1. Contribution from the Department of Chemistry, the Institute for Biophysical Dynamics & the James Franck Institute, the University of Chicago, Chicago, Illinois 60637, Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Amherst, Massachusetts 01003, Manuel Lujan, Jr. Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Materials Research Department, Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark