About the Activation Energies of the Main and Secondary Relaxations in Cured Styrene Butadiene Rubber

This article studies the influence of the network structure on the activation energies of the alpha and beta relaxations in vulcanized styrene butadiene rubber, SBR. A cure system based on sulphur and TBBS (N-t-butyl-2-benzothiazole sulfenamide) was used in the formulation of several compounds cured at 433 K. The activation energies were evaluated from internal friction (loss tangent) data of the compounds using an automated subresonant forced pendulum in a wide frequency range and between 80 K and 273 K. The internal friction data of the samples reveal two transitions, alpha and beta, characterized by the temperatures T-alpha and T-beta, due to the glass transition and the phenyl group rotation of the copolymer, respectively. Although T-alpha increases at higher crosslink density, it shows also a dependence with the amount of polysulphide and monosulphide linkages present in the samples. The highest activation energy for this process is obtained for the samples with high crosslink density and 30% of monosulphides in this structure. In the case of the beta-relaxation, there is a pronounced change in the activation energy between the uncured and the cured samples. The type of structure formed during vulcanization has an important effect in the activation energy of the segmental mode-process. In the case of the (beta-process, the cis-trans isomerization that takes place during vulcanization in the butadiene part of the SBR, might be the cause of conformational changes in the surrounding of the phenyl rings that affect the energy barrier associated to the phenyl rotation. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2361-2367, 2009