1. Laboratoire de Physique des Polymères, CP-223, Université Libre de Bruxelles, Bld. du Triomphe, B-1050, Brussels, Belgium, Chemistry Department, Catholic University of Leuven, Celestijnenlaan 200F, 3001 Heverlee-Leuven, Belgium, and Netherlands Organization for Scientific Research (NWO), DUBBLE-CRG/ESRF, B.P. 220, F-38043 Grenoble Cedex, France