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Preparation and characterizations of asymmetric sulfonated polysulfone membranes by wet phase inversion method
European Polymer Journal 45 (2009) 1293–1301
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Preparation and characterizations of asymmetric sulfonated polysulfone membranes by wet phase inversion method Shih-Hsiung Chen a,*, Rey-May Liou a, Yen-Yi Lin a, Cheng-Lee Lai a, Juin-Yih Lai b a b
Department of Environmental Engineering and Science, Chia-Nan University of Pharmacy and Science, Tainan 717, Taiwan Department of Chemical Engineering, Chung Yuan University, Chung Li 320, Taiwan
a r t i c l e
i n f o
Article history: Received 1 December 2007 Received in revised form 17 October 2008 Accepted 7 November 2008 Available online 28 November 2008
Keywords: Sulfonated polysulfone Asymmetric Membrane Pervaporation
a b s t r a c t This paper presents an original approach to prepare the asymmetric sulfonated polysulfone membranes by using wet phase inversion method and their applications for dehydrating a water/ethanol mixture by pervaporation. The separation performances of sulfonated membranes were strongly affected by the degree of sulfonation and the degree of swelling of membranes. The substitution degree of sulfonic group enhanced the permselectivity of sulfonated polysulfone membranes by increasing the hydrophilicity of polymer backbone. Based on the observations of membrane morphology and light transmittance measurements, the degree of sulfonation of polysulfone presented less influence on the membrane formation pathway and the final structure of membrane in wet phase inversion process. It was also found that the sulfonated membranes showed well hydrophilic properties and facilitated water adsorption in the membranes. The sorption and permeation properties also showed that the permselectivity of asymmetric membrane was dominated by the permeate diffusion rather than the permeate sorption in the skin layer. The high separation performance of pervaporation membrane can be achieved by phase inverse method with sulfonated polysulfone. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction To remove water from water/organic mixture by pervaporation process becomes more attractive than traditional separation processes in industrial applications. The preparation and modification of dehydration membrane attracted much more attentions for their applications in pervaporation dehydration process. Many works were focused on the improving of separation performance of pervaporation membrane by considering the permeate sorption and diffusion properties of separation membranes. The water-permselective pervaporation membranes usually focus on the improvement of the sorption selectivity rather than diffusion selectivity of permeates. Highly water-permselective membranes [1–4] can be achieved by increasing either the sorption selectivity or * Corresponding author. Tel.: +886 6 2660028; fax: +886 6 2669090. E-mail address: [email protected] (S.-H. Chen). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.11.030
the diffusion selectivity of water to ethanol. The Introduction of hydrophilic moiety in polymer matrix to enhance the hydrophilic properties of membrane is an effective method. The ionic membranes were widely applied to the dehydration of organic mixtures by pervaporation process [5–8]. The well-known cation-exchange pervaporation membrane, Nafion, showed a strong interaction between water and the ionic group of membrane, and also presented a good mechanical property and well enhancement on the water sorption. However, the low selectivity of water to alcohol or a low permeation rate of the modified membrane was still a limitation for pervaporation application [4,9]. For improving the disadvantages of Nafion membranes, some modifying technologies were used to overcome those disadvantages, such as chemical modification [9–10], bilayer composite [11], and blending method [12]. The permselectivity of modified Nafion membranes were improved by using the above methods but the water
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permeation rate usually fast decreased with increasing the degree of modification. Polyelectrolyte and alginate materials are the other candidates for preparing the high performance pervaporation membranes. Composite membrane with ultra thin polyelectrolyte separation layer shows a good separation performance [13–14]. The separation performance of polyelectrolyte membrane showed a strong dependence on the charge density in the membrane. Alginate is the other possible material to prepare a high performance pervaporation membrane [15–17]. The alginate composite membrane presented an outstanding performance for the dehydration of organic solution. The ioniccrosslinking alginate membranes were crosslinked with divalent and trivalent ions for improving their separation performance. Alginate membrane crosslinked with Ca2+ showed the highest pervaporation performance in terms of permeation flux and separation factor for the dehydration of ethanol–water and isopropanol–water mixtures [18–19]. Though the alginate membranes present a good pervaporation performance with a certain modified method, the weak mechanical properties and stability of alginate membrane are still a challenge to overcome. The asymmetric polysulfone with a defect free skin were prepared in our previous study [20]. It was found that the separation performance of asymmetric polysulfone membrane strongly depends on the polarity of adding solvent in polysulfone/N-methyl-2-pyrrolidone/water ternary system. In this study, the non-polar solvent was also added in sulfonated polysulfone/N-methyl-2-pyrrolidone/water ternary system for enhancing the defect-free skin formation. The pervaporation characteristics of sulfonated membranes as well as the degree of sulfonation dependence of the sulfonated membrane were discussed for the dehydration of ethanol–water mixture. The relationship between the microstructure change of sulfonated membranes and pervaporation properties is also discussed in this study. The effect of feed composition and operating conditions was studied by measuring the pervaporation properties. 2. Experimental 2.1. Materials UdelÒ Polysulfone P-3500 was obtained from Amoco Performance Products and Merck Chemical Co. supplied chlorosulfonic acid, chloroform, N-methyl-2-pyrrolidone (NMP) and ethanol. 2.2. Membrane preparation The sulfonated polysulfone was prepared from the direct sulfonation method by adding chlorosulfonic acid [21,22]. The sulfonation of polysulfone was achieved by means of chlorosulfonic acid and then soap in deionic water at room temperature for 2 days. The degree of sulfonation was determined by the elementary analysis as our previous report [22]. Then the 18 wt% sulfonated polysulfone was dissolved in NMP with 8 vol% chloroform additive at room temperature. The polymer solution was casted onto a glass plate to a predetermined thickness of 350 lm using a Gardner Knife. The membrane was dried
at room temperature for 30 min, then peeled off and immersed in distilled water for 12 h. Then the sulfonated polysulfone membrane was dried in vacuum oven for 24 h before the sorption and pervaporation measurements. 2.3. Pervaporation experiment A traditional pervaporation process was used [23]. In pervaporation, the feed solution of 90 wt% ethanol was in direct contact with the membrane and was kept at 25 °C. The effective membrane area was 10.2 cm2. The down stream pressure was maintained at about 5–8 Torr. The permeation rate was determined by measuring the weight of permeates. The compositions of the feed solution permeate, and solution adsorbed in the membranes was measured by gas chromatography (GC, China Chromatography). The separation factor, aA/B, was calculated by the formula:
aA=B ¼ ðY A =Y B Þ=ðX A =X B Þ where XA, XB and YA, YB are the weight fractions of A and B in the feed and permeate, respectively (A being the more permeative species). 2.4. Sorption measurements Due to the porous structure of asymmetric membranes, the results of sorption measurement of dense symmetric membrane will apply to illustrate the sorption behavior of asymmetric sulfonated membranes. The dense sulfonated membranes were immersed in the ethanol–water mixture for 24 h at 25 °C. They were subsequently blotted between tissue papers to remove the excess solvent and placed in the left half of a twin tube set-up. The system was evacuated while the tube was heated with hot water for 30 min and the right tube was cooled in liquid nitrogen. The composition of the condensed liquid with the right tube was determined by G.C. The separation factor of sorption was calculated by:
asorp ¼ ðY w =Y e Þ=ðX w =X e Þ where Xe, Xw and Ye, Yw are the weight fractions of ethanol and water in the feed and membranes, respectively. 2.5. Contact angle measurements The contact angle of water was measured with a FACE contact angle meter CA-D type (Kyowa Interface Science Co. Ltd.). 2.6. Swelling measurement Due to the porous structure of asymmetric sulfonated membranes, the degree of swelling of dense membranes is used to descript the swelling behavior on the skin layer of sulfonated membranes. The degree of swelling of dense sulfonated membranes were determined in distilled water and in aqueous ethanol solution at 25 °C. The weight of dry membrane (Wdry) was first determined. After equilibrium with water or ethanol solution, the fully swollen membrane was wiped with tissue paper and weighed. Since
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the ethanol evaporated very fast, it is difficult to read the real weight directly. The weight of the membrane was measured every 5 s and plotted as a function of time for 30 s after wiping dry. The weight at time zero could be extrapolated and was taken as swollen weight (Wwet) of the membrane. The degree of swelling was calculated by following equation:
Degree of swellingð%Þ ¼ ðW wet W dry Þ=W dry 100%
2.7. Scanning electron microscope (SEM) and atomic force microscope (AFM) Hitachi S4100 SEM and DI 5000 AFM (Digital Instrument, DI 5000) was used to observe the surface and cross-section structures of asymmetric membrane. In SEM studies membranes samples were immersed in liquid nitrogen and then fractured membranes for preparing samples. The SEM samples were then deposited with gold using a sputter coater. 2.8. Light scattering measurement Light transmission experiments have bee preformed to measure the time of onset of liquid–liquid demixing in phase inversion. For the detail experimental setup and procedures, one can refer to the work of Mulder et al. [24]. 3. Results and discussion 3.1. Characteristics of sulfonated polysulfone membranes In sulfonated polysulfone/NMP/water system, the chloroform was added in casting solution to delay the demixing rate of casting solution during the wet phase inversion process. The effect of degree sulfonation on the morphology of sulfonated membranes were shown in Fig. 1. As shown in Fig. 1, the dense skin layer and fingerlike porous sublayer were found in all the range of sulfonation membranes with 8 vol% chloroform addition in casting solution. Thought the similar skin layer structure can be found in those asymmetric membranes. But the finger type sub layer structure grows and forms a macrovoid structure with increasing the degree of substitution of sulfonic group in polysulfone. It is indicted that the amount of grafting sulfonic group on polysulfone grows up the macro pore structure in the sublayer of asymmetric membrane. Pekny et al. [25] indicated that the growth of macrovoid is dominated by both of drag and buoyancy force of inflow demixing fluid in macrovoid during the period of demixing of casting solution. The buoyancy force showed the more influence in the case of low gravity or microgravity. In the normal-gravity (1-g) conditions, the drag force plays the most important role to determine the final pore structure of asymmetric membranes. In the case sulfonated polysulfone/polysulfone blend membranes [26], the sub layer macrovoids and skin layer structure differs significantly from that of pure polysulfone membranes. It was proposed that a competition or parallel processes of poly-
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mer-rich/polymer-lean phase separation and polymer– polymer phase separation during the membrane formation. They also concluded that the liquid–liquid phase separation seems to control the macrovoid distribution, and the polymer–polymer phase separation shaped the void form and wall morphology. The two-phase separations would thus have similar time scale. In this investigation, it can be seem that the polar–polar interaction between casting solution and coagulant (water) increased with increasing the degree of sulfonation of polysulfone in this sulfonated polysulfone/NMP/water system. Therefore, the less drag force of the higher degree of sulfonation casting solution were expected due to the polar–polar interaction between casting solution and coagulant (water). The low drag force in bulk casting solution induced the coagulant water more easy to flow into the initial macrovoid and grows the macrovoid. As shown in Fig. 1, the growth of macrovoid in phase inversion process presents a strong dependence on the degree of sulfonation of polysulfone in casting solution. As can be seen in Fig. 1, the clearly skin layer and typically finger type sublayer structure can be observed in the SEM images of these sulfonated membranes. Generally, permselectivity of asymmetric membranes were determined by the skin layer structure. In wet phase inverse process, the skin layer structure is determined by the demixing behavior during the phase inverse process. The skin layer plays an important role to determine the permselectivity of those sulfonated membranes. By considering the demixing behavior of casting solution, the delay demixing thickens the skin layer in wet phase inversion process. If the demixing behavior was delayed by the various degree of sulfonation of polysulfone, the skin layer thickness will be thickening with the delay demixing behavior in phase inversion process. Therefore, it can be expected that the thickness of skin layer should be affected by the different demixing behavior in phase inversion. The light transmittances measurement was applied to detect the demixing behavior in coagulation bath [20,27,28]. The light transmittance measurements were made for clarifying the demixing behavior of sulfonated polymer solution in water bath at 25 °C. As shown in Fig. 2, the light transmittance measurements of casting solutions were made in water bath at 25 °C. The similarly decrease in light transmittance indicated that the phase inversion of casting solution were almost done in first minute in water bath. The similar light transmittance behavior of those casting solutions implied that the polar difference of sulfonated polymers did not significantly change the demixing behavior. On the other hand, the similar demixing time in water bath will be expected to obtain the similar skin layer structure of those sulfonated membranes. Fig. 3 showed the effect of sulfonic group substitution of polymer on the skin layer thickness of asymmetric membranes. The skin layer thickness slightly increased with increasing the degree of substitution of sulfonated membranes. Those results are consistent with the expectation from light transmittance measurement. It is implied that the change of polymeric polarity in casting solution showed a less influence on the demixing behavior and final thickness of skin layer. Those results also implied that
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Fig. 1. Scanning electron micrographs of cross-section of membrane with different substitution molar ratio of sulfonic group to polysulfone unit (a) 0.25%, (b) 0.42%, (c) 0.58%, (d) 0.75%, (e) 0.92%.
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0.2 Degree of sulfonation (SO3H/PSF unit) 0.25 0.42 0.58 0.75 0.92
8
Skin layer thickeen (μm)
Transmittance (%)
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6
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0
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Time (sec) Fig. 2. Light transmittance measurement of sulfonated casting solution in coagulation bath at 25 °C. (a) 0.25, (b) 0.42, (c) 0.58, (d) 0.75, (e) 0.92.
permeate transport through the membranes will be almost own the same barrier in those sulfonated membranes. However, there are many factors influence permeates transporting through the pervaporation membranes. The polymer packing in the dense skin layer is one of the
0 0.2
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Mole ratio (SO3H/PSF unit) Fig. 3. Effect of substitution molar ratio of sulfonic group to polysulfone unit on the skin layer thickness of sulfonated membrane (a) 0.25, (b) 0.42, (c) 0.58, (d) 0.75, (e) 0.92.
importance factors to dominate the permeation flux of pervaporation membranes. Although the same skin layer thickness was observed with SEM images on those sulfonated membranes. To observe the polymer packing structure on the top layer is helpful to clarify its influence on
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the permeation behavior of sulfonation membranes. The atomic force microscope observations on membrane’s surface were made to investigate the polymer nodule packing on the skin layer, as shown in Fig. 4. It can be observed that the roughness on the skin layer of sulfonated membranes increased with increasing the sulfonation degree of polymer. The changes in roughness of the sulfonated membranes were affected by the sulfonation degree of polysulfone. Although the light transmittance measurements and SEM images observation indicated that there is less influences on the thickness of skin layer. But the sulfonation of polysulfone still induced the morphology change in membrane surface formation. The sulfonated polysulfone may be formed a hydrogen bonding between the sulfonic groups in polymer solution. Therefore, the polymeric chain interaction leads to an enhancement on the viscosity of casting solution and polymer entanglement via a different degree of sulfonation on polysulfone, especially in the polymer rich phase during the phase inversion process. Table 1 showed the effect of degree of sulfonation on the surface roughness of sulfonated membranes. It can
Table 1 Surface roughness values of the asymmetric sulfonated membranes. Degree of sulfonation
Rms (nm)
Ra (nm)
Rmax (nm)
0.25 0.42 0.58 0.92
3.91 5.62 8.24 7.78
3.10 4.52 6.28 5.81
42.65 37.96 60.80 62.21
be seen that the roughness of sulfonated membranes increased with increasing the degree of sulfonation of polysulfone membranes. The roughness change evidenced that the polymer chains interaction become stronger with enriching the polymer concentration in the phase inversion process. The size of polymer nodules and dense skins with the different polymer packing density were affected during the phase inversion in coagulation bath with the increase in degree of sulfonation of polysulfone. Therefore, the sulfonation of polysulfone showed the less change on the macrovoid formation and their skin layer thickness. But the polymer–polymer interaction presented a remark-
Fig. 4. Atomic force micrographs of surface of sulfonated membrane with different substitution molar ratio of sulfonic group to polysulfone unit (a) 0.25, (b) 0.42, (c) 0.58, (d) 0.75, (e) 0.92.
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able influence on the polymer packing on the surface skin layer of asymmetric membranes. 3.2. Water contact angle measurement Fig. 5 illustrates the variation of the contact angles of sulfonated polysulfone membranes. The water contact angle was used as the index of hydrophilicity of sulfonated membranes. The lower water contact angle implied the more the hydrophilic on the surface of sulfonated membrane. As shown in Fig. 5, it can be clearly observed that the contact angle of polysulfone membranes is much higher than the values of sulfonated membranes. Moreover, the contact angles of sulfonated membranes decreased with increasing the degree of sulfonation. Thus it is indicated that the sulfonic group lead-in is helpful to improve the hydrophilicity of polysulfone membranes. Based on the concept of polar–polar interaction, the sulfonic group on the polysulfone membrane benefited to the surface polarity and decreased the water contact angle. In the case of ethanol solution in feed, the polar properties may be helpful for the permselectivity of water molecular transporting through the sulfonated membranes. Based on the above results, they implied that the permeation rate of the sulfonated membranes may be decreased with the higher degree of sulfonation because of the closer polymer packing on the skin layer. On the other hand, the increase in polarity of sulfonated membrane indicated that the water adsorption of sulfonated membrane became easier than that of pure polysulfone membrane with increasing the degree of substitution of sulfonic group on the polymer backbone. 3.3. Effect of sulfonic group substitution on pervaporation properties The effect of sulfonic group substitution on permeation flux and separation factor of sulfonated membranes by pervaporation separation is shown in Fig. 6. It can be seen
that both of the permeation flux and the separation factor increased with increasing sulfonic group substitution on polysulfone at 90% ethanol in feed. As the respect of the observation on the skin layer, the increase in permeation flux is opposite the observation on the morphology of skin layer. As shown in Fig. 1, the sulfonic-substituted membrane presents the same skin layer thickness and almost similar support layer structure. Though the AFM observation indicated that the polymer packing are closer with sulfonic substation in those membranes. Those results did not imply the increase in permeation flux. One of the possible factors to enhance the permeation flux is the increase in swelling property of sulfonated membranes. Due to the permeate swelling the top skin layer, the loosen polymer packing on the top layer was formed the more free volume for permeate to transport through the membranes. In this study, the results of swelling measurement of dense symmetric membrane will apply to illustrate the swelling behavior of skin layer on sulfonated membranes [22]. Fig. 7 showed the effect of the degree of substitution of polysulfone on the swelling properties of dense sulfonated membranes with 90% ethanol solution. It can be seen that the degree of swelling increased with the increase in sulfonation degree of polysulfone. These results evidenced that the increase in permeation flux contributed by the swelling properties of sulfonated membranes. As the increase in permeation flux, permeate coupling effect usually induced the loss of permselectivity of membranes. In this study, the loss of permselectivity did not observed but the sulfonation enlarged the permselectivity of sulfonated membranes. The degree of swelling of sulfonated membranes increased with increasing the degree of sulfonation of membranes. The swelling properties of sulfonated membranes indicated skin layer will be swelled in the feed solution. Thus the swelling skin layer may be enhanced the permeation flux and lose their separation factor. As can be seen in Fig. 6, the separation factor of sulfonated mem800
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Separation factor (α)
Permeation flux (g/m2hr)
Contact angle (θ)
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Mole ratio (SO3H+/PSF unit) Fig. 5. Effect of the degree of substitution on the water contact angle of sulfonated membranes.
Fig. 6. Effect of the degree of substitution of sulfonated membranes on the pervaporation performance for 90 wt% ethanol solution in feed at 25 °C.
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Diffusion selectivity (DH O /DEthanol)
20
Degree of Swelling (%)
20
15
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0 0.2
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1
Mole rate (SO3H/PSF unit)
10
16 8
12 6 8
4 4
0 0.2
Solubility selectivity (SH O /SEthanol)
25
2 0.4
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1
Mole ratio (SO3H/PSF unit)
Fig. 7. Effect of the degree of substitution on the degree of swelling of sulfonated membranes in 90 wt% ethanol solution at 25 °C.
Fig. 8. Effect of the degree of substitution on the sorption selectivity and diffusion selectivity of sulfonated membranes for 90 wt% ethanol solution in feed at 25 °C.
brane did not decrease but it increased with the degree of sulfonation of those membranes. It is well known that the permeation behavior was almost dominated by the diffusion behavior of permeate in the pervaporation membrane for separating the ethanol/water mixture. Therefore, the understanding of diffusion behavior of permeates in solfonated membrane is helpful for further clarifying the real transport behavior in membranes. Based on the sorption–diffusion mechanism, the diffusion selectivity (ad) can be defined as the ratio of permeation selectivity (ap) and sorption selectivity (as): [29], ap = ad as. In order to calculated the diffusion selectivity through a pervaporation membrane, the permeate content in sulfonated membrane were measured and the sorption selectivity were calculated based on the sorption test. The sorption selectivity of water to ethanol was also calculated by dividing the amount of water content by the amount of ethanol content in sulfonated membranes. Fig. 8 shows that the sorption selectivity of water to ethanol increased with decreasing the substitution of sulfonic group in the membrane. The diffusion selectivity (ad) and sorption selectivity (as) were calculated by above equation. It can be seen that the sorption selectivity slightly decreased and the diffusion selectivity increased as the degree of substitution is increased. The separation factor (permeation selectivity) and sorption selectivity shows a different trend with increasing the content of sulfonic group in polysulfone membranes. The different behavior on the permeate permeation from sorption may be contributed by the permeate diffusion control in the sulfonated membrane which was caused by the interaction between permeate and membrane. Therefore, the increase in permeation rate was due to the high degree of swelling of sulfonated polysulfone membrane which was caused by the hydration effect of sulfonic group with water and ethanol. The high degree of swelling induced a higher permeate content in membranes. Due to the molecular size difference between water and ethanol, the water mole-
cules own a higher diffusion rate than those of ethanol. On the other hand, the hydration of sulfonated membrane increased with increasing the degree of sulfonation. Therefore, the higher degrees of sulfonation enriched water content in membranes and sieving effect of permeates preferred more water molecular diffusing through the membranes. Thus both of the two effects contributed the enhancement on the diffusion selectivity of sulfonated membranes. It was indicated that separation factor of sulfonate membrane were dominated by the permeate diffusion behavior superior than the sorption behavior. 3.4. Pervaporation separation index Effect of ethanol concentration on the PSI value of asymmetric sulfonated polysulfone membrane was shown in Fig. 9. It can be seen that the PSI value of sulfonated membranes was enhanced by the increase in degree of sulfonation. Because of the less change in permeation rate, the increase in PSI value mainly contributed by the enhancement on diffusivity selectivity of sulfonated membranes, which was contributed by the hydration effect in the membrane. It was indicated that the increase in degree of swelling lead to an enhancement in diffusion selectivity of sulfonated membranes. This result implied that the permselectivity of sulfonated membranes were strongly affected by the permeate swelling in the feed solution. 3.5. Effect of feed ethanol concentration on the pervaporation performance Generally, the swelling degree of membranes dominated the free volume for permeate transport through the membrane. It was implied that the change of feed solution concentration lead to a different swelling properties of sulfonated membranes. Fig. 10 showed the effect of feed ethanol concentration on the permeation flux and separation factor of sulfonated membranes. It can be found that
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60000
PSI
50000
40000
30000
20000
0
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Mole ratio (SO3H/PSF unit) Fig. 9. Effect of the degree of substitution on the pervaporation separation index of sulfonated membranes for 90 wt% ethanol solution in feed at 25 °C.
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Separation factor (α)
Permeation flux (g/m2hr)
the permeation flux slightly decreased with feed ethanol concentration. On the other hand, the increase in feed ethanol concentration increased the separation factor of sulfonated membranes. It is indicated the permselectivity is strongly affected by the feed concentration due to the increase in ethanol concentration. The decrease in permeation flux can be contributed by the decrease hydration effect of water molecules in membranes. The less hydration and low degree of swelling of sulfonated membrane induced an increased in sieving effect in top skin layer and showed a larger separation factor. As shown in previous results, the polymer chains were swelled in higher concentration ethanol solution and offered an excess pathway for permeates to transport through the membrane. On the other hand, the siev-
0 100
Feed Ethanol composition (%) Fig. 10. Effect of the feed ethanol composition on the pervaporation performance of sulfonated membrane with degree of sulfonation 0.92 at 25 °C.
ing effect of swelling membrane also determined the permeate transport through the sulfonated membranes. Thus, it is interesting to clarify that the effect of feed ethanol concentration on the sorption and diffusion behavior of sulfonated membranes. Fig. 11 showed the effect of feed ethanol concentration on the sorption and diffusion selectivity of sulfonated membranes. It can be seen that the sorption selectivity increased with increasing the feed ethanol concentration but the diffusion selectivity first slightly decreased and rapidly increased with the feed ethanol concentration. It was found that the swelling increase did not deteriorate the sorption selectivity of sulfonated membranes. It is proposed that the swelling membrane enlarge the transport path of permeates and the polymer–permeate interaction also play an important role on the adsorbing permeate into sulfonated membrane. It is believed that the sulfonic group played a key role to adsorb the water molecular into membranes. Therefore, the hydration of sulfonic group in membranes contributed the superior water uptake in membrane than the ethanol. Therefore, the sorption selectivity increased with increasing the ethanol concentration in feed solution. On the other hand, the permeate diffusion behavior was dominated by the transport paths in the membranes. Therefore, the enlargement of free volume of membrane increased the diffusion rate both of water and ethanol. If the path way is too small to pass through for water and ethanol molecular, the permeate partition will play the key role to determine the diffusion rate of permeate. As shown in Fig. 10, it was indicated that the water is the superior adsorption permeate in sulfonated membrane and the partition of water in membrane increased with increasing with the feed concentration of ethanol. Based on the result of water sorption in various ethanol concentrations in feed, it was found that the hydration of the water played an important role to determine the permselectivity of sulfonated membranes. 20
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Solubility selectivity (SH2O /SEthanol )
70000
Diffusion selectivity (DH2O /DEthanol )
1300
0 100
Feed Ethanol composition (%) Fig. 11. Effect of the feed ethanol composition on the sorption selectivity and diffusion selectivity of sulfonated membranes with degree of sulfonation 0.92 at 25 °C.
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4. Conclusion This paper is successful to prepare the asymmetric sulfonated polysulfone membranes by using wet phase inversion method and showed the good performance for dehydrating water/ethanol mixture by pervaporation. The similar light transmittance of those casting solutions implied that the polarity change of modified polymer did not strongly affect the demixing behavior. On the other hand, the less demixing time in water bath will be lead to a thin skin layer of sulfonated membranes. Base on the AFM observation, the roughness on the skin layer of sulfonated membranes decreased with increasing the sulfonation degree of polymers. It is indicated that the sulfonation affect the surface’s roughness of sulfonated membranes. The substitution of sulfonic group on polysulfone enhanced both of the permeation flux and the separation factor at 90% ethanol in feed. The sulfonated membranes showed well hydrophilic properties and benefit both of their swelling properties and permeate permeation flux. The sorption and permeation measurement showed that the permselectivity of asymmetric membrane was dominated by the permeate diffusion control rather than the permeate sorption in the dense skin layer. The high performance of pervaporation membrane was prepared by phase inverse method by using the polymer sulfonation method. Acknowledgements The authors wish to thank the National Science Council of Republic of China (NSC-93-2216-E-041-003) Taiwan, ROC for their financial support in this research. The authors also want to thank the Center-of-Excellence Program on Membrane Technology, the Ministry of Education, Taiwan, ROC for their financial and technology support in this research. References [1] Xu T, Liu Z, Yang W. Fundamental studies of a new series of anion exchange membranes:membrane prepared from poly(2, 6-dimethyl1, 4-phenylene oxide) (PPO) and triethylamine. J Membr Sci 2005;249:183–91. [2] Yamauchi A, Shin Y, Shinozaki M, Kawabe M. Membrane characteristics of composite collodion membrane IV. Transport properties across blended collodion/Nafion membrane. J Membr Sci 2000;170:1–7. [3] Choi JH, Moon SH. Structural change of ion-exchange membrane surfaces under high electric fields and its effects on membrane properties. J Colloid Interface Sci 2003;265:93–100. [4] Komkova EN, Stamatialis DF, Strathmann H, Wessling M. Anionexchange membranes containing diamines: preparation and stability in alkaline solution. J Membr Sci 2004;244:25–34. [5] Choi EY, Strathmann H, Park JM, Moon SH. Characterization of nonuniformly charged ion-exchange membrane separated by plasmainduced graft polymerization. J Membr Sci 2006;268:165–74. [6] Kujawski W, Staniszewski M, Nguyen TQ. Transport parameters of alcohol vapors through ion-exchange membranes. Sep Purif Technol 2007;57:476–82.
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[7] Rao PS, Smith B, Sridhar S, Krishnaiah A. Preparation and performance of poly(vinyl alcohol)/polyethyleneimine blend membranes for the dehydration of 1, 4-dioxane by pervaporation: comparison with glutaraldehyde cross-linked membranes. Sep Purif Technol 2006;48:244–54. [8] Schwarz HH, Malsch G. Polyelectrolyte membranes for aromatic– aliphatic hydrocarbon separation by pervaporation. J Membr Sci 2005;247:143–52. [9] Kusumocahyo SP, Sudoh M. Dehydration of acetic acid by pervaporation with charged membranes. J Membr Sci 1999;161: 77–83. [10] Jiang JS, Greenberg DB, Fried JR. Pervaporation of methanol from a triglyme solution using a Nafion membrane: 2 concentration polarization. J Membr Sci 1997;132:263–70. [11] Ames RL, Way JD, Elizabeth AB. Dehydration of nitric acid using perfluoro carboxylate ionomer membranes. J Membr Sci 2005;249: 65–73. [12] DeLuc NW, Elab YA. NafionÒ/poly(vinyl alcohol) blends: effect of composition and annealing temperature on transport properties. J Membr Sci 2006;282:217–24. [13] Krasemann L, Tieke B. Ultrathin self-assemble polyelectrolyte membranes for pervaporation. J Membr Sci 1998;150:23–30. [14] Toutianoush A, Krasemann L, Tieke B. Polyelectrolyte multilayer membranes for pervaporation separation of alcohol/water mixtures. Colloid Surface A 2002;198–200:881–9. [15] Naidu BV K, Rao KSVK, Aminabhavi TM. Pervaporation separation of water + 1, 4-dioxane and water + tetrahydrofuran mixtures using sodium alginate and its blend membranes with hydroxyethylcellulose-A comparative study. J Membr Sci 2005;260:131– 141. [16] Moon GY, Pal R, Huang RYM. Novel two-ply composite membranes of chitosan and sodium alginate for the pervaporation dehydration of isopropanol and ethanol. J Membr Sci 1999;156: 17–27. [17] Wang XP, Li N, Wang WZ. Pervaporation properties of novel alginate composite membranes for dehydration of organic solvents. J Membr Sci 2001;193:85–95. [18] Yang G, Zhang L, Peng T, Zhong W. Effects of Ca2+ bridge crosslinking on structure and pervaporation of cellulose/alginate blend membranes. J Membr Sci 2000;175:53–60. [19] Smith B, Suhany D, Sridhar S, Ramakrishn M. Separation of organic– organic mixtures by pervaporation – a review. J Membr Sci 2004;241:1–21. [20] Chen SH, Liou RM, Lai JY, Lai CL. Effect of the polarity of additional solvent on membrane formation in polysulfone/N-methyl-2pyrrolidone/water ternary system. Euro Polym J 2007;43: 3997–4007. [21] Zschocke P, Quellmalz D. Novel ion exchange membranes based on an aromatic polysulfone. J Membr Sci 1985;22:325–32. [22] Chen SH, Yu KC, Lin SS, Chang DJ, Liou RM. Pervaporation separation of water/ethanol mixture by sulfonated polysulfone membrane. J Membr Sci 2001;183:29–36. [23] Lee KR, Lai JY. Dehydration of acetic acid–water mixture by pervaporation with a modified poly(4-methyl-1-pentent) membrane. J Polym Res 1994;1:247–54. [24] Mulder M. Basic principles of membrane technology. Dordrecht: Kluwer Academic Publishers; 1991. p. 94–5. [25] Pekny MR, Zartman J, Krantz WB, Greenberg AR, Todd P. Flowvisualization during macrovoid pore formation in dry-cast cellulose acetate membranes. J Membr Sci 2003;211:71–90. [26] Blanco JF, Sublet J, Nguyen QT, Schaetzel P. Formation and morphology studies of different polysulfones-based membranes made by wet phase inversion process. J Membr Sci 2006;283: 27–37. [27] Kim JH, Lee KH. Effect of PEG additive on membrane formation by phase inversion. J Membr Sci 1998;138(2):153–63. [28] Luo RL, Young TH, Sun YM. Structure formation and characterization of EVAL membranes with cosolvent of isopropanol and water. Polymer 2003;44(1):157–66. [29] Huang RYM. Pervaporation separation process. Amsterdam: Elsevier Science publishers B. V.; 1991. 397–398.
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Preparation and characterizations of asymmetric sulfonated polysulfone membranes by wet phase inversion method Shih-Hsiung Chen a,*, Rey-May Liou a, Yen-Yi Lin a, Cheng-Lee Lai a, Juin-Yih Lai b a b
Department of Environmental Engineering and Science, Chia-Nan University of Pharmacy and Science, Tainan 717, Taiwan Department of Chemical Engineering, Chung Yuan University, Chung Li 320, Taiwan
a r t i c l e
i n f o
Article history: Received 1 December 2007 Received in revised form 17 October 2008 Accepted 7 November 2008 Available online 28 November 2008
Keywords: Sulfonated polysulfone Asymmetric Membrane Pervaporation
a b s t r a c t This paper presents an original approach to prepare the asymmetric sulfonated polysulfone membranes by using wet phase inversion method and their applications for dehydrating a water/ethanol mixture by pervaporation. The separation performances of sulfonated membranes were strongly affected by the degree of sulfonation and the degree of swelling of membranes. The substitution degree of sulfonic group enhanced the permselectivity of sulfonated polysulfone membranes by increasing the hydrophilicity of polymer backbone. Based on the observations of membrane morphology and light transmittance measurements, the degree of sulfonation of polysulfone presented less influence on the membrane formation pathway and the final structure of membrane in wet phase inversion process. It was also found that the sulfonated membranes showed well hydrophilic properties and facilitated water adsorption in the membranes. The sorption and permeation properties also showed that the permselectivity of asymmetric membrane was dominated by the permeate diffusion rather than the permeate sorption in the skin layer. The high separation performance of pervaporation membrane can be achieved by phase inverse method with sulfonated polysulfone. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction To remove water from water/organic mixture by pervaporation process becomes more attractive than traditional separation processes in industrial applications. The preparation and modification of dehydration membrane attracted much more attentions for their applications in pervaporation dehydration process. Many works were focused on the improving of separation performance of pervaporation membrane by considering the permeate sorption and diffusion properties of separation membranes. The water-permselective pervaporation membranes usually focus on the improvement of the sorption selectivity rather than diffusion selectivity of permeates. Highly water-permselective membranes [1–4] can be achieved by increasing either the sorption selectivity or * Corresponding author. Tel.: +886 6 2660028; fax: +886 6 2669090. E-mail address: [email protected] (S.-H. Chen). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.11.030
the diffusion selectivity of water to ethanol. The Introduction of hydrophilic moiety in polymer matrix to enhance the hydrophilic properties of membrane is an effective method. The ionic membranes were widely applied to the dehydration of organic mixtures by pervaporation process [5–8]. The well-known cation-exchange pervaporation membrane, Nafion, showed a strong interaction between water and the ionic group of membrane, and also presented a good mechanical property and well enhancement on the water sorption. However, the low selectivity of water to alcohol or a low permeation rate of the modified membrane was still a limitation for pervaporation application [4,9]. For improving the disadvantages of Nafion membranes, some modifying technologies were used to overcome those disadvantages, such as chemical modification [9–10], bilayer composite [11], and blending method [12]. The permselectivity of modified Nafion membranes were improved by using the above methods but the water
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permeation rate usually fast decreased with increasing the degree of modification. Polyelectrolyte and alginate materials are the other candidates for preparing the high performance pervaporation membranes. Composite membrane with ultra thin polyelectrolyte separation layer shows a good separation performance [13–14]. The separation performance of polyelectrolyte membrane showed a strong dependence on the charge density in the membrane. Alginate is the other possible material to prepare a high performance pervaporation membrane [15–17]. The alginate composite membrane presented an outstanding performance for the dehydration of organic solution. The ioniccrosslinking alginate membranes were crosslinked with divalent and trivalent ions for improving their separation performance. Alginate membrane crosslinked with Ca2+ showed the highest pervaporation performance in terms of permeation flux and separation factor for the dehydration of ethanol–water and isopropanol–water mixtures [18–19]. Though the alginate membranes present a good pervaporation performance with a certain modified method, the weak mechanical properties and stability of alginate membrane are still a challenge to overcome. The asymmetric polysulfone with a defect free skin were prepared in our previous study [20]. It was found that the separation performance of asymmetric polysulfone membrane strongly depends on the polarity of adding solvent in polysulfone/N-methyl-2-pyrrolidone/water ternary system. In this study, the non-polar solvent was also added in sulfonated polysulfone/N-methyl-2-pyrrolidone/water ternary system for enhancing the defect-free skin formation. The pervaporation characteristics of sulfonated membranes as well as the degree of sulfonation dependence of the sulfonated membrane were discussed for the dehydration of ethanol–water mixture. The relationship between the microstructure change of sulfonated membranes and pervaporation properties is also discussed in this study. The effect of feed composition and operating conditions was studied by measuring the pervaporation properties. 2. Experimental 2.1. Materials UdelÒ Polysulfone P-3500 was obtained from Amoco Performance Products and Merck Chemical Co. supplied chlorosulfonic acid, chloroform, N-methyl-2-pyrrolidone (NMP) and ethanol. 2.2. Membrane preparation The sulfonated polysulfone was prepared from the direct sulfonation method by adding chlorosulfonic acid [21,22]. The sulfonation of polysulfone was achieved by means of chlorosulfonic acid and then soap in deionic water at room temperature for 2 days. The degree of sulfonation was determined by the elementary analysis as our previous report [22]. Then the 18 wt% sulfonated polysulfone was dissolved in NMP with 8 vol% chloroform additive at room temperature. The polymer solution was casted onto a glass plate to a predetermined thickness of 350 lm using a Gardner Knife. The membrane was dried
at room temperature for 30 min, then peeled off and immersed in distilled water for 12 h. Then the sulfonated polysulfone membrane was dried in vacuum oven for 24 h before the sorption and pervaporation measurements. 2.3. Pervaporation experiment A traditional pervaporation process was used [23]. In pervaporation, the feed solution of 90 wt% ethanol was in direct contact with the membrane and was kept at 25 °C. The effective membrane area was 10.2 cm2. The down stream pressure was maintained at about 5–8 Torr. The permeation rate was determined by measuring the weight of permeates. The compositions of the feed solution permeate, and solution adsorbed in the membranes was measured by gas chromatography (GC, China Chromatography). The separation factor, aA/B, was calculated by the formula:
aA=B ¼ ðY A =Y B Þ=ðX A =X B Þ where XA, XB and YA, YB are the weight fractions of A and B in the feed and permeate, respectively (A being the more permeative species). 2.4. Sorption measurements Due to the porous structure of asymmetric membranes, the results of sorption measurement of dense symmetric membrane will apply to illustrate the sorption behavior of asymmetric sulfonated membranes. The dense sulfonated membranes were immersed in the ethanol–water mixture for 24 h at 25 °C. They were subsequently blotted between tissue papers to remove the excess solvent and placed in the left half of a twin tube set-up. The system was evacuated while the tube was heated with hot water for 30 min and the right tube was cooled in liquid nitrogen. The composition of the condensed liquid with the right tube was determined by G.C. The separation factor of sorption was calculated by:
asorp ¼ ðY w =Y e Þ=ðX w =X e Þ where Xe, Xw and Ye, Yw are the weight fractions of ethanol and water in the feed and membranes, respectively. 2.5. Contact angle measurements The contact angle of water was measured with a FACE contact angle meter CA-D type (Kyowa Interface Science Co. Ltd.). 2.6. Swelling measurement Due to the porous structure of asymmetric sulfonated membranes, the degree of swelling of dense membranes is used to descript the swelling behavior on the skin layer of sulfonated membranes. The degree of swelling of dense sulfonated membranes were determined in distilled water and in aqueous ethanol solution at 25 °C. The weight of dry membrane (Wdry) was first determined. After equilibrium with water or ethanol solution, the fully swollen membrane was wiped with tissue paper and weighed. Since
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the ethanol evaporated very fast, it is difficult to read the real weight directly. The weight of the membrane was measured every 5 s and plotted as a function of time for 30 s after wiping dry. The weight at time zero could be extrapolated and was taken as swollen weight (Wwet) of the membrane. The degree of swelling was calculated by following equation:
Degree of swellingð%Þ ¼ ðW wet W dry Þ=W dry 100%
2.7. Scanning electron microscope (SEM) and atomic force microscope (AFM) Hitachi S4100 SEM and DI 5000 AFM (Digital Instrument, DI 5000) was used to observe the surface and cross-section structures of asymmetric membrane. In SEM studies membranes samples were immersed in liquid nitrogen and then fractured membranes for preparing samples. The SEM samples were then deposited with gold using a sputter coater. 2.8. Light scattering measurement Light transmission experiments have bee preformed to measure the time of onset of liquid–liquid demixing in phase inversion. For the detail experimental setup and procedures, one can refer to the work of Mulder et al. [24]. 3. Results and discussion 3.1. Characteristics of sulfonated polysulfone membranes In sulfonated polysulfone/NMP/water system, the chloroform was added in casting solution to delay the demixing rate of casting solution during the wet phase inversion process. The effect of degree sulfonation on the morphology of sulfonated membranes were shown in Fig. 1. As shown in Fig. 1, the dense skin layer and fingerlike porous sublayer were found in all the range of sulfonation membranes with 8 vol% chloroform addition in casting solution. Thought the similar skin layer structure can be found in those asymmetric membranes. But the finger type sub layer structure grows and forms a macrovoid structure with increasing the degree of substitution of sulfonic group in polysulfone. It is indicted that the amount of grafting sulfonic group on polysulfone grows up the macro pore structure in the sublayer of asymmetric membrane. Pekny et al. [25] indicated that the growth of macrovoid is dominated by both of drag and buoyancy force of inflow demixing fluid in macrovoid during the period of demixing of casting solution. The buoyancy force showed the more influence in the case of low gravity or microgravity. In the normal-gravity (1-g) conditions, the drag force plays the most important role to determine the final pore structure of asymmetric membranes. In the case sulfonated polysulfone/polysulfone blend membranes [26], the sub layer macrovoids and skin layer structure differs significantly from that of pure polysulfone membranes. It was proposed that a competition or parallel processes of poly-
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mer-rich/polymer-lean phase separation and polymer– polymer phase separation during the membrane formation. They also concluded that the liquid–liquid phase separation seems to control the macrovoid distribution, and the polymer–polymer phase separation shaped the void form and wall morphology. The two-phase separations would thus have similar time scale. In this investigation, it can be seem that the polar–polar interaction between casting solution and coagulant (water) increased with increasing the degree of sulfonation of polysulfone in this sulfonated polysulfone/NMP/water system. Therefore, the less drag force of the higher degree of sulfonation casting solution were expected due to the polar–polar interaction between casting solution and coagulant (water). The low drag force in bulk casting solution induced the coagulant water more easy to flow into the initial macrovoid and grows the macrovoid. As shown in Fig. 1, the growth of macrovoid in phase inversion process presents a strong dependence on the degree of sulfonation of polysulfone in casting solution. As can be seen in Fig. 1, the clearly skin layer and typically finger type sublayer structure can be observed in the SEM images of these sulfonated membranes. Generally, permselectivity of asymmetric membranes were determined by the skin layer structure. In wet phase inverse process, the skin layer structure is determined by the demixing behavior during the phase inverse process. The skin layer plays an important role to determine the permselectivity of those sulfonated membranes. By considering the demixing behavior of casting solution, the delay demixing thickens the skin layer in wet phase inversion process. If the demixing behavior was delayed by the various degree of sulfonation of polysulfone, the skin layer thickness will be thickening with the delay demixing behavior in phase inversion process. Therefore, it can be expected that the thickness of skin layer should be affected by the different demixing behavior in phase inversion. The light transmittances measurement was applied to detect the demixing behavior in coagulation bath [20,27,28]. The light transmittance measurements were made for clarifying the demixing behavior of sulfonated polymer solution in water bath at 25 °C. As shown in Fig. 2, the light transmittance measurements of casting solutions were made in water bath at 25 °C. The similarly decrease in light transmittance indicated that the phase inversion of casting solution were almost done in first minute in water bath. The similar light transmittance behavior of those casting solutions implied that the polar difference of sulfonated polymers did not significantly change the demixing behavior. On the other hand, the similar demixing time in water bath will be expected to obtain the similar skin layer structure of those sulfonated membranes. Fig. 3 showed the effect of sulfonic group substitution of polymer on the skin layer thickness of asymmetric membranes. The skin layer thickness slightly increased with increasing the degree of substitution of sulfonated membranes. Those results are consistent with the expectation from light transmittance measurement. It is implied that the change of polymeric polarity in casting solution showed a less influence on the demixing behavior and final thickness of skin layer. Those results also implied that
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Fig. 1. Scanning electron micrographs of cross-section of membrane with different substitution molar ratio of sulfonic group to polysulfone unit (a) 0.25%, (b) 0.42%, (c) 0.58%, (d) 0.75%, (e) 0.92%.
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permeate transport through the membranes will be almost own the same barrier in those sulfonated membranes. However, there are many factors influence permeates transporting through the pervaporation membranes. The polymer packing in the dense skin layer is one of the
0 0.2
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Mole ratio (SO3H/PSF unit) Fig. 3. Effect of substitution molar ratio of sulfonic group to polysulfone unit on the skin layer thickness of sulfonated membrane (a) 0.25, (b) 0.42, (c) 0.58, (d) 0.75, (e) 0.92.
importance factors to dominate the permeation flux of pervaporation membranes. Although the same skin layer thickness was observed with SEM images on those sulfonated membranes. To observe the polymer packing structure on the top layer is helpful to clarify its influence on
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the permeation behavior of sulfonation membranes. The atomic force microscope observations on membrane’s surface were made to investigate the polymer nodule packing on the skin layer, as shown in Fig. 4. It can be observed that the roughness on the skin layer of sulfonated membranes increased with increasing the sulfonation degree of polymer. The changes in roughness of the sulfonated membranes were affected by the sulfonation degree of polysulfone. Although the light transmittance measurements and SEM images observation indicated that there is less influences on the thickness of skin layer. But the sulfonation of polysulfone still induced the morphology change in membrane surface formation. The sulfonated polysulfone may be formed a hydrogen bonding between the sulfonic groups in polymer solution. Therefore, the polymeric chain interaction leads to an enhancement on the viscosity of casting solution and polymer entanglement via a different degree of sulfonation on polysulfone, especially in the polymer rich phase during the phase inversion process. Table 1 showed the effect of degree of sulfonation on the surface roughness of sulfonated membranes. It can
Table 1 Surface roughness values of the asymmetric sulfonated membranes. Degree of sulfonation
Rms (nm)
Ra (nm)
Rmax (nm)
0.25 0.42 0.58 0.92
3.91 5.62 8.24 7.78
3.10 4.52 6.28 5.81
42.65 37.96 60.80 62.21
be seen that the roughness of sulfonated membranes increased with increasing the degree of sulfonation of polysulfone membranes. The roughness change evidenced that the polymer chains interaction become stronger with enriching the polymer concentration in the phase inversion process. The size of polymer nodules and dense skins with the different polymer packing density were affected during the phase inversion in coagulation bath with the increase in degree of sulfonation of polysulfone. Therefore, the sulfonation of polysulfone showed the less change on the macrovoid formation and their skin layer thickness. But the polymer–polymer interaction presented a remark-
Fig. 4. Atomic force micrographs of surface of sulfonated membrane with different substitution molar ratio of sulfonic group to polysulfone unit (a) 0.25, (b) 0.42, (c) 0.58, (d) 0.75, (e) 0.92.
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able influence on the polymer packing on the surface skin layer of asymmetric membranes. 3.2. Water contact angle measurement Fig. 5 illustrates the variation of the contact angles of sulfonated polysulfone membranes. The water contact angle was used as the index of hydrophilicity of sulfonated membranes. The lower water contact angle implied the more the hydrophilic on the surface of sulfonated membrane. As shown in Fig. 5, it can be clearly observed that the contact angle of polysulfone membranes is much higher than the values of sulfonated membranes. Moreover, the contact angles of sulfonated membranes decreased with increasing the degree of sulfonation. Thus it is indicated that the sulfonic group lead-in is helpful to improve the hydrophilicity of polysulfone membranes. Based on the concept of polar–polar interaction, the sulfonic group on the polysulfone membrane benefited to the surface polarity and decreased the water contact angle. In the case of ethanol solution in feed, the polar properties may be helpful for the permselectivity of water molecular transporting through the sulfonated membranes. Based on the above results, they implied that the permeation rate of the sulfonated membranes may be decreased with the higher degree of sulfonation because of the closer polymer packing on the skin layer. On the other hand, the increase in polarity of sulfonated membrane indicated that the water adsorption of sulfonated membrane became easier than that of pure polysulfone membrane with increasing the degree of substitution of sulfonic group on the polymer backbone. 3.3. Effect of sulfonic group substitution on pervaporation properties The effect of sulfonic group substitution on permeation flux and separation factor of sulfonated membranes by pervaporation separation is shown in Fig. 6. It can be seen
that both of the permeation flux and the separation factor increased with increasing sulfonic group substitution on polysulfone at 90% ethanol in feed. As the respect of the observation on the skin layer, the increase in permeation flux is opposite the observation on the morphology of skin layer. As shown in Fig. 1, the sulfonic-substituted membrane presents the same skin layer thickness and almost similar support layer structure. Though the AFM observation indicated that the polymer packing are closer with sulfonic substation in those membranes. Those results did not imply the increase in permeation flux. One of the possible factors to enhance the permeation flux is the increase in swelling property of sulfonated membranes. Due to the permeate swelling the top skin layer, the loosen polymer packing on the top layer was formed the more free volume for permeate to transport through the membranes. In this study, the results of swelling measurement of dense symmetric membrane will apply to illustrate the swelling behavior of skin layer on sulfonated membranes [22]. Fig. 7 showed the effect of the degree of substitution of polysulfone on the swelling properties of dense sulfonated membranes with 90% ethanol solution. It can be seen that the degree of swelling increased with the increase in sulfonation degree of polysulfone. These results evidenced that the increase in permeation flux contributed by the swelling properties of sulfonated membranes. As the increase in permeation flux, permeate coupling effect usually induced the loss of permselectivity of membranes. In this study, the loss of permselectivity did not observed but the sulfonation enlarged the permselectivity of sulfonated membranes. The degree of swelling of sulfonated membranes increased with increasing the degree of sulfonation of membranes. The swelling properties of sulfonated membranes indicated skin layer will be swelled in the feed solution. Thus the swelling skin layer may be enhanced the permeation flux and lose their separation factor. As can be seen in Fig. 6, the separation factor of sulfonated mem800
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Mole ratio (SO3H+/PSF unit) Fig. 5. Effect of the degree of substitution on the water contact angle of sulfonated membranes.
Fig. 6. Effect of the degree of substitution of sulfonated membranes on the pervaporation performance for 90 wt% ethanol solution in feed at 25 °C.
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Diffusion selectivity (DH O /DEthanol)
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Fig. 7. Effect of the degree of substitution on the degree of swelling of sulfonated membranes in 90 wt% ethanol solution at 25 °C.
Fig. 8. Effect of the degree of substitution on the sorption selectivity and diffusion selectivity of sulfonated membranes for 90 wt% ethanol solution in feed at 25 °C.
brane did not decrease but it increased with the degree of sulfonation of those membranes. It is well known that the permeation behavior was almost dominated by the diffusion behavior of permeate in the pervaporation membrane for separating the ethanol/water mixture. Therefore, the understanding of diffusion behavior of permeates in solfonated membrane is helpful for further clarifying the real transport behavior in membranes. Based on the sorption–diffusion mechanism, the diffusion selectivity (ad) can be defined as the ratio of permeation selectivity (ap) and sorption selectivity (as): [29], ap = ad as. In order to calculated the diffusion selectivity through a pervaporation membrane, the permeate content in sulfonated membrane were measured and the sorption selectivity were calculated based on the sorption test. The sorption selectivity of water to ethanol was also calculated by dividing the amount of water content by the amount of ethanol content in sulfonated membranes. Fig. 8 shows that the sorption selectivity of water to ethanol increased with decreasing the substitution of sulfonic group in the membrane. The diffusion selectivity (ad) and sorption selectivity (as) were calculated by above equation. It can be seen that the sorption selectivity slightly decreased and the diffusion selectivity increased as the degree of substitution is increased. The separation factor (permeation selectivity) and sorption selectivity shows a different trend with increasing the content of sulfonic group in polysulfone membranes. The different behavior on the permeate permeation from sorption may be contributed by the permeate diffusion control in the sulfonated membrane which was caused by the interaction between permeate and membrane. Therefore, the increase in permeation rate was due to the high degree of swelling of sulfonated polysulfone membrane which was caused by the hydration effect of sulfonic group with water and ethanol. The high degree of swelling induced a higher permeate content in membranes. Due to the molecular size difference between water and ethanol, the water mole-
cules own a higher diffusion rate than those of ethanol. On the other hand, the hydration of sulfonated membrane increased with increasing the degree of sulfonation. Therefore, the higher degrees of sulfonation enriched water content in membranes and sieving effect of permeates preferred more water molecular diffusing through the membranes. Thus both of the two effects contributed the enhancement on the diffusion selectivity of sulfonated membranes. It was indicated that separation factor of sulfonate membrane were dominated by the permeate diffusion behavior superior than the sorption behavior. 3.4. Pervaporation separation index Effect of ethanol concentration on the PSI value of asymmetric sulfonated polysulfone membrane was shown in Fig. 9. It can be seen that the PSI value of sulfonated membranes was enhanced by the increase in degree of sulfonation. Because of the less change in permeation rate, the increase in PSI value mainly contributed by the enhancement on diffusivity selectivity of sulfonated membranes, which was contributed by the hydration effect in the membrane. It was indicated that the increase in degree of swelling lead to an enhancement in diffusion selectivity of sulfonated membranes. This result implied that the permselectivity of sulfonated membranes were strongly affected by the permeate swelling in the feed solution. 3.5. Effect of feed ethanol concentration on the pervaporation performance Generally, the swelling degree of membranes dominated the free volume for permeate transport through the membrane. It was implied that the change of feed solution concentration lead to a different swelling properties of sulfonated membranes. Fig. 10 showed the effect of feed ethanol concentration on the permeation flux and separation factor of sulfonated membranes. It can be found that
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200
20
0
0
20
40
60
80
Separation factor (α)
Permeation flux (g/m2hr)
the permeation flux slightly decreased with feed ethanol concentration. On the other hand, the increase in feed ethanol concentration increased the separation factor of sulfonated membranes. It is indicated the permselectivity is strongly affected by the feed concentration due to the increase in ethanol concentration. The decrease in permeation flux can be contributed by the decrease hydration effect of water molecules in membranes. The less hydration and low degree of swelling of sulfonated membrane induced an increased in sieving effect in top skin layer and showed a larger separation factor. As shown in previous results, the polymer chains were swelled in higher concentration ethanol solution and offered an excess pathway for permeates to transport through the membrane. On the other hand, the siev-
0 100
Feed Ethanol composition (%) Fig. 10. Effect of the feed ethanol composition on the pervaporation performance of sulfonated membrane with degree of sulfonation 0.92 at 25 °C.
ing effect of swelling membrane also determined the permeate transport through the sulfonated membranes. Thus, it is interesting to clarify that the effect of feed ethanol concentration on the sorption and diffusion behavior of sulfonated membranes. Fig. 11 showed the effect of feed ethanol concentration on the sorption and diffusion selectivity of sulfonated membranes. It can be seen that the sorption selectivity increased with increasing the feed ethanol concentration but the diffusion selectivity first slightly decreased and rapidly increased with the feed ethanol concentration. It was found that the swelling increase did not deteriorate the sorption selectivity of sulfonated membranes. It is proposed that the swelling membrane enlarge the transport path of permeates and the polymer–permeate interaction also play an important role on the adsorbing permeate into sulfonated membrane. It is believed that the sulfonic group played a key role to adsorb the water molecular into membranes. Therefore, the hydration of sulfonic group in membranes contributed the superior water uptake in membrane than the ethanol. Therefore, the sorption selectivity increased with increasing the ethanol concentration in feed solution. On the other hand, the permeate diffusion behavior was dominated by the transport paths in the membranes. Therefore, the enlargement of free volume of membrane increased the diffusion rate both of water and ethanol. If the path way is too small to pass through for water and ethanol molecular, the permeate partition will play the key role to determine the diffusion rate of permeate. As shown in Fig. 10, it was indicated that the water is the superior adsorption permeate in sulfonated membrane and the partition of water in membrane increased with increasing with the feed concentration of ethanol. Based on the result of water sorption in various ethanol concentrations in feed, it was found that the hydration of the water played an important role to determine the permselectivity of sulfonated membranes. 20
20
16
16
12
12
8
8
4
4
0
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Solubility selectivity (SH2O /SEthanol )
70000
Diffusion selectivity (DH2O /DEthanol )
1300
0 100
Feed Ethanol composition (%) Fig. 11. Effect of the feed ethanol composition on the sorption selectivity and diffusion selectivity of sulfonated membranes with degree of sulfonation 0.92 at 25 °C.
S.-H. Chen et al. / European Polymer Journal 45 (2009) 1293–1301
4. Conclusion This paper is successful to prepare the asymmetric sulfonated polysulfone membranes by using wet phase inversion method and showed the good performance for dehydrating water/ethanol mixture by pervaporation. The similar light transmittance of those casting solutions implied that the polarity change of modified polymer did not strongly affect the demixing behavior. On the other hand, the less demixing time in water bath will be lead to a thin skin layer of sulfonated membranes. Base on the AFM observation, the roughness on the skin layer of sulfonated membranes decreased with increasing the sulfonation degree of polymers. It is indicated that the sulfonation affect the surface’s roughness of sulfonated membranes. The substitution of sulfonic group on polysulfone enhanced both of the permeation flux and the separation factor at 90% ethanol in feed. The sulfonated membranes showed well hydrophilic properties and benefit both of their swelling properties and permeate permeation flux. The sorption and permeation measurement showed that the permselectivity of asymmetric membrane was dominated by the permeate diffusion control rather than the permeate sorption in the dense skin layer. The high performance of pervaporation membrane was prepared by phase inverse method by using the polymer sulfonation method. Acknowledgements The authors wish to thank the National Science Council of Republic of China (NSC-93-2216-E-041-003) Taiwan, ROC for their financial support in this research. The authors also want to thank the Center-of-Excellence Program on Membrane Technology, the Ministry of Education, Taiwan, ROC for their financial and technology support in this research. References [1] Xu T, Liu Z, Yang W. Fundamental studies of a new series of anion exchange membranes:membrane prepared from poly(2, 6-dimethyl1, 4-phenylene oxide) (PPO) and triethylamine. J Membr Sci 2005;249:183–91. [2] Yamauchi A, Shin Y, Shinozaki M, Kawabe M. Membrane characteristics of composite collodion membrane IV. Transport properties across blended collodion/Nafion membrane. J Membr Sci 2000;170:1–7. [3] Choi JH, Moon SH. Structural change of ion-exchange membrane surfaces under high electric fields and its effects on membrane properties. J Colloid Interface Sci 2003;265:93–100. [4] Komkova EN, Stamatialis DF, Strathmann H, Wessling M. Anionexchange membranes containing diamines: preparation and stability in alkaline solution. J Membr Sci 2004;244:25–34. [5] Choi EY, Strathmann H, Park JM, Moon SH. Characterization of nonuniformly charged ion-exchange membrane separated by plasmainduced graft polymerization. J Membr Sci 2006;268:165–74. [6] Kujawski W, Staniszewski M, Nguyen TQ. Transport parameters of alcohol vapors through ion-exchange membranes. Sep Purif Technol 2007;57:476–82.
1301
[7] Rao PS, Smith B, Sridhar S, Krishnaiah A. Preparation and performance of poly(vinyl alcohol)/polyethyleneimine blend membranes for the dehydration of 1, 4-dioxane by pervaporation: comparison with glutaraldehyde cross-linked membranes. Sep Purif Technol 2006;48:244–54. [8] Schwarz HH, Malsch G. Polyelectrolyte membranes for aromatic– aliphatic hydrocarbon separation by pervaporation. J Membr Sci 2005;247:143–52. [9] Kusumocahyo SP, Sudoh M. Dehydration of acetic acid by pervaporation with charged membranes. J Membr Sci 1999;161: 77–83. [10] Jiang JS, Greenberg DB, Fried JR. Pervaporation of methanol from a triglyme solution using a Nafion membrane: 2 concentration polarization. J Membr Sci 1997;132:263–70. [11] Ames RL, Way JD, Elizabeth AB. Dehydration of nitric acid using perfluoro carboxylate ionomer membranes. J Membr Sci 2005;249: 65–73. [12] DeLuc NW, Elab YA. NafionÒ/poly(vinyl alcohol) blends: effect of composition and annealing temperature on transport properties. J Membr Sci 2006;282:217–24. [13] Krasemann L, Tieke B. Ultrathin self-assemble polyelectrolyte membranes for pervaporation. J Membr Sci 1998;150:23–30. [14] Toutianoush A, Krasemann L, Tieke B. Polyelectrolyte multilayer membranes for pervaporation separation of alcohol/water mixtures. Colloid Surface A 2002;198–200:881–9. [15] Naidu BV K, Rao KSVK, Aminabhavi TM. Pervaporation separation of water + 1, 4-dioxane and water + tetrahydrofuran mixtures using sodium alginate and its blend membranes with hydroxyethylcellulose-A comparative study. J Membr Sci 2005;260:131– 141. [16] Moon GY, Pal R, Huang RYM. Novel two-ply composite membranes of chitosan and sodium alginate for the pervaporation dehydration of isopropanol and ethanol. J Membr Sci 1999;156: 17–27. [17] Wang XP, Li N, Wang WZ. Pervaporation properties of novel alginate composite membranes for dehydration of organic solvents. J Membr Sci 2001;193:85–95. [18] Yang G, Zhang L, Peng T, Zhong W. Effects of Ca2+ bridge crosslinking on structure and pervaporation of cellulose/alginate blend membranes. J Membr Sci 2000;175:53–60. [19] Smith B, Suhany D, Sridhar S, Ramakrishn M. Separation of organic– organic mixtures by pervaporation – a review. J Membr Sci 2004;241:1–21. [20] Chen SH, Liou RM, Lai JY, Lai CL. Effect of the polarity of additional solvent on membrane formation in polysulfone/N-methyl-2pyrrolidone/water ternary system. Euro Polym J 2007;43: 3997–4007. [21] Zschocke P, Quellmalz D. Novel ion exchange membranes based on an aromatic polysulfone. J Membr Sci 1985;22:325–32. [22] Chen SH, Yu KC, Lin SS, Chang DJ, Liou RM. Pervaporation separation of water/ethanol mixture by sulfonated polysulfone membrane. J Membr Sci 2001;183:29–36. [23] Lee KR, Lai JY. Dehydration of acetic acid–water mixture by pervaporation with a modified poly(4-methyl-1-pentent) membrane. J Polym Res 1994;1:247–54. [24] Mulder M. Basic principles of membrane technology. Dordrecht: Kluwer Academic Publishers; 1991. p. 94–5. [25] Pekny MR, Zartman J, Krantz WB, Greenberg AR, Todd P. Flowvisualization during macrovoid pore formation in dry-cast cellulose acetate membranes. J Membr Sci 2003;211:71–90. [26] Blanco JF, Sublet J, Nguyen QT, Schaetzel P. Formation and morphology studies of different polysulfones-based membranes made by wet phase inversion process. J Membr Sci 2006;283: 27–37. [27] Kim JH, Lee KH. Effect of PEG additive on membrane formation by phase inversion. J Membr Sci 1998;138(2):153–63. [28] Luo RL, Young TH, Sun YM. Structure formation and characterization of EVAL membranes with cosolvent of isopropanol and water. Polymer 2003;44(1):157–66. [29] Huang RYM. Pervaporation separation process. Amsterdam: Elsevier Science publishers B. V.; 1991. 397–398.