1. Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
2. Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901, USA
3. Institute of Physics, Johannes Gutenberg University Mainz, Staudingerweg 7, 55128 Mainz, Germany
4. Leibniz Institut für Polymerforschung Dresden, Hohe Strasse 6, D-01 069 Dresden, Germany