Formation of porous poly(ethylene‐co‐vinyl alcohol) membrane via thermally induced phase separation

Crystalline poly(ethylene‐co‐vinyl alcohol) (EVOH) membranes were prepared by a thermally induced phase separation (TIPS) process. The diluents used were 1,3‐propanediol and 1,3‐butanediol. The dynamic crystallization temperature was determined by DSC measurement. No structure was detected by an optical microscope in the temperature region higher than the crystallization temperature. This means that porous membrane structures were formed by solid–liquid phase separation (polymer crystallization) rather than by liquid–liquid phase separation. The EVOH/butanediol system showed a higher dynamic crystallization temperature and equilibrium melting temperature than those of the EVOH/propanediol system. SEM observation showed that the sizes of the crystalline particles in the membranes depended on the polymer concentration, cooling rate, and kinds of diluents. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2449–2455, 2001     

Effects of cooling temperature and aging treatment on the morphology of nano‐ and micro‐porous poly(ethylene‐co‐vinyl alcohol) membranes by thermal induced phase separation method

Poly(ethylene‐co‐vinyl alcohol) (EVOH 32) / 1,3‐propanediol mixtures are processed by thermally induced phase separation for the formation of porous membranes. The crystallization line was determined both by the cloud‐point and DSC methods. Two precursor solution compositions, four quench temperatures and various aging times were explored. It is found possible to generate both polymer‐crystallization controlled morphologies (for high quenches and/or sufficiently aged dopes), especially globular microporous ones, and novel nano‐scale porous morphologies dominated by intra‐binodal phase separation (for low quenches and limited or no precursor solution aging). Structural characterization of the membranes was accomplished via application of scanning electron microscopy and wide angle X‐ray diffraction. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40374.     

Preparation and characterization of poly(ethylene‐co‐vinyl alcohol) membranes via thermally induced liquid–liquid phase separation

Porous membranes were prepared through the thermally induced phase separation of poly(ethylene‐co‐vinyl alcohol) (EVOH)/glycerol mixtures. The binodal temperature and dynamic crystallization temperature were determined by optical microscopy and differential scanning calorimetry measurements, respectively. It was determined experimentally that the liquid–liquid phase boundaries were shifted to higher temperatures when the ethylene content in EVOH increased. For EVOHs with ethylene contents of 32–44 mol %, liquid–liquid phase separation occurred before crystallization. Cellular pores were formed in these membranes. However, only polymer crystallization (solid–liquid phase separation) occurred for EVOH with a 27 mol % ethylene content, and the membrane morphology was the particulate structure. Scanning electron microscopy showed that the sizes of the cellular pores and crystalline particles in the membranes depended on the ethylene content in EVOH, the polymer concentration, and the cooling rate. Furthermore, the tendency of the pore and particle sizes was examined in terms of the solution thermodynamics of the binary mixture and the crystallization kinetics. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 853–860, 2003     

Novel phase inversion process for symmetric membrane formation through thermal quenching of polymer solution in same solvent

A new and useful form of phase inversion for the formation of porous polymeric membranes is presented herein. As in the case of thermally induced phase separation (TIPS), this new form involves only two components (polymer and solvent) and a thermal quench; here the quench is accomplished via immersion in a cold bath of the micromolecular component (solvent) of the dope. Ιn terms of a fixed‐pressure two‐component phase diagram the quench is a non‐vertical one. We will refer to the new method as cold‐solvent induced phase separation (CIPS). In the present work we study mainly the poly(ethylene‐co‐vinyl alcohol)/1,3‐propanediol system which leads to bi‐continuous structures stemming from a combination of liquid‐liquid demixing and crystallization. In addition, we compare with the case of the Nylon‐l2/formic acid system that we have briefly considered before and study further herein; the consequences of the TIPS to CIPS shift of method are different for the two systems, and the two situations are representative of two general possibilities. We also report general properties such as porosity, tensile strength, water permeation flux, and crystallinity of the produced poly(ethylene‐co‐vinyl alcohol) membranes. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42282.     

Fabrication and characterization of maleic anhydride grafted polypropylene membranes with high antifouling properties

In this study, maleic anhydride grafted polypropylene microporous flat‐sheet membranes were prepared via a thermally induced phase separation method with a mixture of dibutyl phthalate and dioctyl phthalate as a diluent. The effects of the polymer composition and coagulation bath temperature on the morphology and performance of the fabricated membranes were investigated. The hydrophilicity results of the membranes demonstrated that membrane modification reduced the water contact angle by about 45°, whereas the pure water flux was enhanced about four times. The antifouling behavior of the fabricated membranes was also investigated in a membrane bioreactor. The results show that the pure water flux, membrane pore size, and porosity decreased, whereas the antifouling performance was improved with increasing polymer concentration and decreasing bath temperature. Finally, the results reveal that the removal efficiency of contaminates was independent on the membrane characterization and was done exclusively through biological removal. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43857.     

Influence of the coagulation‐bath temperature on the phase‐separation process of poly(vinylidene fluoride)‐graft‐poly(N‐isopropylacrylamide) solutions and membrane structures

A poly(vinylidene fluoride)‐graft‐poly(N‐isopropylacrylamide) (PVDF‐g‐PNIPAAm) copolymer was synthesized, and flat‐sheet membranes were prepared via the phase‐inversion method with N,N‐dimethylformamide (DMF) as the solvent and water as the coagulation bath. The effects of the coagulation‐bath temperature on poly(vinylidene fluoride) (PVDF)/DMF/water and PVDF‐g‐PNIPAAm/DMF/water ternary systems were studied with phase diagrams. The results showed that the phase‐separation process could be due to the hydrophilicity/hydrophobicity of poly(N‐isopropylacrylamide) at low temperatures, and the phase‐separation process was attributed to crystallization at high temperatures. The structures and properties of the membranes prepared at different coagulation‐bath temperatures were researched with scanning electron microscopy, porosity measurements, and flux measurements of pure water. The PVDF‐g‐PNIPAAm membranes, prepared at different temperatures, formed fingerlike pores and showed higher water flux and porosity than PVDF membranes. In particular, a membrane prepared at 30°C had the largest fingerlike pores and greatest porosity. The water flux of a membrane prepared in a 25°C coagulation bath showed a sharp increase with the temperature increasing to about 30°C. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Preparation of a poly(vinyl chloride) ultrafiltration membrane through the combination of thermally induced phase separation and non‐solvent‐induced phase separation

To regulate the polymer–diluent interaction and control the viscosity of the casting solution, diphenyl ketone (DPK) and a N,N‐dimethylacetamide/N,N‐dimethylformamide mixture were selected as a combined diluent. Poly(vinyl chloride) (PVC) utlrafiltration membranes, which had sufficient mechanical properties for their practical applications because of their bicontinuous spongy structure, were prepared by a combined process of thermally induced phase separation and non‐solvent‐induced phase separation. The phase‐separation mechanism was analyzed. In an air bath, the cast nascent solution immediately transformed into a transparent gel, and liquid–liquid phase separation was induced by a sudden drop in the temperature before crystallization. An ice–water bath was used to coagulate the membrane. The effects of the DPK and PVC concentrations on the membrane structures and performances were mainly investigated. The results show that an increase in the DPK content made the membrane pores change from fingerlike to spongy. Fully spongy pores formed, and the pores size decreased with increasing PVC concentration. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42953.     

Polyamide/polyacrylonitrile thin film composites as forward osmosis membranes

Thin film composites (TFCs) as forward osmosis (FO) membranes for seawater desalination application were prepared. For this purpose, polyacrylonitrile (PAN) as a moderately hydrophilic polymer was used to fabricate support membranes via nonsolvent‐induced phase inversion. A selective thin polyamide (PA) film was then formed on the top of PAN membranes via interfacial polymerization reaction of m‐phenylenediamine and trimesoyl chloride (TMC). The effects of PAN solution concentration, solvent mixture, and coagulation bath temperature on the morphology, water permeability, and FO performance of the membranes and composites were studied. Support membranes based on low PAN concentrations (7 wt %), NMP as solvent and low coagulation bath temperature (0 °C) demonstrated lower thickness, thinner skin layer, more porosity, and higher water permeability. Meanwhile, decreasing the PAN solution concentration lead to higher water permeance and flux and lower reverse salt flux, structural parameter, and tortuosity for the final TFCs. Composites made in N,N‐dimethylformamide presented lower permeance and flux for water and salt and higher salt rejection, structural parameter, and tortuosity. FO assay of the composites showed lower water permeance values in saline medium comparing to pure water. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44130.     

Effect of the ethylene content of poly(ethylene-co-vinyl alcohol) on the formation of microporous membranes via thermally induced phase separation

Porous poly(ethylene-co-vinyl alcohol) (EVOH) membranes were prepared via thermally induced phase separation. The effect of the EVOH ethylene content on the membrane morphology and solute rejection property was investigated. For EVOHs with ethylene contents of 27–44 mol %, polymer crystallization (solid–liquid phase separation) occurred, and the membrane morphology was the particulate structure. However, the liquid–liquid phase separation occurred before crystallization for EVOH with a 60 mol % ethylene content. Cellular pores were formed in this membrane. For the particulate membranes, higher solute rejection and lower water permeance were obtained for EVOH with a lower ethylene content. The membrane formed by the liquid–liquid phase separation showed a sharper solute rejection change with a change in the solute radius than the particulate membranes did. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2583–2589, 2001     

Effects of the preparation conditions on ethylene/vinyl acetate membrane morphology with the use of scanning electron microscopy

In this research, the effects of preparation conditions, including the coagulation bath temperature, polymer solution composition, preliminary drying time, and thickness of cast polymeric films, on the morphology of ethylene/vinyl acetate copolymer membranes were investigated with scanning electron microscopy and nitrogen gas permeability tests. Flat sheet membranes were prepared through a thermal–wet phase‐inversion method. Scanning electron microscopy pictures showed asymmetric structures for some of the membranes. It was also observed that the porosity of the membranes decreased with an increase in the temperature of the coagulation bath and the solvent evaporation period. When the concentration of the polymer solution was increased from 5 to 12 wt %, the nucleation and growth of the solvent‐rich phase replaced the nucleation and growth of the polymer‐rich phase. With an increase in the thickness of the cast polymeric films, the number of macrovoids increased in the membranes. The nitrogen gas permeability of the developed membranes was in good agreement with the scanning electron microscopy results. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Preparation and characterization of nanoporous polysulfone membranes with high hydrophilic property using variation in CBT and addition of tetronic‐1107 surfactant

Asymmetric polysulfone (PSF) membranes were prepared from PSF, Tetronic‐1107, and 1‐methyl‐2‐pyrrolidone (NMP) via immersion precipitation. Pure water was used as the gelation media. The effects of coagulation bath temperature (CBT) (0 and 25°C), and addition of Tetronic‐1107 on the morphology, wettability, and pure water permeation flux (PWF) of the prepared membranes were studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle measurements, and experimental set up. The contact angle measurements demonstrated that the hydrophilicities of the nanoporous PSF membranes were significantly enhanced by addition of a small amount of Tetronic‐1107 surfactant in the casting solution, along with using the lower CBT. It was also found that addition of Tetronic‐1107 in the casting solution along with increasing the CBT from 0 to 25°C incites formation of bigger pores on the top surface and results in formation of membranes with higher thickness and more porous structure in the sublayer. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Investigation of membrane preparation condition effect on the PSD and porosity of the membranes using a novel image processing technique

A totally computerized image processing program package is developed to analyze the SEM images of membrane surface and cross‐section. Pore size distribution and porosity of the fabricated membranes are determined using the proposed image processing procedure. Furthermore, effect of coagulation bath temperature on the morphology and mechanical properties (such as tensile strength, strain break, tensile energy absorbent, and tensile stiffness) of Polysulfone (PSf) membranes are investigated. The results reveal that the mechanical properties are higher when N‐methyl‐2‐pyrrolidone (NMP) is used as solvent. Also, an increase in the coagulation bath temperature caused a monotonous increase in the mean pore size value of Dimethylformamide (DMF)‐based membranes. However, mean pore size curve has a maximum when NMP is used as solvent. Also, porosity of the fabricated membranes increased when coagulation bath temperature increased. For the NMP‐base membranes, pore's diameter was in the range of 0–5 μm. However, DMF‐based membranes have pore size value of smaller than 1 μm when the precipitation medium is kept at 8°C. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39899.     

Gating Characteristics of Thermo‐Responsive Membranes with Grafted Linear and Crosslinked Poly(N‐isopropylacrylamide) Gates

Thermo‐responsive porous membranes with grafted linear and crosslinked poly(N‐isopropylacrylamide) (PNIPAM) gates are successfully prepared at temperatures above and below the lower critical solution temperature (LCST) of PNIPAM by using a plasma‐induced grafting polymerization method, and the effects of operation pressure and grafting temperature on the thermo‐responsive gating characteristics of the prepared membranes are investigated systematically. The fluxes of water through the grafted membranes increase simply with increasing the operation pressure no matter whether the environmental temperature is 40 °C or 25 °C. Under high operation pressure (e.g., higher than 0.14 MPa), the grafted linear PNIPAM gates deform to a certain extent, whereas the grafted crosslinked PNIPAM gates do not deform. For both membranes with grafted linear and crosslinked PNIPAM gates, the membranes prepared at 25 °C (below the LCST of PNIPAM) show larger thermo‐responsive gating coefficients than those prepared at 40 °C (above the LCST of PNIPAM), which results from different distributions of grafted PNIPAM gates in the membrane pores. When the PNIPAM gates are grafted at 25 °C, the grafted layer near the membrane surface is much thicker than that inside the membrane pores; on the other hand, when the PNIPAM gates are grafted at 40 °C, the grafted layer is homogeneously formed throughout the whole pore length. Both linear and crosslinked grafted PNIPAM gates in the membrane pores exhibit stable and repeatable thermo‐responsive “open‐close” switch performances under the operation pressure of 0.26 MPa. The results in this study provide valuable guidance for designing, fabricating, and operating thermo‐responsive gating membranes with desirable performances.     

Effect of thermal crosslinking on the properties of sulfonated poly(phenylene sulfone)s as proton conductive membranes

The synthesis and characterization of crosslinked aromatic polymer membranes with high ion exchange capacity (IEC) values are reported. Through aromatic nucleophilic substitution polycondensation and the subsequent sulfonation reaction, the highly sulfonated polymers SPPSU‐2S and SPPSU‐4S with high molecular weight (M_n = 138–145 kDa, M_w = 200–279 kDa) and well‐defined structures were synthesized. By solution casting and thermal annealing treatment, flexible crosslinked membranes with high solvent insolubility were obtained. The membranes exhibited mechanical and chemical stability as confirmed by dynamic mechanical analysis (DMA) and conductivity measurement. The crosslinked SPPSU‐4S membrane with IEC = 3.20 meq/g showed the highest proton conductivity of 0.163 S/cm at 120 °C, 90% RH, and improved thermal stability compared with its precursor (uncrosslinked) membrane. The results show that simple annealing method could improve significantly membranes properties of highly sulfonated aromatic polymers. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44218.     

Polyphosphazene membranes. III. Solid‐state characterization and properties of sulfonated poly[bis(3‐methylphenoxy)phosphazene]

Poly[bis(3‐methylphenoxy)phosphazene] was sulfonated in a solution with SO₃ and solution‐cast into 100–200‐μm‐thick membranes from N,N‐dimethylacetamide. The degree of polymer sulfonation was easily controlled and water‐insoluble membranes were fabricated with an ion‐exchange capacity (IEC) as high as 2.1 mmol/g. For water‐insoluble polymers, there was no evidence of polyphosphazene degradation during sulfonation. The glass transition temperature varied from −28°C for the base polymer to −10°C for a sulfonated polymer with an IEC of 2.1 mmol/g. The equilibrium water swelling of membranes at 25°C increased from near zero for a 0.04‐mmol/g IEC membrane to 900 % when the IEC was 2.1 mmol/g. When the IEC was < 1.0 mmol/g, SO₃ attacked the methylphenoxy side chains at the para position, whereas sulfonation occurred at all available aromatic carbons for higher ion‐exchange capacities. Differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized microscopy showed that the base polymer, poly[bis(3‐methylphenoxy)phosphazene], was semicrystalline. For sulfonated polymers with a measurable IEC, the 3‐dimensional crystal structure vanished but a 2‐dimensional ordered phase was retained. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 387–399, 1999     

Thermal and dielectric properties of electrospun fiber membranes from polyimides with different structural units

In this work, electrospinning technique was used to prepare low dielectric constant membranes. First, three kinds of polyimide (PI) fiber membranes were fabricated by electrospinning of poly(amic acid) (PAA) solutions which are from polycondensation of 4,4′‐oxidianiline (ODA) and three dianhydrides, pyromellitic dianhydride (PMDA), 2,2′‐bis(3,4‐dicarboxyphenyl) hexafluropropane dianhydride (6FDA) and 1,2,4,5‐cyclohexanetetracarboxylic dianhydride(HPMDA), followed by imidization at higher temperature. The relationship of the fiber morphology, thermostability and dielectric properties of the membranes with the polymer structure were discussed. Under the same conditions, PAAs with more flexible structure are easier to form low viscosity solution and fabricate high pore fraction membranes which are low dielectric constant materials. Under the coupling effect of fluorine‐containing groups and contribution of pores, the dielectric constant of 6FDA‐containing PI is lowered to 1.21 at 1 KHz with lower dielectric loss which accords with the calculated one. Also the 5% weight loss temperature of the three kinds of PIs is all higher than 400°C. The formed electrospun membranes are thermostable low dielectric constant materials. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43081.     

Cellulose acetate reverse osmosis membranes: Optimization of preparation parameters

Asymmetric cellulose acetate based membranes usually employed in reverse osmosis as well as in separations in aqueous systems can possibly be applied in the so‐called salinity process of energy generation. For these applications, membranes with a relatively high water permeability (sometimes also called water flux) and low salt permeability (or high salt rejection) are required. In this study the authors present the optimization of such membranes, which concerns the preparation parameters. The membranes studied were prepared from a solution whose composition were previously optimized.⁴ The authors concluded that the optimum preparation parameters are as follows: thickness of the liquid film of 100 μm; 30 s allowed for evaporation of the solvent; and temperature of coagulation bath of 0–4°C and 80–85°C as annealing temperature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 134–139, 2007     

Asymmetric cellulose acetate dialysis membranes: Synthesis,characterization, and performance

Cellulose acetate (CA) is highly comparable to other synthetic polymer materials and is effective in the hemodialysis process. In this work, asymmetric CA membranes were synthesized with the phase‐inversion method. CA with a molecular weight of 52,000, poly(ethylene glycol) (PEG) with a molecular weight of 400, and 1‐methyl‐2‐pyrrolidone (NMP) were used as the polymer, additive, and solvent, respectively. The effects of the CA and PEG concentrations and coagulation bath temperature (CBT) on the morphology, pure water permeability (PWP), insulin/human serum albumin (HSA) transmission, and finally thermal and chemical stability of the prepared membranes were determined and investigated. In general, increasing the PEG concentration and CBT and reducing the CA concentration resulted in increased PWP and insulin/HSA transmission. Also, these variations facilitated the formation of macrovoids in the membrane sublayer. On the other hand, increasing the PEG and CA concentrations and reducing CBT resulted in increased thermal and chemical stability of the prepared membranes. Also, ratios of 15.5/10/74.5 and 17.5/10/72.5 (w/w) for the CA/PEG/NMP casting solutions and their immersion into coagulation baths with CBTs of 0 and 25°C, respectively, resulted in the preparation of membranes that had not only optimum sieving properties and higher PWP but also thermal and chemical stability better than that of conventional CA hemodialysis membranes. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The impact of nonpolar coagulation bath‐immiscible liquid additives on the polyethersulfone membranes structure and performance

In this study, the impact of several normal alkanes as additives on the structural parameters and the morphologies of flat sheet PES membranes were investigated. The asymmetric membranes were fabricated by phase inversion method. Also n‐hexane, n‐decane, and n‐tetradecane were used as additives. The effects of different concentrations of these additives were investigated by producing the ternary phase diagrams. Further the membranes were characterized by means of solute transport test, contact angle, pure water flux, porosity measurement, and scanning electronic microscopy. It was expected that two distinct phase inversions to be occurred since both the polymer and the additives were insoluble in the coagulation bath. Observations revealed that n‐hexane and n‐decane were less effective compared with n‐tetradecane in terms of binodal shifting toward PES/NMP axis. In conclusion, it was revealed that the coagulation bath immiscible‐liquid additives had a major impact on the membranes structure in order to achieve the enhanced properties. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44509.     

Preparation of a cellulose acetate/organic montmorillonite composite porous ultrafine fiber membrane for enzyme immobilization

Four types of fibrous membranes based on cellulose acetate (CA)—CA membranes with nonporous fibers, CA/organic montmorillonite (O‐MMT) membranes with nonporous fibers, CA membranes with porous fibers, and CA/O‐MMT membranes with porous fibers—were prepared by electrospinning, and then, they were used for enzyme immobilization. The surface morphologies of the composite fibrous membranes were investigated with scanning electron microscopy and transmission electron microscopy. The optimum pH was 3.5 for all of the immobilized enzymes, and the optimum temperature was 50 °C. Compared with the free enzyme, the immobilized enzyme showed better stability for pH and temperature changes. Moreover, the addition of O‐MMT and the pores on the fibers improved the storage stability and the operational stability. Among the four kinds of fibrous membranes, the CA/O‐MMT membranes with porous fibers showed the best stability for the immobilized enzymes. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43818.