Properties of liquid crystal epoxy thermosets cured in a magnetic field

Two liquid crystal diepoxides [1,4-phenylene bis(4-(2,3-epoxypropoxy)benzoate) (A) and 4,4'-diglycidyloxydiphenyl (B)] were cured with a diamine [4,4'-diaminobiphenyl (C)] in the nematic phase to obtain liquid crystalline thermoset (LCT) materials. The systems were studied using different curing conditions and different ratios of the compounds. The mechanical properties were investigated by stress/strain experiments to determine ultimate properties (break strength and elongation at break) and dynamic mechanical thermal analysis (DMTA) to determine small strain properties as a function of temperature. An A/C mole ratio of 4/1 gave materials with the best combination of high strength and percent elongation. The addition of a small amount of diepoxide B to the system (with a mole ratio of A/B = 4/1) improves the mechanical properties. Dynamic mechanical thermal analysis shows that increasing diamine content or curing time increases the cross-linking density. Macroscopic orientation of the LCTs was achieved by curing the mixtures in the presence of a magnetic field. The samples show that the tensile modulus, break strength, elongation at break, and the storage modulus below the glass transition all increase by a factor of about 2.