Development of polyaniline-modified polysulfone nanocomposite membrane
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In the present investigation, polyaniline (PANI) nanoparticles were used to improve the separation figures of merit of polysulfone (PSu) membrane. Polyaniline nanoparticles were dispersed into polysulfone matrix for the development of PSu/PANI nanocomposites through solution blending. A wet phase inversion method was used to fabricate a flat sheet polysulfone (PSu) and PSu/PANI nanocomposite membranes. The structure and characteristic properties of the membranes were investigated in terms of the surface and cross-section morphologies, roughness, and hydrophilicity, which were interpreted by scanning electron microscope, atomic force microscope, and water contact angle meter, respectively. Apart from these, the uniform dispersion of polyaniline nanoparticles (<20 nm) into polysulfone matrix was ascertained by transmission electron microscope. Compared with polysulfone membrane, PSu/PANI nanocomposite membranes had more hydrophilicity and smooth surface, and honeycomb cross-section structure. Therefore, the nanoparticles inclusion in the polysulfone membrane showed a significant effect on hydrophilic property as well as membrane morphology, which resulted in improvement of the permeability characteristics of polysulfone membrane.
KeywordsSeparation figures of merit Hydrophilic surface Phase inversion method Permeability characteristics
Today, membrane technology is the leading technology for all kinds of separation techniques due to its intrinsic characteristics such as operational simplicity, environmental impact, high selectivity and permeability for the transport of specific components (Mulder and Mulder 1996). It holds a significant commercial impact in several areas including water and wastewater treatment (Ashraf 2009), chemical, metallurgical and petrochemical-related industries, food industries and bioseparation areas (Baker 2004). A recent advance in membrane technology is the focus on the development of membrane materials which are the key determinants of separation performance and water productivity (Lee et al. 2011; Ulbricht 2006; Nunes and Peinemann 2010). The materials used in membrane production are generally constructed from durable polysulfone and its derivatives, the majority of which are hydrophobic. Due to hydrophobic nature of PSu material, conventional PSu membranes easily suffer serious membrane fouling and have low permeation flux, making it undesirable in the long-lasting filtration process (Higuchi et al. 1991; Ghosh and Hoek 2009). As a consequence, the surface of these membranes must be rendered hydrophilic through the addition of wetting agent, e.g. hydrophilic polymers and/or by chemical modification of the membrane structure prior to use in aqueous separations. The approaches currently used for membrane surface modification include coating, surface graft polymerization, exposure to irradiation, interfacial polymerization, and etc (Deng et al. 2011; Dong et al. 2009; Patel et al. 2009; Hegde et al. 2011; He et al. 2009). However, all approaches have extra processing steps to meet the goal for membrane surface modification. Breakthroughs effects of nanocomposites on the membrane performance that have been reported in the field of water and wastewater treatment include fouling mitigation, improvement of permeate quality and flux enhancement (Anadão et al. 2010; Jeong et al. 2007; Jadav and Singh 2009; Diallo et al. 2009; Hoek and Ghosh 2009).
Interest has recently developed in conducting polymers as materials for the nanocomposite membranes. Conducting polymers are a new class of polymers with unique chemical and electrochemical properties. Thermally and chemically stable conducting polymers have attracted much interest to membrane materials since they elicit the possibility of both exploiting the chemical and physical attributes of the polymer and incorporating their electronic and electrochemical properties to enhance the separation features. Also they can be used either as deposited films or nanoparticles that can be easily bound with polymers or be firmly fixed on substrate membranes through different chemical reactions. Among the family of conducting polymers, polyaniline is one of the most useful since it is environmentally stable in both its doped, conducting form and in its de-doped, insulating form. Polyaniline is also unique among conducting polymers in that it has a very simple acid/base doping/dedoping chemistry (facile and reversible electrochemistry, where the polymer can be oxidized and reduced with simultaneous change in properties). For example, undoped form of polyaniline is hydrophobic, while in the doped form polyaniline has been shown to be hydrophilic. Together, these properties make polyaniline a promising candidate for liquid separations. Several membrane systems based on polyaniline have been significantly investigated for last few decades (Liu et al. 1994; Rebattet et al. 1995; Wen and Kocherginsky 1999; Bhadra et al. 2009; Mansouri and Burford 1994; Gupta et al. 2006; Feldheim and Elliott 1992; Fan et al. 2008a, b; Deligöz 2007; Sivakumar et al. 2006). However, polyaniline has been generally categorized as an intractable polymer because of the difficulty in solution processing. The subject of many publications focused on the improvement of its processability in the past several years, such as substitution of aromatic ring of polyaniline with -CH, -OCH, -SO, or long alkyl chain that leads to higher solubility in organic solvents and even in water. Simultaneously, undoped polyaniline (emeraldine salt form) shows better solubility in common nonpolar or weakly polar organic solvents and surprisingly desired bulk polymers (such as polysulfone) are also dissolved in these solvents. Therefore, the development of polyaniline nanocomposites and blends via solution blending is expected to make a critical impact in fabricating higher performance membranes with increased permeability, selectivity. In the present study, a highly processible form of polyaniline (undoped polyaniline) was used as nanoparticles to develop polysulfone nanocomposites. The dispersion of polyaniline to polysulfone casting solutions was accomplished by blending process which has advantages over other membrane developing methods as it can produce surface-modified membranes via a single-step casting procedure. The novelty of this study is that all improvements (such as hydrophilicity, membrane roughness, mechanical stability and selectivity) in modified membrane are obtained at low loading of polyaniline (≤0.5 mass%). In addition, the synthesis route of a doped PSu/PANI nanocomposite membrane is different from others already mentioned in literature.
Materials and reagents
Double distilled aniline (Aldrich), hydrochloric acid 37% (HCl) (R&M Chemicals), 1-methyl-2-pyrrolidone (NMP) (Aldrich), ammonium hydroxide 30% (R&M Chemicals) and ammonium peroxydisulphate (NH4)2S2O8 (APS, R&M Chemicals) were used as received. Polysulfone (Aldrich, Typical Mw = 35,000), acetone (Bendosen laboratory chemicals), methanol (R&M chemicals) distilled water. Polyethylene glycol (PEG, molecular weight 600–10,000) was of chemical grade and obtained from Merck. Sodium hydroxide (R&M chemicals) and sodium chloride (Sigma-Aldrich) were of chemical grade and used without any further purification.
Synthesis of polyaniline
Polyaniline was synthesized (conventional route in aqueous medium using HCl as dopant ion) by chemical polymerization of aniline using ammonium persulfate as an oxidant (Huang and Kaner 2004). According to this route, aniline monomer, acid dopant and oxidizing agents were taken in the molecular ratio of 1:1:1.1 aniline monomer was added into the 1N aqueous solution of hydrochloric acid and stirred to get aniline–acid complex and kept in the freeze to attain the reaction temperature of 0–5°C. In another beaker (NH4)2S2O8 was dissolved in distilled water for an initiator of aniline monomer. A fixed volume of initiator was added drop wise to a 250-ml round bottom flask containing 1 N HCl and double distilled aniline (5 ml) with slow and continuous stirring maintained at a constant temperature. After an induction period of 30 min, the color of the solution changed from brown to green, which confirmed the polymerization of aniline in conducting form. Polymerization was further continued for 12 h at room temperature. The green precipitate of polyaniline was filtered, and washed several times with distilled water, methanol, and then dried in vacuum for 72 h at 60°C. Hereafter the green PANI powder was converted to an insulating dark blue PANI powder by deprotonation with NH4OH and then dried.
Fabrication of doped/undoped PSu/PANI nanocomposite membranes
Scanning electron microscopy (SEM Gamm: modol supra 55vp-Zeiss) was used to observe the surface and cross-section morphologies of membranes. The membranes were frozen before prior to SEM observation, followed by gold sputtering (Biorad Polaron Division) (Philips SEMEDAX; XL40; PW6822/10) with the potentials of 5 kV at different magnifications.
The uniform distribution of polyaniline nanoparticles into polysulfone host were ascertained by transmission electron micrographs (TEM Philips: cm12 Transmission Electron Microscope). 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. All measurements were performed on dry membrane samples under ambient atmospheric conditions and the membrane surfaces were imaged in tapping mode. In addition, ATR-FTIR spectra for the composite membranes were taken on BRUBER–VERTEX 80.
Contact angles were measured with a contact angle meter (contact angle meter 110 VAC, 50/60 Hz) by sessile drop method that is where a water droplet was placed onto the membrane surface, using a syringe, and the values reported were averages of five separate measurements on the same membrane.
Permeability experimental setup
All the water permeability experiments were carried out using SEPA ST Stirred Cell (Osmonics USA). A circular membrane disk with a diameter of 0.049 m (49 mm) was placed at the bottom of the filtration module, the active top layer towards the feed solution. The membrane was sealed between a Teflon o-ring and a stainless steel porous support. In this way, the membrane surface had an active area of 14.2 cm2. The feed solution was magnetically stirred with a Teflon-coated stirrer bar. The maximum volume capacity of the cell was 0.3 L and the maximum operating pressure was 10 bars. The stirring speed was fixed at 400 rpm. Pressure at the feed side was supplied by an inert gas N2. Preliminary permeation experiment was conducted for ultrapure at various ranges of separation pressure (1–6 bars) and temperature at 25 ± 2°C.
Reagent A: 5% (w/v) BaCl2 in 1 N HCl (100 ml)
Reagent B: 2% (w/v) KI diluted 10 times + 1.27 g I2.
Result and discussions
Membrane supporting characterizations
Characterizations are important aspect to understand the performance of materials under service conditions. Progress in characterizing membranes by several advanced equipments such as atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) opens exciting possibilities for the evaluation of membranes to delineate the membrane surface and cross-section morphologies which generally affect membrane performance efficiency. Another important characterization of membrane is mechanical properties that involve membrane behavior under stress and play a major role in membrane performance. Mechanical properties of membranes were studied on Hounsfield H100KS.
FTIR-ATR spectra analysis
Mechanical properties of PSu and PSu/PANI nanocomposite membranes
PSu/PANI nanocomposite membrane
Membrane thickness (mm)
Tensile speed (mm/min)
Gauge length (mm)
Elongation at max. (%)
Mechanical properties that involve membrane behavior under stress play a major role in membrane performance. As a matter of fact, polyaniline alone shows poor mechanical properties. Therefore, it is mostly used either as deposited film or nanofillers (nanoparticles, nanofibers, etc). The tensile strength at break and elongation at break were measured according to the ASTM D412 standard method by a tensile test machine. It was observed that mechanical properties of membrane enhanced after inclusion of polyaniline. Table 1 shows the mechanical properties comparison between polysulfone and composites membranes.
Wettability of membrane surface
Contact angle and roughness of PSu and PSu/PANI nanocomposite membranes
Value of contact angle, θ (°)
Ra (nm) (5 μm × 5 μm)
Rq (nm) (5 μm × 5 μm)
Salt rejection and MWCO of PSu and PSu/PANI membranes
Rejection rate (%) of NaCl
Undoped PSu/PANI nanocomposite
Doped PSu/PANI nanocomposite
A hydrophilic modification in polysulfone membranes is one of the important concern in the quest for advances and improvements in membrane technology. Concerning this, polyaniline nanoparticles, which enable to change the membrane surface properties during doping/dedoping process were used as nanofillers for the hydrophilic modification of polysulfone membrane. As a result, it was found that polyaniline not only increased the separation characteristics, but also altered the membrane morphology and surface roughness. Therefore, polyaniline nanoparticles inclusion in the polysulfone membrane showed a significant effect on hydrophilic property, which induced the improvement of permeability characteristics of polysulfone membrane.
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