1. Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136, Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, Reactor Research Facility, University of MissouriColumbia, Columbia, Missouri 65211, and Corporate Research Laboratory, Exxon Research & Engineering Company, Route 22 East, Clinton Township, Annandale, New Jersey 08801