Effect of the introduction of methoxy branches on low-temperature relaxations and fracture toughness of epoxide resins

Abstract Some epoxide resins with or without methoxy branches were cured with an aromatic diamine. A well defined relaxation was observed near room temperature for all cured resins with methoxy branches, independent of the epoxide-resin structure. The relaxation was denoted here as the β′-relaxation. It is suggested that the β′-relaxation can be attributed to suppression of the motion of the hydroxy ether group due to the interaction between this group and the methoxy branch. The effect of the β′-relaxation on the fracture toughness of cured epoxide resins was studied by comparing the stress intensity factor K c of a spiro-ring type epoxide resin containing a methoxy branch with that of a bisphenol A type resin. The value of K c in the former system is considerably higher above the β′-relaxation temperature than that of the latter system. This is explained by the increase in the plastic deformation region at the crack front with increase in temperature caused by the presence of the β′-relaxation.