1. Polymer Materials Program, Department of Chemical Engineering, Princeton University, Princeton, NJ 08544
2. Current address: Bell Laboratories, Whippany, NJ 07981
3. Ministry of Defence, Royal Armament Research and Development Establishment (Waltham Abbev), Essex EN9 IBP, United Kingdom
4. Current address: Department of Mechanical Engineering, Imperial College, London University, London, SW7 2BX United Kingdom