S S symmetry

Review Porous Polyvinyl Alcohol Membranes: Preparation Methods and Applications

Andreas A. Sapalidis Institute of Nanoscience and Nanotechnology, National Centre for Scientific Research Demokritos, Agia Paraskevi, 153 41 Attikis, Greece; [email protected]



Received: 10 May 2020; Accepted: 3 June 2020; Published: 5 June 2020


Abstract: Polymeric membrane technology is a constantly developing field in both the research and industrial sector, with many applications considered nowadays as mature such as desalination, wastewater treatment, and hemodialysis. A variety of polymers have been used for the development of porous membranes by implementing numerous approaches such as phase inversion, electrospinning, sintering, melt-spinning and cold-stretching, 3D printing, and others. Depending on the application, certain polymer characteristics such as solubility to non-toxic solvents, mechanical and thermal stability, non-toxicity, resistance to solvents, and separation capabilities are highly desired. Poly (vinyl alcohol) (PVA) is a polymer that combines the above-mentioned properties with great film forming capabilities, good chemical and mechanical stability, and tuned hydrophilicity, rendering it a prominent candidate for membrane preparation since the 1970s. Since then, great progress has been made both in preparation methods and possible unique applications. In this review, the main preparation methods and applications of porous PVA based membranes, along with introductory material are presented.

Keywords: poly (vinyl alcohol); polymeric membranes; porous materials; separation applications

1. Introduction Polymeric membranes are used in a vast array of applications; predominantly in water treatment, gas separations, medical, industrial, fuel cells, and others [1–10]. The membrane’s purpose is to act as a selective barrier for species of interest. In order for that to be achieved, a membrane must have a certain structure. Typically, these can be either porous, having openings of a desired diameter acting as a sieve, or non-porous, where the chemical nature of the polymeric matrix dominates molecular diffusion, favoring the passage of a certain component against other(s). Membrane performance typically is a trade-off between permeability, which is the normalized flux of the desired component and the selectivity, which is defined as the ratio of the compositions of the species of interest in the permeate side. This means that it is challenging to achieve both high fluxes accompanied with high selectivity. Membrane structure plays a vital role in controlling these factors; dense structures exhibit high selectivity but low permeability (disproportional to the membrane’s thickness) while porous structures are characterized by high fluxes and lower selectivity. The combination of these two cases is considered as the ideal scenario: an asymmetric structure comprising of a highly porous substrate, providing flux and mechanical strength, and a thin dense layer providing the desired selectivity. This asymmetry is highly desirable but many challenges remain still on how to achieve controlled structures with desired properties and up-scalable capabilities. Common polymers used for the membrane preparation include cellulose acetate (CA), polyethersulfone (PES), polysulfone (PSf), poly(vinylidene fluoride) (PVDF), polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), poly-(1,4-phenylene ether), ether sulfone (PPEES), sulfonated poly(ether ether ketone) (SPEEK), poly(p-phenylene sulfide) (PPS), polypropylene (PP), polycarbonate (PC), poly(dimethyl siloxane) (PDMS), polyether block amide (PEBAX), polyimides (PI), ultra-high molecular weight polyethylene (UHMWPE), and others. Several polymer characteristics are taken into consideration in

Symmetry 2020, 12, 960; doi:10.3390/sym12060960 www.mdpi.com/journal/symmetry Symmetry 2020, 12, 960 2 of 22 regard to specific membrane applications [11–15] such as thermal, chemical, and mechanical stability as well as hydrophobicity/hydrophilicity. For example, hydrophilic polymers such as poly (vinyl alcohol) (PVA) are the choice of preference when it comes to pervaporation [16] or treating oil containing wastewater [17].

Properties of Poly(Vinyl Alcohol) Synthetic routes for the preparation of poly (vinyl alcohol) have been reported since the 1920s from Herrmann and Haehnel [18] by saponifying poly (vinyl esters) and in the 1930s by Herrmann, Haehnel, and Berg [19] by transesterification. Since then, various preparation routes have been proposed, while in the industrial scale, the majority of PVA is produced via the polymerization of vinyl esters or ethers (usually vinyl acetate), with subsequent saponification or transesterification. Hydrolysis of poly (vinyl acetate), (PVAc) to PVA can be performed in solution, suspension, or emulsion with alkaline or acidic catalysts. The preferred process is transesterification in methanol in the presence of catalytic amounts of sodium methoxide [20]. In reality, PVA is a copolymer consisting of hydroxyl and acetyl units remaining from the incomplete hydrolysis of the parent poly (vinyl acetate). Therefore, polymers with greater than 50% hydroxyl groups are considered as poly (vinyl alcohols) while the ones with more than 50% acetyl groups are poly (vinyl acetates). The percentage of OH– (%mol) groups in the final product is also called degree of hydrolysis (DH) and the properties of PVA are greatly dependent on that. Typical commercial degrees of hydrolysis are 70–72%mol, 87–89%mol and 99+%mol (fully hydrolyzed). Solubility of PVA is greatly dependent on the DH with the fully hydrolyzed grades being more difficult to dissolve, typically in hot water, than the lower hydrolysis grades producing more viscous aqueous solutions. This phenomenon is attributed to the strong hydrogen intramolecular bonds that form in the fully hydrolyzed grades than the lower ones [21]. PVA is also soluble in polar solvents like N-methyl-2-pyrrolidone [22], dimethyl sulfoxide, diethylenetriamine, formamide, N,N-dimethylformamide, and hexamethylphosphoric triamide, glycerol (hot), and piperazine while it is not soluble in lower alcohols, tetrahydrofuran, dioxane, ethylene glycol formal, ketones, esters, carboxylic acids, and concentrated aqueous solutions [23]. PVA’s melting point is approximately 220–230 ◦C for fully hydrolyzed grades and 180–190 ◦C for partially hydrolyzed ones. A similar effect is observed in glass transition temperature, which ranges from 65–85 ◦C while the decomposition temperature is 220–250 ◦C. The crystal structure of PVA was first reported by Bunn [24] in 1948 and confirmed by studies conducted by Assender and Windle [25,26] using x-ray, and Takahashi [27] neutron diffraction. o The crystal cell is monoclinic (a = 7.81 Å, b = 2.52 Å, c = 5.51 Å, α = γ = 90 , β = 91.7◦)[25] with the intramolecularSymmetry 2020, 12 hydrogen, x FOR PEER bondsREVIEW responsible for the chain arrangements as can be seen3 of in 25 Figure 1.

Figure 1.FigureThe 1. crystal The crystal structure structure of polyof poly (vinyl (vinyl alcohol)alcohol) as as proposed proposed by byBunn. Bunn. Repr Reprintedinted with permission with permission from [25from]. Copyright [25]. Copyright 1998 1998 Elsevier Elsevier B.V. B.V.

Annealing affects the crystal structure and crystallinity, resulting in increased levels of the latter, as can been seen in Figure 2 Crystal formation is also altered in the presence of nanoparticles [28], especially with hydrophilic ones like clays [29,30], as seen in Figure 3, where the addition of bentonite clay induced an additional crystal formation.

Figure 2. X-ray powder diffraction traces from films annealed at different temperatures. Reprinted with permission from [25]. Copyright 1998 Elsevier B.V.

The reported density for the amorphous part of PVA is pa = 1.269 g/cc, while for the crystalline part it is pc = 1.345 g/cc [31,32].

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Figure 1. The crystal structure of poly (vinyl alcohol) as proposed by Bunn. Reprinted with permission Symmetryfrom2020 [25]., 12 ,Copyright 960 1998 Elsevier B.V. 3 of 22

Annealing affects the crystal structure and crystallinity, resulting in increased levels of the latter, Annealing affects the crystal structure and crystallinity, resulting in increased levels of the latter, as can been seen in Figure 2 Crystal formation is also altered in the presence of nanoparticles [28], as can been seen in Figure2 Crystal formation is also altered in the presence of nanoparticles [ 28], especially with hydrophilic ones like clays [29,30], as seen in Figure 3, where the addition of bentonite especially with hydrophilic ones like clays [29,30], as seen in Figure3, where the addition of bentonite clay induced an additional crystal formation. clay induced an additional crystal formation.

FigureFigure 2.2. X-rayX-ray powderpowder didiffractionffraction tracestraces fromfrom filmsfilms annealedannealed atat didifferentfferent temperatures.temperatures. ReprintedReprinted withwith permissionpermission fromfrom [[25].25]. CopyrightCopyright 19981998 ElsevierElsevier B.V.B.V.

The reported density for the amorphous part of PVA is pa = 1.269 g/cc, while for the crystalline The reported density for the amorphous part of PVA is pa = 1.269 g/cc, while for the crystalline part it is pc = 1.345 g/cc [31,32]. Symmetrypart it2020 is ,p 12c =, x1.345 FOR PEER g/cc REVIEW[31,32]. 4 of 25

Figure 3. (a) Differential scanning callorimetry curves in the melting region. (b) Fraction of the heats Figureof fusion 3. (a) forDifferential the two scanning phases (closed callorimetry symbols: curves bulklike in the phase, melting open region. symbols: (b) Fraction clay induced of the heats phase). of fusionReproduced for the by two permission phases (closed from [33 symbols:]. Copyright bulklike 2011 phase, Wiley Periodicals,open symbols: Inc. clay induced phase). Reproduced by permission from [33]. Copyright 2011 Wiley Periodicals, Inc. Additionally, the great film forming, adhesion, good mechanical and optical properties,

O2Additionally,barrier capability the great in low film humidity forming, [33adhesion,–35], non-toxicity good mechanical [36,37], and biocompatibility optical properties, and partial O2 barrierbiodegradability capability [in38 ,low39], andhumidity relatively [33–35], easy nanocomposite non-toxicity preparation[36,37], biocompatibility capabilities [40 and] render partial PVA a biodegradabilityvaluable candidate [38,39], material and relatively in diverse easy and nanocomposite demanding applications. preparation capabilities [40] render PVA a valuableDue candidate to these properties, material in PVA diverse has foundand demanding its way in aapplications. wide spectrum of technological fields ranging fromDue biomedical to these properties, to construction. PVA has Indicative found its applications way in a wide include spectrum fibers, of water technological soluble packaging, fields rangingprotective from colloid biomedical in emulsion to construction. and suspension Indica polymerization,tive applications adhesives, include sizing, fibers, paper water industry, soluble tissue packaging,engineering protective [41], wound colloid dressing in emulsion [42] contact and lenses,suspension drug deliverypolymerization, systems [43adhesives,–46], and sizing, orthopedics paper [47 ]. industry,Practical tissue engineering applications [41], of poly(vinyl wound dressing alcohol) based[42] contact membranes lenses, require drug delivery that it should systems be crosslinked[43–46], andprior orthopedics to use in order[47]. to retain the structure and mechanical properties, especially in water related processes; Practical applications of poly(vinyl alcohol) based membranes require that it should be crosslinked prior to use in order to retain the structure and mechanical properties, especially in water related processes; studies have also shown that crosslinking can induce minimum effects on the thermal and mechanical properties, or even their deterioration [48]. Crosslinking can be performed via various chemical or physical paths [49–51]. Chemical crosslinking requires di-functional crosslinking reagents [49] that typically include glutaraldehyde [52,53], as can be seen in Figures 4(a) and 4(b), acetaldehyde, formaldehyde, and other monoaldehydes. The crosslinking reaction takes place in an acidic environment with sulfuric and hydrochloric acids being the common chemicals used. A list of common crosslinkers used on PVA can be seen in

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Table 1. Crosslinkers used on poly (vinyl alcohol) (PVA), adapted from [49].

Glutaraldehyde Formaldehyde Symmetry 2020, 12, 960 Citric acid Boric acid 4 of 22 Tetraethoxysilane Malic acid Poly(acrylic acid) Glyoxal studies have also shown that crosslinkingGenipin can induce minimumPEG diacylchloride effects on the thermal and mechanical properties, or even theirTerephthaldegyde deterioration [48 ]. CrosslinkingMalonic can be performedacid via various chemical or physical paths [49–51].Sulfur Chemical -succinic crosslinking acid requires di-functional Acetaldehyde crosslinking reagents [49] that Acrolein and methacrolein Fumaric acid typically include glutaraldehyde [52,53], as can be seen in Figure4a,b, acetaldehyde, formaldehyde, Urea formaldehyde/H2SO4 1,2-Dibromoethane and other monoaldehydes. The crosslinking reaction takes place in an acidic environment with sulfuric Divinyl sulfone γ-Glycidoxypropyltrimethoxysilane and hydrochloric acidsMaleic being acid the and common anhydride chemicals used. Trimesoyl A list of chloride common crosslinkers used on PVA can be seen in Toluene diisocyanate Glycidyl acrylate

FigureFigure 4. Crosslinking 4. Crosslinking reaction reaction between between ( a()a) poly poly (vinyl(vinyl alcohol) alcohol) and and (b ()b glutaraldehyde.) glutaraldehyde. Reproduced Reproduced with permission from [53]. Copyright 2007 Elsevier B.V. with permission from [53]. Copyright 2007 Elsevier B.V.

Table1. Crosslinkers used on poly (vinyl alcohol) (PVA), adapted from [ 49]. Since some of these crosslinking agents are considered toxic, proper actions should be taken in order that no residues exist in the final material. Crosslinking reactions of PVA with some commonly used acids [54] can be seen in Figure5. Other crosslinking methods include freeze /thaw method [16], heat treatment [55], and γ-irradiation [56].

Table 1. Crosslinkers used on poly (vinyl alcohol) (PVA), adapted from [49].

Glutaraldehyde Formaldehyde Citric acid Boric acid Tetraethoxysilane Malic acid Poly(acrylic acid) Glyoxal Genipin PEG diacylchloride Terephthaldegyde Malonic acid Sulfur -succinic acid Acetaldehyde Acrolein and methacrolein Fumaric acid Urea formaldehyde/H2SO4 1,2-Dibromoethane Divinyl sulfone γ-Glycidoxypropyltrimethoxysilane Maleic acid and anhydride Trimesoyl chloride Toluene diisocyanate Glycidyl acrylate

As a membrane material, PVA is considered attractive due to the following advantages: hydrophilicity, water permeability, good mechanical properties, thermal and chemicals resistance, anti-fouling potential, and film forming ability. On the negative side, PVA is permeable to ions, has a high degree of swelling, compacts under pressure, and shows low flux when it is highly crosslinked [40]. Symmetry 2020, 12, 960 5 of 22 Symmetry 2020, 12, x FOR PEER REVIEW 7 of 25

Figure 5. Cross-linking reactions between (a) citric acid, (b) maleic acid and (c) poly (acrylic acid) Figure 5. Cross-linking reactions between (a) citric acid, (b) maleic acid and (c) poly (acrylic acid) and and PVA. Reproduced with permission from [54]. Copyright 2017 WILEY-VCH Verlag GmbH & Co. PVA. Reproduced with permission from [54]. Copyright 2017 WILEY-VCH Verlag GmbH & Co. In this context, dense (non-porous) PVA has been used in various membrane preparations for many As a membrane material, PVA is considered attractive due to the following advantages: years. The main applications include the implementation of PVA based membranes in pervaporation [57] hydrophilicity, water permeability, good mechanical properties, thermal and chemicals resistance, for the dehydration of ethanol [58–63], isopropanol [64], acetic acid [65], ethylene glycol mixtures [66], anti-fouling potential, and film forming ability. On the negative side, PVA is permeable to ions, has direct methanol fuel cells [67], reverse osmosis desalination [68], CO separation [69], as a polymer a high degree of swelling, compacts under pressure, and shows low flux2 when it is highly crosslinked electrolyte membrane [70,71], biodiesel synthesis [72], ion exchange [73], and others. [40]. Furthermore, poly (vinyl alcohol) has attracted a lot of research interest as a membrane for In this context, dense (non-porous) PVA has been used in various membrane preparations for separations, especially in water treatment due to its low biofouling tendency [74], a known issue that many years. The main applications include the implementation of PVA based membranes in affects various membrane processes [75]. pervaporation [57] for the dehydration of ethanol [58–63], isopropanol [64], acetic acid [65], ethylene 2.glycol Polymeric mixtures Porous [66], Structuresdirect methanol fuel cells [67], reverse osmosis desalination [68], CO2 separation [69], as a polymer electrolyte membrane [70,71], biodiesel synthesis [72], ion exchange [73], and others.Over the last 60 years or so, there has been great progress in several techniques that have been developedFurthermore, for the preparation poly (vinyl of alcohol) porous polymerichas attracted membranes a lot of both research on industrial interest andas a lab membrane scale [76– 79for] includingseparations, (a) especially phase inversion in water alsotreatment called due phase to its separation low biofouling (PS), tendency (b) track-etching, [74], a known (c) stretching, issue that (d)affects sintering, various (e) membrane electrospinning, processes and [75]. interfacial polymerization [80]. In parallel, the effects of nanoparticle incorporation during membrane preparation have also been extensively studied for both performance2. Polymeric andPorous structure Structures control [81,82]. In the following paragraphs, the main techniques for obtaining porous structures and how they wereOver applied the for last the 60 case years of or PVA so, willthere be has described. been great progress in several techniques that have been developed for the preparation of porous polymeric membranes both on industrial and lab scale [76– 2.1.79] including Phase Inversion (a) phase Method inversion also called phase separation (PS), (b) track-etching, (c) stretching, (d) sintering, (e) electrospinning, and interfacial polymerization [80]. In parallel, the effects of Phase inversion can be generally described as the process by which a homogeneous polymer nanoparticle incorporation during membrane preparation have also been extensively studied for solution becomes separated in a polymer rich and in a polymer lean phase in a controlled manner [83]. both performance and structure control [81,82]. The polymer rich phase will eventually become the membrane’s solid skeleton, while from the residual In the following paragraphs, the main techniques for obtaining porous structures and how they were applied for the case of PVA will be described.

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2.1. Phase Inversion Method Phase inversion can be generally described as the process by which a homogeneous polymer solution becomes separated in a polymer rich and in a polymer lean phase in a controlled manner [83]. The polymer rich phase will eventually become the membrane’s solid skeleton, while from the residual lean phase, the pores will be created. Structures obtained by this method typically result in aSymmetry sponge2020 or ,finger-like12, 960 macropores (i.e., pore diameter > 50 nm) [84]. 6 of 22 Phase inversion, also called demixing or precipitation, is the general term of the process and can be divided in the following sub categories [85,86]: lean phase, the pores will be created. Structures obtained by this method typically result in a sponge or Non solvent induced phase separation (NIPS), also referred to as immersion precipitation, in which finger-like macropores (i.e., pore diameter > 50 nm) [84]. the addition of a non-solvent in the polymer solution results in a miscibility gap, according to the Phase inversion, also called demixing or precipitation, is the general term of the process and can ternary diagram of the system as shown in Figure 6 [87]. NIPS is the primary technique for the be divided in the following sub categories [85,86]: production of asymmetric structures and polymeric membranes on an industrial scale, resulting in Non solvent induced phase separation (NIPS), also referred to as immersion precipitation, in which pore size diameters ranging from hundreds of nanometers to 10+ μm. Typically, the polymeric the addition of a non-solvent in the polymer solution results in a miscibility gap, according to the ternary solution is casted in the appropriate support (e.g., a non-woven fabric [88]) and is afterward dipped diagram of the system as shown in Figure6[ 87]. NIPS is the primary technique for the production in a non-solvent bath, resulting in the immediate solid membrane formation. of asymmetric structures and polymeric membranes on an industrial scale, resulting in pore size Evaporation induced PS (EIPS), in which the polymer is dissolved in a mixture of a volatile solvent diameters ranging from hundreds of nanometers to 10+ µm. Typically, the polymeric solution is casted and a less volatile non-solvent. As solvent evaporation starts, the composition of the solution is in the appropriate support (e.g., a non-woven fabric [88]) and is afterward dipped in a non-solvent changing in favor of the non-solvent and eventually results in polymer precipitation, forming an bath, resulting in the immediate solid membrane formation. asymmetric skinned membrane.

Figure 6. SchematicSchematic representation representation of of different different poss possibleible thermodynamic thermodynamic regions regions for for a a ternary ternary semicrystalline polymericpolymeric system. system. Reproduced Reproduced with with permission permission from from [87 ].[87]. Copyright Copyright Elsevier Elsevier B.V. 2015.B.V. 2015. Evaporation induced PS (EIPS), in which the polymer is dissolved in a mixture of a volatile solvent and a less volatile non-solvent. As solvent evaporation starts, the composition of the solution is Vapor-induced PS (VIPS) was first introduced by Zsigmondy and Bachmann [89] in 1918 and then changing in favor of the non-solvent and eventually results in polymer precipitation, forming an developed further by Elford in 1937 [90]. In the VIPS process, the casted membrane is placed in a asymmetric skinned membrane. controlled environment chamber containing non-solvent vapors, which penetrate from the vapor Vapor-induced PS (VIPS) was first introduced by Zsigmondy and Bachmann [89] in 1918 and then phase to the polymer solution and induce phase inversion [91–93]. In general, VIPS is regarded as a developed further by Elford in 1937 [90]. In the VIPS process, the casted membrane is placed in a more controllable technique with the tradeoff of longer production time requirements. controlledTemperature environment induced chamber PS (TIPS), containing in which non-solvent the temperature vapors, of which the polymer penetrate solution from the is vaporchanged phase so thatto the the polymer polymer solution solubility and in induce a solvent phase chan inversionges, eventually [91–93 ].leading In general, to precipitation. VIPS is regarded as a more controllable technique with the tradeoff of longer production time requirements. Temperature induced PS (TIPS), in which the temperature of the polymer solution is changed so that the polymer solubility in a solvent changes, eventually leading to precipitation.

2.2. Electrospinning Electrospinning (electrostatic + spinning) is a technique used to create polymer nanofibers from polymeric solutions ranging in diameter from less than 3 nm to 1 µm with the aid of an electrostatic Symmetry 2020, 12, x FOR PEER REVIEW 9 of 25

2.2. Electrospinning Symmetry 2020, 12, 960 7 of 22 Electrospinning (electrostatic + spinning) is a technique used to create polymer nanofibers from polymeric solutions ranging in diameter from less than 3 nm to 1 μm with the aid of an electrostatic forceforce (Figure(Figure7 )[7) 94[94,95].,95]. AlthoughAlthough notnot thethe mostmost popularpopular porousporous materialmaterial manufacturingmanufacturing process,process, electrospinningelectrospinning has has evolved, evolved, especially especially in in the the last last few few years, years, into into aa promisingpromising manufacturingmanufacturing process process ofof membrane membrane technology technology called called electrospun electrospun nanofibrous nanofibrous membranes membranes (ENMs). (ENMs). ByBy carefully carefully selecting selecting thethe proper proper fiber fiber diameter, diameter, consecutive consecutive layering layering and and packingpacking density,density, a a porous porous asymmetric asymmetric structure structure cancan bebe thereforetherefore shaped shaped with with membrane membrane characteristicscharacteristics suchsuch asashigh highair air permeability permeabilityand andtargeted targeted liquidliquid entryentry andand bubblebubble point point pressure. pressure. ElectrospinningElectrospinning waswas introducedintroduced asas aa meanmean ofof processingprocessing textiletextile yarns yarns inin the the 1930s 1930s [ 96[96],], though though it it originates originates inin the the studies studies of of Lord Lord Rayleigh, Rayleigh, and and since since then then it it waswas used used in in a a variety variety ofof materialmaterial preparationpreparation [[97]97] includingincluding amongamong others,others, medicalmedical applicationsapplications [[98],98], protectiveprotective textiles, textiles, [99 [99,100],,100], and and filtration filtration membranes membranes [101 [101].]. The The main main advantages advantages of ENMs of ENMs is their is hightheir surfacehigh surface area and area porosity, and porosity, open structure, open structure, controllable controllable manufacturing, manufacturing, and nanocomposite and nanocomposite capability. Typicallycapability. ENMs Typically are produced ENMs are as produced thin layers, as whichthin layers, depending which on depending the transmembrane on the transmembrane pressure of thepressure intended of the application intended canapplication require thecan needrequire of athe support need of (usually a support a non-woven (usually a fabric)non-woven in order fabric) to providein order mechanical to provide stability.mechanical stability.

Figure 7. Schematic diagram of setup of electrospinning apparatus. (a) Typical vertical setup Figure 7. Schematic diagram of setup of electrospinning apparatus. (a) Typical vertical setup and (b) and (b) horizontal setup of electrospinning apparatus. Reproduced with permission from [94]. horizontal setup of electrospinning apparatus. Reproduced with permission from [94]. Copyright Copyright 2010 Elsevier Inc. 2010 Elsevier Inc. A special category of PVA based porous materials is the development of hydrogels that have been studiedA forspecial many category years, mainly of PVA for based biomedical porous applications materials is [ 39the]. Althoughdevelopment not intendedof hydrogels to function that have as membranes,been studied their for preparationmany years, routes mainly can for be adaptedbiomedical to obtain applications porous polymeric[39]. Although structures not asintended supports to andfunction separating as membranes, elements [102 their]. preparation routes can be adapted to obtain porous polymeric structures as supports and separating elements [102]. 3. Porous Poly(Vinyl Alcohol) Preparation Methods 3. Porous Poly(Vinyl Alcohol) Preparation Methods Chae et al. [103] reported a phase inversion method to prepare porous PVA membranes using waterChae as the et solvent al. [103] and reported isopropyl a phase alcohol inversion as the non-solvent. method to Theprepare membrane porous was PVA composed membranes of packed using microsphereswater as the assolvent a result and of crystallizationisopropyl alcohol with as its th degreee non-solvent. depending The on membrane the solvent was/nonsolvent composed ratio. of Thepacked preparation microspheres route was as as follows:a result 1 mLof ofcrystallization the PVA aqueous with solution its degree (with concentration depending ofon 2, 4,the 6, 8,solvent/nonsolvent or 10% PVA) was poured ratio. The into pr a polystyreneeparation route (PS) Petriwas as dish follows: filled with 1 mL 20 of mL the isopropanol. PVA aqueous To performsolution the(with crosslinking concentration of the of formed 2, 4, 6, 8, structure, or 10% PVA) 3 mL was of gludaraldehyde poured into a polystyrene (4 wt%) and (PS) 0.5 Petri mL of dish hydrochloric filled with acid20 mL (35–37%) isopropanol. was added To perform in the Petri the crosslinking dish and left of for the 12 formed h. The scanningstructure, electron3 mL of microscopygludaraldehyde (SEM) (4 imageswt%) and of the 0.5 prepared mL of hydrochloric samples can acid be seen (35–37%) in Figure was8. added in the Petri dish and left for 12 h. The scanning electron microscopy (SEM) images of the prepared samples can be seen in Figure 8.

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Figure 8. Surface and cross-sectional scanning electron microscopy (SEM) images of the matrix as Figure 8. Surface and cross-sectional scanning electron microscopy (SEM) images of the matrix as a a function of the PVA solution concentration: (a) 2, (b) 4, (c) 6, (d) 8, and (e) 10% PVA solutions. function of the PVA solution concentration: (a) 2, (b) 4, (c) 6, (d) 8, and (e) 10% PVA solutions. (f) The (f) The thickness of the matrix as a function of the PVA solution concentration. (g) Graph showing thickness of the matrix as a function of the PVA solution concentration. (g) Graph showing the the porosity as a function of the PVA solution concentration. Reproduced with permission from [64]. porosity as a function of the PVA solution concentration. Reproduced with permission from [64]. Copyright 2014 ACS publications. Copyright 2014 ACS publications. The PVA chains were found to be self-aggregated into a porous structure under the influence of dipoleThedipole PVA chains interactions were found in a low-polarity to be self-aggregated solvent (Figure into a 9porous), while structure the porosities under ofthe the influence matrices of − prepareddipole−dipole using interactions 4 10% PVA in solutions a low-polarity were estimated solvent (Figure at 67.6 9),14.9% while the porosities of the matrices − − preparedKim using and 4 Lee−10% [ 104PVA] solutions reported were the estimated preparation at 67.6 of− integrally14.9% skinned asymmetric PVA (99+% hydrolyzed, Mw. 31,000–51,000) membrane structures by using 2-propanol as the non-solvent and a mixture of N-methyl-2-pyrrolidone and water as a cosolvent membrane structure from asymmetric having a dense layer to uniform porous was found to be dependent on the ratio between NMP and water. As the NMP ratio increased, the structure was driven toward the asymmetric structures. The preparation route that followed was: 100 g of 10 wt% PVA solution in the cosolvent mixture was prepared by mixing at 80 ◦C for 12 h. The resulting solutions were cast on a glass plate at 25 ◦C with a casting knife and submerged into an isopropanol bath for 20 min at 25 ◦C. Ahmad et al. [105] prepared PVA (88% hydrolyzed, 88,000 Mw) asymmetric membranes using deionized water as the solvent and a mixture of sodium hydroxide (4.0 wt%) and sodium sulfate (8.0 wt%) as a coagulant in similar way with previous studies [106]. The prepared membrane was then crosslinked using glutaraldehyde (10 g/L), sodium sulfate (45 g/L), and sulfuric acid (5 g/L) for 0.5, 1.0, 1.5, and 2 h of reaction time. Although not clear from the electron microscopy observation, by increasing the crosslinking reaction time, the pore size distribution became narrower while the average pore size was also reduced, as depicted in Figures 10 and 11. The membranes were evaluated for their pure water flux with the results indicating that there is no clear relationship between pore size and water permeability, rather, a relationship between the hydrophilicity and the water flux.

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Figure 9. PolarityPolarity of of the the binary binary solvent solvent mixtures mixtures and and mo morphologyrphology of of the the PVA PVA matrix prepared at each solvent polarity. Reproduced with permission from [[64].64]. Copyright 2014 ACS publications.

KimWang and et al. Lee [107 [104]] studied reported the influence the preparation of (a) chemical of integrally crosslinking skinned with glutaraldehyde asymmetric PVA and sulfuric (99+% hydrolyzed,acid, and (b) Mw. heating, 31,000–51,000) as post treatment membrane methods structures in porous by using PVA 2-propanol membranes. as the The non-solvent casting solution and a wasmixture a 10 of wt%N-methyl-2-pyrrolidone PVA (99% hydrolysis, and polymerizationwater as a cosolvent degree, membrane 1750) and structure 0.5 wt% from poly(ethylene asymmetric havingglycol) a (Mw. dense 10,000). layer to Afteruniform degassing porous was (50 ◦foundC for to 12 be h), dependent the solution on wasthe ratio cast between onto a glass NMP plate, and water.and immediately As the NMP immersed ratio increased, in acetone, the structure where itwas remained driven toward for 30 the min. asymmetric A crosslinking structures. solution The preparationcomprised withroute glutaraldehyde that followed was: (3 wt%) 100 andg of sulfuric10 wt% acidPVA (5solution wt%) inin athe saturated cosolvent sodium mixture sulfate was preparedsolution (27.5%, by mixing 1 g/100 at 80 g) °C was for used. 12 h. PVA The membranesresulting solutions were immersed were cast into on thea glass crosslinking plate at 25 solution °C with at a25 casting◦C for knife different and timessubmerged (5, 10, into and an 30 isopropanol min), washed bath with for deionized 20 min at water,25 °C. and dried with acetone. For theAhmad heat-treatment, et al. [105] prepared it was performed PVA (88% athydrol 120 ◦yzed,C for 88,000 different Mw) durations asymmetric (one, membranes two, and using three deionizedhours). The water membranes as the solvent were evaluated and a mixture on their of sodi waterum permeation hydroxide flux,(4.0 wt%) morphology, and sodium and mechanicalsulfate (8.0 wt%)properties. as a coagulant Chemical in crosslinking similar way did with not haveprevious any majorstudies eff [106].ect on The the membrane’sprepared membrane structure, was whereas then crosslinkedheat treatments using increased glutaraldehyde crystallinity (10 g/L), and thussodium changed sulfate their (45 morphology.g/L), and sulfuric acid (5 g/L) for 0.5, 1.0, 1.5,Chuang and 2 et h al.of [reaction108] prepared time. asymmetricAlthough not PVA clear (Mw from 74,800) the electron membranes microscopy (Figure observation,12) and studied by increasingthe effect of the including crosslinking dextran reaction (Mw 12,000) time, and the poly pore (vinyl size pyrrolidone)distribution (Mwbecame 10,000) narrower in the membranewhile the averagecasting solution.pore size Membraneswas also reduced, were as prepared depicted using in Figures water 10 as and the solvent,11. The membranes and Na2SO were4/KOH evaluated/H2O as forthe their coagulant pure water medium flux Findings with the indicateresults indicating that when that PVP there was is introduced no clear relationship in the solution, between compact pore sizestructures and water were permeability, favored, while rather, the addition a relationship of dextran between induced the the hydrophilicity pore formation and on the the water membrane’s flux. top layer.

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FigureFigure 10. Pore 10. Pore size size distribution distribution of of crosslinked crosslinked PVA membranes atat different different crosslinking crosslinking durations. durations. ReproducedReproduced with with permission permission from from [78 [78].]. Copyright Copyright ©© 20112011 Elsevier Elsevier B.V. B.V.

The membranes were evaluated on their ultrafiltration capabilities with respect to the rejection of dextran and PVP. The same group also studied the role of adding acetic acid in the structure of PVA membranes [109]. Acetic acid containing aqueous PVA solutions were immersed in a Na2SO4/KOH/H2O coagulation medium with the findings suggesting an apparent effect of the acidic acid in the overall morphology as a consequence of the filtration performance of the prepared membranes. The thickness of the dense layer was decreased by the increased acidic acid content. A reason for this observation can be found in + the increased amount of H3O , affecting the acid–base equilibrium during coagulation.

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Figure 11.11.E Effectsffects of of crosslinking crosslinking time time toward toward the PVA the membrane PVA membrane surface. Reproducedsurface. Reproduced with permission with permissionfrom [78]. Copyright from [78].© Copyright2011 Elsevier © 2011 B.V. Elsevier B.V. Symmetry 2020, 12, x FOR PEER REVIEW 14 of 25 Wang et al. [107] studied the influence of (a) chemical crosslinking with glutaraldehyde and sulfuric acid, and (b) heating, as post treatment methods in porous PVA membranes. The casting solution was a 10 wt% PVA (99% hydrolysis, polymerization degree, 1750) and 0.5 wt% poly(ethylene glycol) (Mw. 10,000). After degassing (50 °C for 12 h), the solution was cast onto a glass plate, and immediately immersed in acetone, where it remained for 30 min. A crosslinking solution comprised with glutaraldehyde (3 wt%) and sulfuric acid (5 wt%) in a saturated sodium sulfate solution (27.5%, 1 g/100 g) was used. PVA membranes were immersed into the crosslinking solution at 25 °C for different times (5, 10, and 30 min), washed with deionized water, and dried with acetone. For the heat-treatment, it was performed at 120 °C for different durations (one, two, and three hours). The membranes were evaluated on their water permeation flux, morphology, and mechanical properties. Chemical crosslinking did not have any major effect on the membrane’s structure, whereas heat treatments increased crystallinity and thus changed their morphology. Chuang et al. [108] prepared asymmetric PVA (Mw 74,800) membranes (Figure 12) and studied the effect of including dextran (Mw 12,000) and poly (vinyl pyrrolidone) (Mw 10,000) in the membrane casting solution. Membranes were prepared using water as the solvent, and Na2SO4/KOH/H2O as the coagulant medium Findings indicate that when PVP was introduced in the solution, compact structures were favored, while the addition of dextran induced the pore formation on the membrane’s top layer. The membranes were evaluated on their ultrafiltration capabilities with respect to the rejection of dextran and PVP. Figure 12. SEM images of a PVA membrane: (a) top layer and (b) cross-section. Reproduced with Figure 12. SEM images of a PVA membrane: (a) top layer and (b) cross-section. Reproduced with permission from [108]. Copyright 2000 Elsevier Science Ltd. permission from [108]. Copyright 2000 Elsevier Science Ltd.

The same group also studied the role of adding acetic acid in the structure of PVA membranes [109]. Acetic acid containing aqueous PVA solutions were immersed in a Na2SO4/KOH/H2O coagulation medium with the findings suggesting an apparent effect of the acidic acid in the overall morphology as a consequence of the filtration performance of the prepared membranes. The thickness of the dense layer was decreased by the increased acidic acid content. A reason for this observation can be found in the increased amount of H3O+, affecting the acid–base equilibrium during coagulation. In a study by M’Barki et al. [110], porous membranes were prepared by temperature induced phase serration with a PVA of 72% DH selected due to its low cloud point temperature (Tcp, 47 °C for 10 wt% polymer). The membrane microstructure resulted from phase separation mechanisms occurring by spinodal decomposition. Authors studied the binary PVA/water phase diagram (Figure 13) as well as the crosslinking kinetics in order to determine the desired conditions for membrane preparation.

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In a study by M’Barki et al. [110], porous membranes were prepared by temperature induced phase serration with a PVA of 72% DH selected due to its low cloud point temperature (Tcp, 47 ◦C for 10 wt% polymer). The membrane microstructure resulted from phase separation mechanisms occurring by spinodal decomposition. Authors studied the binary PVA/water phase diagram (Figure 13) as well as the crosslinking kinetics in order to determine the desired conditions for membrane preparation. Symmetry 2020, 12, x FOR PEER REVIEW 15 of 25

FigureFigure 13. 13.Experimental Experimental binary binary phase phase diagram diagram of PVA72–waterof PVA72–water solution solution showing showing the cloudthe cloud point point curve, thecurve, spinodal the temperatures spinodal temperatures and the binodals and the for binodals several concentrations. for several concen Pathstrations. (1) and Paths (2) lead (1) to and the (2) formation lead of denseto the membranes.formation ofPath dense (3) membrane induces thes. formationPath (3) induces of porous the membraneformation thanksof porous to themembrane crosslinking thanks reaction to in thethe diphasiccrosslinking region. reaction Reproduced in the diphasic with permission region. Reproduced from [110]. Copyrightwith permission 2014 Elsevier from [110]. B.V. Copyright 2014 Elsevier B.V. In order to produce the porous membranes, hydrocloric or sulfuric acid was added to the casting solutionIn (PVA order 10 to wt% produce and the gludaraldehyde porous membranes, 0.5 wt%) hydrocloric and after or 1 sulfuric min of stirring, acid was it added was cast to the onto casting a glass solution (PVA 10 wt% and gludaraldehyde 0.5 wt%) and after 1 min of stirring, it was cast onto a substrate that was placed in a heated support of the required temperature under a controlled relative glass substrate that was placed in a heated support of the required temperature under a controlled humidity environment for 15 min. Afterward, the prepared membrane was removed and dried at relative humidity environment for 15 min. Afterward, the prepared membrane was removed and 60 C. Results indicated that the crosslinking control was the key step to obtaining porous membrane dried◦ at 60 °C. Results indicated that the crosslinking control was the key step to obtaining porous morphology (Figure 14). membrane morphology (Figure 14). Modeling studies of the PVA/water system that have also been performed by the same group [111] suggested that simulations showed that the initial solution thickness could have a significant influence on the membrane formation dynamics as well as volatile catalyzer components such as hydrochloric acid. Additional thermodynamic analysis providing useful estimations for the membrane formation of the PVA/water/DMSO mixtures was performed by Young and Chuang [112] by using the Flory–Huggins ternary solution theory, indicating optimum ratios favoring system demixing. Interesting approaches for creating porous structures are by utilizing microfluidics [113] or supercritical CO2 as a component in polymeric solutions [114]. Studies conducted by Reverchon et al. [115,116] reported the preparation of PVA membranes by supercritical CO2 assisted phase inversion. The authors reported their findings with respect to preparation conditions such as polymer concentration, ethanol to CO2 ratio, temperature, and pressure. Several morphologies found to be achievable with macropores from 0.5–4 µm and a top layer that is either dense or porous, as can be seen in Figures 15 and 16.

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FigureFigure 14. 14.Eff ectEffect of temperatureof temperature on theon membranethe membrane structure structure made made with with hydrochloric hydrochloric acid asacid the as catalyst. the Reproducedcatalyst. Reproduced with permission with permission from [110 ].from Copyright [110]. Copyright 2014 Elsevier 2014 Elsevier B.V. B.V.

TheModeling gas foaming studies method of the wasPVA/water used by system Narkkun that et have al. [ 117also] tobeen produce performed L-arginine by the functionalizedsame group [111] suggested that simulations showed that the1 initial solution thickness could have a significant PVA (average molecular weight 130,000 g mol− and 99% hydrolyzed) with CO2 introduced from influence on the membrane formation dynamics as well as volatile catalyzer components such as the thermal decomposition of sodium bicarbonate (NaHCO3) at 130 ◦C. The membranes exhibited a structurehydrochloric with anacid. average pore size of 32–56 µm depending on the amount of grafted L-arginine.

Symmetry 2020, 12, x FOR PEER REVIEW 17 of 25 Symmetry 2020, 12, x FOR PEER REVIEW 17 of 25 Additional thermodynamic analysis providing useful estimations for the membrane formation of the PVA/water/DMSOAdditional thermodynamic mixtures analysis was performed providing by us Youngeful estimations and Chuang for the [112] membrane by using formation the Flory– of the PVA/water/DMSO mixtures was performed by Young and Chuang [112] by using the Flory– Huggins ternary solution theory, indicating optimum ratios favoring system demixing. Huggins ternary solution theory, indicating optimum ratios favoring system demixing. Interesting approaches for creating porous structures are by utilizing microfluidics [113] or Interesting approaches for creating porous structures are by utilizing microfluidics [113] or supercritical CO2 as a component in polymeric solutions [114]. Studies conducted by Reverchon et al. supercritical CO2 as a component in polymeric solutions [114]. Studies conducted by Reverchon et al. [115,116] reported the preparation of PVA membranes by supercritical CO2 assisted phase inversion. [115,116] reported the preparation of PVA membranes by supercritical CO2 assisted phase inversion. TheThe authors authors reported reported their their findings findings withwith respectrespect to preparation conditionsconditions suchsuch as as polymer polymer concentration, ethanol to CO2 ratio, temperature, and pressure. Several morphologies found to be concentration, ethanol to CO2 ratio, temperature, and pressure. Several morphologies found to be Symmetry 2020, 12, 960 14 of 22 achievableachievable with with macropores macropores from from 0.5–4 0.5–4 μ μmm and and aa toptop layer that isis eithereither dense dense or or porous, porous, as as can can be be seenseen in Figuresin Figures 15 15 and and 16. 16.

Figure 15. PVA membrane surface obtained at P = 200 bar, T = 45 °C and 10% (w/w), with an ethanol: Figure 15. PVA membrane surface obtained at P = 200 bar, T = 45 °C and 10% (w/w), with an ethanol: FigureCO 15.2 ratioPVA of membrane 40:60, w/wsurface (A) and obtained 60:40, w/ atw P(B=). 200Reproduced bar, T = 45with◦C permi and 10%ssion (w from/w), with[115]. an Copyright ethanol: CO2 CO2 ratio of 40:60, w/w (A) and 60:40, w/w (B). Reproduced with permission from [115]. Copyright ratio of2008 40:60, Elsevier.w/w ( A) and 60:40, w/w (B). Reproduced with permission from [115]. Copyright 2008 Elsevier. 2008 Elsevier.

FigureFigure 16. E 16.ffect Effect of polymerof polymer concentration concentration on the the PVA PVA membrane membrane section section at P = at 250P =bar,250 T = bar, 55 °CT and= 55 ◦C with an ethanol: CO2 ratio of 60:40, w/w, (A) 10% (w/w) and (B) 20% (w/w). Reproduced with andFigure with 16. an Effect ethanol: of polymer CO2 ratio concentration of 60:40, w on/w the,(A PVA) 10% membrane (w/w) and section (B) 20% at P( =w 250/w). bar, Reproduced T = 55 °C and with permission from [115]. Copyright 2008 Elsevier. permissionwith an ethanol: from [115 CO].2 Copyright ratio of 60:40, 2008 Elsevier.w/w, (A) 10% (w/w) and (B) 20% (w/w). Reproduced with permission from [115]. Copyright 2008 Elsevier. Wu etThe al. gas [118 foaming] reported method the was preparation used by Narkkun of a et composite al. [117] to produce PVA ultrafiltration L-arginine functionalized membrane for PVA (average molecular weight 130,000 g mol−1 and 99% hydrolyzed) with CO2 introduced from the the treatmentThe gas of foaming oily water method by crosslinking was used by PVANarkkun to a et mixed al. [117] cellulose to produce ester L-arginine microfiltration functionalized membrane thermal decomposition of sodium bicarbonate (NaHCO3) at 130 °C. The membranes exhibited a and the results indicated that the prepared membrane−1 had excellent antifouling properties against oil. PVAstructure (average with molecular an average weight pore 130,000size of 32–56 g mol μm and depending 99% hydrolyzed) on the amount with of CO grafted2 introduced L-arginine. from the thermalTreatmentWu decomposition et ofal. oil[118]/water reported of emulsionssodium the preparation bicarbonate was also ofstudied (NaHCO a composite by3) meansat PVA 130 ofultrafiltration°C. highly The membranes porous membrane electrospun exhibited for the PVAa membranesstructuretreatment with (crosslinking of anoily average water by with pore crosslinking GAsize inof acetone)32–56 PVA μ tom a by dependingmixed Wang cellulose et on al. the [ester119 amount] micr usingofiltration of a grafted variety membrane L-arginine. of PVA and grades. The ultrafiltrationtheWu results et al. indicated [118] experiments reported that the the prepared were preparation performed membrane of a in hadcomposite PVA excellent coated PVA antifouling sca ultrafiltrationffolds properties with membrane a hydrogel against oil. for layer the of ~1.8treatmentµm (FigureTreatment of oily 17) water andof oil/water compared by crosslinking emulsions to a Pebax PVA was alsoto 1074 a studiemixed coatingd celluloseby with means the ester of PVAhighly micr water ofiltrationporous flux electrospun reaching membrane 130PVA and LMH andthe Pebaxmembranes results 57 indicated LMH. (crosslinking that the with prepared GA in membraneacetone) by had Wang excellent et al. [119] antifouling using a propertiesvariety of PVA against grades. oil. CitricTheTreatment ultrafiltration acid crosslinking of oil/water experiments stability emulsions were of electrospun performed was also PVAfibersstudie in PVAd coatedby have means scaffolds been of proposedhighly with porous a hydrogel as a promising electrospun layer of alternative ~1.8 PVA tomembranes GA [120] while (crosslinking Truong et with al. [ 54GA] reported in acetone) a comparison by Wang et of al. crosslinking [119] using stabilitya variety by of usingPVA citricgrades. acid (FigureThe ultrafiltration 18), maleic acid, experiments and PAA towere PVA performed (Mw 100,000). in PVA The coated prepared scaffolds membranes with a hydrogel where evaluated layer of ~1.8 in two potential applications: (a) for metal uptake in aqueous systems and (b) ammonia adsorption after decorating the membranes with a metal organic framework copper benzene-1,3,5-tricarboxylate (HKUST-1). Preparation of PVA hydrogels by the freezing and thawing technique was first reported by Peppas [121,122] in the 1970s. Recently, Li and coworkers [123] reported a macroporous PVA hydrogel with improved mechanical properties due to the addition of Agarose (AG) as a pore forming agent. The study claims that the AG-interacting water hybrids can facilitate the formation of ice particles and thereby the macropores. By introducing AG, the crystallinity of macroporous PVA hydrogels is improved and the hydrogel network is enhanced by the hydrogen bonds, strengthening the mechanical stiffness. Symmetry 2020, 12, x FOR PEER REVIEW 18 of 25

μm (Figure 17) and compared to a Pebax 1074 coating with the PVA water flux reaching 130 LMH andSymmetry Pebax2020 57, 12 LMH., 960 15 of 22

Figure 17. (Left) SEM images of electrospun PVA membranes with various molecular weights but Figuresimilar 17. degrees (Left of) SEM hydrolysis: images (Aof) electrospun 98% hydrolyzed, PVA Mwmembranes 13,000–23,000 with gvarious/mol (electrospun molecular fromweights 24 wt%but similarsolution); degrees (B) 98% of hydrolyzed,hydrolysis: Mw(A) 98% 78,000 hydrolyzed, g/mol (from Mw 11 wt%13,000–23,000 solution); g/mol and (C (electrospun) 98–99% hydrolyzed, from 24 wt%Mw 85,000–124,000solution); (B) g98%/mol hydrolyzed, (from 9 wt% Mw solution). 78,000( Upperg/mol right(from) Typical11 wt% SEM solution); cross-sectional and (C) image98–99% of hydrolyzed,PVA nanofibrous Mw composite85,000–124,000 membrane. g/mol ((fromLower 9 rightwt%) Relationssolution). of(Upper permeate right flux) Typical and solute SEM rejection cross- sectionalof the nanofibrous image of PVA composite nanofibrous membranes composite with the membrane. degree of ( crosslinkingLower right) in Relations the PVA hydrogelof permeate coating flux

andfor separation solute rejection of oil/ waterof the emulsionnanofibrous (feed composite pressure: membranes 100 psi; temperature: with the degree 30–35 ◦ofC). crosslinking Reproduced in with the PVApermission hydrogel from coating [119]. for Copyright separation 2006 of Elsevier. oil/water emulsion (feed pressure: 100 psi; temperature: 30– 35 °C). Reproduced with permission from [119]. Copyright 2006 Elsevier. Porous films have been also reported to be produced by SiO2 etching. According to Lee and Wey [124], theyCitric dispersed acid thecrosslinking nanoparticles stability in a PVAof electrospun solution that PVA were fibers cast have onto abeen glass proposed with a 250 asµ am promising thickness alternativeovernight, andto GA the [120] next while day, theTruong dried et membraneal. [54] reported was peeled a comparison off and annealedof crosslinking for 2 hstability at 160 by◦C usingunder citric vacuum. acid (Figure A 2 M NaOH18), maleic solution acid, wasand usedPAA toto etchPVA the (Mw SiO 100,000).2, resulting The in prepared adjustable membranes pore size whereporous evaluated membranes. in two potential applications: (a) for metal uptake in aqueous systems and (b) ammonia adsorption after decorating the membranes with a metal organic framework copper benzene-1,3,5-tricarboxylate (HKUST-1).

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FigureFigure 18. 18.SEM SEM images images of of citric citric acid–PVA acid–PVA membranes membranes beforebefore and after water water i immersion.mmersion. Reproduced Reproduced withwith permission permission from from [54 [54].]. Copyright Copyright 2017 2017 WILEY-VCH WILEY-VCH VerlagVerlag GmbHGmbH && Co.Co.

4. ConclusionsPreparation and of Future PVA hydrogels Prospective by the freezing and thawing technique was first reported by PeppasMembranes, [121,122] especially in the 1970s. those Recently, developed Li an ford watercoworkers treatment, [123] reported are expected a macroporous to be of research PVA hydrogel with improved mechanical properties due to the addition of Agarose (AG) as a pore and industrial focus for many years to come due to factors like climate change, environmental forming agent. The study claims that the AG-interacting water hybrids can facilitate the formation of pollution, and population growth. PVA based porous membranes will continue to be developed ice particles and thereby the macropores. By introducing AG, the crystallinity of macroporous PVA by incorporating functional nanomaterials and by finding innovative ways of pore structure control hydrogels is improved and the hydrogel network is enhanced by the hydrogen bonds, strengthening like microfluidics. Currently, PVA is one of the polymers of choice when it comes to dehydration the mechanical stiffness. via the pervaporation method. Various types of commercially available PVA membranes exist in Porous films have been also reported to be produced by SiO2 etching. According to Lee and Wey the[124], market they with dispersed the aim the to nanoparticles provide solutions in a PVA for so dehydrationlution that were or methanol cast onto removal a glass with from a mixtures250 μm ofthickness volatile organic overnight, compounds, and the next a trend day, thatthe dried is expected membrane to continue. was peeled Crosslinking off and annealed density for as well2 h at as the crosslinkers will be in the forefront of pervaporation research, aiming eventually at increased 160 °C under vacuum. A 2 M NaOH solution was used to etch the SiO2, resulting in adjustable pore processsize porous thermal membranes. efficiency. Hydrophilic membranes will also be the center of research attention for ultrafiltration applications, mainly due to their high achievable fluxes, low fouling, and oil rejection. Based4. Conclusions on the above, and PVAFuture is Prospective currently used on a commercial scale as the selective layer-coating in various polymeric membranes based on polymers such as PVDF and PSf. Implementation of green Membranes, especially those developed for water treatment, are expected to be of research and preparation routes alongside the vast progress in the synthesis of water compatible and ecofriendly industrial focus for many years to come due to factors like climate change, environmental pollution, nanoparticlesand population in the growth. preparation PVA of PVAbased based porous membranes membranes can contributewill continue to the to ever be growingdeveloped concern by ofincorporating toxic solvent functional use by the nanomaterials industry. Nanoparticle and by findin addition,g innovative together ways with of pore new structure preparation control routes, like canmicrofluidics. boost the properties Currently, and PVA durability is one of of the PVA polymers membranes, of choice introducing when it comes them to into dehydration new and via exciting the fields.pervaporation In order for method. PVA to Various be a successful types of commercially replacement available material PVA of the membranes conventional exist used in the polymers, market a serieswith the of improvements aim to provide should solutions be achieved for dehydration including or mechanicalmethanol removal compression from mixtures resistance, of consistent volatile selectivityorganic undercompounds, different a trend operating that environments,is expected to low continue. fouling, Crosslinking and improved density mechanical as well properties. as the Intensivecrosslinkers research will currentlybe in the forefront undertaken of pervaporation by groups around research, the aiming world exploringeventually the at increased unique properties process ofthermal PVA ensures efficiency. successful Hydrophilic solutions membranes for the above-mentioned will also be aspects.the center of research attention for ultrafiltration applications, mainly due to their high achievable fluxes, low fouling, and oil rejection. Funding:Based onThis the research above, received PVA is no currently external funding.used on a commercial scale as the selective layer-coating in Conflictsvarious of polymeric Interest: The membrane author declaress based no on conflicts polymers of interest. such as PVDF and PSf. Implementation of green preparation routes alongside the vast progress in the synthesis of water compatible and ecofriendly nanoparticles in the preparation of PVA based membranes can contribute to the ever growing concern of toxic solvent use by the industry. Nanoparticle addition, together with new preparation routes, can boost the properties and durability of PVA membranes, introducing them into new and exciting fields. In order for PVA to be a successful replacement material of the conventional used polymers, a series of improvements should be achieved including mechanical compression

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References

1. Lee, A.; Elam, J.; Darling, S. Membrane materials for water purification: Design, development, and application. Environ. Sci. Water Res. Technol. 2016, 2, 17–42. [CrossRef] 2. Bernardo, P.; Drioli, E.; Golemme, G. Membrane gas separation: A review/state of the art. Ind. Eng. Chem. Res. 2009, 48, 4638–4663. [CrossRef] 3. Sanders, D.F.; Smith, Z.P.; Guo, R.; Robeson, L.M.; McGrath, J.E.; Paul, D.R.; Freeman, B.D. Energy-efficient polymeric gas separation membranes for a sustainable future: A review. Polymer 2013, 54, 4729–4761. [CrossRef] 4. Ockwig, N.; Nenoff, T. Membranes for Hydrogen Separation. Chem. Rev. 2007, 107, 4078–4110. [CrossRef] [PubMed] 5. Chen, X.; Li, J. Bioinspired by cell membranes: Functional polymeric materials for biomedical applications. Mater. Chem. Front. 2020, 4, 750–774. [CrossRef] 6. Shao, P.; Huang, R.Y.M. Polymeric membrane pervaporation. J. Membr. Sci. 2007, 287, 162–179. [CrossRef] 7. Ulbricht, M. Advanced functional polymer membranes. Polymer 2006, 47, 2217–2262. [CrossRef] 8. Yin, J.; Deng, B.B. Polymer-matrix nanocomposite membranes for water treatment. J. Membr. Sci. 2015, 479, 256–275. [CrossRef] 9. Karami, P.; Khorshidi, B.; McGregor, M.; Peichel, J.T.; Soares, J.B.P.; Sadrzadeh, M. Thermally stable thin film composite polymeric membranes for water treatment: A review. J. Clean. Prod. 2020, 250, 119447. [CrossRef] 10. Wen, Y.; Yuan, J.; Ma, X.; Wang, S.; Liu, Y. Polymeric nanocomposite membranes for water treatment: A review. Environ. Chem. Lett. 2019, 17, 1539–1551. [CrossRef] 11. Miller, D.J.; Dreyer, D.R.; Bielawski, C.W.; Paul, D.R.; Freeman, B.D. Surface Modification of Water Purification Membranes. Angew. Chem. Int. Ed. 2017, 56, 4662. [CrossRef][PubMed] 12. Diban, N.; Aguayo, A.T.; Bilbao, J.; Urtiaga, A.; Ortiz, I. Membrane Reactors for in Situ Water Removal: A Review of Applications. Ind. Eng. Chem. Res. 2013, 52, 10342–10354. [CrossRef] 13. Marcos-Madrazo, A.; Casado-Coterillo, C.; García-Cruz, L.; Iniesta, J.; Simonelli, L.; Sebastián, V.; Encabo-Berzosa, M.D.M.; Arruebo, M.; Irabien, Á. Preparation and Identification of Optimal Synthesis Conditions for a Novel Alkaline Anion-Exchange Membrane. Polymers 2018, 10, 913. [CrossRef][PubMed] 14. Merle, G.; Wessling, M.; Nijmeijer, K. Anion exchange membranes for alkaline fuel cells: A review. J. Membr. Sci. 2011, 377, 1–35. [CrossRef] 15. Ahmadi, M.; Janakiram, S.; Dai, Z.; Ansaloni, L.; Deng, L. Performance of Mixed Matrix Membranes

Containing Porous Two-Dimensional (2D) and Three-Dimensional (3D) Fillers for CO2 Separation: A Review. Membranes 2018, 8, 50. [CrossRef] 16. Semenova, S.; Ohya, H.; Soontarapa, K. Hydrophilic membranes for pervaporation: An analytical review. Desalination 1997, 110, 251–286. [CrossRef] 17. Ismail, N.H.; Salleh, W.N.W.; Ismail, A.F.; Hasbullah, H.; Yusof, N.; Aziz, F.; Jaafar, J. Hydrophilic polymer-based membrane for oily wastewater treatment: A review. Sep. Purif. Technol. 2020, 233, 116007. [CrossRef] 18. Herrmann, W.O.; Haehnel, W. Chem. Forschungsgemeinschaft; DE-OS 450 286; American Chemical Society: New York, NY, USA, 1924. 19. Herrmann, W.O.; Haehnel, W.; Berg, H. Chem. Forschungsgemeinschaft, DE642531. Chem. Abstr. 1937, 31, 59059. 20. Hallensleben, M.L.; Fuss, R.; Mummy, F. Polyvinyl Compounds, Others, in Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co: Weinheim, Germany, 2015; pp. 1–23. [CrossRef] 21. Briscoe, B.; Luckham, P.; Zhu, S. The effects of hydrogen bonding upon the viscosity of aqueous poly (vinyl alcohol) solutions. Polymer 2000, 41, 3851–3860. [CrossRef] 22. Patel, P.; Rodriguez, F.; Moloney, G. N-methyl-2-pyrrolidone as a solvent for poly (vinyl alcohol). J. Appl. Polym. Sci. 1979, 23, 2335–2342. [CrossRef] 23. Barton, A.M. Handbook of Polymer-Liquid Interaction Parameters and Solubility Parameters; CRC Press: Boca Raton, FL, USA, 1990; p. 386. 24. Bunn, C.W. Crystal Structure of Polyvinyl Alcohol. Nature 1948, 161, 929–930. [CrossRef] 25. Assender, H.E.; Windle, A.H. Crystallinity in poly (vinyl alcohol). 1. An X-ray diffraction study of atactic PVOH. Polymer 1998, 39, 4295–4302. [CrossRef] Symmetry 2020, 12, 960 18 of 22

26. Assender, H.E.; Windle, A.H. Crystallinity in poly (vinyl alcohol). 2. Computer modelling of crystal structure over a range of tacticities. Polymer 1998, 39, 4303–4312. [CrossRef] 27. Takahashi, Y. Neutron Structure Analysis of Poly (vinyl Alcohol). J. Polym. Sci. Part B Polym. Phys. 1997, 35, 193–198. [CrossRef] 28. Strawhecker, K.E.; Manias, E. AFM of Poly (vinyl alcohol) Crystals Next to an Inorganic Surface. Macromolecules 2001, 34, 8475–8482. [CrossRef] 29. Strawhecker, K.E.; Manias, E. Structure and Properties of Poly (vinyl alcohol)/Na+ Montmorillonite Nanocomposites. Chem. Mater. 2000, 12, 2943–2949. [CrossRef] 30. Lee, J.; Lee, K.J.; Jang, J. Effect of silica nanofillers on isothermal crystallization of poly (vinyl alcohol): In-situ ATR-FTIR study. Polym. Test. 2008, 27, 360–367. [CrossRef] 31. Hassan, C.M.; Peppas, N.A. Structure and Application of Poly (Vinyl Alcohol) Hydrogels by Conventional Crosslinking or by Freezing/Thawing Methods in Advances in Polymer Science; Springer: Berlin/Heidelberg, Germany, 2000; Volume 153, pp. 37–65. 32. Sakurada, I.; Nukushina, Y.; Sone, Y. Study on the Swelling of Polyvinyl Alcohol (II) Behavior of Crystalline Region in Swelling, ibid. Chem. High Polym. 1955, 12, 510. 33. Sapalidis, A.; Katsaros, K.; Steriotis, T.; Kanellopoulos, N. Properties of poly (vinyl alcohol)—Bentonite clay nanocomposite films in relation to polymer–clay interactions. J. Appl. Polym. Sci. 2012, 123, 1812–1821. [CrossRef] 34. Lim, M.; Kim, D.; Seo, J. Enhanced oxygen-barrier and water-resistance properties of poly (vinyl alcohol) blended with poly (acrylic acid) for packaging applications. Polym. Int. 2016, 65, 400–406. [CrossRef] 35. Gaume, J.; Wong-Wah-Chung, P.; Rivaton, A.; Thérias, S.; Gardette, J.L. Photochemical behavior of PVA as an oxygen-barrier polymer for solar cell encapsulation. RSC Adv. 2011, 1, 1471–1481. [CrossRef] 36. Sheskey, P.J.; Cook, W.G.; Camble, C.G. (Eds.) Handbook of Pharmaceutical Excipients, 8th ed.; Pharmaceutical Press: London, UK; American Pharmacists Association: Washington, DC, USA, 2017; pp. 758–760. 37. DeMerlis, C.C.; Schoneker, D.R. Review of the oral toxicity of polyvinyl alcohol (PVA). Food Chem. Toxicol. 2003, 41, 319–326. [CrossRef] 38. Nihed Ben Halima. Poly (vinyl alcohol): Review of its promising applications and insights into biodegradation. RSC Adv. 2016, 6, 39823–39832. [CrossRef] 39. Julinová, M.; Vaˇnharová, L.; Jurˇca, M. Water-soluble polymeric xenobiotics—Polyvinyl alcohol and polyvinylpyrrolidon—And potential solutions to environmental issues: A brief review. J. Environ. Manag. 2018, 228, 213–222. [CrossRef][PubMed] 40. Sapalidis, A.; Sideratou, Z.; Panagiotaki, K.N.; Sakellis, E.; Kouvelos, E.; Papageorgiou, S.; Katsaros, F. Fabrication of Antibacterial Poly(Vinyl Alcohol) Nanocomposite Films Containing Dendritic Polymer Functionalized Multi-Walled Carbon Nanotubes. Front. Mater. 2018, 5, 11. [CrossRef] 41. Kumar, A.; Han, S. PVA-based hydrogels for tissue engineering: A review. Int. J. Polym. Mater. Polym. Biomater. 2017, 66, 159–182. [CrossRef] 42. Chen, K.-Y.; Lin, Y.-S.; Yao, C.-H.; Li, M.-H.; CheLin, J. Synthesis and Characterization of Poly (vinyl alcohol) Membranes with Quaternary Ammonium Groups for Wound Dressing. J. Biomater. Sci. Polym. Ed. 2010, 21, 429–443. [CrossRef] 43. Kayal, S.; Ramanujan, R.V. Doxorubicin loaded PVA coated iron oxide nanoparticles for targeted drug delivery. Mater. Sci. Eng. C 2010, 30, 484–490. [CrossRef] 44. Pramod Kumar, T.M.; Umesh, H.M.; Shivakumar, H.G.; Ravi, V.; Siddaramaiah. Feasibility of Polyvinyl Alcohol as a Transdermal Drug Delivery System for Terbutaline Sulphate. J. Macromol. Sci. Part A Pure Appl. Chem. 2007, 44, 583–589. [CrossRef] 45. Teodorescu, M.; Bercea, M.; Morariu, S. Biomaterials of PVA and PVP in medical and pharmaceutical applications: Perspectives and challenges. Biotechnol. Adv. 2019, 37, 109–131. [CrossRef] 46. Horiike, S.; Yumoto, K.; Matsuzawa, S.; Yamaura, K. Properties of low temperature swelling hydrogels prepared with partially saponified PVA and drug release using the hydrogels. Polym. Adv. Technol. 2003, 14, 422–427. [CrossRef] 47. Baker, M.I.; Walsh, S.P.; Schwartz, Z.; Boyan, B.D. A review of polyvinyl alcohol and its uses in cartilage and orthopedic applications. J. Biomed. Mater. Res. Part B 2012, 100B, 1451–1457. [CrossRef][PubMed] 48. Krumova, M.; López, D.; Benavente, R.; Mijangos, C.; Pereña, J.M. Effect of crosslinking on the mechanical and thermal properties of poly(vinyl alcohol). Polymer 2000, 41, 9265–9272. [CrossRef] Symmetry 2020, 12, 960 19 of 22

49. Bolto, B.; Tran, T.; Hoang, M.; Xie, Z. Crosslinked poly (vinyl alcohol) membranes. Prog. Polym. Sci. 2009, 34, 969–981. [CrossRef] 50. Otsuka, E.; Suzuki, A. Swelling properties of physically cross-linked PVA gels prepared by a cast-drying method. Prog. Colloid Polym. Sci. 2009, 136, 121–126. 51. Peppas, N.A. Hydrogels in Medicine and Pharmacy, Polymers; CRC: Boca Raton, FL, USA, 1987; Volume 2, pp. 1–48. 52. Kim, K.; Lee, S.; Han, N. Kinetics of crosslinking reaction of PVA membrane with glutaraldehyde. Korean J. Chem. Eng. 1994, 11, 41–47. [CrossRef] 53. Mansur, H.S.; Sadahira, C.M.; Souza, A.N.; Mansur, A.A.P. FTIR spectroscopy characterization of poly (vinyl alcohol) hydrogel with different hydrolysis degree and chemically crosslinked with glutaraldehyde. Mater. Sci. Eng. C 2008, 28, 539–548. [CrossRef] 54. Truong, Y.B.; Choi, J.; Mardel, J.; Gao, Y.; Maisch, S.; Musameh, M.; Kyratzis, I.L. Functional Cross-Linked Electrospun Polyvinyl Alcohol Membranes and Their Potential Applications. Macromol. Mater. Eng. 2017, 302, 1700024. [CrossRef] 55. Amanda, A.; Mallapragada, S.K. Comparison of protein fouling on heat-treated poly(vinyl alcohol), poly(ether sulfone) and regenerated cellulose membranes using diffuse reflectance infrared Fourier transform spectroscopy. Biotechnol. Prog. 2001, 17, 917–923. [CrossRef] 56. Nikolic, V.; Krkljes, A.; Popovic, Z.; Lausevic, Z.; Miljanic, S. On the use of gamma irradiation crosslinked PVA membranes in hydrogen fuel cells. Electrochem. Commun. 2007, 9, 2661–2665. [CrossRef] 57. Yang, G.; Xie, Z.; Doherty, C.M.; Cran, M.; Ng, D.; Gray, S. Understanding the transport enhancement of poly (vinyl alcohol) based hybrid membranes with dispersed nanochannels for pervaporation application. J. Membr. Sci. 2020, 603, 118005. [CrossRef] 58. Uragami, T.; Okazaki, K.; Matsugi, H.; Miyata, T. Structure and Permeation Characteristics of an Aqueous Ethanol Solution of Organic Inorganic Hybrid Membranes Composed of Poly (vinyl alcohol) − and Tetraethoxysilane. Macromolecules 2002, 35, 9156–9163. [CrossRef] 59. Selim, A.; Toth, A.J.; Fozer, D.; Haaz, E.; Valentínyi, N.; Nagy, T.; Keri, O.; Bakos, L.P.; Szilágyi, I.M.; Mizsey, P. Effect of silver-nanoparticles generated in poly (vinyl alcohol) membranes on ethanol dehydration via pervaporation. Chin. J. Chem. Eng. 2019, 27, 1595–1607. [CrossRef] 60. Yamasaki, A.; Iwatsubo, T.; Masuoka, T.; Mizoguchi, K. Pervaporation of ethanol/ water through a poly (vinyl alcohol)/cyclodextrin (PVA/CD) membrane. J. Membr. Sci. 1994, 89, 111–117. [CrossRef] 61. Bolto, B.; Hoang, M.; Xie, Z. A review of membrane selection for the dehydration of aqueous ethanol by pervaporation. Chem. Eng. Process 2011, 50, 227–235. [CrossRef] 62. Cai, W.; Cheng, X.; Chen, X.; Li, J.; Pei, J. Poly (vinyl alcohol)-Modified Membranes by Ti3C2T x for Ethanol Dehydration via Pervaporation. ACS Omega 2020, 5, 6277–6287. [CrossRef] 63. Li, G.; Zhang, W.; Yang, J.; Wang, X. Time-dependence of pervaporation performance for the separation of ethanol/water mixtures through poly (vinyl alcohol) membrane. J. Colloid Interface Sci. 2007, 306, 337–344. [CrossRef] 64. Rachipudi, P.S.; Kariduraganavar, M.Y.; Kittur, A.A.; Sajjan, A.M. Synthesis and characterization of sulfonated-poly (vinyl alcohol) membranes for the pervaporation dehydration of isopropanol. J. Membr. Sci. 2011, 383, 224–234. [CrossRef] 65. Dmitrenko, M.E.; Penkova, A.V.; Missyul, A.B.; Kuzminova, A.I.; Markelov, D.A.; Ermakov, S.S.; Roizard, D. Development and investigation of mixed-matrix PVA-fullerenol membranes for acetic acid dehydration by pervaporation. Sep. Purif. Technol. 2017, 187, 285–293. [CrossRef] 66. Guo, R.; Hu, G.; Li, B.; Jiang, Z. Pervaporation separation of ethylene glycol/water mixtures through surface crosslinked PVA membranes: Coupling effect and separation performance analysis. J. Membr. Sci. 2007, 289, 191–198. [CrossRef] 67. Kim, D.S.; Park, H.B.; Rhim, J.W.; Lee, Y.M. Preparation and characterization of crosslinked PVA/SiO2 hybrid membranes containing sulfonic acid groups for direct methanol fuel cell applications. J. Membr. Sci. 2004, 240, 37–48. [CrossRef] 68. Chen, C.T.; Chang, Y.J.; Chen, M.C.; Tobolsky, A.V. Formalised poly (vinyl alcohol) membranes for reverse osmosis. J. Appl. Polym. Sci. 1973, 17, 789–796. [CrossRef] 69. Torstensen, J.Ø.; Helberg, R.M.L.; Deng, L.; Gregersen, Ø.W.; Syverud, K. PVA/nanocellulose nanocomposite membranes for CO2 separation from flue gas. Int. J. Greenh. Gas Control 2019, 81, 93–102. [CrossRef] Symmetry 2020, 12, 960 20 of 22

70. Yang, J.M.; Chiang, C.Y.; Wang, H.Z.; Yang, C.C. Two step modification of poly (vinyl alcohol) by UV radiation with 2-hydroxy ethyl methacrylate and sol–gel process for the application of polymer electrolyte membrane. J. Membr. Sci. 2009, 341, 186–194. [CrossRef] 71. Yang, J.M.; Wang, H.Z.; Yang, C.C. Modification and characterization of semicrystalline poly (vinyl alcohol) with interpenetrating poly(acrylic acid) by UV radiation method for alkaline solid polymer electrolytes membrane. J. Membr. Sci. 2008, 322, 74–80. [CrossRef] 72. Guerreiro, L.; Pereira, P.M.; Fonseca, I.M.; Martin-Aranda, R.M.; Ramos, A.M.; Dias, J.M.L.; Oliveira, R.; Vital, J. PVA embedded hydrotalcite membranes as basic catalysts for biodiesel synthesis by soybean oil methanolysis. Catal. Today 2010, 156, 191–197. [CrossRef] 73. Higa, M.; Kakihana, Y.; Sugimoto, T.; Toyota, K. Preparation of PVA-Based Hollow Fiber Ion-Exchange Membranes and Their Performance for Donnan Dialysis. Membranes 2019, 9, 4. [CrossRef][PubMed] 74. Rana, D.; Matsuura, T. Surface Modifications for Antifouling Membranes. Chem. Rev. 2010, 110, 2448–2471. [CrossRef][PubMed] 75. Bar-Zeev, E.; Passow, U.; Castrillón, S.R.V.; Elimelech, M. Transparent Exopolymer Particles: From Aquatic Environments and Engineered Systems to Membrane Biofouling. Environ. Sci. Technol. 2015, 49, 691–707. [CrossRef][PubMed] 76. Strathmann, H.; Giorno, L.; Drioli, E. Basic Aspects in Polymeric Membrane Preparation. In Comprehensive Membrane Science and Engineering, vol. 1: Basic Aspects of Membrane Science and Engineering; Drioli, E., Giorno, L., Eds.; Elsevier: Amsterdam, The Netherlands, 2010; pp. 91–112. 77. Tan, X.; Rodrigue, D. A Review on Porous Polymeric Membrane Preparation. Part I: Production Techniques with Polysulfone and Poly (Vinylidene Fluoride). Polymers 2019, 11, 1160. [CrossRef] 78. Tasselli, F. Membrane Preparation Techniques. In Encyclopedia of Membranes; Drioli, E., Giorno, L., Eds.; Springer: Berlin/Heidelberg, Germany, 2014. 79. Wang, D.M.; Lai, J.Y. Recent advances in preparation and morphology control of polymeric membranes formed by nonsolvent induced phase separation. Curr. Opin. Chem. Eng. 2013, 2, 229–237. [CrossRef] 80. Lalia, B.S.; Kochkodan, V.; Hashaikeh, R.; Hilal, N. A review on membrane fabrication: Structure, properties and performance relationship. Desalination 2013, 326, 77–95. [CrossRef] 81. Ursino, C.; Castro-Muñoz, R.; Drioli, E.; Gzara, L.; Albeirutty, M.H.; Figoli, A. Progress of Nanocomposite Membranes for Water Treatment. Membranes 2018, 8, 18. [CrossRef][PubMed] 82. Porel, S.; Singh, S.; Harsha, S.; Narayana Rao, D.; Radhakrishnan, T.P. Nanoparticle-Embedded Polymer: In Situ Synthesis, Free-Standing Films with Highly Monodisperse Silver Nanoparticles and Optical Limiting. Chem. Mater. 2005, 17, 9–12. [CrossRef] 83. Guillen, G.R.; Pan, Y.; Li, M.; Hoek, E.M.V. Preparation and characterization of membranes formed by nonsolvent induced phase separation: A review. Ind. Eng. Chem. Res. 2011, 50, 3798–3817. [CrossRef] 84. Baker, R.W. Membrane Technology and Applications; John Wiley & Sons Ltd: Chichester, UK, 2004. 85. Figoli, A.; Simone, S.; Drioli, E. Polymeric Membranes. In Membrane Fabrication; Hilal, N., Ismail, A.F., Wright, C.J., Eds.; CRC Press Taylor & Francis Group: Boca Raton, FL, USA, 2015; pp. 3–44. 86. Mulder, M.H.V. Membrane preparation. In Encyclopedia of Separation Science: Phase Inversion Membranes; Academic Press: London, UK, 2000; pp. 3331–3346. 87. Keshavarz, L.; Khansary, M.A.; Shirazian, S. Phase diagram of ternary polymeric solutions containing nonsolvent/solvent/polymer: Theoretical calculation and experimental validation. Polymer 2015, 73, 1–8. [CrossRef] 88. Xia, L.; Zhang, Q.; Zhuang, X.; Zhang, S.; Duan, C.; Wang, X.; Cheng, B. Hot-Pressed Wet-Laid Polyethylene Terephthalate Nonwoven as Support for Separation Membranes. Polymers 2019, 11, 1547. [CrossRef] 89. Zsigmondy, R.; Bachmann, W.Z. Uber neue filter. Anorg. U. Allgem. Chem 1918, 103, 119–128. [CrossRef] 90. Elford, W.J. Principles governing the preparation of membranes having graded porosities. The properties of “GRADOCOL” membranes as ultrafilters Trans. Faraday Soc. 1937, 33, 1094–1104. [CrossRef] 91. Ismail, N.; Venault, A.; Mikkola, J.-P.; Bouyer, D.; Drioli, E.; Kiadeh, N.T.H. Investigating the potential of membranes formed by the vapor induced phase separation process. J. Membr. Sci. 2020, 597, 117601. [CrossRef] 92. Matsuyama, H.; Teramoto, M.; Nakatani, R.; Maki, T. Membrane formation via phase separation induced by penetration of nonsolvent from vapor phase. I. Phase diagram and mass transfer process. J. Appl. Polym. Sci. 1999, 74, 159–170. [CrossRef] Symmetry 2020, 12, 960 21 of 22

93. Lee, H.J.; Jung, B.; Kang, Y.S.; Lee, H. Phase separation of polymer casting solution by nonsolvent vapor. J. Membr. Sci. 2004, 245, 103–112. [CrossRef] 94. Bhardwaj, N.; Kundu, S.C. Electrospinning: A fascinating fiber fabrication technique. Biotechnol. Adv. 2010, 28, 325–347. [CrossRef][PubMed] 95. Reneker, D.H.; Yarin, A.L.; Fong, H.; Koombhongse, S. Bending instability of electrically charged liquid jets of polymer solutions in electrospinning. J. Appl. Phys. 2000, 87, 4531–4547. [CrossRef] 96. Formhals, A. Process and Apparatus for Preparing Artificial Threads. U.S. Patent 1,975,504A, 2 October 1934. 97. Thavasi, V.; Singh, G.; Ramakrishna, S. Electrospun nanofibers in energy and environmental applications. Energy Environ. Sci. 2008, 1, 205–221. [CrossRef] 98. Song, W.; Mitchell, G.R.; Burugapalli, K. Electrospinning for Medical Applications. In Electrospinning: Principles, Practice and Possibilities; Geoffrey, R.M., Ed.; The Royal Society of Chemistry: Cambridge, UK, 2015; pp. 214–252. 99. Davis, F.J.; Mohan, S.D.; Ibraheem, M.A. Introduction. In Electrospinning: Principles, Practice and Possibilities; Geoffrey, R.M., Ed.; The Royal Society of Chemistry: Cambridge, UK, 2015; pp. 1–21. 100. Haghi, A.K. Electrospinning of Nanofibers in Textiles; CRC Press, Taylor & Francis Group: Boca Raton, FL, USA, 2012. 101. Renuga, G.; Kaur, S.; Ma, Z.; Chan, C.; Ramakrishna, S.; Matsuura, T. Electrospun nanofibrous filtration membrane. J. Membr. Sci. 2006, 281, 581–586. 102. Dai, W.S.; Barbari, T.A. Hydrogel membranes with mesh size asymmetry based on the gradient crosslinking of poly (vinyl alcohol). J. Membr. Sci. 1999, 156, 67–79. [CrossRef] 103. Chae, S.-K.; Mun, C.H.; Noh, D.-Y.; Kang, E.; Lee, S.-H. Simple Fabrication Method for a Porous Poly (vinyl alcohol) Matrix by Multisolvent Mixtures for an air-Exposed Model of the Lung Epithelial System. Langmuir 2014, 30, 12107–12133. [CrossRef] 104. Kim, S.-G.; Lee, K.H. Poly (vinyl alcohol) membranes having an integrally skinned asymmetric structure. Mol. Cryst. Liquid Cryst. 2009, 512, 32–39. [CrossRef] 105. Ahmad, A.L.; Yusuf, N.M.; Ooi, B.S. Preparation and modification of poly (vinyl) alcohol membrane: Effect of crosslinking time towards its morphology. Desalination 2012, 287, 35–40. [CrossRef] 106. Kim, K.-J.; Lee, S.-B.; Han, N.W. Effects of the degree of crosslinking on properties of poly (vinyl alcohol) membranes. Polym. J. 1993, 25, 1295–1302. [CrossRef] 107. Wang, Y.-L.; Yang, H.; Xu, Z.-L. Influence of post-treatments on the properties of porous poly (vinyl alcohol) membranes. J. Appl. Polym. Sci. 2008, 107, 1423–1429. [CrossRef] 108. Chuang, W.Y.; Young, T.H.; Chiu, W.Y.; Lin, C.Y. The effect of polymeric additives on the structure and permeability of poly (vinyl alcohol) asymmetric membranes. Polymer 2000, 41, 5633–5641. [CrossRef] 109. Chuang, W.-Y.; Young, T.-H.; Chiu, W.-Y. The effect of acetic acid on the structure and filtration properties of poly (vinyl alcohol) membranes. J. Membr. Sci. 2000, 172, 241–251. [CrossRef] 110. M’barki, O.; Hanafia, A.; Bouyer, D.; Faur, C.; Sescousse, R.; Delabre, U.; Blot, C.; Guenoun, P.; Deratani, A.; Quemener, D.; et al. Greener method to prepare porous polymer membranes by combining thermally induced phase separation and crosslinking of poly(vinyl alcohol) in water. J. Membr. Sci. 2014, 458, 225–235. [CrossRef] 111. Bouyer, D.; M’barki, O.; Pochat-Bohatier, C.; Faur, C.; Petit, E.; Guenoun, P. Modeling the membrane formation of novel PVA membranes for predicting the composition path and their final morphology. Aiche J. 2017, 63, 3035–3047. [CrossRef] 112. Young, T.-H.; Chuang, W.-Y. Thermodynamic analysis on the cononsolvency of poly (vinyl alcohol) in water–DMSO mixtures through the ternary interaction parameter. J. Membr. Sci. 2002, 210, 349–359. [CrossRef] 113. Colosi, C.; Marco Costantini, M.; Barbetta, A.; Pecci, R.; Bedini, R.; Dentini, M. Morphological Comparison of PVA Scaffolds Obtained by Gas Foaming and Microfluidic Foaming Techniques. Langmuir 2013, 29, 82–91. [CrossRef]

114. Zhou, C.; Wang, P.; Li, W. Fabrication of functionally graded porous polymer via supercritical CO2 foaming. Compos. Part B Eng. 2011, 42, 318–325. [CrossRef] 115. Reverchon, E.; Cardea, S.; Schiavo Rappo, E. Membranes formation of a hydrosoluble biopolymer (PVA)

using a supercritical CO2-expanded liquid. J. Supercrit. Fluids 2008, 45, 356–364. [CrossRef] 116. Reverchon, E.; Cardea, S.; Rapuano, C. Formation of poly-vinyl-alcohol structures by supercritical CO2. J. Appl. Polym. Sci. 2007, 104, 3151–3160. [CrossRef] Symmetry 2020, 12, 960 22 of 22

117. Narkkun, T.; Boonying, P.; Yuenyao, C.; Amnuaypanich, S. Green synthesis of porous polyvinyl alcohol membranes functionalized with L-arginine and their application in the removal of 4-nitrophenol from aqueous solution. J. Appl. Polym. Sci. 2019, 136, 47835. [CrossRef] 118. Wu, C.; Li, A.; Li, L.; Zhang, L.; Wang, H.; Qi, X.; Zhang, Q. Treatment of oily water by a poly(vinyl alcohol) ultrafiltration membrane. Desalination 2008, 225, 312–321. [CrossRef] 119. Wang, X.; Fang, D.; Yoon, K.; Hsiao, B.S.; Chu, B. High performance ultrafiltration composite membranes based on poly(vinyl alcohol) hydrogel coating on crosslinked nanofibrous poly(vinyl alcohol) scaffold. J. Membr. Sci. 2006, 278, 261–268. [CrossRef] 120. Nataraj, D.; Reddy, R.; Reddy, N. Crosslinking electrospun poly (vinyl) alcohol fibers with citric acid to impart aqueous stability for medical applications. Eur. Polym. J. 2020, 124, 109484. [CrossRef] 121. Peppas, N.A. Turbidimetric studies of aqueous poly (vinyl alcohol) solutions. Makromol. Chem. 1975, 176, 3433–3440. [CrossRef] 122. Stauffer, S.R.; Peppas, N.A. Poly (vinyl alcohol) hydrogels prepared by freezing-thawing cyclic processing. Polymer 1992, 33, 3932–3936. [CrossRef] 123. Li, H.; Wu, C.-W.; Wang, S.; Zhang, W. Mechanically strong poly (vinyl alcohol) hydrogel with macropores and high porosity. Mater. Lett. 2020, 266, 127504. [CrossRef] 124. Lee, J.-T.; Wey, M.-Y. PVA/Pt/N-TiO2/SrTiO3 porous films with adjustable pore size for hydrogen production under simulated sunlight. J. Colloid Interface Sci. 2020, 573, 158–164. [CrossRef]

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