1. Institut Charles Sadron, CNRS-ULP, 67083 Strasbourg Cedex, France, Departement of Physical and Colloid Chemistry, Wageningen Agricultural University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands, Polymer Laboratory, University of Moscow, Moscow, Russia, and Departement de Fisica Fondamental, Facultat de Fisica, Universitat de Barcelona, Barcelona, Spain