Facile synthesis and properties of a cation exchange membrane with bifunctional groups prepared by pre-irradiation graft copolymerization

A new type of a cation exchange membrane named ETFE-g-poly(AA-co-SSS) with bifunctional groups was synthesized by a one-step method. Its preparation by an electron beam-induced pre-irradiation grafting method and the effects of reaction temperature, monomer concentration, pH value of the grafting solution, storage time and temperature of the irradiated poly(ethylene-alt-tetrafluoroethylene) (ETFE) films on the grafting yield were studied. A total concentration of 2 mol L−1 of monomers was found to be beneficial for acrylic acid (AA) and sodium styrene sulfonate (SSS) co-grafting onto the ETFE films. Infrared spectroscopic analysis of the grafted membrane confirmed the existence of sulfonate and carboxylic acid groups. The contact angle of the grafted membrane decreased from 94.3 to 46.7° with the increase in grafting yield. The higher the grafting yield, the faster the response and recovery rate with respect to humidity. AFM images showed that the diameter of the grafted chains on the surface of ETFE membranes was about 30 nm. The voltage of the grafted membrane was stable after 100 cycles of charge–discharge; thus, the prepared membranes have great potentials to be used as separators in secondary batteries.


Introduction
Traditional cation exchange membranes are mostly prepared by physical blending or chemical copolymerization. Since 1971, the DuPont company has produced commercial ion exchange membranes named Naon membranes with sulfonate groups. The membranes have low resistance and high stability, but their degradation rate is accelerated when the cathodic potential is relatively high. 1 In addition, Naon membranes also have the defects of high cost, low conductivity and poor resistance to methanol at high temperature. 2 Radiation-induced gra polymerization is a promising method to introduce desirable properties onto polymers because of its simplicity in controlling the graing parameters. Monomer or monomer mixtures with functional groups are graed onto ready-made lms so that the lm shaping process can be excluded as the preparation begins with a polymer already in the lm form. 3 All membranes synthesized by the radiation-induced graing technique have special advantages over other membranes prepared by blending or polymerization modication. Membranes prepared by this technique have wide applications in proton exchange membrane fuel cells, 4 secondary batteries, 5,6 and treatment of wastewater. 7 Although various efforts have been made to prepare highperformance membranes for ion exchange, 8,9 so far, only few radiation-graed ion exchange membranes have been successfully applied in batteries or fuel cells for large-scale commercialization because of a few key challenges; these include (1) radiation degradation of polyolen and monomer polymerization initiated by radicals produced by radiation, and (2) the high cost of certain starting materials and irradiation facility. In China, for commercial development of ion exchange membranes, the membranes are mainly prepared by copolymerization induced by a chemical initiator. Researchers engaged in the membrane manufacturing technology have a background in polymer chemistry. In the next 5 to 10 years, we strongly believe that with continuous research in this eld, great progress will be made towards the manufacture of radiation-graed cation membranes and their applications in secondary batteries or fuel cells.
We have reported that the addition of acid plays a signicant role in radiation-induced graing of vinyl monomers such as sodium styrene sulfonate (SSS) and acrylic acid (AA) onto polyethylene. 10 Based on our research, Nasef et al. further prepared proton exchange membranes containing only sulfonic acid groups by adding an aqueous acid solution to the graing mixture. 11 However, membranes with -SO 3 H groups can swell in water to a great extent due to their strong hydrophilicity, whereas membranes containing -COOH groups, which are weakly acidic and less hydrophilic, swell to a lesser extent in water. Patent applications and research done by Henkensmeier et al. have demonstrated that membranes having two different ion-exchange groups perform better than membranes having one type of an ion-exchange group. 12,13 Our attempt aims at synthesizing membranes having more than one type of ionexchange group so as to achieve better performance in secondary batteries or humidity sensors. In our previous study, we carried out graing of AA and SSS comonomers onto high density polyethylene (HDPE) by the pre-irradiation technique, and we studied the effects of various reaction parameters on the degree of graing. 14, 15 We used electron spin-resonance spectroscopy to study the stability of radicals formed on HDPE and poly(ethylene-alt-tetrauoroethylene) (ETFE) lms by electron beam irradiation. The results indicated that the trapped radicals at the crystalline region of ETFE were more stable than the radicals of HDPE under the same absorption doses and storage conditions. A further improvement is expected by changing the matrix from HDPE to partially uorinated ETFE, which is commercially available for approximately 3V per m 2 . In addition, our research team has been devoted to the development of low-cost and anti-degradation ion-exchange membranes by radiation-induced graing methods. To improve the stability of the modied polymer, a partially uorinated lm has been chosen as the polymer matrix. 16 Several advantages over per-uorinated lms are identied such as superior mechanical properties, 17 faster graing kinetics due to higher monomer compatibility and less radiation-induced damage. 18,19 Shkolnik et al. reported the difficulties in direct graing of sodium styrene sulfonate (SSS) onto uoropolymers by radiation-induced graing because of the incompatibility between the highly ionized sulfonic acid groups and its hydration sphere and the hydrophobic polymer matrix. 20 Hence, a two-step graing method was reported for the preparation of ion-exchange membranes bearing sulfonic acid groups. [21][22][23][24] First, styrene or substituted styrene monomers was graed onto the polymer lms and then, the graed membrane was immersed into chlorosulfonic acid or sulfuric acid at a higher temperature to introduce sulfonic groups into the benzene ring. [25][26][27] The physical strength of the membranes deteriorated because of strongly acidic reaction conditions. Therefore, to simplify the preparation process and to improve the mechanical properties of the obtained membranes, we designed a comonomer gra system to directly introduce -SO 3 H and -COOH groups onto uorinated lms by a one-step graing method. Hence, a detailed study on the inuence of graing parameters was performed for the graing of acrylic acid (AA) and SSS onto ETFE lms. The introduction of both -COOH and -SO 3 H groups onto ETFE lms further improved the swelling property and sensitivity to humidity of the graed membranes.

Materials
ETFE lms of 50 mm were purchased from Nowofol GmbH (Siegsdorf, Germany). Acrylic acid (AA) was obtained from Sinopharm Chemical Reagent Co. Ltd. AA was puried by reduced pressure distillation to remove any inhibitors. Sodium styrene sulfonate (SSS) was bought from Zibo Jinyuelong Chemical Ltd and used without purication. Commercial battery separator was bought from Shanghai Shilong Hi-Tech. Ltd.

Equipment
A 2 MeV accelerator (GJ-2, Shanghai Xianfeng Electric Machinery Plant) was used to irradiate ETFE lms. The structure and morphology of ungraed and graed membranes were characterized by FT-IR spectroscopy (IR200, Nicolit) and AFM (Multimode NanoscopeIIIa, Bruker), respectively. The pH value of the solution was determined with a pH meter (Seven compact™, Mettler Toledo Co., Ltd.).

Experimental method
2.3.1 Graing procedure. ETFE lms were cut into rectangles (length: 6 cm; width: 4 cm). All lms were washed with acetone and then dried in a vacuum oven until a constant weight was obtained. Then, the ETFE lms were packaged into self-sealing plastic bags, which were lled with high purity nitrogen. Irradiation of ETFE lms was carried out on driving devices of an electron accelerator with an electron beam intensity of 1 mA. The irradiated lms together with polypropylene non-woven fabrics were rolled into cylindrical shapes; then, the lms were immersed into the graing solution that was prepared at the given concentrations and deaerated by purging high purity nitrogen. The graing reaction was carried out in a constant-temperature water bath for a xed time. The preparation scheme is shown in Scheme 1.
2.3.2 Measurement of total graing yield (G t ). Aer the reaction was completed, the graed membranes were removed Scheme 1 Illustration for preparation of a bifunctional cation exchange membrane.
from the graing solution and washed with distilled water to remove homopolymers and copolymers attached to the membranes. Aer being dried in a vacuum oven at 40 C until a constant weight, G t was calculated as follows: here, W 0 and W g are the weights of the ungraed and graed membranes, respectively. Before graing, the thickness of the ETFE lm was 50 mm. The thickness of the graed membranes varied with G t . For example, the thickness was 128 mm for G t ¼ 98.3%. 2.3.3 Measurement of graing yield of SSS onto ETFE (G s ). The graed membranes were immersed into 1 mol L À1 HCl solution, which was stirred with a magnetic stirrer until -SO 3 Na groups on the membranes transformed to -SO 3 H. Aer being removed from the HCl solution, the membranes were washed with distilled water until the pH of the distilled water was 7. The membranes were then placed in 5% NaCl solution for 24 hours while being stirred. H + on the membranes exchanged with Na + from the solution. -COOH on the graed membranes ionized and generated H + , but this can be ignored because the other functional -SO 3 H groups on the graed membranes were strong acids, and H + obtained from the ionization of -SO 3 H could inhibit the ionization of -COOH. The replaced H + was titrated with an NaOH standard solution. G s can be calculated according to the following formula: here, C NaOH is the concentration of NaOH (mol L À1 ), and V NaOH is the volume of NaOH (mL).

Results and discussion
3.1 The inuence of reaction time on G t and G s From Fig. 1, we can see that G t and G s increase with the increasing graing time at xed monomer concentration and molar ratio. During the graing time from 1.5 to 4.5 h, G t increases linearly with the reaction time. At the beginning, the graing reaction occurs on the surface of the ETFE membranes and is controlled by the monomeric diffusion rate. AA is graed onto ETFE and then, the hydrophilicity of ETFE membranes improves; therefore, both AA and SSS react with the radicals formed on the gra chains. Due to the increase in membrane swelling and monomer diffusion rate, different thicknesses of the ETFE lms can be nally introduced for the AA and SSS monomers. 28,29 3.2 The inuence of monomer concentration on G t From Fig. 2, we can see that G t increases from 2.9 to 84.6% when the monomer concentration varies from 0.8 to 3.2 mol L À1 at a xed molar ratio. High monomer concentrations accelerate the diffusion rate of the monomers into the ETFE matrix so that more monomers can react with the radicals formed on ETFE by irradiation. However, it was reported that the graing yield increased with the increasing monomer concentration to a certain value aer which the graing yield dropped with the increasing monomer concentration. 30 In our graing system, on one hand, SSS has limited solubility in the graing solution; on the other hand, the homopolymerization or the copolymerization rate of monomers is slower than the graing rate when monomer concentrations are lower than 3.2 mol L À1 .
3.3 The inuence of reaction temperatures at different graing times on G t Fig. 3 shows that G t increases with an increase in temperature. G t is less than 10% at 40 C although the graing reaction continues for 5 hours, which indicates that the diffusion rate of monomers to the ETFE matrix is very slow at 40 C; therefore, the graing reaction has long induction period under low temperature. When the graing temperature is higher than 50 C, G t clearly increases with the increasing temperature due to greater membrane swelling and faster monomeric diffusion rate. Aer the graing reaction is continued for 4.5 h, G t almost levels off for all temperatures. Aer the introduction of  hydrophilic -COOH and -SO 3 Na functional groups, the graed membrane gradually swells in the graing solution, which makes the gra chains highly movable within ETFE. In this case, the termination rate of the growing chains by mutual combination is accelerated. 31 A similar tendency has also been reported for the graing of styrene onto ETFE-based lms. 32

The inuence of pH values of the graing solution on G t
Additives such as acids and metal salts were used in the radiation graing system. Acidic additives seemed to have a very important effect on the graing yield. Different acids were added to the graing solution to improve the graing yield and lower the cost of the prepared functional materials.
To enhance the graing yield, the addition of inorganic acids is applicable to many graing systems. 33,34 The relationship between G t and pH is shown in Fig. 4. The original pH value of the solution without adding HCl was 2.67. We obtained a pH value of 4.27 by dripping 0.1 mol L À1 NaOH solution into the graing system. It was found that G t was 49.8% at pH of 0.49, and it dropped to 12.9% when pH was 4.27. We found that the viscosity of the solution at pH ¼ 0.49 was higher than that of the solution at pH ¼ 4.27 when the graed membranes were removed from the solution. The movement of the graed chain with high molecular weight was restricted due to high viscosity of the graing solution. Meanwhile, the monomers and their homopolymers with low molecular weights could freely diffuse and react with the growing graed chains. The propagation rate of the graed chains was faster than their termination rate; thus, G t increased rapidly at pH ¼ 0.49. The increase in graing yield in simultaneous irradiation graing systems by adding an inorganic acid was concluded based on the assumption that the addition of acid accelerated hydrogen abstraction by the monomers from nearby polymer molecules. 35 For the preirradiation graing system, research work done by Garnett indicated that the acid enhancement in the graing yield was due to a partitioning effect. 36 3.5 The inuence of storage time of irradiated membranes on G t ETFE lms irradiated with an electron beam in a nitrogen atmosphere were stored at À19 C for different storage times and reacted with monomers at 80 C. The relationship between G t and storage time is shown in Fig. 5. G t was 50.0% when the graing reaction was carried out immediately aer irradiation. Under the same reaction conditions, the G t values dropped to 43.8% and 31.9% during 6 and 15 days of storage times, respectively. The trapped radicals formed in the ETFE lm decayed with longer storage times. The lower the storage temperature, the longer is the lifetime of the radicals. Gupta et al. found that the graing yield of an FEP lm remained almost unchanged aer 118 days of storage time at a temperature of À60 C. 37

FTIR test of graed membranes
Fig . 6 shows the FTIR spectra of irradiated ETFE lm (a), ETFE lm graed with AA having 98.3% graing yield (b) and ETFE lm graed with AA and SSS with 141% graing yield (c).
Comparing the spectrum (a) with (b), a broad absorption peak between 3200 and 3650 cm À1 is seen, which is a characteristic peak for -OH stretching vibrations of AA. The absorption peak at 1732 cm À1 corresponds to the C]O groups of AA. For the spectrum of ETFE-g-poly(AA-co-SSS) membrane (c) compared with spectrum (a), the absorption peak at 842 cm À1 is the characteristic peak of a substituent attached to the para-position of benzene ring. The peak at 1044 cm À1 is due to a symmetrical stretching vibration of S]O. 38 All these new adsorption peaks indicate that AA and SSS have been successfully graed onto the ETFE lm.

AFM test of graed membranes
The surface roughness and topography of original ETFE and graed membranes were tested by an Atomic Force Microscope (AFM) equipped with the nanoscope analysis soware (Multimode NanoscopeIIIa) at room temperature. The AFM test was performed in a tapping mode, and a measurement of roughness of the 3-D surface was estimated. Fig. 7 shows AFM images for the surface morphology of the irradiated ETFE lm (a) and the ETFEg-poly(AA-co-SSS) membrane with G t ¼ 39.6% (b). Comparing the image (a) with (b), we inferred that the surface of the membrane became thicker and rougher because of the formation of graed chains on the ETFE surface. The diameter of the graed chains measured from the 3-D images was about 30 nm.

The inuence of graing yield on contact angles
From Fig. 8, we can see that water contact angles rst decrease with an increase in G t and then, they almost level off with a further increase in G t . The contact angle decreases from 94.3 to 46.7 when G t rises from 6.3% to 50%. At the beginning of the graing reaction, the hydrophilic graed chains, which contain -COOH and -SO 3 H groups, only cover part of the surface of the membranes. When G t is higher than 50%, the graing chains are uniformly distributed on the surface of the graed ETFE, due to which the contact angle remains almost unchanged. When G t is higher than 50%, the gra reaction mainly occurs at a different depth of ETFE lms. The hydrophilicity of the cation exchange membranes has a signicant effect on their ionic conductivity. If the graing yield is low or only parts of the ETFE lm are graed, ion-exchange groups are distributed inhomogeneously on the membrane. Furthermore, the hydrophilicity of the membrane also inuences other performance characteristics, for example, compatibility, dimensional stability and resistance to methanol at high temperature when the   Fig. 8 Relationship between G t and water contact angle.
membrane is used in a proton exchange fuel cell. To prepare cation exchange membranes with long-term stability, hydrophilicity of the membrane should be optimized by changing the graing conditions.

Electrochemistry performance of graed membrane
The graed ETFE membrane with G t ¼ 135% and a commercial battery separator were separately assembled within batteries, and their electrochemistry performances were tested by charging and discharging cycles. The battery electrode was wrapped by a graed membrane or a commercial battery separator and then soaked into 6 mol L À1 KOH solution. Aer activating for 8 h, the electrochemistry performance of the assembled batteries was tested. Fig. 9(a) and (b) show parts of the charge and discharge spectra of the graed membrane and the commercial separator, respectively. The whole test was performed at an electric current of 66 mA for 200 h. For the graed membrane, the charging voltage dropped from 1.66 V to 1.60 V. For the commercial separator, the charging voltage reduced from 1.83 V to 1.77 V. A lower charging voltage indicates that the resistance of the graed membrane is lower than that of the commercial separator under the same test conditions. Low resistance is very important for graed membranes used as battery separators, because the lower the lm resistance, the greater the current when the battery is discharging. Furthermore, aer 100 cycles of charging and discharging, the charging voltage was still relatively stable. Thus, the prepared ETFE-g-poly(AA-co-SSS) membranes have great potentials to be used as separators in secondary batteries.

The relationship between G t and alkali absorption property
The membranes with G t ¼ 0, 2.6, 3.3, 5.1, 10.0, 20.0, 33.9 and 45.5% were soaked in 30% NaOH solution for 24 h at 25 C and then taken out. Aer water on the surface of membranes was wiped off, the area and weight of the membrane were measured. The relationship between the increase in the percentage of weight or area is shown in Fig. 10. Aer the membranes were swollen in 30% NaOH solution, the area and weight of the membranes clearly increased with increasing G t . Alkali absorption property is another key factor for evaluating the performance of ion-exchange membranes as separators of alkaline batteries. Because the membranes synthesized in this research contained both strong and weak acid groups when compared with membranes with only carboxylic acid groups, the alkali absorption rate was signicantly improved.

Inuence of environmental humidity on the resistance of graed membranes
The humidity sensitivity curves of ETFE-g-poly(AA-co-SSS) membranes with different G t values were measured at 25 C at the following humidity values: 5%, 31%, 51%,79%, and 98%. From Fig. 11, we can see that the resistance of the graing membranes (R) decreased with the increase in environmental humidity. At the same humidity, the resistance of the membrane with G t ¼ 68.6% was lower than that of the membrane with G t ¼ 59.1%. When G t increased, more -COOH and -SO 3 Na groups graed onto ETFE membranes were ionized at high humidity. When the humidity was 98%, the membrane resistance dropped by 4 orders of magnitude compared with  Paper that at humidity of 5%. All the results indicated that the graing membranes with hydrophilic -COOH and -SO 3 Na groups responded quickly to the change in humidity. Therefore, the prepared membrane with bifunctional groups can be used as a humidity sensitive material of a humidity sensor.

Response and recovery curves of graed membranes
The graed membrane was rst placed in a container with 5% humidity until the resistance reached equilibrium; then, it was transferred to another container with 98% humidity. We found that the resistance decreased sharply in a short time and then leveled off. In the meantime, we recorded the resistance values and obtained the rst half of a log R-t curve, which is called the response curve. Then, the membrane was taken out of the container with 98% humidity and placed back into the container with 5% humidity. The membrane resistance rapidly increased and nally reached equilibrium. The second half of the log R-t curve was named as the recovery curve. The entire curve including the response and recovery processes is shown in Fig. 12; we can see that the membrane with 68.6% graing yield responded and recovered more rapidly than the membrane with 59.1% graing yield. In comparison with the result reported by Sangthumchai, 39 the total response and recovery time of our prepared membrane with bifunctional groups -COOH and -SO 3 Na were shorter than those of a membrane that contained only -SO 3 Na groups. Due to weak interactions between the -COOH groups and absorbed H 2 O, the adsorbed H 2 O was easily desorbed; thus, the total response and recovery time became shorter.

Conclusion
Radiation-graed cation exchange membranes were prepared by a one-step method to investigate the effect of bifunctional groups on the properties of the membranes. New absorption peaks assigned to C]O and S]O indicated that the monomers AA and SSS were graed onto ETFE membranes. When the graing reaction was carried out at 80 C, the highest graing rate and G t were obtained. The addition of HCl was an effective method for the enhancement of G t and G s . When irradiated ETFE lms were kept at À19 C, the decay rate of trapped radicals was very slow; thus, G t dropped from 50.0 to 43.8% during a period of 144 h storage time.
Aer the introduction of -COOH and -SO 3 Na groups, the graing membranes not only exhibited excellent hydrophilicity but also showed high alkali uptake values. The resistance of the membranes decreased by 4 orders of magnitude at high humidity and therefore, the graing membranes can be used as humidity sensitive materials for humidity sensors. The membranes with bifunctional groups displayed shorter response and recovery times compared to the ones containing only -SO 3 H groups. The charging voltage of the graed membrane was lower than that of a commercial separator that only contained carboxylic acid groups. Furthermore, the charging voltage was still relatively stable aer 100 cycles; therefore, the graing membranes have great potentials to be used as separators in secondary batteries.

Conflicts of interest
There are no conicts to declare.