Influence of crosslink density and chain flexibility on mechanical properties of model epoxy networks

Fully cured epoxy networks having different architectures were prepared by introducing either a monoamine acting as a chain extender or an aliphatic epoxy monomer. As expected an increase in the amount of the difunctional amine leads to a decrease in the crosslink density whereas the introduction of an aliphatic epoxy comonomer leads to more flexible chains between crosslinks. Values of the Young modulus and the dependence of the yield stress as a function of temperature were related to the occurrence and amplitude of secondary relaxations. The fracture toughness at room temperature was clearly associated with the glass transition temperature as a consequence of plastic deformation, rather than with chain scission.