1. Key Laboratory of Polymer Materials of Gansu Province, Key Laboratory of Eco-functional Polymer Materials of the Ministry of Education, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, China
2. College of Science, Gansu Agricultural University, Lanzhou, Gansu 730070, China
3. State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, Gansu 730070, China