Critical shear rates and structure build-up at rest in MWCNT suspensions

In this paper, we quantitatively analyze the extent of structure build-up at rest in carbon nanotubes (CNTs) dispersed in an epoxy by a set of transient flow measurements. Stress overshoots appeared at very small deformations during forward and reverse flow measurements by providing some rest time between the two consecutive flows in opposite directions, during which the suspensions structure was reconstructed. The rest time required to build-up the structure completely was much longer than the values reported in literature (about 1 h). Moreover, unlike fiber suspensions or some nano-composites, it was shown that the Brownian motion plays an important role in the structure build-up of the CNT suspensions in the absence of flow. We observed critical shear rates at low and intermediate concentrations above which some nanotube entanglements broke down; this resulted in lower elasticity of the suspensions and partial structure build-up at rest during transient flow reversal measurements. This phenomenon and the possible mechanisms of structure evolution during flow and rest were further investigated by comparing the experimental results with the predictions of a structural model. (C) 2012 Published by Elsevier B.V.