Rheological and morphological properties of graphene-epoxy nanocomposites

In this paper the rheological and morphological properties of an epoxy resin filled with graphene-based nanoparticles have been investigated. Two samples of partially exfoliated graphite (pEG) and carboxylated partially exfoliated graphite (CpEG), differing essentially for the content of carboxylated groups, are used. The percentage of exfoliated graphite is slightly different for the two samples: 56% for pEG and and 60% for CpEG. Exfoliated graphite is prepared using traditional acid intercalation followed by a sudden treatment at high temperature (900°C). The epoxy matrix is prepared by mixing a tetrafunctional precursor with a reactive diluent which produces a significant decrease in the viscosity of the epoxy precursor so that the dispersion step of nanofillers in the matrix can easily occur. The hardener agent, the 4,4-diaminodiphenyl sulfone (DDS), is added at a stoichiometric concentration with respect to all the epoxy rings. The inclusion of the pEG and CpEG samples in the formulated epoxy mixture significantly modifies the rheological behaviour of the mixture itself. The epoxy mixture, indeed, shows a Newtonian behavior; on the contrary the complex viscosity of the nanocomposites clearly shows a shear thinning behavior at 3 wt % of pEG content and at 0.75 wt% of CpEG content. The increase in complex viscosity with the increasing of pEG and CpEG content is mostly caused by a dramatic increase in the storage modulus of the nanocomposites. All the graphene-based epoxy mixtures are cured by a two-stage curing cycles: a first isothermal stage is carried out at the lower temperature of 125°C for 1 hour and, then, a second isothermal stage at the higher temperature of 200°C for 3 hours. The different morphology shown by the two pEG and CpEG samples is consistent with the difference in the percentage of exfoliation degree and well correlates with the rheological behavior of investigated graphene-epoxy nanocomposites.