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Recent advances in preparation and morphology control of polymeric membranes formed by nonsolvent induced phase separation Da-Ming Wang1,3 and Juin-Yih Lai2,3 Nonsolvent induced phase separation (NIPS) is the most widely used method to prepare membranes for separation processes. Even though it has been used for about five decades, NIPS still attracts much attention nowadays. In the present article, we first discuss some recent advances in the fundamentals of NIPS, including the mechanism of membrane formation and the selection of polymer, solvent, and nonsolvent. We will also review the recent researches about modification of membranes by blending copolymers or inorganic particles in the casting solution. The blending modification technique has drawn extensive attention because it can accomplish membrane fabrication and modification in one step. In addition, reviews are given for the development of three novel morphology control techniques based on NIPS: phase separation micromolding (a method combining lithography and NIPS), the ‘breath figure’ method (a phase separation process induced by water vapor), and a technique based on the combination of NIPS and self-assembly of block copolymers. Addresses 1 Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 2 Department of Chemical Engineering, Chung Yuan University, ChungLi, Taiwan 3 R&D Center for Membrane Technology, Chung Yuan University, Chung-Li, Taiwan Corresponding author: Wang, Da-Ming ([email protected]
Current Opinion in Chemical Engineering 2013, 2:229–237 This review comes from a themed issue on Separation engineering Edited by WS Winston Ho and Kang Li For a complete overview see the Issue and the Editorial Available online 4th May 2013 2211-3398/$ – see front matter, # 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.coche.2013.04.003
Introduction A membrane is a thin interface that acts as a selective barrier between two phases, to regulate the permeation of substances in contact with it so that separation of the substances can be accomplished. For preparation of membranes, the most widely used method is a technique called nonsolvent induced phase separation (NIPS). In the process nonsolvent for polymer is introduced to a homogeneous polymer solution to demix the solution into two phases: a polymer-rich phase that turns into the www.sciencedirect.com
matrix of membranes after the precipitation of the solution, and a polymer-poor phase that forms membrane pores after it is removed from the precipitated solution. Phase separation can be induced in several ways, as demonstrated in Figure 1: by directly immersing a polymer solution into a nonsolvent bath, the wet method or called immersion precipitation; by evaporating a polymer solution for a time period and then immersing it in nonsolvent, the dry/wet method; by evaporating a polymer solution that contains low-volatility nonsolvent and high-volatility solvent to result in an increase in the nonsolvent/solvent ratio and phase separation; or by absorbing nonsolvent vapor (usually water vapor) from the ambient air, the vapor-induced-phase-separation (VIPS) process. During the membrane formation process, because of the solvent evaporation, nonsolvent absorption, or exchange of solvent and nonsolvent, a polymer concentration gradient occurs across the casting solution. Therefore, the technique can form membranes with asymmetric structure. For example, with the dry/wet method, a membrane with a thin dense skin on top of porous substructures can be prepared. Loeb and Sourirajan  adopted the process to prepare asymmetric cellulose acetate membranes for reverse osmosis (RO), which made the commercialization of RO possible: the thin skin layer provided high salt rejection and economically acceptable flux and the porous substrate gave suitable mechanical strength. Dry/wet methods similar to the Loeb–Sourirajan technique have been developed to prepare membranes with thin skins for gas separation and pervaporation processes. Asymmetric membranes, with thin skins on top of porous substructures, can also be prepared by the wet method. But, without solvent evaporation, the skins formed may have defects and are usually not as dense as those of the membranes prepared by the dry/wet method. Most of the commercial ultrafiltration membranes are prepared by the wet method. In addition, the VIPS process can be adopted to prepare microfiltration membranes. An advantage of NIPS is that it can be easily modified to prepare hollow-fiber membranes, which have high membrane packing density. Figure 2 is a schematic illustration of the hollow-fiber spinning process. Polymer solution (dope) is extruded through the annular region of a spinneret while bore fluid (a nonsolvent) is forced through the inner capillary of the spinneret to form the inner bore of Current Opinion in Chemical Engineering 2013, 2:229–237
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Casting Casting solution Polymer/solvent/additives
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Preparation of membranes by nonsolvent induced phase separation.
fibers. The polymer solution is extruded into a coagulation (nonsolvent) bath for NIPS to occur, forming porous structure in the fiber walls. NIPS is widely used not only to prepare polymer hollow fibers but also to fabricate inorganic and organic–inorganic hybrid hollowfiber membranes. Even though NIPS has been used to prepare membranes for about five decades, it still attracts much attention nowadays. Several review papers [2,3,4] published recently gave an overview of the development and applications of NIPS. In the present article, we concentrate only on the most recent development on membrane formation mechanism, blending modification of membranes, and Current Opinion in Chemical Engineering 2013, 2:229–237
novel morphology control techniques based on NIPS. We will first discuss the recent research results about the fundamentals of NIPS, focusing on the phase separation mechanism, the coarsening of phase-separated domains, and the formation of macrovoids. In addition to the recent advances in the fundamentals, the research trends on membrane modification and morphology control are included. Modification of membranes is usually needed to improve the separation performance of membranes. Blending modification, which accomplishes membrane fabrication and modification in one step by blending modifying moieties in casting solution, is probably the most economical method to modify membranes. Two categories of additives for blending modification are discussed: www.sciencedirect.com
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Air gap region
Coagulation bath Current Opinion in Chemical Engineering
Hollow-fiber spinning with nonsolvent induced phase separation.
copolymers and inorganic particles. About the novel morphology control techniques, reviews are given below on the ‘phase separation micromolding’ method (a method combining lithography and NIPS) and ‘breath figure’ method (a VIPS process) which have been used to prepare membranes with patterned structure. Also included in the present paper is the recent development of an interesting technique based on the combination of NIPS and selfassembly of block copolymers, an efficient method to prepare highly uniform nanoporous membranes.
Recent advances in fundamentals of membrane preparation Discussed in the section are recent research results about the formation mechanism of membranes prepared by NIPS. Also included are the research trends about selection of solvent, nonsolvent, and polymers for membrane preparation. Membrane formation mechanism
A widely used approach to investigate the relationship between membrane structure and its formation process is to construct a ternary phase diagram of the mixture of polymer, solvent and nonsolvent, providing the thermodynamics fundamentals of the phase separation behavior of casting solution, and then to track the composition change during membrane formation by a composition path on the phase diagram, describing the mass transfer associated with the membrane formation. Theoretical calculation of the composition paths has been shown to be a useful tool to reason how mass transfer affects membrane morphology [5–7]. However, due to the difficulty in the measurement of composition change in the casting solution during membrane formation, few experwww.sciencedirect.com
imental works were performed to directly verify the theoretical calculations. In the last decade, measurement of the composition change in the casting solution has been proposed by using magnetic resonance imaging (MRI) , near infrared (IR) , and Fourier transform infrared (FTIR) spectroscopy microscope . But, the methods cannot be used for in situ measurement and can only be used under limited conditions (such as slow exchange of solvent and nonsolvent). In situ measurement of the composition change during membrane formation still remains a challenge. Another experimental method for studying the membrane formation process is to observe the phase separation of polymer solution through optical microscopes. The method, though cannot provide quantitative data such as the change of composition during membrane formation, can give important qualitative information about where and when the phase separation occurs and how voids grow in membranes . Most of the research on membrane formation presumed that the phase separation mechanism associated with NIPS was nucleation and growth. But, recent research results provided evidence supporting that spinodal decomposition could also occur, explaining why membranes with lacy (bi-continuous) structure (giving highly interconnected pores) can be formed . Another interesting research topic is how the coarsening of phasedomains influences the membrane separated morphology. Data were obtained showing that the lacy structure formed after spinodal decomposition could evolve to cellular-like if domain coarsening was not interrupted [10,12]. The effect of domain coarsening on membrane morphology was strongly related to how fast the demixed solution gelled after phase separation. Therefore, the effect was more obvious for membranes prepared by VIPS or by immersion precipitation with weak nonsolvent as the coagulant, since in such processes after phase separation the demixed solution gelled slowly. Furthermore, when semicrystalline polymers were used to prepare membranes, the interplay of polymer crystallization, phase separation, and domain coarsening could give interesting morphology and even superhydrophobicity [13,14]. Another interesting research topic related to membrane formation is the mechanism of the formation of macrovoids, which may weaken the mechanical strength of membranes. Early investigation indicated that macrovoids were initiated by instantaneous demixing of casting solution. But recent works [11,15,16] pointed out that the mechanism might be more complicated. Macrovoids could be initiated by viscous fingering, a hydrodynamically unstable phenomenon occurs when a fluid with high viscosity is in contact with a less viscous fluid [16,17]. And the competition of viscous fingering and phase separation played an important role in the formation of Current Opinion in Chemical Engineering 2013, 2:229–237
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macrovoids . Recent publications suggested that thinner cast film (for flat-sheet membranes) [18,19] or a smaller spinneret annulus gap (for hollow-fiber membranes)  helps to suppress the formation of macrovoids. More researches are still needed to unify the theories of macrovoid formation presented in the literature. Selection of solvent, nonsolvent, and polymer
The choice of solvent–nonsolvent pairs can have dramatic effects on membrane morphology and separation performance. A rule of thumb is that the higher mutual affinity between solvent and nonsolvent usually gives more porous membranes. For more detailed discussion about the effect of solvent and nonsolvent on membrane morphology, one can refer to a recent review paper . Some of the recent publications stressed on the effect of solvent quality on membrane morphology. By using 2pyrrolidone, a low-quality solvent for polysulfone, highly porous polysulfone membranes with interconnected pores were successfully prepared, which cannot be accomplished by using a solvent with higher solubility such as n-methylpyrrolidinone . For semicrystalline polymer such as poly(vinylidene fluoride), the polymer dissolution temperature could have dramatic effect on solvent quality and hence had influence on membrane morphology [21,22]. To reduce the impact on environment, it has been studied using ionic liquids to replace traditional organic solvents for membrane preparation . Adopting supercritical carbon dioxide as polymer nonsolvent also draws attention [24,25]. To enhance membrane stability for long-term operation or harsh separation conditions, polymers with good chemical and thermal stabilities have been used to prepare membranes. Fluoropolymers, such as poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoridehexafluoropropylene) (PVDF-HFP), have been used to prepare membranes for microfiltration, ultrafiltration, pervaporation, and membrane distillation [4,26–29]. Polyimide membranes  have been developed for gas separation [31–33], pervaporation [34,35], and solventresistant nanofiltration processes [36–38]. Polybenzimidazole [39,40], polyaniline [41,42], poly (phthalazinone ether sulfone) , and polyphenylsulfone  have also been used to prepare membranes by using NIPS.
Blending modification Modification of membranes prepared from commercial polymers is usually performed to further improve their separation performance. Surface coating and grafting are widely used to modify membranes; but, they are not discussed here since they are not directly related to the NIPS process. We focus on blending modification, in which additives are blended in the casting solution to accomplish membrane fabrication and modification in one step. Polymer additives, such as polyethylene glycol Current Opinion in Chemical Engineering 2013, 2:229–237
(PEG) and polyvinylpyrrolidone (PVP), have been used to produce commercial membranes. The additives function as both the pore-forming agents to enhance membrane porosity and the hydrophilicity enhancers to mitigate membrane fouling. One can refer to a recent review on NIPS  for more details about the commonly used additives and their roles in membrane formation. We here concentrate on two types of additives that have been studied extensively recently: the amphiphilic copolymers for anti-fouling application, and the inorganic particles for preparation of hybrid (mixed-matrix) membranes. Copolymer additives
Additives of amphiphilic copolymers have hydrophilic and hydrophobic segments. During membrane formation, the hydrophilic segments tend to migrate toward the membrane surface or the pore (polymer-poor domains) surface to reduce the interfacial energy, and the entanglement of the hydrophobic segments with the chains of membrane polymer helps to anchor the hydrophilic segments , as depicted in Figure 3. The hydrophilic segments of the copolymer additives usually comprise PEG, zwitterionics, or other hydrophilic moieties, and the hydrophobic segments usually have high compatibility with the bulk membrane polymer. Listed in Table 1 are some examples of copolymer additives used for various polymer membranes. Commercial PEO–PPO–PEO (poly(ethylene oxide)– poly(propylene oxide)–poly(ethylene oxide)) tri-block copolymers (Pluronic) are also good blending modification additives for preparation of antifouling membranes Figure 3
Immersion in water
(a) Phase separation
(b) Phase separation hydrophilic segment hydrophobic segment Current Opinion in Chemical Engineering
Schematic presentation of the migration of amphiphilic-copolymer additives. (a) A polymer solution phase separates to form a porous membrane after being immersed in water. (b) A polymer solution containing amphiphilic-copolymer additives phase separates to form a porous membranes with the additives migrating to the membrane and pore surfaces. www.sciencedirect.com
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Table 1 Examples of copolymer additives used for various polymer membranes Membranes
PMMA–PEO copolymers PEG containing PU PVDF-graft-PEGMA PVDF-graft-PDMAEMA
   
PSF-graft-POEM PSF–POEM block copolymer
Citric acid grafted PU
PAN–PMPDSAH copolymer PAN-graft-PEG
Fluorinated PU-based polymer PLA–PEG–PLA triblock copolymer
PVDF, poly(vinylidene fluoride); PMMA, poly(methyl methacrylate); PEO, poly(ethylene oxide); PEG, poly(ethylene glycol); PU, polyurethane; PEGMA, poly(ethylene glycol) methyl ether methacrylate; PDMAEMA, poly(dimethyl aminoethyl methacrylate); PSF, polysulfone; POEM, poly(ethylene glycol) methyl ether methacrylate; PES, polyethersulfone; PAN, polyacrylonitrile; PMPDSAH, poly((3-(methacryloylamino)propyl)-dimethyl(3-sulfopropyl) ammonium hydroxide); PEI, polyetherimide; PLA, poly(lactic acid).
[56–58]. An interesting publication shows that copolymers synthesized from the Pluronic tri-block copolymers can be used as blending additives to prepare membranes exhibiting antifouling performances for oily foulants .
carbon-dioxide selective membranes can be prepared by blending zeolite or MOF (metal organic framework) in polymer membranes [72,73]. The blending of inorganic particles can be further extended to preparation of inorganic hollow fibers. Suspensions for spinning, containing polymer (as the binder), inorganic particles, solvent and additives, are prepared and extruded out of a spinneret into a coagulant bath to form hollow-fiber precursors. Phase separation of the polymer suspensions introduces porous structure into the walls of the hollow-fiber precursors. Afterwards the precursors are placed in an oven to perform a hightemperature sintering process. The organic components (polymer and additives) are then removed from the precursors by thermal degradation and gasification in the high temperature environment. Meanwhile, the inorganic particles contained in the precursors are fused into a porous fiber form by sintering. One can refer to a recent review article  for more discussion about the preparation of inorganic hollow-fiber membranes by NIPS.
Novel processes for morphology control Three novel morphology control techniques based on NIPS are discussed below: phase separation micromolding, the ‘breath figure’ method, and a method combining NIPS and self-assembly of block copolymers. Phase separation micromolding
Interesting results have been published recently showing that the anti-fouling performance of the additive-blended membranes depended on how the membranes were formed. The VIPS process was shown to be able to effectively enhance the anti-fouling performance, which was believed to be related to the slow polymer precipitation rate that could give the copolymer additives more time to perform self organization [60,61]. More works are still needed to clarify how copolymer additives migrate during membrane formation, and how the microphase separation of copolymers interacts with the nonsolventinduced phase separation. Inorganic particles
Inorganic particles were blended in polymer membranes to improve membrane hydrophilicity and to mitigate membrane fouling. The inorganic particles studied include titanium oxide , aluminum oxide , zirconium oxide , silicon oxide , zinc oxide , and carbon nanotube . It has also been shown that the blending of mesoporous silica , nano-kaolinite , fluorine , or ferrous chloride  can enhance the mechanical strength of membranes. Dense organic–inorganic hybrid membranes, usually called mixed matrix membranes, can have good gas separation performance. One can refer to a recent review paper  for the development of the preparation of mixed matrix hollowfiber membranes by NIPS. It has also been shown that www.sciencedirect.com
Phase separation micromolding is a newly developed method to prepare membranes with patterned surface [75–77]. Polymer solution was cast on a substrate with a designed pattern, usually a silicon wafer with patterned structure fabricated by photolithography. Phase separation of the polymer solution was then induced either by immersing the solution with the substrate in a nonsolvent bath (the wet method) or by exposing it to humid air (VIPS). The obtained porous membranes have a surface pattern that is a negative replica of the substrate. The membranes with patterned structure could have a lot of potential applications: microfluidic devices, scaffolds for tissue engineering, microsieves, and porous molds for microcontact printing . And the method could also be used to prepare a surface with two-tier hierarchical structure that had superhydrophobic property . Recent research focused on using a poly(dimethylsiloxane) (PDMS) replica mold as the substrate instead of silicon wafer  and using VIPS to precipitate the polymer solution instead of the wet method . In addition, it has been shown that the membranes with patterned surface prepared by the method can effectively reduce the deposition of microbial cells on them . Honeycomb porous structure by the ‘breath figure’ method
Polymer films with honeycomb structure, a two-dimensional array of hexagonal pores, can be fabricated by water Current Opinion in Chemical Engineering 2013, 2:229–237
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droplet condensation accompanied with fast solvent evaporation in humid air [81–84]. A schematic diagram is given in Figure 4 to illustrate the process. The solvent used was volatile and immiscible with water. The evaporation of solvent cooled down the surface, inducing condensation of water vapor onto the surface. Because the solvent was immiscible with water, phase separation occurred on the surface to form nuclei of water droplets that grew into hexagonal pores. The method is usually called the ‘breath figure’ method and can be categorized as a VIPS process. The breath figure method has been widely used to prepare polymer films with honeycomb surface structure , but the method was seldom used to prepare porous membranes because the hexagonal pores were not through-pores across the films. Recent researches shed light on resolving such a problem: by casting a very thin film of polymer solution on ice, a thin polymer membrane with through-pores was formed by the method; the membrane was then transferred on to a porous substrate to form a thin selective layer for microfiltration [86,87]. The selective layer was highly porous and the pore size was very uniform.
water vapor condensation
Combination of NIPS and self-assembly of block copolymers
A challenge in membrane preparation is to prepare membranes with uniform pore size, which can give high membrane selectivity for mixtures with different particle (molecular) sizes . An emerging technique to prepare membranes with uniform pores in the range of 10–100 nm is to use the self-assembling nature of block copolymers to form ordered structures in nanometer scale [89,90,91]. Usually a dense microphase segregated block-copolymer template is formed first, and an etching or selectivedissolution process is followed to remove one of the blocks to form uniform pores . But, a simpler method is needed for commercial-scale membrane manufacturing. Recently, an interesting technique, called ‘selfassembly and non-solvent induced phase separation’, has been developed to form nanoporous membranes with uniform pore size [92,93]. Block copolymers were dissolved in carefully selected solvent mixtures to allow the formation of assembly of copolymer micelles in the solution. The well-ordered morphology of the copolymer micelles was then stabilized and frozen by the dry/wet phase separation method, leading to mesoporous membranes with extremely high pore densities and uniformity. Several key strategies of membrane manufacturing were identified: solvent quality should be carefully adjusted to form the needed self-assembly pattern , addition of metal ions to form metal–polymer complexation [94,95] or addition of low-quality solvent for the micelle core blocks  can greatly stabilize the micelle structure in the solution, and the solvent mixture should contain a volatile component to promote a fast rise of polymer concentration and solution viscosity before NIPS . The ‘self-assembly and non-solvent induced phased separation’ method is a promising technique to prepare highly uniform nanoporous membranes in commercial scale.
Current Opinion in Chemical Engineering
Schematic presentation of the ‘breath-figure’ method. (a) Water vapor condenses on the surface of polymer solution because of the surface cooling caused by solvent evaporation. (b) Water droplets form after the condensation. (c) Porous surface forms after the polymer precipitation and water removal. Current Opinion in Chemical Engineering 2013, 2:229–237
Recent research results indicate that some of our previous understanding about NIPS needs polishing, especially about the phase separation mechanism, the coarsening of phase-separated domains, and the formation of macrovoids. Development of experimental techniques to measure the composition change and to study the structure evolution during membrane formation, which still remains a challenge, is needed to unify the theories about membrane formation presented in the literature. A research trend of NIPS is to use it to prepare membranes containing copolymers, which can serve as blending additives for membrane modification or as bulk membrane polymers that can form uniformly ordered structures in nanometer scale. Most of the research focused on preparation of membranes containing copolymers and evaluation of their separation performance. More works are needed to clarify how copolymers segregate during membrane formation, and how the microphase separation www.sciencedirect.com
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of copolymers interacts with NIPS. Also, NIPS of polymer solutions containing inorganic particles has attracted much attention, for preparation of organic–inorganic hybrid membranes or precursors of inorganic membranes. In this regard, more fundamental studies are needed to clarify the underlying mechanism of how the phase separation process affects the distribution of particles in the membranes and how the inorganic particles influence the structure of the prepared membranes (precursors). In addition, novel morphology control techniques based on NIPS, such as phase separation micromolding and the ‘breath figure’ method, have been developed to prepare membranes with patterned structure. The techniques are promising to prepare membranes for some niche and specific applications.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
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