Mechanical properties and cross-link density of styrene-butadiene model composites containing fillers with bimodal particle size distribution

Mechanical properties and cross-link density of model composites being solution styrene-butadiene rubbers filled with different amounts of nanosized silica particles or mixtures of nanosized silica particles and micrometer-sized borosilicate glass particles are studied. The cross-link density of the rubber matrix is measured based on a double-quantum NMR pectroscopy method. Shear data show that reinforcement and dissipation G '' in the rubber plateau range depend systematically on the total surface area of the filler per unit composite. Different contributions to reinforcement due to hydrodynamic effects, "filler network", glassy polymer layer, and "occluded rubber" are quantified based on a comparison of linear response measurements with strain sweeps performed at different temperatures. The results show a percolation threshold at silica volume fractions of about 0.15. The load-carrying capacity of the "filler network" decreases significantly with temperature. This may indicate the existence of a glassy polymer layer on the surface of the filler particles which softens several ten degrees above the bulk T-g of the rubber matrix. Two regimes are found in the dissipation above T-g which both depend systematically on the surface area of the filler system: A strongly frequency-dependent dissipation regime with power-law behavior is observed in G ''(omega) at temperatures up to 70 K above the bulk T-g, and a nearly frequency-independent G '' regime dominates at higher temperatures. The molecular nature and importance of this finding for tire applications are discussed