1. Physics Department, University of Fribourg, CH-1700 Fribourg, Switzerland, Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark, Department of Physics and Astronomy, The University of Edinburgh, Edinburgh EH9 3JZ, U.K., Polymer Institute, ETH Zürich, CH-8092 Zürich, Switzerland, and Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland