1. Max-Planck Institut für Polymerforschung, Ackermannweg 10, Postfach 3148, D-55021 Mainz, Germany, ISIS Facility, Rutherford Appleton Laboratory, Didcot, U.K., and Center for Education and Research on Macromolecules, University of Liège, Sart-Tilman B6, B-4000 Liège, Belgium