1. Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lódz, Poland, Max-Planck-Institut für Polymerforschung, Postfach 3148, D-55021 Mainz, Germany, and Humboldt University Berlin, Department of Physics, Invalidenstrasse 110, D-10115 Berlin, Germany