1. Beijing National Laboratory for Molecular Sciences; Key Laboratory of Polymer Chemistry and Physics of Ministry of Education; College of Chemistry; Peking University; Beijing 100871 P. R. China
2. School of Chemical Engineering and Technology; Tianjin University; Tianjin 300072 P. R. China
3. State Key Laboratory for Mechanical Behavior of Materials; Xi'an Jiaotong University; Xi'an 710049 P. R. China
4. Institut Charles Sadron; CNRS-Université de Strasbourg; Strasbourg 67034 France