Effect of flow history on linear viscoelastic properties and the evolution of the structure of multiwalled carbon nanotube suspensions in an epoxy

This paper analyzes the effect of flow history on the linear viscoelastic properties of suspensions of multiwalled carbon nanotubes in an epoxy as well as the evolution of the suspension microstructure under small deformations for different concentrations and temperatures. The effect of the flow history on the microstructure is interpreted in the light of the variation of the rheological percolation threshold, which is shown to increase with the pre-shear rate. After cessation of the shear flow, the storage modulus increased with time revealing the build-up of the structure. By decreasing the pre-shear rate, the resulting storage modulus increased and the relative increase of the storage modulus with respect to the pre-shear rate was more pronounced at lower concentrations. The rate of increase in the storage modulus drastically increased with the concentration and temperature, while its variation with respect to the pre-shear rate depended on the concentration. In dilute suspensions, it decreased dramatically by increasing the rate of pre-shear, revealing a slower structure build-up while it remained almost intact in more concentrated suspensions. The increase in kinetics of structure build-up with temperature suggests the importance of Brownian forces in the absence of flow regardless of concentration or applied pre-shear rate. (C) 2011 The Society of Rheology. [DOI: 10.1122/1.3523628]