Interfacial interactions-controlled thermoelasticity and stress relaxation behavior of synthetic rubber/organoclay nanocomposites

Synthetic rubber/organoclay nanocomposites with weak (nonchemical) interfacial interactions (reference sample RS) and a sample of identical composition with strong (chemical) interfacial interactions (test sample TS) were characterized by x-ray scattering and stretching calorimetry techniques. Strain amplification proved to be a common mechanism of reinforcement of the rubber matrix by spatial aggregates of nanoparticles in both RS and TS samples. The breakdown of the initial infinite clusters of nanoparticles into small, isolated clusters at high extensions of the RS manifested itself as a strain dependence of the strain amplification factor concomitant to the generation of the large excess exothermal heat effects of external friction between nanoparticles. The strain-invariant strain amplification factor for the TS, combined with much smaller excess exothermal heat effects of external friction between nanoparticles, were regarded as evidence for the survival of the initial infinite clusters of nanoparticles even at high extensions. Long relaxation times and high amplitudes of stress relaxation for the RS suggested a mechanism of structural relaxation involving large-scale displacements of isolated clusters of nanoparticles within the rubber matrix. Shorter relaxation times, significantly smaller relaxation amplitudes and their regular decrease with the fixed prestrain for the TS were explained by the finite extensibility of chemically bonded rubber chain strands in the interstitial space between neighboring nanoparticles, thus reducing the eventual structural rearrangements to the small-scale displacements of nanoparticles within the infinite clusters.