Microdeformation of a polydomain, smectic liquid crystalline thermoset

The microdeformation and fracture mechanisms of a densely crosslinked, polydomain, smectic liquid crystalline thermoset (LCT) were investigated in order to gain some insight into its interesting mechanical properties (e.g. improved fracture toughness). A difunctional LC epoxy monomer, diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene (DGDHMS), was crosslinked with the tetrafunctional, aromatic crosslinker, 4,4′-methylene dianiline (MDA) to produce the LCT. Thermoset films (30 μm in thickness) were bonded to copper grids, strained in tension, and observed under the polarizing optical microscope. A new type of microdeformation and fracture mechanism was observed for the smectic LCT. At small strains, numerous microcracks formed which were oriented at various angles to the straining direction. Many smaller isolated and interconnected defects (of the order of a single LC domain, ∼1 μm in size) surrounded and emanated from the crack tips. At larger strains, the microcracks propagated slowly and stably until reaching a critical size of ∼250 μm, at which time they began to widen and change direction, indicative of plastic deformation which extended over many LC domains. Film failure occurred through microcrack interconnection. A fracture mechanism for the LCT is proposed based on microscopic voiding near the crack tip through the failure of individual LC domains.