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Diffusion dialysis membranes with semi-interpenetrating network for alkali recovery
Journal of Membrane Science 451 (2014) 18–23
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Diffusion dialysis membranes with semi-interpenetrating network for alkali recovery Rui Liu, Liang Wu n, Jiefeng Pan, Chenxiao Jiang, Tongwen Xu n CAS Key Laboratory of Soft Matter Chemistry, Lab of Functional Membranes, School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 July 2013 Received in revised form 9 September 2013 Accepted 13 September 2013 Available online 27 September 2013
Cation exchange membranes based on semi-interpenetrating network (sIPN) are prepared for diffusion dialysis (DD) using poly(vinylidene difluoride) (PVDF) and sodium p-styrenesulfonate (SSS) as starting materials. Five membranes have been prepared by varying the content of PVDF and dosage of crosslinking agent. Ion exchange capacity (IEC), water uptake (WR), swelling resistance and mechanical property of the membranes have been measured. These membranes show excellent thermal and alkali stability due to the high-performance PVDF matrix and sIPN morphology. The membranes have been successfully applied to alkali recovery. The base dialysis coefficients (UOH) are in the range of 0.0008– 0.0061 m/h and the separation factors (S) in the range of 12.0–90.3 at 25–65 1C. Effect of PVDF content and cross-linking degree on ion permeability and selectivity has been discussed. & 2013 Elsevier B.V. All rights reserved.
Keywords: Cation exchange membrane Diffusion dialysis Alkali recovery Semi-interpenetrating network
1. Introduction Alkali waste water, generated by paper, leather, printing and drying, tungsten ore smelting and man-made fiber industries [1], brings serious pollution to the environment. Direct discharge of the waste water will cause water contamination, kill animals and plants, and threaten human health [1]. To date, several methods are available to treat the alkali waste water, including neutralization with acids, concentration and burning (in paper industries) and membrane-related technology [1]. Among these methods, diffusion dialysis (DD) is the most potential one as it is low-cost and environment-friendly. DD is a spontaneous separation process based on ion diffusion from high concentration to low concentration. No additives and external driving force are needed therein. DD has been widely used in recoveries of inorganic acids including sulfuric acid (H2SO4) [2], hydrochloric acid (HCl) [3] and nitric acid (HNO3) [4] and some organic acids [5,6], etc. However, its application in alkali recovery is far away from expectation, due to the lack of high-performance cation exchange membranes. In addition to high base permeability and selectivity, alkali resistance and thermal stability are also important properties required for membranes used in practical DD process [7]. Therefore, developing membranes with good stability and alkali resistance is the key of application of DD in alkali recovery.
n
Corresponding authors. Tel.: þ 86 551 360 1587; fax: þ 86 551 360 1592. E-mail addresses: [email protected] (L. Wu), [email protected] (T. Xu).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.09.021
One solution is to choose a material with high stability as membrane matrix. Poly(vinylidene difluoride) (PVDF) is widely used as membrane material for its excellent thermal and mechanical properties and good chemical resistance [8]. It has been used to prepare various functional membranes, such as hollow fiber membranes [9,10], ion exchange membranes [8,11] and ultrafiltration membranes [12,13]. However, hydrophobicity of PVDF membranes is a serious problem when applied to DD. For this reason, blending with hydrophilic polymers, e.g. poly(sodium p-styrenesulfonate) (PSSS), is commonly employed to improve the hydrophilicity. However, poor miscibility behavior of blending components usually results in serious loss of the hydrophilic component during DD process. Recent publications have shown that construction of semi-interpenetrating polymer network (sIPN) represents a cost-effective way to improve the compatibility between the blend polymers [14,15]. The sIPN is composed of a cross-linked polymer network and a linear polymer immobilized in the cross-linked network. Two polymer chains blended through sIPN technology show good compatibility and the blend materials usually present high mechanical property and good stability [16]. In this work, sIPN is prepared through in situ synthetic pathway. PVDF polymer chains are immobilized homogeneously in the PSSS network. The preparation procedure is illustrated in Fig. 1. Membranes prepared this way are expected to present good performance, including (1) Excellent thermal and alkali stability benefited from PVDF matrix and sIPN morphology; (2) good hydrophilicity inherited from hydrophilic PSSS network; (3) relatively good swelling resistance at high temperature. The sIPN morphology is favorable to membrane dimensional stability [16].
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Table 1 Compositions of five membrane solutions.
Fig. 1. Forming procedure of semi-interpenetrating network (sIPN).
Hydrophobic PVDF chains in this structure also contribute to enhancing the membrane swelling resistance at high temperature. In this work, membranes with different component ratios and cross-linking degree have been prepared. Ion exchange capacity, water uptake, thermal stability, mechanical property and alkali stability of the membranes are characterized. Their application performance in DD process for alkali recovery is also tested. Effect of component ratio and cross-linking degree on properties and performance of membranes is discussed in detail.
2. Experimental 2.1. Materials Poly(vinylidene difluoride) (PVDF) was purchased from Inner Mongolia 3F-Wanhao Fluorine Chemical, Co., Ltd. Divinylbenzene (DVB) was purchased from J&K Scientific Ltd. N,N-dimethylformamide (DMF) and azobisisobutyronitrile (AIBN) were supplied by Sinopharm Chemical Reagent Co., Ltd. AIBN was purified by recrystallization. Sodium p-styrenesulfonate (SSS) was purchased from XZL Bio-Technology Co., Ltd. 2.2. Preparation of the membranes PVDF and SSS were dissolved in 60 mL DMF and the solution was stirred at 60 1C for 2 h to ensure the two components were completely dissolved. After that, DVB and AIBN were added into the solution. Then the solution was stirred at 80 1C for 5 h, under the protection of nitrogen. After reaction, the solution was poured onto a glass plate and dried at 60 1C for 10 h. Then the membrane was obtained and named PxSy-z (x stands for the weight of PVDF in grams, y for the weight of SSS in grams, and z for the volume of DVB in milliliters). Five membranes have been prepared by varying the dosage of reagents and their compositions are presented in Table 1. Our preliminary test shows that, to obtain good membraneforming property and ion-exchange performance, dosage of PVDF is in the range of 3–5 g, when dosage of SSS is 3 g. Dosage of DVB is no more than 0.5 mL to prevent the gel formation during the cross-linking process. Besides, the second and fourth sets of experiments are designed, in which the dosage of DVB, in proportion to dosage of PVDF, is decreased compared to other three sets, to investigate the effect of cross-linking degree on membrane properties. 2.3. Characterization of the membranes 2.3.1. Ion exchange capacity (IEC), water uptake (WR) and swelling resistance in 65 1C hot water Dry membrane samples were accurately weighed and then immersed in 1 mol/L HCl for 12 h to convert to H þ form. The samples were washed by distilled water. Then the samples were immersed in 1 mol/L NaCl for 8 h. The content of HCl produced by
Membrane
PVDF (g)
SSS (g)
AIBN (g)
DVB (mL)
DMF (mL)
P5S3-0.5 P4S3-0.4 P4S3-0.5 P3S3-0.3 P3S3-0.5
5 4 4 3 3
3 3 3 3 3
0.03 0.03 0.03 0.03 0.03
0.5 0.4 0.5 0.3 0.5
60 60 60 60 60
ion exchange was determined through titration with 0.03 mol/ L NaOH. Water uptake was measured by immersing the membrane samples in distilled water for 1 day at 25 1C. All the samples were dried and weighed before the test. After immersion, surfaces of the samples were wiped and the samples were then weighed. WR was calculated as follows: WR ¼
Ww Wd 100% Wd
where Wd is the dry weight of the membranes and Ww is the wet weight of the membranes after immersion in distilled water at 25 1C. Swelling resistance was evaluated by membrane water uptake at 65 1C. Dry membranes were weighed and then immersed in 65 1C hot water. The membranes were taken out at different times and their WR values were recorded. 2.3.2. Thermal stability and mechanical property Thermal stability of the membranes was characterized by thermogravimetric analysis (TGA) using a TG–DTA analyzer (SDT Q600) under N2 flow, with a heating rate of 10 1C/min. Tensile strength (TS) and elongation at break (Eb) were measured by a dynamic mechanical thermal analyzer (DMA Q800) at room temperature. 2.3.3. Alkali stability Dry membrane samples were weighed and then immersed in 65 1C NaOH (2.0 mol/L) for 60 h, then the samples were washed to be neutral and their dry weights were measured. All the membrane samples were washed by distilled water to remove unreacted SSS before test. The ratio of remaining weight and original weight was recorded. IEC and WR at 25 1C of these treated membranes were also measured to evaluate the alkali stability. 2.4. Diffusion dialysis (DD) Diffusion dialysis (DD) test was carried out using a twocompartment cell at different temperatures. The membrane was fixed between the two compartments and the effective area was about 4.5 cm2. The permeate side of the cell was filled with 100 mL mixture solution of NaOH (1.0 mol/L) and Na2WO4 (0.1 mol/L). The diffusate side was filled with 100 mL distilled water. All the membrane samples were conditioned in the feed solution for 1 h before test. The test was allowed for 1 h. Then the concentrations of NaOH in both sides were measured by titration with HCl, and the concentration of Na2WO4 in diffusate side was determined by thiocyanate spectrophotometeric method. The separation factor (S) with respect to one species over another is defined as the ratio of dialysis coefficients (U) of the two species. U was calculated by the following formula: U¼
M AtΔC
ð1Þ
20
R. Liu et al. / Journal of Membrane Science 451 (2014) 18–23
Fig. 2. Ion exchange capacity (IEC) and water uptake (WR) at 25 1C of the membranes. Fig. 3. Swelling resistance in 65 1C hot water of the membranes.
where M is the amount of component transported in moles, A is the effective area in square meters, t is the time in hours, ΔC is the logarithm average concentration between the two chambers in moles per cubic meters and can be calculated as below ΔC ¼
C 0f ðC tf C td Þ ln ½C 0f =ðC tf C td Þ
ð2Þ
where C 0f and C tf are the feed concentrations at time 0 and t, C td is the dialysate concentration at time t.
3. Results and discussion 3.1. IEC, WR and swelling resistance in 65 1C hot water IEC and WR values of these five membranes are shown in Fig. 2. The IEC values, in the range of 1.13–1.70 mmol/g, show a trend of increase when content of PVDF decreases. With the same ratio of PVDF and SSS, increasing amount of cross-linking agent has no effect on IEC values (from P4S3-0.4 to P4S3-0.5), or just enhances the values in small increments (from P3S3-0.3 to P3S3-0.5). With the same dosage of cross-linking agent, WR values increase from 18.9% to 65.9% when the content of PVDF is decreasing. Hydrophobicity is the main problem of PVDF-based membranes when applied to diffusion dialysis. Cross-linked polymer network of PSSS effectively improves the hydrophilicity of PVDF membranes according to these WR values. Unlike IEC, WR is greatly influenced by cross-linking degree. WR values show an obvious downtrend with an increasing amount of cross-linking agent because the polymer network is more compact at higher cross-linking degree. Swelling resistance of these membranes is shown in Fig. 3. After immersion in 65 1C hot water for 36 h, WR values of these five membranes reach the steady state. WR values of these membranes increase not so much compared to that at 25 1C, indicating good stability in hot water. The sIPN morphology plays an important role in enhancing the membrane swelling resistance at high temperature. PVDF immobilized in sIPN benefits the membrane stability due to its hydrophobicity. Cross-linking of PSSS polymer chains also contributes to the membrane stability in hot water despite the good hydrophilicity of PSSS. High crosslinking degree is favorable to the stability of membranes. From P3S3-0.3 to P3S3-0.5, the values of WR reduce from 83.2% to 55.3%. 3.2. Thermal stability and mechanical property TGA graphs of five membranes are shown in Fig. 4. Initial decomposition temperature (IDT) and thermal degradation temperature (Td, defined as the temperature at 5% weight loss)
Fig. 4. TGA graphs of the membranes.
Table 2 Initial decomposition temperature (IDT) and thermal decomposition temperature (Td) of the membranes. Membrane
P5S3-0.5
P4S3-0.4
P4S3-0.5
P3S3-0.3
P3S3-0.5
IDT (1C) Td (1C)
454 467
435 459
456 466
433 459
454 465
obtained based on TGA graphs are given in Table 2. Weight loss before 200 1C is due to evaporation of water and should not be taken into consideration. IDT values of membranes are in the range of 433–456 1C and Td is in the range of 459–467 1C. High IDT and Td values show good thermal stability of these membranes. The thermal stability benefits, on the one hand, from the high-performance PVDF matrix; on the other, from the advantageous sIPN structure. With the same cross-linking degree, the varying ratio of PVDF and SSS has little effect on membrane thermal stability. Decreasing crosslinking degree leads to a reduction of thermal stability. IDT values of P4S3-0.4 and P3S3-0.3 decrease obviously compared to that of P4S3-0.5 and P3S3-0.5, respectively. Tensile strength (TS) and elongation at break (Eb) values of membranes are shown in Table 3. According to the TS and Eb values, these membranes present high tensile strength, but relatively low toughness. With the same dosage of cross-linking agent, TS values first decrease from 32.02 MPa to 28.68 MPa and then increase from 28.68 MPa to 40.11 MPa, as PVDF content decreases. The downtrend appears first caused by decreasing content of
R. Liu et al. / Journal of Membrane Science 451 (2014) 18–23
21
Table 3 Tensile strength (TS) and elongation at break (Eb) of the membranes. Membrane
P5S3-0.5
P4S3-0.4
P4S3-0.5
P3S3-0.3
P3S3-0.5
TS (MPa) Eb (%)
32.02 11.80
25.93 9.00
28.68 5.95
23.54 5.39
40.11 4.76
Table 4 Weight maintenance of the membranes after immersion in 65 1C NaOH solution. Membrane
P5S3-0.5
Weight maintenance (%) 90.9
P4S3-0.4 P4S3-0.5 P3S3-0.3
P3S3-0.5
86.1
92.6
97.4
91.0
Fig. 5. Ion exchange capacity (IEC) and water uptake (WR) at 25 1C of the membranes treated by alkali.
PVDF, while the following increase trend is due to the structure effect. With the same cross-linking degree, decreasing dosage of PVDF is favorable to the homogeneity of sIPN structure and the compatibility of the two polymers, which is advantageous to the membrane mechanical property. Therefore, P3S3-0.5 presents high strength despite its low PVDF content. With the same ratio of PVDF and SSS, high cross-linking degree is advantageous to membrane strength. TS values of P4S3-0.5 and P3S3-0.5 are higher than that of P4S3-0.4 and P3S3-0.3, respectively. Decreasing PVDF content will cause adecline of membrane toughness. On the other hand, the more compact structure of PSSS network formed at higher cross-linking degree is also disadvantageous to the membrane toughness. Effects of decreasing PVDF content and increasing cross-linking degree on Eb values are consistent. Therefore, Eb values decline monotonously from P5S3-0.5 to P3S3-0.5. 3.3. Alkali stability Weight maintenance values of these membranes recorded after immersion in 2.0 mol/L NaOH at 65 1C for 60 h are shown in Table 4. All the membranes maintained integrity in hot NaOH solution, but they turned black and brittle. Pure PVDF membrane underwent the same treatment maintained 99.4% of its original weight and its strength remained good. Blending with PSSS leads to decline of alkali stability. PVDF chains released hydrogen fluoride (HF) during immersion in hot alkali, causing the weight loss of the membranes [17]. Besides, PSSS network also underwent degradation and weight loss since weight maintenance values of these membranes are lower than that of pure PVDF membrane. However, these results are still better than that of poly(vinyl alcohol) (PVA) hybrid membranes for DD [18]. The sIPN morphology benefits chemical resistance and remedies the decline of alkali stability of the blending material to some extent. The polymer network partly degraded in hot alkali solution, but the main chain structure still maintained integrity according to the high weight maintenance, indicating the potential application of these membranes for DD at high temperature. With the same cross-linking degree, the weight maintenance increases first and then decreases as the dosage of PVDF decreases. As mentioned before, decreasing content of PVDF benefits the structure homogeneity. The structure effect has primary effect on alkali stability from P5S3-0.5 to P4S30.5, while content of PVDF begins to play the leading role from P4S3-0.5 to P3S3-0.5. High cross-linking degree is favorable to the stability of membranes, so the weight maintenance of P4S3-0.5 and P3S3-0.5 are higher than that of P4S3-0.4 and P3S3-0.3, respectively. IEC and WR of the treated membranes at 25 1C were also measured and the results are shown in Fig. 5. WR values generally increase after alkali treatment, while IEC values just remain about the same. Loss of functional groups due to the degradation of PSSS
Fig. 6. Dialysis coefficients of sodium hydroxide (NaOH) of the membranes at different temperatures.
network may cause the decrease of IEC, but as the PVDF chains also have a weight loss the IEC values can remain unchanged. The degradation of PSSS network leads to the decrease of cross-linking degree and the decline of structure stability, which result in the increase of WR values, despite the loss of functional groups. However, the membrane degradation under strong alkali condition at high temperature is just to a small extent according to the weight maintenance and does not cause great damage to membrane performance based on the IEC values. There is no significant loss in functional groups and the membrane retains its main structure after alkali treatment. Therefore, these membranes present fairly good alkali stability and can be used in DD process at high temperature. 3.4. Diffusion dialysis (DD) 3.4.1. Dialysis coefficients (UOH and UWO4) Diffusion dialysis tests using these membranes have been carried out at different temperatures (25 1C, 45 1C and 65 1C), and the dialysis coefficients of the base (OH ) and tungstate (WO4 2 ) (UOH and UWO4) are shown in Figs. 6 and 7. UOH values are in the range of 0.0008–0.0061 m/h. These values are not so high compared to that of PVA-based hybrid membranes. UOH values of PVA hybrid membranes can reach 0.0095–0.0208 m/h at 20–40 1C [18], and some even reach 0.010–0.042 m/h at 35–55 1C [19]. However, this result is still acceptable considering the special properties of membranes in this work. Unlike the hydrophilic PVA chains containing many –OH groups, PVDF is highly hydrophobic and contributes little to base permeability. Addition of SSS improves hydrophilicity of the membranes and
22
R. Liu et al. / Journal of Membrane Science 451 (2014) 18–23
Fig. 7. Dialysis coefficients of sodium tungstate (Na2WO4) of the membranes at different temperatures.
facilitates transport of OH , but –SO3 is strongly repulsive to OH , leading to low UOH values. Although the base permeability is relatively low, these PVDF-based membranes can be applied to high temperatures thanks to the good stability. The membranes have been successfully applied to alkali recovery at 65 1C in this work, while the PVA-based hybrid membranes can only be used at lower temperatures since they are less stable [18]. UOH and UWO4 values show a general trend of increase with increasing temperature. Large diffusion coefficient at high temperature facilitates the ion permeability. Moreover, membrane water uptake is increasing at high temperature, making it much easier for OH and WO4 2 to transport through the interstitial area of the polymer network. UOH values generally increase with decreasing content of PVDF at three temperatures, due to increasing IEC and WR values. However, P4S3-0.5 has a lower base permeability compared to that of P5S3-0.5 at 25 1C and 45 1C. It is because decreasing content of PVDF leads to a more homogeneous and compact structure of P4S3-0.5 than that of P5S3-0.5, which has a negative effect on base permeability. Increasing the water uptake of polymer network at high temperature reduces the impact of structure and IEC begins to play the leading role in OH transport, so UOH value of P4S3-0.5 is higher than that of P5S3-0.5 at 65 1C. UWO4 values show the same increase trend with UOH values, when PVDF content is decreasing. P4S3-0.5 and P3S3-0.5 are two exceptions to this trend. The UWO4 value of P3S3-0.5 is lower than that of P5S3-0.5 and P4S3-0.4 at 25 1C, and UWO4 value of P4S3-0.5 is lower than that of P5S3-0.5 in the whole temperature range. It could also be explained by structure effect as mentioned in UOH analysis. Cross-linking degree also has significant effect on ion transport. UOH and UWO4 values of P4S3-0.5 and P3S3-0.5 are generally lower than that of P4S3-0.4 and P3S3-0.3 at three temperatures, respectively. With the same ratio of PVDF and SSS, membranes with high cross-linking degree have a more compact structure, making it more difficult for OH and WO4 2 to transport. Almost all the UOH and UWO4 values follow this rule except for the UWO4 value of P3S3-0.5 at 65 1C, which is higher than that of P3S3-0.3 at the same temperature. This is due to the weakening of structure effect at high temperature and the high IEC of P3S3-0.5 is beneficial to ion transport.
3.4.2. Separation factor (S) Separation factors (S) defined as the ratio of UOH and UWO4 are shown in Fig. 8. The S values, in the range of 12.0–90.3, are better
Fig. 8. Separation factors (S) of the membranes at different temperatures.
than that of some PVA-based hybrid membranes (23.8–54.4 at 20 1C) [18]. The S values usually decrease with increasing temperature. When the temperature increases, diffusivity of OH is enhanced as well as that of WO4 2 , but in different degrees of increase. Increase of WO4 2 diffusion is more significant as its diffusivity is very low at lower temperature for its large volume. Therefore, the S values will drop at high temperature [7]. For this reason, the S values of some PVA-based membranes with a high selectivity at low temperature (52.3–95.7 at 35 1C) reduce to less than 30 at 55 1C [19]. In this work, the S values of these membranes do not change much as the temperature increases, except for P3S3-0.5. In particular, P4S3-0.5 has the S value of 57.6 even at a temperature as high as 65 1C. Membrane structure is the main reason for this result. As mentioned before, hydrophobic PVDF and sIPN structure effectively enhance the membrane swelling resistance at high temperature. When temperature is increasing, the membrane just swells in a small degree. Hydrophobic PVDF polymer chains that immobilized in the PSSS network also play a negative role in ion transport. This unique structure restrains the large increase of WO4 2 diffusion efficiently, even with enhanced diffusivity at high temperature. P3S3-0.5 is an exception to this rule. As its content of PVDF is lowest and cross-linking degree is highest among these five membranes, P3S3-0.5 may have a most compact structure, proven by the low UOH and high S value at 25 1C. Contribution of swelling at high temperature to WO4 2 transport is more significant than that of the other four membranes, leading to the rapid decrease of S values as temperature increases. With the same ratio of PVDF and SSS, increasing the amount of cross-linking agent will enhance the S values of membranes. The S values of P4S3-0.5 and P3S3-0.5 are generally higher than that of P4S3-0.4 and P3S3-0.3 respectively, due to the more compact structure formed at higher cross-linking degree.
4. Conclusions Cation exchange membranes with sIPN structure have been prepared using PVDF and SSS as starting materials through in situ synthetic pathway. The membranes show excellent thermal stability (Td: 459–467 1C) and alkali stability after immersion in 2.0 mol/L NaOH at 65 1C for 60 h (weight maintenance: 86.1–97.4%). The membranes have been successfully applied to DD process for alkali recovery at different temperatures and possess reasonable base permeability. Especially, membrane selectivity generally
R. Liu et al. / Journal of Membrane Science 451 (2014) 18–23
changes little in the whole temperature range due to the advantageous structure of sIPN, indicating the potential application of DD at high temperature. Hence, ion exchange membrane based on sIPN has a promising prospect in DD application for its good thermal stability, mechanical property and alkali resistance.
[8]
[9]
Acknowledgments This project was supported in part by National High Technology Research and Development Program 863 (2012AA03A608), the National Science Foundation of China (Nos. 51273185, 21106140, and 21025626) and the Programs of Anhui Province for Science and Technology (No. 11010202157). References [1] J.Y. Luo, C.M. Wu, T.W. Xu, Y.H. Wu, Diffusion dialysis-concept, principle and applications, Journal of Membrane Science 366 (2011) 1–16. [2] J.K. Jeong, M.S. Kim, B.S. Kim, S.K. Kim, W.B. Kim, J.C. Lee, Recovery of H2SO4 from waste acid solution by a diffusion dialysis method, Journal of Hazardous Materials 124 (2005) 230–235. [3] J. Xu, S.G. Lu, D. Fu, Recovery of hydrochloric acid from the waste acid solution by diffusion dialysis, Journal of Hazardous Materials 165 (2009) 832–837. [4] S.J. Lan, X.M. Wen, Z.H. Zhu, F. Shao, C.L. Zhu, Recycling of spent nitric acid solution from electrodialysis by diffusion dialysis, Desalination 278 (2011) 227–230. [5] A. Narebska, M. Staniszewski, Separation of carboxylic acids from carboxylates by diffusion dialysis, Separation Science and Technology 43 (2008) 490–501. [6] E.G. Akgemci, M. Ersoz, T. Atalay, Transport of formic acid through anion exchange membranes by diffusion dialysis and electro-electro dialysis, Separation Science and Technology 39 (2004) 165–184. [7] X.L. Xiao, C.M. Wu, P. Cui, J.Y. Luo, Y.H. Wu, T.W. Xu, Cation exchange hybrid membranes from SPPO and multi-alkoxy silicon copolymer: preparation,
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Diffusion dialysis membranes with semi-interpenetrating network for alkali recovery Rui Liu, Liang Wu n, Jiefeng Pan, Chenxiao Jiang, Tongwen Xu n CAS Key Laboratory of Soft Matter Chemistry, Lab of Functional Membranes, School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 July 2013 Received in revised form 9 September 2013 Accepted 13 September 2013 Available online 27 September 2013
Cation exchange membranes based on semi-interpenetrating network (sIPN) are prepared for diffusion dialysis (DD) using poly(vinylidene difluoride) (PVDF) and sodium p-styrenesulfonate (SSS) as starting materials. Five membranes have been prepared by varying the content of PVDF and dosage of crosslinking agent. Ion exchange capacity (IEC), water uptake (WR), swelling resistance and mechanical property of the membranes have been measured. These membranes show excellent thermal and alkali stability due to the high-performance PVDF matrix and sIPN morphology. The membranes have been successfully applied to alkali recovery. The base dialysis coefficients (UOH) are in the range of 0.0008– 0.0061 m/h and the separation factors (S) in the range of 12.0–90.3 at 25–65 1C. Effect of PVDF content and cross-linking degree on ion permeability and selectivity has been discussed. & 2013 Elsevier B.V. All rights reserved.
Keywords: Cation exchange membrane Diffusion dialysis Alkali recovery Semi-interpenetrating network
1. Introduction Alkali waste water, generated by paper, leather, printing and drying, tungsten ore smelting and man-made fiber industries [1], brings serious pollution to the environment. Direct discharge of the waste water will cause water contamination, kill animals and plants, and threaten human health [1]. To date, several methods are available to treat the alkali waste water, including neutralization with acids, concentration and burning (in paper industries) and membrane-related technology [1]. Among these methods, diffusion dialysis (DD) is the most potential one as it is low-cost and environment-friendly. DD is a spontaneous separation process based on ion diffusion from high concentration to low concentration. No additives and external driving force are needed therein. DD has been widely used in recoveries of inorganic acids including sulfuric acid (H2SO4) [2], hydrochloric acid (HCl) [3] and nitric acid (HNO3) [4] and some organic acids [5,6], etc. However, its application in alkali recovery is far away from expectation, due to the lack of high-performance cation exchange membranes. In addition to high base permeability and selectivity, alkali resistance and thermal stability are also important properties required for membranes used in practical DD process [7]. Therefore, developing membranes with good stability and alkali resistance is the key of application of DD in alkali recovery.
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Corresponding authors. Tel.: þ 86 551 360 1587; fax: þ 86 551 360 1592. E-mail addresses: [email protected] (L. Wu), [email protected] (T. Xu).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.09.021
One solution is to choose a material with high stability as membrane matrix. Poly(vinylidene difluoride) (PVDF) is widely used as membrane material for its excellent thermal and mechanical properties and good chemical resistance [8]. It has been used to prepare various functional membranes, such as hollow fiber membranes [9,10], ion exchange membranes [8,11] and ultrafiltration membranes [12,13]. However, hydrophobicity of PVDF membranes is a serious problem when applied to DD. For this reason, blending with hydrophilic polymers, e.g. poly(sodium p-styrenesulfonate) (PSSS), is commonly employed to improve the hydrophilicity. However, poor miscibility behavior of blending components usually results in serious loss of the hydrophilic component during DD process. Recent publications have shown that construction of semi-interpenetrating polymer network (sIPN) represents a cost-effective way to improve the compatibility between the blend polymers [14,15]. The sIPN is composed of a cross-linked polymer network and a linear polymer immobilized in the cross-linked network. Two polymer chains blended through sIPN technology show good compatibility and the blend materials usually present high mechanical property and good stability [16]. In this work, sIPN is prepared through in situ synthetic pathway. PVDF polymer chains are immobilized homogeneously in the PSSS network. The preparation procedure is illustrated in Fig. 1. Membranes prepared this way are expected to present good performance, including (1) Excellent thermal and alkali stability benefited from PVDF matrix and sIPN morphology; (2) good hydrophilicity inherited from hydrophilic PSSS network; (3) relatively good swelling resistance at high temperature. The sIPN morphology is favorable to membrane dimensional stability [16].
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Table 1 Compositions of five membrane solutions.
Fig. 1. Forming procedure of semi-interpenetrating network (sIPN).
Hydrophobic PVDF chains in this structure also contribute to enhancing the membrane swelling resistance at high temperature. In this work, membranes with different component ratios and cross-linking degree have been prepared. Ion exchange capacity, water uptake, thermal stability, mechanical property and alkali stability of the membranes are characterized. Their application performance in DD process for alkali recovery is also tested. Effect of component ratio and cross-linking degree on properties and performance of membranes is discussed in detail.
2. Experimental 2.1. Materials Poly(vinylidene difluoride) (PVDF) was purchased from Inner Mongolia 3F-Wanhao Fluorine Chemical, Co., Ltd. Divinylbenzene (DVB) was purchased from J&K Scientific Ltd. N,N-dimethylformamide (DMF) and azobisisobutyronitrile (AIBN) were supplied by Sinopharm Chemical Reagent Co., Ltd. AIBN was purified by recrystallization. Sodium p-styrenesulfonate (SSS) was purchased from XZL Bio-Technology Co., Ltd. 2.2. Preparation of the membranes PVDF and SSS were dissolved in 60 mL DMF and the solution was stirred at 60 1C for 2 h to ensure the two components were completely dissolved. After that, DVB and AIBN were added into the solution. Then the solution was stirred at 80 1C for 5 h, under the protection of nitrogen. After reaction, the solution was poured onto a glass plate and dried at 60 1C for 10 h. Then the membrane was obtained and named PxSy-z (x stands for the weight of PVDF in grams, y for the weight of SSS in grams, and z for the volume of DVB in milliliters). Five membranes have been prepared by varying the dosage of reagents and their compositions are presented in Table 1. Our preliminary test shows that, to obtain good membraneforming property and ion-exchange performance, dosage of PVDF is in the range of 3–5 g, when dosage of SSS is 3 g. Dosage of DVB is no more than 0.5 mL to prevent the gel formation during the cross-linking process. Besides, the second and fourth sets of experiments are designed, in which the dosage of DVB, in proportion to dosage of PVDF, is decreased compared to other three sets, to investigate the effect of cross-linking degree on membrane properties. 2.3. Characterization of the membranes 2.3.1. Ion exchange capacity (IEC), water uptake (WR) and swelling resistance in 65 1C hot water Dry membrane samples were accurately weighed and then immersed in 1 mol/L HCl for 12 h to convert to H þ form. The samples were washed by distilled water. Then the samples were immersed in 1 mol/L NaCl for 8 h. The content of HCl produced by
Membrane
PVDF (g)
SSS (g)
AIBN (g)
DVB (mL)
DMF (mL)
P5S3-0.5 P4S3-0.4 P4S3-0.5 P3S3-0.3 P3S3-0.5
5 4 4 3 3
3 3 3 3 3
0.03 0.03 0.03 0.03 0.03
0.5 0.4 0.5 0.3 0.5
60 60 60 60 60
ion exchange was determined through titration with 0.03 mol/ L NaOH. Water uptake was measured by immersing the membrane samples in distilled water for 1 day at 25 1C. All the samples were dried and weighed before the test. After immersion, surfaces of the samples were wiped and the samples were then weighed. WR was calculated as follows: WR ¼
Ww Wd 100% Wd
where Wd is the dry weight of the membranes and Ww is the wet weight of the membranes after immersion in distilled water at 25 1C. Swelling resistance was evaluated by membrane water uptake at 65 1C. Dry membranes were weighed and then immersed in 65 1C hot water. The membranes were taken out at different times and their WR values were recorded. 2.3.2. Thermal stability and mechanical property Thermal stability of the membranes was characterized by thermogravimetric analysis (TGA) using a TG–DTA analyzer (SDT Q600) under N2 flow, with a heating rate of 10 1C/min. Tensile strength (TS) and elongation at break (Eb) were measured by a dynamic mechanical thermal analyzer (DMA Q800) at room temperature. 2.3.3. Alkali stability Dry membrane samples were weighed and then immersed in 65 1C NaOH (2.0 mol/L) for 60 h, then the samples were washed to be neutral and their dry weights were measured. All the membrane samples were washed by distilled water to remove unreacted SSS before test. The ratio of remaining weight and original weight was recorded. IEC and WR at 25 1C of these treated membranes were also measured to evaluate the alkali stability. 2.4. Diffusion dialysis (DD) Diffusion dialysis (DD) test was carried out using a twocompartment cell at different temperatures. The membrane was fixed between the two compartments and the effective area was about 4.5 cm2. The permeate side of the cell was filled with 100 mL mixture solution of NaOH (1.0 mol/L) and Na2WO4 (0.1 mol/L). The diffusate side was filled with 100 mL distilled water. All the membrane samples were conditioned in the feed solution for 1 h before test. The test was allowed for 1 h. Then the concentrations of NaOH in both sides were measured by titration with HCl, and the concentration of Na2WO4 in diffusate side was determined by thiocyanate spectrophotometeric method. The separation factor (S) with respect to one species over another is defined as the ratio of dialysis coefficients (U) of the two species. U was calculated by the following formula: U¼
M AtΔC
ð1Þ
20
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Fig. 2. Ion exchange capacity (IEC) and water uptake (WR) at 25 1C of the membranes. Fig. 3. Swelling resistance in 65 1C hot water of the membranes.
where M is the amount of component transported in moles, A is the effective area in square meters, t is the time in hours, ΔC is the logarithm average concentration between the two chambers in moles per cubic meters and can be calculated as below ΔC ¼
C 0f ðC tf C td Þ ln ½C 0f =ðC tf C td Þ
ð2Þ
where C 0f and C tf are the feed concentrations at time 0 and t, C td is the dialysate concentration at time t.
3. Results and discussion 3.1. IEC, WR and swelling resistance in 65 1C hot water IEC and WR values of these five membranes are shown in Fig. 2. The IEC values, in the range of 1.13–1.70 mmol/g, show a trend of increase when content of PVDF decreases. With the same ratio of PVDF and SSS, increasing amount of cross-linking agent has no effect on IEC values (from P4S3-0.4 to P4S3-0.5), or just enhances the values in small increments (from P3S3-0.3 to P3S3-0.5). With the same dosage of cross-linking agent, WR values increase from 18.9% to 65.9% when the content of PVDF is decreasing. Hydrophobicity is the main problem of PVDF-based membranes when applied to diffusion dialysis. Cross-linked polymer network of PSSS effectively improves the hydrophilicity of PVDF membranes according to these WR values. Unlike IEC, WR is greatly influenced by cross-linking degree. WR values show an obvious downtrend with an increasing amount of cross-linking agent because the polymer network is more compact at higher cross-linking degree. Swelling resistance of these membranes is shown in Fig. 3. After immersion in 65 1C hot water for 36 h, WR values of these five membranes reach the steady state. WR values of these membranes increase not so much compared to that at 25 1C, indicating good stability in hot water. The sIPN morphology plays an important role in enhancing the membrane swelling resistance at high temperature. PVDF immobilized in sIPN benefits the membrane stability due to its hydrophobicity. Cross-linking of PSSS polymer chains also contributes to the membrane stability in hot water despite the good hydrophilicity of PSSS. High crosslinking degree is favorable to the stability of membranes. From P3S3-0.3 to P3S3-0.5, the values of WR reduce from 83.2% to 55.3%. 3.2. Thermal stability and mechanical property TGA graphs of five membranes are shown in Fig. 4. Initial decomposition temperature (IDT) and thermal degradation temperature (Td, defined as the temperature at 5% weight loss)
Fig. 4. TGA graphs of the membranes.
Table 2 Initial decomposition temperature (IDT) and thermal decomposition temperature (Td) of the membranes. Membrane
P5S3-0.5
P4S3-0.4
P4S3-0.5
P3S3-0.3
P3S3-0.5
IDT (1C) Td (1C)
454 467
435 459
456 466
433 459
454 465
obtained based on TGA graphs are given in Table 2. Weight loss before 200 1C is due to evaporation of water and should not be taken into consideration. IDT values of membranes are in the range of 433–456 1C and Td is in the range of 459–467 1C. High IDT and Td values show good thermal stability of these membranes. The thermal stability benefits, on the one hand, from the high-performance PVDF matrix; on the other, from the advantageous sIPN structure. With the same cross-linking degree, the varying ratio of PVDF and SSS has little effect on membrane thermal stability. Decreasing crosslinking degree leads to a reduction of thermal stability. IDT values of P4S3-0.4 and P3S3-0.3 decrease obviously compared to that of P4S3-0.5 and P3S3-0.5, respectively. Tensile strength (TS) and elongation at break (Eb) values of membranes are shown in Table 3. According to the TS and Eb values, these membranes present high tensile strength, but relatively low toughness. With the same dosage of cross-linking agent, TS values first decrease from 32.02 MPa to 28.68 MPa and then increase from 28.68 MPa to 40.11 MPa, as PVDF content decreases. The downtrend appears first caused by decreasing content of
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Table 3 Tensile strength (TS) and elongation at break (Eb) of the membranes. Membrane
P5S3-0.5
P4S3-0.4
P4S3-0.5
P3S3-0.3
P3S3-0.5
TS (MPa) Eb (%)
32.02 11.80
25.93 9.00
28.68 5.95
23.54 5.39
40.11 4.76
Table 4 Weight maintenance of the membranes after immersion in 65 1C NaOH solution. Membrane
P5S3-0.5
Weight maintenance (%) 90.9
P4S3-0.4 P4S3-0.5 P3S3-0.3
P3S3-0.5
86.1
92.6
97.4
91.0
Fig. 5. Ion exchange capacity (IEC) and water uptake (WR) at 25 1C of the membranes treated by alkali.
PVDF, while the following increase trend is due to the structure effect. With the same cross-linking degree, decreasing dosage of PVDF is favorable to the homogeneity of sIPN structure and the compatibility of the two polymers, which is advantageous to the membrane mechanical property. Therefore, P3S3-0.5 presents high strength despite its low PVDF content. With the same ratio of PVDF and SSS, high cross-linking degree is advantageous to membrane strength. TS values of P4S3-0.5 and P3S3-0.5 are higher than that of P4S3-0.4 and P3S3-0.3, respectively. Decreasing PVDF content will cause adecline of membrane toughness. On the other hand, the more compact structure of PSSS network formed at higher cross-linking degree is also disadvantageous to the membrane toughness. Effects of decreasing PVDF content and increasing cross-linking degree on Eb values are consistent. Therefore, Eb values decline monotonously from P5S3-0.5 to P3S3-0.5. 3.3. Alkali stability Weight maintenance values of these membranes recorded after immersion in 2.0 mol/L NaOH at 65 1C for 60 h are shown in Table 4. All the membranes maintained integrity in hot NaOH solution, but they turned black and brittle. Pure PVDF membrane underwent the same treatment maintained 99.4% of its original weight and its strength remained good. Blending with PSSS leads to decline of alkali stability. PVDF chains released hydrogen fluoride (HF) during immersion in hot alkali, causing the weight loss of the membranes [17]. Besides, PSSS network also underwent degradation and weight loss since weight maintenance values of these membranes are lower than that of pure PVDF membrane. However, these results are still better than that of poly(vinyl alcohol) (PVA) hybrid membranes for DD [18]. The sIPN morphology benefits chemical resistance and remedies the decline of alkali stability of the blending material to some extent. The polymer network partly degraded in hot alkali solution, but the main chain structure still maintained integrity according to the high weight maintenance, indicating the potential application of these membranes for DD at high temperature. With the same cross-linking degree, the weight maintenance increases first and then decreases as the dosage of PVDF decreases. As mentioned before, decreasing content of PVDF benefits the structure homogeneity. The structure effect has primary effect on alkali stability from P5S3-0.5 to P4S30.5, while content of PVDF begins to play the leading role from P4S3-0.5 to P3S3-0.5. High cross-linking degree is favorable to the stability of membranes, so the weight maintenance of P4S3-0.5 and P3S3-0.5 are higher than that of P4S3-0.4 and P3S3-0.3, respectively. IEC and WR of the treated membranes at 25 1C were also measured and the results are shown in Fig. 5. WR values generally increase after alkali treatment, while IEC values just remain about the same. Loss of functional groups due to the degradation of PSSS
Fig. 6. Dialysis coefficients of sodium hydroxide (NaOH) of the membranes at different temperatures.
network may cause the decrease of IEC, but as the PVDF chains also have a weight loss the IEC values can remain unchanged. The degradation of PSSS network leads to the decrease of cross-linking degree and the decline of structure stability, which result in the increase of WR values, despite the loss of functional groups. However, the membrane degradation under strong alkali condition at high temperature is just to a small extent according to the weight maintenance and does not cause great damage to membrane performance based on the IEC values. There is no significant loss in functional groups and the membrane retains its main structure after alkali treatment. Therefore, these membranes present fairly good alkali stability and can be used in DD process at high temperature. 3.4. Diffusion dialysis (DD) 3.4.1. Dialysis coefficients (UOH and UWO4) Diffusion dialysis tests using these membranes have been carried out at different temperatures (25 1C, 45 1C and 65 1C), and the dialysis coefficients of the base (OH ) and tungstate (WO4 2 ) (UOH and UWO4) are shown in Figs. 6 and 7. UOH values are in the range of 0.0008–0.0061 m/h. These values are not so high compared to that of PVA-based hybrid membranes. UOH values of PVA hybrid membranes can reach 0.0095–0.0208 m/h at 20–40 1C [18], and some even reach 0.010–0.042 m/h at 35–55 1C [19]. However, this result is still acceptable considering the special properties of membranes in this work. Unlike the hydrophilic PVA chains containing many –OH groups, PVDF is highly hydrophobic and contributes little to base permeability. Addition of SSS improves hydrophilicity of the membranes and
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Fig. 7. Dialysis coefficients of sodium tungstate (Na2WO4) of the membranes at different temperatures.
facilitates transport of OH , but –SO3 is strongly repulsive to OH , leading to low UOH values. Although the base permeability is relatively low, these PVDF-based membranes can be applied to high temperatures thanks to the good stability. The membranes have been successfully applied to alkali recovery at 65 1C in this work, while the PVA-based hybrid membranes can only be used at lower temperatures since they are less stable [18]. UOH and UWO4 values show a general trend of increase with increasing temperature. Large diffusion coefficient at high temperature facilitates the ion permeability. Moreover, membrane water uptake is increasing at high temperature, making it much easier for OH and WO4 2 to transport through the interstitial area of the polymer network. UOH values generally increase with decreasing content of PVDF at three temperatures, due to increasing IEC and WR values. However, P4S3-0.5 has a lower base permeability compared to that of P5S3-0.5 at 25 1C and 45 1C. It is because decreasing content of PVDF leads to a more homogeneous and compact structure of P4S3-0.5 than that of P5S3-0.5, which has a negative effect on base permeability. Increasing the water uptake of polymer network at high temperature reduces the impact of structure and IEC begins to play the leading role in OH transport, so UOH value of P4S3-0.5 is higher than that of P5S3-0.5 at 65 1C. UWO4 values show the same increase trend with UOH values, when PVDF content is decreasing. P4S3-0.5 and P3S3-0.5 are two exceptions to this trend. The UWO4 value of P3S3-0.5 is lower than that of P5S3-0.5 and P4S3-0.4 at 25 1C, and UWO4 value of P4S3-0.5 is lower than that of P5S3-0.5 in the whole temperature range. It could also be explained by structure effect as mentioned in UOH analysis. Cross-linking degree also has significant effect on ion transport. UOH and UWO4 values of P4S3-0.5 and P3S3-0.5 are generally lower than that of P4S3-0.4 and P3S3-0.3 at three temperatures, respectively. With the same ratio of PVDF and SSS, membranes with high cross-linking degree have a more compact structure, making it more difficult for OH and WO4 2 to transport. Almost all the UOH and UWO4 values follow this rule except for the UWO4 value of P3S3-0.5 at 65 1C, which is higher than that of P3S3-0.3 at the same temperature. This is due to the weakening of structure effect at high temperature and the high IEC of P3S3-0.5 is beneficial to ion transport.
3.4.2. Separation factor (S) Separation factors (S) defined as the ratio of UOH and UWO4 are shown in Fig. 8. The S values, in the range of 12.0–90.3, are better
Fig. 8. Separation factors (S) of the membranes at different temperatures.
than that of some PVA-based hybrid membranes (23.8–54.4 at 20 1C) [18]. The S values usually decrease with increasing temperature. When the temperature increases, diffusivity of OH is enhanced as well as that of WO4 2 , but in different degrees of increase. Increase of WO4 2 diffusion is more significant as its diffusivity is very low at lower temperature for its large volume. Therefore, the S values will drop at high temperature [7]. For this reason, the S values of some PVA-based membranes with a high selectivity at low temperature (52.3–95.7 at 35 1C) reduce to less than 30 at 55 1C [19]. In this work, the S values of these membranes do not change much as the temperature increases, except for P3S3-0.5. In particular, P4S3-0.5 has the S value of 57.6 even at a temperature as high as 65 1C. Membrane structure is the main reason for this result. As mentioned before, hydrophobic PVDF and sIPN structure effectively enhance the membrane swelling resistance at high temperature. When temperature is increasing, the membrane just swells in a small degree. Hydrophobic PVDF polymer chains that immobilized in the PSSS network also play a negative role in ion transport. This unique structure restrains the large increase of WO4 2 diffusion efficiently, even with enhanced diffusivity at high temperature. P3S3-0.5 is an exception to this rule. As its content of PVDF is lowest and cross-linking degree is highest among these five membranes, P3S3-0.5 may have a most compact structure, proven by the low UOH and high S value at 25 1C. Contribution of swelling at high temperature to WO4 2 transport is more significant than that of the other four membranes, leading to the rapid decrease of S values as temperature increases. With the same ratio of PVDF and SSS, increasing the amount of cross-linking agent will enhance the S values of membranes. The S values of P4S3-0.5 and P3S3-0.5 are generally higher than that of P4S3-0.4 and P3S3-0.3 respectively, due to the more compact structure formed at higher cross-linking degree.
4. Conclusions Cation exchange membranes with sIPN structure have been prepared using PVDF and SSS as starting materials through in situ synthetic pathway. The membranes show excellent thermal stability (Td: 459–467 1C) and alkali stability after immersion in 2.0 mol/L NaOH at 65 1C for 60 h (weight maintenance: 86.1–97.4%). The membranes have been successfully applied to DD process for alkali recovery at different temperatures and possess reasonable base permeability. Especially, membrane selectivity generally
R. Liu et al. / Journal of Membrane Science 451 (2014) 18–23
changes little in the whole temperature range due to the advantageous structure of sIPN, indicating the potential application of DD at high temperature. Hence, ion exchange membrane based on sIPN has a promising prospect in DD application for its good thermal stability, mechanical property and alkali resistance.
[8]
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Acknowledgments This project was supported in part by National High Technology Research and Development Program 863 (2012AA03A608), the National Science Foundation of China (Nos. 51273185, 21106140, and 21025626) and the Programs of Anhui Province for Science and Technology (No. 11010202157). References [1] J.Y. Luo, C.M. Wu, T.W. Xu, Y.H. Wu, Diffusion dialysis-concept, principle and applications, Journal of Membrane Science 366 (2011) 1–16. [2] J.K. Jeong, M.S. Kim, B.S. Kim, S.K. Kim, W.B. Kim, J.C. Lee, Recovery of H2SO4 from waste acid solution by a diffusion dialysis method, Journal of Hazardous Materials 124 (2005) 230–235. [3] J. Xu, S.G. Lu, D. Fu, Recovery of hydrochloric acid from the waste acid solution by diffusion dialysis, Journal of Hazardous Materials 165 (2009) 832–837. [4] S.J. Lan, X.M. Wen, Z.H. Zhu, F. Shao, C.L. Zhu, Recycling of spent nitric acid solution from electrodialysis by diffusion dialysis, Desalination 278 (2011) 227–230. [5] A. Narebska, M. Staniszewski, Separation of carboxylic acids from carboxylates by diffusion dialysis, Separation Science and Technology 43 (2008) 490–501. [6] E.G. Akgemci, M. Ersoz, T. Atalay, Transport of formic acid through anion exchange membranes by diffusion dialysis and electro-electro dialysis, Separation Science and Technology 39 (2004) 165–184. [7] X.L. Xiao, C.M. Wu, P. Cui, J.Y. Luo, Y.H. Wu, T.W. Xu, Cation exchange hybrid membranes from SPPO and multi-alkoxy silicon copolymer: preparation,
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