Synthesis and improved mechanical properties of twin-mesogenic epoxy thermosets using siloxane spacers with different lengths

A series of twin-mesogenic epoxy resins with siloxane (di-, tri-, and tetra-siloxane) chain spacers of different lengths were successfully synthesized. Results showed that the melting point of the twin-mesogenic epoxy monomers decrease by more than 50 degrees C due to the change of siloxane spacer length. In particular, the tri- and tetra-incorporated siloxane twin-mesogenic epoxy monomers showed a liquid crystalline (LC) phase below 100 degrees C during the cooling process. Epoxy thermosets with isotropic and smectic (S-m) LC phases were then prepared using an aromatic amine and the dependence on curing temperature was investigated. Furthermore, the relationship between the ordered network chain structure and mechanical properties was investigated. Results were compared with isotropic and S-m LC phases. The tensile strain and fracture energy in the S-m type LC thermosets increased without a decrease in the tensile modulus, and this occurred in all systems irrespective of siloxane spacer length. The maximum value of the fracture energy (283 kJ m(-2)) reached by LC thermosets with increased spacer lengths was twice as large as that using the alkyl chain spacer (144 kJ m(-2)). (c) 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47891.