Yielding and fracture in crosslinked epoxies

From a comparison of tensile properties of two epoxy networks crosslinked, respectively, with an aromatic diamine or an anhydride and from the available literature data on the fracture properties of thermosets, it has been tried to establish structure-property relationships in this field. Attention was focused on possible specific aspects of the crosslinked polymers. Surprisingly, the crosslink density does not seem to play a crucial direct role in fracture. In contrast, there is a relation between toughness and T_g: Aliphatic networks of low T_g are generally tougher than aromatic ones. The local mobility plays a very important role because it partly controls the value of Poisson's ratio (which determines the relative importance of dilatant stresses and shear stresses responsible, respectively, for brittle rupture and yielding) and, presumably, the activation volume of yielding (which partly determines the broadness of the temperature interval of ductility).