Recent advances in hydrophilic modification and performance of polyethersulfone (PES) membrane via additive blending

The blending of additives in the polyethersulfone (PES) matrix is an important approach in the membrane industry to reduce membrane hydrophobicity and improve the performance (flux, solute rejection, and reduction of fouling). Several (hydrophilic) modifications of the PES membrane have been developed. Given the importance of the hydrophilic modification methods for PES membranes and their applications, we decided to dedicate this review solely to this topic. The types of additives embedded into the PES matrix can be divided into two main categories: (i) polymers and (ii) inorganic nanoparticles (NPs). The introduced polymers include polyvinylpyrrolidone, chitosan, polyamide, polyethylene oxide, and polyethylene glycol. The introduced nanoparticles discussed include titanium, iron, aluminum, silver, zirconium, silica, magnesium based NPs, carbon, and halloysite nanotubes. In addition, the applications of hydrophilic PES membranes are also reviewed. Reviewing the research progress in the hydrophilic modification of PES membranes is necessary and imperative to provide more insights for their future development and perhaps to open the door to extend their applications to other more challenging areas.


Introduction
Polyethersulfone (PES) is a recognized polymeric material, which is widely employed in the fabrication of membranes for various applications. Due to its high glass transition temperature (225 C), and amorphous and transparent properties, PES possesses a high mechanical and hydrolytic stability, thermal and chemical resistance, and outstanding oxidative characteristics, 1 making it ideal for the preparation of asymmetric membranes with different surfaces and pore sizes. [1][2][3] Asymmetric PES membranes are generally prepared via a phaseseparation method. The nal membrane properties and performance are inuenced by the composition (additives, concentration, and solvent), temperature of the doping solution, the non-solvent or the mixture of non-solvents, and the coagulation bath or the environment. 4 The risk of the fouling effect due to the high hydrophobicity of PES, especially in protein-contacting applications and aqueous ltrations, limits their wide applications. 2,3 Numerous research studies have reported efforts to enhance the hydrophilicity of the PES membrane surface. 1,5 Basically, the water contact angle (WCA) formed between the membrane-liquid boundary and liquid-gas tangent is generally employed to evaluate the hydrophilic properties of the membrane. 6 Commercial PES membranes are hydrophobic in nature with high mechanical, chemical, and thermal stability. 7 Usually, these membranes possesses high WCA values and are prone to solute adsorption from various feed streams. It has been well documented that membranes with hydrophilic surfaces are less prone to the fouling effect with microorganisms and organic substances due to: (i) a decreased interaction between the membrane surface and foulant, and (ii) no interaction of hydrogen bonds in the boundary layer between water and the membrane interface. [8][9][10] The repulsion of water molecules away from the surface of the hydrophobic PES membrane is a spontaneous process with increasing entropy, and therefore foulant molecules have a tendency to dominate the boundary layer and adsorb onto the PES membrane surface. However, a modied PES membrane with a hydrophilic chain and high surface tension can enhance the formation of hydrogen bonds with the surrounding water molecules. This hydrogen bonding can reduce or prevent the adhesion of foulants on the surface of the PES membrane. 11,12 The membrane WCA is related to the zeta potential, surface roughness, and functional groups. 13,14 Ref. 15 and 16 demonstrated that an improved membrane hydrophilicity can be favored by increasing the density of the surface hydrophilic-group, including -NH 2 and -OH.
Numerous studies on PES membranes have been carried out with the aim to enhance the hydrophilicity and performance, including through an improvement in their preparation process (blending) and by surface modication of the nascent membranes. In surface modication, a hydrophilic layer is formed on the existing PES membrane surface, which can then aid the prevention of contact between the solute and membrane surface, thus reducing the membrane fouling effect. The surface modication can be classied based on two categories, namely chemical or physical modication. In chemical modi-cation, the PES membrane surface is modied through covalent bonding interactions. In this procedure, PES chains are rst activated by chemical reaction, followed by the graing with hydrophilic additives. The use of surface modication may render the hydrophilicity permanent, but may, however, lead to degradation of the PES chains on the membrane surface. 17 In practice, these methods usually require caustic chemicals, which limits their wide use and long-term stability in membrane applications. Physical modication signies that the hydrophilic modiers exist on the PES membrane surface via physical interaction. Here, the blending approach is a versatile and convenient procedure under mild conditions to enhance the hydrophilicity and performance of PES membranes. 18,19 Blending is a process in which two (or more) inorganic and/or organic materials are physically mixed to obtain the required properties on the membrane. This introduction can be achieved by adding polymer material and inorganic nanollers into the casting solution. Table 1 presents the advantages and disadvantages of both approaches. Since most of these additives are hydrophilic in nature, they are able to increase the hydrophilicity of the resulting membranes and thus can reduce the fouling effects. Other advantages of blending with hydrophilic additives include an increase in the water ux (WF) due to the enlarged effective membrane surface area and the introduction of additional functional groups. 20

Embedding polymer materials
In this approach, hydrophilic organic polymers are dissolved in PES solution. The materials most commonly used include polyvinyl pyrrolidone (PVP), chitosan (CS), polyamide, polyethylene oxide (PEO), and polyethylene glycol (PEG) derivatives due to their reasonable price and high compatibility with PES. 21 Table 2 shows the progress reported in recent studies on polymeric addition in PES membranes.
One general issue of blending with a polymer is the elution of this polymer and poor compatibility with the PES matrix. 17 To address this issue, some researchers have looked into the use of amphiphilic copolymers as well as amphiphilic copolymers containing PES chains with hydrophobic parts as modiers. Amphiphilic modiers contain hydrophilic and hydrophobic properties, which means they are able to interact with the hydrophobic PES polymer, which is totally insoluble in water, and are also able to interact with hydrophobic PES polymers. The hydrophobic chains guarantees the compatibility with the host PES polymer, while the hydrophilic chains are enriched onto the membrane pore during phase inversion due to a segregation effect, thus, providing a high coverage of hydrated side chains anchored by a hydrophobic backbone entangled with the PES bulk that is water insoluble. 30 membrane performance properties can be achieved, such as a higher solute rejection, fouling resistance, and permeability. 32 Table 3 show a summary of hydrophilic PES-amphiphilic copolymer blend membranes.
Generally, it has been well documented that these copolymers have better compatibility with the PES bulk, and could be used as modiers to enhance the hydrophilicity, antifouling properties, and performance of PES membranes. [41][42][43][44] Embedding inorganic materials Apart from introducing polymers and copolymers, inorganic NPs are another promising modier. The addition of inorganic NPs with the PES matrix has become an attractive approach for the fabrication of polymeric membranes and has captured much attention in recent times. [45][46][47][48][49][50] Much of the bulk of the research has been carried out on the preparation of composite PES-inorganic membranes by the addition of inorganic NPs. For instance, the presence of dispersed inorganic NPs in the membrane matrix has been reported to improve the membrane performance and properties, particularly by: (a) increasing the permeability due to the larger effective membrane surface area of NPs; (b) inducing a membrane with the functional properties of the nanomaterials; 20 (c) enhancing the mass transfer in the membrane pre-evaporation process; 51 (d) improving a membrane's hydrophilicity as well as fouling resistance properties; 46,52 (e) improving the thermal and mechanical properties. [53][54][55][56][57][58] To date, many types of inorganic materials have been incorporated as additives in the PES matrix, including titanium dioxide, silicon dioxide, carbon nanotubes, halloysite nanotubes, manganese oxide, cellulose nanocrystals, graphene oxide, silver NPs, zirconia, zinc oxide, alumina, and metal-organic frameworks. However, there are two ways to introduce these NPs into the PES membrane during the preparation process: blending them in a coagulation bath or in the polymer solution. Compared to the blending of nanollers in a coagulation bath, blending the nanoparticles in the polymer solution has been the dominant method. The discussion below introduces the incorporation of inorganic additives in the PES matrix.

Embedding titanium dioxide NPs
Titanium dioxide (TiO 2 ) has been the major focus of quite a signicant number of studies in recent and past years, due to its photocatalytic effects, which aid in killing bacteria and decomposing organic chemicals, relative cheapness, chemical stability, optical property, and non-toxicity. 49,[59][60][61][62][63][64][65] As one of the most investigated NPs, when TiO 2 NPs are dispersed in the PES matrix, the membrane hydrophilicity and antifouling ability can be enhanced. On this basis, lots of effort has been devoted to investigating the effect of TiO 2 NPs to improve the PES membrane hydrophilicity. For instance, ref. 66 Table 1 Advantages and disadvantages of incorporating polymer and inorganic additives in the PES membrane matrix

Membrane synthesis approach Advantages Disadvantages
Blending with a polymer (a) Miscibility in common solvents (a) Tendency toward physical and/or chemical aging (b) Flexible to incorporate (b) Leaching during the preparation and operation process, which may reduce the efficiency (c) Poor compatibility in the polymer matrix Blending with an inorganic material (a) High chemical resistance (a) More expensive than equivalent polymeric ones (b) Improve thermal stability (b) Defect-free commercial-scale inorganic PES membranes are difficult to manufacture (c) The resultant membrane combines the advantages of the organic and inorganic parts (c) Stability of the doped form is a big issue due to its nano-size (d) Can be easily incorporated (d) Very expensive formulations (e) Provides an enhanced surface that allows multiple functional groups to be added on the membrane surface (e) Non-uniform dispersion of NPs in the polymer matrix (f) Aggregation phenomenon (g) Weak interaction with the polymer matrix (h) Leaching of NPs during the operation process (i) Uncontrollable pore size (j) Poor dissolution in various organic solvents

Embedding silica (SiO 2 ) NPs
The addition of SiO 2 NPs has been investigated intensively and proven ideal as an additive for PES membranes due to their many useful properties, such as ne suspendability in aqueous solution, relatively environmentally inert, being thermally and chemically stable with a large surface area (SA), and highly miscible. 72  NPs graed with N-halamine were obtained via a three-step reaction process (Fig. 1). Their result showed an improved hydrophilicity with a WCA of 70.6 using 5 wt% of modied NPs, which was lower than the neat membrane WCA of 90.7 .

Embedding zinc oxide NPs
Zinc oxide (ZnO) is another compound that has excellent electrical, optical, chemical, and mechanical properties, including antimicrobial activity. [78][79][80] With its low cost and increased surface-to-volume ratio, ZnO is a potential NP that could meet the demand for the fabrication of a lower-cost and efficient membrane.  . In their study, CZN was prepared through a facile onestep homogeneous co-precipitation method at a low temperature ( Fig. 2). Their results showed an improve hydrophilicity of 65.5 against 70.2 for the neat membrane at an optimal content of CZN (0.2 wt%).

Embedding zirconium dioxide NPs
Zirconium dioxide (ZrO 2 ), or zirconia, is a white crystalline oxide of zirconium with excellent chemical stability, melting point, good mechanical properties, and strong anti-corrosion. Zirconia membranes are known to be chemically more stable than alumina and titania PES membranes, and are more suitable for liquid phase applications under harsh conditions. 88

Embedding silver nanoparticles
Within the wide range of commercially available nanoscale materials, silver nanoparticles (AgNPs) have also received a great deal of attention. AgNPs have unique properties (such as extremely large surface-to-volume ratio, antimicrobial, optical, and electrical properties), making the NP able to serve as a sustained local supply of Ag + ions in membranes, and so it can prevent bacterial and solute adhesion onto the membrane surface. [94][95][96] Several preparation techniques have been reported for the synthesis of silver NPs; notable examples include photochemical methods, gamma irradiation, laser ablation, microwave processing, electron irradiation, biological synthetic methods, and chemical reduction. 95 The effects of AgNPs on the nal membrane hydrophilicity have been investigated in many studies. A study by ref. 97

Embedding graphene oxide NPs
Graphene oxide (GO), a two-dimensional carbon material, has received tremendous attention due to the presence of abundant O 2 -containing functional groups (such as carboxyl, epoxy, hydroxyl, and carbonyl groups), fantastic chemical stability, high strength, superior electron transport, low thickness, high exibility and a negatively charged surface, innocuity, high surface area, and good miscibility with polymers. [100][101][102][103][104][105][106][107][108][109][110][111][112][113] The existence of these groups makes GO possess good hydrophilicity, is easy to be modied, and has the capability of dispersion in water to yield a prolonged, stable suspension. 114 All these factors make it more appropriate for the hydrophilic modication of PES membranes. 115,116 On this basis, a large amount of work has been devoted to developing GO/PES NC membranes and their summaries are shown in Table 4.

Embedding CNTs
Carbon nanotubes (CNTs) are a member of the fullerene structural family and consist of six-membered carbon rings in the honeycomb lattice relative to the axis of the nanotubes (NT). 122 The pioneer discovery of CNTs by Lijima 123 has opened up new directions for many applications. CNTs have the ability to interact and alter the physico-chemical properties of the membrane. 124 This property coupled with their high specic surface area with low density, exceptional mechanical properties, nanoscale dimensions and highly precise diameters, high thermal stability, very low frictional coefficients on their internal surface, high strength-to weight ratio, formation of highly porous structures, and chemical stability makes CNTs a promising candidate for complementing or substituting conventional NPs in the fabrication of new generation nanocomposite membranes. [125][126][127][128][129][130][131][132][133][134][135][136][137][138] The excellent mechanical properties of CNTs arise from the presence of C-C bonds in the graphite layer, which are most probably the strongest chemical bonds known in nature. CNTs can be synthesized either as a series of shells of different diameters spaced around a common axis, called multi-walled carbon nanotubes, MWCNTs (consisting of up to 10-100 carbon shells), or as singular tubes, called single-walled carbon nanotubes (SWCNTs). 139 The former are of particular interest over the latter due to their availability in larger quantities and relatively low cost as a result of their more advanced stage in commercial production.
The most crucial problem when using CNTS is the poor dissolution and dispersion of synthesized CNTs in various organic solvents and different polymers as well as their weak interaction with the polymer matrix. [140][141][142][143] Moreover during CNTs preparation, the presence of metal catalytic particles and   57 190 amorphous carbon, as impurities, could add an additional burden to the intended application. 144 These factors are important in the utilization of additives in polymer composites as well as CNTs. 145,146 Therefore, the purication and functionalization of CNTs could be established to negate the hydrophobic nature of CNTs and to broaden their promising scope. For this reason, different linking groups, e.g., -NH 2 , -SO 3 H, -COOH, -OH, or -CONH 2 could be introduced to the CNTs surface to facilitate linking different metal clusters to the nanotubes surface via polymer wrapping, covalent attachment (graing), and non-covalent attachment (adsorbing). 122,[146][147][148][149][150][151][152][153][154][155][156][157][158][159] The amine (NH 2 ) group has a wealth of chemistry and high reactivity with many chemicals, such as polymers. [160][161][162] Aer modication, they become soluble in different solvents, as well as contain functional groups, which turn them into a multidisciplinary materials in other applications. The functionalization by chemical oxidation of CNTs is the most commonly used method, which breaks the sp 2 hybrid carbon bonds on the sidewalls, and attaches carboxyl/hydroxyl groups to the CNTs. 163 Functionalized CNTs can enhance the properties of PES membranes by increasing the hydrophilicity and surface charge of the membrane top layer, [147][148][149][150][151]164,165 which will inuence the permeability and reduce fouling. [147][148][149][150][151][164][165][166][167][168][169] An increase in the surface charge will raise the Donnan exclusion effect and Review electrostatic interactions, which will result in an improved rejection of salt and an increase in hydrophilicity, which will provide better fouling resistance. 170,171 To date, several authors have shown the successful preparation of CNT-blended PES membranes. The summary of their results are presented in Table  5.

Embedding halloysite nanotubes
Halloysite nanotubes (HNTs) are a kind of naturally occurring aluminosilicate (Al 2 Si 2 O 5 (OH) 4 $2H 2 O) with a hollow nanotubular structure, 191,192 regular open-ending pores, as well as a great deal of hydroxyls on their surface. 193 HNTs can easily be dispersed in a polymer matrix, even at high loading due to their tubular shape, low density of hydroxyl functional groups, and well-crystallized structure. [194][195][196][197] In contrast with other NPs, HNTs can be obtained easily and are much cheaper. 198,199 HNTs own a low charge density, which means they cannot affect the membrane potential when they are embedded into the polymer matrix. 193 Recently, HNTs have been used as a new type of ller for PES to improve the properties and performance of the composites. For instance, ref. 200 synthesized a HNTs loaded with copper ions (Cu 2+ -HNTs) by the chemical modication of HNTs, which were then incorporated in the PES matrix to produce Cu 2+ -HNTs/PES MMM, which signicantly resulted in an improvement of membrane hydrophilicity, with WCA decreasing from 84.9 to 69.8 for 3 wt% of Cu 2+ -HNTs. Ref. 193 presented a sulfonated halloysite nanotubes (HNTs-SO 3 H)/PES membrane. To prepare highly crosslinked HNTs-SO 3 H, styrene was graed onto HNTs surface via distillation-precipitation polymerization and then sulfonated with concentrated sulfuric acid. Fig. 6 shows a schematic illustration of the overall preparation process of HNTs-SO 3 H. The control PES membrane presented the highest contact angle of 83.5 , which was decreased to 58.3 when 3 wt% HNTs-SO 3 H was introduced. In another study by ref. 201, sodium 4-styrene sulfonate was graed onto HNTs surfaces via surface-initiated atom transfer radical polymerization, as shown in Fig. 7, which was then introduced in the PES matrix to prepared negatively charged nanoltration membranes. WCA was observed to decrease from 83.5 to 56.6 at 3 wt%.
Ref. 202 reported a PES hybrid membrane containing HNTs graed with 2-methacryloyloxyethyl phosphorylcholine (MPC).    introduced HNTs-chitosan-Ag nanoparticles (HNTs-CS@Ag) into the PES matrix. Prior to blending, the HNTs-CS@Ag were synthesized by chemically modifying HNTs with chitosan, and then mixing with silver nitrate for complexing the silver ions, and nally the silver NPs were formed using sodium tetrahydroborate as a reducing agent. Fig. 9 presents the reaction principle for preparing the HNTs-CS@Ag NPs. The hybrid membranes were shown to be more hydrophilic, with the optimum membrane displaying the lowest contact angle of 55 when the content of HNTs-CS@Ag amounted to 3 wt%.
Ref. 204 presented polyethersulfone (PES) ultraltration membrane by incorporating dextran graed HNTs (HNTsdextran). Fig. 10 presents the basic reactions of the modied HNTs. The results indicated that the surface hydrophilicity of the membranes was signicantly improved aer adding HNTsdextran. The WCA of the pristine PES membrane amounted to 90.8 , while the WCA of the hybrid membrane with the modied HNTs-dextran content of 3% was 58.3 . Embedding metal-organic frameworks Metal-organic frameworks (MOFs) are organic-inorganic hybrid solids with innite, uniform framework structures built from inorganic metal (or metal-containing cluster) nodes and organic linkers. [205][206][207][208] MOFs are zeolite-like structures but they do not have the limitations of zeolites in terms of the material's chemistry. MOFs are recently attracting a lot of attention as potential additive materials for MMMs, owing to their extraordinary porosity (as high as 50% of the crystal volume), high surface area (ranging from 1000 to 10 000 m 2 g À1 ), affinity for certain molecules, capability for functionalization, low density (0.2-1 g cm À3 ), tunable chemical composition, and exible structure. [209][210][211][212] MOFs have regular and highly harmonic pore structures and they play a very vital role in increasing the hydrophilic property of the membrane. [213][214][215][216][217][218][219][220][221][222][223] To date, different types of MOFs, including zeolitic imidazolate framework (ZIF), ZIF-8, [Zn(oba)(4-bpdh) 0.5 ]$(DMF) 1.5 (TMU-5), UiO-66, matériauxs de l'Institut Lavoisier, have been developed and introduced in the PES matrix to prepare a MMM. Their potentials have been well recognized both experimentally and computationally. 120,[224][225][226][227][228][229][230] The emerging zirconium MOFs (Zr-MOFs) has exhibited exceptionally high chemical and thermal stability. 231 Ref. 224 reported a novel hydrophilic PES/ TMU-5 UF membrane synthesized by blending with TMU-5. The growth of TMU-5 upon silk ber was achieved by sequential dipping in alternating baths of aqueous Zn(NO 3 ) 2 $6H 2 O and DMF solution of 4-bpdh and (H 2 oba) under an ultrasound bath. They found that upon the addition of 0.1 wt% NPs, the hydrophilicity was enhanced, with WCA declining from 67.2 to 57.5 . In another study by ref. 225, a two-dimensional zeolitic imidazolate framework with a leaf-shaped morphology (ZIF-L) was synthesized in zinc salt and 2-methylimidazole aqueous solution and then doped in the PES matrix to prepare the MMM. Upon the introduction of 0.5 wt% loading of NPs, the WCA declined slightly to 62.72 as compared to the neat membrane of 67.72 .

Other inorganic materials
Ref. 232

Applications of the hydrophilic PES membrane
With the increasing demand for functional hydrophilic membrane materials, a great deal of attention has been focused on the development of hydrophilic PES membranes. Due to their interaction with water, the use of a hydrophilic PES membrane has found use in various applications, such as desalination, water treatment, wastewater treatment, textile applications, and protein purications. In fact, hydrophilic PES membrane modication via blending is a simple approach to overcoming the performance trade-off and minimizing membrane fouling. A signicant number of works have shown that enhancing the hydrophilicity of the PES membrane will result in a reduction of membrane fouling as well as leading to performance improvement. Tables 6 and 7 show summaries of the applications of hydrophilic PES membranes.

Conclusions and future prospects
Seemingly the permanent hydrophilic modication of PES membranes can be achieved by blending with organic and/or inorganic materials. Furthermore, there is no denying the fact that the amount of data available today on the hydrophilic enhancement of PES membranes via blending is a stepping stone to upgrading PES membranes to new heights. Some of the conclusions drawn from this comprehensive review are listed as follows: To achieve an improved surface hydrophilicity and performance, many factors need to be considered in the overall process of composite membrane preparation, such as precise control over the functional groups, uniformity, and reproducibility. For instance, the functional groups on CNTs have the ability to be converted to membrane functional groups and can change the surface hydrophilicity and performance of the PES membrane. Therefore, more functional groups on CNTs are expected to reveal more signicant changes in membrane hydrophilicity and performance. However, there is also a need for comprehensive investigation concerning the use and inuence of multiple-modied SWCNTs and MWCNTs on PES NC membranes characteristics to verify the efficiency of PES modication of CNTs on the surface hydrophilization of PES membranes. Furthermore, the production costs of carbon nanotubes are quite high. Thus, further work should investigate and address the economic aspects so that their potentials for commercial scale can be realized.
In the case of blending with inorganic materials, the interaction between PES and NPs is specic and the nal membrane hydrophilicity and performance depends on such interaction. Therefore, the effectiveness of hydrophilicity will depend on the location of NPs in the membrane matrix because the location of NPs can change the diffusivity in the PES matrix. The surface energy and concentration are other important factors that can affect NPs dispersion and location, which could lead to NPs aggregation on the surface of the PES NC membrane. NPs aggregation will mean that the effectiveness of surface hydropilicity will be reduced during intended applications. To decrease the surface energy or improve the dispersion of NPs in the PES matrix, the surface modication of the NPs by graing with a polymer can be an effective method.
The use of a variety of functional and synthetic materials (i.e., lyotropic liquid crystals, aquaporins) will improve the hydrophilicity, enable the highest permeation rates, as well as keep the doors open for research and development in the eld of multifunctional, high-performance, and antifouling PES membranes.
Although, the combination of two or three additives can be more complex in terms of the environmental drawbacks and cost effectiveness, these could lead to multifunctional PES membranes that are of great interest for 'future hydrophilic PES membranes'. Comparison with the existing ones to determine their adaptability and sustainability for commercial purposes will be the next step.
With a hydrophilic PES membrane, it should be mentioned that solute adsorption is reduced at the produced hydrophilic surfaces, but is never completely prevented. Therefore, it is expected that membrane surface hydrophilicity can be tuned for specic applications through the discussed methods, although they still need to be developed further in such a way that they allow even more and better environmentally friendly control over other modication methods.
Finally, but also very important, is the processing ability and economic cost. Generally, the cost is a major concern in the commercialization of membrane technology. Some hydrophilic PES membranes might produce a better quality of permeate and solute removal but the operating costs may be higher. Thus, the cost associated with the synthesis and incorporation of these additives needs to be addressed at the earliest for their development from the laboratory to a commercial-applicable scale.

Conflicts of interest
We declare that there is no conict of interest in this work.