DYNAMIC STRUCTURE FACTORS DUE TO RELAXATION PROCESSES IN GLASS-FORMING POLYMERS

We report neutron spin echo experiments on the momentum (Q) and time ( t)-dependent dynamic structure factor S(Q, t) from the glass-forming p olymers polyisobutylene (PIB) and polybutadiene (PB). Performing measu rements in a Q-range encompassing the first and second structure facto r peaks, we are able to separate inter and intrachain relaxations and to assign the secondary Johari-Goldstein beta-relaxation to an intrach ain relaxation. While for PB this process exhibits average motional am plitudes of about 1.5 Angstrom, the underlying jump distances for PIE are much smaller (0.6 Angstrom). The structural alpha-relaxation is ch aracterized as an interchain process displaying anomalous diffusion. I ts temperature dependence thereby agrees with that of the viscous flow . The data bear evidence that a polymer segment undergoes primary and secondary relaxation in a statistically independent way. (C) 1998 Else vier Science B.V. All rights reserved.