1. Institute of Macromolecular Chemistry, Heyrovský Sq. 2, 16206 Prague 6, Czech Republic
2. CNRS, Centre de Recherche Paul-Pascal, 115 Avenue du Docteur-Schweitzer, 33600 Pessac, France
3. Laboratoire Léon Brillouin, CEA Saclay, 91191 Gif-sur-Yvette, France