Dynamic mechanical relaxation of cross-linked styrene-butadiene polymers containing free chains: Possibility of reptation

The structure of styrene-butadiene rubbers (SBRs) consists of chains attached to cross-links of relatively small number density. These rubbers show an α-relaxation process that is attributed to segmental motions of the attached chains. To investigate how an SBRʹs rubber-like behavior would change when free chains interpenetrate its structure, we prepared one SBR of mol wt 120 × 103 containing 3–4% free chains of identical mol wt, and studied its dynamic mechanical relaxation. For 0.1 Hz frequency measurements, the α-relaxation peak appeared at 210 K, and a second relaxation peak, which we refer to as αR-relaxation, at 300 K. A third relaxation of small magnitude was resolved at temperatures between the αR- and the α-relaxation peaks, and its origin was discussed in relation to other studies. Spectral features of the αR-and the α-relaxations were found similar. We investigated the merits of the time-temperature superposition of the α- and of the αR-relaxations. The αR relaxation vanished when the free chains were extracted from the SBR by swelling-extraction procedure. The features of α-relaxation can be interpreted as segmental motions of the SBR, but the features of the αR-relaxation is be interpretable only in terms of tube model for diffusion of free chains in a lightly cross-linked structure. We estimated the mol wt of these free chains and the mass between entanglements, and then used these quantities to calculate, by using the Doi-Edwards formalism, the relaxation time of the αR-process at 293 K. This relaxation time agrees with the experimental value. The αR-relaxation seems consistent with the tube model for diffusion of free chains in the cross-linked structure of SBR.