Relationships between Nanofiller Morphology and Viscoelastic Properties in CNF/Epoxy Resins

This paper investigates the rheological behavior of Epoxy-Amine Resins/Carbon Nanofibers (CNFs) dispersions and its correlation with the nanofiller morphology. The use of the reactive diluent 1,4-butandiol diglycidyl ether (BDE) into the tetraglycidylmethylene dianiline (TGMDA) liquid epoxy precursor has proven to be a key in reducing the viscosity of the epoxy matrix. The effect of nanoadditives on the oscillatory shear behavior of the un-cured epoxy precursor matrix in the liquid state was studied. These nanofillers consist of carbon nanofibers as made and after high heat treatment temperature. The inclusion of as made CNFs at 0.5 wt % content has no influence on the Newtonian rheological behavior of the epoxy precursor. The morphological investigation indicated that the as made nanofibers show a tendency to bend and have functionalized surfaces which determine a large epoxy layer thickness around the nanofibers. Due to these events, the as made CNFs are further apart in the epoxy liquid precursor and, consequently, the rheological percolation network at 0.5 wt % is not formed. Conversely, when the heat-treated CNFs are used, the rheological results of the epoxy/0.5 wt % CNF dispersion indicated a transition from liquid-like to solid-like behavior at low frequencies showing that in this case the 0.5 wt % content of heat-treated CNFs is higher than the rheological percolation threshold. Indeed, the heat-treated CNFs exhibit an increase in their “equivalent length” and a smaller thickness of the TBD layer around the nanofibers so that these heat-treated CNFs can easily form a percolation network.