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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.33 No.3 pp.110-126
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2023.33.3.110

Studies of the Membrane Formation Techniques and Its Correlation with Properties and Performance: A Review

Kumari Nikita*, Chivukula Narayana Murthy**, Sang Yong Nam*,***†
*Research Institute for Green Energy Convergence Technology, Gyeongsang National University, Jinju 52828, Korea
**Applied Chemistry Department, Faculty of Technology and Engineering, The Maharaja Sayajirao University of Baroda, Vadodara 390001, India
***Department of Materials Engineering and Convergence Technology, Gyeongsang National University, Jinju 52828, Korea
†Corresponding author(e-mail: walden@gnu.ac.kr; http://orcid.org/0000-0002-6056-2318)
June 21, 2023 ; June 21, 2023 ; June 23, 2023

Abstract


In this review, the approaches, properties, and elements involved in the formation of polymeric membranes for various materials are discussed. The present research emphasizes the proficiency in several membrane formation processes such phase inversion, interfacial polymerization, stretching, track etching, and electrospinning. Additionally, the obstacles and applicability of various application manufacturing processes are addressed. Various polymeric membranes are reviewed with regard to significant surface properties such as surface roughness, surface tension, surface charge and surface functional group. Additional enhancements of popular membrane formation processes like phase inversion and interfacial polymerization are required to ensure advancements in membrane efficiency. Analysing the possibilities of modern manufacturing practices like track etching and electrospinning is also crucial.


polymeric membranes , formation processes , surface properties , phase inversion

막 형성 기술 및 특성과의 상관관계 연구 및 성능: 리뷰

쿠마리 니키타*, 치부쿨라 나라야나 머티**, 남상용*,***†
*경상대학교 그린에너지융합기술연구소
**더 마라하자 사야이라오 대학교 기술공학부 응용화학과
***경상대학교 나노신소재융합공학과

초록


이 총설에서는 다양한 소재를 이용한 고분자 분리막의 제조를 위한 제조방식, 특성과 여러가지 인자들에 대해서 논의하고자 한다. 분리막 제조방식은 상전이, 계면중합, 연신, 트랙에칭 그리고 전기방사 같은 방법을 주로 강조하여 설명하 고자 하며, 추가적으로 다양한 응용에 따른 제조방식에 대한 한계나 응용성에 대해서도 설명하고자 한다. 또한 다양한 고분자 분리막의 표면거칠기, 표면장력, 표면전하와 표면의 기능성 작용기 같은 표면특성에 대해서도 정리하였으며, 막성능의 향상을 위하여 상전이법이나 계면중합 같은 일반적인 분리막 제조공정에서 필요한 추가적인 향상방법을 나타내었다. 트랙에칭이나 전기방사와 같은 새로운 제조방법의 가능성에 대해서도 분석하였다.


    1. Introduction

    Using filtration process, a solid component is eliminated from a fluid stream mostly because of variations in the sizes of the fluid and particle populations. Typically, the term “filtration” signifies the process of removing particles from a liquid or gaseous stream[1]. For the selective separation of mixtures of gases and multicomponent solutions, membrane filtration expands the possibilities of solid-liquid separations to include colloidal particles, macromolecules, and dissolved solutes[ 2]. Membranes have many benefits for molecular separations, including the following: (a) not requiring for a phase transition of the solute or the carrier solvent; (b) outstanding selectivity and productivity; and (c) no requirement for regeneration of solid or liquid sorbents. As a result, membranes are commonly employed in numerous significant chemical, biological, and environmental fields.

    During the fabrication of membranes, a variety of synthetic substances, like ceramics, glasses, metals, or polymers, can be used. The purpose of the process is to produce a structure of membranes with morphology that is specifically suited for a separation. Phase inversion, interfacial polymerization, electrospinning, sintering, stretching, track-etching, template-leaching, and dip-coating are a few approaches to preparation that can be used to fabricate a membrane from a specific substance with the appropriate membrane morphology [3]. The most popular methods of making polymeric membranes are phase inversion, interfacial polymerization, stretching, track-etching, and electrospinning [4]. The effectiveness of the manufactured membrane, its characteristics, and its cost for production all influence the procedure selection. Thermodynamic circumstances affect the membrane's structure and efficiency, while selectivity and the pressure differential throughout the membrane affect thermodynamic efficiency. Selectivity of the membrane rises along with permeation as thermodynamic efficiency grows[5].

    This review offers information on several membrane production methods and the factors that determine the performance of the resulting membrane, such as feed characteristics and surface properties.

    2. Membrane Formation Techniques

    2.1. Phase inversion process

    The phase inversion technique involves converting a polymer's solid form into a solution or molten form under highly controlled parameters. The controlled atmosphere reveals the perfect parameters for the fabrication of the membrane. Thermodynamics and kinetics are the two main driving forces behind phase inversion manufacturing processes. The interchange of constituents among a solvent and a non-solvent is the main reason of the phase inversion. Various techniques, such as immersion precipitation, thermally induced phase separation, evaporation induced phase separation and vapor induced phase separation, can be used for the manufacturing[6].

    2.1.1. Immersion precipitation

    In the technique of immersion precipitation, a polymer solution is cast on an appropriate substrate and submerged in a coagulation bath having a non-solvent, where an interchange of the solvent and non-solvent occurs and the membrane is precipitated. Therefore, the solvent and non-solvent used in this process need to be miscible. Fig. 1 provides a schematic representation of the procedures following the immersion of a polymer solution in a non-solvent environment. In contrast to the non-solvent, which diffuses into the cast film at flux J1, the solvent diffuses into the coagulation bath at flux J2. Once a certain period of time has passed, the solvent and non-solvent are exchanged until the solution becomes thermodynamically unstable and demixing occurs. Finally, an asymmetrically structured solid polymeric membrane is produced. In most cases, J2 > J1 “skin” ultrafiltration (UF) membranes with pore sizes of 10~300 are developed, whereas J2 = J1 is mostly used to produce microfiltration (MF) membranes having pore sizes of 0.2~0.5 m. One of the biggest and most significant achievements in desalination for membrane technologies was the fabrication of the first high-flux anisotropic acetate cellulose (CA) ROmembranes by Loeb and Sourirajan[7].

    The ability to alter the membrane's pore structure, particularly its cross-section morphology, by choosing the right polymer, solvents and non-solvents, additives, precipitation time, bath temperature, and other factors throughout immersion precipitation is now well understood[ 8-14]. In addition to a casting polymer's chemical composition, the concentration of the polymer plays a crucial role in the development of membranes through immersion precipitation. A higher polymer content in the casting fluid results in membranes having smaller pores and lower porosity. The propensity to generate sponge-like formations is increased in this situation while the development of macro voids is reduced. In contrast to reverse osmosis (RO) membranes, which are normally made from casting solutions having polymer concentrations of greater than 20 wt%, UF membranes can be manufactured in the polymer concentration ranging from 12 to 20 weight percent[15]. The appearance and characteristics of formed membranes are significantly influenced by the choice of solvent or non-solvent medium. During membrane casting, aprotic solvents are usually chosen because they lack hydrogen atoms that could participate in hydrogen bonds. It is preferred to use an aprotic polar solvent, like dimethyl formamide, dimethyl acetamide, N-methyl-2-pyrrolidone or dimethyl sulfoxide, for prompt demixing (fast precipitation), which results in anisotropic membranes having significant porosity [16]. An additive can speed up the phase inversion procedure, act as a pore former, or enhance the viscosity of the solution. Several inorganic and high molecular weight organic (like polyvinyl pyrrolidone (PVP) or poly(ethylene glycol (PEG)) additives to casting solution are frequently utilized in order to enhance the membrane structure and performance[17,18]. For instance, various studies[19-23] looked into the impact of LiCl addition on membrane development. At low LiCl concentrations of 2.5 wt%, Fontananova et al.'s research showed that incorporating LiCl to the polyvinyledine fluoride (PVDF)/dimethylacetamide dope increased the flux of the casted membranes, but at high LiCl concentrations of 7.5%, it reduced macrovoid creation and lowered the flux of the membrane permeation[ 19]. When casting solutions of poly(amic acid) (PAA) with N-methyl-2-pyrrolidone, Lee et al. found comparable outcomes[20]. For the PAA/N-methyl-2- pyrrolidone structure, they observed that by raising the LiCl concentration, the solution viscosity may be increased to the level when macro void generation is inhibited and the growth of a highly porous structure is encouraged. The changes in the thermodynamic and kinetic characteristics of the polymer dope combination prior to and following the addition of LiCl were thought to be related to the aforementioned facts. According to Saljoughi et al., an enhanced pure water flux was obtained, as PVP content increased from 0 to 1.5 wt% in the prepared film due to the formation of macro void in the membrane sub-layer[21]. Subsequently was shown that the water flux decreased due to the lesser macro void formation as PVP concentration increased from 1.5 to 3, 6 and 9 wt%. The PES membrane containing PVP has a larger water flux and a smaller water contact angle than the pristine polyether sulfone (PES) membrane, as shown by Wang et al.[22]. Ochoa et al. studied, as the PVP level in the casting solution was 10 wt%, the contact angle dropped by 16% and the increase in the UF PES membrane permeability when PVP is added to the casting solution without significantly altering selectivity[23].

    New advances in membrane characteristics, such as higher strength and modulus, brought about by the significant interfacial interactions the nanoparticles have with the surrounding polymer matrix have made the utilization of inorganic nanoparticles as additives to polymeric membranes increasingly popular[24]. New research by Ng et al. provides a thorough overview of polymeric membranes containing metal/metal oxide nanoparticles[25]. By incorporating nanoparticles in the casting fluid, Zodrow et al. produced polysulfone membranes containing Ag nanoparticles (1~70 nm) through phase inversion process[26]. It was demonstrated that pure polysulfone membranes and membranes impregnated with 0.9 wt% Ag nanoparticles had comparable permeabilities and surface charges. But pure polysulfone membranes were far more hydrophilic, with a 10% decrease in contact angle. Ag nanoparticle insertion was shown to have no discernible impact on the membrane's morphology. Yan et al., who fabricated PVDF membranes using nano-sized Al2O3 particles in casting solutions containing dimethylacetamide, reported identical outcomes[27]. It was discovered that higher Al2O3 contents in the casting solution, ranging from 0 to 2%, had enhanced water permeate flux because they improved the membrane's hydrophilicity. The cross-section, surface and inner pore arrangements of the membrane architectures were not impacted by the incorporation of nano-sized Al2O3, in accordance with scanning electron microscopy (SEM) images. Conventional asymmetric structure with finger-like pores was present in both pure PVDF and PVDF Al2O3 membranes.

    However, Yang et al. demonstrated that TiO2 nanoparticles added to TiO2/PS membranes made from 18 wt% PS solution in N, N-dimethylacetamide with N-methyl-2-pyrrolidinone have a significant impact upon the membrane morphology[28]. The cross-section morphologies of membranes indicating that as the amount of TiO2 is increased, the thickness of the skin layer enhances. At low TiO2 levels, macro voids expand and become run through, but at larger additive dosages (3 wt%), they are reduced. Membranes having 1-2 wt% TiO2, and more tiny pores are present than in the neat polysulfone (PSF) membrane. Although boosting the development of bigger pores (50-70 nm) brought on by the nanoparticle aggregation, resulting in a bimodal pore distribution, through incorporating extra TiO2 (3%) to the casting solution. Due to the existence of bigger pores, the mean pore radius of the membrane having 1-2 wt% TiO2 concentration dropped at low TiO2 concentration and then improved at increasing TiO2 concentration. Such outcomes show that the PS matrix may be made more porous and have a greater number of tiny pores by including suitable TiO2 nanoparticles. Thus, it is possible to considerably enhance the flux across these membranes. Additionally, it was demonstrated that the incorporation of TiO2 nanoparticles results in a drop in contact angle from 85° for pristine PS membrane to 41~52° over TiO2/polystyrene (PS) membranes, illustrating the fact that the inclusion of TiO2 improves the membrane's hydrophilicity as some hydrophilic TiO2 nanoparticles adsorb to and adhere to the membrane surface.

    This should be noted, nevertheless, that excessive nanoparticle accumulation, which leads to a poor dispensability in the casting solution, is one of the variables which restricts the inclusion of nanoparticles through polymeric membranes. Additionally, to reduce potential (eco) contamination impacts, strict oversight and control of the nano-particles emanating through the altered membranes are required.

    2.1.2. Controlled evaporation precipitation

    Solvent evaporation is an alternate approach of phase separation. By combining polymeric components with solvent, a homogeneous solution is developed. It is then dispersed across a porous support after being poured on it. The solvent utilized in the arrangement evaporates when it dries in the air. Across the porous support, the rest of the components generate a polymeric thin layer. Utilizing the appropriate solvents, the pore size of the film can be modified. Employing several organic solvents, Nguyen et al. produced PS, PVDF, polyvinyl acetamide (PVAc), and polyvinyl chloride (PVC) microporous membranes and investigated the way various solvents affected the surface structure and pore dimensions[29]. Microporous polystyrene films have been developed by Kim et al. utilizing PEG as the pore forming agent[30]. Variable polystyrene/ PEG ratios and various PEG molecular weights were used for modifying the membranes' 5~12 μm range pore size. Employing this approach, Zhao et al. generated silicon rubber microporous membranes[31]. Through adjusting the temperature of the casting assembly and the liquid paraffin level, the pore dimension and morphology of the membranes were modified. The development of porous silicon membranes that had a variety pore dimension and morphology was depicted in Fig. 2.

    2.1.3. Thermal induced phase separation

    In this method, a polymer and solvent are mixed together to form the membrane. The elevated temperatures lead the solvent to evaporate, allowing the solution to solidify into films. The technique used generates a membrane comprising a fibrous framework and a tangled polymer matrix[32]. These approaches are commercially available and are utilized to produce membranes for usage in industrial and medicinal contexts. Thinner microporous membranes can be made using the thermally induced phase separation (TIPS) approach. Non-solvent induced phase separation (NIPS) takes place whenever the solvent is hydrophilic. These days TIPS and NIPS are used in conjunction with each other. TIPS offer a variety of benefits, including membranes that have an uncomplicated procedure, substantial porosity, few flaws and simple repeatability. The physio-chemical characteristics of the membranes in TIPS are governed by the solvent utilized. The solvent must therefore have little volatility as well as excellent thermal resistance in order to be appropriate for this method. To synthesize a polyvinylidene fluoride membrane with outstanding mechanical properties and a confined pore size distribution, acetyl tri-butyl citrate is used as a solvent through TIPS[33]. Dimethyl sulfone is used as a diluent in this process in order to generate polyacrylonitrile membranes. According to investigations on the membrane, a greater amount or cooling speed of polyacrylonitrile improves both the elongation and tensile characteristics of the membrane [34]. In an effort to produce a hydrophilic rough-surfaced membrane having cellular pores using polyethylene and polyethylene glycol as co-polymers liquid- liquid TIPS, a polyethylene with a high density membrane is constructed[35].

    2.2. Interfacial Polymerization

    The most significant technique for producing NF and thin-film composite (TFC) RO membranes commercially is interfacial polymerization (IP). With the fabrication of the first interfacially polymerized TFC membranes, Cadotte et al. made a significant advancement in the efficiency of membranes for RO functions [36, 37]. The initially proposed procedure for IP included immersing a microporous polysulfone matrix in an aqueous solution of a polymeric amine before dipping the membrane inside a di-isocyanate in hexane solution. Following that, the membrane was heated to 110 °C to cross-link it[36]. When compared to an integrally- skinned asymmetric cellulose acetate membrane, the consequent TFC polyurea membrane demonstrated greater salt rejection and more water flux[37]. Multiple TFC membranes have been produced effectively as a result of the IP technique's advantageous properties in separately modifying the skin layer's and the microporous substrate layer's characteristics[38,39]. The intrinsic morphological features and constitution of the barrier membrane surface are influenced by a number of variables, including reaction duration, solvent kind, post-treatment parameters and monomer content [39-41]. The majority of IP-produced RO and NF membranes have a thinner PA film on the surface of the membrane substrate. The two most often utilized reactive monomers to fabricate a functional PA layer in RO/NF membranes are m-phenylenediamine (MPD) and trimesoyl chloride (TMC) as shown in Fig. 3.

    Triethylenetetramine[42], p-phenylenediamine[43], piperazine[ 44], N-(2-aminoethyl)-piperazine[45], and N-N'- diaminopiperazine[46] are additional amine monomers utilized in the fabrication of TFC PA membranes. For the IP approach employed for manufacturing TFC membranes, new monomers have recently been proposed[ 46]. The manufactured membrane has a surface that is more smooth or improved hydrophilicity due to the presence of additional functional groups in such monomers, making them useful for enhancing the antifouling feature of the membranes. For the development of TFC RO membranes containing MPD, Li et al. produced two novel tri- and tetrafunctional biphenyl acid chlorides (3,4',5-biphenyl triacyl chloride and 3,3',5,5'- biphenyl tetraacyl chloride)[47]. A unique RO composite membrane made from 5-isocyanatoisophthaloyl chloride and MPD was reported by Liu et al. in their paper from the same year[48]. Along with looking into new IP monomers, reactive organic modifiers have been added to TMC or MPD solutions in an attempt to optimize the IP technique. These modifiers can interact with the process and are incorporated within the functional barrier layer, which enhances the surface characteristic and fouling rebellion of the produced RO membranes. The chlorine resistance of TFC PA membranes towards oxidative damage has been improved via multiple attempts[49,50]. The efficacy of chlorine resilience is in the sequence of aromatic, cycloaliphatic, and aliphatic diamines, respectively. It has been observed that chemically altering of PA layer by the application of diamine moieties may considerably improve the chlorine resilience of the membrane[51]. TFC polyester and polyesteramide membranes were developed as well using the IP approach, in addition to TFC PA membranes[52,53]. The addition of ester linkage improved the membrane's resistance to oxidation, which considerably improved the membrane's sensitivity to chlorine damage.

    2.3. Stretching

    Extrusion and stretching are utilized to produce the microporous membranes that are frequently used in MF, MD and UF. Stretching initially emerged for the manufacture of polymer membranes in the 1970s, and various companies regulated the patented technology. Membranes composed of poly ethylene (PE) and poly propylene (PP) are typically produced by Celgard® for application in storage of energy[54]. In order to generate porous membranes employing homopolymers, stretching process is a no solvent method. Polyethylene, polypropylene, and poly tetrafluoroethylene (PTFE) are the most commonly utilized polymers for porous membranes[ 55]. In this approach, a polymer is heated to its melting point and then quickly pulled downwards while being dispensed, which results in an aligned polymeric chain. To develop axial networks, stretching is initially performed under cold conditions. It is then followed by stretching under hot conditions, typically between 130 and 145 °C, to boost the ultimate pore configuration[ 56]. The manufacturing conditions employed influence the physical characteristics and pore dimensions[ 57]. Uniaxial and biaxial stretching are two distinct classifications for stretching orientation. Biaxial stretching is used for producing membranes made of nanofiber-rich polyethylene. The above approach is preferable because it produces a membrane containing fibers and involves stretching in two distinct directions at a temperature closer to the melting point of polyethylene[ 58]. In polymeric films, stretching is mostly used to create pores or increase the dimension of existing pores[59]. To expand the pores, the polymeric layer is irradiated and put through an etching and rinsing procedure[60]. Key highlights of this approach include control over thickness of membrane along with excellent mechanical characteristics.

    2.4. Track etching

    In this method, a nonporous polymeric film is exposed to reactive heavier ions, causing linear degraded tracks to develop throughout the exposed polymeric surface. Fig. 4 depicts the layout for a single ion-irradiation system. Track etching is a technique for fabricating membranes that originated by Nucleopore Corporation. The primary benefit of this method above others is the ability to regulate a broad range in the arrangement of pore size and density across the surface [61]. Track etched membranes are able to be employed for a variety of purposes by adjusting the pore arrangements. The etching period and irradiation temperature time are determining the changes in pore arrangements. This method is employed for producing membranes made of nano-porous graphene-polyethylene terephthalate using NaOH to serve as a solvent. Etching takes place within 7 and 16 minutes when the object is submerged in a solution of sodium hydroxide (1.5 mol L-1) at 80 °C or 50 °C. The outcomes are highly contradictory given that the average pore size is 400 nm[62]. Chemical parameters, optimizing temperature and the limiting elements membrane resistance and ion selectivity will result in a limited size distribution[ 63]. According to the data collected, ion radiation significantly impacts the density and development of pores in track-etched membranes[64]. The advantages of track etching technologies include steady particle flux, excellent stability, affordability and significant porosity but little overlapping[65]. The functionality of track etched membranes is straightforward, and they may be grown down from single pores to several pores[62].

    2.5. Electrospinning

    The fabrication of porous membranes for use in desalination and filtration is possible using the process of electrospinning[63-65]. The visual representation of electrospinning is shown in Fig. 5. With the help of this approach, polymeric films having a greater surface area are produced. This process operates by attempting to balance out the polymeric solution’s lower surface tension forces with a powerful mutually repulsive forces[66]. The electrospinning device’s capillary tube has been loaded with the polymer solution, in atmospheric condition. The solution is held by surface tension at its end, which is also exposed to an electric field, leading to in an electric charge on the outermost layer. Surface tension will be dissipated by electric forces once the charge reaches its threshold. The polymer is then left over together with the capillary tip and the collector as the solvent evaporation takes place. When fabricating membranes for needleless classifications, electrospinning is carried out more quickly, producing membranes with wider nanofibers than those produced by conventional methods[67]. A hydrophobic electrospun membrane featuring large pores is manufactured by generating a PVDF composite membrane employing the electrospinning technology. The ultimate membrane has a positive impact on the distillate stream’s turbulent conditions along with excellent gas permeability[68]. The distinguishing feature is the targeted pore generation and applicability in numerous membrane separation domains such as, nano generators, Li-ion batteries and tissue engineering[69]. Excellent thermal stability and better performance are provided by the electrospinning process[70]. This approach is utilized in filtration, distillation, and drug release owing to its affordable price and ease of production[ 71].

    3. Elements Affecting the Performance of the Membranes

    3.1. Surface features of the membranes

    3.1.1. Surface charge

    Evaluations of the zeta potential contribute to in determining the surface charge. The voltage disparity among the membrane surface and the liquid that is submerged is known as surface charge. The feed water properties, such as ion quantities, pH and the amount of organic substances, affect the charge variability. The preceding equation (1) represents the logarithmic relationship, which provides the link among surface charge σ and feeds content Cf[72].

    ln | σ | = a + b   ln C f
    (1)

    Where the salt's nature determines the values of the parameters a and b. Tetra ethylene pentamine (TEPA) with PVDF and PES represents a pair that shows positive charge for membranes (Fig. 6).

    Polymers, such as cellulose acetate propionate (CAP), cellulose acetate (CA), cellulose acetate butyrate (CAB) etc., display negative charges[73,74]. The wettability characteristics of the membrane is significantly influenced by the polymers utilized in its manufacture[75]. It has been proven through investigation that membranes having a pH of 3 or below will have a negative charge, as well as that negative charge is often reduced at lower pH levels[76]. Following a rise in surface energy, the membrane becomes more hydrophilic. Co-ions with identical charges will effectively separate when the surface charge is positive. Fouling results in reduced performance when there are distinct charges among molecules and membranes[ 77]. This shows that the separation technique is directly affected by surface charges.

    3.1.2. Surface roughness

    It has been found that rougher membrane surfaces are more appropriate for contaminants to adhere to, leading to faster fouling speeds. Using the atomic force microscopy (AFM) analysis, Vrijenhoek et al. demonstrated unambiguously that particles selectively collect in the valleys of rough membranes, causing valley clogging triggering an even more significant flux drop compared to smoother membranes[78]. Bowen et al. measured the contact pressure among a colloidal silica probe and a rough membrane layer employing an AFM adhesion force analysis process[79]. The colloid's electrostatic repulsion from the membrane surface was found to be substantially decreased by membrane surface roughness, and the valley regions showed stronger adhesion forces. Nevertheless, although both hydrophobicity and membrane roughness had an impact on colloidal fouling, Boussu et al. showed that membrane hydrophobicity appeared to be more important for causing fouling[80]. When comparing smooth, hydrophilic semiaromatic piperazine-based PA membranes against much hydrophobic m-phenylene-diamine-deived completely aromatic membranes, Park et al.'s observations also revealed lesser fouling possibility for the former[81]. The stronger repulsive acid-base interaction with highly hydrophilic poly(piperazine) membranes is the factor that causes the increased anti-fouling behavior, according to these researchers. Regarding the matter concerning how surface roughness affects membrane flux, conflicting results have also been reported: increasing surface roughness can result in greater flux, decreased flux, or no effect at all. In filtration experiments using thin film polyamide RO membranes, Kwak and Ihm failed to identify a linear correlation among membrane surface roughness and flux[82]. Stamatialis et al. observed that the smaller the surface roughness of cellulose acetate (CA) and cellulose acetate butyrate membranes, the lesser the flux and greater the rejection while sodium chloride filtration experiment[83]. But no justification for these outcomes has been presented. Ramon et al. modelled the relationship among rough coating films and the underlying porous supporting membrane in an effort to better understand the function that membrane shape serves in movement across composite systems[84]. They explained that when surface roughness developed along with generating thin areas in the coating film, basically lowering the base film thickness, the permeability of the film improves through surface roughness; subsequently, if roughness is developed on top of an unaltered base film thickness, the permeability of the film declines as roughness enhances. The thinner parts called 'valleys' offer locally higher flux “hot spots” compared to the thicker parts called peaks' if surface roughness is at the expense of base film thickness; thus, these areas of high flux may be points of beginning for organic and colloidal accumulation in addition to development of mineral scale. It should be remembered that membrane surface roughness is not a constant value and can fluctuate depending on the environment to which the membrane is introduced. As an instance, it was discovered that water adsorption on TriSep X20 membrane might alter the surface roughness of the membrane by as much as 35%[85]. Furthermore, while examining various membrane regions, a variation in surface roughness of up to 33% was found. While developing connections among roughness and membrane efficiency measures, such significant elements are frequently neglected. It is important to emphasize that various membrane surface features should be considered with assessing membrane fouling for complex feed systems. A smooth hydrophilic surface free of any carboxylic moieties might be the optimum membrane choice if feed comprises both positively and negatively charged contaminants, according to Jin et al.[86]. In comparison to the membrane with no a poly vinyl alcohol (PVA) coating, the membrane that had a hydrophilic coating demonstrated much improved fouling tolerance towards alginate.

    3.1.3. Surface tension

    Another of the crucial thermodynamic characteristics that reveals details regarding the corresponding surface activity of constituents is surface tension. The following formula (2) can be used to compute surface tension[ 87]:

    S u r f a c e   t e n s i o n = W 2 π r B F ( r B V 1 3 )
    (2)

    Whereas W represents the dropped weight, rB indicates the capillary tip diameter, and F (rB/V1/3) represents a correction factor depending on the extent to which the capillary tip radius resembles the cubic root of the drop volume. The binding force which maintains the structural integrity of cell membranes called membrane surface tension. It has a significant impact on hydrating membrane surfaces. According to Goh et al., hydrophobic membranes are produced through polymers with lower surface tension, while hydrophilic membranes typically have a strong surface tension in order to establish H bonds between them and the neighbouring H2O molecule[88]. With an experiment, Lee et al. demonstrated that membranes with a solution which has a comparatively small surface tension (3.5% NaCl including 0.1 mM sodium dodecyl sulphate (SDS)) and a high degree of attraction among the key and membrane surface have a decreased contact angle (i.e., from 156.7 to 117.1)[89]. Weak surface tension and impurities in the feed solution can both quickly hydrate the membrane. Lower-surface-tension feed will make the membrane more difficult to wet while working less effectively when it comes to permeation. There exist a weak relationship between surface tension and temperature is also shown, thus the CA of the membrane should be tested at high temperatures[ 90].

    3.1.4. Surface functional groups

    The operation of a membrane is governed by the existence of different functional groups on its outermost layer. For instance, Ti4+ from TiO2 nanoparticles attach with oxygen in the PVA/CA membrane because of an existence of hydroxyl group (OH) within the composite; as a result, the leaching of nanoparticles from its surface is eliminated[91]. Another study described, the development of hydrogen bonds among molecules is increased by a rise in the number of acetyl (RCH3CO) groups. As a result, it produces a wettable substance that has significant surface energy and CA values[92]. Functional groups such as carboxyl, hydroxyl, amino and methyl bind to molecules through hydrogen bonds, which improves the bonding of polymers. The compatible nature of the matrix and the homogeneous dispersion of solvents are both improved by the existence of oxide, acidic and amine groups[93].

    3.2. Polymer Crystallinity

    Primarily for nonporous membranes, the crystallinity of polymers is a key factor in influencing their strength and permeation. The chain interactions, chain elasticity and polymer's molecular weight significantly affect the crystalline structure and glass transition temperature[ 94]. The majority of polymers are typically semi-crystalline, consisting of both amorphous and crystalline forms. Since there are significant intermolecular bonds, like hydrogen bonding in with respect to PVA, the polymer chains are arranged in an orderly manner when the material is in the crystalline phase. The amorphous phase, or a random arrangement of molecular chains, connects the polymer's crystal fragments. The tightly packed nature of the polymers' crystal fragments prohibits liquid from flowing inside of them, therefore liquid movement occurs via the amorphous regions. The polymers' crystallinity and coefficient of diffusion can be best explained as given in equation (3)[95].

    D i = D i , 0 ( ϕ c n B )
    (3)

    Wherein B indicates constant, c represents the percentage of crystalline substance, and n is an exponential component (n <1), is the coefficient of diffusion at zero concentration.

    To increase the hydrophobicity and limit membrane swelling, Gholap et al. grafted N-tertiary butyl acrylamide onto PVA chains. The membranes are subjected to heat to aid in crystallinity, thereby decreasing the permeation flux across the membranes[96]. The relationship among the water diffusivity and the PVA's free volume has been addressed by Lue et al. Water diffusivity is improved due to the incorporation of silica to PVA, which additionally reduces the membrane's crystallinity and improves free volume hole size and density[97]. According to Yu et al., adding SiO2 to PVDF hollow fiber membranes at higher amounts caused a change in the crystal structure from the -phase to the β-phase and impeded PVDF mobility. The membrane's transport abilities suffered as a result. Yet, the mechanical, UF, thermal and antifouling characteristics were enhanced by the PVDF-SiO2 membrane containing 3 wt% tetraethyl orthosilicate (a precursor to SiO2)[98]. The efficiency of the PS UF membranes was examined by Peng et al. with respect to the degree of cross-linking of the PVA coatings. The pure water permeability of PVA is improved, and the crystal structure of the PVA film is reduced, as the degree of cross-linking increases[99]. The impact of plasticizing microfibrillated cellulose with glycerol upon the water sorption, diffusion factor, and morphology of polymer films was also studied. Water molecule diffusion is often improved by the incorporation of plasticizers because they make the polymer chains in plasticized material more mobile[100-102].

    4. Conclusion

    The present review addressed the correlations among polymer membrane fabrication, surface characteristics, morphology and efficiency. It was demonstrated that the formation of membranes has resulted in significant advancement to this point. The production of sustainable membranes featuring strong resistance to chlorine attack, excellent mechanical properties, reduced thickness of the membrane barrier layer, and substantial flux, nevertheless, remains an issue. Additional research is required to optimize existing membrane manufacturing procedures and additionally establish novel fabrication methods in order to assure advancement in these areas. The effort to get the physio-chemical characteristics, rejection ratio, and surface morphology is not adequately explained although an abundance of study. It may be possible to develop the membrane more effectively and affordably by taking systematic steps to the fabrication of the membrane components and determining the process elements.

    Acknowledgements

    This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2020R1A6A1A03038697) and this paper was supported by Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE) (P0017310, Human Resource Development Program for Industrial Innovation(global))

    Figures

    MEMBRANE_JOURNAL-33-3-110_F1.gif

    Schematic representation of a membrane/coagulation bath interface. J1, J2 are the non-solvent and the solvent flux respectively.

    MEMBRANE_JOURNAL-33-3-110_F2.gif

    SEM images of the surface morphologies of the porous silicon rubber membranes prepared at different liquid paraffin concentrations (a) 10, (b) 15, (c) 20, (d) 25, (e) 30 and (f) 40 wt% (Reprint with permission of Zhao et al., 2013, American Chemical Society[31]).

    MEMBRANE_JOURNAL-33-3-110_F3.gif

    Reaction scheme followed in the formation of thin film polyamide membrane from m-phenylenediamine (MPD) and trimesoyl chloride (TMC) through IP.

    MEMBRANE_JOURNAL-33-3-110_F4.gif

    Visual representation of single ion irradiation system utilized in the formation of track etched membrane.

    MEMBRANE_JOURNAL-33-3-110_F5.gif

    Schematic depicting the electrospinning process of polymer solution.

    MEMBRANE_JOURNAL-33-3-110_F6.gif

    Zeta potential values of various polymeric membranes.

    Tables

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