositions. Polyethyleneglycol 600 was employed as a nonsolvent additive in various concentrations to the casting solution to improve the ultrafiltration performance of the resulting membranes. The total polymer concentration, cellulose acetate, and sulfonated polysulfone polymer blend composition, additive concentration, and its compatibility with polymer blends were optimized. The membranes prepared were characterized in terms of compaction, pure water flux, membrane resistance, and water content. The compaction takes place within 3–4 h for all the membranes. The pure water flux is determined largely by the composition of sulfonated polysulfone and concentration of additive. Membrane resistance is inversely proportional to pure water flux, and water content is proportional to pure water flux for all the membranes. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1749–1761, 2002     

Study on Preparation and Performance of Modified Polyvinyl Chloride-co-vinyl Acetate Microfiltration Membranes

In this paper, flat-sheet microfiltration membranes were prepared from modified polyvinyl chloride-vinyl acetate (VC-co-VAc) material with hydroxyl group, VC-co-VAc-OH, by phase inversion technique. The influences of casting solution composition (polymer concentration, additive types, and content) and preparation conditions (coagulation temperature, evaporation time of solvent, and the relative humidity in the environment) on pure water flux, retention, and pore size distribution were discussed. The results showed that casting solution composition and the membrane preparation conditions all have significant impacts on the performances of the membrane: on the one hand, in order to prepare membranes with high flux, either decrease the polymer concentration, or increase the PEG molecular weight or the coagulation temperature or the relative humidity can achieve it; on the other hand, with the increase of additive content, the pure water flux increased quickly, then decreased dramatically, while the effect of evaporation time were the opposite. And these results were confirmed by the SEM images. Furthermore, the microfiltration membranes, which were prepared under optimized conditions, were used to treat synthetic oilfield polymer-flooding wastewater, and good experimental results were obtained.     

Clay-hyperbranched epoxy/polyphenylsulfone nanocomposite membranes

Nanocomposite membranes containing polyphenylsulfone (PPSU) and a clay modified with a hyperbranched epoxy (HBE) were prepared by blending of modified montmorillonite (m-MMT) with a polymer solution using phase inversion method. The hyperbranched epoxy synthesized by polycondensation reaction of bisphenol A and triethanolamine with epichlorohydrin was grafted to amine-functionalized MMT by reaction between the epoxy groups of hyperbranched epoxy and the amine groups on the MMT surface. In this way, the m-MMT was exfoliated into single layers of nanoparticles in a solvent medium and the polymer chains were intercalated into m-MMT layers. The aim was to study the effect of this additive on the membrane separation efficiency. For this purpose, pure water flux, fouling, and pigment and heavy metal rejection were measured by a home-made dead end filtration cell and the performance of the prepared membranes was investigated. Hydrophilicity of the nanocomposite membranes was specified by water contact angle measurements. Degree of dispersion of additive into the polymer matrix and membrane morphology were studied by FESEM. Membrane surface area, pore size, and volume were evaluated by BET. The results indicated that the surface hydrophilicity increased after incorporation of m-MMT. Furthermore, the water permeability, salt rejection, and antifouling resistance of PPSU membranes were improved significantly. Membrane with 3 wt% m-MMT showed the best performance compared to other membranes.     

Dehydration of Isopropanol by Pervaporation Using Aromatic Polyetherimide Membranes

Abstract

Aromatic polyetherimide membranes were prepared by the phase inversion method and tested for the pervaporation separation of water from isopropanol with emphasis on the breaking of azeotropic composition and the dehydration of high concentrations of isopropanol. It was found that the membrane selectivity was enhanced by partial evaporation of the solvent in the cast polymer films prior to the gelation step during membrane formation. The membrane performance was shown to be dependent on the feed concentration and the operating temperature. At a feed temperature of 25°C and a permeate pressure of 133 Pa, separation factors of 173 and 384 were achieved for the dehydration of isopropanol solutions at 0.68 (azeotropic composition) and 0.96 mole fractions isopropanol, respectively, with reasonably high permeation rates. The utility of the membranes for the proposed separation was demonstrated; however, these membranes were not prepared under optimized conditions and thus a continuous study is required to rationalize the effects of membrane preparation parameters on membrane performance.     

Preparation and characterization of poly (vinyl chloride)-blend-poly (carbonate) heterogeneous cation exchange membrane: Investigation of solvent type and ratio effects

In this research polyvinylchloride/polycarbonate blend heterogeneous cation exchange membranes were prepared by solution casting technique using cation exchange resin powder as functional groups agent. Tetrahydrofuran (THF) and dimethylformamide (DMF) were utilized as solvents. The effect of solvent type and ratio (THF/DMF mixture) on properties of prepared membranes was studied. SEM and SOM images showed relatively uniform particle distribution and also uniform surface for the membranes. Images showed that at high DMF ratio decrease of polymer conformation with particles surface reduces the compatibility of polymer-particle. The membrane ion exchange capacity and permeability were enhanced initially by increase of DMF ratio up to 5% (v/v) in casting solution and then they began to decrease with more DMF ratio. Results showed that membrane potential, transport number, selectivity and thermal stability all were decreased by DMF ratio increasing. Conversely, membrane water content, specific surface area and roughness showed opposite trends. Membrane electrical resistance initially declined by increase in DMF content up to 15% (v/v) and then it began to increase. The increase of electrolyte concentration also led to decrease in membrane transport number and selectivity. Membrane with (95:5) (v/v) solvent ratio (THF:DMF) exhibited more appropriate performance compared to others.     

Preparation of porous aluminium oxide (Al2O3) hollow fibre membranes by a combined phase-inversion and sintering method

Al₂O₃ hollow fibre membranes were prepared by a combined phase-inversion and sintering method. An organic binder solution (dope) containing suspended aluminium oxide (Al₂O₃) powders, either in mono size or a distributed size, is spun to a hollow fibre precursor, which is then sintered at elevated temperatures. In spinning the hollow fibre precursor, polyethersulfone (PESf), N-methyl-2-pyrrolidone (NMP) and polyvinyl pyrrolidone (PVP) were used as a polymer binder, a solvent and an additive, respectively. The Al₂O₃ hollow fibre membranes prepared were characterized using a scanning electron microscope (SEM) and gas permeation techniques. Effects of Al₂O₃ particle size and size distribution, the sintering temperature and Al₂O₃/PESf ratio on the structure and performance of the resulting membranes were studied extensively. The prepared Al₂O₃ hollow fibre membranes retains its asymmetric structure (mainly resulted from the phase inversion technique) even after the sintering process. Preparation of the Al₂O₃ hollow fibre membrane with a high mechanical strength and moderate permeation characteristics is feasible if the Al₂O₃ powders with a distributed particle size in the spinning (dope) solution is employed.     

Preparation and characterization of asymmetric polysulfone membrane for separation of oxygen and nitrogen gases

Defect‐free skinned asymmetric gas separation membranes were prepared by a dual bath coagulation method using a wet phase inversion technique. The membranes were cast from polysulfone solution in different solvents such as: dimethyl‐formamid, 1‐methyl‐2‐pyrrolidone, N‐N‐dimethyl‐acetamide (DMAC), and tetrahydrofuran. The mixtures of water/iso‐propanol (IPA), water/propanol, water/ethanol (EtOH), and water/methanol (MeOH) with volume ratio of 80/20 were used as the first coagulation bath. This led to the formation of a dense skin top layer. Distillated water was used as the second coagulation bath. The influences of several experimental variables, such as thickness of the membrane, polymer concentration, type of solvent and nonsolvent, immersion time in IPA 20%, and second coagulation bath temperature on skin layer and sublayer were elucidated. For preparing membrane with higher permeance, the influence of internal nonsolvents and addition of polyvinylpyrrolidone (PVP) as additive were investigated. The membrane performance was tested in terms of gas permeance and selectivity for O₂/N₂ separation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Identification of optimum synthesis conditions for a novel anion exchange membrane by response surface methodology

The Response Surface Methodology (RSM) was employed for the optimization of the synthesis conditions of an anion exchange membrane. A novel chlorinated‐polypropylene heterogeneous anion exchange membrane was made via phase inversion. A nonionic surfactant was incorporated into the composition as an additive to enhance the membrane properties. The membrane performance was measured in terms of ion exchange capacity (IEC) and permselectivity. An experimental design was used to quantify the effects of variables including the ratio of resin/polymer, the ratio of additive/total solid, and the ratio of solvent/polymer, on IEC and permselectivity. For each function, a quadratic model was developed to correlate the relationship between variables and the response. The results demonstrated the accuracy of the two models. The anion exchange membrane with the best combination of a high IEC and high permselectivity was synthesized with a solvent/polymer ratio of 18.63 (v/w), resin/polymer ratio of 1 (w/w), and additive/total solid ratio of 0.02 (w/w). © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39888.     

Experimental analysis and simulation of the gas transport in dense Hyflon AD60X membranes: Influence of residual solvent

The present paper discusses the tendency of solution-cast Hyflon^® AD membranes to retain unexpectedly high amounts of solvent, the possible reasons of this phenomenon and its effect on the membrane performance. Dense membranes, prepared by solution-casting and subsequent evaporation, showed large differences in their thermal, mechanical and transport properties, depending on the residual solvent content. Complete solvent removal required heating under vacuum up to well above the glass transition temperature. Analysis of the permeability, diffusion and solubility coefficients of six permanent gases showed that plasticization by the residual solvent reduces the permselectivity and increases the permeability.Data of solution-cast membranes after complete solvent removal compare well with those of a melt-pressed sample. Experimental gas transport parameters were confronted with simulated data, obtained by the Gusev-Suter Transition-State Theory (TST) method and by molecular dynamics (MD) simulations. ¹H High Resolution Magic Angle Spinning Nuclear Magnetic Resonance spectroscopic analysis of the residual solvent in the polymer matrix did not reveal a particular interaction between polymer and solvent, suggesting that the solvent retention is mainly diffusion controlled.