DEFORMATION OF A POLYDOMAIN, LIQUID-CRYSTALLINE EPOXY-BASED THERMOSET

Liquid crystalline thermosets (LCT's) were prepared by curing a difunc tional LC epoxy monomer, diglycidyl ether of 4,4'-dihydroxy-alpha-meth ylstilbene, with the tetrafunctional cross-linker 4,4'-methylenedianil ine (MDA). A commercial, non-LC epoxy monomer of similar starting mole cular-weight was also cured with MDA to produce an isotropic thermoset for comparison. Dynamic mechanical analysis revealed reduced glassy m oduli, increased stiffness in the rubbery state, and broadened and low ered glass transitions far the LCT's compared to the isotropic thermos et based on the non-LC monomer. At room temperature, the true stress v ersus true strain curves of the LCT's under uniaxial compression showe d no strain softening region, substantial plastic deformation (epsilon (f) approximate to 50%), and increased strain hardening compared to th e isotropic thermoset. LCT's with a smectic type of local order exhibi ted bulk, homogeneous plastic yielding, which led to slow, stable crac k propagation and an increased fracture toughness (G(Ic) = 1.62 kJ/m(2 ), K-Ic = 1.59 MPa.m(1/2).