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Preparation and characterizations of novel zwitterionic membranes
Journal of Membrane Science 325 (2008) 495–502
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Preparation and characterizations of novel zwitterionic membranes Junsheng Liu a,b , Yin Zhan a , Tongwen Xu b,∗ , Guoquan Shao a a b
Key Laboratory of Membrane Materials & Processes, Department of Chemical and Materials Engineering, Hefei University, 373 Huangshan Road, Hefei 230022, China Laboratory of Functional Membranes, School of Chemistry and Materials Science, University of Science and Technology of China (USTC), 96 Jinzhai Road, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 4 July 2008 Received in revised form 14 August 2008 Accepted 16 August 2008 Available online 22 August 2008 Keywords: Zwitterionic membranes Zwitterionic copolymers Free radical polymerization Ion-exchange membrane Piezodialysis
a b s t r a c t A novel route was proposed for preparation of zwitterionic membranes, and it comprises (1) free radical polymerization between glycidylmethacrylate (GMA) and acrylic acid (AA) monomers and (2) subsequent ring-opening of epoxide and quaternary amination with trimethylamine hydrochloride. The reaction products were confirmed by FTIR spectra. The cation-exchange capacities (CIECs) and anionexchange capacities (AIECs) of these zwitterionic membranes are within the range of (5.4–9.3) × 10−2 and (30.6–11.1) × 10−2 mmol g−1 , respectively. In addition, the CIEC increases as AA content increases, but the AIEC generally decreases. TGA and DrTGA measurements reveal that their thermal stabilities can arrive at as high as 380 ◦ C. The water uptake is independent of pH. As for membrane potential and streaming potential, they both suggest that these membranes possess the characteristics of anion-exchange membranes and no IEP is observed. Permeation experiments of mixed KCl and glucose solution indicate that the transport behavior of ions is dependent on the membrane’s charge density. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Zwitterionic membranes – containing both anionic and cationic groups in their polymer backbones – have attracted considerable attention due to their pendant-side structures [1–3] and excellent anti-fouling property [4]. Such type of membranes can be potentially applied in such fields as non-linear optical systems [5,6], ionic conduction [7], biomedical systems [8], and chemical separation [4,9]. Among them, the application in chemical industry as ion-exchange membranes has drawn escalating interests in recent years. Since the zwitterionic membranes can prevent the permeation of counter-ion via electrostatic interactions, they are expected to separate and recover valuable metals from industrial waste chemicals and contaminated water [3,10,11]. Currently, various novel routes are developed to prepare zwitterionic polymers [4,12–17]. These routes include (1) ring-opening of lactone reagents, such as 1,3-propanesultone, 1,4-butanesultone or 1,4-butyrolactone [4,12,13], (2) copolymerization between the zwitterionic monomers via free radical polymerization [14–16], and (3) aqueous polymerization between N-carboxymethylN,N-dimethyl-N-allylammonium (CDMA) and acrylic acid (AA) monomer solution to prepare the zwitterionic CDMA/AA copolymer in the presence of cross-linker N,N -methylenebisacrylamide (NMBA) [17]. To date, however, the investigation of the zwitterionic membranes derived from these polymers is insufficient.
∗ Corresponding author. Tel.: +86 551 3601587; fax: +86 551 3601592. E-mail address: [email protected] (T. Xu). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.08.028
As part of our current interest in the investigation of ionexchange membranes, the syntheses and characterizations of novel hybrid zwitterionic polymers and membranes have been reported [9,18–21]. The advantages of such zwitterionic polymers or membranes stem from the pendant-side structure of ion pairs, which are higher in thermal stability and can be used in piezodialysis to separate inorganic salt from mixed electrolyte and non-electrolytes (i.e., desalination). Our continuing interest in such zwitterionic membranes stimulates us to make further efforts. In the previous articles [9,18,20], a series of novel hybrid zwitterionic polymers and membranes were synthesized successfully via sol–gel reaction and zwitterionic process to create ion pairs in the polymer chains. Herein, a new approach is designed for the preparation of zwitterionic membranes and their separation performances will be evaluated by the separation of mixed KCl and glucose solution. Compared with the previous studies [9,18–21], special attention will be paid to the novel route for membrane preparation: free radical polymerization and subsequent ring-opening of epoxide in the prepared copolymer chains. The membranes prepared using such route will exhibit different performances from those previously prepared ones. 2. Experimental 2.1. Materials Glycidylmethacrylate (GMA) and acrylic acid (AA) monomers were purchased from Shanghai Chemical Reagent Co. Ltd. (China). GMA was purified by vacuum distillation (around 2 mmHg) at
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Table 1 Composition of the investigated zwitterionic membranes
2.3. Sample characterizations
Membrane
GMA (mol)
AA (mol)
TMAH (mol)
A B C D
0.1 0.1 0.1 0.1
0.0 0.05 0.1 0.2
0.3 0.3 0.3 0.3
130 ◦ C prior to use. Azobisisobutyronitrile (AIBN) was dissolved in warm methanol (35 ◦ C), re-crystallized in an ice bath, and then dried in a vacuum oven at room temperature prior to use. Other reagents were used as received. Trimethylamine hydrochloride (TMAH) was prepared from trimethylamine and hydrochloric acid (the molar ratio of trimethylamine and hydrochloric acid = 1:1) at 80–90 ◦ C for 30–40 min [22].
2.2. Preparation of zwitterionic membranes Zwitterionic membranes were prepared by free radical polymerization between GMA and AA and subsequent ring-opening of epoxide in the copolymer chains. The procedure is described briefly as follows. Firstly, AIBN was dissolved in 0.1 mol of GMA monomer solution (AIBN can be uniformly dissolved in GMA monomer solution, therefore no other solvent was used in this step), and then they were mixed together with a pure AA monomer and stirred vigorously for 1 h to form a homogeneous solution. Secondly, the solution was coated on non-woven fabric, clamped between two glass plates and placed in an oven to initiate free radical polymerization at 70 ◦ C for 24 h. Finally, the above-prepared base membranes were immersed in a trimethylamine hydrochloride (TMAH) solution for a further 24 h to carry out the ring-opening of epoxide. The unreacted monomer could be removed by the TMAH solution. After that, the prepared membranes were purged with deionized water for 4 h to get rid of the amine impurity. The zwitterionic membranes can thus be obtained. The composition designed for these membranes are listed in Table 1, and the reaction route is shown in Scheme 1.
Scheme 1. The designed reaction route for preparation of zwitterionic membranes: Step 1 is the free radical polymerization between GMA and AA monomers; Step 2 is the ring-opening of epoxide in the copolymer chains.
FTIR spectra of products were obtained using a Bruker Vector22 Fourier transform infrared spectrometer in the region of 4000–400 cm−1 . TGA and DrTGA (the differential of TGA) analyses were performed using a Shimadzu TGA-50H thermogravimetry analyzer under a N2 -flow at a heating rate of 10 ◦ C min−1 (from 30 to 700 ◦ C). The anion-exchange capacities (AIECs) of these zwitterionic membranes were determined by the Mohr method [20], in which the membranes were firstly washed with deionized water for 24 h so as to remove the impurity. The influence of amine impurities on the titration results can thus be highly reduced. Then they were converted to the Cl− form in 0.1 M NaCl aqueous solution, finally back titrated with 0.01N AgNO3 aqueous solution to determine the AIECs. The cation-exchange capacities (CIECs) of these membranes were determined by the similar method, in which the membranes were converted to the Na+ form in 0.1 M aqueous Na2 SO4 solution; and then back titrated with 0.01 M aqueous NaOH solution. The effective membrane area for both AIEC and CIEC measurements was 10.2 cm2 . To reduce the experimental errors, the final results were the mean values of three independent measurements. Water uptake (WR ) was determined by using the conventional process and described in detail in our earlier work [9]. The setup used for measuring membrane potential (MP) was similar to that described in the literature [23,24]. Reversible Ag/AgCl electrodes, placed on both sides of the membrane, were used to measure the electrical potential difference through a digital electrometer (UT30B, China). The effective membrane area was 10.2 cm2 . During testing, the Ag/AgCl electrode on the right cell was contacted with the cathode of the digital electrometer. The right cell was filled with an aqueous KCl solution of the concentration (C0 ) ranging from 0.001 to 0.010 mol dm−3 . On the other hand, the Ag/AgCl electrode on the left cell was contacted with the anode and the left cell was filled with an aqueous KCl solution of the concentration equal to 10C0 . The concentration difference of the aqueous KCl solution in left and right cells was fixed at 10 times so as to produce an ion-concentration gradient for each measurement. Pure water flux of these investigated zwitterionic membranes was determined using a self-made dead-end membrane module [25], which can be described as follows: the pressure difference used in this testing was around 0–1.5 MPa, and the effective membrane area was about 10.2 cm2 ; the total volume of water across the membranes was collected at a given time interval (1–2 min). To reduce the experimental errors, the mean values of thrice testing were selected as the final results. The pure water flux can be calculated as: F = V/(At P), where V is the total volume of water permeated during the experiment; A represents the membrane area; t denotes the operation time; and P is the pressure difference across the membranes. The setup used for measurement of streaming potential (SP) was similar to that described in a previous paper [23]. For this measurement, 0.01 mol dm−3 aqueous KCl solution was used. Separation performances of the zwitterionic membranes were evaluated using an aqueous solution containing electrolyte and non-electrolyte. The setup was described in the literature [26]. The left cell was filled with mixed aqueous solution containing KCl (of different concentrations) and glucose (2 g dm−3 ); meanwhile the right cell was filled with deionized water. Thus it will generate an ion-concentration gradient on both sides of the tested membrane. The concentration of electrolyte on the permeate side (right cell) was measured by using a digital conductivity meter (YEW Model SC-51, Japan). Our preliminary tests showed that, if the concentration of KCl varies from 0.01 to 0.1 mol dm−3 , there exists linear dependence between the conductivity and the concentration of KCl
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Fig. 2. TGA curves of the zwitterionic membranes A–D.
Fig. 1. FTIR spectra of products. (a–d) Products of Step 1, (e) The product of sample (d) derived from of Step 2.
and the linear regression ratio R = 0.9995. The content of glucose on the permeate side was determined by iodimetry [27]. The effective membrane area was 10.2 cm2 . 3. Results and discussion 3.1. Synthesis of zwitterionic membranes As mentioned in the experimental section, the zwitterionic membranes were synthesized via free radical polymerization and subsequent ring-opening of epoxide. Both glycidylmethacrylate (GMA) and acrylic acid (AA) monomers all have unsaturated double bonds in their polymer backbones, so they can be polymerized in the presence of initiator AIBN. As shown in Scheme 1, Step 1 is a free radical polymerization between GMA and AA to produce copolymer precursor. To avoid the rapid self-polymerization of AA monomer, the initiator AIBN was firstly mixed with GMA monomer solution. The composition of the main product in this step depends on the molar ratio of GMA to AA monomer. Step 2 is the ring-opening of epoxide. In this step, the epoxide ring in the copolymer backbone performs the ringopening reaction to produce –N+ (CH3 )3 groups in the copolymer precursor, forming a pendant-side structure. Such copolymer precursor simultaneously contains –COOH and –N+ (CH3 )3 groups, and thus the membrane prepared from such copolymer precursor is zwitterionic.
of the epoxide ring (ring-breathing) while the asymmetric deformation was observed at 730 cm−1 [28,29]. In addition, it can be found that the intensity of the band at 1726 cm−1 increases with an increase in C O content in the samples, which is caused by the grafting of AA. Comparing the curve e (membrane D from the reaction of Step 2) with the curves a–d (products from Step 1), the band at 730 cm−1 disappears and the intensity of peak at 1260–1240 cm−1 decreases dramatically, indicating the opening of epoxide rings. Meanwhile, the band at 3450 cm−1 corresponding to the stretching vibration of –OH groups becomes broader, suggesting an increase of –OH groups in the membranes; which indirectly reveals the occurrence of quaternary amination reaction in Step 2. Based on these changes in absorption peaks, it can be deduced that the ring-opening of epoxide and quaternary amination have occurred during the membrane preparation. 3.3. TGA and DrTGA thermal analyses To investigate the thermal stability and degradation process of these zwitterionic membranes, TGA and DrTGA analyses were performed and presented in Figs. 2 and 3. As shown in Fig. 2, it is interesting to find that the TGA curve for membrane A, which is free of carboxylic groups (curve a), differs from those for membranes B–D, which contain different con-
3.2. FTIR spectra To verify the reactions described in Scheme 1 and investigate the bonding coupling behavior of the samples, FTIR spectra of the step products were probed and shown in Fig. 1. Since the FTIR spectra of the products in Step 2 are similar, only the FTIR spectrum of membrane D is presented in this report. As shown in Fig. 1 (curves a–d), the strong absorption peak near 1726 cm−1 is the stretching vibration of carbonyl group in acrylic acid (C O ). The large band at 3400 cm−1 is the stretching vibration of –OH groups, which may be from the partial ringopening of epoxy groups [22]. The absorption peaks at 2950 cm−1 are ascribed to the C–H-stretching of CH3 and CH2 groups. The peak at 1260–1240 cm−1 can be attributed to the symmetric deformation
Fig. 3. DrTGA curves of the zwitterionic membranes A–D.
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tents of carboxylic groups (curves b–d). Obviously, for membrane A, two thermal degradation steps can be observed; however, for membranes B–D, there are four thermal degradation steps. Corresponding to these degradation steps, several endothermic peaks can be discovered in DrTGA curves. For membrane A, the weight loss below 250 ◦ C is primarily caused by evaporation of organic impurities and the removal of the bonded water in membrane A. The sharp weight loss in the range of 250–400 ◦ C can be attributed to the degradation of organic chains. The slight weight loss beyond 400 ◦ C is primarily the further degradation of organic chains. With regard to membranes B–D, TGA curves have shown similar change trends except that the weight loss decreases slightly as the content of carboxylic groups increases in these membranes. The sharp weight loss below 377 ◦ C is primarily caused by the removal of the bonded water and the breaking of organic chains. A steady weight loss over the temperature range from 377 to 496 ◦ C is attributed to the decomposition of organic groups—chiefly carboxylic groups. The slight weight loss beyond 496 ◦ C can be ascribed to further degradation of organic groups and breaking of copolymer chains. The theoretical explanation on such trends can be attributed to the introduction of ion pairs into the copolymer chains, which favor the increase in Coulombic attraction between the molecules of prepared zwitterionic membranes. As a result, the weight loss of the produced membranes decreases accordingly. Fig. 3 illustrates the DrTGA curves of membranes A–D. For different membranes, several endothermic peaks (i.e., weight loss peak) can be recognized, which correspond to the weight loss stages as demonstrated in TGA curves (cf. Fig. 2). According to the comparison between these endothermic peaks, the first one gradually shifts to higher temperature (from 315, 377, and 389 to 395 ◦ C for membranes A–D, respectively). This finding suggests that the thermal stability of zwitterionic membranes A–D increases slightly with an increase in AA content. To explain the above phenomena, special attention will be given to the electrostatic effect of ion pairs on the performance of zwitterionic membranes. After the ring-opening of epoxide in the copolymer chains, the –N+ (CH3 )3 groups are thus produced. As a result, the positively and negatively charged groups will coexist in the copolymer backbone and be arrayed as pendant-side structure. Such type of zwitterionic molecular structure will contribute to the electrostatic effect. Meanwhile, the Coulombic attraction between the molecular chains increases as the content of carboxylic groups increases, leading to the creation of intimate molecular chains and an increase in thermal stability. Consequently, the position of the first endothermic peak in Fig. 3 shifts to higher temperature. It should be noted that, for zwitterionic membranes B–D, there exists an endothermic peak below 100 ◦ C in DrTGA curves, especially for membrane D. This endothermic peak should be attributed to evaporation of physically absorbed water on the membrane surface due to the strong hydrophilicity of –COOH groups. This is because –COOH groups can form hydrogen bonds with H2 O to increase its hydrophilicity; therefore, there exists physically absorbed water around –COOH groups. In contrast, no such endothermic peak (<100 ◦ C) can be observed in the DrTGA curve of membrane A. This is because membrane A is free of –COOH groups, and thus has less physically absorbed water on its surface; which is confirmed by the water uptake (Fig. 4) data presented below.
Fig. 4. Plot of water uptakes (WR ) of the zwitterionic membranes A–D vs pH.
of (5.4–9.3) × 10−2 mmol g−1 , suggesting that the prepared membranes contain cation-exchange groups [9,25]. Meanwhile, the anion-exchange capacities (AIECs) of these zwitterionic membranes are in the range of (30.6–11.1) × 10−2 mmol g−1 , indicating that these membranes also contain anion-exchange groups [23]. These findings reveal that the membranes prepared from the copolymerization of acrylic acid (AA) and glycidylmethacrylate (GMA) have both cation- and anion-exchange groups; further corroborating the occurrence of ring-opening of epoxide in the copolymer chains. It is interesting to find in Table 2 that the CIECs and AIECs exhibit opposite trends: CIEC increases but AIEC decreases except that of membrane C. The reason lies in the difference in charge density of ionic groups. If the membrane composition is considered, such different change trends can be easily understood. As shown in Table 1, the number of the carboxylic groups in membranes A–D increases as AA content increases. Consequently, their CIECs increase with an increase in –COOH content. Whereas, for AIECs, the relative content of epoxide groups in these membranes decreases as AA content increases. This is because the membrane’s weight increases and thus the content of anion-exchange groups is reduced, leading to a decrease in anion-exchange capacity. Accordingly, the AIECs generally exhibit a downward trend. As for membrane C, it has the lowest AIEC value, which is mainly related to its molecular structure. As shown in Table 1 and Scheme 1, the molar ratio of –COOH to –N+ (CH3 )3 groups in membrane C is equal to 1:1, and the cationic and anionic groups are arrayed symmetrically in the copolymer chains, resulting in a relatively strong attraction between them. Accordingly, anion-exchange behavior of membrane C is weakened. With regard to membranes A, B, and D, however, asymmetric arrangement will be created due to the inequality of cationic and anionic groups in their copolymer chains, leading to a relatively weak repulsion between the ionic groups.
Table 2 The cation- and anion-exchange capacities of the prepared zwitterionic membranes Membrane
CIECs (mmol g−1 ) Practical
3.4. Ion-exchange capacity (IEC) Cation-exchange capacities (CIECs) and anion-exchange capacities (AIECs) of the prepared membranes are shown in Table 2. It can be seen that the cation-exchange capacities (CIECs) of the three types of zwitterionic membranes are within the range
A B C D
– 5.41 × 10−2 7.56 × 10−2 9.28 × 10−2
AIECs (mmol g−1 ) a
Theoretical – 2.22 3.83 6.01
Theoreticala
Practical −2
30.65 × 10 12.39 × 10−2 9.78 × 10−2 11.08 × 10−2
5.29 4.44 3.83 3.00
a Assuming complete conversion of epoxy groups in the zwitterionic copolymers to –N+ (CH3 )3 – after the occurrence of ring-opening reaction.
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Consequently, membranes A, B, and D are higher in AIEC than membrane C. Furthermore, as shown in Table 2, the CIEC values are lower than the AIEC values. Several factors might be responsible for such trend. One is related to the partial dissociation of –COOH groups due to its weaker acidity, resulting in relatively lower amount of cation-exchange. Another can be ascribed to the electrostatic effect between the anionic and cationic groups, which will reduce or block the cationic exchange due to the stronger basicity of –N+ (CH3 )3 groups. The third factor can be attributed to the chain length and distribution of ionic groups. As shown in Scheme 1, the –COOH groups are embedded on the main chain of the copolymer backbone, and the C–C chain is shorter (from the C on –COOH groups to the C on the main chain of copolymer backbone). The –N+ (CH3 )3 groups are situated on the pendant-side chain, and the N–C chain is longer (from the N on –N+ (CH3 )3 groups to the C on the main chain of copolymer backbone). Such array of –N+ (CH3 )3 groups will strengthen the repulsion of H+ ions to the negative charge point of –COOH groups and tend to decrease CIECs. It should be noted that comparing with the theoretical values (cf. Table 2), the practical IECs data are much lower. Several reasons might be responsible for such trend. One can be ascribed to partial GMA units taking part in ring-opening of epoxide and quaternary amination with trimethylamine hydrochloride, leading to some epoxide ring in the epoxy groups remaining intact. Another is related to the unwanted reaction between –OH and –Si–OH or water with epoxy groups as well as the cross-linking of epoxy groups during the heating process. These undesirable reactions will consume a quantity of epoxy groups in the prepared zwitterionic copolymers without producing ion-exchange groups, resulting in the relatively lower ion-exchange capacities (IECs) as reported by Wu et al. [22]. In addition, the formation of electroneutrality between the carboxylic and quaternary amine groups via ionic bonds will also lower the IEC data. 3.5. Water uptake (WR ) To have an insight into the hydrophilicity of the above-prepared zwitterionic membranes at different pH values, the dependence of WR on pH values was determined and showed in Fig. 4. Obviously, if the experimental errors are considered, pH has little effect on water uptake. However, at the same pH value, it can be observed that WR increases with the elevating content of –COOH groups in membranes A–D. Such change trend is consistent with the theoretical one. The interpretation is as follows. It is well accepted that the hydrophilicity of a membrane is dependent on the quantity and category of ionic groups. Since membranes A–D contain ionic groups in their backbones, these ionic groups will affect their affinities for water due to the fabrication of hydrogen bonding between the ionic groups and water. Specially, –COOH groups are stronger in hydrophilicity than –N+ (CH3 )3 groups, which were proved by the TGA and DrTGA measurements (cf. Figs. 2 and 3). Hence, a zwitterionic membrane containing a higher amount of –COOH groups will exhibit larger hydrophilicity than those with a higher amount of –N+ (CH3 )3 groups. By comparing the amount of ionic groups in membranes A–D, it can be found that the amount of –COOH groups in these membranes increases in the following order: membranes A < B < C < D. Whereas, the quantity of –N+ (CH3 )3 groups decreases from membranes A to D except that membrane C has a little lower value than D, i.e., membrane D has the highest amount of –COOH groups and membrane A contains the highest amount of –N+ (CH3 )3 groups but free –COOH groups. Consequently, the hydrophilicity of membrane D (curve d, Fig. 4) is much larger than that of membrane A (curve a, Fig. 4). The results suggest that the hydrophilicity of
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Fig. 5. Plot of membrane potentials (MP) of zwitterionic membranes A–D vs ionconcentration gradient. The lower concentration of KCl solution, C0 , varies from 0.001 to 0.010 mol dm−3 .
zwitterionic membranes can be highly increased by increasing the content of cation-exchange groups. 3.6. Pure water flux To examine the effect of water flux on ion transport in the aqueous KCl solution, pure water fluxes of the investigated zwitterionic membranes were determined. The results were (14.05, 7.36, 5.26, and 4.42) × 10−5 l (m−2 Pa−1 h−1 ) for membranes A–D, respectively. It can be noted that the water flux reveals a downward trend, suggesting that electrolyte solution across membranes A–D will follow such trend [23]. These data can be used to explain the trends of ion transport and membrane potential as shown below. 3.7. Membrane potential (MP) Membrane potential (MP) is a parameter containing the information of ion-transport, so it is taken as one characterization of zwitterionic membranes. Fig. 5 shows the results. For all the membranes and in all the concentrations, the membrane potentials are positive, i.e., these membranes behave like an anion-exchange membrane. Two facts are responsible for the results. Although the zwitterionic membranes contain both cation- and anion-exchange groups, quaternary amine groups are of strong base while –COOH is of weak acid, and thus it ends up behaving like an anion-exchange membrane in terms of MP. Another explanation can be attributed to the difference of net charge between the cationic and anionic groups [30]. As shown in Table 2, AIECs are much higher than CIECs; hence, the membranes will produce net positive charge in the matrix. At the same concentration, MP increases from membranes A to D. If the content of –COOH groups is considered, MP should decrease from membranes A to D with an increase in CIEC since CIEC contributes to negative membrane potential values. A reasonable explanation is that, in our case, CIEC does not play a decisive role on MP because its value is much lesser than AIEC. On the other hand, the rate of ion transport becomes the dominating factor and contributes to such increase in MP at a given concentration. As listed in Section 3.6, pure water flux of these explored membranes decreases from membranes A to D [(14.05, 7.36, 5.26 and 4.42) × 10−5 l m−2 Pa−1 h−1 for membranes A–D, respectively], which gives an indirect evidence to demonstrate the decreasing
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Fig. 6. Plot of streaming potentials of zwitterionic membranes A–D vs pH.
trend in membrane pore size and pore density as reported [23]. Thus, the rate of ion transport decreases from membranes A to D due to convection flow, leading to a larger potential difference between the left and right cells. To sum up, MP demonstrates the opposite trend to that of pure water flux. The effect of external electrolyte solution on MP seems complicated for a given membrane. These potential curves are zig-zag-shaped: rising at lower concentrations with the maximal values at the concentration of about 0.050 mol dm−3 , declining at higher concentrations, but finally rising slightly. It is well known that for single-charged membrane, i.e., cation- and anion-exchange membranes, the absolute value of MP decreases to zero with increasing C0 according to Teorell–Meyer–Sievers (TMS) theory [31–35]. Obviously, the zwitterionic membranes with both negatively and positively charged groups show completely different trends in MP. Though it is difficult to explain such phenomenon within our present knowledge, we still propose the following interpretations. As well known, in a zwitterionic membrane equilibrating with external electrolyte solution, the potential is determined by many factors, such as fixed group content and type, the relative ratio of cationic to anionic groups, and the distribution of the charge groups [32,33]. The trend is too complex to be predicted. But what can be confirmed is that, as the concentration of external electrolyte increases, MP increases in the case of cation-exchange membranes (from negative to positive) and decreases in the case of anion-exchange membranes (from positive to negative) [32]. At low concentrations, an increase in MP depends on the cationic groups; at high concentrations, a decrease in MP depends on the anionic groups in the membrane matrix. Even though CIEC is relatively low, its effect on membrane potential cannot be neglected when the concentration of external solution is also low. 3.8. Streaming potential (SP) vs pH values Since the above-prepared zwitterionic membranes contain both cationic and anionic groups in their copolymer chains, there should exist an isoelectric point (IEP). Such IEP can be determined by streaming potential (SP) [34,35]. Fig. 6 shows the plot of these two variables. Membranes A–D always exhibit positive SP values, and no IEP can be recognized within the pH range of 2–12, suggesting that these zwitterionic membranes behave like anion-exchange membranes [23]. The results are consistent with the membrane potential as discussed above. Meanwhile, for each membrane, the SP curve
exhibits an upward trend at lower pH values up to pH 5.0 and downward trend at higher pH values. At the same pH value, however, SP does not increase with an increase in the content of carboxylic groups, which is different from of the results of MP (Fig. 5). Several factors might be responsible for such trends. One is related to the net charge of the ionic groups in the membranes as discussed above [30]. Another can be ascribed to the Coulombic repulsion between the ionic groups. As shown in Scheme 1, –N+ (CH3 )3 groups primarily are on the pendant-side chain, but –COOH groups are on the main chain and adjacent to –N+ (CH3 )3 groups. The –N+ (CH3 )3 groups will thus prevent the ion-exchange of cations due to the effect of co-ion repulsion. As a result, the cation-exchange behavior will be weakened [33]. In contrast, the anion-exchange behavior is not affected because –COOH groups have shorter chains. The third factor can be attributed to the partial dissociation of –COOH groups, which will act as buffer as pH varies. –COOH groups (anionic groups) are easier to be neutralized, but –N+ (CH3 )3 groups (cationic groups) are insensitive to the change in pH due to the structure of quaternary amine groups as described [2]. The disappearance of IEP might be ascribed to the neutralization of net negative charge (cationic groups) by the positive charges (anionic groups) since AIECs are much higher than CIECs (Table 2). In addition, the position of –COOH and –N+ (CH3 )3 groups and the repulsion of both cationic and anionic ions to the negative charge sites might also be responsible [20]. This finding is in agreement with the observation reported by Nonaka et al. [24], who proposed that the disappearance of IEP within the pH range of 2–12 is due to the shielding of charges of the amphoteric polymer membrane by addition of electrolytes, such as Na2 HPO4 and citric acid. 3.9. Separation of electrolyte from non-electrolyte To investigate the separation performance through these zwitterionic membranes, permeation experiment was conducted. Since the ion transport behaviors are similar as the concentration of KCl varies in the mixed solution, only two testing results (KCl, 0.01 and 0.025 mol dm−3 ; glucose, 2 g dm−3 ) are presented as examples in Fig. 7. As shown in Fig. 7a–b, the permeate concentration in the right side increases as time elapses. Notably, the permeation reaches balance within 10 min in the case of membrane A, indicating the swift transport of K+ and Cl− ions. For membrane B, the balance time is shortened with an increase in the feed concentration of KCl. For example, when KCl concentration in the feed is 0.01 mol dm−3 , the balance time is estimated to be 70 min (cf. Fig. 7a). Nonetheless, when KCl concentration is 0.025 mol dm−3 , the balance time reduces to around 40 min (cf. Fig. 7b). In addition, the permeate flux decreases from membranes A to D. Charge density and membrane pore size are two factors contributing to the above results. As presented in Table 2, the net charge density decreases from membranes A to D (30.65, 6.98, 2.12 and 1.80 × 10−2 mmol g−1 from membrane A to D), leading to distinctive ion fluxes. Membrane A has the largest net charge density, so the ion transport across membrane A will be faster than that across other membranes. Consequently, the balance time for membrane A is shorter than that of membranes B–D. As for membrane B, it is lower in net charge density than membrane A, the balance time for membrane B will be longer than that for membrane A. Meanwhile, such balance time shortens with an increase in the feed concentration of KCl. As concerns membranes C and D, their net charge densities are close and much lower than that of membranes A and B; hence, the balance time is longer than that for membranes A and B. Furthermore, the difference in membrane pore size and pore density also influences the rate of ion transport across these mem-
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(right cell), it can be found that the content of non-electrolyte, glucose in the right cell has the same values; whereas the concentration of electrolyte, KCl in the right cell has different values. These findings reveal that it is the electrical difference that causes salt transport. 4. Conclusions
Fig. 7. Plot of KCl concentration Ci on permeate side vs permeation time. The feed concentration of KCl is: (a) 0.01 mol dm−3 or (b) 0.025 mol dm−3 .
A series of novel zwitterionic membranes were prepared using free radical polymerization and subsequent ring-opening of epoxide. The reaction was mainly conducted between the GMA and AA monomers, and then followed by quaternary amination with TMAH. The CIECs of these zwitterionic membranes are within the range of (5.4–9.3) × 10−2 and their AIECs are in the range of (30.6–11.1) × 10−2 mmol g−1 . Meanwhile, the CIECs increase with an increase in AA content (from membranes A to D), but the AIECs decrease due to the difference in the content of ionic groups in these membranes. TGA and DrTGA measurements indicate that the thermal stabilities of these zwitterionic membranes can reach 380 ◦ C and increase as –COOH content increases. Water uptake is independent of pH. Both membrane potential and streaming potential reveal that these membranes behave anion-exchange-like membranes and no IEP can be observed. The disappearance of IEP can be ascribed to the shielding of weak acid groups by strong basic groups as well as the electrostatic repulsion between the electrolyte and ionic groups grafted on the copolymer chains. When these membranes are used to separate salt from mixed electrolyte and non-electrolyte, higher net charge density will favor the transport of both anions and cations across these membranes due to the existence of simultaneous anionic and cationic groups in the membrane matrix, in which anions or cations can be preferentially transferred, respectively. Further studies on the separation of electrolyte from protein across these zwitterionic membranes are under way. Acknowledgements
branes. As discussed in Section 3.6, from membranes A to D, the water flux is (14.05, 7.36, 5.26 and 4.42) × 10−5 l m−2 Pa−1 h−1 for membranes A–D, respectively. Accordingly, the rate of ion transport reduces from membranes A to D. It should be noted that, for membranes A–D, there exists a slight difference of KCl content in the permeate side, i.e., the KCl content for membranes B–D is higher than that for membrane A when equilibrium time is reached. This can be ascribed to the difference of performances between zwitterionic membranes and positively charged membranes. In comparison, zwitterionic membranes are more suitable for ion enrichment, i.e., the ions can be preferentially transferred across such membrane from the mixed electrolyte and non-electrolyte solution by means of pressure dialysis (or piezodialysis) [9,36]. Although there exists a difference in permeate flux for various membranes, the final concentration of KCl in the permeate side will reach balance and approach to half of the concentration in the feed side. As for the diffusion of glucose, electrical difference has little effect on the transport of glucose since it is in the form of molecule. The contents of glucose on the permeate side are all approximately 0.68 g dm−3 when the permeation time is 20 min for membranes A–D (feed: KCl, 0.01, and 0.05 mol dm−3 ; glucose, 2 g dm−3 ). This indicates that the non-electrolyte transport is independent of the charge density of the prepared zwitterionic membranes. Moreover, it should be pointed out that it seems that both the membrane porosity and the electrical difference can impact the salt permeability. However, if we make a comparison between the contents of electrolyte and non-electrolyte in the permeate side
Financial supports from the Natural Science Foundation of China (No. 20576130), Significant and Key Foundations of Educational Committee of Anhui Province (Nos. ZD2008002 and KJ2008A072), and National Basic Research Program of China (973 program, No. 2009CB623403) are greatly appreciated. Special thanks will be given to Dr. Huang C.H. for proofreading the English. References [1] J. Cardoso, R. Manrique, M. Albores-Velasco, A. Huanosta, Synthesis, characterization, and thermal and dielectric properties of three different methacrylate materials with zwitterionic pendant groups, J. Polym. Sci., Part B: Polym. Phys. 35 (1997) 479–488. [2] A.B. Lowe, C.L. McCormick, Synthesis and solution properties of zwitterionic polymers, Chem. Rev. 102 (2002) 4177–4189. [3] S. Kudaibergenov, W. Jaeger, A. Laschewsky, Polymeric betaines: synthesis, characterization, and application, Adv. Polym. Sci. 201 (2006) 157–224. [4] Q. Sun, Y.L. Su, X.L. Ma, Y.Q. Wang, Z.Y. Jiang, Improved antifouling property of zwitterionic ultrafiltration membrane composed of acrylonitrile and sulfobetaine copolymer, J. Membr. Sci. 285 (2006) 299–305. [5] P. Innocenzi, E. Miorin, G. Brusatin, A. Abbotto, L. Beverina, G.A. Pagani, M. Casalboni, F. Sarcinelli, R. Pizzoferrato, Incorporation of zwitterionic push–pull chromophores into hybrid organic–inorganic matrixes, Chem. Mater. 14 (2002) 3758–3766. [6] G. Brusatin, P. Innocenzi, A. Abbotto, L. Beverina, G.A. Pagani, M. Casalboni, F. Sarcinelli, R. Pizzoferrato, Hybrid organic–inorganic materials containing poled zwitterionic push–pull chromophores, J. Eur. Ceram. Soc. 24 (2004) 1853–1856. [7] W.J. Liang, C.P. Wu, C.Y. Hsu, P.L. Kuo, Synthesis, characterization, and protonconducting properties of organic–inorganic hybrid membranes based on polysiloxane zwitterionomer, J. Polym. Sci., Part A: Polym. Chem. 44 (2006) 3444–3453.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Preparation and characterizations of novel zwitterionic membranes Junsheng Liu a,b , Yin Zhan a , Tongwen Xu b,∗ , Guoquan Shao a a b
Key Laboratory of Membrane Materials & Processes, Department of Chemical and Materials Engineering, Hefei University, 373 Huangshan Road, Hefei 230022, China Laboratory of Functional Membranes, School of Chemistry and Materials Science, University of Science and Technology of China (USTC), 96 Jinzhai Road, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 4 July 2008 Received in revised form 14 August 2008 Accepted 16 August 2008 Available online 22 August 2008 Keywords: Zwitterionic membranes Zwitterionic copolymers Free radical polymerization Ion-exchange membrane Piezodialysis
a b s t r a c t A novel route was proposed for preparation of zwitterionic membranes, and it comprises (1) free radical polymerization between glycidylmethacrylate (GMA) and acrylic acid (AA) monomers and (2) subsequent ring-opening of epoxide and quaternary amination with trimethylamine hydrochloride. The reaction products were confirmed by FTIR spectra. The cation-exchange capacities (CIECs) and anionexchange capacities (AIECs) of these zwitterionic membranes are within the range of (5.4–9.3) × 10−2 and (30.6–11.1) × 10−2 mmol g−1 , respectively. In addition, the CIEC increases as AA content increases, but the AIEC generally decreases. TGA and DrTGA measurements reveal that their thermal stabilities can arrive at as high as 380 ◦ C. The water uptake is independent of pH. As for membrane potential and streaming potential, they both suggest that these membranes possess the characteristics of anion-exchange membranes and no IEP is observed. Permeation experiments of mixed KCl and glucose solution indicate that the transport behavior of ions is dependent on the membrane’s charge density. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Zwitterionic membranes – containing both anionic and cationic groups in their polymer backbones – have attracted considerable attention due to their pendant-side structures [1–3] and excellent anti-fouling property [4]. Such type of membranes can be potentially applied in such fields as non-linear optical systems [5,6], ionic conduction [7], biomedical systems [8], and chemical separation [4,9]. Among them, the application in chemical industry as ion-exchange membranes has drawn escalating interests in recent years. Since the zwitterionic membranes can prevent the permeation of counter-ion via electrostatic interactions, they are expected to separate and recover valuable metals from industrial waste chemicals and contaminated water [3,10,11]. Currently, various novel routes are developed to prepare zwitterionic polymers [4,12–17]. These routes include (1) ring-opening of lactone reagents, such as 1,3-propanesultone, 1,4-butanesultone or 1,4-butyrolactone [4,12,13], (2) copolymerization between the zwitterionic monomers via free radical polymerization [14–16], and (3) aqueous polymerization between N-carboxymethylN,N-dimethyl-N-allylammonium (CDMA) and acrylic acid (AA) monomer solution to prepare the zwitterionic CDMA/AA copolymer in the presence of cross-linker N,N -methylenebisacrylamide (NMBA) [17]. To date, however, the investigation of the zwitterionic membranes derived from these polymers is insufficient.
∗ Corresponding author. Tel.: +86 551 3601587; fax: +86 551 3601592. E-mail address: [email protected] (T. Xu). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.08.028
As part of our current interest in the investigation of ionexchange membranes, the syntheses and characterizations of novel hybrid zwitterionic polymers and membranes have been reported [9,18–21]. The advantages of such zwitterionic polymers or membranes stem from the pendant-side structure of ion pairs, which are higher in thermal stability and can be used in piezodialysis to separate inorganic salt from mixed electrolyte and non-electrolytes (i.e., desalination). Our continuing interest in such zwitterionic membranes stimulates us to make further efforts. In the previous articles [9,18,20], a series of novel hybrid zwitterionic polymers and membranes were synthesized successfully via sol–gel reaction and zwitterionic process to create ion pairs in the polymer chains. Herein, a new approach is designed for the preparation of zwitterionic membranes and their separation performances will be evaluated by the separation of mixed KCl and glucose solution. Compared with the previous studies [9,18–21], special attention will be paid to the novel route for membrane preparation: free radical polymerization and subsequent ring-opening of epoxide in the prepared copolymer chains. The membranes prepared using such route will exhibit different performances from those previously prepared ones. 2. Experimental 2.1. Materials Glycidylmethacrylate (GMA) and acrylic acid (AA) monomers were purchased from Shanghai Chemical Reagent Co. Ltd. (China). GMA was purified by vacuum distillation (around 2 mmHg) at
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Table 1 Composition of the investigated zwitterionic membranes
2.3. Sample characterizations
Membrane
GMA (mol)
AA (mol)
TMAH (mol)
A B C D
0.1 0.1 0.1 0.1
0.0 0.05 0.1 0.2
0.3 0.3 0.3 0.3
130 ◦ C prior to use. Azobisisobutyronitrile (AIBN) was dissolved in warm methanol (35 ◦ C), re-crystallized in an ice bath, and then dried in a vacuum oven at room temperature prior to use. Other reagents were used as received. Trimethylamine hydrochloride (TMAH) was prepared from trimethylamine and hydrochloric acid (the molar ratio of trimethylamine and hydrochloric acid = 1:1) at 80–90 ◦ C for 30–40 min [22].
2.2. Preparation of zwitterionic membranes Zwitterionic membranes were prepared by free radical polymerization between GMA and AA and subsequent ring-opening of epoxide in the copolymer chains. The procedure is described briefly as follows. Firstly, AIBN was dissolved in 0.1 mol of GMA monomer solution (AIBN can be uniformly dissolved in GMA monomer solution, therefore no other solvent was used in this step), and then they were mixed together with a pure AA monomer and stirred vigorously for 1 h to form a homogeneous solution. Secondly, the solution was coated on non-woven fabric, clamped between two glass plates and placed in an oven to initiate free radical polymerization at 70 ◦ C for 24 h. Finally, the above-prepared base membranes were immersed in a trimethylamine hydrochloride (TMAH) solution for a further 24 h to carry out the ring-opening of epoxide. The unreacted monomer could be removed by the TMAH solution. After that, the prepared membranes were purged with deionized water for 4 h to get rid of the amine impurity. The zwitterionic membranes can thus be obtained. The composition designed for these membranes are listed in Table 1, and the reaction route is shown in Scheme 1.
Scheme 1. The designed reaction route for preparation of zwitterionic membranes: Step 1 is the free radical polymerization between GMA and AA monomers; Step 2 is the ring-opening of epoxide in the copolymer chains.
FTIR spectra of products were obtained using a Bruker Vector22 Fourier transform infrared spectrometer in the region of 4000–400 cm−1 . TGA and DrTGA (the differential of TGA) analyses were performed using a Shimadzu TGA-50H thermogravimetry analyzer under a N2 -flow at a heating rate of 10 ◦ C min−1 (from 30 to 700 ◦ C). The anion-exchange capacities (AIECs) of these zwitterionic membranes were determined by the Mohr method [20], in which the membranes were firstly washed with deionized water for 24 h so as to remove the impurity. The influence of amine impurities on the titration results can thus be highly reduced. Then they were converted to the Cl− form in 0.1 M NaCl aqueous solution, finally back titrated with 0.01N AgNO3 aqueous solution to determine the AIECs. The cation-exchange capacities (CIECs) of these membranes were determined by the similar method, in which the membranes were converted to the Na+ form in 0.1 M aqueous Na2 SO4 solution; and then back titrated with 0.01 M aqueous NaOH solution. The effective membrane area for both AIEC and CIEC measurements was 10.2 cm2 . To reduce the experimental errors, the final results were the mean values of three independent measurements. Water uptake (WR ) was determined by using the conventional process and described in detail in our earlier work [9]. The setup used for measuring membrane potential (MP) was similar to that described in the literature [23,24]. Reversible Ag/AgCl electrodes, placed on both sides of the membrane, were used to measure the electrical potential difference through a digital electrometer (UT30B, China). The effective membrane area was 10.2 cm2 . During testing, the Ag/AgCl electrode on the right cell was contacted with the cathode of the digital electrometer. The right cell was filled with an aqueous KCl solution of the concentration (C0 ) ranging from 0.001 to 0.010 mol dm−3 . On the other hand, the Ag/AgCl electrode on the left cell was contacted with the anode and the left cell was filled with an aqueous KCl solution of the concentration equal to 10C0 . The concentration difference of the aqueous KCl solution in left and right cells was fixed at 10 times so as to produce an ion-concentration gradient for each measurement. Pure water flux of these investigated zwitterionic membranes was determined using a self-made dead-end membrane module [25], which can be described as follows: the pressure difference used in this testing was around 0–1.5 MPa, and the effective membrane area was about 10.2 cm2 ; the total volume of water across the membranes was collected at a given time interval (1–2 min). To reduce the experimental errors, the mean values of thrice testing were selected as the final results. The pure water flux can be calculated as: F = V/(At P), where V is the total volume of water permeated during the experiment; A represents the membrane area; t denotes the operation time; and P is the pressure difference across the membranes. The setup used for measurement of streaming potential (SP) was similar to that described in a previous paper [23]. For this measurement, 0.01 mol dm−3 aqueous KCl solution was used. Separation performances of the zwitterionic membranes were evaluated using an aqueous solution containing electrolyte and non-electrolyte. The setup was described in the literature [26]. The left cell was filled with mixed aqueous solution containing KCl (of different concentrations) and glucose (2 g dm−3 ); meanwhile the right cell was filled with deionized water. Thus it will generate an ion-concentration gradient on both sides of the tested membrane. The concentration of electrolyte on the permeate side (right cell) was measured by using a digital conductivity meter (YEW Model SC-51, Japan). Our preliminary tests showed that, if the concentration of KCl varies from 0.01 to 0.1 mol dm−3 , there exists linear dependence between the conductivity and the concentration of KCl
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Fig. 2. TGA curves of the zwitterionic membranes A–D.
Fig. 1. FTIR spectra of products. (a–d) Products of Step 1, (e) The product of sample (d) derived from of Step 2.
and the linear regression ratio R = 0.9995. The content of glucose on the permeate side was determined by iodimetry [27]. The effective membrane area was 10.2 cm2 . 3. Results and discussion 3.1. Synthesis of zwitterionic membranes As mentioned in the experimental section, the zwitterionic membranes were synthesized via free radical polymerization and subsequent ring-opening of epoxide. Both glycidylmethacrylate (GMA) and acrylic acid (AA) monomers all have unsaturated double bonds in their polymer backbones, so they can be polymerized in the presence of initiator AIBN. As shown in Scheme 1, Step 1 is a free radical polymerization between GMA and AA to produce copolymer precursor. To avoid the rapid self-polymerization of AA monomer, the initiator AIBN was firstly mixed with GMA monomer solution. The composition of the main product in this step depends on the molar ratio of GMA to AA monomer. Step 2 is the ring-opening of epoxide. In this step, the epoxide ring in the copolymer backbone performs the ringopening reaction to produce –N+ (CH3 )3 groups in the copolymer precursor, forming a pendant-side structure. Such copolymer precursor simultaneously contains –COOH and –N+ (CH3 )3 groups, and thus the membrane prepared from such copolymer precursor is zwitterionic.
of the epoxide ring (ring-breathing) while the asymmetric deformation was observed at 730 cm−1 [28,29]. In addition, it can be found that the intensity of the band at 1726 cm−1 increases with an increase in C O content in the samples, which is caused by the grafting of AA. Comparing the curve e (membrane D from the reaction of Step 2) with the curves a–d (products from Step 1), the band at 730 cm−1 disappears and the intensity of peak at 1260–1240 cm−1 decreases dramatically, indicating the opening of epoxide rings. Meanwhile, the band at 3450 cm−1 corresponding to the stretching vibration of –OH groups becomes broader, suggesting an increase of –OH groups in the membranes; which indirectly reveals the occurrence of quaternary amination reaction in Step 2. Based on these changes in absorption peaks, it can be deduced that the ring-opening of epoxide and quaternary amination have occurred during the membrane preparation. 3.3. TGA and DrTGA thermal analyses To investigate the thermal stability and degradation process of these zwitterionic membranes, TGA and DrTGA analyses were performed and presented in Figs. 2 and 3. As shown in Fig. 2, it is interesting to find that the TGA curve for membrane A, which is free of carboxylic groups (curve a), differs from those for membranes B–D, which contain different con-
3.2. FTIR spectra To verify the reactions described in Scheme 1 and investigate the bonding coupling behavior of the samples, FTIR spectra of the step products were probed and shown in Fig. 1. Since the FTIR spectra of the products in Step 2 are similar, only the FTIR spectrum of membrane D is presented in this report. As shown in Fig. 1 (curves a–d), the strong absorption peak near 1726 cm−1 is the stretching vibration of carbonyl group in acrylic acid (C O ). The large band at 3400 cm−1 is the stretching vibration of –OH groups, which may be from the partial ringopening of epoxy groups [22]. The absorption peaks at 2950 cm−1 are ascribed to the C–H-stretching of CH3 and CH2 groups. The peak at 1260–1240 cm−1 can be attributed to the symmetric deformation
Fig. 3. DrTGA curves of the zwitterionic membranes A–D.
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tents of carboxylic groups (curves b–d). Obviously, for membrane A, two thermal degradation steps can be observed; however, for membranes B–D, there are four thermal degradation steps. Corresponding to these degradation steps, several endothermic peaks can be discovered in DrTGA curves. For membrane A, the weight loss below 250 ◦ C is primarily caused by evaporation of organic impurities and the removal of the bonded water in membrane A. The sharp weight loss in the range of 250–400 ◦ C can be attributed to the degradation of organic chains. The slight weight loss beyond 400 ◦ C is primarily the further degradation of organic chains. With regard to membranes B–D, TGA curves have shown similar change trends except that the weight loss decreases slightly as the content of carboxylic groups increases in these membranes. The sharp weight loss below 377 ◦ C is primarily caused by the removal of the bonded water and the breaking of organic chains. A steady weight loss over the temperature range from 377 to 496 ◦ C is attributed to the decomposition of organic groups—chiefly carboxylic groups. The slight weight loss beyond 496 ◦ C can be ascribed to further degradation of organic groups and breaking of copolymer chains. The theoretical explanation on such trends can be attributed to the introduction of ion pairs into the copolymer chains, which favor the increase in Coulombic attraction between the molecules of prepared zwitterionic membranes. As a result, the weight loss of the produced membranes decreases accordingly. Fig. 3 illustrates the DrTGA curves of membranes A–D. For different membranes, several endothermic peaks (i.e., weight loss peak) can be recognized, which correspond to the weight loss stages as demonstrated in TGA curves (cf. Fig. 2). According to the comparison between these endothermic peaks, the first one gradually shifts to higher temperature (from 315, 377, and 389 to 395 ◦ C for membranes A–D, respectively). This finding suggests that the thermal stability of zwitterionic membranes A–D increases slightly with an increase in AA content. To explain the above phenomena, special attention will be given to the electrostatic effect of ion pairs on the performance of zwitterionic membranes. After the ring-opening of epoxide in the copolymer chains, the –N+ (CH3 )3 groups are thus produced. As a result, the positively and negatively charged groups will coexist in the copolymer backbone and be arrayed as pendant-side structure. Such type of zwitterionic molecular structure will contribute to the electrostatic effect. Meanwhile, the Coulombic attraction between the molecular chains increases as the content of carboxylic groups increases, leading to the creation of intimate molecular chains and an increase in thermal stability. Consequently, the position of the first endothermic peak in Fig. 3 shifts to higher temperature. It should be noted that, for zwitterionic membranes B–D, there exists an endothermic peak below 100 ◦ C in DrTGA curves, especially for membrane D. This endothermic peak should be attributed to evaporation of physically absorbed water on the membrane surface due to the strong hydrophilicity of –COOH groups. This is because –COOH groups can form hydrogen bonds with H2 O to increase its hydrophilicity; therefore, there exists physically absorbed water around –COOH groups. In contrast, no such endothermic peak (<100 ◦ C) can be observed in the DrTGA curve of membrane A. This is because membrane A is free of –COOH groups, and thus has less physically absorbed water on its surface; which is confirmed by the water uptake (Fig. 4) data presented below.
Fig. 4. Plot of water uptakes (WR ) of the zwitterionic membranes A–D vs pH.
of (5.4–9.3) × 10−2 mmol g−1 , suggesting that the prepared membranes contain cation-exchange groups [9,25]. Meanwhile, the anion-exchange capacities (AIECs) of these zwitterionic membranes are in the range of (30.6–11.1) × 10−2 mmol g−1 , indicating that these membranes also contain anion-exchange groups [23]. These findings reveal that the membranes prepared from the copolymerization of acrylic acid (AA) and glycidylmethacrylate (GMA) have both cation- and anion-exchange groups; further corroborating the occurrence of ring-opening of epoxide in the copolymer chains. It is interesting to find in Table 2 that the CIECs and AIECs exhibit opposite trends: CIEC increases but AIEC decreases except that of membrane C. The reason lies in the difference in charge density of ionic groups. If the membrane composition is considered, such different change trends can be easily understood. As shown in Table 1, the number of the carboxylic groups in membranes A–D increases as AA content increases. Consequently, their CIECs increase with an increase in –COOH content. Whereas, for AIECs, the relative content of epoxide groups in these membranes decreases as AA content increases. This is because the membrane’s weight increases and thus the content of anion-exchange groups is reduced, leading to a decrease in anion-exchange capacity. Accordingly, the AIECs generally exhibit a downward trend. As for membrane C, it has the lowest AIEC value, which is mainly related to its molecular structure. As shown in Table 1 and Scheme 1, the molar ratio of –COOH to –N+ (CH3 )3 groups in membrane C is equal to 1:1, and the cationic and anionic groups are arrayed symmetrically in the copolymer chains, resulting in a relatively strong attraction between them. Accordingly, anion-exchange behavior of membrane C is weakened. With regard to membranes A, B, and D, however, asymmetric arrangement will be created due to the inequality of cationic and anionic groups in their copolymer chains, leading to a relatively weak repulsion between the ionic groups.
Table 2 The cation- and anion-exchange capacities of the prepared zwitterionic membranes Membrane
CIECs (mmol g−1 ) Practical
3.4. Ion-exchange capacity (IEC) Cation-exchange capacities (CIECs) and anion-exchange capacities (AIECs) of the prepared membranes are shown in Table 2. It can be seen that the cation-exchange capacities (CIECs) of the three types of zwitterionic membranes are within the range
A B C D
– 5.41 × 10−2 7.56 × 10−2 9.28 × 10−2
AIECs (mmol g−1 ) a
Theoretical – 2.22 3.83 6.01
Theoreticala
Practical −2
30.65 × 10 12.39 × 10−2 9.78 × 10−2 11.08 × 10−2
5.29 4.44 3.83 3.00
a Assuming complete conversion of epoxy groups in the zwitterionic copolymers to –N+ (CH3 )3 – after the occurrence of ring-opening reaction.
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Consequently, membranes A, B, and D are higher in AIEC than membrane C. Furthermore, as shown in Table 2, the CIEC values are lower than the AIEC values. Several factors might be responsible for such trend. One is related to the partial dissociation of –COOH groups due to its weaker acidity, resulting in relatively lower amount of cation-exchange. Another can be ascribed to the electrostatic effect between the anionic and cationic groups, which will reduce or block the cationic exchange due to the stronger basicity of –N+ (CH3 )3 groups. The third factor can be attributed to the chain length and distribution of ionic groups. As shown in Scheme 1, the –COOH groups are embedded on the main chain of the copolymer backbone, and the C–C chain is shorter (from the C on –COOH groups to the C on the main chain of copolymer backbone). The –N+ (CH3 )3 groups are situated on the pendant-side chain, and the N–C chain is longer (from the N on –N+ (CH3 )3 groups to the C on the main chain of copolymer backbone). Such array of –N+ (CH3 )3 groups will strengthen the repulsion of H+ ions to the negative charge point of –COOH groups and tend to decrease CIECs. It should be noted that comparing with the theoretical values (cf. Table 2), the practical IECs data are much lower. Several reasons might be responsible for such trend. One can be ascribed to partial GMA units taking part in ring-opening of epoxide and quaternary amination with trimethylamine hydrochloride, leading to some epoxide ring in the epoxy groups remaining intact. Another is related to the unwanted reaction between –OH and –Si–OH or water with epoxy groups as well as the cross-linking of epoxy groups during the heating process. These undesirable reactions will consume a quantity of epoxy groups in the prepared zwitterionic copolymers without producing ion-exchange groups, resulting in the relatively lower ion-exchange capacities (IECs) as reported by Wu et al. [22]. In addition, the formation of electroneutrality between the carboxylic and quaternary amine groups via ionic bonds will also lower the IEC data. 3.5. Water uptake (WR ) To have an insight into the hydrophilicity of the above-prepared zwitterionic membranes at different pH values, the dependence of WR on pH values was determined and showed in Fig. 4. Obviously, if the experimental errors are considered, pH has little effect on water uptake. However, at the same pH value, it can be observed that WR increases with the elevating content of –COOH groups in membranes A–D. Such change trend is consistent with the theoretical one. The interpretation is as follows. It is well accepted that the hydrophilicity of a membrane is dependent on the quantity and category of ionic groups. Since membranes A–D contain ionic groups in their backbones, these ionic groups will affect their affinities for water due to the fabrication of hydrogen bonding between the ionic groups and water. Specially, –COOH groups are stronger in hydrophilicity than –N+ (CH3 )3 groups, which were proved by the TGA and DrTGA measurements (cf. Figs. 2 and 3). Hence, a zwitterionic membrane containing a higher amount of –COOH groups will exhibit larger hydrophilicity than those with a higher amount of –N+ (CH3 )3 groups. By comparing the amount of ionic groups in membranes A–D, it can be found that the amount of –COOH groups in these membranes increases in the following order: membranes A < B < C < D. Whereas, the quantity of –N+ (CH3 )3 groups decreases from membranes A to D except that membrane C has a little lower value than D, i.e., membrane D has the highest amount of –COOH groups and membrane A contains the highest amount of –N+ (CH3 )3 groups but free –COOH groups. Consequently, the hydrophilicity of membrane D (curve d, Fig. 4) is much larger than that of membrane A (curve a, Fig. 4). The results suggest that the hydrophilicity of
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Fig. 5. Plot of membrane potentials (MP) of zwitterionic membranes A–D vs ionconcentration gradient. The lower concentration of KCl solution, C0 , varies from 0.001 to 0.010 mol dm−3 .
zwitterionic membranes can be highly increased by increasing the content of cation-exchange groups. 3.6. Pure water flux To examine the effect of water flux on ion transport in the aqueous KCl solution, pure water fluxes of the investigated zwitterionic membranes were determined. The results were (14.05, 7.36, 5.26, and 4.42) × 10−5 l (m−2 Pa−1 h−1 ) for membranes A–D, respectively. It can be noted that the water flux reveals a downward trend, suggesting that electrolyte solution across membranes A–D will follow such trend [23]. These data can be used to explain the trends of ion transport and membrane potential as shown below. 3.7. Membrane potential (MP) Membrane potential (MP) is a parameter containing the information of ion-transport, so it is taken as one characterization of zwitterionic membranes. Fig. 5 shows the results. For all the membranes and in all the concentrations, the membrane potentials are positive, i.e., these membranes behave like an anion-exchange membrane. Two facts are responsible for the results. Although the zwitterionic membranes contain both cation- and anion-exchange groups, quaternary amine groups are of strong base while –COOH is of weak acid, and thus it ends up behaving like an anion-exchange membrane in terms of MP. Another explanation can be attributed to the difference of net charge between the cationic and anionic groups [30]. As shown in Table 2, AIECs are much higher than CIECs; hence, the membranes will produce net positive charge in the matrix. At the same concentration, MP increases from membranes A to D. If the content of –COOH groups is considered, MP should decrease from membranes A to D with an increase in CIEC since CIEC contributes to negative membrane potential values. A reasonable explanation is that, in our case, CIEC does not play a decisive role on MP because its value is much lesser than AIEC. On the other hand, the rate of ion transport becomes the dominating factor and contributes to such increase in MP at a given concentration. As listed in Section 3.6, pure water flux of these explored membranes decreases from membranes A to D [(14.05, 7.36, 5.26 and 4.42) × 10−5 l m−2 Pa−1 h−1 for membranes A–D, respectively], which gives an indirect evidence to demonstrate the decreasing
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Fig. 6. Plot of streaming potentials of zwitterionic membranes A–D vs pH.
trend in membrane pore size and pore density as reported [23]. Thus, the rate of ion transport decreases from membranes A to D due to convection flow, leading to a larger potential difference between the left and right cells. To sum up, MP demonstrates the opposite trend to that of pure water flux. The effect of external electrolyte solution on MP seems complicated for a given membrane. These potential curves are zig-zag-shaped: rising at lower concentrations with the maximal values at the concentration of about 0.050 mol dm−3 , declining at higher concentrations, but finally rising slightly. It is well known that for single-charged membrane, i.e., cation- and anion-exchange membranes, the absolute value of MP decreases to zero with increasing C0 according to Teorell–Meyer–Sievers (TMS) theory [31–35]. Obviously, the zwitterionic membranes with both negatively and positively charged groups show completely different trends in MP. Though it is difficult to explain such phenomenon within our present knowledge, we still propose the following interpretations. As well known, in a zwitterionic membrane equilibrating with external electrolyte solution, the potential is determined by many factors, such as fixed group content and type, the relative ratio of cationic to anionic groups, and the distribution of the charge groups [32,33]. The trend is too complex to be predicted. But what can be confirmed is that, as the concentration of external electrolyte increases, MP increases in the case of cation-exchange membranes (from negative to positive) and decreases in the case of anion-exchange membranes (from positive to negative) [32]. At low concentrations, an increase in MP depends on the cationic groups; at high concentrations, a decrease in MP depends on the anionic groups in the membrane matrix. Even though CIEC is relatively low, its effect on membrane potential cannot be neglected when the concentration of external solution is also low. 3.8. Streaming potential (SP) vs pH values Since the above-prepared zwitterionic membranes contain both cationic and anionic groups in their copolymer chains, there should exist an isoelectric point (IEP). Such IEP can be determined by streaming potential (SP) [34,35]. Fig. 6 shows the plot of these two variables. Membranes A–D always exhibit positive SP values, and no IEP can be recognized within the pH range of 2–12, suggesting that these zwitterionic membranes behave like anion-exchange membranes [23]. The results are consistent with the membrane potential as discussed above. Meanwhile, for each membrane, the SP curve
exhibits an upward trend at lower pH values up to pH 5.0 and downward trend at higher pH values. At the same pH value, however, SP does not increase with an increase in the content of carboxylic groups, which is different from of the results of MP (Fig. 5). Several factors might be responsible for such trends. One is related to the net charge of the ionic groups in the membranes as discussed above [30]. Another can be ascribed to the Coulombic repulsion between the ionic groups. As shown in Scheme 1, –N+ (CH3 )3 groups primarily are on the pendant-side chain, but –COOH groups are on the main chain and adjacent to –N+ (CH3 )3 groups. The –N+ (CH3 )3 groups will thus prevent the ion-exchange of cations due to the effect of co-ion repulsion. As a result, the cation-exchange behavior will be weakened [33]. In contrast, the anion-exchange behavior is not affected because –COOH groups have shorter chains. The third factor can be attributed to the partial dissociation of –COOH groups, which will act as buffer as pH varies. –COOH groups (anionic groups) are easier to be neutralized, but –N+ (CH3 )3 groups (cationic groups) are insensitive to the change in pH due to the structure of quaternary amine groups as described [2]. The disappearance of IEP might be ascribed to the neutralization of net negative charge (cationic groups) by the positive charges (anionic groups) since AIECs are much higher than CIECs (Table 2). In addition, the position of –COOH and –N+ (CH3 )3 groups and the repulsion of both cationic and anionic ions to the negative charge sites might also be responsible [20]. This finding is in agreement with the observation reported by Nonaka et al. [24], who proposed that the disappearance of IEP within the pH range of 2–12 is due to the shielding of charges of the amphoteric polymer membrane by addition of electrolytes, such as Na2 HPO4 and citric acid. 3.9. Separation of electrolyte from non-electrolyte To investigate the separation performance through these zwitterionic membranes, permeation experiment was conducted. Since the ion transport behaviors are similar as the concentration of KCl varies in the mixed solution, only two testing results (KCl, 0.01 and 0.025 mol dm−3 ; glucose, 2 g dm−3 ) are presented as examples in Fig. 7. As shown in Fig. 7a–b, the permeate concentration in the right side increases as time elapses. Notably, the permeation reaches balance within 10 min in the case of membrane A, indicating the swift transport of K+ and Cl− ions. For membrane B, the balance time is shortened with an increase in the feed concentration of KCl. For example, when KCl concentration in the feed is 0.01 mol dm−3 , the balance time is estimated to be 70 min (cf. Fig. 7a). Nonetheless, when KCl concentration is 0.025 mol dm−3 , the balance time reduces to around 40 min (cf. Fig. 7b). In addition, the permeate flux decreases from membranes A to D. Charge density and membrane pore size are two factors contributing to the above results. As presented in Table 2, the net charge density decreases from membranes A to D (30.65, 6.98, 2.12 and 1.80 × 10−2 mmol g−1 from membrane A to D), leading to distinctive ion fluxes. Membrane A has the largest net charge density, so the ion transport across membrane A will be faster than that across other membranes. Consequently, the balance time for membrane A is shorter than that of membranes B–D. As for membrane B, it is lower in net charge density than membrane A, the balance time for membrane B will be longer than that for membrane A. Meanwhile, such balance time shortens with an increase in the feed concentration of KCl. As concerns membranes C and D, their net charge densities are close and much lower than that of membranes A and B; hence, the balance time is longer than that for membranes A and B. Furthermore, the difference in membrane pore size and pore density also influences the rate of ion transport across these mem-
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(right cell), it can be found that the content of non-electrolyte, glucose in the right cell has the same values; whereas the concentration of electrolyte, KCl in the right cell has different values. These findings reveal that it is the electrical difference that causes salt transport. 4. Conclusions
Fig. 7. Plot of KCl concentration Ci on permeate side vs permeation time. The feed concentration of KCl is: (a) 0.01 mol dm−3 or (b) 0.025 mol dm−3 .
A series of novel zwitterionic membranes were prepared using free radical polymerization and subsequent ring-opening of epoxide. The reaction was mainly conducted between the GMA and AA monomers, and then followed by quaternary amination with TMAH. The CIECs of these zwitterionic membranes are within the range of (5.4–9.3) × 10−2 and their AIECs are in the range of (30.6–11.1) × 10−2 mmol g−1 . Meanwhile, the CIECs increase with an increase in AA content (from membranes A to D), but the AIECs decrease due to the difference in the content of ionic groups in these membranes. TGA and DrTGA measurements indicate that the thermal stabilities of these zwitterionic membranes can reach 380 ◦ C and increase as –COOH content increases. Water uptake is independent of pH. Both membrane potential and streaming potential reveal that these membranes behave anion-exchange-like membranes and no IEP can be observed. The disappearance of IEP can be ascribed to the shielding of weak acid groups by strong basic groups as well as the electrostatic repulsion between the electrolyte and ionic groups grafted on the copolymer chains. When these membranes are used to separate salt from mixed electrolyte and non-electrolyte, higher net charge density will favor the transport of both anions and cations across these membranes due to the existence of simultaneous anionic and cationic groups in the membrane matrix, in which anions or cations can be preferentially transferred, respectively. Further studies on the separation of electrolyte from protein across these zwitterionic membranes are under way. Acknowledgements
branes. As discussed in Section 3.6, from membranes A to D, the water flux is (14.05, 7.36, 5.26 and 4.42) × 10−5 l m−2 Pa−1 h−1 for membranes A–D, respectively. Accordingly, the rate of ion transport reduces from membranes A to D. It should be noted that, for membranes A–D, there exists a slight difference of KCl content in the permeate side, i.e., the KCl content for membranes B–D is higher than that for membrane A when equilibrium time is reached. This can be ascribed to the difference of performances between zwitterionic membranes and positively charged membranes. In comparison, zwitterionic membranes are more suitable for ion enrichment, i.e., the ions can be preferentially transferred across such membrane from the mixed electrolyte and non-electrolyte solution by means of pressure dialysis (or piezodialysis) [9,36]. Although there exists a difference in permeate flux for various membranes, the final concentration of KCl in the permeate side will reach balance and approach to half of the concentration in the feed side. As for the diffusion of glucose, electrical difference has little effect on the transport of glucose since it is in the form of molecule. The contents of glucose on the permeate side are all approximately 0.68 g dm−3 when the permeation time is 20 min for membranes A–D (feed: KCl, 0.01, and 0.05 mol dm−3 ; glucose, 2 g dm−3 ). This indicates that the non-electrolyte transport is independent of the charge density of the prepared zwitterionic membranes. Moreover, it should be pointed out that it seems that both the membrane porosity and the electrical difference can impact the salt permeability. However, if we make a comparison between the contents of electrolyte and non-electrolyte in the permeate side
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