Preparation of Low fouling Polyethersulfone Membranes by Simultaneously Phase Separation and Redox Polymerization

This paper presents preparation of low fouling PES membranes by non solvent induced phase separation (NIPS) coupled with redox polymerization. The membrane characterization included water permeability, morphology structure (by SEM) and surface chemistry (by FTIR). Water permeability measurements showed thatthe membranes have water permeability within the range 10-50 L/h.m2.bar. Addition of PEG dan PEGMA intopolymer solution increased water permeability, whereas blending redox initiator and crosslinker, MBAA in polymer solution decreased water permeability. Surface morfology of membranes by SEM showed that unmodified PES membrane had smaller pore size than PEG or PEGMA modified PES membranes. Furthermore, PES-PEG or PES-PEGMA membranes modified by blending with redox initiator and MBAA as crosslinker showed smaller pore size than unmodified membrane. FTIR analysis showed that all membranes have typical spectraof PES polymer; however no additional peak was observed forthe membranes prepared with addition of PEG/PEGMA, initiator redox and also crosslinker. The addition of PEG/PEGMA, redox initiator and crosslinker resulted in membranes with high rejection and an acceptable flux as well as more stable due to relatively high fouling resistance.


Introduction
UF membrane technology has been applied in various industries such as water and wastewater treatment, beverage and food processing, medicine and pharmaceutical industry [1][2][3][4]. But along with the increasing application of ultrafiltration, fouling which causing significant loss of performance with respect to flux and often selectivity is still the biggest problem. Furthermore, fouling can also shorten the membrane life.
Eventhough several methods to control fouling of the membrane have been developed [5,6], but in many cases fouling is determined by the membrane itself [7,8]. Therefore, preparation of low fouling membranes is urgently needed. The hydrophobic caharacter of PES polymer has driven modification of PES membrane. Three different approaches have been developed to increase the hydrophilicity of PES membranes, namely (i) modification of polymer membranes (pre-modification) [9] (ii) mixing the polymer membranes with additives [10][11]  preparation (post-modification) [12]. However, each approach has drawbacks such as takes a long time, stability of additives in the polymer matrix is low, and also some treatments can cause changes in membrane structure. Integrationof NIPS and chemical modification is the best solution becauseit is simple and effective way to make low fouling membranes with high stability. Development of NIPS modified by redox polymerization is expected to be able to synthesis low fouling UF membrane in single process with high stability of hydrophilic character.
In general, the objective of this research is to synthesis low fouling UF membranes by using NIPS integrated with redox polymerization. In particular, this research investigates the effects of additives, initiator redox and crosslinking agent on the characteristics and stability of the resulting membranes. In addition, fouling behaviour is also investigated.

Membrane preparation
PES polymer was dissolved in NMP. PEG and PEGMA were then added to the polymer solution. In addition, oxidizing agent (TEMED) and MBAA were also added into polymer solution. The homogenous polymer solution was left without stirring until no bubbles is observed. The polymer solution was cast with a thickness of 200 μm using a steel casting knife on a glass substrate. Thereafter, the proto-membrane was solidified by immersing in a coagulation bath containing water and reducting agent (APS) for one hour.The resulting membranes was washed and soaked in the water for 24 h before drying.

Membrane characterization
Membrane characterization included water permeability, morphology structure (by SEM) and surface chemistry (by FTIR). Measurement of water permeability followed our previous publication [18]. The top surface morphology of the membrane was observed by using a Quanta 400 FEG (FEI) environmental scanning electron microscope (ESEM). Before observation, sputtering to coat the outer surface of the sample with gold/palladium should be conducted. The membrane surface chemistry was analyzed by using the Varian 3100 Fourier transform infrared spectroscopy (FTIR) Excalibur series.

Water permeability
Measurement of water permeability was performed for unmodified and modified membranes.The effects of PEG, PEGMA, redox initiators and crosslinking agent were investigated. The results are depicted in Figure 1 The water permeability data presented in Figure 1 suggestthat the resulted membranes are in the range of UF membranes, which have water permeabilityof 1050 L/h.m 2 .bar [1]. Addition of PEG and PEGMA increased water permeability. PEG is a hydrophilic polymer,which will increase the hydrophilic character of PES membrane. I addition, the presence of PEG into polymer solution may favor the formation of larger pores and porosity, resulting an increase of water flux. This is in agreement with the results obtained by previous authors [13][14][15][16]. Similar phenomenon was obtained using PEGMA as the additive and in agrement with the result obtained by Susanto et al. [12].
Blending redox initiator in polymer solution decreased water permeability of modified membrane. The presence of initiator may stimulate the chemical reaction which may reduce pore size. Similar phenomenon is also seen for MBAA addition in the polymer solution. The addition ofcrosslinker may form a network structure. This result is in agreement with the experiment conducted by Peeva et al. [17]. Crosslinking with MBAA yielded denser hydrogel layers on the porous base membrane and consequently decrease water permeability. Figures 2, 3 and 4 show surface morphology of unmodified PES and PEG/PEGMA modified PES membranes. It is observed that all membranes had pore sizes within the nanometer range. Comparing the unmodified PES and modified PES membranes by PEG or PEGMA addition, it is seen that all membranes had significant difference in pore size. Similar with previous authors, unmodified PES membrane had smaller pore size than PEG or PEGMA modified PES membranes [14,18]. These results support previous explanation about water permeability of membranes.PES-PEG or PES-PEGMA membranes modified by blending with redox initiator and MBAA as crosslinker showed smaller pore size than unmodified membrane. Redox initiator generated radicals on the membrane surface to create new functional groups on the membrane surfaces [19].    C=O vibration in the ester molecule, which is located at 1714 cm −1 . Such overlapping band is also found in previous publications by Susanto and Ulbricht [18] and Belfer et al. [20].

Conclusion
Modification of PES UF membranes by redox polymerization using hydrophilic monomers (PEG/PEGMA), initiator redox and also MBAA as crosslinker has been successfully developed. Water permeability measurment suggested that all membranes should be UF membrane. Addition of PEG dan PEGMA increased water permeability. By contrastblending redox initiator and crosslinker into polymer solution decreased water permeability. Surface morfology of membranes indicated thatthe pore sizes are within the nanometer range. Unmodified PES membrane had smaller pore size than PEG or PEGMA modified PES membrane . Furthermore, PES-PEG or PES-PEGMA membranes modified by redox initiator and MBAA showed smaller pore size than unmodified membrane. FTIR analysis showed that no additional peak was observed for the modified membranes. The fouling resistance of unmodified and modified membranes are under evaluation.