LOCAL MOTIONS OF CIS-1,4-POLYBUTADIENE IN THE MELT - A QUASI-ELASTIC NEUTRON-SCATTERING STUDY

Local motions of cis-1,4-polybutadiene in the melt have been investigated by quasielastic neutron scattering in the time range 10(-13) to 10(-10) s. We found two modes in this time range. The characteristic time of the faster mode is on the order of 10(-12) s, and the activation energy is congruent-to 0.5 kcal/mol. On the other hand, the slower mode has a characteristics time of about 5 x 10(-11) s with an activation energy of congruent-to 2.5 kcal/mol, which almost corresponds to the height of one energy barrier separating rotational isomeric states (2-3 kcal/mol). The faster and slower modes are assigned to damped vibrational motion and local conformational transition, respectively. The latter is analyzed in terms of a jump diffusion model with the damped vibrational motions on the jump sites. The physical picture of the model is very similar to the results of the dynamic simulation by Helfand and co-workers, which have showed that isolated conformational transitions occur as well as cooperative transitions. The observed conformational transition rate is compared with the Kramers' rate theory, and the spatial scales of the motions are discussed based on the molecular structure of cis-1,4-polybutadiene.