Viscoelastic behavior of suspensions of reduced graphene oxide nanoparticles in epoxy

The viscoelastic behavior of dilute suspensions of reduced graphene oxide (RGO) nanoparticles in a liquid SU8 epoxy dissolved in gamma butyrolactone was investigated at RGO concentrations in the range from 0.16 to 1.07 vol. %. Our results revealed that the addition of only 1 vol. % of RGO can increase the complex viscosity of the composites by up to 3 orders of magnitude, which is crucial for processing purposes. A marked shear thinning behavior with a yield stress was, moreover, found to emerge at low RGO concentrations for sonicated homogenized suspensions. This behavior was attributed to the partial polymerization of the epoxy and bonding between the epoxy rings and the residual functional groups on the RGO induced by the energy intensive ultrasound process that is key to achieve a good dispersion of the nanoparticles, in combination with the formation of a gel-like, percolated nanocomposite network. The percolation threshold and the aspect ratio of the RGO particles were derived from the analysis of the elastic shear modulus of the suspensions and found to be equal to 9 x 10(-4) and 500, respectively. We attribute this extremely with low percolation threshold to the restriction of the polymer chain mobility within the well dispersed, high aspect ratio RGO network.