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Nanocomposites under tensile strain : anisotropic and heterogeneous dynamics.

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At a very low volume fraction (< 1%), nanoparticles do not modify significantly the mechanical or dynamical properties of nanocomposites. It follows that nanoparticles can be used as tracers or probes for revealing the dynamics of strained polymer chains. Researchers of the Laboratory, in collaboration with other scientists of Grenoble and of two foreign Universities, have investigated the dynamics of stretched elastomer chains by means of X-Ray Photon Correlation Spectroscopy (XPCS). The two-time correlation functions determined in the directions parallel and perpendicular to that of the tensile strain, display anisotropic and heterogeneous dynamics.

The probe "particles" (< 1%) used in this work are fractal aggregates of pyrogenic silica or carbon black. They are dispersed in an elastomer. The correlation functions determined in the directions parallel and perpendicular to the tensile strain (8%) show that the probe particle dynamics is slower in the direction of elongation than in the perpendicular one. This result indicates that the mobility of the stretched chains is lower than that of the un-stretched chains (perpendicular direction). In both cases, the relaxation time varies as q-1. This scaling law is characteristic of soft glassy materials and often associated with heterogeneous dynamics. The occurrence of a peak in the variation of the variance of the correlation functions shows that it is the case. The height of this peak varies as a power law of q. It appears that this variation depends on the morphology of the "particles" contrary to what is expected for probe particles whose dynamics is supposed to be controlled only by that of the strained polymer chains. This result reveals a modification of the dynamics of the chains in the vicinity of the fractal aggregates. In fact, this phenomenon is well known, although still unclear, in the case of reinforced elastomers in which the volume fraction of nanoparticles is much higher (30 to 40%). It is related to what is named “bound” (or “occluded”) polymer that plays a key role in the dynamic mechanical properties of these materials (including tires). The present study shows that XPCS will permit to go one step further in the analysis of the formation and the characteristics of this "interphase" which depends on the morphology of the reinforcing particles, their interaction with the matrix and the mechanical stress level.

Voir en ligne : Anisotropic and heterogeneous dynamics in stretched elastomer nanocomposites. F. Ehrburger-Dolle, I. Morfin, F. Bley, F. Livet, G. Heinrich, Y. Chushkin and M. Sutton, Soft Matter, 2019, 15, 3796 – 3806.