Microdeformation of a polydomain, smectic liquid crystalline thermoset

The microdeformation and fracture mechanisms of a densely crosslinked, poly domain, smectic liquid crystalline thermoset (LCT) were investigated in ord er to gain some insight into its interesting mechanical properties (e.g. im proved fracture toughness). A difunctional LC epoxy monomer, diglycidyl eth er of 4,4'-dihydroxy-alpha-methylstilbene (DGDHMS), was crosslinked with th e tetrafunctional, aromatic crosslinker, 4,4'-methylene dianiline (MDA) to produce the LCT. Thermoset films (30 mu m in thickness) were bonded to copp er grids, strained in tension, and observed under the polarizing optical mi croscope. A new type of microdeformation and fracture mechanism was observe d for the smectic LCT. At small strains, numerous microcracks formed which were oriented at various angles to the straining direction. Many smaller is olated and interconnected defects (of the order of a single LC domain, simi lar to 1 mu m in size) surrounded and emanated from the crack tips. At larg er strains, the microcracks propagated slowly and stably until reaching a c ritical size of similar to 250 mu m, at which time they began to widen and change direction, indicative of plastic deformation which extended over man y LC domains. Film failure occurred through microcrack interconnection. A f racture mechanism for the LCT is proposed based on microscopic voiding near the crack tip through the failure of individual LC domains. (C) 2000 Kluwe r Academic Publishers.