Porous Polyvinyl Alcohol Membranes: Preparation Methods and Applications

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.

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.   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.   [25]. Copyright 1998 Elsevier 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. 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]. The reported density for the amorphous part of PVA is p a = 1.269 g/cc, while for the crystalline part it is p c = 1.345 g/cc [31,32].  Reproduced by permission from [33]. Copyright 2011 Wiley Periodicals, Inc.
Due to these properties, PVA has found its way in a wide spectrum of technological fields ranging from biomedical to construction. Indicative applications include fibers, water soluble packaging, protective colloid in emulsion and suspension polymerization, adhesives, sizing, paper industry, tissue engineering [41], wound dressing [42] contact lenses, drug delivery systems [43][44][45][46], and orthopedics [47].
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][50][51]. Chemical crosslinking requires di-functional Additionally, the great film forming, adhesion, good mechanical and optical properties, O 2 barrier capability in low humidity [33][34][35], non-toxicity [36,37], biocompatibility and partial biodegradability [38,39], and relatively easy nanocomposite preparation capabilities [40] render PVA a valuable candidate material in diverse and demanding applications.
Due to these properties, PVA has found its way in a wide spectrum of technological fields ranging from biomedical to construction. Indicative applications include fibers, water soluble packaging, protective colloid in emulsion and suspension polymerization, adhesives, sizing, paper industry, tissue engineering [41], wound dressing [42] contact lenses, drug delivery systems [43][44][45][46], and orthopedics [47].
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][50][51]. Chemical crosslinking requires di-functional crosslinking reagents [49] that typically include glutaraldehyde [52,53], as can be seen in Figure 4a,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 Symmetry 2020, 12, x FOR PEER REVIEW 6 of 25    Table 1. 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 Figure 5. Other crosslinking methods include freeze/thaw method [16], heat treatment [55], and γ-irradiation [56]. 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]. 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].
Furthermore, poly (vinyl alcohol) has attracted a lot of research interest as a membrane for separations, especially in water treatment due to its low biofouling tendency [74], a known issue that affects various membrane processes [75].

Polymeric Porous Structures
Over the last 60 years or so, there has been great progress in several techniques that have been developed for the preparation of porous polymeric membranes both on industrial and lab scale [76][77][78][79] including (a) phase inversion also called phase separation (PS), (b) track-etching, (c) stretching, (d) sintering, (e) electrospinning, and interfacial polymerization [80]. In parallel, the effects of nanoparticle incorporation during membrane preparation have also been extensively studied for both performance and structure control [81,82].
Furthermore, poly (vinyl alcohol) has attracted a lot of research interest as a membrane for separations, especially in water treatment due to its low biofouling tendency [74], a known issue that affects various membrane processes [75].

Polymeric Porous Structures
Over the last 60 years or so, there has been great progress in several techniques that have been developed for the preparation of porous polymeric membranes both on industrial and lab scale [76][77][78][79] including (a) phase inversion also called phase separation (PS), (b) track-etching, (c) stretching, (d) sintering, (e) electrospinning, and interfacial polymerization [80]. In parallel, the effects of nanoparticle incorporation during membrane preparation have also been extensively studied for both performance and structure control [81,82].
In the following paragraphs, the main techniques for obtaining porous structures and how they were applied for the case of PVA will be described.

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 Symmetry 2020, 12, 960 6 of 22 lean phase, the pores will be created. Structures obtained by this method typically result in a sponge or finger-like macropores (i.e., pore diameter > 50 nm) [84].
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]: Non solvent induced phase separation (NIPS), also referred to as immersion precipitation, in which the addition of a non-solvent in the polymer solution results in a miscibility gap, according to the ternary diagram of the system as shown in Figure 6 [87]. NIPS is the primary technique for the production of asymmetric structures and polymeric membranes on an industrial scale, resulting in pore size diameters ranging from hundreds of nanometers to 10+ µm. Typically, the polymeric solution is casted in the appropriate support (e.g., a non-woven fabric [88]) and is afterward dipped in a non-solvent bath, resulting in the immediate solid membrane formation.
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 a sponge or finger-like macropores (i.e., pore diameter > 50 nm) [84].
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]: Non solvent induced phase separation (NIPS), also referred to as immersion precipitation, in which the addition of a non-solvent in the polymer solution results in a miscibility gap, according to the ternary diagram of the system as shown in Figure 6 [87]. NIPS is the primary technique for the production of asymmetric structures and polymeric membranes on an industrial scale, resulting in pore size diameters ranging from hundreds of nanometers to 10+ μm. Typically, the polymeric solution is casted in the appropriate support (e.g., a non-woven fabric [88]) and is afterward dipped in a non-solvent bath, resulting in the immediate solid membrane formation.
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 changing in favor of the non-solvent and eventually results in polymer precipitation, forming an asymmetric skinned membrane. Vapor-induced PS (VIPS) was first introduced by Zsigmondy and Bachmann [89] in 1918 and then developed further by Elford in 1937 [90]. In the VIPS process, the casted membrane is placed in a controlled environment chamber containing non-solvent vapors, which penetrate from the vapor phase to the polymer solution and induce phase inversion [91][92][93]. In general, 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. 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 changing in favor of the non-solvent and eventually results in polymer precipitation, forming an asymmetric skinned membrane.
Vapor-induced PS (VIPS) was first introduced by Zsigmondy and Bachmann [89] in 1918 and then developed further by Elford in 1937 [90]. In the VIPS process, the casted membrane is placed in a controlled environment chamber containing non-solvent vapors, which penetrate from the vapor phase to the polymer solution and induce phase inversion [91][92][93]. In general, 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.

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  [94,95]. Although not the most popular porous material manufacturing process, electrospinning has evolved, especially in the last few years, into a promising manufacturing process of membrane technology called electrospun nanofibrous membranes (ENMs). By carefully selecting the proper fiber diameter, consecutive layering and packing density, a porous asymmetric structure can be therefore shaped with membrane characteristics such as high air permeability and targeted liquid entry and bubble point pressure. Electrospinning was introduced as a mean of processing textile yarns in the 1930s [96], though it originates in the studies of Lord Rayleigh, and since then it was used in a variety of material preparation [97] including among others, medical applications [98], protective textiles, [99,100], and filtration membranes [101]. The main advantages of ENMs is their high surface area and porosity, open structure, controllable manufacturing, and nanocomposite capability. Typically ENMs are produced as thin layers, which depending on the transmembrane pressure of the intended application can require the need of a support (usually a non-woven fabric) in order to provide mechanical stability.

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 force ( Figure 7) [94,95]. Although not the most popular porous material manufacturing process, electrospinning has evolved, especially in the last few years, into a promising manufacturing process of membrane technology called electrospun nanofibrous membranes (ENMs). By carefully selecting the proper fiber diameter, consecutive layering and packing density, a porous asymmetric structure can be therefore shaped with membrane characteristics such as high air permeability and targeted liquid entry and bubble point pressure. Electrospinning was introduced as a mean of processing textile yarns in the 1930s [96], though it originates in the studies of Lord Rayleigh, and since then it was used in a variety of material preparation [97] including among others, medical applications [98], protective textiles, [99,100], and filtration membranes [101]. The main advantages of ENMs is their high surface area and porosity, open structure, controllable manufacturing, and nanocomposite capability. Typically ENMs are produced as thin layers, which depending on the transmembrane pressure of the intended application can require the need of a support (usually a non-woven fabric) in order to provide mechanical stability. A special category of PVA based porous materials is the development of hydrogels that have been studied for many years, mainly for biomedical applications [39]. Although not intended to function as membranes, their preparation routes can be adapted to obtain porous polymeric structures as supports and separating elements [102].

Porous Poly(Vinyl Alcohol) Preparation Methods
Chae et al. [103] reported a phase inversion method to prepare porous PVA membranes using water as the solvent and isopropyl alcohol as the non-solvent. The membrane was composed of packed microspheres as a result of crystallization with its degree depending on the solvent/nonsolvent ratio. The preparation route was as follows: 1 mL of the PVA aqueous solution (with concentration of 2, 4, 6, 8, or 10% PVA) was poured into a polystyrene (PS) Petri dish filled with 20 mL isopropanol. To perform the crosslinking of the formed structure, 3 mL of gludaraldehyde (4 wt%) and 0.5 mL of hydrochloric acid (35-37%) was 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. A special category of PVA based porous materials is the development of hydrogels that have been studied for many years, mainly for biomedical applications [39]. Although not intended to function as membranes, their preparation routes can be adapted to obtain porous polymeric structures as supports and separating elements [102].

Porous Poly(Vinyl Alcohol) Preparation Methods
Chae et al. [103] reported a phase inversion method to prepare porous PVA membranes using water as the solvent and isopropyl alcohol as the non-solvent. The membrane was composed of packed microspheres as a result of crystallization with its degree depending on the solvent/nonsolvent ratio. The preparation route was as follows: 1 mL of the PVA aqueous solution (with concentration of 2, 4, 6, 8, or 10% PVA) was poured into a polystyrene (PS) Petri dish filled with 20 mL isopropanol. To perform the crosslinking of the formed structure, 3 mL of gludaraldehyde (4 wt%) and 0.5 mL of hydrochloric acid (35-37%) was 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. The PVA chains were found to be self-aggregated into a porous structure under the influence of dipole−dipole interactions in a low-polarity solvent (Figure 9), while the porosities of the matrices prepared using 4−10% PVA solutions were estimated at 67.6−14.9% The PVA chains were found to be self-aggregated into a porous structure under the influence of dipole−dipole interactions in a low-polarity solvent (Figure 9), while the porosities of the matrices prepared using 4−10% PVA solutions were estimated at 67.6−14.9% Kim and Lee [104] reported the preparation of integrally 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. Kim and Lee [104] reported the preparation of integrally 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. 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 Na 2 SO 4 /KOH/H 2 O 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.
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 Na 2 SO 4 /KOH/H 2 O 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 H 3 O + , affecting the acid-base equilibrium during coagulation. Symmetry 2020, 12, x FOR PEER REVIEW 13 of 25 Figure 11. Effects of crosslinking time toward the PVA membrane surface. Reproduced with permission from [78]. Copyright © 2011 Elsevier B.V.
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. 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 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. In order to produce the porous membranes, hydrocloric or sulfuric acid was added to the casting solution (PVA 10 wt% and gludaraldehyde 0.5 wt%) and after 1 min of stirring, it was cast onto a glass substrate that was placed in a heated support of the required temperature under a controlled relative humidity environment for 15 min. Afterward, the prepared membrane was removed and dried at 60 °C. Results indicated that the crosslinking control was the key step to obtaining porous membrane morphology (Figure 14). In order to produce the porous membranes, hydrocloric or sulfuric acid was added to the casting solution (PVA 10 wt% and gludaraldehyde 0.5 wt%) and after 1 min of stirring, it was cast onto a glass substrate that was placed in a heated support of the required temperature under a controlled relative humidity environment for 15 min. Afterward, the prepared membrane was removed and dried at 60 • C. Results indicated that the crosslinking control was the key step to obtaining porous 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 CO 2 as a component in polymeric solutions [114]. Studies conducted by Reverchon et al. [115,116] reported the preparation of PVA membranes by supercritical CO 2 assisted phase inversion. The authors reported their findings with respect to preparation conditions such as polymer concentration, ethanol to CO 2 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. 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. The gas foaming method was used by Narkkun et al. [117] to produce L-arginine functionalized PVA (average molecular weight 130,000 g mol −1 and 99% hydrolyzed) with CO 2 introduced from the thermal decomposition of sodium bicarbonate (NaHCO 3 ) at 130 • C. The membranes exhibited a structure with an average pore size of 32-56 µm depending on the amount of grafted L-arginine.
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.  The gas foaming method was used by Narkkun et al. [117] to produce L-arginine functionalized PVA (average molecular weight 130,000 g mol −1 and 99% hydrolyzed) with CO2 introduced from the thermal decomposition of sodium bicarbonate (NaHCO3) at 130 °C. The membranes exhibited a structure with an average pore size of 32-56 μm depending on the amount of grafted L-arginine.
Wu et al. [118] reported the preparation of a composite PVA ultrafiltration membrane for the treatment of oily water by crosslinking PVA to a mixed cellulose ester microfiltration membrane and the results indicated that the prepared membrane had excellent antifouling properties against oil.
Treatment of oil/water emulsions was also studied by means of highly porous electrospun PVA membranes (crosslinking with GA in acetone) by Wang et al. [119] using a variety of PVA grades. The ultrafiltration experiments were performed in PVA coated scaffolds with a hydrogel layer of ~1.8 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.  The gas foaming method was used by Narkkun et al. [117] to produce L-arginine functionalized PVA (average molecular weight 130,000 g mol −1 and 99% hydrolyzed) with CO2 introduced from the thermal decomposition of sodium bicarbonate (NaHCO3) at 130 °C. The membranes exhibited a structure with an average pore size of 32-56 μm depending on the amount of grafted L-arginine.
Wu et al. [118] reported the preparation of a composite PVA ultrafiltration membrane for the treatment of oily water by crosslinking PVA to a mixed cellulose ester microfiltration membrane and the results indicated that the prepared membrane had excellent antifouling properties against oil.
Treatment of oil/water emulsions was also studied by means of highly porous electrospun PVA membranes (crosslinking with GA in acetone) by Wang et al. [119] using a variety of PVA grades. The ultrafiltration experiments were performed in PVA coated scaffolds with a hydrogel layer of ~1.8 Wu et al. [118] reported the preparation of a composite PVA ultrafiltration membrane for the treatment of oily water by crosslinking PVA to a mixed cellulose ester microfiltration membrane and the results indicated that the prepared membrane had excellent antifouling properties against oil.
Treatment of oil/water emulsions was also studied by means of highly porous electrospun PVA membranes (crosslinking with GA in acetone) by Wang et al. [119] using a variety of PVA grades. The ultrafiltration experiments were performed in PVA coated scaffolds with a hydrogel layer of 1.8 µm ( Figure 17) and compared to a Pebax 1074 coating with the PVA water flux reaching 130 LMH and Pebax 57 LMH. Citric acid crosslinking stability of electrospun PVA fibers have been proposed as a promising alternative to GA [120] while Truong et al. [54] reported a comparison of crosslinking stability by using citric acid (Figure 18), maleic acid, and PAA to PVA (Mw 100,000). The prepared membranes where evaluated 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. μm ( Figure 17) and compared to a Pebax 1074 coating with the PVA water flux reaching 130 LMH and Pebax 57 LMH. Citric acid crosslinking stability of electrospun PVA fibers have been proposed as a promising alternative to GA [120] while Truong et al. [54] reported a comparison of crosslinking stability by using citric acid (Figure 18), maleic acid, and PAA to PVA (Mw 100,000). The prepared membranes where evaluated 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). Porous films have been also reported to be produced by SiO 2 etching. According to Lee and Wey [124], they dispersed the nanoparticles in a PVA solution that were cast onto a glass with a 250 µm thickness overnight, and the next day, the dried membrane was peeled off and annealed for 2 h at 160 • C under vacuum. A 2 M NaOH solution was used to etch the SiO 2 , resulting in adjustable pore size porous membranes. 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.
Porous films have been also reported to be produced by SiO2 etching. According to Lee and Wey [124], they dispersed the nanoparticles in a PVA solution that were cast onto a glass with a 250 μm thickness overnight, and the next day, the dried membrane was peeled off and annealed for 2 h at 160 °C under vacuum. A 2 M NaOH solution was used to etch the SiO2, resulting in adjustable pore size porous membranes.

Conclusions and Future Prospective
Membranes, especially those developed for water treatment, are expected to be of research and industrial focus for many years to come due to factors like climate change, environmental pollution, and population growth. PVA based porous membranes will continue to be developed by incorporating functional nanomaterials and by finding innovative ways of pore structure control like microfluidics. Currently, PVA is one of the polymers of choice when it comes to dehydration via the pervaporation method. Various types of commercially available PVA membranes exist in the market with the aim to provide solutions for dehydration or methanol removal from mixtures of volatile organic compounds, a trend that is expected to continue. Crosslinking density as well as the crosslinkers will be in the forefront of pervaporation research, aiming eventually at increased process thermal 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. Based on the above, PVA is 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 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

Conclusions and Future Prospective
Membranes, especially those developed for water treatment, are expected to be of research and industrial focus for many years to come due to factors like climate change, environmental pollution, and population growth. PVA based porous membranes will continue to be developed by incorporating functional nanomaterials and by finding innovative ways of pore structure control like microfluidics. Currently, PVA is one of the polymers of choice when it comes to dehydration via the pervaporation method. Various types of commercially available PVA membranes exist in the market with the aim to provide solutions for dehydration or methanol removal from mixtures of volatile organic compounds, a trend that is expected to continue. Crosslinking density as well as the crosslinkers will be in the forefront of pervaporation research, aiming eventually at increased process thermal 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. Based on the above, PVA is 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 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 resistance, consistent selectivity under different operating environments, low fouling, and improved mechanical properties. Intensive research currently undertaken by groups around the world exploring the unique properties of PVA ensures successful solutions for the above-mentioned aspects.
Funding: This research received no external funding.