Segregation behavior of a tackifier in styrene-butadiene rubber under a temperature gradient

Structure development under a temperature gradient was studied using a miscible blend of styrene-butadiene rubber (SBR) and a tackifier, an oligomeric copolymer comprising mainly styrene and α-methyl styrene (AMS). AMS was found to be miscible with SBR at an AMS content of up to 30 parts per hundred rubber (phr) (23 wt.%) at temperatures below 120 °C. A blend sheet with a thickness of 1 mm was placed in a compression-molding machine, where the top and bottom plates were maintained at different temperatures, such as 120 °C/80 °C and 100 °C/60 °C. After 30 min, the AMS contents on both surfaces were characterized. The AMS content on the low temperature side was high, and vice versa, with no phase separation. Furthermore, the phase diagram of the SBR/AMS blends as a function of the blend composition and temperature was examined. The system was found to show lower critical solution temperature behavior, suggesting that the Flory–Huggins interaction parameter increases with temperature. Therefore, at low temperature, blends containing large amounts of AMB must have a low free energy, which may result in the different compositions of the surfaces after exposure to a temperature gradient. The segregation behavior of Styrene-Butadiene Rubber (SBR) and poly(α-methylstyrene) (AMS) under a temperature gradient is investigated in detail. SBR and AMS are found to be miscible at an AMS content of 30 phr (23 wt.%), as confirmed by dynamic mechanical analysis. After treatment under a temperature gradient, AMS is observed to segregate toward the surface exposed to the lower temperature, as detected by Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR). The underlying mechanism is further elucidated based on the phase diagram of the SBR/AMS system.