1. Max Planck Institute for Polymer Research, P.O. Box 3148, 55128 Mainz, Germany, Section of Solid State Physics, University of Athens, GR-157 84 Athens, Greece, Department of Chemical and Biological Engineering, University of WisconsinMadison, Wisconsin 53706, and Department of Materials Science, University of Crete and FO.R.T.H., P.O. Box 1527, GR-711 10 Heraklion, Greece