1. Key Laboratory of Polymer Ecomaterials; Changchun Institute of Applied Chemistry; Chinese Academy of Sciences; Changchun 130022 P. R. China
2. University of Chinese Academy of Sciences; Beijing 100039 P. R. China
3. School of Life Sciences; University of Science and Technology of China; Hefei 230026 P. R. China
4. Department of Materials Science and Engineering; University of Illinois at Urbana-Champaign; Urbana IL 61801 USA