SYNTHESIS, PHASE-BEHAVIOR, AND CURING STUDIES OF BISACETYLENE RIGID-ROD THERMOSETS

We have synthesized a series of rigid-rod bisacetylene thermosets from 4-ethynylbenzoyl chloride and various substituted dihydroxy aromatic compounds. The resulting thermoset monomers can show no melting transition, a crystal-to-nematic transition, or a crystal-to-isotropic transition, depending on the substituent and the central aromatic group. Molecular modeling was used to explain this phase behavior. The observed transitions are determined by a balance between molecular linearity, substituent bulkiness, and intermolecular interactions. Optical microscopy was used to observe phase behavior during thermal curing. Nematic monomers can be cured in the liquid-crystalline melt to give a cross-linked solid that retains the nematic order. Qualitatively, the phase behavior during cure follows a generalized nonequilibrium phase diagram for liquid-crystal thermosets. Quantitative measurements of the curing kinetics were performed using Raman spectroscopy. Comparisons between isomeric compounds which melted into either a nematic phase or an isotropic phase show that the curing reaction occurs faster in the nematic phase by up to 126%.