In this work, styrene-butadiene rubber (SBR) was thermally aged at 130 °C with ageing period up to 4 days. The relaxation behavior was studied by dynamic mechanical analysis (DMA) in temperature scanning mode at multifrequencies and the crosslinking network was characterized by swelling method. Glass transition temperature (Tg) and activation energies (Ea) during transition evaluated by Starkweather method were found to increase after ageing. Time-temperature superposition (TTS) procedure was performed to obtain master curve and shift factors (αT). The derived αTs were fitted by William-Landel-Ferry (WLF) and Vogel-Fulcher-Tamman (VFT) equations. WLF fitting parameters confirmed decreased free volume fraction (f) and thermal expansion coefficient (αf) near Tg versus ageing time. Temperature dependent relaxation properties were examined by VFT fitting parameter D. It was found that the aged SBR displayed more brittle behavior, becoming more and more deviated from Arrhenius fashion. This phenomenon was illustrated by the enhanced intermolecular coupling and severe crosslinking during ageing.

Thermal oxidative ageing process of commercial available styrene–butadiene random copolymer (SBR) has been studied by the combination use of in-situ collected dynamic FTIR spectrum and two dimensional correlation analysis. The modification on the viscoelastic properties were studied by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). It was found that the degradation took place predominantly in the aliphatic segment at the allylic position, giving rise to a maximum absorption at 1725 cm−1 in the carbonyl stretching region. The initiation process occurring in the aromatic parts produced acetophenone end groups as evidenced by the shoulder peak at 1679 cm−1. A series of oxygenated species were identified in the dynamic spectrum including anhydrides, lactones, peracids, aliphatic ketones and unsaturated carbonyl species. However, the band of unsaturated carbonyl species at 1695 cm−1 varied nonmonotonously, first increase sharply and then decreased. Concerning the concentration of double bonds, a detailed analysis revealed the different evolving trend between benzylic and aliphatic vinyls. The former showed an increment whereas the later were found to decrease, resulting in the broadening at 1638 cm−1. Furthermore, the relative generation rates of different carbonyls were ranked by two dimensional correlation analysis together with two shoulder peaks at 1713 and 1679 cm−1 identified subtly in the asynchronous spectrum. On the basis of the above spectroscopic results, the mechanisms of thermal degradation were proposed.The relaxation behavior of thermally degraded SBR was studied by DMA and DSC. Both of the two techniques evidenced an increasing and broadening Tg, suggesting the crosslinking reaction dominated thermal degradation. After checking the normalized spectra, it is found that all of the relaxation curves (irrespective of storage modulus, loss modulus or loss tangent) get widen over ageing. Two parameters, steepness (S) and wideness index (W) were defined in normalized storage modulus plot to make a quantitative discussion. The S reduced whereas the W rose up after degradation. The broader glass transition zone after ageing was attributed to the more heterogeneous relaxation environment and the wider size distribution of motion units participating in the transition.