1. Contribution from BASF AG, ZKM/A, B1, D-67056 Ludwigshafen, Germany, Institute of Macromolecular Compounds of the Russian Academy of Sciences, Bolshoi prospect 31, 199004 St. Petersburg, Russia, and Institut für Physikalische Chemie, Johannes Gutenberg-Universität, Welderweg 15, D-55099 Mainz, Germany