Rheology of electrostatically tethered nanoplatelets and multi-walled carbon nanotubes in epoxy
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2015 Elsevier Ltd. All rights reserved. Multi-walled carbon nanotubes (MWCNTs) dispersed using -zirconium phosphate (ZrP) nanoplatelets in a glassy epoxy have previously been reported to tremendously improve modulus, strength and ductility in a manner that could not be explained based on continuum-level descriptions of particle-level reinforcement. Several other recent studies have shown similar synergistic improvements in bulk properties for low concentrations of co-dispersed plate-like and rod-like nanoparticles. In this work, we have used rheology as a tool to probe the microstructure and interactions for the ZrP/MWCNT hybrid system in an uncured epoxy pre-polymer fluid. The results are compared to the behavior of model suspensions containing the individually dispersed nanoparticles to more clearly differentiate between particle-level and mesostructural interactions. The linear viscoelastic results show that the ZrP-assisted dispersion approach fully disentangles and stabilizes the MWCNTs in the epoxy fluid. When subjected to large amplitude oscillatory shear or steady shear, the hybrid suspensions show pronounced strain-hardening and shear thickening behaviors, respectively, that were not observed with suspensions containing either individual component. The non-linear behavior is attributed to the direct tethering of the exfoliated ZrP nanoplatelets to the disentangled MWCNTs, which are proposed to serve as mechanically robust junctions. In the solid state, this hybrid network retains sufficient mechanical integrity to delocalize stress concentrations near flaws and cracks in the epoxy matrix, and is responsible for the enormous strengthening behavior observed in this system. The present study provides several potential design considerations for preparation of high-performance nanocomposite systems containing a co-dispersion of interacting nanoparticles.
