Nonentropic Reinforcement in Elastomer Nanocomposites

We use an innovative combination of measurements to study reinforcement in a series of SBR elastomers filled with various amounts of submicrometric precipitated silica. While mechanical measurements give access to the overall response of the nanocomposite material, measurements of the chain segment average orientation induced upon uniaxial stretching give selective access to the response of the elastomer matrix only. Average segment orientation is measured by X-ray scattering. Reinforcement effects are analyzed in terms of the enhancement ratio of the mechanical modulus or induced segmental orientation in a reinforced sample over the corresponding quantity measured in the pure matrix. Cross-link densities are measured independently by NMR to account for possible impact of fillers on the cross-link density. It is demonstrated that in filled materials the orientational enhancement ratio does not decrease significantly as temperature increases, while the mechanical reinforcement ratio decreases as temperature increases, as it is known already. Also, the mechanical reinforcement ratio increases considerably as the silica fraction increases beyond a threshold, which.is generally attributed to percolation or onset of filler networking, while the orientational reinforcement ratio qualitatively follows a Guth and Gold type of variation, associated solely with the geometrical (or hydrodynamical) local strain amplification contribution. Comparison Of both mechanical and orientational responses thus allows discriminating and quantifying rigid network contribution from strain amplification contribution to reinforcement as a function of either temperature or filler volume fraction.