1. Contribution from TNO Science and Industry, De Rondom 1, P. O. Box 6235, 5600 HE Eindhoven, The Netherlands, Philips Research Laboratories, Professor Holstlaan 4, 5656 AA, Eindhoven, The Netherlands, Department of Polymer Technology, Faculty of Chemistry and Chemical Engineering, Eindhoven University of Technology, P. O Box 513, 5600 MB Eindhoven, The Netherlands, and Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands