A design of composite hollow fiber membranes with tunable performance and reinforced mechanical strength

A new design of hollow fiber membranes with high mechanical strength, great surface area per volume ratio and tunable filtration performance is presented. This newly developed hollow fiber membrane was produced by an intensified production process, in which the processes of thermally induced phase separation (TIPS), non‐solvent induced phase separation (NIPS), and interfacial polymerization (IP) were combined. PVDF (polyvinylidene difluoride) hollow fiber membranes (produced by TIPS) were used as support substrates. Afterwards, PES (polyethersulfone) (made by NIPS) and PA (polyamide) layers (manufactured by IP) were coated one by one. The pure water permeability, molecular weight cut off (MWCO), salt rejection, tensile stress together with surface and cross‐sectional morphology indicate that the properties of the hollow fiber membranes can be easily adjusted from microfiltration‐like to nanofiltration‐like membranes only by varying the presence of the IP step and the concentration in the PES layer in the production system. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41247.     

Thermally induced phase separation and electrospinning methods for emerging membrane applications: A review

In this review, thermally induced phase separation (TIPS) and electrospinning methods for preparation of fluoropolymer membranes are assessed, particularly for the polyvinylidene fluoride (PVDF) and polyethylene chlorotrifluoroethylene membranes. This review focuses on controlling the membrane morphology from the thermodynamic and kinetic perspectives to understand the relationship between the membrane morphology and fabrication parameters. In addition, the current status of the nonsolvent induced phase separation (NIPS) method and the combined NIPS‐TIPS (N‐TIPS) method, which is a new emerging fabrication method, are discussed. The past literature data are compiled and an upperbound curve (permeability vs. tensile strength) is proposed for the TIPS‐prepared PVDF membranes. Furthermore, the key parameters that control and determine the membrane morphology when using the electrospinning method are reviewed. Exploiting the unique advantages of the electrospinning method, our current understanding in controlling and fine‐tuning the PVDF crystal polymorphism (i.e., β‐phase) is critically assessed. © 2015 American Institute of Chemical Engineers AIChE J, 62: 461–490, 2016     

Fabrication of cellulose acetate ultrafiltration membrane with diphenyl ketone via thermally induced phase separation

With diphenyl ketone as diluent, cellulose acetate (CA) ultrafiltration (UF) membrane with a bicontinuous structure was prepared via thermally induced phase separation (TIPS) method. The liquid–liquid phase separation region of CA/diphenyl ketone system was measured and the maximum corresponding polymer concentration was approximately 53 wt %. The effects of polymer concentration, coarsening time and coarsening temperature on the morphologies, and mechanical properties of CA membranes were investigated systematically. As the polymer concentration increased from 15 to 30 wt %, the bicontinuous structure could be obtained and the tensile strength of CA membranes increased from 3.92 to 30.17 MPa. With the increase of coarsening time, the thickness of dense skin layer and the asymmetry of cross‐section reduced. However, excess coarsening rendered the membrane morphology evolved from a bicontinuous structure to a cellular structure. When the coarsening time was 5 min, the bicontinuous structure in cross‐section showed good interconnectivity and the dense skin layer exhibited a thin thickness of 2 μm. The fabricated CA hollow fiber UF membrane exhibited a high tensile strength of 31.00 MPa and rejection of 96.10% for dextran 20 kDa. It is indicated that diphenyl ketone is a competitive diluent to prepare CA membranes with excellent performance via TIPS. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42669.     

Preparation and properties of poly (vinylidene fluoride) membranes via the low temperature thermally induced phase separation method

Tensile strength is of paramount importance to poly (vinylidene fluoride) (PVDF) membranes in expanding their industrial application. In this paper, porous PVDF membranes with higher tensile strength were prepared by the low temperature thermally induced phase separation (LT-TIPS) method. The effects of mixed diluents (MD) composition on the morphology, polymorphism, and tensile strength of such prepared flat sheet membranes were investigated. The competition of membrane formation mechanisms between the nonsolvent induced phase separation (NIPS) and TIPS was demonstrated by observing the membrane morphology in the LT-TIPS process. It was found that the tensile strength was improved by suppressing the formation of finger-like macrovoids and spherulitic morphologies through adjusting the composition of MD. PVDF crystallized into α phase for all the investigated cases, and as the MD became poorer, the total crystallinity increased slightly. Based on these experimental results, PVDF hollow fiber membranes were fabricated via LT-TIPS. The influences of MD composition and polymer concentration on the morphology, water permeability and tensile strength of the formed hollow fiber membranes were studied. The properties of optimized hollow fiber membranes associated with the surface and cross-section morphologies were promising and the performance can be further enhanced in future work.     

Preparation of PVDF/poly(tetrafluoroethylene‐co‐vinyl alcohol) blend membranes with antifouling propensities via nonsolvent induced phase separation method

Poly(vinylidene fluoride) (PVDF) was blended with a new amphiphilic copolymer, poly(tetrafluoroethylene‐co‐vinyl alcohol) [poly(TFE‐VA)], via non‐solvent induced phase separation (NIPS) method to make membranes with superior antifouling properties. The effects of the VA/TFE segment ratio of the copolymer and the copolymer/PVDF blend ratio on the properties of the prepared membranes were studied. Membranes with similar water permeabilities, surface pore sizes, and rejection properties were prepared and used in bovine serum albumin (BSA) filtrations with the same initial water flux and almost the same operating pressure, to evaluate the sole effect of membrane material on fouling propensity. While the VA/TFE segment ratio strongly affected the membrane antifouling properties, the effects of the copolymer/PVDF blending ratio were not so drastic. Membrane surface hydrophilicity increased, and BSA adsorption and fouling decreased upon blending a small amount of amphiphilic copolymer with a high VA/TFE segment ratio with PVDF (copolymer/PVDF blending ratio 1:5). © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43780.     

Formation of an interconnected lamellar structure in PVDF membranes with nanoparticles addition via solid‐liquid thermally induced phase separation

Novel microporous membranes were prepared via thermally induced solid‐liquid (S‐L) phase separation of mixtures containing poly(vinylidene fluoride) (PVDF)/diphenyl ketone (DPK)/nanoparticles [such as montmorillonite (MMT) and polytetrafluoroethylene (PTFE)] in diluted systems with a mass ratio of 29.7/70/0.3 wt %. The crystallization and melting characteristics of these diluted systems were investigated by polarizing optical microscopy (POM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and wide angle X‐ray diffraction (WAXD). The nanoparticle structure and the interaction between PVDF chains and nanoparticle surfaces determined the crystallization behavior and morphology of the PVDF membrane. The addition of MMT and PTFE had a significant nucleation enhancement on the crystallization of PVDF accompanied by S‐L phase separation during the thermally induced phase separation (TIPS) process. It was observed that an interconnected lamellar structure was formed in these two membranes, leading to a higher tensile strength compared with that of the reference membrane without nanoparticles addition. Additionally, addition of MMT facilitates the fiber‐like β phase crystal formation, resulting in the highest elongation at break. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Formation of a bicontinuous structure membrane of polyvinylidene fluoride in diphenyl ketone diluent via thermally induced phase separation

The polyvinylidene fluoride (PVDF)‐diphenyl ketone (DPK) mixture was studied as a new system to prepare PVDF membranes via thermally induced phase separation (TIPS). The phenomena of liquid–liquid phase separation was found in this mixture when the temperature of mixture was decreasing and the PVDF concentration was less than 30 wt %. Using DPK as diluent, PVDF membrane with bicontinuous structure was obtained without necessity to add a nonsolvent or a stretching process further. The phase diagram of PVDF‐DPK system was also constructed to help investigate the effect of PVDF concentration and coarsening temperature on morphology of resulting membrane. The experiments showed that high coarsening temperatures and low PVDF concentrations resulted in the formation of the large pore size membrane. The strength of the wet membrane was decreasing with decreasing PVDF concentration. On condition that the PVDF concentration was larger than 30 wt %, thermally induced solid–liquid separation occurred and bicontinuous structure disappeared. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Engineering design of outer‐selective tribore hollow fiber membranes for forward osmosis and oil‐water separation

Outer‐selective thin‐film composite (TFC) hollow fiber membranes offer advantages like less fiber blockage in the feed stream and high packing density for industrial applications. However, outer‐selective TFC hollow fiber membranes are rarely commercially available due to the lack of effective ways to remove residual reactants from fiber's outer surface during interfacial polymerization and form a defect‐free polyamide film. A new simplified method to fabricate outer‐selective TFC membranes on tribore hollow fiber substrates is reported. Mechanically robust tribore hollow fiber substrates containing three circular‐sector channels were first prepared by spinning a P84/ethylene glycol mixed dope solution with delayed demixing at the fiber lumen. The thin wall tribore hollow fibers have a large pure water permeability up to 300 L m^?2 h^?1 bar^?1. Outer‐selective TFC tribore hollow fiber membranes were then fabricated by interfacial polymerization with the aid of vacuum sucking to ensure the TFC layer well‐attached to the substrate. Under forward osmosis studies, the TFC tribore hollow fiber membrane exhibits a good water flux and a small flux difference between active‐to‐draw (i.e., the active layer facing the draw solution) and active‐to‐feed (i.e., the active layer facing the feed solution) modes due to the small internal concentration polarization. A hyperbranched polyglycerol was further grafted on top of the newly developed TFC tribore hollow fiber membranes for oily wastewater treatment. The membrane displays low fouling propensity and can fully recover its water flux after a simple 20‐min water wash at 0.5 bar from its lumen side, which makes the membrane preferentially suitable for oil‐water separation. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4491–4501, 2015     

Fabrication of poly(vinylidene fluoride) membrane via thermally induced phase separation using ionic liquid as green diluent

Ionic liquid(IL), 1-butyl-3-methylimidazolium hexafluorophosphate([BMIM]PF6) as a new and environmentally friendly diluent was introduced to prepare poly(vinylidene fluoride)(PVDF) membranes via thermally induced phase separation(TIPS). Phase diagram of PVDF/[BMIM]PF6 was measured. The effects of polymer concentration and quenching temperature on the morphologies, properties, and performances of the PVDF membranes were investigated. When the polymer concentration was 15 wt%, the pure water flux of the fabricated membrane was up to nearly 2000 L·m~(-2)·h~(-1), along with adequate mechanical strength. With the increasing of PVDF concentration and quenching temperature, mean pore size and water permeability of the membrane decreased. SEM results showed that PVDF membranes manufactured by ionic liquid(BMIm PF6) presented spherulite structure. And the PVDF membranes were represented as β phase by XRD and FTIR characterization. It provides a new way to prepare PVDF membranes with piezoelectric properties.     

Crystallization behavior of PVDF in PVDF‐DMP system via thermally induced phase separation

The crystallization behavior of PVDF (poly (vinylidene) fluoride) in PVDF‐dimethylphthalate(DMP) system was investigated in the liquid–liquid (L–L) phase separation region, solid–liquid (S–L) phase separation region and different quenching conditions via thermally induced phase separation (TIPS). Differential scanning calorimetry (DSC) indicated the crystallinity of PVDF in PVDF‐DMP system increased in the early stage of phase separation and polymer‐rich phase crystallized completely in the late stage of phase separation. The scanning electron microscopy (SEM) showed the different quenching temperatures had effects on the spherulite size of polymer rich phase and the ultimate membrane structure in the different phase separation regions. The wide angle X‐ray diffraction (WAXD) was used to quantify the crystal structure of PVDF in PVDF‐DMP system. The α‐phase PVDF was obtained when the system quenched to different temperatures above 40°C, and the area of diffraction peaks changed when quenching temperatures changed. While the β‐phase PVDF was formed when PVDF‐DMP system was quenched form liquid nitrogen and crystallized for 24 h in 25°C water bath. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3714–3719, 2006     

Recognition properties of poly(vinylidene fluoride) hollow‐fiber membranes modified by levofloxacin‐imprinted polymers

Through the use of thermal polymerization, poly(vinylidene fluoride) (PVDF) hollow‐fiber membranes modified by a thin layer of molecularly imprinted polymers (MIPs) were developed for the selective separation of levofloxacin. To demonstrate the changes induced by thermal polymerization, PVDF hollow‐fiber membranes with different modification degrees by repeated polymerization were weighed. The total weight of the imprinted membranes increased by 14 μg/cm² after a five‐cycle polymerization. An increase in the membrane weight indicated the deposition of an MIP layer on the external surface of PVDF hollow‐fiber membranes during each polymerization cycle, which was also characterized by scanning electron microscopy. MIP membranes with different degrees of surface modification provided highly selective binding of levofloxacin. Both hollow‐fiber MIP membranes and nonimprinted membranes showed enhanced adsorption of levofloxacin and ofloxacin gradually with an increase in the modification degrees of PVDF hollow‐fiber membranes to a maximum value followed by a decrease. These results indicate that thermal polymerization indeed produces an MIP layer on the external surface of PVDF hollow‐fiber membranes and that it is feasible to control the permeability by repeated polymerization cycles. Different solvent systems in the permeation experiments were used to understand the hydrophobic interaction as one of the results of the binding specificity of MIP membranes. Selective separation was obtained by multisite binding to the template via ionic, hydrogen‐bond, and hydrophobic interactions. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Effect of diluent on poly(ethylene‐co‐vinyl alcohol) hollow‐fiber membrane formation via thermally induced phase separation

Poly(ethylene‐co‐vinyl alcohol) hollow‐fiber membranes with a 44 mol % ethylene content were prepared by thermally induced phase separation. A mixture of 1,3‐propanediol and glycerol was used as the diluent. The effects of the ratio of 1,3‐propanediol to glycerol in the diluent mixture on the phase diagram, membrane structure, and membrane performance were investigated. As the ratio increased, the cloud point shifted to lower temperatures, and the membrane structure changed from a cellular structure due to liquid–liquid phase separation to a particulate structure due to polymer crystallization. Better pore connectivity was obtained in the hollow‐fiber membrane when the ratio of 1,3‐propanediol to glycerol was 50:50, and the membrane showed about 100 times higher water permeability than the membrane prepared with pure glycerol. For the prepared hollow‐fiber membrane, the solute 20 nm in diameter was almost rejected. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 219–225, 2005     

Effects of air‐cooling on skin cells of hollow‐fiber membranes prepared via thermally induced phase separation

A new coarsening model was established to describe the growth of skin droplets on a hollow‐fiber membrane prepared by liquid–liquid thermally induced phase separation. Forced air‐cooling was performed to trigger the phase separation. The heat and mass transfer processes of the skin layer are considered simultaneously. With the aid of a transfer model and data from the phase diagram, the skin cell sizes could be calculated. The effects of air velocity, air temperature, and dimensions of the hollow fiber on the skin cell sizes of the isotactic polypropylene–di‐n‐butyl phthalate–dioctyl phthalate system were investigated. The growth of the skin droplets follows an exponential relationship with time, and the exponent increases with the decrease in the polymer content. All exponents are close to 1/3. The air temperature plays less important roles on the skin cell sizes as compared with those of the air velocity and fiber dimension. Calculated values of cell sizes agree well with the experimental values. POLYM. ENG. SCI., 55:1661–1670, 2015. © 2014 Society of Plastics Engineers     

Structure control of asymmetric poly(vinyl butyral)‐TiO2 composite membrane prepared by nonsolvent induced phase separation

Poly(vinyl butyral) (PVB)‐TiO₂ composite hollow fiber membranes were prepared via nonsolvent induced phase separation (NIPS). The membrane had a skin layer on both the outer and inner surface at the initial stage after membrane preparation. However, the outer surface became porous with the passage of time, as the polymer in the membrane's outer surface was decomposed by the photocatalysis of TiO₂. The initial water permeability increased with the increase of TiO₂ content. Furthermore, for all the membranes, as time elapsed the water permeabilities increased and became constant after about 15 days, which was in accordance with the alteration on the membrane's outer surface. Despite decomposition of the polymer on the outer surface, particle rejection hardly changed because the inner surface kept the original structure. Thus, addition of TiO₂ to the membrane is a useful way to improve water permeability while maintaining particle rejection. The clear asymmetric structure with both porous structure at the outer surface and skin layer at the inner surface was achieved by the addition of TiO₂. Therefore, the addition of TiO₂ is a new method for achieving the high porosity at the outer surface of the hollow fiber membrane. In addition, tensile strength and elasticity kept constant over time and were higher than those of original PVB membranes. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Preparation of polypropylene microfiltration membranes via thermally induced (solid–liquid or liquid–liquid) phase separation method

Polypropylene (PP) hollow fiber microfiltration membranes with excellent performance were successfully prepared from the PP‐binary diluent system via thermally induced liquid–liquid (L–L) phase separation method. The binary diluent consisted of myristic acid and diphenyl carbonate. The effects of the binary diluent on phase separation and membrane structure were systematically investigated. With the decrease in the weight ratio of myristic acid to diphenyl carbonate, the Flory–Huggins interaction parameter between PP and the binary diluent became more positive, and the mechanism of phase separation changed from solid–liquid (S–L) to L–L. This resulted in the membrane structure changing from spherulitic to bicontinuous. Moreover, as the weight ratio of myristic acid to diphenyl carbonate decreased from 11/9 to 2/3, the L–L phase separation region kept enlarging while the viscosity of the whole system became higher. The pore size of the cross‐section increased due to the longer coarsening time while the surface pore size decreased due to the higher viscosity of the system. The bulk porosity of resultant PP membranes was mostly higher than 70% and pure water flux were generally larger than 650 L m^?2 h^?1. In addition, the PP hollow fiber microfiltration membrane possessed excellent mechanical properties (tensile strength of 3.47 MPa and elongation of 118%) and good separation performance (rejection to PEO (M_w = 1000 kDa) of 94.6%) when the weight ratio of myristic acid to diphenyl carbonate was 2/3. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42490.     

Electrospun fiber membranes from blends of poly(vinylidene fluoride) with fouling‐resistant zwitterionic copolymers

Nonwoven super‐hydrophobic fiber membranes have potential applications in oil–water separation and membrane distillation, but fouling negatively impacts both applications. Membranes were prepared from blends comprising poly(vinylidene fluoride) (PVDF) and random zwitterionic copolymers of poly(methyl methacrylate) (PMMA) with sulfobetaine methacrylate (SBMA) or with sulfobetaine‐2‐vinylpyridine (SB2VP). PVDF imparts mechanical strength to the membrane, while the copolymers enhance fouling resistance. Blend composition was varied by controlling the PVDF‐to‐copolymer ratio. Nonwoven fiber membranes were obtained by electrospinning solutions of PVDF and the copolymers in a mixed solvent of N,N‐dimethylacetamide and acetone. The PVDF crystal phases and crystallinities of the blends were studied using wide‐angle X‐ray diffraction and differential scanning calorimetry (DSC). PVDF crystallized preferentially into its polar β‐phase, though its degree of crystallinity was reduced with increased addition of the random copolymers. Thermogravimetry (TG) showed that the degradation temperatures varied systematically with blend composition. PVDF blends with either copolymer showed significant increase of fouling resistance. Membranes prepared from blends containing 10% P(MMA‐ran‐SB2VP) had the highest fouling resistance, with a fivefold decrease in protein adsorption on the surface, compared to homopolymer PVDF. They also exhibited higher pure water flux, and better oil removal in oil–water separation experiments. © 2018 Society of Chemical Industry     

Effect of the exposure time on the structure and performance of hydrophobic polydimethylsiloxane–poly(vinylidene fluoride) membranes via a non‐solvent‐induced phase separation process in a clean room

The objective of this study was to investigate the effects of the exposure time on the properties and permeability of polydimethylsiloxane (PDMS)–poly(vinylidene fluoride) (PVDF) blend hydrophobic microporous membranes, which were fabricated via a non‐solvent‐induced phase separation process at 25 °C and 60% relative humidity in a clean‐room circumstance. For the prepared PDMS–PVDF membranes, the membrane morphologies were observed by scanning electron microscopy. Crystalline structures were observed by X‐ray diffraction. Pore structures were analyzed by membrane porosity and mean pore size. Hydrophobicity was measured by contact angle measurement, and the mechanical properties were characterized by tensile strength testing. Our study results show that with increasing exposure time from 10 to 110 s, all of the membranes showed a similar pore structure: a spongelike substrate layer with a thin realm of fingerlike structures under the top surface. Phase separation between PDMS and PVDF occurred. The membrane porosity and mean pore radius decreased, and the membrane thickness increased. The membrane hydrophobicity decreased, and the mechanical properties first increased and then decreased. In addition, vacuum membrane distillation experiments were conducted. With the increase in the exposure time from 10 to 110 s, the membrane permeate flux decreased from 16.54 to 6.65 kg m⁻²·h⁻¹, and the salt rejection was higher than 99.9%. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43842.     

Porous polyphenylene sulfide membrane with high durability against solvents by the thermally induced phase‐separation method

Porous polyphenylene sulfide membranes were prepared as new solvent‐resistant membranes by the thermally induced phase‐separation (TIPS) method. Porous structures were either formed by solid–liquid phase separation (polymer crystallization) or liquid–liquid phase separation. The effects of solvents, cooling rates, and polymer concentrations on the porous structures were investigated. Various characteristics of pore structure can be obtained with suitable diluents and cooling rates using the TIPS method. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2959–2966, 2006     

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.     

Preparation of PVDF/CaCO3 composite hollow fiber membrane via a thermally induced phase separation method

The spinnabiliy of polyvinylidene fluoride (PVDF)/CaCO₃ microparticle dope solution were explored via a thermally induced phase separation process, and composite hollow fiber (CHF) membranes were prepared successfully. The experiment results showed that the self‐supporting property of CHF got improved after adding CaCO₃ at low concentration of PVDF, so the control of spinning process became easy. The effects of CaCO₃ on the structure and properties of hollow fiber membrane were investigated in terms of morphology, water flux, porosity, breakage strength, and crystallization. The results indicated that CHF presented a uniform bicontinuous structure. The permeability and porosity of CHF increased obviously when CaCO₃ was removed by acid, while the breakage strength decreased slightly. CHF presented a good drying stability. POLYM. COMPOS., 34:1204–1210, 2013. © 2013 Society of Plastics Engineers