1. Department of Physics, University of Ioannina, and Foundation for Research and Technology-Hellas (FORTH), Biomedical Research Institute (BRI), P.O. Box 1186, GR-45110 Ioannina, Greece, Max-Planck-Institut für Polymerforschung, D-55021 Mainz, Germany, and Department of Chemistry, University of Athens, Panepistimiopolis, Zografou, GR-15771 Athens, Greece