alteration of polyethersulphone membranes through uv ... · with non-solvent induced phase...

Arabian Journal of Chemistry (2013) xxx, xxx–xxx

King Saud University

Arabian Journal of Chemistry

www.ksu.edu.sawww.sciencedirect.com

REVIEW

Alteration of polyethersulphone membranes through

UV-induced modification using various materials: A brief

review

Law Yong Ng, Akil Ahmad, Abdul Wahab Mohammad *

Research Centre for Sustainable Process Technology (CESPRO), Department of Chemical and Process Engineering, Facultyof Engineering and Built Environment, University Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia

Received 6 September 2012; accepted 14 July 2013

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KEYWORDS

Nanoparticle;

UV irradiation;

Monomers;

UV-grafting;

Hydrophilicity;

Surface modification

Corresponding author. Tel.-mail addresses: nglawyong@

ohammad).

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Abstract Polyethersulphone (PES) membranes have been widely applied in various separation

applications such as microfiltration, ultrafiltration and nanofiltration. This has occurred as these

membranes are easy to form, have good mechanical strength and good chemical stability (resistant

to acidic or alkaline conditions) due to the presence of aromatic hydrocarbon groups in the struc-

ture. PES membranes are commonly fabricated through the phase inversion method due to the sim-

plicity of the process. However, PES membranes are generally hydrophobic, which usually requires

them to be modified before application. In most cases, these methods can reduce the hydrophobicity

of the membrane surface and thus reduce membrane fouling during application. This review will

further discuss the recently developed UV-induced modifications of PES membranes. The UV-

induced grafting method is easy to apply to existing PES membranes, with or without the need

for a photo-initiator. Additionally, nanoparticles entrapped in PES membranes subsequently

exposed to UV-irradiation have been reported to possess photo-catalytic activity. However, UV-

irradiation methods still require special care in order to produce membranes with the best perfor-

mance.ª 2013 Production and hosting by Elsevier B.V. on behalf of King Saud University.

8921 6410; fax: +60 3 8921 6148.om (L.Y. Ng), [email protected] (A. Ahmad), [email protected], [email protected] (A.W.

Saud University.

g by Elsevier

ng by Elsevier B.V. on behalf of King Saud University.

7.009

g, L.Y. et al., Alteration of polyethersulphone membranes through UV-induced modificationiew. Arabian Journal of Chemistry (2013), http://dx.doi.org/10.1016/j.arabjc.2013.07.009

mailto:[email protected]




http://dx.doi.org/10.1016/j.arabjc.2013.07.009


http://www.sciencedirect.com/science/journal/18785352



2 L.Y. Ng et al.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002. Common PES membrane fabrication methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3. PES membrane modification methods applied in previous studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004. UV-induced modification of PES membranes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.1. UV-grafting using monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.2. Nanoparticle modification of PES membranes with UV irradiation (deposition and entrapment methods) . . . . . . 00

5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

1. Introduction

In the last few decades, with increasing population and urban-isation, the huge amount of wastewater produced has become

a great concern for many countries. These concerns need amore efficient and high quality technique for the treatmentof wastewater. Polymeric membranes are the most recognised

materials used in various separation processes (Robeson,2012) due to their effective separation performance and lowerproduction or maintenance costs in comparison with inorganicmaterials such as ceramic and metallic-based membranes.

Types of well-known polymeric membrane materials usedand investigated in depth include polysulphone (Abu-Thabitet al., 2010; Homayoonfal et al., 2010; Li et al., 2012; Namvar

et al., 2013; Prakash et al., 2008; Shah and Murthy, 2013; Sue-yoshi et al., 2012; Yoo et al., 2003), cellulose nitrate (Shieh andChung, 2000; Soylak and Cay, 2007; Sun et al., 2007), polyvi-

nyl chloride (Hu et al., 2011; Wenjuan et al., 2011; Xia et al.,2010, 2011; Zhang et al., 2009), polyvinylidene fluoride (DeGusseme et al., 2011; Feng et al., 2008; Puspitasari et al.,

2010; Souzy et al., 2012; Venault et al., 2012b), polyvinyl alco-hol (Han et al., 2008; Liu et al., 2013b; Singha et al., 2009; Wuet al., 2006; Zhang et al., 2006), cellulose acetate (Lv et al.,2007; Sossna et al., 2007; Zavastin et al., 2010), polyethersul-

phone (Cao et al., 2010; Fang et al., 2009; Qian et al., 2009;Rahimpour and Madaeni, 2010; Susanto et al., 2009; Zhaoet al., 2011), and so forth. Most of the time, these polymeric

materials are suitable to be applied in pressure-driven applica-tions, for instance reverse osmosis, nanofiltration, ultrafiltra-tion, microfiltration, dialysis and in some circumstances

pervaporation.To be used for long-term pressure-driven liquid-based sep-

aration processes, polymeric membrane materials should pos-sess properties such as excellent mechanical or tensile

strength, good anti-fouling resistance, high selectivity, highpermeability and good control of the pore size distributionover the entire membrane surface area (Akar et al., 2013). This

reduces the production and maintenance costs over the longterm (Strathmann, 2001) and ensures sustainability. Thus,researchers have been working on modifying and improving

various polymeric membranes for better tensile strength(Linet al., 2009; Seol et al., 2007), better fouling resistance (Liet al., 2009; Liu et al., 2009; Mansourpanah et al., 2009; Peeva

et al., 2010), higher rejection or selectivity (Ben-David et al.,2010; Cano-Odena et al., 2011; Li et al., 2007; Malaisamyet al., 2011) and higher flux or permeability (Shen et al.,2011a; Susanto et al., 2009; Yu et al., 2010; Zou et al.,

Please cite this article in press as: Ng, L.Y. et al., Alteration of polusing various materials: A brief review. Arabian Journal of Chem

2011). Polymeric membranes have some physical and chemicaldisadvantages in comparison to inorganic materials such aslow resistance towards oxidising agents and pH-dependent

performance (Van Wagner et al., 2009; Wang et al., 2011; Zhaiet al., 2003), and they usually cannot withstand long-termexposure to a high temperature which causes deterioration ofthe polymeric structures. Polymeric membranes which are

resistant to acids (Tanninen et al., 2004), bases, solvents (Van-dezande et al., 2008), oxidising agents (such as hydrogen per-oxide, chlorine (Glater et al., 1994), fluorine, and so on),

pressure-induced compaction, and extreme temperature (Deli-goz and Yilmazoglu, 2011; Jun et al., 2011) could be an addedbenefit and advantage for polymeric membranes to be well-

commercialised and applied in various fields.Polyethersulphone (PES) membranes have been focused on

in this review due to their recently increased popularity in thepolymeric membrane research field. PES membranes have been

fabricated, applied and modified for microfiltration, ultrafiltra-tion and nanofiltration purposes.

In microfiltration (MF) application, researchers employed

gamma-ray irradiation during the grafting of PES powder withacrylic acid (Deng et al., 2008). They prepared the membranesusing the modified PES powder with different degrees of graft-

ing (DG) through phase inversion method. They confirmedthat the water permeability and porosity of the producedMF membranes increased with an increase in DG. Besides,

they also reported that the properties of produced MF mem-branes (using modified PES powder) were highly dependenton the pH value of the aqueous solution. The increased degreesof swelling and decreased flux were reasoned with enhanced

ionisation of grafted polyacrylic acid side chains in elevatedpH value of aqueous solution. PES MF membranes were alsoproduced using N-methyl-2-pyrrolidone (NMP) and 2-meth-

oxy ethanol (2-ME) non-solvent as an additive in the castingsolution (Shin et al., 2005). In this specific work, vapour-in-duced phase inversion and non-solvent induced phase inver-

sion were employed during the membrane preparation. Thenon-solvent additive and the exposure time at the relativehumidity of 74% were found to have significant effects to-wards the membrane porosity. High performance PES MF

membranes with a high flux and stable hydrophilic propertywere also produced using induced phase separation coupledwith non-solvent induced phase separation method (Susanto

et al., 2009). It was observed that the non-solvent content(tri-ethylene glycol) and the exposure time were important toobtain high-flux membranes, whereas the Pluronic was impor-

tant in improving the membrane surface hydrophilicity.

yethersulphone membranes through UV-induced modificationistry (2013), http://dx.doi.org/10.1016/j.arabjc.2013.07.009


Solvent is measured, heated to a desired temperature and stirred continuously

PES is added to the solvent and stirred to obtain a well-mixed solution

PES solution isstored at room temperature for a period of time to release bubbles

PES solution is poured onto a glass plate and cast at a predetermined thickness

The glass plate with PES solution is then immersed in a coagulation bath, which is usually but not necessarily water

The membranes are stored in clean water to completely remove the solvent

Storage of the membranes

Figure 1 Common PES fabrication steps.

Alteration of polyethersulphone membranes through UV-induced modification using various materials: 3

To be used in the UF process, PES membranes were pro-duced using phase inversion process and casting solution con-taining PES, N,N-dimethylformamide (DMF) as solvent and

polyethylene glycol (PEG) of different molecular weights (Idriset al., 2007). It was observed that higher molecular weight ofPEG can increase the membrane porosity and pure water per-

meability. Besides, PEG concentration in the casting solutionwas found to have significant effect towards the membranesolution flux and solute separation capability. PES UF mem-

branes were also fabricated using various kinds of solventssuch as dimethyl sulphoxide (DMSO), NMP, and DMF(Arthanareeswaran and Starov, 2011). Researchers observedthat the membrane pure water permeability produced using

solvent DMSO was the highest, followed by NMP andDMF. Researchers claimed that the formation of PES mem-brane active separation layer was due to the thermodynamic

properties of the mixture and membrane formation kinetics.Besides, the diffusion rate of the solvent and non-solvent candetermine the structures of macro-porous bottom sub-layer

of the PES membranes (Lau et al., 2013). Researchers also pro-duced PES-silver UF membranes through phase inversionmethod using polyvinylpyrollidone as dispersant in the casting

solution (Basri et al., 2011). The distribution and entrapmentof the silver particles in the membrane structure contributedto good anti-bacterial property and showed minimum silverloss during operation. PES UF membranes were also improved

using acrylonitrile and acrylic acid, followed by membrane sur-face grafting using bovine serum albumin (Fang et al., 2009).The PES membrane produced using this modification tech-

nique has good blood compatibility while having increasedwater flux. Instead of flat-sheet membranes, PES hollow-fibremembranes were prepared using dry-wet spinning method

which was followed by heat-treatment process (Gholamiet al., 2003). The heat-treated PES hollow-fibre membraneswere found to have increased solute separation without visible

changes in the hollow-fibre dimension.Besides MF and UF, PES membranes also have been

widely produced purposely for NF applications. PES NFmembranes were previously produced using various amounts

of polyvinylpyrrolidone (Ismail and Hassan, 2007). Research-ers successfully increased the membrane fluxes in the presenceof additive but also recorded a decline in the salt rejection

capability. The presence of the additive also contributed to areduction in the effective membrane thickness. PES NF mem-branes were also produced by blending PES and polyimide in

DMF solvent in the presence of self-produced modifiers(Mansourpanah et al., 2010). In this work, researchers claimedthat the solution flux decline was reduced when the self-pro-duced modifier was introduced into the polymeric membrane

structure. Researchers also tried to use PES NF membranesto remove the ammonia–nitrogen from the aquaculture system(Ali et al., 2010). The PES NF membranes were produced

using various shear rates during the membrane casting stage.The best shear rate in terms of highest flux and rejection abilitywas determined to be 200 s�1 with 68% of the ammonia–nitro-

gen removal. To understand the salt rejection property in PESNF membranes, researchers also conducted zeta-potentialmeasurements using single salt solutions (Na2SO4 and KCl)

at different pHs and concentrations (Ismail and Hassan,2007). They observed that when the membrane was used atits iso-electric point, there is no preferential adsorption of chlo-ride ions or potassium ions when KCl solution was used. Thus,


they postulated that in this case only the dissociation ofsulphonic acids from the PES membrane surface will affect

the membrane electrostatic properties.The PES molecular structure which consists of two benzene

rings connected by alternate sulphonyl (SO2) groups and ether(–O–) linkages, contributed to the greater chemical stability of

these compounds (Guan et al., 2005; Richard Bowen et al.,2001). Chemical stability means that common PES membranescan be continuously exposed to solutions of pH 1–13, which is

an added advantage in cleaning processes. The existence ofaromatic groups might also improve the physical propertiesof PES membranes (Guan et al., 2005; Li et al., 2002).

2. Common PES membrane fabrication methods

PES-based membranes, which are usually produced through

the phase inversion technique, involve controlled conversionof a polymeric solution from the liquid phase into the solidphase. PES membranes are commonly fabricated according

to several steps as shown in Fig. 1. In the first step of PESmembrane preparation, a solvent is commonly used, such asN-methyl-2-pyrrolidone (NMP) (Maximous et al., 2009) ordimethylacetamide (DMAC) (Liu et al., 2013a; Rahimpour

et al., 2010b, c; Razmjou et al., 2011), which is heated and stir-red continuously until the solution reaches a homogeneoustemperature. In the next step, PES beads are added in slowly

until all the beads are completely dissolved in the solvent.The homogeneous solution is left for a few hours in order tocompletely release any bubbles, commonly designated as the

degassing process. Next, the casting of the PES membranesis done using a casting machine or hand casting with a prede-termined thickness. Casting of the membrane is followed by

immersing the PES polymeric solution into gelation or coagu-lation media. The coagulation bath is usually composed ofwater as the major component, but sometimes a combinationof few components is used. For instance, a coagulation bath



Figure 2 Molecular structures of (a) non-modified PES structure

and (b) sulphonated PES structure (Adapted from (Klaysom et al.,

2011) with permission).

4 L.Y. Ng et al.

may use a combination of isopropanol (30%) and water (70%)

or a mixture of water/isopropanol/acrylic acid or 2-hydroxy-ethylmethacrylate (Rahimpour et al., 2010c). Normally, PESmembranes should remain in the gelation media or water bath

to facilitate the removal of residual solvent. The final stage inPES membrane fabrication is membrane storage. Membranesare usually pressed between sheets of filter paper at ambienttemperature during the drying process (Rahimpour et al.,

2010b, c) or immersed in water with the addition of formalde-hyde (or sometimes methanol) to prevent bacterial growth.

Based on the PES membrane fabrication method as de-

scribed above, alteration or adjustment of the PES membranescan be easily done through modification of the chemical com-position or fabrication conditions during the membrane fabri-

cation process. Additionally, alterations or adjustments ofPES-based membranes can also be done after the formationof the membranes, which usually involves surface modifica-

tion. Common surface modification methods included surfacecoating, surface grafting or surface chemical reactions (Vena-ult et al., 2012a).

3. PES membrane modification methods applied in previous

studies

PES has turned out to be an important polymeric material

(Villaverde-de-Saa et al., 2012) in membrane fabrication.

Figure 3 PES carboxylation (Adapted fro


However, the hydrophobicity of PES membranes contributesto lower membrane performance and leads to poor anti-foul-ing properties and lower membrane permeability, which seri-

ously limit the lifespan and applications of PES membranes.Surface modification methods of PES membranes, whetherchemical or physical, in order to reduce membrane surface

hydrophobicity, have become the focus of research in recentyears. One of the membrane surface modification methodsintroduced was through the sulphonation reaction (Klaysom

et al., 2011). This method has successfully introduced the neg-atively charged sulphonic acid group onto the PES membranesurface through the sulphonation reaction. The molecularstructures of the non-modified and sulphonated PES are dis-

played in Fig. 2. Sulphonation is a reaction where the cross-linking between polymeric membranes (usually polysulphoneor PES) occurs with concentrated sulphuric or sulphonic-based

acids. PES membranes generated by sulphonation are morehydrophilic, have better selectivity and are resistant to fouling(Barona et al., 2007; Klaysom et al., 2011; Lau and Ismail,

2009; Rahimpour et al., 2010a).Besides, another chemical modification method introduced

to modify the PES membrane surface was through carboxyla-

tion (Wang et al., 2011). Carboxylation is a practice where acarboxyl group is introduced in the polymeric membrane ma-trix (Sajitha et al., 2002; Wang et al., 2011). In PES, the car-boxyl group would become a substituent to replace the

hydrogen atom at an aromatic hydrocarbon. In the carboxyl-ation method, the PES molecules can be acetylated and oxi-dised at specific temperatures as described in Fig. 3. Another

chemical modification method which is similar to carboxyla-tion and sulphonation is the nitration process. Similarly, thenitration process is used to introduce amine functional groups

to polymeric membranes to modify the membrane material(Botvay et al., 1999; Conceicao et al., 2009).

Another method which has been applied in the PES mem-

brane surface modification is plasma treatment. In membranesurface application, plasma treatment implies the alteration ofmembrane performance by plasma which is produced afterionisation of water vapour or gases (argon, carbon dioxide,

oxygen, nitrogen, hydrogen gas and so on) through electricaldischarge at an elevated frequency (Groning et al., 1996; Kullet al., 2005; Pal et al., 2008ab; Steen et al., 2002; Tyszler et al.,

2006; Wavhal and Fisher, 2002). Plasma would then be al-lowed to flow perpendicularly along the membrane surface.

Incorporation of the additives into the PES membrane

matrices offers another efficient way to improve the membranecharacteristics (Arthanareeswaran et al., 2010; Basri et al.,2010, 2011; Maximous et al., 2010; Ng et al., 2011, 2013;Rahimpour et al., 2008a; Shen et al., 2011b; Yuliwati and

m (Wang et al., 2011) with permission).



Figure 4 Molecular structure of monomers applied in PES

membrane modification (Adapted from (Rahimpour, 2011) with

permission).


Ismail, 2011; Zhao et al., 2008). Incorporation of additives intopolymeric matrices could alter the porosity and hydrophilicityof polymeric membranes. Additives added into the polymeric

membrane matrix could be inorganic nanoparticles such as sil-ica (Huang et al., 2012), titanium dioxide (Ngang et al., 2012),silver (Diagne et al., 2012; Zhang et al., 2012), zirconium diox-

ide, selenium and copper (Akar et al., 2013). Besides, poly-meric materials also have been added as blended materials inpolymer-based membranes such as Pluronic, polyethylene gly-

col, polyvinylpyrrolidone, and so on.Research has been carried on in search of a better mem-

brane modification method to improve the PES-based mem-branes in the past few years (Zhao et al., 2013). Amongst all

the modification methods suggested, UV-induced modificationof PES membranes has attracted much attention in the recentyears due to several reasons which will be discussed in the fol-

lowing section.

4. UV-induced modification of PES membranes

Another familiar membrane alteration method uses UV radia-tion, which has acquired popularity due to the ease of the mod-ification process (Peeva et al., 2012). UV radiation also has

been used in UV photo-grafting (Rahimpour, 2011) modifica-tions. UV-induced modification has been applied in many re-cent studies (Bilongo et al., 2010; Homayoonfal et al., 2010;

Yu et al., 2010; Zhou et al., 2010). Details on UV-inducedmodification and the effects on membrane performance willbe discussed in the following sections.

The reasons for performing membrane modification varied

and depend on the application and suitability in certain fields.Generally, membrane modification often improves certainproperties of the membrane, but these are compensated for

with a reduction in certain aspects of membrane performance.Thus, the discussion here focuses more on how the UV-in-duced modification variables affect the performance of modi-

fied PES membranes. It is worthwhile to build on previousresearch to achieve further improvements in modification pro-cesses in the future.

In general, UV-induced modification can be divided intotwo categories: UV-grafting using monomers and UV-inducedmodification using nanoparticles. The following discussion fo-cuses on how other researchers have modified PES membranes

using UV irradiation, some of the improvements achievedthrough this method and significant observations which maybe useful for future investigations. PES has been focused on

in this review as previous research has revealed that PES is eas-ier to modify through UV irradiation (Kaeselev et al., 2001).PES is much more sensitive towards the UV source in compar-

ison to the other polymeric membranes such as polysulphonemembranes in UV-induced polymerisation and PES canachieve the desired degree of grafting with less energy usage.

4.1. UV-grafting using monomers

In a recent paper (Rahimpour, 2011), a variety of monomerswas used (which are considered hydrophilic) with various

weight percentages: 2-hydroxyethylmethacrylate (HEMA)and acrylic acid (AA) as monomers of acrylic, and ethylenediamine (EDA) and 1,3-phenylenediamine (mPDA) as amino

monomers for the alteration of PES membrane surfaces


(Mw = 58,000 g/mol). During the modification process, thePES membranes were immersed briefly into the amino and ac-rylic monomer solutions. Next, the membranes immersed in

the monomer solution were irradiated by a UV source. Themolecular structures of the applied hydrophilic monomersare shown in Fig. 4. PES membranes were generated by the

phase immersion procedure. The doping solution was madeby adding PES beads to the solvent in the presence of a poreforming agent (in this specific research, this was polyvinylpyr-

rolidone). The coagulation bath mixture was a combination of80% water and 20% 2-propanol by volume. The PES mem-branes had been customised by coating the membrane surfaceby a dipping procedure. The membranes were then irradiated

by a UV source. In the membrane surface coating process,the membranes were immersed briefly (half an hour) in theamino and acrylic monomer solutions (5 wt.% EDA, 1 and

6 wt.% AA, HEMA and mPDA). The immersed membraneswere positioned in a Plexiglas (PMMA) tube and irradiatedby a 160 W UV source (wavelength 259 nm, intensity

24.3 mW/cm2) for 5 min. The membranes that had been ex-posed to UV radiation were rinsed with clean water for the re-moval of excess or unused hydrophilic monomers. Subsequent

to rinsing, the drying process of the as-generated or modifiedPES membranes was done at ambient temperature. In orderto determine the degree of modification (abbreviated asDM), Eq. (1) was used:

DMð%Þ ¼ ðW2 �W1Þ=W1 ð1Þ

where the weights of the membrane prior and subsequent tothe grafting process are W1 and W2, respectively.

It has been reported that the hydrophobicity on the modi-

fied PES membrane surface is reduced when exposed to UV-irradiation due to the existence of hydrophilic monomers(Rahimpour, 2011). However, the unchanged PES membrane

surface displayed the highest hydrophobicity. In particular,the HEMA (6 wt.%) customised membrane showed the bestor lowest hydrophobicity amongst the customised membranes.

This study summarised that the reduction in the hydrophobic-ity of customised membranes irradiated by UV and graftedwith amino and acrylic monomers can be used to demonstratethe polymerisation of the monomers with the PES membrane

surface. The grafting of these monomers through UV irradia-



Figure 5 AFM images of the membrane surface for (a) the unmodified PES membrane, (b) PES membrane grafted with AA (6 wt.%),

(c) PES membrane grafted with HEMA (6 wt.%), (d) PES membrane grafted with mPDA (6 wt.%) and (e) PES membrane grafted with

EDA (5 wt.%) (Adapted from (Rahimpour, 2011) with permission).

6 L.Y. Ng et al.

tion increased the hydrophilicity of the membrane surface.

This experiment used AFM analysis to distinguish those PESmembrane surfaces with and without modification. The cap-tured AFM images of the membrane surface are shown in

Fig. 5. The AFM scan area used in this study was


10 · 10 lm. Rougher surfaces were observed for the PES mem-

branes polymerised with the hydrophilic monomers (refer Ta-ble 1 for surface roughness values).

To examine the consequences of the polymerisation of

monomers onto the PES membrane through UV-irradiation,



Figure 6 Milk water fluxes produced by a PES membrane

following surface polymerisation with hydrophilic monomers

(Adapted from (Rahimpour, 2011) with permission).

Figure 7 Milk protein rejections by PES membranes following

surface polymerisation with hydrophilic monomers (Adapted from

(Rahimpour, 2011) with permission).

Table 1 Surface roughness recorded for unmodified mem-

brane and membranes modified with hydrophilic monomers.

Membrane Roughness (nm)

Sa Sq Sz

Unmodified

2.64 3.34 22.6

Modified with:

1 wt.% AA – – –

6 wt.% AA

38 50.4 328

1 wt.% HEMA – – –

6 wt.% HEMA

87.9 113 747

1 wt.% mPDA – – –

6 wt.% mPDA

70.8 87.4 545

5 wt.% EDA

40.7 51.9 398

Surface roughness parameters of the membranes are expressed in

terms of:

Mean roughness (Sa),

Root mean square of the Z data (Sq), and

Mean different between the five highest peaks and lowest valleys

(Sz).

(Adapted from (Rahimpour, 2011) with permission).


non-skim milk and pure water filtration were done using theoriginal and customised PES membranes (Rahimpour, 2011).

The PES membranes showed a reduction in pure water fluxfor any concentration of the monomers polymerised onto themembrane surface by UV-irradiation. An elevated concentra-

tion of monomers produced membranes with a greater declinein pure water flux. The explanation given was that the mono-mer chains polymerised onto the PES membranes formed nar-

rower pores (a higher monomer concentration producednarrower surface mean pore sizes (Rahimpour, 2011). Basedon Fig. 6, the photo-grafted PES membrane produced lowermilk water fluxes. It was obvious that an increase in monomer

concentration led to lower milk-water fluxes. It has been pro-posed that membranes grafted with monomers and with higherDM values produce membranes with reduced milk-water and

pure water fluxes.Monomer-grafted PES membranes had better milk protein

rejection as shown in Fig. 7 (Rahimpour, 2011). Additionally,

a higher monomer concentration led to higher rejection. Thiswas explained by the observation that the reduction in poresize increased protein rejection.

In 2002, a similar research was used to investigate the con-

sequences of UV-irradiation on PES with and without anymodifying agents (Kaeselev et al., 2002). The modifying agentsused were 2-acrylamidoglycolic acid monohydrate (AAG), N-

vinyl-2-pyrrolidine (NVP) and 2-acrylamido-2-methyl-1-pro-panesulphonic acid (AAP). Characterisation using FTIR andXPS was performed for the determination of grafting in this

study. AAG showed a better grafting tendency with the PESmembrane compared to AAP or NVP. The PES ultrafiltrationmembranes were also tested for separation performance using

a dextran solution. Without any monomers, the irradiated PESmembrane showed reduced solution flux when the energy ofirradiation was 200 mJ/cm2. However, the highest rejectionof dextran was achieved when the UV irradiation energy in


the absence of monomers was 200 mJ/cm2. However, whenthe intensity was increased from 200 to 300 mJ/cm2, dextran

solution fluxes were found to be increased along with a drasticreduction in dextran rejection. This confirmed that a deteriora-tion of membrane separation performance occurred when ex-

cess UV energy was used in the membrane modification.This could provide useful information for researchers whenchoosing their UV irradiation energy range during the mem-brane modification process. Modification of membranes with

the addition of monomers indicated an increase in the separa-tion performance. However, all membranes displayed a drasticreduction in solution fluxes and further improvements are

required.In 2010, there was a similar study on the modification of

nanofiltration membranes (NFPES10, supplied by Hoechst

Company) using acrylic acid and N-vinylpyrrolidone (Semanet al., 2012). The researchers claimed that a shorter exposuretime to the UV source was not sufficient for the N-vinylpyrroli-

done to be grafted with the PES membrane when the concen-tration of the monomer was too low. However, when theN-vinylpyrrolidone concentration was higher and exposed tothe UV source for a longer period, higher water permeability



8 L.Y. Ng et al.

was observed. The explanation given was that this might bedue to trunk polymer scission, exhaustion of the monomersfrom the membrane surface or detachment of previously

grafted monomers from the membrane surface. Moreover,the degree of grafting (DG) results revealed that the value ofDG was in a linear relationship with the UV irradiation dura-

tion up to a maximum point, and then started to reduce withprolonged irradiation times. Over-irradiation could break themonomers that had already been grafted on the PES mem-

brane, thus leading to lower DG values. Another observationfrom this research was that the concentration of monomers re-quired might depend on the pH of the separation process. Forinstance, a high acrylic acid concentration was required for

grafting when the modified membrane was designated to beused in an acidic environment in the presence of humic acidsat pH 3. Additionally, a moderate N-vinylpyrrolidone concen-

tration performed well for separation processes in a neutralenvironment at pH 7.

These studies have shown that the type of monomer used in

membrane grafting not only alters the membrane structure andmembrane characteristics, but also determines the membrane’ssuitability to be used for different solutions or environments,

whether acidic, neutral or alkaline. Irradiation intensity, dura-tion and distance also might play a significant and importantrole on the monomer grafting process or degree of grafting.The selection of monomers to be used in membrane surface

modification through UV-irradiation should be considered inview of all these parameters as they are dependent on variousfactors related to membrane performance.

For instance, UV-induced surface modification usingmonomers can improve the hydrophilicity or wettability ofthe PES membrane surface. One of the studies published in

2004 revealed that customised PES membrane filtration perfor-mance varied on the type of monomers used (Taniguchi andBelfort, 2004). This work categorised the monomers according

to their charge property, such as neutrally charged, e.g. 2-hydroxyethyl methacrylate (HEMA) and N-2-vinyl-pyrroli-done (NVP); weakly charged, e.g. 2-acrylamidoglycolic acid(AAG) and acrylic acid (AA), and strongly negatively charged,

e.g. 2-acrylamido-2-methyl-1-propanesulphonic acid (AMPS)and 3-sulphopropyl methacrylate (SPMA). Thus, in this study,the researchers concluded that the type of monomer could af-

fect the filtration performance and would exhibit a differentdegree of grafting. Based on this study, the monomers shouldbe selected according to the application where the membranes

would be used for better separation performance and anti-fouling properties (Mansourpanah and Momeni Habili, 2013).

4.2. Nanoparticle modification of PES membranes with UVirradiation (deposition and entrapment methods)

Nanoparticles have been incorporated into the membrane ma-trix as done in our previous work (Ng et al., 2011), where the

silica nanoparticles and polysulphone membrane compositionhad been modified and optimised accordingly. Through theincorporation of silica nanoparticles into the polysulphone

membrane matrix using blending method during the dope for-mulation, we observed an increase in membrane permeabilityand real salt rejection. This observation might be attributed

to the surface charges of the silica nanoparticles which re-pulsed the permeation of ions.


Besides, silver chloride nanoparticles also were used to pro-duce composite membrane through in situ micro-emulsionpolymerisation (Wu et al., 2013). Researchers claimed that

the AgCl nanoparticles can stay in a well-dispersed mannerin the produced membranes. In this section, another newlydeveloped method to incorporate the nanoparticles into the

membrane matrices using UV-irradiation is discussed.In another study (Rahimpour et al., 2008b), PESmembranes

with entrapped and non-entrapped TiO2 nanoparticles at vari-

ous concentrations were fabricated by the phase immersionmethod. Membrane separation performance was investigatedbased on milk retention and water fluxes. The coating of thenanoparticles was conducted on a PES membrane prepared by

immersing the membrane briefly into a nanoparticle colloidalsuspension and UV-irradiated with various adjustments. Thenanoparticle-covered membranes were modified by briefly

immersing the pure membrane in various percentages of tita-nium dioxide nanoparticle suspensions for various times. Dis-tilled water was used for the nanoparticle suspension by

adding a suitable concentration of titanium dioxide nanoparti-cles. Thewell-mixed nanoparticle suspensionswere prepared be-fore use for this application. Finally, distilled water was used to

rinse the membranes which were then exposed to the UV sourcefor a variable period of time. The outcome regarding flux perfor-mance for pure and UV-irradiated nanoparticle-entrappedmembranes is shown in Fig. 8. The researchers reported that

the UV-irradiated nanoparticle-entrapped PES membraneshowed better solution (milk) flux in comparison to the purePES membrane and the nanoparticle-entrapped membrane

without UV irradiation. The researchers also reported thatUV irradiation did not cause degradation of the PESmembranemorphology. Furthermore, the nanoparticle-entrapped mem-

brane and the UV-irradiated membranes exhibited a similarprotein retention ability, which signifies that the integrity ofthe membrane had been conserved.

When a substance whose electrical conductivity is interme-diate between that of a metal and an insulator, also referred toas semi-conductor, is exposed to energy equivalent to or largerthan the band gap energy, under normal circumstances, an

electron can be moved to the conduction band from the capac-ity band (Rahimpour et al., 2008b). Consequently, pairs ofelectrons and holes can be produced on the surface of the

semi-conductor. Due to the fact that titanium dioxide is a kindof semi-conductor, the electrons produced due to UV-irradia-tion causes O2 molecules from the surroundings to generate the

superoxide radical anion (O�2 ). After being irradiated by a UVsource, the holes produced react with the surrounding watervapour to produce OH radicals. Impurities such as organiccompounds are then disintegrated and eliminated due to these

strong oxidant reagents. The pathway of the photo-catalysishas been proposed as shown in Fig. 9.

The next observable fact was the super-hydrophilicity in

this research (Rahimpour et al., 2008b). In this situation,formed holes and electrons would react in a dissimilar manner.The electrons produced due to UV-irradiation have a tendency

to decrease the oxidation number of titanium from titanium(IV) to the titanium (III) state, and oxidised O�2 anions areproduced as a result of the holes generated. As proposed, oxy-

gen vacancies will be generated on the membrane surface whenoxygen atoms are released. The vacant locations will be takenup, and adsorbed OH groups can be produced from the sur-rounding water molecules on the membrane surface. This fur-



Figure 8 Pure PES and UV-irradiated nanoparticle-entrapped

PES membrane flux performance using non-skim milk (Adapted

from (Rahimpour et al., 2008b) with permission).Figure 10 Super-hydrophilicity process (Adapted from (Rahim-

pour et al., 2008b) with permission).


ther reduces the hydrophobicity of the membrane. The super-hydrophilicity process is shown in Fig. 10.

Nanoparticle-entrapped PES membranes which were ex-

posed to UV-irradiation exhibited elevated flux which can becontributed to by photo-catalysis and super-hydrophilicity(Rahimpour et al., 2008b). TiO2 nanoparticles are destructive

to organic matter as they generate strong oxidant reagents.This limits the accumulation of proteins, fats and any other or-ganic matter present in the non-skim milk on the surface of the

membrane. Flux was therefore improved by limiting mem-brane fouling.

Another method used is coating the surface of a polymeric

membrane with inorganic nanoparticles (Rahimpour et al.,2008b). The immersion of the membrane in a TiO2 nanoparticlesuspension was done for one hour at ambient temperature. Next,water was used to flush the membrane irradiated by a UV source

(160 W) for half an hour. A significant change in contact angleswas used to reveal that the nanoparticles had been deposited onthe surface of the membrane. Fig. 11 shows the SEM images of

the control and TiO2-coated membranes. Agglomeration ofnanoparticles was observed in this study. Deposition of nanopar-ticles on the membrane surface was explained by the existence of

OH bonds in the membrane structure. Deposition of nanoparti-cles on the membrane surface occurred by forming a strong bondwith the membrane and the inhibited removal of nanoparticles

from the membrane surface.

Figure 9 Proposed pathway of photo-catalysis (Adapted from

(Rahimpour et al., 2008b) with permission).


Higher flux decline was observed with membranes notcoated with nanoparticles when the membranes were testedwith a non-skim milk solution. This observation was claimedby the authors as an improvement in the membrane antifoul-

ing characteristics after irradiation with a UV source and coat-ing with TiO2 nanoparticles (Rahimpour et al., 2008b).However, differences in the amount of TiO2 did not signifi-

cantly alter the membrane antifouling properties. This mayhave occurred due to pore blockage by the TiO2 nanoparticleswith elevated loadings. Additionally, increased irradiation time

also led to performance deterioration of the nanoparticle-deposited membrane. This likely occurred due to the forma-tion of too many aggregated radicals on the membrane surface

after exposure to the UV source at elevated energy and for anextended duration without any monomers. Peroxide groupscan be produced on the membrane surface as a result of the re-sponse between OH radicals. Membrane performance after

prolonged exposure to a UV source was diminished, probablydue to alterations to the membrane surface. Moreover, it wasfound that prolonged membrane immersion in the nanoparti-

cle colloidal suspension produces pore blockages which actu-ally reduce membrane performance. In addition, increasedUV source energy could also deteriorate the membrane perfor-

mance, as previously reported.In another similar research work, TiO2 nanoparticles were

deposited onto the poly(vinylidene fluoride)/sulphonatedpolyethersulphone membrane which followed by UV-irradia-

tion to activate the photo-catalytic property (Rahimpouret al., 2012). The researchers used scanning electron micro-scope to confirm the presence of the nanoparticles on the mem-

branes. Besides, they found that the nanoparticles’ modifiedmembrane showed better anti-fouling property when testedusing bovine serum albumin solution. Lastly, they also re-

ported that this modification method can produce membranewith better anti-bacterial property.

In brief, the deposition of nanoparticles onto the membrane

surface might contribute to improved antifouling properties.However, the suitability of this method in other separationprocesses is still worth further investigation besides focusingon ultrafiltration applications. Furthermore, controlling the



Figure 11 SEM photos of (a) the control membrane surface and

(b) the 0.03 wt.% TiO2-coated PES membrane surface (Adapted

from (Rahimpour et al., 2008b) with permission).

10 L.Y. Ng et al.

parameters of this process is still a questionable issue; optimi-sation might be the solution for this problem.

5. Summary

PES membranes have been successfully employed in diverse fil-tration processes and applications. PES is recognised as an

excellent material in the manufacture of membranes as it ischemically resistant, possesses good mechanical propertiesand is thermally stable. PES membranes have been categorised

as hydrophobic membranes in comparison with other types ofmembranes such as cellulose acetate, polyamide, polyimide,and so forth. The hydrophobicity of the membrane has been

claimed by some researchers to reduce membrane fouling incertain separation processes. However, in some studies, modi-fication of PES can significantly improve the membrane anti-

fouling properties by reducing the hydrophobicity of themembrane (Mockel et al., 1999). The modification of PESmembranes should be thoroughly investigated in the near fu-ture for better incorporation into various applications and

purposes. PES membranes have been well-commercialised forthe desalination process. However, in view of the progressivedevelopments and research done on the forward osmosis appli-

cation, PES membranes might be offered as a new kind ofmaterial for forward osmosis membranes. The requirementsfor a forward osmosis membrane, such as high salt rejection,

high water permeability, low concentration polarisation, good


chemical and thermal stability, are the characteristics pos-sessed by PES membranes, except that PES membranes stillneed modification to improve their antifouling performance.

Thus, the discussion above can be referred to for membranemodification selection, especially involving UV irradiationmodifications.

Strongly charged monomers, for example, could be effec-tive in the desalination process where ion separation is themain concern. The repulsion and attraction of ions in solution

due to the charged membrane could prevent the ions frompassing through the membrane (Donnan effect), thus improv-ing the separation performance. Neutral or weakly chargedmembranes, however, could be appropriate for neutral solute

separation where the charge of the membrane deterioratesthe filtration performance.

In view of the easier operability of UV-induced surface mod-

ification, various modification conditions have been studiedusing polyethersulphone and polysulphone membranes. In gen-eral, when the monomer concentration increases during UV-in-

duced grafting, membrane rejection will increase accordinglyuntil a maximum concentration, which is highly dependent onthe type of monomer, the UV-source intensity, the base mem-

brane materials and the degree of grafting. Membrane separa-tion performance will start to deteriorate when the monomerconcentration exceeds themaximumconcentration due to excesspore plugging. Consequently, the concentration of monomers

used in surface modification is one of the main concerns; thisparameter should be thoroughly investigated and controlledfor all UV-induced surface modifications to obtain the opti-

mised performance such as the greatest rejection of solutes, high-est permeability, lowest fouling tendency due to solutes and thuslower operating pressure or energy consumption.

Besides monomers, nanoparticles also have been used andincorporated into PES membranes. So, UV-grafting of nano-particles onto the PES membrane surface, as discussed above,

could provide an alternative route to modification which mightcontribute to higher ion rejection during the salt filtration pro-cess. When nanoparticles are deposited onto the membranesurface, the charges possessed by the nanoparticles could inhi-

bit ions from passing though the separation layer. Addition-ally, by using certain nanoparticles such as titanium dioxideor silver nanoparticles, the membrane anti-bio-fouling proper-

ties are increased, possibly since nanoparticles are destructiveto organic matter by the generation of strong oxidant reagents.This could be useful for future nanofiltration membrane mod-

ification where it can be used to lessen the fouling tendency ofthe nanofiltration membrane and simultaneously increase therejection of the membranes towards freely moving ions inaqueous solution by imposing repulsion in the presence of

the surface charges of the nanoparticles on the membrane sur-face. However, the concentration of nanoparticles to be used insurface coating is also a critical issue that needs much investi-

gation and attention as excess nanoparticles could cause poreblocking in polymeric membranes, thus reducing membranepermeability.

Acknowledgement

The authors of this work wish to gratefully acknowledge thefinancial support for this work by the UKM Research Grant(DIP) through the project No. DIP-2012-01.




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