1. Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup-si, Jeollabuk-do 580-185, Republic of Korea
2. Department of Chemistry, Sogang University, Mapo-gu, Seoul 121-742, Republic of Korea
3. Department of Polymer Science and Engineering, Chungnam National University, Yuseong-gu, Daejeon 305-764, Republic of Korea