The Strain Dependent Dielectric Behaviour of Carbon Black Filled Natural Rubber
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The behaviour of filled elastomer materials is of practical importance in many fields of engineering science. The exact mechanisms that result in increases in the physical behaviour such as the modulus, strength, damping or toughness of the resulting composite materials is still being widely debated in the academic literature. In this work, a new and novel approach has been developed to study the reinforcing mechanisms of such composite systems using dielectric spectroscopic measurements of the filled elastomer materials. The materials used are mostly Natural Rubber (NR) filled with either carbon black (CB) or silica. Initially broadband dielectric spectroscopy (ranging from around 0.1 Hz to 0.3 THz) on NR/CB composites was developed using a wide range of different technologies that included impedance measurements, microwave testing and quasi-optical free space measurements. The carbon black makes a large contribution to the permittivity measured using the impedance method as a consequence of filler percolation effects. At higher frequencies, tested using either of the other two technologies, the measured permittivity shows an increase almost proportional to the filler volume fraction. By examining different types of carbon black, this behaviour was found to be independent of the filler surface area. The polymer dynamics of NR filled with either CB or silica has also been studied in detail using temperature-domain impedance spectroscopy. A technique has been developed to measure the bulk glass transition temperature, Tg from the alpha relaxations. The bulk Tg of the filled NR compounds is, to within the accuracy of the experiment, almost totally unchanged by the incorporation of the filler materials. This suggests that the polymer dynamics of the NR chains around the filler surface are either very similar in behaviour to the bulk matrix polymer or at least they have too small a volume to be measured as a change in the bulk properties. An entirely new technique was developed to measure the permittivity of natural rubber filled with either CB or silica under strain. This required the building of a simultaneous II dielectric and mechanical spectroscopy facility. Both the real and imaginary part of the permittivity decrease when the strain is larger than 1% for all the filled samples. This decrease comes from the deactivation of the dipoles around the filler-polymer interface with strain. The electrical permittivity versus strain behaviour follows a very similar softening effect to the mechanical Payne and Mullins effects albeit to a much greater extent and at a different level of strain. This allows an independent measure to be made of changes in the filler network structure with strain and this facilitates a better understanding of the actual network structures that might exist. The dielectric behaviour versus strain relationship presents an opportunity to develop novel strain sensors. The prototype capacitance strain sensors made using a carbon black filled NR exhibited a much larger sensitivity when compared to other recently reported sensors.
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