1. a Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea
2. b Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea
3. c Department of Chemical Engineering, Kyungpook National University, Daegu 702-701, Korea
4. d Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea