Investigation of anisotropic epoxy-amine thermosets synthesised in a magnetic field

Bis{4-[2-(2,3-epoxypropyl)ethoxy]benzoate}-1,4-phenylene and bis[4-(2,3-epoxypropoxy)benzoate]-methyl-1,4-phenylene liquid crystalline diepoxy monomers were crosslinked with various diamines in a magnetic field. X-ray scattering was used for mesophase identification and to determine the nematic order parameter of the resulting thermosets, which were cured under various temperature and magnetic field strength conditions. All thermosets exhibited nematic or smectic A (SmA) mesophases. The thermosets obtained from aliphatic diamines exhibited a very low degree of orientation. This phenomenon was assigned to their high reactivity, inducing a very fast crosslinking reaction that prevents the alignment of the mesogens. On the other hand, very high degrees of orientation along the magnetic field axis were observed for the SmA thermosets. In this case, the smectic period was smaller than the length of the epoxy monomer unit, which might be due to a staggered packing of the mesogenic cores. Negative longitudinal thermal expansion coefficients were measured below and above the glass transition temperature for the materials cured in a magnetic field.