1. School of Chemical Engineering and Materials Science, Chung-Ang University, 221 Huksuk-dong, Dongjak-gu, Seoul, 156-756, Korea, Department of Conservative Dentistry and Dental Research Institute, College of Dentistry, Seoul National University. 28-2 Yongon-dong, Jongno-gu, Seoul, Korea, and Department of Polymer Science and Engineering, Dankook University, Korea