Merging of the α and β relaxations in polybutadiene: A neutron spin echo and dielectric study

Abstract

The local dynamics of 1,4 polybutadiene below and above the merging of the α and β relaxations have been investigated by combining neutron spin echo (NSE) and dielectric spectroscopy. The study of the dynamic structure factor measured by NSE over a wide momentum transfer range allows us to characterize the α relaxation as an interchain process while the β relaxation originates from mainly intrachain motions. At temperatures below the merging, the dynamic structure factor can be described by a superposition of elemental processes for the β relaxation as obtained from dielectric spectroscopy. The elemental motions behind this process can be related to rotational jumps of the chain building blocks around their center of mass. Furthermore, we have been able to consistently describe the dynamic structure factor above the merging of the α and β relaxations by assuming that both processes are statistically independent. In the framework of this scenario a procedure for analyzing the dielectric response in the α-β merging region has been developed. Its application to the dielectric data allows us to describe the dielectric response in this region on the basis of the low temperature behavior of the α and β processes and without considering any particular change in the relaxation mechanism of these processes. The temperature dependence found for the relaxation time of the α process follows now the viscosity, a masked feature in the experimental data due to the merging process. In this way, we have been able to consistently describe the relaxation of both, the polarization and the density fluctuations, by using the same scenario, i.e., independent α and β processes, and considering the same functional forms and temperature dependences of the characteristic times of the two processes. © 1996 The American Physical Society.