1. Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands; DSM Research, P.O. Box 18, 6160 MD, Geleen, The Netherlands; IPTME, Loughborough University, Leicestershire LE11 3TU, United Kingdom; and Unité de Physique et de Chimie des Hauts Polymères, Université Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium