1. Department of Polymer Technology, Eindhoven University of Technology, Helix HEO 0.26, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, FOM-Institute AMOLF, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands, Philips Research Laboratories, Professor Holstlaan, 5656 AA Eindhoven, The Netherlands, and Dutch Polymer Institute, Eindhoven University of Technology, HEO 0.22, P.O. Box 513, 5600 MB Eindhoven, The Netherlands