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The influences of micro and nanoparticles on relaxation kinetics were studied using Differential Scanning Calorimetry and Dielectric Spectroscopy. The samples are composed of Quartz and/or Organically Modified Montmorillonite dispersed in an epoxy matrix. The heat capacity step, normalized to epoxy quantity, did not show significant variations, contrary to the glass transition temperature, which decreased for the nanostructured microcomposite. The main ? relaxation parameters, fragility index and relaxation time at the glass transition temperature combined to show that, even though molecular movements are hindered by the presence of particles acting as obstacles, the global mobility of the molecular chains is increased because reticulation of the epoxy has been prevented. The local β relaxation, associated with crankshaft motions of the hydroxylether groups, is not affected by micro and nanoparticles. The modification of the γ relaxation associated with the ending epoxy groups suggests that different interfacial interactions occur with nanoparticles/epoxy and microparticles/epoxy. The relaxation parameters will be situated in the context of the dielectric breakdown results. Dielectric breakdown strengths are more modified by inhomogeneity in sample preparation than by addition of organically modified Montmorillonite to the composite material. [ABSTRACT FROM PUBLISHER]