[1] Philips Research Laboratories, P.O. Box 500145, 52085 Aachen, Germany
[2] Johann Wolfgang Goethe University, Frankfurt, Germany
[3] Max Planck Inst. F. Polymerphysik, Mainz, Germany
[4] Philips Research Laboratories, Aachen, Germany
[5] Philipps University of Marburg, Germany
[6] Bonn University
[7] Philips Research Laboratories