Reinforcement in Natural Rubber Elastomer Nanocomposites: Breakdown of Entropic Elasticity

Abstract

We propose an approach based on the combination of different techniques in order to discriminate various reinforcement effects in vulcanized natural rubber elastomers with various cross-link densities, filled with carbon black or silica: mechanical response, independent measurements of the cross-link density by proton multiple-quantum NMR, and measurements of chain segment orientation under stretching by X-ray scattering. We show that, while the modulus measured in dynamical mechanical measurements decreases as the strain amplitude increases (Payne effect), the response of the elastomer matrix in terms of average chain segment orientation under stretching measured by X-ray scattering stays constant. The amplification of average chain segment orientation is comparable to the amplification of modulus measured at medium/large strain amplitude. By analyzing the deviations with respect to the behavior of the pure unfilled elastomer matrix, we show that the contribution due to strain amplification effects in the elastomer matrix can be selectively distinguished. We show that the mechanical response at medium/large strains is essentially driven by strain amplification effects, while, in the linear regime, there is a strong additional reinforcement which is not related to the properties of the elastomer matrix. Hypothesis on the origin of this additional reinforcement are suggested and discussed.