1. Key Laboratory of Mesoscopic Chemistry of MOE, Department of Polymer Science and Technology, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China
2. Physics Department and Shanghai Key Laboratory of Magnetic Resonance, East China Normal University, Shanghai 200062, P. R. China
3. College of Material Science and Engineering, Polymer Institute, South China University of Technology, Guangzhou, Guangdong 510640, P. R. China