1. Institut für Physikalische Chemie, Universität Wien, Währingerstrasse 42, A 1090 Vienna, Austria, Department of Chemical Physics, Faculty of Chemical Technology, Slovak Technical University, Radlinskeho 9, 812 37 Bratislava, Slovakia, Theoretische Polymerphysik, Universität Freiburg, Rheinstrasse 12, D-79104 Freiburg i.Br., Germany, and P. N. Lebedev Physical Institute of Academy of Sciences of Russia, Leninsky Prospect 53, Moscow 117924, Russia