1. Laboratoire Léon Brillouin, UMR12, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France, Laboratory of Polymer Chemistry, University of Helsinki, PB 55, FIN-00014 HY, Finland, and Service de Physique de l’Etat Condensé, URA 2464, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France