Deformation of a polydomain, liquid crystalline epoxy-based thermoset

Liquid crystalline thermosets (LCT's) were prepared by curing a difunctional LC epoxy monomer, diglycidyl ether of 4,4'-dihydroxy-alpha-methylstilbene, with the tetrafunctional cross-linker 4,4'-methylenedianiline (MDA). A commercial, non-LC epoxy monomer of similar starting molecular-weight was also cured with MDA to produce an isotropic thermoset for comparison. Dynamic mechanical analysis revealed reduced glassy moduli, increased stiffness in the rubbery state, and broadened and lowered glass transitions far the LCT's compared to the isotropic thermoset based on the non-LC monomer. At room temperature, the true stress versus true strain curves of the LCT's under uniaxial compression showed no strain softening region, substantial plastic deformation (epsilon(f) approximate to 50%), and increased strain hardening compared to the isotropic thermoset. LCT's with a smectic type of local order exhibited bulk, homogeneous plastic yielding, which led to slow, stable crack propagation and an increased fracture toughness (G(Ic) = 1.62 kJ/m(2), K-Ic = 1.59 MPa.m(1/2).