Mechanical fatigue and dielectric relaxation of carbon black/polymer composites

Abstract

Using a home-built experimental setup, we investigate the carbon black (CB) volume fraction-resolved, time-resolved, and frequency-resolved spectra of the room temperature (well above the glass transition temperature of the matrix) effective complex permittivity of well characterized CB filled ethylene butylacrylate copolymer samples that are submitted to a uniaxial tension. We focus here on three samples with CB volume fraction less, near, and well away the percolation threshold at about 8 vol %. Our primary observation is that the temporal evolution of the real and imaginary parts of the effective permittivity is distinctly different for samples containing a CB volume fraction below and above percolation threshold. For samples containing a CB volume fraction below and close to the percolation threshold, and at a given frequency, ε′ and ε″ remain constant over the time scale of our measurements. For the sample containing a CB volume fraction above the percolation threshold the evolution of ε′ and ε″ is different in the low and high elongation ratio regimes. At low strain, the temporal evolution of permittivity during aging under stress shows a logarithmic growth phase followed by a logarithmic decay phase, whereas for sufficiently large strain the permittivity behaviors coincide with those observed below the percolation threshold. To explain the issues involved, we argue that the phenomenology for physical aging in these materials is related to the change in the mesostructure, formed by the heterogeneous three-dimensional interconnected network of polymer and of aggregates (or agglomerates) of CB particles, as the composite is stretched. Indeed, it is possible to argue qualitatively that below the percolation threshold the time-independent permittivity behavior is reminiscent to the elasticity network properties of the polymeric matrix. The reorientation and breakdown of the CB aggregates are believed to be crucial for the physical aging understanding above the percolation threshold and low strain. An examination of the surface and volume morphological evolutions of these materials under the action of a mechanical stress at the microscale by scanning electron microscopy and atomic force microscopy indicates that aging during a few hundreds of hours, even at a moderate strain, generates voids and cracks that are aligned along the stretching direction. The overall behavior is compared to what occurs in another type of filled polymer system, i.e., plastoferrites, for which it was recently discovered that two characteristic time scales are required to describe physical aging.