Fracture toughness and molecular structure of unfilled epoxy adhesives

The cohesive, mode I (tensile cleavage) fracture energy (or fracture toughness), G_Ic, of bulk tapered double cantilevered beam (TDCB) samples of a series of three epoxy thermoset networks was determined using a linear elastic fracture mechanics (LEFM) analysis. Networks of different crosslink density were obtained by mixing various amounts of an aromatic epoxy novolac and an aliphatic epoxy and crosslinking with an imidazole catalyst. Brittle, stick-slip fracture was observed for all formulations, with G_Ic increasing as the amount of aliphatic epoxy increased. However, fracture surface morphologies exhibited evidence of increasing plastic deformation as G_Ic increased. In the investigation of structure-property relationships for this series of thermoset networks, G_Ic was found to be inversely related to both network crosslink density and glass transition temperature (T_g). It was also found that the room temperature frequency of the glassy state transition (-transition) increased as fracture toughness increased.