Performance Improvement of Polysulfone Ultrafiltration Membrane

Article pubs.acs.org/IECR

Performance Improvement of Polysulfone Ultrafiltration Membrane Using Well-Dispersed Polyaniline−Poly(vinylpyrrolidone) Nanocomposite as the Additive Song Zhao,†,‡,§ Zhi Wang,*,†,‡,§ Xin Wei,†,‡,§ Boran Zhao,†,‡,§ Jixiao Wang,†,‡,§ Shangbao Yang,†,§ and Shichang Wang†,§ †

Chemical Engineering Research Center, School of Chemical Engineering and Technology, ‡State Key Laboratory of Chemical Engineering, and §Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, P.R. China ABSTRACT: Well-dispersed polyaniline−poly(vinylpyrrolidone) (PANI−PVP) nanocomposite was synthesized through dispersion polymerization and then used as a novel additive to prepare a polysulfone (PSf)/PANI−PVP nanocomposite membrane via immersion precipitation process. During membrane formation, a portion of PVP acted as a pore-forming agent while another PANI−PVP nanocomposite, combined by hydrogen bonds between carbonyl groups of PVP and N-hydrogen groups of PANI, acted as a hydrophilic modification agent. The addition of PANI−PVP nanocomposite increased membrane surface pore size, porosity, and hydrophilicity. Pure water fluxes of PSf/PANI−PVP nanocomposite membranes were 1.8−3.5 times that of PSf membrane with a slight change of bovine serum albumin (BSA) rejection. The membrane antifouling property was examined by the cross-flow ultafiltration using BSA solution as the model system. The results of flux decline behavior and flux recovery ratio showed that PSf/PANI−PVP nanocomposite membranes had an excellent antifouling property. Compared with PSf/PVP membranes prepared using PVP as the additive, PSf/PANI−PVP nanocomposite membranes processed higher pure water flux and better hydrophilicity, antifouling property, and stability.

1. INTRODUCTION Polysulfone (PSf) ultrafiltration (UF) membrane has been widely used in ultrafiltration and as a support layer of reverse osmosis membranes due to its excellent heat resistance, chemical stability, and mechanical properties.1−4 However, the hydrophobic characteristic of PSf often causes serious membrane fouling, which deteriorates membrane performance and shortens membrane life.2−7 Thus, an increase in membrane hydrophilicity is considered to be a suitable way to diminish membrane fouling. Many approaches to improve the hydrophilicity of PSf UF membrane have been investigated, including surface coating, surface grafting, and blending hydrophilic additives.3,8−12 Among these methods, blending hydrophilic additives has been widely studied since it is simple, relatively low-cost, and effective in improving membrane permeability and antifouling property.13 Water-soluble polymers, such as poly(vinyl pyrrolidone) (PVP) and poly(ethylene glycol) (PEG), have been extensively used as additives during the preparation of ultrafiltration membranes via phase inversion method, which may be due to their good solubility in water and organic solvent, low toxicity, high complexing ability, and good film-forming characteristics.14 Generally, these water-soluble polymers are favorable to the increase of membrane surface pore size and porosity due to their pore-forming effect.10,12,15 However, most water-soluble polymers are easily lost during membrane formation and usage, which makes it difficult to significantly improve membrane hydrophilicity.10,16,17 Nanomaterials are of great scientific interest as they are effectively a bridge between bulk materials and atomic or molecular structure.18 Recently, nanocomposite UF membranes © 2012 American Chemical Society

prepared by blending nanomaterials, such as silver nanoparticles, silica nanoparticles, titania nanoparticles, polyaniline (PANI) nanofibers, and carbon nanotubes, to the casting solution have attracted much attention due to the significant improvement of membrane permeability, antifouling property, and mechanical property.11,19−25 When adding the appropriate content of nanomaterials, membrane porosity together with the number of small pores increase and membrane pores in the cross-section structure become run through. Moreover, unlike water-soluble polymers, nanomaterials as the additive could stably exist in the prepared membrane, which is favorable to the stability of membrane hydrophilicity. Adding the low range of nanomaterials could provide a significant improvement in membrane permeability and antifouling property.25 However, nanomaterials have the tendency to agglomerate due to their very large specific surface area. The agglomeration reduces the efficiency of nanomaterials in improving membrane performance, which negatively affects the effective utilization of nanomaterials in nanocomposite membranes.21 Thus, solving the agglomeration problem of nanomaterials is of great significance to the performance improvement and the industrial production of nanocomposite UF membrane. Generally, two methods have been tried to overcome the agglomeration of nanomaterials, that is, dispersion of nanomaterials by mechanical methods (such as sonication and grind) and surface modification of nanomaterials by chemical Received: Revised: Accepted: Published: 4661

November 1, 2011 March 6, 2012 March 8, 2012 March 8, 2012 dx.doi.org/10.1021/ie202503p | Ind. Eng. Chem. Res. 2012, 51, 4661−4672

Industrial & Engineering Chemistry Research

Article

methods.21,26 In the former approach, a long-time and vigorous sonication is required to break the strong intrananomaterials interaction. The nanomaterials may agglomerate again after letting the casting solution stand for some time. In the latter approach, nanomaterials are chemically modified or functionalized to minimize intrananomaterials interaction and improve their dispersion. Razmjou et al. chemically modified titania nanoparticles with aminopropyltriethoxysilane to overcome the agglomeration. The incorporation of modified nanoparticles into polyethersulfone ultrafiltration membranes showed a significant improvement in antifouling behavior.21 PANI is one of the most promising conducting polymers due to its easy synthesis, good environmental stability, and thermal stability. It has been used as a membrane material or an additive to prepare membranes for gas separation, pervaporation, electrodialysis, and ultrafiltration.11,27−32 The typical synthetic method for PANI is chemical oxidative polymerization, including interfacial polymerization, solution polymerization, and dispersion polymerization.14,27,33,34 In our previous study, PANI nanofibers were synthesized by chemical oxidative polymerization and then dispersed in the casting solution through sonication and vigorous stirring.11 The addition of PANI nanofibers improved membrane permeability and antifouling property. However, the dispersion process of PANI nanofibers using mechanical methods not only consumed much time and energy but also made it difficult to obtain a homogeneous casting solution. In the procedure of dispersion polymerization, water-soluble polymer (such as PVP) is often used as an effective steric stabilizer such that it can prevent the precipitation of polymerized aniline and make finely dispersed PANI in solution.14,35−37 After polymerization, the precipitating PANI nanomaterials get attached (either grafted or adsorbed) to the steric stabilizer.37 For example, the cohesion of PANI and PVP in PANI−PVP nanocomposite was illustrated through the hydrogen bonding between the Nhydrogen of PANI and the carbonyl group of PVP.14,36 According to this approach, well-dispersed PANI−PVP nanocomposite could be obtained through one-step synthesis. In this work, well-dispersed PANI−PVP nanocomposite was synthesized through a one-step facile procedure and used as the additive to modify PSf UF membrane. There are several reasons for using PANI−PVP nanocomposite as a novel additive. First, both PVP and PANI were reported to be effective additives to increase membrane permeability and antifouling property. Second, PANI−PVP nanocomposite could be well dispersed in organic solvents, such as N-methyl-2-pyrrolidone (NMP), which made it easy to prepare homogeneous and stable casting solution and accordingly convenient to prepare nanocomposite UF membrane. Third, PANI−PVP nanocomposite could stably exist in the prepared membrane. In view of these considerations, PSf/PANI−PVP nanocomposite membrane was prepared via an immersion precipitation process using PANI−PVP nanocomposite as the additive. The effects of well-dispersed PANI−PVP nanocomposite on membrane structure and performance were analyzed in depth. Attenuated total reflectance Fourier transform infrared spectroscopy (ATRFTIR) and scanning electron microscopy (SEM) were used to investigate membrane surface chemical composition and surface and cross-sectional morphology, respectively. Dynamic water contact angle measurement was used to characterize membrane surface hydrophilicity and pore hydrophilicity. Membrane pure water flux, protein [including bovine serum albumin (BSA), egg albumin (EA), and trypsin] rejections, and

antifouling properties were measured through UF experiments. Surface chemical composition and water contact angle were used as indicators to evaluate membrane stability.

2. EXPERIMENTAL PROCEDURES 2.1. Materials. PSf was purchased from Dalian Polysulfone Plastic Limited Co. (Dalian, China) and used as membrane material. Aniline was purchased from Kewei Chemical Reagent Co. Ltd. (Tianjin, China) and distilled under reduced pressure to purify it before use. BSA (molecular weight 67 kDa) was electrophoresis pure and purchased from Zhengjiang Hightechnology Co. (Tianjin, China). EA (molecular weight 45 kDa), trypsin (molecular weight 23 kDa), and PVP (K30) were supplied by Aladdin Reagent Co. (Shanghai, China). Hydrochloric acid (HCl), ammonium peroxydisulfate (APS), NMP, and acetone purchased from Kewei Chemical Reagent Co. Ltd. (Tianjin, China) were of analytical grade and used as received. Pure water having a conductivity of less than 12 μs/cm was produced by a reverse osmosis system. 2.2. Preparation and Characterization of PANI−PVP Nanocomposite. PANI−PVP nanocomposite was synthesized through a dispersion polymerization method using APS as the oxidant and PVP as the steric stabilizer in aqueous HCl medium at 0 °C, according to the procedure mentioned in the references.36,38 In a typical procedure, aqueous PVP was prepared by dissolving 4 g of PVP in 35 mL of pure water, and 0.7 mL of aniline was then added to aqueous PVP with stirring for about 4 h at 0 °C. Aqueous APS, prepared by dissolving 0.45 g of APS and 4 mL of HCl in 11 mL of pure water, was dropwise added to the above solution with continuous stirring. Polymerization of aniline was allowed to proceed at 0 °C for 12 h. After that, a green aqueous dispersion was obtained. PANI− PVP nanocomposite was then isolated by precipitating the dispersion with excess acetone, washing with water and acetone repeatedly, and then drying at 60 °C for about 6 h. Fourier transform infrared spectroscopy (FTIR) and ultraviolet−visible (UV−vis) spectroscopy were used to confirm the chemical composite of PANI−PVP nanocomposite. FTIR spectrum in the range of 2000−500 cm−1 was measured on a spectrometer (FTS-6000, Bio-Rad) using KBr pressed disks. UV−vis spectrum was recorded between 250 and 1000 nm using a UV−vis spectrophotometer (TU-1810DPC). The sample for UV−vis analysis was the dispersion of PANI−PVP nanocomposite in pure water. Transmission electron microscopy (TEM, JEM-100 CX, JEOL) was used to observe the morphology and the dispersion of PANI−PVP nanocomposite. The sample was prepared by dipping an aqueous dispersion of PANI−PVP nanocomposite into copper meshes and then drying them at room temperature. 2.3. Preparation of PSf/PANI−PVP Nanocomposite Membrane. Membranes were prepared via an immersion precipitation process. To prepare the casting solution, a certain amount of PANI−PVP nanocomposite was well-dispersed in NMP with stirring. Then, PSf was added into the above dispersion and vigorous stirring was carried out at room temperature for 12 h to ensure the dissolution of PSf. The prepared casting solution was left still for about 12 h to allow a complete release of bubbles. After that, it was cast onto a glass plate using a stainless steel knife to get a casting film of 260 μm thickness, exposed to atmosphere (temperature 24 ± 1 °C, relative humidity 27 ± 2%) for 30 s, and then immersed into a coagulation bath of pure water. The membrane preparation process was performed in a constant temperature chamber 4662

dx.doi.org/10.1021/ie202503p | Ind. Eng. Chem. Res. 2012, 51, 4661−4672


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where mw is the mass of wet membrane sample, md is the mass of dry state membrane sample, and A, L, and ρ are the sample area, the sample thickness, and pure water density, respectively. In order to evaluate membrane hydrophilicity, the dynamic contact angle of the membrane was measured using a sessile drop method with a contact angle analyzer (OCA15EC, Dataphysics). A 2.0 μL droplet of pure water was placed onto the membrane surface using a syringe, and then the changes of the contact angles were recorded as a function of the drop age and then analyzed by SCA 202 software (Dataphysics). 2.6. Membrane Performance Characterization. The pure water flux of the membrane with an effective area of 19.3 cm2 was tested using a cross-flow UF experimental apparatus.11 Initially, membrane compaction was carried out at 0.30 MPa transmembrane pressure (TMP) for 30 min. Then, the pure water flux was measured at 0.20 MPa TMP, 20 ± 1 °C, and 0.22 m/s cross-flow velocity. The pure water flux was calculated using eq 2

(GT-TH-S-64G) to control the temperature and humidity. Prior to the test, the prepared membranes were kept in pure water for more than 12 h to remove residual solvent. For all the casting solutions, the content of PSf to total casting solution was kept constant at 15 wt %. The contents of PANI−PVP nanocomposite and NMP in the casting solutions added up to 85 wt %, with varying PANI−PVP nanocomposite contents from 0 to 1.5 wt %. The prepared membranes were designated as M-0, M-0.1, M-0.5, M-1.0, and M-1.5 with 0, 0.1, 0.5, 1.0, and 1.5 wt % PANI−PVP nanocomposite contents in the casting solutions, respectively. 2.4. Characterization of the Casting Solution. The thermodynamic property of the casting solution was studied by cloud-point titration measurement with a constant stirring speed at 45 °C. Nonsolvent (pure water) was titrated using microsyringe into the casting solution until the solution revealed a cloudy feature. The mass of nonsolvent consumed was recorded as the cloud point. Each cloud point of the casting solution was measured three times and the average calculated. The viscosity of the casting solution was investigated using a rotating viscometer (Brookfield LVDV-C) at 25 °C with a rotating rate of 20 rpm. Generally, nanomaterials are easy to agglomerate and precipitate in the highly viscous casting solution after being allowed to stand for several months. Thus, the dispersion of nanomaterials has a close relationship with the stability of the casting solution, which is one of the critical issues from a practical point of view. In this work, the casting solution with PANI−PVP nanocomposite was allowed to stand for about 6 months to evaluate its stability. 2.5. Membrane Structure Characterization. An ATRFTIR spectrometer (FTS-6000, Bio-Rad) was used to study membrane surface chemical composition. Each ATR-FTIR spectrum was collected at 2 cm−1 resolution over the range from 2000 to 800 cm−1. The membrane sample was dried at 40 °C before the analysis. The top surface and cross-sectional morphologies of membranes were observed by SEM (Nova NanoSEM430, FEI). Before SEM analysis, the membrane samples for top surface morphology observation were dehydrated through graded ethanol series and then dried at room temperature.39 After that, it was cut into appropriate size and sputter-coated with gold. The membrane samples for cross-sectional morphology observation were cut into an appropriate size, fractured in liquid nitrogen, and then sputter-coated with gold. Membrane surface pore size and pore size distribution were determined by the analysis of SEM surface images using ImageJ software (1.38×, National Institute of Health, http://rsb.info. nih.gov/ij). For each sample, SEM surface images were taken from random locations. At least two hundred membrane pores were selected from three SEM surface images to get an average surface pore size and pore size distribution. The membrane porosity was determined by the mass loss of wet membrane after drying. The membrane sample being wetted thoroughly was mopped with water on the surface and weighed under wet status. Then, the membrane sample was dried until a constant mass was obtained. Porosity, ε, i.e., the ratio of pore volume to geometrical volume, for the membranes was obtained by eq 1 ε=

(m w − md)/ρ AL

Jw =

V AΔt

(2)

where Jw (L m−2 h−1) is the pure water flux, V (L) is the volume of permeated water, A (m2) is the effective membrane area, and Δt (h) is the permeation time. The protein rejections including BSA rejection, EA rejection, and trypsin rejection of the membranes were tested at 0.16 MPa TMP using 1.0 g/L BSA, EA, and trypsin aqueous solutions, respectively. The protein concentrations in the feed solution and the permeate solution were measured using the UV−vis spectrophotometer, at a wavelength of 280 nm. The protein rejection (%R) was calculated using eq 3 ⎛ Cp ⎞ %R = ⎜1 − ⎟ × 100 Cf ⎠ ⎝

(3)

where Cp and Cf are BSA, EA, or trypsin concentrations in the permeate solution and the feed solution, respectively. The membrane antifouling property was studied as follows. First, pure water flux of the membrane, Jw1 (L m−2 h−1), was tested under the condition of 0.16 MPa TMP, 20 ± 1 °C. Then, 0.8 g/L BSA aqueous solution was fed into the filtration system. After BSA ultrafiltration for 90 min, the membrane was flushed with pure water for 10 min and then the pure water flux of the membrane after being flushed, Jw2 (L m−2 h−1), was measured. The flux recovery ratio (FRR) was calculated using eq 4 to evaluate the antifouling property of the membrane J FRR (%) = w2 × 100 Jw1

(4)

2.7. Performance Comparison between PSf/PANI− PVP Nanocomposite Membrane and PSf/PVP Membrane. PSf/PVP membranes were prepared using PVP as the additive via the immersion precipitation process shown in section 2.3. Membrane performances including pure water flux, BSA rejection, and stability were compared between PSf/ PANI−PVP nanocomposite membranes and PSf/PVP membranes. The stability of hydrophilic additive in the prepared membrane is one of the important parameters for evaluating membrane practicality and durability.17 In this work, membrane stability test was investigated with the aid of ATR-FTIR

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in the visible region, the peaks are due to the PANI in PANI− PVP nanocomposite. The spectrum shows three peaks at 348, 420, and 783 nm, which are in accordance with the spectrum of PANI−PVP nanocomposite reported in the references.14,40 The first peak is assigned to the π→π* transition of the benzenoid ring. The second and third absorption bands are related to the doping level and the formation of polaron.41 Figure 3 shows TEM image of PANI−PVP nanocomposite. It can be seen that PANI−PVP nanocomposite is well-

spectrometry and dynamic water contact angle measurement according to the method reported by Susanto.17

3. RESULTS AND DISCUSSION 3.1. Characterization of PANI−PVP Nanocomposite. Figure 1 shows FTIR spectra of PVP and PANI−PVP

Figure 1. FTIR spectra of (a) PVP and (b) PANI−PVP nanocomposite.

nanocomposite. The FTIR spectrum of PVP shows the characteristic absorption band at 1688 cm−1 due to the carbonyl absorption. The FTIR spectrum of PANI−PVP nanocomposite shows the characteristic absorption peaks of PVP and PANI, which is similar to the spectrum of PANI−PVP nanocomposite reported in Palaniappan’s paper.14 The carbonyl absorption band at 1688 cm−1 in the spectrum of PVP shifted to 1676 cm−1 in the spectrum of PANI−PVP nanocomposite, which is attributed to the intermolecular hydrogen bonding between the carbonyl group of PVP and N-hydrogen of PANI.14,36 The bands at 1570 and 1492 cm−1 can be attributed to CC stretching for the quinoid (Q) ring and benzenoid (B) ring of PANI, respectively.14,38 The band at 1288 cm−1 is ascribed to the C−N stretching mode of the benzenoid ring of PANI and N−H imide stretching of PVP.14,38 The bands at 1141 and 825 cm−1 are due to the aromatic C−H bending in the plane and out of the plane of the aromatic ring.14 The UV−vis spectrum of PANI−PVP nanocomposite is shown in Figure 2. Since PVP does not have an absorption peak

Figure 3. TEM image of PANI−PVP nanocomposite.

dispersed and exhibits nanorod shape with an average diameter of 34.5 nm and an average length of 91.2 nm. 3.2. Casting Solution Property and Membrane Formation Process. As shown in Figure 4a, the cloud point curves gradually approach the solvent axis with increasing PANI−PVP nanocomposite contents, indicating that the addition of PANI−PVP nanocomposite decreases the nonsolvent tolerance of the casting solution and works in favor of the enhancement in the demixing of the casting solution thermodynamically. As exhibited in Figure 4b, the viscosities of the casting solutions increase with increasing PANI−PVP nanocomposite content, which can be explained by the following reasons. (1) The addition of PANI−PVP nanocomposite led to the decrease of NMP content, and thus the PSf concentration in the casting solution increased; (2) A solubility parameter distance, Ra, between PSf and NMP was calculated as 5.36 MPa1/2, while Ra between PVP and NMP was calculated as 4.18 MPa1/2 according to the data reported.42,43 The lower Ra between PVP and NMP indicates that PVP in PANI−PVP nanocomposite might have a strong interaction with NMP, which decreased the effective solvating power of NMP for PSf.44,45 These factors resulted in the intermolecular and intramolecular aggregations or entanglements of PSf chains and then caused the increase of the casting solution viscosity.46 During the casting solution preparation, PANI−PVP nanocomposite was easily dispersed. Moreover, after letting the casting solution stand for about 6 months, no precipitate was found in the bottom of the casting solution, which indicated that the casting solution with PANI−PVP nanocomposite has fine dispersion and long-term stability. Figure 5 shows a schematic illustration of the role of PANI− PVP nanocomposite during membrane formation. During phase separation, PSf coagulated immediately to form a membrane matrix and hydrophilic PANI−PVP nanocomposite

Figure 2. UV−vis spectrum of PANI−PVP nanocomposite. 4664



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Figure 4. Cloud point phase diagram (a) and viscosity (b) of the casting solutions with different PANI−PVP nanocomposite contents.

Figure 5. Schematic illustration of the role of PANI−PVP nanocomposite as pore-forming agent and hydrophilic modification agent during membrane formation.

migrated spontaneously to the film/water interface to decrease the interface energy. A portion of molecular PVP having weak interaction with PANI in PANI−PVP nanocomposite could combine with NMP in the casting solution due to the good solubility of PVP in NMP. This portion of PVP would diffuse into the coagulation bath along with NMP during membrane formation, which could be considered as a pore-forming agent. Another portion of PVP attaching on PANI−PVP nanocomposite would be embedded into the membrane matrix. In order to decrease interface energy, hydrophilic PANI−PVP nanocomposite would enrich near the membrane top surface, acting as a hydrophilic modification agent. 3.3. Membrane Structure. Figure 6 shows ATR-FTIR spectra of PSf membrane and PSf/PANI−PVP nanocomposite membrane. Compared with that of PSf membrane, ATR-FTIR spectrum of PSf/PANI−PVP nanocomposite membrane has an obvious adsorption band at 1670 cm−1, which is assigned to the carbonyl absorption of PVP. Compared with the FTIR spectra of PVP and PANI−PVP nanocomposite, the shift in the carbonyl absorption band to a lower frequency may be attributed to a stronger hydrogen bonding between PVP and PANI in PSf/PANI−PVP nanocomposite membrane than that in PANI−PVP nanocomposite.14,36 As mentioned in section 3.2, PVP having a weak interaction with PANI in PANI−PVP nanocomposite would be washed away during membrane formation while PVP having strong interaction with PANI in PANI−PVP nanocomposite would reside in the prepared membrane. Besides, the shift of the carbonyl absorption band to a lower frequency may also result from the interaction between the pyrrolidone groups of PVP and the sulfone groups of PSf.44

Figure 6. ATR-FTIR spectra of PSf membrane and PSf/PANI−PVP nanocomposite membrane.

Figure 7 shows the SEM images of top surface and crosssectional morphologies of the membranes with different PANI−PVP nanocomposite contents. Visualization of top surface morphology presents that all the membranes have fine pore structure with dimensions in the nanometer range. Figure 8 shows the surface pore diameter distribution of each membrane. It can be seen that the surface pore diameter distributions of all of the membranes are concentrated in the range of 3−15 nm. The surface pore diameter distributions of PSf/PANI−PVP nanocomposite membranes are clearly shifted toward large pore diameter values compared with that of PSf membrane. The average surface pore sizes were quantitatively measured, and the results are shown in Table 1. It can be found that membrane surface pore sizes increase first and then decrease with increasing PANI−PVP nanocomposite content, and the average surface pore sizes of PSf/PANI−PVP nanocomposite membranes display nearly 18%−31% increase compared with that of PSf membrane. Table 1 also shows that all of PSf/PANI−PVP nanocomposite membranes have higher porosity than PSf membrane. Moreover, with increasing PANI−PVP nanocomposite content, membrane porosity increases first and then decreases. The changes of membrane surface pore size and porosity can be explained by three reasons: (1) the addition of PANI−PVP nanocomposite decreased the thermodynamic stability of the casting solution and thus induced the phase separation at a low polymer concentration, resulting in the formation of large surface pore size and high porosity;47 (2) a portion of PVP was leached out of the casting film with the exchange of solvent−nonsolvent 4665



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Figure 7. SEM images of the membranes with different PANI−PVP nanocomposite contents: (a) top surface morphology and (b) cross-sectional morphology.

typical structure of ultrafiltration membrane prepared via the immersion precipitation process. With increasing PANI−PVP nanocomposite content, the macrovoids increasein number and become larger, and the pore walls among macrovoids become looser with some channel-like pores. It has been reported that the addition of PVP in the casting solution could accelerate the phase separation and thus enlarge the macrovoids while the addition of PANI in the casting solution could improve pore interconnection due to the migration behavior during membrane formation.11,49 In this work, it seems that the changes of membrane structure are the combination of the effects of PVP and PANI. That is, the addition of PANI−PVP nanocomposite results in the formation of both large macrovoids and well interconnected pores. Generally, nucleated droplets of the polymer lean phase were responsible for the

during the phase separation and acted as a pore-former during membrane formation, which may be another reason for the increase of membrane surface pore size and porosity;10,16,48 and (3) with increasing PANI−PVP nanocomposite content, the increased viscosity of the casting solution hinders the exchange between solvent and nonsolvent. Under the highly viscous casting solution, solvent’s outdiffusion from the casting solution is favored over nonsolvent’s indiffusion into the solution because of its barrier effect against nonsolvent. The delayed polymer coagulation on the surface region of the casting solution caused the formation of a dense top layer with relatively fewer and smaller-sized pores.15 As shown in Figure 7, all of the membranes exhibit an asymmetric structure consisting of a dense top layer and a porous sublayer with fully developed macrovoids, which is the 4666



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Figure 8. Surface pore diameter distribution of the membranes with different PANI−PVP nanocomposite contents: (a) M-0, (b) M-0.1, (c) M-0.5, (d) M-1.0, and (e) M-1.5.

membranes. It can be found that at the drop age of 0 s, water contact angles of the membranes decrease with increasing PANI−PVP nanocomposite content. M-1.5 shows the initial water contact angle of 54°, which is nearly 25° lower than that of PSf membrane. These results indicated that the addition of PANI−PVP nanocomposite significantly improved membrane surface hydrophilicity and water wettability. Moreover, for all the membranes, water contact angle has an apparent declining trend with increasing the drop age. Water contact angle of PSf membrane decreases slightly while those of PSf/PANI−PVP nanocomposite membranes decrease rapidly with drop ages. Moreover, the decreasing rate of water contact angles increases with increasing PANI−PVP nanocomposite content. At the drop age of 120 s, water contact angles of M-0, M-0.1, M-0.5, M-1.0, and M-1.5 are 75°, 64°, 52°, 43°, and 27°, respectively. These results indicated that the addition of PANI−PVP nanocomposite improved not only membrane surface hydrophilicity but also pore hydrophilicity. The hydrophilicity improvement is a comprehensive result involving the component changes as well as structure changes.17,51 In this work, the improvement of membrane hydrophilicity may be due to the changes of hydrophilic constitution and membrane surface pore size and porosity. Hydrophilic PANI−PVP nanocomposite embedded in the prepared membrane results in the formation of hydrophilic surface as well as hydrophilic pores (see section3.2). The high surface pore size and porosity PSf/PANI−PVP nanocomposite membranes might facilitate water diffusion into the membrane pore and thus further improve membrane hydrophilicity.52 The addition of PANI−PVP nanocomposite has improved membrane hydrophilicity more significantly than that of PANI nanomaterials reported in our previous papers.11,28 This may be due to the well dispersion of PANI−PVP nanocomposite and the immobilization of PVP in PANI−PVP nanocomposite. 3.4. Membrane Performance. Figure 10 shows the timedependent pure water fluxes of the membranes during membrane compaction. It can be seen that the pure water fluxes of membranes suffer gradual decrease during membrane compaction. Compaction is defined as a compression of membrane structure under a transmembrane pressure difference causing a decrease in membrane permeability.53 Usually,

Table 1. Summary of Average Surface Pore Size, Porosity, and Flux Recovery Ratio (FRR) Value of the Membranes membranes M-0 M-0.1 M-0.5 M-1.0 M-1.5

average surface pore size (nm) 6.2 7.3 7.8 8.1 7.5

± ± ± ± ±

1.8 2.1 2.6 2.7 2.2

porosity (%) 66.2 72.9 77.4 82.0 78.4

± ± ± ± ±

0.6 0.9 1.0 1.3 1.2

FRR value (%) 59.5 69.2 76.4 80.5 84.1

± ± ± ± ±

2.9 2.1 1.5 2.7 2.3

initiation of macrovoids.50 The addition of PANI−PVP nanocomposite decreased the thermodynamic stability of the casting solution and led to the rapid formation of nucleated droplets of the polymer lean phase. These nuclei could further expand due to the diffusional flow of solvent into the nuclei, resulting in the formation of lots of macrovoids in PSf/PANI− PVP nanocomposite membranes. The channel-like pores in the pore walls between macrovoids might be caused by the rapid demixing rate and the migration behavior of PANI−PVP nanocomposite during membrane formation.11 Dynamic water contact angle measurement has been commonly used to assess membrane hydrophilicity. Figure 9 shows the time dependence of water contact angles of the

Figure 9. Time dependence of water contact angles of the membranes. 4667



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Figure 11a shows the pure water fluxes of the membranes with different PANI−PVP nanocomposite contents. With increasing PANI−PVP nanocomposite content, pure water fluxes of the membranes increase first and then decrease. Pure water fluxes of M-0.1, M-0.5, M-1.0, and M-1.5 are 1.8, 2.8, 3.5, and 3.2 times that of M-0, respectively. This can be interpreted by the changes of membrane structure shown in section3.3. When PANI−PVP nanocomposite content increased from 0 to 1.0 wt %, membrane surface pore size, porosity, macrovoids, pore interconnection, and hydrophilicity increased, which greatly weakened the resistance of water permeating through the membranes and thus increased membrane permeability. When PANI−PVP nanocomposite content increased from 1.0 to 1.5 wt %, membrane surface pore size and porosity decreased, which may be the reason that the pure water flux of M-1.5 was lower than that of M-1.0. Figure 11b shows protein rejections of the membranes including BSA, EA, and trypsin rejections. With increasing PANI−PVP nanocomposite content, BSA rejection changes little while EA rejection decreases from 97.4% to 92.7% and trypsin rejection decreases from 67.6% to 39.7%. The dramatic decrease of trypsin rejection may be due to the low molecular weight of proteins and the increasing surface pore size of PSf/ PANI−PVP nanocomposite membranes compared with PSf membrane. Figure 12 shows flux decline behavior of the membranes during BSA ultrafiltration. It can be seen that fluxes of all the membranes decrease rapidly at the initial BSA ultrafiltration, which may be due to the absorption and deposition of BSA on membrane surface and pore. Compared with PSf membrane, PSf/PANI−PVP nanocomposite membranes have slow flux decline rates. After 90 min of BSA ultrafiltration, all PSf/ PANI−PVP nanocomposite membranes show higher fluxes than PSf membrane. Fluxes of M-0.1, M-0.5, M-1.0, and M-1.5 are 1.6, 2.0, 2.3, and 2.2 times that of M-0, respectively. These results suggested that PSf/PANI nanocomposite membranes could not be easily adsorbed by protein molecule during BSA ultrafiltration, which resulted from the better hydrophilicity of PSf/PANI−PVP nanocomposite membranes than PSf membrane. As shown in Table 1, FRR values of PSf/PANI−PVP nanocomposite membranes are higher than that of PSf membrane. With increasing PANI−PVP nanocomposite content, FRR values of the membranes increase gradually, having a maximum of 84.1% for M-1.5. Generally, the molecular weight cutoff (MWCO) of the membrane is defined as the lowest molecular weight of a solute that has a rejection of 95%.57 According to protein rejections of the membranes, the

Figure 10. The time-dependent pure water fluxes of the membranes during membrane compaction at 0.30 MPa TMP.

this compression reflects a reduction of membrane thickness due to mechanical deformation and is best represented as a compressive strain.17,53−56 Peterson et al. investigated membrane compaction using an ultrasonic time domain reflectometry (UTDR) technique, and the results indicated that the membrane with high porosity suffered greater initial compression strain than that with low porosity. In addition, the time required to approach the respective limiting strain values increases with increasing membrane porosity.55 In this work, the flux of PSf membrane decreases from 233 to 199 L m−2 h−1 during membrane compaction. Compared with PSf/ PANI−PVP nanocomposite membranes, PSf membrane shows a slight flux decrease, which may be due to its low porosity. After about 30 min of membrane compaction, the pure water fluxes of the membranes reach a steady value. The steady pure water fluxes are 84.3%, 81.1%, 72.3%, 66.3%, and 69.3% of the initial pure water fluxes for M-0, M-0.1, M-0.5, M-1.0, and M1.5, respectively. The decrease of pure water fluxes during membrane compaction becomes severe with the addition of PANI−PVP nanocomposite, which is probably due to the presence of lots of macrovoids in the sublayer of PSf/PANI− PVP nanocomposite membranes.10 Similar results can also be seen when PVP and PEG were used as the additives.17 However, the decreased degrees of pure water fluxes with the addition of PANI−PVP nanocomposite (50%, which was obtained through our experiment). This may be due to the good rigidity structure of PANI and the stable existence of PANI−PVP nanocomposite in the prepared membranes.

Figure 11. (a) Pure water fluxes of the membranes, TMP = 0.20 MPa. (b) BSA and EA rejections of the membranes; protein concentration = 1.0 g/ L, TMP = 0.16 MPa. 4668



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Figure 12. (a) Flux decline behavior of the membranes during BSA ultrafiltration and (b) J/J0 decline behavior of the membranes during BSA ultrafiltration (J is the flux during BSA ultrafiltration, and J0 is the initial flux during BSA ultrafiltration); protein concentration = 0.8 g/L, TMP = 0.16 MPa.

approximate MWCOs for M-0, M-0.1, M-0.5, M-1.0, and M-1.5 are obtained as 43, 45, 49, 53, and 56 kDa, respectively. The results of flux decline behavior and FRR values indicated that PSf/PANI−PVP nanocomposite membranes had better antifouling property than PSf membrane. Generally, membrane fouling is attributed to surface adsorption, pore adsorption, and pore blocking. Surface adsorption or pore adsorption was reversible membrane fouling, which could be easily removed by water flushing, while pore blocking was irreversible membrane fouling that could not be easily removed. There are two main factors influencing membrane antifouling property, i.e., membrane surface hydrophilicity and surface pore size. On one hand, hydrophilic membrane surface could not be easily adsorbed by protein molecules. On the other hand, the membrane with large surface pore size easily suffered more serious pore blocking or pore adsorption than that with small surface pore size.58,59 Therefore, it is reasonable to deduce that the more hydrophilic the membrane surface is and the smaller the surface pore size is, the better the membrane antifouling property is. In this work, FRR values of the membranes increased with increasing PANI−PVP nanocomposite content, indicating that they were mainly influenced by membrane hydrophilicity, as shown in Figure 9. 3.5. Performance Comparison between PSf/PANI− PVP Nanocomposite Membranes and PSf/PVP Membranes. The pure water fluxes of PSf/PVP membranes with 0.5, 1.0, and 1.5 wt % PVP are 285 ± 19, 349 ± 23, and 375 ± 18 L m−2 h−1, respectively. As shown in Figure 11a, the pure water fluxes of M-0.5, M-1.0, and M-1.5 are 508 ± 22, 635 ± 38, and 580 ± 24 L m−2 h−1, respectively, which are much higher than those of PSf/PVP membranes. PSf/PANI−PVP nanocomposite membranes and PSf/PVP membranes have similar BSA rejection in the range of 96.5%−98.3%. BSA ultrafiltration experiment shows that PSf/PVP membranes with 0.5−1.5 wt % PVP have 70%−74% FRR values, while PSf/ PANI−PVP nanocomposite membranes with 0.5−1.5 wt % PANI−PVP nanocomposite have 76%−84% FRR values. This indicates that PSf/PANI−PVP nanocomposite membranes have better antifouling property than PSf/PVP membranes. This may be because PSf/PANI−PVP nanocomposite membranes have better hydrophilicity and stability than PSf/PVP membranes, which can be found in Table 2. The water contact angles of PSf/PANI−PVP nanocomposite membranes (before testing) were lower and decreased faster with drop ages than those of PSf/PVP membranes (before testing). After BSA

Table 2. Dynamic Water Contact Angles of PSf/PVP Membranes and PSf/PANI−PVP Nanocomposite Membranes dynamic water contact angle (deg) (0 → 120 s) membranes PSf/PVP (0.5 wt %) membrane PSf/PVP (1.0 wt %) membrane PSf/PVP (1.5 wt %) membrane M-0.5 M-1.0 M-1.5

before test 65.3 63.9 59.6 64.0 57.9 53.6

→ → → → → →

60.5 59.1 52.6 52.2 43.4 26.6

after testa 71.7 68.9 67.1 65.4 60.4 59.7

→ → → → → →

66.4 64.2 60.9 56.8 51.4 28.7

a

Note that the test includes BSA ultrafiltration and pure water flushing.

ultrafiltration and water flushing, water contact angles of membranes increased due to the loss of PVP and membrane fouling. Water contact angles of PSf/PANI−PVP nanocomposite membranes increased about 1°−8° while water contact angles of PSf/PVP membranes increased about 5°−9°. The performance of membrane prepared with a hydrophilic additive will strongly depend on the stability of the additive in the prepared membrane during membrane formation and usage.17 For PSf/PVP membrane, it was reported that above 90% of PVP was washed out during membrane formation.16 Residual PVP in the prepared membranes also suffered great loss during membrane usage.16,17 With the loss of hydrophilic additives, membrane hydrophilicity would decrease and membrane performance including permeability and antifouling property would worsen. In this work, the stability of PVP in membranes was expected to be improved through the immobilization of PVP in PANI− PVP nanocomposite. To prove it, the surface chemical composites of PSf/PVP membrane and PSf/PANI−PVP nanocomposite membrane were measured through ATRFTIR spectrometry. As shown in Figure 13, the spectra of PSf/PVP membranes have an obvious adsorption band at 1676 or 1678 cm−1, while the spectra of PSf/PANI−PVP nanocomposite membranes have an obvious adsorption band at 1672 or 1670 cm−1. The peak areas were calculated as 177, 226, 257, and 270 for A1, A2, A3 and A4, respectively. These results indicated that PSf/PANI−PVP nanocomposite membranes processed more PVP than PSf/PVP membranes with the same additive content. It is worth mentioning that PVP contents in the casting solution of PSf/PVP membranes are much larger 4669



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4. CONCLUSIONS Well-dispersed PANI−PVP nanocomposite was synthesized through a facile one-step procedure and then used as the additive to prepare PSf/PANI−PVP nanocomposite membrane via the immersion precipitation process. PANI−PVP nanocomposite could be easily and stably dispersed in the casting solution. The addition of PANI−PVP nanocomposite enhanced membrane surface pore size, porosity, and hydrophilicity. An ultrafiltration experiment showed that pure water fluxes of PSf/PANI−PVP nanocomposite membranes were 1.8−3.5 times that of PSf membrane with slight changes of BSA rejection and EA rejection. PSf/PANI−PVP nanocomposite membranes had better antifouling property than PSf membrane, including slower flux decline rates and higher FRR values after water flushing. Moreover, the results of a performance comparison study showed that PSf/PANI−PVP nanocomposite membranes had much better hydrophilicity, higher pure water flux, and better antifouling property than PSf/PVP membranes. The membrane stability study indicated that a portion of PVP could be immobilized in PSf/PANI−PVP nanocomposite membranes during membrane formation and usage. In summary, well-dispersed PANI−PVP nanocomposite is an effective additive to improve ultrafiltration membrane performance, and PSf/PANI−PVP nanocomposite membrane has great potential for industry production and practical application.

Figure 13. ATR-FTIR spectra of the prepared membranes: (a) PSf/ PVP (0.5 wt %) membrane, (b) M-0.5, (c) PSf/PVP (1.0 wt %) membrane, and (d) M-1.0.

than that in the casting solution of PSf/PANI−PVP nanocomposite membranes. Thus, it can be deduced that PANI− PVP nanocomposite could immobilize PVP and reduce the loss of PVP during membrane formation. In order to investigate the stability of PVP in the prepared membranes during membrane usage, membrane samples were immersed into water (40 °C) for 10 days and measured through ATR-FTIR spectrometry. All the membranes have the typical bands of PSf at 1578 cm−1. Thus, according to the method shown in ref 17, the ratios of peak heights at 1578 and 1676 cm−1 (H1676/H1578) and at 1578 and 1672 cm−1 (H1672/ H1578) were calculated to evaluate the stability of PVP in PSf/ PVP membrane and PSf/PANI−PVP nanocomposite membrane, respectively. As shown in Figure 14, H1676/H1578 and

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 22 27404533. Fax: +86 22 27404496. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by grants from the National Natural Science Foundation of China (No. 20836006), the Major State Basic Research Development Program of China (973 Program, No. 2009CB623405), the Science & Technology Pillar Program of Tianjin (No. 10ZCKFSH01700), and the Program of Introducing Talents of Discipline to Universities (No. B06006).

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Figure 14. Stability test investigated by measuring the ATR-FTIR absorbance of the typical bands: (a) PSf/PANI−PVP nanocomposite membrane and (b) PSf/PVP membrane.

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Performance Improvement of Polysulfone Ultrafiltration Membrane

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