Microstructure of Composite Materials Using Small-Angle Neutron Scattering and Contrast Variation: Approaches toward Understanding the Structure of Carbon Black-Elastomer Mixtures

Abstract

Mixed composites of rubber and carbon black are of fundamental importance to rubber technology, and neutron scattering provides a unique view of structure in these interesting and complex systems. This is due, in part, to the nature of the interaction of the neutron with nuclei, which is a function of the specific nucleus, not on atomic number, per se. This allows neutrons to be used to probe fluctuations in chemical and isotopic composition and density in a manner that is a distinct from x-rays or electrons. This aspect of neutron scattering has been used extensively in the study of multicomponent systems, even when components have elements of similar atomic numbers. The large difference in scattering from some isotopes—hydrogen and deuterium being the most important example—can be used to enhance fluctuations in these systems and thus to probe structures that are not accessible by other methods. We describe the use of small-angle neutron scattering in carbon black, elastomer and carbon black elastomer mixtures using the method of contrast variation. In this approach the sample is suspended in solvents containing different proportions of perdeuterated and protonated molecules. The variation in scattering observed as a function of solvent scattering power gives information on the distribution of components within the sample. Our first experiences using this approach show the potentials of the technique. The use of solvents near the average scattering length density of the carbon black allows observation of non-uniformity in carbon black particles that has not been previously noted. Further, we have established that the shape of the scattering curves in N762 carbon black can be explained as scattering from spherical particles of size and polydispersity measured by electron microscopy. In N121, in which the particle sizes are smaller than N762, contribution to the scatter from the carbon black particle interactions is observable. In gels of solvent extracted carbon black and elastomer we find that the scattering is dominated by carbon black density fluctuations. We find further that the elastomer coats the carbon black particles, at least in N762. There are differences in the association of the carbon blacks in solvent extracted mixtures compared to the carbon blacks suspended in cyclohexane. There may be changes, as well, in the elastomer conformation in the mixtures compared to the polymer alone in cyclohexane. These differences are small in gels containing low concentrations of polyisoprene and carbon black, but are quite profound in a gel containing styrene butadiene rubber with high concentrations of components. Further measurements and modelling of die data will be needed to come to a more satisfactory picture of carbon black morphology and elastomer conformation in the solvent extracted mixtures.