1. Center for Macromolecular Engineering Department of Chemistry Carnegie Mellon University Pittsburgh PA 15213 USA
2. Department of Mechanical Engineering Carnegie Mellon University Pittsburgh PA 15213 USA
3. Carl Zeiss X‐Ray Microscopy Pleasanton CA 94588 USA
4. School of Materials Science and Engineering University of Science and Technology Beijing 30 Xueyuan Road Beijing 100083 P. R. China