Tensile stress relaxation and recovery behavior of a cross-linked EPDM rubber matrix loaded with different fillers

The complexity of the mechanical behavior of filled elastomers can be partly attributed to the fact that theduration of an applied strain plays a crucial role. In order to bring new insights into this still incompletely solved problem,we look for relationships between the macroscopic mechanical relaxation and the relaxation of the filler particles at the nano-to mesoscale. To this end, X-ray photon correlation spectros-copy (XPCS) in homodyne and heterodyne configurationscombined with tensile stress relaxation is employed. The paper is devoted to the study of the role of the filler−filler and the filler−matrix interactions in a cross-linked elastomer on the aging mechanisms under strain. Thefillers investigated are carbon black, as an example of strong filler−matrix interactions, and hydroxylated silica for which thefiller−filler interaction is strong (H-bonds). Homodyne XPCS correlation reveals features of jammed systems (compressed exponential and ballistic motion) for both systems. The exponents characterizing the aging of the homodyne relaxation times are not the same in the carbon black and in the silicafilled samples. For both systems, the decrease of the particle velocity determined by heterodyne detection with aging time follows a power law. The silica sample is characterized by a slow decrease of the velocity during aging. For the carbon black sample, the velocity remains small and decreases faster than for the silica sample. The reverse is observed for the behavior of the tensile force.