Interactions of Stober silica with natural rubber under the influence of coupling agents, studied by 1H NMR T2 relaxation analysis

H-1 NMR T-2 relaxation has proved to be a successful technique for the study of rubberfiller interactions in carbon black filled rubbers. For the present study, the interactions between NR and pure silica as well as silicas grafted with various coupling agents were investigated using 1H NMR T2 relaxation. Monodisperse spherical silica particles were used, prepared according to a method by Stober, and afterward treated with the coupling agents prior to mixing into NR. Depending on the type of coupling agent and the degree of condensation, grafting densities of coupling agent on the silica range from a minimum of 62% to 100%. NMR shows that the highest amount of immobilized rubber chains on the silica surface occurs for pure silica: about 15%. Grafting of the silica leads to a decrease of the fraction of immobilized rubber chains, dependent on the chemical structure of the coupling agent and its ability to physically or chemically interact with NR: 13% immobilized rubber for a mercapto-functional coupling agent, which is expected to react with the NR unsaturation, to about 1% for a coupling agent with only a propyl group extending into the rubber matrix, unable to react. The immobilization of rubber chains to the silica surface corresponds to the formation of a physical network, which is strong enough to remain intact at 100 degreesC, even when swollen in a good solvent.