- Email: [email protected]
Study on poly-electrolyte membrane of crosslinked PTFE by radiation-grafting
Nuclear Instruments and Methods in Physics Research B 208 (2003) 424–428 www.elsevier.com/locate/nimb
Study on poly-electrolyte membrane of crosslinked PTFE by radiation-grafting Kohei Sato a
a,*
, Shigetoshi Ikeda b, Minoru Iida b, Akihiro Oshima Yoneho Tabata b,c, Masakazu Washio a
a,b
,
Advanced Research Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan b RAYTECH Corporation, Sousyu Building 402, 4-40-13, Takadanobaba, Shinjuku, Tokyo 169-0075, Japan c Professor Emeritus, The University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-8656, Japan
Abstract Polymer electrolyte fuel cell membrane based on crosslinked polytetrafluoroethylene (PTFE) [RX-PTFE] has been processed by radiation-grafting with reactive styrene monomers by c-rays under atmospheric circumstances, and the characteristic properties of the obtained membranes have been studied. The grafting yields of styrene monomer onto RX-PTFE, which have various crosslinking densities, were in the range of 5–100%. At the reaction period of 24 h, the grafting yields for RX-PTFE with low crosslinking density, which was reacted at 60 °C, achieved 94%. As a tendency, the lower grafting temperature gives higher grafting ratio of styrene onto RX-PTFE. Moreover, the yields of subsequent sulfonation for all samples were close to 100%. Mechanical properties were decreased with increasing grafting yields; especially the membrane with higher grafting yields was brittle. Ion exchange capacity of sulfonated RX-PTFE reached 1.1 meq/g while maintaining the mechanical properties. Ó 2003 Elsevier B.V. All rights reserved. PACS: 61.82.Pv Keywords: Polymer electrolyte fuel cell; Crosslinked PTFE; Radiation-grafting; Styrene monomer; Poly-electrolyte membrane
1. Introduction Fuel cell has attracted attention as a low exhaust new generation dynamo. Especially Polymer electrolyte fuel cells (PEFC) using proton exchange membranes (PEM) have attracted much attention for stationary and electric vehicle applications. Ready-made perfluorosulfonic acid mem-
*
Corresponding author. Tel.: +81-3-5286-3893; fax: +81-33205-0723. E-mail address: [email protected] (K. Sato).
branes have been developed as a PEM. However there are still several problems such as insufficient mechanical properties and low thermal resistance. Recently, it has been demonstrated that polytetrafluoroethylene (PTFE) can be crosslinked by irradiation in its molten state around 340 5 °C under an oxygen-free atmosphere [1–4]. The RXPTFE shows remarkable improvements in radiation resistance and mechanical properties, such as yield strength and YoungÕs modulus, compared with those of non-crosslinked PTFE [1,3–6]. In this study, an attempt has been made PEM by radiation-induced grafting of styrene onto RX-PTFE
0168-583X/03/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)00898-X
K. Sato et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 424–428
films at various temperatures and subsequently sulfonation. The relationship between the grafting yield and crosslinking dose has been investigated as well as the effect of the various grafting conditions on the grafting yields. 2. Experimental PTFE called G-192 was supplied from Asahi Glass Co. Ltd. in a sheet of 0.5 mm thickness. The molecular weight was about 1.0 107 from the determination of heat of crystallization using SuwaÕs equation [7]. The radiation crosslinking was made by electron beam (EB) irradiation around 335 °C 3 °C in argon gas atmosphere, as described in our previous paper [1,3,6]. Original and obtained crosslinked PTFE (RXPTFE) films (crosslinking doses of 50, 100, 500, 2000 kGy are called RX50, RX100, RX500 and RX2000, respectively) were cut into strips and irradiated with a dose of 30 kGy using c-rays from a 60 Co source at room temperature in air. Irradiated film was immersed in styrene monomer in glass ample and grafted at various temperatures (60–120 °C) under vacuum (102 Torr). After the reaction, films were removed from the ampoule and washed with carbon-tetrachloride to remove the homopolymer adhering to the surface of grafted films. The grafting yield was determined as the weight gain according to the following equation: Grafting yield ð%Þ ¼
425
measured by ESR. c-irradiation was carried out at room temperature under vacuum and in air. The higher the crosslinking density, the higher the initial rate increases in the radical yields. For original PTFE, the yields of radicals are proportional to cdose. However, for RX-PTFE, it tends to saturate above 30 kGy. Thus, c-irradiation with a dose of 30 kGy was used for the grafting experiments, because it was thought that the trapped radicals would react more efficiently with reactive styrene monomer. Radical yields of both samples irradiated at room temperature in air and under vacuum were almost the same. Fig. 1 shows grafting yields as a function of reaction period for styrene grafting onto RXPTFE films with a crosslinking dose of 50 kGy at various temperatures. For all temperatures, grafting yields show a rapid increase initially, and then tend to saturate above 5 h approximately. For reaction temperature 80 °C, the initial rate of grafting was higher than that of the other temperatures, while the grafting yield was finally lower compared to that at 60 °C. Such behavior could be explained by the following reasons. The reason for the acceleration of grafting yields might be due to the diffusion ratio of styrene monomer into the
Wg W0 100; W0
where Wg and W0 are the weight of grafted sample and initial weight of original PTFE film, respectively. The grafted films were sulfonated with a mixture of chlorosulfonic acid and carbon tetrachloride (20:80 vol%) at room temperature, and ionic conductivity of the membranes were measured. FT-IR, ESR measurements and tensile test were carried out for original and grafted PTFE. 3. Results and discussion The yields of free radicals trapped at room temperature in RX-PTFE for various c-dose were
Fig. 1. Grafting yields as a function of reaction period for styrene grafting onto RX-PTFE with a crosslinking dose of 50 kGy film (RX50) at various temperatures.
426
K. Sato et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 424–428
interior of RX-PTFE matrix that would be enhanced at higher reaction temperatures, which increases the availability of the monomer to the radical sites. On the contrary, inhibition of grafting yields can be explained by the fact that radical annihilation also would be enhanced at higher temperatures. Thus, the grafting yield at 60 °C is higher than that for other temperatures due to competitive reactions between grafting and elimination of radicals. Fig. 2 shows grafting yields for various temperatures of styrene grafting onto RX-PTFE film as a function of crosslinking dose. Reaction periods were 24 h. Maximum grafting yields are obtained around the crosslinking dose of 50–500 kGy and crosslinking dose of 2 MGy shows very low grafting yields. The high dose crosslinked PTFE has higher crosslinking density and has limited molecular motion. Hence, styrene monomer has not sufficiently diffused into the sample, and most of styrene has been homopolymerized. Fig. 3 shows grafting yields of styrene as a function of reaction temperature onto different RX-PTFE films with reaction periods of 24 h. Grafting yields decreased as temperature increased except for RX500 and RX2000. From ESR mea-
Fig. 2. Grafting yields for various temperatures of styrene grafting onto RX-PTFE film as a function of crosslinking dose. Reaction periods were 24 h.
Fig. 3. Grafting yields of styrene as a function of reaction temperature onto various RX-PTFE films (RX50, RX100, RX500, RX2000) and PTFE (original) with reaction periods of 24 h.
surements of temperature dependence [8–10], it has been found that most radicals were trapped in an amorphous region, and the mobility of polymer chains at the higher temperatures would be enhanced. Under such circumstances, the termination of primary radicals at higher temperatures becomes predominant. Therefore, when the styrene monomer was grafted onto RX-PTFE at the higher temperatures, the annihilation rate of radicals would have been faster than diffusion rate of styrene monomer into inner matrix. As a result, the grafting might react only at the surface, hence, the grafting yields at the higher temperatures were higher than that of lower temperatures. On the other hand, samples with very high crosslinking dose were more affected by increasing diffusion of the styrene than acceleration of radical annihilation at higher temperatures. Fig. 4 shows FT-IR spectra obtained for RXPTFE and grafted RX-PTFE. The presence of aromatic rings due to styrene grafting was confirmed by the @C–H stretching vibration at 3050 cm1 and the C–C in-plane stretching vibrations at 1500 and 1600 cm1 , respectively. C–H out of plane bending overtone and combination band
K. Sato et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 424–428
Fig. 4. FT-IR spectra obtained for RX-PTFE and styrene grafting onto RX-PTFE with a crosslinking dose of 100 kGy film, Grafting yield: 81.5%.
427
Results of the tensile test of grafted PTFE are shown in Table 1. Although grafted RX-PTFE with the higher crosslinking dose (i.e. grafted RX500) and grafted RX-PTFE with higher grafting yields (i.e. 24 h reaction) showed higher tensile strength, the RX-PTFE with high grafting yields was brittle and did not have toughness. The obtained membranes with higher grafting yields are very close to a property of polystyrene by styrene grafting onto matrix. Using the grafted films, sulfonation was carried out, at about 100%. Ion exchange capacity (IEC) of the obtained membrane (RX100, styrene grafting yield: 83%) was 1.1 (meq/g). The obtained IEC value is almost the same as the ready-made perfluorosulfonic acid membranes. Although the high ionic conductivity membrane obtained was a sulfonated high grafted RX-PTFE one, mechanical properties would have been deteriorated.
4. Conclusions patterns were in the region around 1660–2000 cm1 . The absorption bands at 2800–2900 and 2900–3000 cm1 are assigned to the symmetric and the asymmetric stretching of CH2 group, respectively.
PEFC membrane based on RX-PTFE have been processed by radiation-grafting with reactive styrene monomers by c-rays in air at room temperature, and the characteristic properties of the obtained membranes have been studied. The
Table 1 Tensile strength of styrene grafting onto RX-PTFE films (RX100, RX500) and PTFE (original)a Sample notation
EB crosslinking dose (kGy)
Grafting condition c-rays irradiation
Orignal
0
No irradiation 30 kGy, RT, air
RX100
100
No irradiation 30 kGy, RT, air
RX500
500
No irradiation 30 kGy, RT, air
a
Reaction temperature: 60 °C.
Grafting yields (%)
Tensile strength (MPa)
0 16 24
– 0.0 21.4 34.1
5.0 3.2 3.0 Broken
0 16 24
– 0.0 34.9 81.5
18.9 12.5 12.9 23.0
0 16 24
– 0.0 35.7 53.4
25.6 17.9 18.9 21.1
Reaction periods (h)
428
K. Sato et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 424–428
grafting yields of styrene monomer onto RXPTFE, which have various crosslinking densities, were in the range of 5–100%. Maximum grafting yields are obtained for samples with crosslinking dose of 50–500 kGy, while crosslinking doses of 2 MGy show very low grafting yields. At the reaction period of 24 h, the grafting yield for RX-PTFE with low crosslinking density, which was reacted at 60 °C, was 94%. As a tendency, lower grafting temperatures give higher grafting ratios onto RXPTFE. Sulfonation ratio was about 100%, and IEC of sulfonated RX-PTFE was 1.1 meq/g while maintaining the mechanical properties. Acknowledgements The authors acknowledge Prof. Y. Katsumura and Dr. C. Matsuura for c-ray irradiation experiments and various discussions. The authors also acknowledge Prof. Y. Hama and Research associate T. Oka for FT-IR experiments. The development of the new PEM using RX-PTFE was supported by projects of ‘‘Research and Develop-
ment of Polymer Electrolyte Fuel Cell’’ in the New Energy and Industrial Technology Development Organization (NEDO).
References [1] A. Oshima, Y. Tabata, T. Seguchi, in: Proceedings of 14th International Symposium on Florin Chemistry, Yokohama, Japan, July, 1994. [2] J. Sun, Y. Zhang, X. Zhong, X. Zhu, Radiat. Phys. Chem. 44 (1994) 655. [3] A. Oshima, Y. Tabata, H. Kudoh, T. Seguchi, Radiat. Phys. Chem. 45 (1995) 269. [4] Y. Tabata, A. Oshima, K. Takashika, T. Seguchi, Radiat. Phys. Chem. 48 (1996) 563. [5] Y. Tabata, A. Oshima, Macromol. Symp. 143 (1999) 337. [6] A. Oshima, S. Ikeda, E. Katoh, Y. Tabata, Radiat. Phys. Chem. 62 (2001) 39. [7] T. Suwa, M. Takehisa, S. Machi, J. Appl. Polym. Sci. 17 (1973) 3253. [8] A. Oshima, T. Seguchi, Y. Tabata, Radiat. Phys. Chem. 50 (1997) 601. [9] A. Oshima, T. Seguchi, Y. Tabata, Polym. Int. 48 (1999) 996. [10] A. Oshima, T. Seguchi, Y. Tabata, Radiat. Phys. Chem. 55 (1999) 61.
Study on poly-electrolyte membrane of crosslinked PTFE by radiation-grafting Kohei Sato a
a,*
, Shigetoshi Ikeda b, Minoru Iida b, Akihiro Oshima Yoneho Tabata b,c, Masakazu Washio a
a,b
,
Advanced Research Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan b RAYTECH Corporation, Sousyu Building 402, 4-40-13, Takadanobaba, Shinjuku, Tokyo 169-0075, Japan c Professor Emeritus, The University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-8656, Japan
Abstract Polymer electrolyte fuel cell membrane based on crosslinked polytetrafluoroethylene (PTFE) [RX-PTFE] has been processed by radiation-grafting with reactive styrene monomers by c-rays under atmospheric circumstances, and the characteristic properties of the obtained membranes have been studied. The grafting yields of styrene monomer onto RX-PTFE, which have various crosslinking densities, were in the range of 5–100%. At the reaction period of 24 h, the grafting yields for RX-PTFE with low crosslinking density, which was reacted at 60 °C, achieved 94%. As a tendency, the lower grafting temperature gives higher grafting ratio of styrene onto RX-PTFE. Moreover, the yields of subsequent sulfonation for all samples were close to 100%. Mechanical properties were decreased with increasing grafting yields; especially the membrane with higher grafting yields was brittle. Ion exchange capacity of sulfonated RX-PTFE reached 1.1 meq/g while maintaining the mechanical properties. Ó 2003 Elsevier B.V. All rights reserved. PACS: 61.82.Pv Keywords: Polymer electrolyte fuel cell; Crosslinked PTFE; Radiation-grafting; Styrene monomer; Poly-electrolyte membrane
1. Introduction Fuel cell has attracted attention as a low exhaust new generation dynamo. Especially Polymer electrolyte fuel cells (PEFC) using proton exchange membranes (PEM) have attracted much attention for stationary and electric vehicle applications. Ready-made perfluorosulfonic acid mem-
*
Corresponding author. Tel.: +81-3-5286-3893; fax: +81-33205-0723. E-mail address: [email protected] (K. Sato).
branes have been developed as a PEM. However there are still several problems such as insufficient mechanical properties and low thermal resistance. Recently, it has been demonstrated that polytetrafluoroethylene (PTFE) can be crosslinked by irradiation in its molten state around 340 5 °C under an oxygen-free atmosphere [1–4]. The RXPTFE shows remarkable improvements in radiation resistance and mechanical properties, such as yield strength and YoungÕs modulus, compared with those of non-crosslinked PTFE [1,3–6]. In this study, an attempt has been made PEM by radiation-induced grafting of styrene onto RX-PTFE
0168-583X/03/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)00898-X
K. Sato et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 424–428
films at various temperatures and subsequently sulfonation. The relationship between the grafting yield and crosslinking dose has been investigated as well as the effect of the various grafting conditions on the grafting yields. 2. Experimental PTFE called G-192 was supplied from Asahi Glass Co. Ltd. in a sheet of 0.5 mm thickness. The molecular weight was about 1.0 107 from the determination of heat of crystallization using SuwaÕs equation [7]. The radiation crosslinking was made by electron beam (EB) irradiation around 335 °C 3 °C in argon gas atmosphere, as described in our previous paper [1,3,6]. Original and obtained crosslinked PTFE (RXPTFE) films (crosslinking doses of 50, 100, 500, 2000 kGy are called RX50, RX100, RX500 and RX2000, respectively) were cut into strips and irradiated with a dose of 30 kGy using c-rays from a 60 Co source at room temperature in air. Irradiated film was immersed in styrene monomer in glass ample and grafted at various temperatures (60–120 °C) under vacuum (102 Torr). After the reaction, films were removed from the ampoule and washed with carbon-tetrachloride to remove the homopolymer adhering to the surface of grafted films. The grafting yield was determined as the weight gain according to the following equation: Grafting yield ð%Þ ¼
425
measured by ESR. c-irradiation was carried out at room temperature under vacuum and in air. The higher the crosslinking density, the higher the initial rate increases in the radical yields. For original PTFE, the yields of radicals are proportional to cdose. However, for RX-PTFE, it tends to saturate above 30 kGy. Thus, c-irradiation with a dose of 30 kGy was used for the grafting experiments, because it was thought that the trapped radicals would react more efficiently with reactive styrene monomer. Radical yields of both samples irradiated at room temperature in air and under vacuum were almost the same. Fig. 1 shows grafting yields as a function of reaction period for styrene grafting onto RXPTFE films with a crosslinking dose of 50 kGy at various temperatures. For all temperatures, grafting yields show a rapid increase initially, and then tend to saturate above 5 h approximately. For reaction temperature 80 °C, the initial rate of grafting was higher than that of the other temperatures, while the grafting yield was finally lower compared to that at 60 °C. Such behavior could be explained by the following reasons. The reason for the acceleration of grafting yields might be due to the diffusion ratio of styrene monomer into the
Wg W0 100; W0
where Wg and W0 are the weight of grafted sample and initial weight of original PTFE film, respectively. The grafted films were sulfonated with a mixture of chlorosulfonic acid and carbon tetrachloride (20:80 vol%) at room temperature, and ionic conductivity of the membranes were measured. FT-IR, ESR measurements and tensile test were carried out for original and grafted PTFE. 3. Results and discussion The yields of free radicals trapped at room temperature in RX-PTFE for various c-dose were
Fig. 1. Grafting yields as a function of reaction period for styrene grafting onto RX-PTFE with a crosslinking dose of 50 kGy film (RX50) at various temperatures.
426
K. Sato et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 424–428
interior of RX-PTFE matrix that would be enhanced at higher reaction temperatures, which increases the availability of the monomer to the radical sites. On the contrary, inhibition of grafting yields can be explained by the fact that radical annihilation also would be enhanced at higher temperatures. Thus, the grafting yield at 60 °C is higher than that for other temperatures due to competitive reactions between grafting and elimination of radicals. Fig. 2 shows grafting yields for various temperatures of styrene grafting onto RX-PTFE film as a function of crosslinking dose. Reaction periods were 24 h. Maximum grafting yields are obtained around the crosslinking dose of 50–500 kGy and crosslinking dose of 2 MGy shows very low grafting yields. The high dose crosslinked PTFE has higher crosslinking density and has limited molecular motion. Hence, styrene monomer has not sufficiently diffused into the sample, and most of styrene has been homopolymerized. Fig. 3 shows grafting yields of styrene as a function of reaction temperature onto different RX-PTFE films with reaction periods of 24 h. Grafting yields decreased as temperature increased except for RX500 and RX2000. From ESR mea-
Fig. 2. Grafting yields for various temperatures of styrene grafting onto RX-PTFE film as a function of crosslinking dose. Reaction periods were 24 h.
Fig. 3. Grafting yields of styrene as a function of reaction temperature onto various RX-PTFE films (RX50, RX100, RX500, RX2000) and PTFE (original) with reaction periods of 24 h.
surements of temperature dependence [8–10], it has been found that most radicals were trapped in an amorphous region, and the mobility of polymer chains at the higher temperatures would be enhanced. Under such circumstances, the termination of primary radicals at higher temperatures becomes predominant. Therefore, when the styrene monomer was grafted onto RX-PTFE at the higher temperatures, the annihilation rate of radicals would have been faster than diffusion rate of styrene monomer into inner matrix. As a result, the grafting might react only at the surface, hence, the grafting yields at the higher temperatures were higher than that of lower temperatures. On the other hand, samples with very high crosslinking dose were more affected by increasing diffusion of the styrene than acceleration of radical annihilation at higher temperatures. Fig. 4 shows FT-IR spectra obtained for RXPTFE and grafted RX-PTFE. The presence of aromatic rings due to styrene grafting was confirmed by the @C–H stretching vibration at 3050 cm1 and the C–C in-plane stretching vibrations at 1500 and 1600 cm1 , respectively. C–H out of plane bending overtone and combination band
K. Sato et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 424–428
Fig. 4. FT-IR spectra obtained for RX-PTFE and styrene grafting onto RX-PTFE with a crosslinking dose of 100 kGy film, Grafting yield: 81.5%.
427
Results of the tensile test of grafted PTFE are shown in Table 1. Although grafted RX-PTFE with the higher crosslinking dose (i.e. grafted RX500) and grafted RX-PTFE with higher grafting yields (i.e. 24 h reaction) showed higher tensile strength, the RX-PTFE with high grafting yields was brittle and did not have toughness. The obtained membranes with higher grafting yields are very close to a property of polystyrene by styrene grafting onto matrix. Using the grafted films, sulfonation was carried out, at about 100%. Ion exchange capacity (IEC) of the obtained membrane (RX100, styrene grafting yield: 83%) was 1.1 (meq/g). The obtained IEC value is almost the same as the ready-made perfluorosulfonic acid membranes. Although the high ionic conductivity membrane obtained was a sulfonated high grafted RX-PTFE one, mechanical properties would have been deteriorated.
4. Conclusions patterns were in the region around 1660–2000 cm1 . The absorption bands at 2800–2900 and 2900–3000 cm1 are assigned to the symmetric and the asymmetric stretching of CH2 group, respectively.
PEFC membrane based on RX-PTFE have been processed by radiation-grafting with reactive styrene monomers by c-rays in air at room temperature, and the characteristic properties of the obtained membranes have been studied. The
Table 1 Tensile strength of styrene grafting onto RX-PTFE films (RX100, RX500) and PTFE (original)a Sample notation
EB crosslinking dose (kGy)
Grafting condition c-rays irradiation
Orignal
0
No irradiation 30 kGy, RT, air
RX100
100
No irradiation 30 kGy, RT, air
RX500
500
No irradiation 30 kGy, RT, air
a
Reaction temperature: 60 °C.
Grafting yields (%)
Tensile strength (MPa)
0 16 24
– 0.0 21.4 34.1
5.0 3.2 3.0 Broken
0 16 24
– 0.0 34.9 81.5
18.9 12.5 12.9 23.0
0 16 24
– 0.0 35.7 53.4
25.6 17.9 18.9 21.1
Reaction periods (h)
428
K. Sato et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 424–428
grafting yields of styrene monomer onto RXPTFE, which have various crosslinking densities, were in the range of 5–100%. Maximum grafting yields are obtained for samples with crosslinking dose of 50–500 kGy, while crosslinking doses of 2 MGy show very low grafting yields. At the reaction period of 24 h, the grafting yield for RX-PTFE with low crosslinking density, which was reacted at 60 °C, was 94%. As a tendency, lower grafting temperatures give higher grafting ratios onto RXPTFE. Sulfonation ratio was about 100%, and IEC of sulfonated RX-PTFE was 1.1 meq/g while maintaining the mechanical properties. Acknowledgements The authors acknowledge Prof. Y. Katsumura and Dr. C. Matsuura for c-ray irradiation experiments and various discussions. The authors also acknowledge Prof. Y. Hama and Research associate T. Oka for FT-IR experiments. The development of the new PEM using RX-PTFE was supported by projects of ‘‘Research and Develop-
ment of Polymer Electrolyte Fuel Cell’’ in the New Energy and Industrial Technology Development Organization (NEDO).
References [1] A. Oshima, Y. Tabata, T. Seguchi, in: Proceedings of 14th International Symposium on Florin Chemistry, Yokohama, Japan, July, 1994. [2] J. Sun, Y. Zhang, X. Zhong, X. Zhu, Radiat. Phys. Chem. 44 (1994) 655. [3] A. Oshima, Y. Tabata, H. Kudoh, T. Seguchi, Radiat. Phys. Chem. 45 (1995) 269. [4] Y. Tabata, A. Oshima, K. Takashika, T. Seguchi, Radiat. Phys. Chem. 48 (1996) 563. [5] Y. Tabata, A. Oshima, Macromol. Symp. 143 (1999) 337. [6] A. Oshima, S. Ikeda, E. Katoh, Y. Tabata, Radiat. Phys. Chem. 62 (2001) 39. [7] T. Suwa, M. Takehisa, S. Machi, J. Appl. Polym. Sci. 17 (1973) 3253. [8] A. Oshima, T. Seguchi, Y. Tabata, Radiat. Phys. Chem. 50 (1997) 601. [9] A. Oshima, T. Seguchi, Y. Tabata, Polym. Int. 48 (1999) 996. [10] A. Oshima, T. Seguchi, Y. Tabata, Radiat. Phys. Chem. 55 (1999) 61.