1. The Polymer Centre, Department of Chemistry, The University of Sheffield, Sheffield S3 7HF, U.K., CCLRC Daresbury Laboratory, Warrington WA4 4AD, U.K., DUBBLE CRG, ESRF, 6 rue Jules Horowitz, BP 220, F-38043 Grenoble Cédex 9, France, and The Polymer Centre, Department of Physics & Astronomy, The University of Sheffield, Sheffield S3 7RH, U.K.