Carbon nanotubes-blended poly(phenylene sulfone) membranes for ultrafiltration applications
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Multi-walled carbon nanotubes (MWCNT) were carboxylated by a chemical method. Poly(phenylene sulfone) (PPSU), MWCNT and functionalized (carboxylated) MWCNT/poly(phenylene sulfone) (PPSU) blend membranes were synthesized via the phase-inversion method. The resultant membranes were then characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM) and contact angle. The FMWCNT blend membranes appeared to be more hydrophilic, with higher pure water flux than did the pure PPSU and MWCNT/PPSU blend membranes. It was also found that the presence of multi-walled carbon nanotubes (MWCNTs) in the blend membranes was an important factor affecting the morphology and permeation properties of the membranes. The model proteins such as trypsin (20 kDa), pepsin (35 kDa), egg albumin (45 kDa) and bovine serum albumin (69 kDa) rejection experiments were carried out under identical operational conditions employing both PPSU and blend membranes. The membranes were also subjected to the determination of molecular weight cut-off (MWCO) using different molecular weights of proteins. During trypsin ultrafiltration, PPSU/MWCNT and PPSU/FMWCNT membranes showed a slower flux decline rate than did the PPSU membrane.
KeywordsPoly(phenylene sulfone) Ultrafiltration Carbon nanotubes Nanocomposite Blend membranes
To date, polysulfone that is a family of thermoplastic polymers has been showing a continuous domination on the membranes of materials for separation technology due to low cost and ease of process in addition to thermal and chemical resistibility. However, these membranes comprising hydrophobic materials, which tend to suffer severe decreases in pure water flux, during operation caused by solute adsorption pore blocking and cake formation (Baker 2000). Membrane fouling, especially, is a serious problem in the case of protein separation because hydrophobic interactions between proteins and the membrane surface induce a non-selective and irreversible adsorption. Easily fouled membranes need frequent chemical cleaning and replacement of the membrane module, giving a sort lifetime, which leads to high utility costs (Pontie et al. 2007; Zularisam et al. 2007; Vrijenhoek et al. 2001; Shi et al. 2007). To circumvent the above drawbacks, it is well known that increasing the membrane hydrophilicity can effectively minimize membrane fouling (Zhao et al. 2010; Ran et al. 2011) though charged membranes can also be used to reduce membrane fouling (Mulder 1997). As such, methods such as surface graft polymerization, chemical grafting, and radiation-induced grafting have been developed in an attempt to increase the surface hydrophilicity of membranes (Deng et al.2009; Abu Seman et al. 2010).
Recently, the new types of membrane have been showing a promising role in improving existing membrane materials. For example, polymeric blend membranes, and polymeric nanocomposite membranes (organic–inorganic), in which organic–inorganic membranes have great attractions for separation applications, because of their excellent mechanical strength and chemical resistance (Savage et al. 2009).
In addition, it is expected that such membranes will have the physicochemical stability of inorganic materials and the membrane-forming properties of polymers. In particular, nano-sized inorganic material-blended composite membranes are attractive because of their enhanced properties, such as high permselectivity, higher hydrophilicity, and enhanced fouling resistance (Liu et al. 2005; Shen et al. 2011; Lee et al. 2011). Literature reports have shown that there is a high potential for carbon nanotubes (CNTs) to improve the material properties of polymers (Kim et al. 2006, 2007; Peng et al. 2007a, b; Sharma et al. 2010; Aroon et al. 2010a, b, c; Cong et al. 2007). CNTs have an exceptionally high aspect ratio in combination with low density, and high strength and stiffness, which makes them a potential candidate as an effective reinforcing additive in polymeric materials.
Surprisingly, poly(phenylene sulfone) (PPSU) which is one the members of the polysulfone family, has superior physical and chemical properties compared to bisphenol-A-modified polysulfones. Only few researchers have studied the applicability of PPSU in fuel cell (Luisa Di Vona et al. 2006, 2008; Sgreccia et al. 2011; Hsiang Weng et al. 2008). However, not much attention was paid to PPSU for UF applications. Hence, the present investigation is aimed to study the utility of PPSU in ultrafiltration applications.
Carbon nanotube (MWCNT)-blended polysulfone ultrafiltration and microfiltration membranes were reported elsewhere. In view of literature, poly(phenylene sulfone) has not yet been used to prepare CNT-blended membranes for protein separation and water treatment. In this present investigation, multi-walled carbon nanotube/poly(phenylene sulfone) and functionalized multi-walled carbon nanotube/poly(phenylene sulfone) blend membranes were prepared and characterized, PPSU/MWCNT and PPSU/MWCNT blend membranes characterized in terms of pure water flux, water content and water contact angle. The enrichment of CNT in the surface of the membrane ascertained through Fourier transform infrared (FTIR) spectroscopy. The surface and cross-sectional morphology were analyzed by SEM. Furthermore, model proteins such as trypsin (20 kDa), pepsin (35 kDa), egg albumin (45 kDa) and bovine serum albumin (69 kDa) rejection experiments were carried out under identical operational conditions employing both the PPSU and blend membranes. The protein rejection efficiency and stable permeate flux were tested to compare the separation behavior of both the ultrafiltration membranes. Then, the fouling decline ratio was determined using trypsin as a model protein.
Radel-5500 Poly (phenylene sulfone) was supplied by Solvay polymers. Analytical grade N-methylpyrrolidone (NMP) from Merck (I) Ltd was used as supplied as a solvent for the nanocomposite blend solution preparation. Carbon nanotubes were procured from Korean Carbon Nanomaterial Technology Co., Ltd. Anhydrous sodium monobasic phosphate and sodium dibasic phosphate heptahydrate were procured from Merck (I) Ltd and were used for the preparation of phosphate buffer solutions in the protein analysis. Proteins, viz., bovine serum albumin (BSA) (69 kDa), from Alpha Laboratories, India; egg albumin (EA) (45 kDa), from Merck (I) Ltd; pepsin (35 kDa) and trypsin (20 kDa) from BDH Chemicals Limited, was used as received. Deionized water was used throughout this study.
Fabrication of membranes
The blend solutions based on PPSU, PPSU/0.5 wt% MWCNT, PPSU/0.5 wt% FMWCNT (total polymer concentration = 17.5 wt%) were prepared by dissolving with different compositions in a solvent, NMP (80 wt%) under constant mechanical stirring at a moderate speed of rotation in a round bottomed flask for 4 h at 40 ºC (before adding PPSU CNT and NMP were sonicated for 4 h to attain uniform dispersion of CNT in NMP). Multi-walled carbon nanotube (MWCNT) was carboxylated by chemical method as reported elsewhere. The homogeneous solution that was obtained was allowed to stand at room temperature for at least 1 day in an airtight condition to get rid of air bubbles. The method of preparation involved is the same as that of the “phase inversion” method employed in earlier works as reported by other researchers (Rahimpour and Madaeni 2007). The casting environment (relative humidity and temperature) was standardized for the preparation of membranes with better physical properties such as the homogeneity, thickness, and smoothness. The membrane-casting chamber was maintained at a temperature of 24 ± 1 °C and a relative humidity of 50 ± 2 %. The casting and gelation conditions were maintained constant throughout, because the thermodynamic conditions would largely affect the morphology and performance of the resulting membranes (Barth et al. 2000). The membranes were casted over a glass plate using a doctor blade. After casting, the solvent present in the cast film was allowed to evaporate for 30 s, and the cast film along with the glass plate was gently immersed in the gelation bath. After 2 h of gelation, the membranes were removed from the gelation bath and washed thoroughly with distilled water to remove all NMP from the membranes. The membrane sheets were subsequently stored in distilled water, containing 0.1 % formalin solution to prevent microbial growth. Perkin Elmer attenuated total reflectance Fourier transform infrared spectroscopy (ATR) FTIR was employed for membrane surface characterization.
Compaction and pure water permeability test
Water content and contact angle measurement
Sessile drop method was employed to measure the contact angle of the membranes. The contact angles were measured at ten places and then average values were reported. Contact Angle Meter 110 VAC was employed to measure the contact angles.
Thermal stability of the membranes
The temperature of degradation was obtained by a thermo gravimetric analyzer with heating rate of 10 °C min−1 (Mettler, Model TA 3000) with a TG 50 thermo balance.
The top surfaces of the CA/PPSU blend membranes were studied under a scanning electron microscope (SEM) (JEOL, Japan). The membranes were cut into pieces of various sizes and mopped with filter paper. These pieces were immersed in liquid nitrogen for 20–30 s and frozen. Frozen bits of the membranes were broken and kept in a desiccator. These dry membrane samples were used for SEM studies. The samples were gold-sputtered for producing electrical conductivity, and photomicrographs of the samples were taken under very high vacuum conditions operating between 5 and 10 kV, depending on the physical nature of the sample. Various SEM images were taken for top surface views of the polymeric membranes. In addition, atomic force microscopy (AFM) (using a Veeco MultiMode SPM with a Nanoscope V controller) was used to characterize the topography of membrane surfaces.
Results and discussion
PPSU, PPSU/MWCNT and PPSU/FMWCNT membranes of 1 mm thickness were prepared by phase-inversion technique. Generally, hydrophilicity and surface structure are main factors, which have strong effects on the transport property of the membrane. The PPSU, PPSU/CNT nanocomposite membranes were characterized in terms of pure water flux, surface and cross-section morphology by SEM, contact angle by sessile drop method and also rejection percentage of proteins was studied and discussed in detail below.
FTIR (Fourier transform infrared spectroscopy)
Compaction and pure water permeability test
At constant operating pressure (414 kPa), the pure water flux of pure PPSU, PPSU/MWCNT and PPSU/MWCNT blend membranes upon compaction was measured for every 1 h. During compaction, the water flux was found to be high initially and declines gradually and reaches a steady state after 2–3 h of compaction. This initial decline in flux might be due to the fact that the membrane pores are being compacted leading to uniform pore size and steady-state water flux.
Water content and contact angle
Contact angle measurement of membranes and water content is considered to be an important parameter for membrane characterization, since the pure water flux of the membrane can be predicted based on these results. Water content of the membranes is an indirect indication of the hydrophilicity and flux behavior of membranes. Membranes were thoroughly washed with distilled water before estimation of water content. The pure PPSU membrane in absence of CNT was found to have water content of 60 %. In the case of PPSU/MWCNT nanocomposite membrane the water content was increased; it was found to be 70 %. Addition of MWCNT increased the immiscible nature of blend due to poor adhesion properties between PPSU and CNT. Further, this leads to increase in void volume of membrane, which results in the formation of bigger size pores. In the case of PPSU/FMWCNT blend membranes, higher water content was observed in comparison with PPSU/MWCNT membranes. This enhancement in water content is due to addition of hydrophilic –COOH moieties in the blend membrane. Water content for the PPSU/FMWCNT was found to be 73 %.
Thermal stability of membranes
Protein rejection studies
Poly(phenylene sulfone) membranes were successfully modified by adding MWCNT and carboxylated MWCNT for the formation of the blend membranes, where MWCNT and FMWCNT resulted in higher separation figures of merit including with water flux and water content and lower hydraulic resistance. The addition of MWCNT and FMWCNT also slightly altered the molecular weight cut-off (MWCO), membrane structure and the mechanical properties of the membranes. The improved surface hydrophilicity, due to surface enrichment of –COOH content, endowed the PPSU/FMWCNT blend membranes with significantly enhanced protein adsorption resistance. We observed that the incorporation of the hydrophilic MWCNT and FMWCNT blend membranes played a major role in improving the flux and performance characteristics of membranes.
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This article is published under license to BioMed Central Ltd. Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited.