1. Polymer Materials and Radiation Group, Department of Chemistry, University of Queensland, St Lucia, Queensland, 4072, Australia DuPont Dow Elastomers L. L. C., DuPont Experimental Station, P.O. Box 80328, Wilmington, Delaware 19880-0328, Japan Atomic Energy Research Institute, Takasaki Radiation Chemistry Research Establishment, Takasaki Gunma 370-12, Japan, Center for Magnetic Resonance, University of Queensland, St Lucia, Queensland, Australia