1. Eindhoven Polymer Laboratories, The Dutch Polymer Institute, Eindhoven University of Technology, P.O. Box 513, NL-5600 MB, Eindhoven, The Netherlands; DSM Research, P.O. Box 18, NL-6160 MD, Geleen, The Netherlands; and Danish Polymer Centre, Risø National Laboratory, DK-4000, Roskilde, Denmark