The α and Johari–Goldstein Relaxations in 1,4-Polybutadiene: Breakdown of Isochronal Superpositioning

Abstract

Dielectric spectra were measured for 1,4-polybutadiene (PBD) at various temperatures and pressures corresponding to a constant value of the relaxation time, τα, for the local segmental dynamics (α-process). Given the relationship of the Johari–Goldstein secondary relaxation to the α-process, it is of interest to determine whether the frequency separation of the Johari–Goldstein secondary relaxation and the α relaxation remains essentially constant under isochronal conditions. We find for PBD this is not the case; the JG relaxation peak moves systematically to higher frequencies at constant τα with increasing temperature and pressure. We show using molecular dynamics simulations that the behavior of PBD, which differs from that reported previously for molecular liquids, is a consequence of the torsional inflexibility of the polymer backbone. This accentuates the effect of constraints from local intermolecular barriers, with consequent deviation in the response of the JG and α relaxations due to their diffe...