Preparation and Characterization of Nanofiltration Membranes Synthesized by Hyperbranched Polyester and Terephthaloyl Chloride (TPC)

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1 Preparation and Characterization of Nanofiltration Membranes Synthesized by Hyperbranched Polyester and Terephthaloyl Chloride (TPC) Preparation and Characterization of Nanofiltration Membranes Synthesized by Hyperbranched Polyester and Terephthaloyl Chloride (TPC) Xiu-Zhen Wei, Jia Yang, and Guo-Liang Zhang* College of Biological and Enviromental Engineering, Zhejiang University of Technology, HangZhou , PR China Received: 25 November 2010, Accepted: 20 May 2011 SUMMARY Thin-film composite (TFC) nanofiltration (NF) membranes were developed by terephthaloyl chloride (TPC) crosslinked the hydroxyl ended groups of hyperbranched polyester (HPE) on polyacrylonitrile (PAN) support. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) indicated that a thin layer of crosslinked HPE molecules were deposited on PAN porous membrane surface successfully. The preparation conditions were also optimized. The separation performance of PAN-HPE-TPC NF membranes are mainly related with the concentration of monomer in the aqueous phase rather than that in the organic phase. Water permeability and salts rejections of the membranes were measured. The flux and rejection of these NF membranes for Na 2 (1 g/l) reached L/m 2 h and 96.5% under 0.6 MPa, respectively. At the same time, the nanofiltration properties were compared with other membranes prepared with hyperbranched polymers. All NF membranes prepared with hyperbranched polymers showed relative high permeate flux. Keywords: Nanofiltration membrane, Hyperbranched polyester, Interfacial polymerization, Flux, Salt rejection 1. INTRODUCTION Nanofiltration (NF) is a kind of pressure driven process whose separation properties are between those of ultrafiltration (UF) and reverse osmosis (RO) membrane. The pore size of NF is typically around 1 nm, which corresponds to dissolving compounds with a molecular weight of about 300. This makes NF suitable for the removal of relatively small organic micro-pollutants, colour from surface water or ground water, and pharmaceutical molecules as well as degradation products from the effluent of biologically treated wastewater. Recently, NF membrane has drawn great attention due to its advantages such as relatively low operating pressures, high fluxes, low * Corresponding author: Tel: ; Fax: address: guoliangz@zjut.edu.cn, xzwei@zjut.edu.cn Smithers Rapra Technology, 2012 operating and maintenance costs 1. Most commercial NF membranes are fabricated via interfacial polymerization (IP), which is the most effective method to prepare the composite membranes. A great advantage of the composite membranes is that the top selective layer and bottom porous substrate of the thin film composite (TFC) membrane can be independently selected, studied and modified to maximize the overall membrane performance 2,3. In the past two decades, some polymeric materials, such as polyamide, sulfonated polyether-sulfone (SPES), polyvinyl alcohol (PVA) derivatives, sulfonated polyphenylene oxide (SPPO), chitosan derivatives, and cellulose acetate (CA) 4-7, have been developed to prepare NF membranes. However, these NF membranes have two obvious drawbacks. One was that the membranes were relatively hydrophobic and thus more prone to organic fouling. The other was that the operation pressure was relatively high. Subsequently, in recent years, studies have been focused on seeking or synthesizing new reactive monomers with special functional groups to prepare the TFC membrane 8,9, such as some new synthesized rigid star amphiphiles polymers 10. Hyperbranched polymers (HBPs) are a new macromolecular species with nearthree-dimensional architecture. Nearly three-dimensional structure and a great number of functional groups are two major characteristics of HBPs 11. Due to their special structures and properties, HBPs have been used as hydrophilic additive to modify the hydrophilicity of ultrafiltration membranes Some crosslinked HBPs are also used as the selective layer of NF membranes 15 and we also fabricated NF membranes with Polymers & Polymer Composites, Vol. 20, No. 3,

2 Xiu-Zhen Wei, Jia Yang, and Guo-Liang Zhang crosslinked hyperbranched polyester (HPE) as the functional layer 16,17. The special structure of HBPs endows these NF membranes with high flux and special salt rejection order. However, the salt rejection was not high enough. Thus, terephthaloyl chloride (TPC) with two degrees of functionality was chosen to replace TMC with three degrees of functionality in this study. We hope that the two dimension structure of TPC could make the loose structure of HBPs relatively dense and increase the salt rejection. Polyacrylonitrile (PAN) membrane was chosen as the porous support in this study, due to that PAN can provide substantial chemical, ph, and solvent resistance and are therefore suitable as supports for preparing TFC membranes functioning as solventstable nanofiltration membrane. The purpose of this study was to optimize the reaction conditions of interfacial polymerization to obtain NF membranes owning high water permeation flux and high salt rejection at the same time. The selective layer s chemical composition was characterized by FTIR-ATR and XPS. The performance of the NF membranes was characterized by the permeation flux and salts rejection. The hydrophilicity of the NF membrane was studied by water contact angle. Simultaneously, NF properties of membranes prepared with different reactive HBPs were compared. 2. EXPERIMENTAL 2.1 Materials All reagents and chemicals were of analytical grade and used as received without further purification. PAN UF Membrane with molecular weight cutoff (MWCO) of Da was supplied by MegaVision Membrane Engineering and Technology, Shanghai, China. Terephthaloyl chloride (TPC) was purchased from Aldrich (USA). Sodium chloride (NaCl), sodium sulfate (Na 2 ), magnesium chloride (MgCl 2 ) and magnesium sulfate (Mg ) was purchased from Shanghai Guoyao Company. HPE (Boltorn H40) was purchased from the Perstorp Specialty Chemicals AB (Sweden). The molecular weight of H40 was 5600 Da; the hydroxyl value was mgkoh/g, which was provided by Perstorp. 2.2 Membrane Preparation A certain amount ( g) of HPE was dissolved in ethanol to form a homogeneous solution, which used as the water phase. A piece of blank PAN-UF membranes were immersed in a water-ethanol mixture (1:1 v/v) for 20 min. The ethanol treated PAN support was immersed in the water phase for 20 min after pre-treated with 0.38% (w/v) sodium dodecyl sulfate (SDS) as a surfactant, and then taken out. When the residual liquid drops on the membrane surface were removed, the membrane was submerged in a solution of terephthaloyl chloride (TPC) in hexane (organic phase). The PAN support was immersed in water phase and organic phase only once. The obtained thin-film composite (TFC) membrane was heat-treated in an oven to attain the desired stability of the formed structure. Finally, the membrane was washed thoroughly with de-ionized water and stored in de-ionized water for further use. 2.3 Membrane Characterization The chemical composition of different membranes were characterized by Fourier transform infrared spectrometer (FTIR-ATR) (Vector 22 FTIR, Bruker Optics, Switzerland) and X-ray photoelectron spectroscopy (XPS, PHI 5000c, Peking-Elmer instruments). The surface and crosssection (fractured in liquid nitrogen) morphologies were observed by a field emission scanning electron microscopy (FE-SEM, JEOL, JSM- 5510LV, Japan) after sputter coated with gold (Hitachi E1020). The hydrophilicity of the membrane top surface was characterized using a contact angle goniometer (OCA20, Data physics, Germany) at room temperature. Before characterization, the samples were dried for 24 h in vacuum at 45 ºC. At least 10 contact angles were averaged to get a reliable value. 2.4 Membrane Performance Evaluation Nanofiltration experiments performed at a flat-sheet cross-flow NF test cell with an active membrane area of 19.6 cm 2. The membrane was stabilized for at least 20 min with deionized water at 0.6 MPa before testing. The PAN-HPE-TPC NF performance was characterized by determining the rejection and flux of individual solutes (Na 2, Mg, NaCl, and MgCl 2 ) from their aqueous solution (1 g/l). The performance tests for the prepared membrane were carried out under 0.6 MPa at 25 C. Both the retentate and permeate were recirculated to the feed tank to maintain a constant feed concentration during the permeation experiments. The filtration characteristics including permeation flux (F) and rejection (R) were determined. The permeation flux, F, is calculated according to formula (1). F=V/(A.t) (1) where V is the total volume of the water or solution permeated during the experiment, A represents the membrane area, and t denotes the operation time. NF rejection is defined as: R= (1-C p /C f ) 100% (2) where R is the percent solute rejection, C p and C f are the concentrations of solute in permeate and feed, respectively. Salt concentration in the feed and permeate were determined by Electrical Conductivity DDS-11D (Shanghai Leici Instrument, China). 262 Polymers & Polymer Composites, Vol. 20, No. 3, 2012

3 Preparation and Characterization of Nanofiltration Membranes Synthesized by Hyperbranched Polyester and Terephthaloyl Chloride (TPC) 3. RESULTS AND DISCUSSION 3.1 FTIR-ATR Study In order to obtain information about the chemical structural changes of the membranes, FTIR-ATR spectra of PAN UF and PAN-HPE-TPC NF membranes were recorded using the FTIR-ATR technique, as shown in Figure 1. The spectra showed a distinct band corresponding to the carbonyl stretching frequency of the ester functionality unique to HPE (characteristic band at 1729 cm -1 ν C=O), which indicated that a thin layer of HPE was deposited onto the surface of the PAN UF membrane. Compared with the PAN UF membrane, the FTIR- ATR spectra of the NF membrane also exhibited evident absorbance signals at 3369 cm -1 (νo-h) and 2921 cm -1 (νc-h). The similar spectra for the PAN UF membrane were not observed. The results suggested that the active skin layer of the PAN-HPE- TPC NF membranes were polyester containing hydroxyl groups. The residual hydrophilic hydroxyl groups would improve the hydrophilicity of the obtained NF membrane. 3.2 XPS Analysis The chemical changes occurred in the PAN-HPE-TPC NF membranes as a result of in situ interfacial polymerization of HPE with TPC were further studied by XPS analysis. The XPS survey spectra for the investigated samples are shown in Figure 2, and the elemental compositions are summarized in Table 1. Compared with virgin PAN UF membrane, the oxygen-containing functional peaks were higher and the nitrogencontaining functional peaks of PAN- HPE-TPC NF membrane were lower. This can be attributed to the active skin layers of PAN-HPE-TPC NF membrane containing carbonyl bonds after interfacial polymerization of HPE and TPC. There is significant change for the composition of PAN-HPE-TPC NF membrane surface. The content Figure 1. FTIR-ATR of PAN-UF and PAN-HPE-TPC NF membranes Figure 2. XPS survey scan of PAN-HPE-TPC NF and PAN UF membranes Table 1. Element content of membrane surface from XPS analysis Sample C (%) O (%) N (%) O/N PAN UF PAN-HPE-TPC NF Polymers & Polymer Composites, Vol. 20, No. 3,

4 Xiu-Zhen Wei, Jia Yang, and Guo-Liang Zhang of oxygen increased from 13.54% to 25.09%, whereas the percentage of nitrogen decreased from 10.94% to 2.24% and the O/N ratio increased from 1.24 to 11.2 due to interfacial polymerization of HPE and TMC. Part oxygen was detected on PAN-UF membrane surface, due to that some CN groups in PAN were hydrolyzed in the process of support membrane preparation. Of course, small amount of nitrogen could also still be detected due to that the selective layer was very thin. The element content changes indicated that PAN membrane surface was covered by a thin layer of crosslinked HPE. 3.3 SEM Image The surface and cross-section morphologies of the UF and NF membrane are characterized by SEM. As shown in Figure 3, the surface of NF membrane was uniform, smooth and dense, no pore structure could be found on the active layer. On the contrary, the surface of the blank PAN membrane had rough surface morphology and there are convex areas on the surface. This phenomenon indicated that, in the process of interfacial polymerization, a thin polyester film would be formed on the porous PAN membrane surface, which would enhance the smooth of the UF membrane and improve the hydrophilicity of the membrane. It could be seen from the cross-section morphologies that the uppermost layer of NF membrane was thicker than that of UF membranes, which indicated that crosslinked HPE composite on the surface of UF membrane surface. We know that the investigation depth of XPS is about 5~25 Å. However, a small amount of nitrogen could still be detected on NF membrane surface. Thus, according to the SEM image Figure 3. SEM images of different membranes, (a) PAN-UF; (b) PAN-HPE-TPC NF; 1: upper surface; 2: cross-section and XPS results, we presumed that the crosslinked HPE were located inside the support membrane pores firstly and then deposited atop the support. The salt rejection is presumably governed by the dense layer. Accordingly, the NF membrane shows a high salt rejection and low water permeability. 3.4 Performance Optimization of PAN-HPE NF Membranes The performance of the composite NF membrane is determined by the chemically structure of the reactive monomer and the preparation conditions of the selective layer. The preparation conditions would play an important role in determining the structure of the interfacial polymerized selective layer and subsequently the membrane performance 3. HPE, we choose, has highly branched structure, which would endow the selective layer with high water permeability in theory. Thus, in order to form an optimized NF membrane, the preparation conditions have to be optimized. 1 g/l Na water solution was used to evaluate the performance of PAN-HPE-TPC composite NF membranes under 0.6 MPa Effect of HPE Dosage First, it is necessary to ensure the concentration of the HPE in the aqueous phase. To optimize the membrane performance, a series of NF membranes were prepared by changing HPE dosage from 0.76 g to 1.9 g. In this study, TPC dosage was fixed at 0.2 g, the reaction time was fixed at 20 min, the heat-treatment temperature was fixed at 80 C and the heat-treatment time was fixed at 20 min. The salt permeation and rejection of different NF membranes was presented in Figure 4. As shown in Figure 4, the decrease of Na 2 permeation flux was cooperated with the increase of Na 2 rejection with the HPE dosage increasing. Na 2 permeation flux decreased from L/m 2 h to 14.1 L/m 2 h, and Na 2 rejection increased 264 Polymers & Polymer Composites, Vol. 20, No. 3, 2012

5 Preparation and Characterization of Nanofiltration Membranes Synthesized by Hyperbranched Polyester and Terephthaloyl Chloride (TPC) from 67.2% to 94.2% when HPE dosage increased form 0.76 g to 1.9 g. The results indicated that the membrane performance was affected significantly by the concentration of reactive monomer in the aqueous phase, which was inconsistent with the report in the literature 18. This result illustrates that the concentrations of HPE improved the formation of thin selective layer on the porous support membrane. With an increase in the dosage of HPE, an apparently denser, higher molecular weight coating would produce and result a distinct decrease in permeation flux and enhanced separation. Considering the permeation salt flux and rejection together, the optimal dosage of HPE is 1.14 g Effect of TPC Dosage To optimize the NF membrane performance, the influence of TPC dosage was investigated on the premise of keeping HPE dosage constant (1.14 g). At the same time, the reaction time was fixed at 20 min, the heattreatment temperature was fixed at 80 C and the heat-treatment time was fixed at 20 min unchangeable. TPC is a type of familiar reagent, which can increase compact degree of the membrane surface. The salt flux and rejection of different membranes was shown in Figure 5. The salt permeation flux decreased from 66.2 L/m 2 h to 32.5 L/m 2 h, and salt rejection increased from 75.4% to 87.6% when the dosage of TPC increased from 0.1 g to 0.25 g. TPC content plays an important role in determining the rate of reaction, which eventually affects the final performance of the membrane 19. When the dosage of TPC is increased, the rate of reaction is increased tremendously, which do rapidly build up the film layer and exert diffusion resistance for further monomer reaction to occur. Since flux is a function of membrane thickness 20, the reduction of flux is an indication of increasing layer thickness. This is why the permeation flux decreased with the increase of the concentration of TPC. Figure 4. Effect of HPE dosage on the NF membrane performance Figure 5. Effect of TPC dosage on the NF membrane performance Effect of CrosslinkingTime The thickness of the selective layer is depended on the crosslinking time and the diffusion rate of monomer through the interface between water phase and organic phase in interfacial polymerization. Thus the crosslinking time plays an important role in determining the extent of reaction 3,21,22. Generally, the top selective layer thickness of the nanofiltration membrane increases with the crosslinking time increasing. As shown in Figure 6, the effect of crosslinking time on membrane separation performance was investigated on the premise of keeping HPE dosage (1.14 g), TPC dosage (0.16 g), heat-treatment temperature (80 ºC) and time (20 min) constant. Na 2 permeation flux decreased from L/m 2 h to 18.4 L/m 2 h, and salt rejection increased from 48.6% to 96%, when crosslinking time increased from 10 min to 50 min. This result may be resulted from the increase of Polymers & Polymer Composites, Vol. 20, No. 3,

6 Xiu-Zhen Wei, Jia Yang, and Guo-Liang Zhang crosslinking degree. With crosslinking time increasing, three-dimensional crosslinked polymer network with high molecular weight would form gradually and result in a dense skin layer on the support PAN-UF membrane surface. When the thickness of the thin layer was enough to prevent the monomer diffusing from one phase into the other phase, the top layer thickness will stop growing and then the separation performance of composite membrane would be almost constant. Thus, for a shorter reaction time, such as 10 min, the extent of crosslinking is lower. As a result, the permeability of the membrane is high and the salt rejection is relatively low. After a certain period of reaction, the flux will almost stay constant because the thickness of the selective layer is almost fixed 23. At the same time, the salt rejection increases slightly although the top layer thickness stops growing due to that some defect on membrane surface was stopped with the proceeding of crosslinking reaction Effect of Heat-treatment Time In order to fabricate NF membrane with perfect performance, heat-treatment Figure 6. Effect of crosslinking time on NF membrane performance Figure 7. Effect of heat-treatment time on NF membrane separation performance was an important process, due to that some defects would disappear and the selective layer structure would be more perfect under suitable heat-treatment process. In this study, the HPE dosage was fixed at 1.14 g, the TPC dosage was fixed at 0.16 g, the reaction time was fixed at 20 min and the heattreatment temperature was fixed at 80 C. The effects of heat-treat time on the permeate flux and salt rejection were investigated under the abovementioned membrane preparation conditions. As shown in Figure 7, when heat-treatment time increased from 10 min to 30 min, the permeation flux decreased from L/m 2 h to 11.4 L/m 2 h, and salt rejection increased from 45.8% to 96.5%. This phenomenon was attributed to the diffusion barrier increased with the prolonging of heat-treatment time due to that top selective layer of membrane encounters a decrease of pore size. Accordingly, as the heat-treatment time prolonged appropriately, such as 30 min, the permeation flux was decreased and salt rejection was increased. However, if the heattreatment time was too long, such as 40 min or 50 min, the permeation flux was increased again, which may be caused by the decomposition of some crosslinked bonds. Meanwhile the different coefficient of thermal expansion between the substrate and selective layer may lead to the split of the active functional layers. The most important is that etherification reaction is an exothermic reaction. With the heat-treatment time prolonging, more and more polyester would hydrolyze to more hydrophilic monomer. What s more, the selective layer may tend to create channeling (defect) at prolonged heat-treatment times 24. That was why the water flux increased and the rejection coefficient decreased as expected Effect of Heat-treatment Temperature The effect of heat-treatment temperature on the performance of the composite NF membrane was investigated on the premise of keeping 266 Polymers & Polymer Composites, Vol. 20, No. 3, 2012

7 Preparation and Characterization of Nanofiltration Membranes Synthesized by Hyperbranched Polyester and Terephthaloyl Chloride (TPC) HPE dosage (1.14 g), TPC dosage (0.16 g), the reaction time (20 min) and the heat-treatment time (20 min) constant. As shown in Figure 8, the flux for PAN-HPE membrane was reduced from 73.5 to 2.1 L/m 2 h and Na 2 rejection was increased from 68.9% to 92.6% when the heat-treatment temperature was increased from 60 C to 110 C. Generally, elevated temperature promotes crosslinking of polymers and shrinkage of the substrate membrane, which results in higher rejection and lower water flux. Besides, the membrane surface formed compact reticular structure as a result of crosslinking, which led to the decrease of pore size and permeate flux. What s more, the extended degree of reaction caused extensive crosslinking and film-growth that resulted in additional diffusion resistance. These explained why the salt rejection enhanced and the water flux declined. higher rejection to sulfate salts ( 2- ) than to chloride salts (Cl - ). Thus, the rejection of Na 2 is higher than that of NaCl and the rejection of Mg is higher than that of MgCl 2. However, the rejection of Na 2 is higher than that of Mg. Sieving and Donnan exclusion did not seem appropriate to apply the same explanation to this phenomenon. It may be the result of Donnan exclusion and pore sieving Table 2. Hydrated ionic radii 29 Figure 8. Effect of heat-treatment temperature on NF membrane performance Ion Cl Na Mg Radius (nm) 3.5 Membrane Nanofiltration Properties Test The nanofiltration properties of the membranes were tested by filtering four different salt solutions, Mg, MgCl 2, Na 2 and NaCl (1 g/l) under the operation pressure of 0.3 MPa and 0.6 MPa, respectively. Figure 9 illustrates the salt rejections under two different operation pressures. The salts rejection order of these NF membranes is Na 2 > Mg >NaCl >MgCl 2. The salt rejection order is similar to the sequence developed by Tang 18 where the negatively charged NF membrane was used and varied from that developed by Yu and Du As we all know, the rejection to salts for a NF membrane depends on both the pore size of the membrane and the electrostatic action between the membrane and the ions in solution 24,28. It can be seen from Table 2, the 2- hydration ion radius of is larger than that of Cl -, and the sieving effect plays an important role. At the same time, the negative charges on membranes reject preferably anions of the higher valence, which results in a Figure 9. Different salt rejection of NF membrane under different operation pressure Polymers & Polymer Composites, Vol. 20, No. 3,

8 Xiu-Zhen Wei, Jia Yang, and Guo-Liang Zhang cooperation. Of course, the special salt rejection behaviour may be related with the three dimension spherical structure of HPE. HPE molecules have highly branched structures and have many hydroxyl groups at the end of the branched arms. Some hydroxyl groups were remained in the membrane after crosslinked with TPC. Parts of hydroxyl groups may migrate into the centre of the pores through flexible alkyl arms. The pendant hydroxyl groups may easily access to and form salt bridge with charged salts. The salt bridge would impose extra steric hindrance for separated salt molecules. Subsequently the diffusive and convective ion transports are retarded. 3.6 Hydrophilicity of Different Membranes Hydrophilicity is one of the most important factors of membrane surface and it can provide additional information on membranes morphologies 30,31. Usually, water contact angle measurements are the most convenient way to assess the hydrophilicity and wetting characteristics of polymer surface. However, such measurements are difficult to interpret for synthetic porous membranes because of capillary forces within pores, membrane contraction in the dry state, heterogeneity, roughness, and restructuring of the surfaces 30,32. Nevertheless, the relative hydrophilicity or hydrophobicity of each membrane can be easily obtained by water contact angle measurement. The time dependence of water contact angle drop on the surface of NF and PAN-UF membranes were shown in Figure 10. The water contact angle of pure PAN-UF membrane was relatively higher than that of NF membrane, which indicated that the NF membrane possessed a relative hydrophilic surface. Both the water contact angle of NF and UF membrane surface reduce gradually with the measurement time prolonging. And the decreasing rate of PAN-UF membrane surface was a little faster than that of NF membrane surface. This was owned to the HPE molecules had a hydrophobic core and many hydrophilic arms and the hydrophobic part would migrate onto membrane surface during dryness process. 3.7 Nanofiltration Properties Comparative In recent years, several NF membranes have been prepared with different hyperbranched polymers (Table 3). It can be seen from Table 3, compared with the traditional NF membranes fabricated with small reactive monomer, NF membranes prepared with HBPs showed relatively low salt rejection. However, HBPs have highly branched structures and a great number of functional terminated groups. Even after crosslinked with crosslinking agent, the elementary structure would not change. The particular structure of HBPs endowed these NF membranes with relatively loose selective layer and high flux. What should be mentioned is that these NF membranes presented special salt rejection order no matter what charge NF membranes was presented. We consider that the special salt rejection order is determined by the special structure of HPE. As we all know, the most important character of HBPs is it s taking many reactive Figure 10. Water contact angle change with time of different membranes Table 3. NF properties of different membranes using crosslinked hyperbranched polymers as selective layer NF membrane Flux (L/(m 2 h MPa)) Rejection (%) Salt rejection order PEI/TMC (MgCl 2 ) MgCl 2 >Mg >Na 2 >NaCl [15] PEI/TPC (MgCl 2 ) MgCl 2 >Mg >Na 2 >NaCl [15 ] HPE/TMC (Na 2 ) Na 2 >Mg >MgCl 2 >NaCl [16] HPE/GA (Na 2 ) Na 2 >NaCl>Mg >MgCl [17] 2 HPE/TPC (Na 2 ) Na 2 >Mg > NaCl > MgCl 2 PAMAEMA/ PSF (MgCl 2 ) MgCl 2 >Mg >NaCl>Na 2 typical positively charged NF [25] SPPESK/PSF (Na 2 ) Na 2 >NaCl>Mg >MgCl 2 typical negatively charged NF [27] 268 Polymers & Polymer Composites, Vol. 20, No. 3, 2012

9 Preparation and Characterization of Nanofiltration Membranes Synthesized by Hyperbranched Polyester and Terephthaloyl Chloride (TPC) groups. Even after crosslinking, some unreacted groups were still remained. Parts of the groups may migrate into the centre of the pores through flexible alkyl arms. The pendant functional groups may easily access to and form salt bridge with charged salts. The salt bridge would impose extra steric hindrance for separated salt molecules, which would lead the NF membranes to illustrate special slat rejection order. As we all know, TMC has three functional groups and TPC has two functional groups. We may infer NF membranes crosslinked with TMC would have relative loose selective layer. Thus, PEI/TMC and HPE/TMC presented higher flux and lower rejection than that of PEI/TPC and HPE/TPC. However, HPE/GA showed relatively high flux and rejection, despite that GA also has two functional groups. It may be due to that GA is a flexible molecule. The flexibility of GA endowed the HPE/ GA NF membrane with a reasonable loose and thickness selective layer. 4. CONCLUSION PAN-HPE-TPC composite NF membranes were prepared via insitu interfacial polymerization of hyperbranched polyester (HPE) and terephthaloyl chloride (TPC). FTIR- ATR and XPS indicated that a thin layer of crosslinked HPE was deposited on the PAN membrane surface. SEM image showed that the surface of NF membrane was smooth and dense, which would endow the NF membrane with relatively good antifouling properties. According to the results, the separation performance of PAN-HPE- TPC NF membranes are mainly related with the change of monomer content in the aqueous phase rather than that in the organic phase and the optimized dosage is 1.14 g for HPE and g for TPC, respectively. The permeate flux decreases drastically when the heat-treat time and temperature are increased, which corresponds to an increase in the salt rejection of the membrane. However, the membrane performance has a break if the heattreatment time is longer than 30 min. The rejection order of these NF membranes to different salts was Na 2 >Mg >NaCl > MgCl 2. The special salt rejection behaviour was related with the three dimension spherical structure of HPE. ACKNOWLEDGEMENTS Financial support from Natural Science Foundation of China (Grant No ) and Zhejiang Province Natural Science Foundation (Grant No.Y ) are gratefully acknowledged. The authors also greatly thank Science Foundation of Zhejiang University of technology (Grant No ). REFERENCE 1. Lu X., Bian X., and Shi L., Preparation and characterization of NF composite membrane, J. Membr. Sci., 210 (2002) Rao A.P., Joshi S.V., Trivedi J.J., Devmurari C.V., and Shah V.J., Structure-performance correlation of polyamide thin film composite membranes: effect of coating conditions on film formation, J. Membr. 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