1. Department of Polymer Materials, School of Materials Science and Engineering Shanghai University Shanghai 200444 P. R. China
2. Mork Family Department of Chemical Engineering and Materials Science University of Southern California 925 Bloom Walk Los Angeles CA 90089‐1211 USA
3. Department of Chemistry University of Southern California 840 Downey Way Los Angeles CA 90089‐0744 USA
4. Department of Biomedical Engineering University of Southern California 3650 McClintock Avenue Los Angeles CA 90089‐1111 USA