Dielectric α- and β-relaxations in uncured styrene butadiene rubber

The development of the dynamic glass transition in styrene-butadiene copolymers has been investigated by dielectric spectroscopy in the frequency range from 10(-2) to 10(6) Hz. Two processes were detected and attributed to the alpha- and beta-relaxations. The alpha relaxation time has a non-Arrhenius temperature behavior that is highly dependent on styrene content while the beta relaxation time shows an Arrhenius behavior with an activation energy that is independent of styrene-content. Furthermore, the shape of the alpha-relaxation is strongly influenced by the styrene content while the shape of beta-relaxation is not. We interpret these results as follows. The observed beta-relaxation is primarily due to local motions of butadiene monomers and therefore not affected by the presence of styrene. The alpha-relaxation, on the other hand, is highly sensitive to the styrene content due to its cooperative character.