1. Department of Materials Science and Engineering and the Materials Science Center, Cornell University, Ithaca, New York 14853
2. Kawasaki Development Center, Japan Polyolefins Company, Ltd., 3−2, Chidori-Cho, Kawasaki-Ku, Kawasaki, Kanagawa, 210, Japan
3. Department of Theoretical and Applied Mechanics and the Materials Science Center, Cornell University, Ithaca, New York 14853