Preparation of sulfonated crosslinked PTFE-graft-poly(alkyl vinyl ether) membranes for polymer electrolyte membrane fuel cells by radiation processing

Journal of Membrane Science 256 (2005) 38–45

Preparation of sulfonated crosslinked PTFEgraft-poly(alkyl vinyl ether) membranes for polymer electrolyte membrane fuel cells by radiation processing Jinhua Chen ∗ , Masaharu Asano, Tetsuya Yamaki, Masaru Yoshida Department of Materials Development, Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan Received 21 December 2004; received in revised form 8 February 2005; accepted 9 February 2005 Available online 11 March 2005

Abstract New polymer electrolyte membranes having sulfonic acid groups for polymer electrolyte membrane fuel cell applications were prepared by simultaneous radiation-induced grafting method. The poly(tetrafluoroethylene) (PTFE) films, crosslinked by electron-beam radiation at molten temperature, were used as substrates for grafting of two alkyl vinyl ether monomers, propyl vinyl ether (nPVE) and isopropyl vinyl ether (iPVE), under controlled grafting conditions followed by sulfonation reactions. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), water contact angle and Fourier transform infrared (FT-IR) were used to characterize the crosslinked PTFE (cPTFE) and grafted cPTFE films. The degree of grafting was found to be dependent on the grafting parameters such as irradiation temperature and Lewis acid catalyst, in which in the presence of Lewis acid catalyst or at a temperature close to the boiling point of each monomer, the grafting reaction significantly accelerated even when the relatively low dose was irradiated. Finally, the grafted cPTFE films were sulfonated in a chlorosulfonic acid solution. In spite of the lower ion-exchange capacity (0.75 mmol/g), the membrane synthesized in this study showed a proton conductivity as high as the Nafion 112. © 2005 Elsevier B.V. All rights reserved. Keywords: Radiation-grafting; Poly(tetrafluoroethylene); Crosslinking; Alkyl vinyl ether; Polymer electrolyte fuel cell membrane

1. Introduction Polymer electrolyte membrane fuel cell (PEMFC), designated as Nafion membrane supplied by Du Pont, is the most promising energy source for automotive and stationary power applications, due to its environmental-friendliness, high efficiency, and high power density [1]. However, the high cost of this membrane limits its full commercialization. In order to reduce the cost of the polymer electrolyte membrane, the development of a novel polymer electrolyte membrane has been actively carried out as alternatives to Nafion, e.g., sulfonated polybenzimidazole (PBI), polyether ether ketone (PEEK), poly(phenylene sulfone) (PSU) and polyimide (PI), and their hybrid composites. The recent ad∗

Corresponding author. Tel.: +81 27 346 9413; fax: +81 27 346 9687. E-mail address: [email protected] (J. Chen).

0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.02.005

vances in these membranes have been published in several reviews [2–6]. However, the problems of these polymer electrolyte membranes are the excessive swelling, the lower proton conductivity and the higher methanol permeability. On the other hand, the synthesis of polymer electrolyte membranes by introduction of the sulfonated graft chain into the high chemical stability fluorinated polymers, such as poly(tetrafluoroethylene) (PTFE), poly(tetrafluoroethyleneco-hexafluoropropylene) (FEP), poly(vinylidene fluoride) (PVDF) and poly(ethylene-co-tetrafluoroethylene) (ETFE), is also one of the interesting methods as another approach [7–18]. The radiation-induced grafting method is the most effective for the introduction of the graft polymer chain, because the molecular chains can propagate through radical species that are generated inside and outside of the film by irradiation.

J. Chen et al. / Journal of Membrane Science 256 (2005) 38–45

In general, grafting of a monomer into PTFE film can be processed using the simultaneous irradiation or the preirradiation method [7,8]. In the case of styrene grafting, the pre-irradiation method was more recommended. However, PTFE is extremely sensitive to high energy radiation that produces chain scission with a very small irradiation dose [19]. Fortunately, the radiation resistance could be improved by the radiation crosslinking of the PTFE at molten temperature [19–24]. In addition, crosslinking structure in the polymer electrolyte membranes can significantly improve their dimensional and chemical stability [7–9]. In this study, we tried to synthesize an alternative electrolyte membrane. For this purpose, the alkyl vinyl ether monomers such as propyl vinyl ether (nPVE) and isopropyl vinyl ether (iPVE) were chosen for the grafting. The PTFE films, crosslinked by electron-beam irradiation at 340 ◦ C in argon gas, were used as the substrates for grafting of the monomers under ␥-ray irradiation in argon gas. The grafted cPTFE was subsequently sulfonated by chlorosulfonic acid in a dichloroethane solution in order to obtain the polymer electrolyte membrane. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), water contact angle measurements, Fourier transform infrared (FT-IR) spectrometer and the detection of the ion-exchange capacity (IEC), proton conductivity and water uptake were used for the characterization.

2. Experimental The crosslinked PTFE (cPTFE) film with a thickness of 50 ␮m was obtained from Nitto Denko Co., Japan. It was crosslinked by electron-beam irradiation at 340 ◦ C in argon gas with irradiation dose in the range of 50–400 kGy. The cPTFE was washed with acetone to remove any impurities on its surface and was stored in a vacuum oven at 40 ◦ C before use. Propyl vinyl ether (nPVE) and isopropyl vinyl ether (iPVE), purchased from Aldrich Chemical Co., Japan, were purified under vacuum distillation just before the grafting reaction. AlCl3 powder purchased from Wako Pure Chemical Ind., Ltd., Japan, was used as a catalyst. Chlorosulfonic acid (ClSO3 H) and dichloroethane were purchased from Aldrich Chemical Co., Japan. The former was used as the sulfonation reagent and the latter was used as its solvent. They were used without further treatment. The thermal analysis was carried out using a Thermo Plus2/DSC8230 and a Thermo Plus2/TG-DTA (Rigaku, Japan). The specimen (about 5 mg) was heated to 650 ◦ C at a heating rate of 10 ◦ C/min. The nitrogen flow rates through the specimen were 75 and 150 ml/min for the DSC and TG analysis, respectively. The cPTFE films crosslinked by 100 kGy dose were used as substrates for grafting. The grafting reaction was carried out under the simultaneous irradiation of the cPTFE and the monomer in an argon gas-filled ampoule. For instance, in a vacuum degassed 100 ml glass ampoule, cPTFE with a size of

39

5 cm × 5 cm, 5 mg AlCl3 powder and 80 ml monomer were added. The argon gas was then bubbled and filled the ampoule. ␥-ray irradiation of the ampoule was performed in Co-60 source facility of Japan Atomic Energy Research Institute (JAERI), with the irradiation rate of 10 kGy/h. After the designed irradiation dose, the film was transferred from the ampoule to a Soxhlet apparatus and was extracted with ethanol for 24 h to remove the ungrafted polymer. After drying to a constant weight in a vacuum at 60 ◦ C, the amount of the grafted polymer in the cPTFE, namely degree of grafting, was obtained as follows: grafting (%) =

Wg − W0 × 100 W0

where Wg and W0 are the weights before and after the grafting reaction, respectively. The contact angle of water on the film was detected using a Contact Angle Meter (model CA-X, Kyowa Interface Science Co. Ltd., Japan) at 25 ◦ C. For each film, at least eight measurements were performed and the average value was calculated. JASCO FT-IR-5300 Spectrometer (Japan Spectroscopic Co. Ltd.) was used for measuring the absorption spectra at resolution of 4 cm−1 for the original cPTFE and the grafted cPTFE films. Sulfonation of the cPTFE-graft-poly(alkyl vinyl ether) film was carried out by directly immersing the film in a 0.2 M chlorosulfonic acid solution of dichloroethane at room temperature for 8 h. After sulfonation, the membrane was washed with acetone and then distilled water. The ion-exchange capacity (IEC) of the polymer electrolyte membrane was determined by acid–base titration. The dried membrane in the protonic form was immersed in 20 ml of NaCl saturated aqueous solution and equilibrated for 24 h. The solution was then titrated with 0.1 M NaOH solution using an automatic titrator (HIRANUMA COM-555). Based on the titration results, the IEC (mmol/g) was calculated: IEC =

0.1VNaOH Wdry

where VNaOH (ml) is the volume of the 0.1 M NaOH solution at pH 7.0, and Wdry (g) is the dry weight of the polymer electrolyte membrane in the protonic form. The proton conductivity of the membrane was measured by impedance spectroscopy using a Solartron 1269 analyzer with an ac perturbation of 10 mV. The samples were hydrated in water overnight and clamped between two Pt electrodes of the apparatus designed in our laboratory. The impedance spectroscopy was measured after wiping off the excess water on the membrane surface. The conductivity (σ, S/cm) was calculated from the impedance data using the relation: σ=

w Rdl

where w (cm) is the distance of the two electrodes, d (cm) is the membrane thickness and l (cm) is the length of the

40

J. Chen et al. / Journal of Membrane Science 256 (2005) 38–45

membrane along the electrode. R ( ) was derived from the low intersect of the high frequency semicircle on a complex impedance plane with the real axis. The water uptake of the polymer electrolyte membrane was determined as follows. First, the polymer electrolyte membrane was immersed in deionized water at 25 ◦ C for 24 h; then the membrane was taken out of the water and weighed after wiping off the excess surface water. The water uptake was estimated using the following equation: water (%) =

Wwater − Wdry Wdry

where Wwater and Wdry are the weights of the membrane in the wet and dry states, respectively.

3. Results and discussion 3.1. Thermal properties of cPTFE film The PTFE is extremely sensitive to high energy radiation producing chain scission even with a low irradiation dose, resulting in a drastic decrease of its mechanical strength and thermal properties. For instance, the tensile strength of the original PTFE film is about 25–30 MPa, while the tensile strength of the radiation grafted PTFE film, such as PTFEgraft-polystyrene film, is only about 4–6 MPa due to the degradation during the pre-irradiation [9]. The low mechanical strength limited their applications in a PEMFC. Fortunately, it was found that the radiation resistance of the PTFE could be improved by crosslinking the PTFE using ␥-ray or electron-beam irradiation at the molten state and in oxygenfree environment. The effect of the irradiation temperature, irradiation dose and irradiation environment on the radiation resistance and mechanical strength of the cPTFE film has been intensively studied [19–24]. However, the thermal stability of the cPTFE film has been less reported. We studied the thermal properties in detail using the DSC and TG instruments. The thermal stability of the cPTFE film was defined by its decomposition temperature, which was obtained from the DSC and TG curves. In addition, the melting point of the cPTFE film was also obtained from the DSC curve. Fig. 1 shows the DSC curves at the temperature ranges of 260–340 and 480–640 ◦ C, containing the melting and decomposition peaks of the cPTFE film, respectively. The crosslinking process was performed under the electron-beam irradiation in argon gas at 340 ◦ C. Irradiation dose for the crosslinking of the cPTFE film was 0, 50, 100, 150, 200 and 400 kGy. The melting point (Tm ) of the cPTFE film was defined as the endothermic peak at the range of 260–340 ◦ C, and the decomposition temperature (Td-DSC ) was defined as the temperature where the DSC curve deviated from the base line at the range of 480–640 ◦ C. The melting points and decomposition temperatures were noted on the DSC curves. The melting point of the cPTFE film decreased with an increase in the irradiation dose for the crosslinking. The melting

Fig. 1. DSC results of the cPTFE films with different irradiation dose for crosslinking.

point of the original PTFE film (0 kGy) was 329 ◦ C. When the irradiation dose for the crosslinking increased to 400 kGy, the melting point decreased to 301 ◦ C. However, the degree of crystallinity of the cPTFE film, in proportion to the peak area of the melting region, was only slightly changed. Therefore, the decrease in the melting point was due to the decrease of the crystal size and the defects in the crystal particle induced by the irradiation [21]. The decrease in the crystal size can also be proved by the appearance change, whereas the original PTFE film was white and nontransparent, and the cPTFE film was colorless and transparent. The decomposition temperature of the cPTFE film from the DSC curve showed the highest value of 528 ◦ C at the dose of 50 kGy. With the dose of 400 kGy, however, the decomposition temperature decreased to 480 ◦ C. Therefore, with low irradiation dose such as 50 and 100 kGy for crosslinking, the thermal stability of the PTFE film was improved. The relatively higher irradiation dose made the cPTFE film more crosslinked, but also produced more defects in the cPTFE film, resulting in a decrease of the decomposition temperature. Zhong et al. reported that the defects in the PTFE crystallite were due to the radiation-induced crosslinking and/or branching in the crystalline region [21]. The thermal stability could also be evaluated by the endothermic area (shaded area) in the temperature range from the decomposition temperature to 560 ◦ C. The smaller the shaded area, the more stable the cPTFE film. Thus, it was concluded that the thermal stability of the PTFE film could be improved by a low dose irradiation such as 50 and 100 kGy at the molten temperature of 340 ◦ C. The TG analysis was also used to evaluate the cPTFE thermal stability. Fig. 2 shows the TG curves in the temperature range of 250–650 ◦ C with different doses of 0, 50, 100, 150, 200 and 400 kGy. The decomposition temperature obtained from the TG curve was defined as the point where the weight loss was 5%. As shown in Fig. 2, the decomposition temperature of the original PTFE film (0 kGy) was 525 ◦ C. With the doses of 50 and 100 kGy, the decomposition temperature was

J. Chen et al. / Journal of Membrane Science 256 (2005) 38–45

41

Fig. 3. Effects of the irradiation dose on the melting point and decomposition temperature of the cPTFE films. Fig. 2. TG results of the cPTFE films with different irradiation dose for crosslinking.

together with the appearance, melting temperature and decomposition temperature of the original PTFE film and the cPTFE films crosslinked by 100 kGy dose were summarized in Table 1. The tensile strength of the cPTFE film somewhat decreased due to the electron-beam irradiation. However, the radiation resistance, defined as the dose at a half value in tensile strength by ␥-rays irradiation under vacuum at room temperature, was significantly improved [23]. The radiation resistance of original PTFE film was only 3.5 kGy while that of the cPTFE was larger than 900 kGy. Therefore, the cPTFE film was possible to be irradiated at room temperature for the grafting reaction.

534 and 524 ◦ C, respectively. However, above the 150 kGy dose, the thermal stability of the cPTFE film was worsened. For instance, with the 400 kGy dose, the decomposition temperature decreased to 485 ◦ C. This result was in good agreement with the result from the DSC curve. The irradiation dose-dependent melting point and decomposition temperature (from TG and DSC curve) are summarized in Fig. 3. The melting point monotonously decreased with an increase in the irradiation dose. The decomposition temperatures from the TG and DSC measurements showed the same tendency to the irradiation dose. With a lower irradiation dose, however, the decomposition temperature from the TG was about 5 ◦ C higher than that from the DSC curve. The difference was due to the different definition of the decomposition temperature. Even then, both the decomposition temperatures from the TG and DSC showed the improved stability of the cPTFE film at the lower irradiation dose. Considering the thermal stability and the crosslinking density, irradiation dose of 100 kGy was more optimum to crosslink the PTFE films. After the electron-beam irradiation at molten temperature, the dimension and density of the cPTFE film were almost not changed. The other important parameters, such as the crystallinity, tensile strength and radiation resistance,

3.2. Grafting of alkyl vinyl ether into cPTFE film The grafting of styrene into PTFE film by radiationinduced grafting has been extensively studied [7,8]. The mechanism of graft polymerization of styrene is a radical process. The graft chain is propagated rapidly from the PTFE radical induced by the pre-irradiation. However, the grafting of the monomer into cPTFE film has been less reported [9,13]. In our previous study, it was found that the cPTFE film was more easily grafted with styrene than the original PTFE film [13]. In this study, the alkyl vinyl ether was used to graft into the cPTFE film for the synthesis of the new electrolyte membrane. In the preliminary experiment, the grafting of nPVE and iPVE into cPTFE film was performed using the

Table 1 Physical properties of the original PTFE film and the cPTFE films crosslinked by 100 kGy irradiation dose Films (50 ␮m)

Original PTFE Crosslinked PTFEd a b c d

Appearance

White nontransparent Colorless transparent

Melting point (◦ C)

Decomposition temperature (◦ C)

DSC

DSC

TG

329 316

508 520

525 524

Crystallinitya (%)

Tensile strengthb (MPa)

Radiation resistancec (kGy)

47.5 50.4

32 27

3.5 >900

Calculated from the heat of crystalliation (J/g) from DSC curve. Determined with the Dumbbell shape specimen according to ASTM D-1822L standard. Dose at a half value in tensile strength by ␥-rays irradiation under vacuum, as in [23]. Crosslinked by electron-beam irradiation in argon gas at 340 ◦ C with the irradiation dose of 100 kGy.

42

J. Chen et al. / Journal of Membrane Science 256 (2005) 38–45

Fig. 4. Effects of the irradiation dose on the grafting of alkyl vinyl ether into the cPTFE film at 25 ◦ C using the simultaneous irradiation method.

pre-irradiation graft method. For instance, cPTFE film was ␥-ray irradiated to produce grafting sites (radical or active ion group). The alkyl vinyl ether was then polymerized from the produced grafting sites. However, the results indicated that both nPVE and iPVE could not graft to the cPTFE film using the pre-irradiation method. It is due to the low activity of the alkyl vinyl ether monomer, which is difficult to be polymerized by the radical process [25]. Therefore, a simultaneous radiation graft was developed for the grafting of nPVE and iPVE into the cPTFE film. The cPTFE film was immersed in the argon gas-bubbled monomer in an ampoule. The ampoule was then irradiated under ␥-ray radiation. It was found that the cPTFE film was swollen during the irradiation. On the other hand, without irradiation, there was no change in both the dimension and weight of the cPTFE film immersed in the monomer even for 1 week. Therefore, it was concluded that the alkyl vinyl ether was really penetrated into the cPTFE film and the graft polymerization was processed under the ␥-ray irradiation. Fig. 4 shows the results of the grafting of nPVE and iPVE into cPTFE films as a function of the irradiation dose at 25 ◦ C. The graftings of nPVE and iPVE into the cPTFE films showed similar relationship between the degree of grafting and the irradiation dose. The degree of grafting linearly increased with an increase in the dose up to 400 kGy, and then increased more quickly as the dose increased. The graft polymerization was suggested to begin at the surface of the film and proceeded internally to the middle via a progressive diffusion of the monomer through the swollen grafted layer [26]. However, it was noted that for a satisfactory degree of grafting amount, such as 30%, the required irradiation dose was as high as 700 kGy. The high irradiation dose may decrease the mechanical strength and thermal stability of the resulting film. To accelerate the graft reaction, promoting the irradiation temperature and addition of the catalyst was considered. Lewis acid is a well-known catalyst for the cationic polymerization. In this study, AlCl3 was chosen for the graft polymerization. As shown in Fig. 5, with ␥-ray irradiation, the grafting of nPVE and iPVE into the cPTFE films quickly

Fig. 5. Effects of the irradiation dose on the grafting of alkyl vinyl ether into the cPTFE film at 25 ◦ C and in the present of AlCl3 using the simultaneous irradiation method.

increased with the increase in the irradiation dose in the presence of AlCl3 catalyst. This process was the so-called radiation-catalytic polymerization [26]. Even with the lower irradiation dose of 30 kGy, the degree of grafting was greater than 30%. It could be explained that the irradiation created a cation site in the cPTFE film, and the cation site immediately initiated the monomer polymerization. The catalyst of AlCl3 activated the cation site at the end of the graft chain and thus accelerated the propagation of the graft chain. On the contrary, without ␥-ray irradiation, both nPVE and iPVE can homopolymerize in the presence of AlCl3 , but the degree of grafting in the cPTFE film was only trace. On the other hand, promoting the irradiation temperature can also accelerate the graft polymerization. Fig. 6 shows the effect of the irradiation temperature on the grafting of nPVE and iPVE into cPTFE film with an irradiation dose of 150 kGy. Both the degree of grafting curves of nPVE and iPVE showed a threshold temperature at 60 and 50 ◦ C, respectively. These temperatures were close to the boiling points of the nPVE and iPVE monomers, 65 and 55 ◦ C, respectively.

Fig. 6. Effects of the irradiation temperature on the grafting of alkyl vinyl ether into the cPTFE film with 150 kGy irradiation dose.

J. Chen et al. / Journal of Membrane Science 256 (2005) 38–45

Fig. 7. TG results of the cPTFE film, grafted cPTFE films and the poly(alkyl vinyl ether) homopolymers. The degrees of grafting of the cPTFE-g-nPVE film and cPTFE-g-iPVE film were 40 and 42%, respectively.

It was considered that the grafting reaction in the cPTFE was a diffusion-controlled process; the enhanced grafting reaction was taken place near the boiling point where the monomer diffusion into cPTFE film increased drastically. Even when the degree of grafting reached 80%, the viscosity of the liquid in the ampoule was relatively lower, indicating that the reaction preferred graft polymerization to homopolymerization. The obtained grafted cPTFE film had a good appearance, and a larger area than the cPTFE film before grafting. 3.3. Properties of the cPTFE-graft-poly(alkyl vinyl ether) film The TG analysis was carried out for the characterization of the thermal properties of the cPTFE-g-nPVE and cPTFE-giPVE films, as well as the cPTFE film and the homopolymers of poly(nPVE) and poly(iPVE). The TG results are shown in Fig. 7. The degrees of grafting of the cPTFE-g-nPVE and cPTFE-g-iPVE films were 40% and 42%, respectively. The initial weight loss of the grafted cPTFE films and the homopolymers at the temperature region less than 200 ◦ C was due to the evaporation of the lower molecule weight oligomers in the samples. The thermal stabilities of the two grafted cPTFE films were lower than that of the cPTFE film. In addition, the grafted cPTFE films showed a two-step decomposition coming from the side graft chain of poly(alkyl vinyl ether) and the cPTFE substrates, at the temperature ranges of 300–500 and 500–580 ◦ C, respectively. The decomposition temperature of the cPTFE chain in grafted cPTFE film was clear lower than that of the original cPTFE film, indicating that there was a stronger interaction between the graft side chain and the cPTFE main chain in the grafted cPTFE films. The contact angle investigation is a useful method for the evaluation of the film surface. Brack et al. used this method to successfully characterize the irradiated, grafted and sul-

43

Fig. 8. Effects of degree of grafting on the water contact angle of the grafted cPTFE films at 25 ◦ C.

fonated membrane by measuring the contact angles of several liquids on the surfaces [15]. It was reported that the wetting and surface properties were varied systematically as a function of degree of grafting and other grafting parameters. In our study, the contact angle of water was determined for the surface evaluation of the grafted PTFE. As shown in Fig. 8, the two polymer-grafted cPTFE films had a similar tendency. That is, the contact angles were almost equal to each other for the identical degree of grafting, and it gradually decreased when increasing the degree of grafting. For example, both contact angles of cPTFE-g-nPVE and cPTFEg-iPVE were about 90◦ and 70◦ when the degrees of grafting were 10% and 50%, respectively. This was due to the similar structure of the two monomers, and the same grafting conditions. The contact angles of the homopolymerized poly(nPVE) and poly(iPVE) films determined under the same conditions, being 66 and 61◦ , respectively, were lower than that of the polymer-grafted cPTFE films. Therefore, the surface of the grafted cPTFE films was composed of the graft chain and the PTFE molecule. As the degree of grafting increased, the ratio of the PTFE molecule decreased, resulting in the decrease of the water contact angle. The poly(nPVE) and poly(iPVE) were grafted inside and outside of the cPTFE film. This was an important advantage for the synthesis of the polymer electrolyte membrane. Fig. 9 shows the FT-IR spectra of the original cPTFE, cPTFE-g-nPVE and cPTFE-g-iPVE films in the range 400–3200 cm−1 . The degree of grafting of the cPTFE-gnPVE film and cPTFE-g-iPVE film are 40% and 42%, respectively. For the original cPTFE, the band at 1785 cm−1 is assigned to double bonds of CF CF due to the irradiation for crosslinking, and the band around 2380 cm−1 is due to the CO2 in air. After alkyl vinyl ether grafting, these two peaks are weakened. On the other hand, the new band around 2960 cm−1 related to CH stretching vibration of the propyl group is presented. Therefore, it is indicated that the nPVE and iPVE monomers have been grafted into the cPTFE films.

44

J. Chen et al. / Journal of Membrane Science 256 (2005) 38–45

Fig. 9. FT-IR spectra of the original cPTFE film, cPTFE-g-nPVE film and cPTFE-g-iPVE film. The degrees of grafting of the cPTFE-g-nPVE film and cPTFE-g-iPVE film were 40 and 42%, respectively.

3.4. Sulfonation of the cPTFE-graft-poly(alkyl vinyl ether) film The cPTFE-graft-poly(alkyl vinyl ether) film was sulfonated to synthesize the polymer electrolyte membrane. The mechanism of the sulfonation of PTFE-graft-polystyrene film in chlorosulfonic acid is well understood. However, in this study, we found that the sulfonation of the cPTFE-graftpoly(alkyl vinyl ether) film underwent a very complex process. In Fig. 10, the IEC of the obtained polymer electrolyte membrane was plotted as a function of the degree of grafting. It was found that both the IECs of the membranes from cPTFE-g-iPVE and cPTFE-g-nPVE films increased with the increase in the degree of grafting. However, even with high degree of grafting, the IEC was always less than 1.1 mmol/g. The cPTFE-g-iPVE film seemed to be more easily sulfonated than that of the cPTFE-g-nPVE film. The IEC was largely lower than that of the calculated values by assuming that one monomer unit substituted with one sulfonic acid group. On the other hand, during the sulfonation reaction, the mem-

Fig. 10. Relationship between the ion-exchange capacity (IEC) and the degree of grafting of the synthesized polymer electrolyte membrane.

Fig. 11. Ion-exchange capacity (IEC) dependence of (a) the proton conductivity and (b) the water uptake of the synthesized polymer electrolyte membrane.

brane appeared a color change from colorless to deep brown, and after sulfonation, the weight of the membrane decreased. Therefore, the lower IEC of the obtained polymer electrolyte membrane was due to the scission of the graft chain as well as the loss of the membrane weight during the sulfonation reaction. The complex reaction process is not yet completely understood. However, the membrane changed from the insulator to a good proton conductor, indicating that the sulfonic acid group was indeed induced into the grafted film. Fig. 11 shows (a) the proton conductivity and (b) the water uptake of the polymer electrolyte membrane as a function of the IEC. Nafion 112 was also plotted for comparison. The IEC dependences of the two type membranes from cPTFE-g-iPVE film and cPTFE-g-nPVE film were similar with each other. In Fig. 11(a), there was a percolation threshold in the range of 0.55–0.70 mmol/g where the proton conductivity drastically increased several orders. At the IEC of 0.91 mmol/g, the sulfonated cPTFE-g-iPVE membrane showed 0.08 S/cm whereas the Nafion 112 showed 0.06 S/cm. On the other hand, as shown in Fig. 11(b), the water uptake at 0.91 mmol/g was around 34 and 30% for the sulfonated cPTFE-g-iPVE membrane and the Nafion 112 membrane, respectively. Therefore, the high proton conductivity of the sulfonated cPTFE-g-iPVE membrane was due to its more hydrophilic than the Nafion 112 membrane. In other words, we could obtain a polymer electrolyte membrane with the same conductivity level as Nafion 112 membrane, but with a relatively lower IEC of 0.75 mmol/g. In Fig. 11(b), it showed that the water uptake of the sulfonated cPTFE-g-iPVE membrane at the IEC 0.75 mmol/g was around 24%, which was clearly lower than that of Nafion 112 membrane of 30%. The lower IEC can keep the electrolyte membrane with a relatively lower swelling, resulting in a more stable electrolyte membrane.

J. Chen et al. / Journal of Membrane Science 256 (2005) 38–45

Different from the crosslinked nonfluorinated polymer electrolyte membrane, which was very brittle when dried out [2–6], the crosslinked polymer electrolyte membrane in this study was mechanically strong and flexible even at the water-free state. This was due to the high flexibility of the base cPTFE film and the grafted poly(alkyl vinyl ether) side chain. In addition, due to the host cPTFE substrate and the relatively lower IEC, the dimensions of the ionomer membrane were very stable. From the view of this point, the sulfonated cPTFE-graft-poly(alkyl vinyl ether) electrolyte membrane was a promising material for application in PEMFCs. 4. Conclusions The PTFE film was successfully crosslinked by electronbeam irradiation at 340 ◦ C. The crosslinking structure in the cPTFE film could be identified by the TG and DSC analyses. The melting point as well as the crystal size of the cPTFE film decreased, and the decomposition temperature slightly increased at the irradiation doses of 50–100 kGy. The alkyl vinyl ether monomers of nPVE and iPVE were successfully grafted into the cPTFE film using the simultaneous radiation-induced grafting method. The grafting linearly increased with the irradiation dose. In the presence of the Lewis acid of AlCl3 or at irradiation temperatures close to the boiling point of each monomer, the grafting reaction accelerated. The water contact angle measurement results showed that the surface of grafted cPTFE film was more hydrophilic than that of the original cPTFE film, but was more hydrophobic than that of the poly(alkyl vinyl ether) homopolymer. The FT-IR results also indicated that the nPVE and iPVE have been grafted into the cPTFE films. The sulfonation of the grafted cPTFE film was performed using chlorosulfonic acid. The IEC was less than 1.1 mmol/g even with a high grafting degree. However, in spite of a lower IEC of 0.75 mmol/g and a lower water uptake around 24%, the conductivity of the obtained electrolyte membrane can reach the same level as Nafion 112. The relatively lower IEC and lower water uptake could make the electrolyte membrane more stable, and thus more promising for the PEMFC applications. References [1] W. Vielstich, H. Gasteiger, A. Lamm, Handbook of Fuel Cells: Fundamentals, Technology Applications, Wiley, New York, 2003. [2] D.J. Jones, J.R. Roziere, Recent advances in the functionalization of polybenzimidazole and polyetherketone for fuel cell applications, J. Membr. Sci. 185 (2001) 41–58. [3] J.A. Kerres, Development of ionomer membranes for fuel cells, J. Membr. Sci. 185 (2001) 3–27. [4] J. Roziere, D.J. Jones, Non-fluorinated polymer materials for proton exchange membrane fuel cells, Annu. Rev. Mater. Res. 33 (2003) 503–555. [5] O. Savadogo, Emerging membranes for electrochemical systems. I. solid polymer electrolyte membranes for fuel cell systems, J. New Mater. Electrochem. Syst. 1 (1998) 47–66.

45

[6] M. Rikukawa, K. Sanui, Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers, Prog. Polym. Sci. 25 (2000) 1463–1502. [7] T.M. Dargaville, G.A. George, D.J. Hill, A.K. Whittaker, High energy radiation grafting of fluoropolymers, Prog. Polym. Sci. 28 (2003) 1355–1376. [8] M.M. Nasef, E.A. Hegazy, Preparation and applications of ion exchange membranes by radiation-induced graft copolymerization of polar monomers onto non-polar films, Prog. Polym. Sci. 29 (2004) 499–561. [9] K. Sato, S. Ikeda, M. Iida, A. Oshima, Y. Tabata, M. Washio, Study on poly-electrolyte membrane of crosslinked PTFE by radiationgrafting, Nucl. Instrum. Method B 208 (2003) 424–428. [10] J.A. Horsfall, K.V. Lovell, Fuel cell performance of radiation grafted sulphonic acid membranes, Fuel Cells 1 (2001) 186–191. [11] M.V. Rouilly, E.R. Kotz, O. Haas, G.G. Scherer, A. Chapiro, Proton exchange membranes prepared by simultaneous radiation grafting of styrene onto Teflon-FEP films, synthesis and characterization, J. Membr. Sci. 81 (1993) 89–95. [12] J.A. Horsfall, K.V. Lovell, Comparison of fuel cell performance of selected fluoropolymer and hydrocarbon based grafted copolymers: incorporating acrylic acid and styrene sulfonic acid, Polym. Adv. Technol. 13 (2002) 381–390. [13] T. Yamaki, M. Asano, Y. Maekawa, Y. Morita, T. Suwa, J. Chen, N. Tsubokawa, K. Kobayashi, H. Kubota, M. Yoshida, Radiation grafting of styrene into crosslinked PTFE films and its sulfonation for fuel cell applications, Radiat. Phys. Chem. 67 (2003) 403–407. [14] M.M. Nasef, H. Saidi, H.M. Nor, Proton exchange membranes prepared by simultaneous radiation grafting of styrene onto poly(tetrafluoroethylene-co-hexafluoropropylene) films. I. Effect of grafting conditions, J. Appl. Polym. Sci. 76 (2000) 220–227. [15] H.P. Brack, M. Wyler, G. Peter, G.G. Scherer, A contact angle investigation of the surface properties of selected proton-conducting radiation-grafted membranes, J. Membr. Sci. 214 (2003) 1–19. [16] M.M. Nasef, H. Saidi, Preparation of crosslinked cation exchange membranes by radiation grafting of styrene/divinylbenzene mixtures onto PFA films, J. Membr. Sci. 216 (2003) 27–38. [17] B. Gupta, F.N. Buchi, G.G. Scherer, A. Chapiro, Crosslinked ion exchange membranes by radiation grafting of styrene/divinylbenzene into FEP films, J. Membr. Sci. 118 (1996) 231–238. [18] F.N. Buchi, B. Gupta, O. Haas, G.G. Scherer, Study of radiationgrafted FEP-g-polystyrene membranes as polymer electrolytes in fuel cells, Electrochim. Acta 40 (1995) 345–353. [19] J. Sun, Y. Zhang, X. Zhong, X. Zhu, Modification of polytetrafluoroethylene by radiation. 1. Improvement in high temperature properties and radiation stability, Radiat. Phys. Chem. 44 (1994) 655–659. [20] E. Katoh, H. Sugisawa, A. Oshima, Y. Tabata, T. Seguchi, T. Yamazaki, Evidence for radiation induced crosslinking in polytetrafluoroethylene by means of high-resolution solid-state 19 F high-speed MAS NMR, Radiat. Phys. Chem. 54 (1999) 165–171. [21] X. Zhong, L. Yi, W. Zhao, J. Sun, Y. Zhang, Radiation-induced crystal defects in PTFE, Polym. Degrad. Stab. 40 (1993) 97–100. [22] A. Oshima, S. Ikeda, E. Katoh, Y. Tabata, Chemical structure and physical properties of radiation-induced crosslinking of polytetrafluoroethylene, Radiat. Phys. Chem. 62 (2001) 39–45. [23] A. Oshima, S. Ikeda, T. Seguchi, Y. Tabata, Improvement of radiation resistance for polytetrafluoroethylene (PTFE) by radiation crosslinking, Radiat. Phys. Chem. 49 (1997) 279–284. [24] U. Lappan, U. Geißler, L. H¨außler, D. Jehnichen, G. Pompe, K. Lunkwitz, Radiation-induced branching and crosslinking of poly(tetrafluoroethylene) (PTFE), Nucl. Instrum. Method B 185 (2001) 178–183. [25] Z.S. Nurkeeva, A.A. Al-Sayed, V.V. Khutoryanskiy, G.A. Mun, S.M. Koblanov, Radiation grafting of vinyl ether of monoethanolamine on polyethylene films, Radiat. Phys. Chem. 65 (2002) 249–254. [26] V.S. Ivanov, Radiation Chemistry of Polymers, VSP, Utrecht, The Netherlands, 1992.