1. The Dutch Polymer Institute/Eindhoven Polymer Laboratories, Eindhoven University of Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands, Department of Physics, University of Ulm, Albert Einstein Allee 11, D-89069 Ulm, Germany, and Department of Physics, University of Bristol, Royal Fort, Tyndall Avenue, Bristol BS8 1TL, England (U.K.)