Influence of graphene nanoplatelets on curing and mechanical properties of graphene/epoxy nanocomposites

The influence of graphene nanoplatelets (GNPs) on the curing of an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) and cross-linked with 4,4′-diaminodiphenylmethane (DDM) was studied. Dynamic mechanical properties and tensile properties of the corresponding graphene/epoxy nanocomposites were obtained. Two compositions 1 and 5 mass% of GNPs were studied. The cross-linking reaction of the epoxy resin is accelerated in dispersions with 5 mass% GNPs. In the presence of GNPs, the curing reaction becomes less exothermic, obtaining less perfect epoxy networks compared to neat epoxy (DGEBA–DDM) thermoset. Accordingly, the glass transition temperatures (Tg) of the nanocomposites are lower than that of the neat epoxy thermoset. This effect is not detected for low content of graphene (1 mass%). Protocol of curing having two isothermal steps leads to more perfect networks than the dynamic curing in the DSC. The Tg reduction is minimized in the samples cured through two isothermal steps. The storage moduli of the nanocomposite containing 5 mass% graphene, both in the glassy (T < Tg) and the rubbery (T > Tg) states, are higher than the ones of neat epoxy thermoset, being most pronounced this effect at T > Tg. Tensile tests confirmed the higher elastic moduli of the nanocomposites; however, a decrease in strain at break and tensile strength was observed for the nanocomposite containing 5 mass% of GNPs. This brittle behavior is consistent with the morphology of the samples studied by scanning electron microscopy.