1. The Polymer Centre, Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK; Department of Physics & Astronomy and Polymer IRC, University of Leeds, Leeds LS2 9JT, UK; Netherlands Organization for Scientific Research (NWO), DUBBLE-CRG/ESRF, B.P. 220, F-38043 Grenoble Cedex, France; and CLRC, Daresbury Laboratory, Daresbury, Warrington, Cheshire, WA4 4AD, UK