Thermodynamic Parameters of Crosslinked Elastomers (BR, SBR and NBR) and Their Blends

Herein, a methodology is employed based on the Flory-Rehner equation for estimating the Flory-Huggins interaction parameter (χ₁₂*) of crosslinked elastomer blends. For this purpose, binary elastomer blends containing polybutadiene rubber (BR), styrene-butadiene rubber (SBR) and nitrile-butadiene rubber (NBR), were prepared in a mixing chamber at a temperature below the activation of the crosslinking agent. Swelling tests with benzene were employed to determine the crosslinked fraction, finding that after 20 min of thermal annealing, the BR and NBR were almost completely crosslinked, while the SBR only reached 60%. Additionally, the BR-SBR blend increased by 2-3 times its volume than its pure components; this could be explained based on the crosslink density. From the mechanical tests, a negative deviation from the rule of mixtures was observed, which suggested that the crosslinking was preferably carried out in the phases and not at the interface. Furthermore, tensile tests and swelling fraction (ϕ_sw) results were employed to determine the average molecular weight between two crosslinking points (M_c), and subsequently χ₁₂*. Calculated χ₁₂* values were slightly higher than those reported in the literature. The calculated thermodynamic parameters for the blends showed positive ΔG_mix values and endothermic behavior, suggesting their immiscible nature.