Aromatic tetra-glycidyl ether versus tetra-glycidyl amine epoxy networks: Influence of monomer structure and epoxide conversion

In this work, two epoxy resins, bis(2,7 glycidyl ether naphthalenediol) methane (NNE), tetraglycidyl diaminodiphenylmethane (TGDDM) and a miscible blend of the two are cured with 4,4′-diaminodiphenylsulfone (DDS) to investigate the differences between glycidyl ether and glycidyl amine crosslinked networks. This work explores in depth, and directly compares for the first time, the structure/property relationships of a highly aromatic and thermally stable glycidyl ether epoxy network with a well-known highly aromatic and thermally stable glycidyl amine epoxy network. The highly aromatic naphthalene motif of NNE imparts improved thermal stability and a significantly higher glass transition temperature, above 340 °C depending on the measurement technique. However, it also produces comparatively lower epoxide conversion, higher moisture ingress and lower flexural properties. These properties are attributed to the NNE having higher free volume and glassy state mobility as suggested by the sub-ambient β relaxations. The glycidyl ether groups react primarily via epoxide amino addition displaying only very minor evidence of homopolymerisation or other side reactions even at 240 °C. This contrasts with the glycidyl amine groups of TGDDM which exhibit a much higher degree of non-epoxy amine addition. Increasing cure conversion also increases the glass transition temperature, fracture toughness and reduced coefficient of thermal expansion. Overall, the results show that despite some reduced mechanical and thermal properties the NNE epoxy resin system is a very thermally stable high temperature epoxy resin based upon glycidyl ether functional groups suitable for a wide range of new applications.