Study of the Morphology and Mechanical Properties of Vulcanized Rubber Functionalized with Sodium Phosphate Esters

Reinforcement of styrene–butadiene rubber/butadiene rubber is achieved by grafting mercapto-functionalized sodium phosphate esters to the rubber during peroxide curing. The size of the anionic phosphate ester moiety is varied by changing the alkyl substituent (ethyl vs octyl) with the intention of modulating the association strength between the grafted ionic dipoles. The concentration of the ionic graft is also varied. These lead to different degrees of reinforcement and dynamic mechanical properties of the vulcanized rubber. The morphologies of ionic aggregates are characterized by transmission electron microscopy (TEM) imaging and X-ray scattering experiments. TEM imaging revealed heterogeneities in the material, which sometimes included the formation of vesicular aggregates with diameters of approximately 20–30 nm. These features are consistent with the radius of gyration calculated from a knee observed in X-ray scattering at scattering vectors below 0.4 nm^–1. Smaller aggregates were analyzed using the Kinning–Thomas liquid-like hard sphere model and were found to be similar in radius (∼1.2–1.3 nm), irrespective of the molar volume of the ionic grafts from which they were constituted. However, the number of aggregates per unit volume within the material and the aggregation number are both profoundly affected by such variations. The structural variations of the ionic graft give rise to substantial changes in the tensile properties of the rubber at room temperature. At high temperatures, the differences in the dynamic moduli substantially diminish among the ionically grafted vulcanizates but remain substantial between the vulcanizates with and without ionic grafts. The equilibrium moduli of all vulcanizates converge at 60 °C. Possible mechanisms responsible for reinforcement are discussed at various temperatures, time scales, and ionic graft concentrations.