1. Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003; Faculté des Sciences, Université du Maine, Avenue O. Messiaen, 72085 Le Mans Cedex 09, France; Department of Physics, Brookhaven National Laboratory, Upton, New York 11973; IBM T. J. Watson Research Center, Yorktown Heights, New York 10598; and IBM Almaden Research Center, San Jose, California 95120