Effect of a Molecular-Order Disrupting Additive on Liquid-Crystal Monomer Properties

Previously we found that temperature-induced molecular-order variance of the monomer system could strongly affect formation of polymer networks, and hence the resulting properties Objectives: The objective was to understand the effects of addition of non liquid-crystal components on cure shrinkage, polymerization kinetics, network structure and mechanical performance in liquid-crystal monomer systems, in which the molecular order was varied as a function of the proportion of the added component.Methods: 2-tert-butyl-1,4-phenylene bis(4-(6-(methacryloyloxy)hexyloxy)benzoate) and selected molecular-order disrupting additive triphenylethylene (TPE) were mixed at TPE concentrations of 0, 2 and 5 wt%. Birefringent phase textures were imaged through crossed-polars on an Axioskop-2 microscope. Changes in degree of conversion (DC) with exposure time were determined with a MIDAC reflectance FTIR. Transverse strength and moduli were measured with an MTS-Instron universal testing machine. Glass transition temperature was measured with a Perkin Elmer DMA-7 and cure shrinkages by density gradient column. Results: The molecular-order of the monomer systems decreased in the nematic and transformed to the isotropic as TPE level reaches 5% at room temperature. Polymerization rate decreased as TPE level increased, while the final conversions were not significantly affected. In addition, there was evidence of delayed polymerization onset, which showed that TPE may act as a radical scavenger. When polymerized from the isotropic state, shrinkage didn't significantly increase as molecular order decreased in liquid crystal monomer admixtures. In addition, the inherent plasticizing property of TPE was found to affect the mechanical performance of mixtures, such as suppressing Tg and slightly impairing mechanical properties of the polymer, when TPE exceeded 2%. Conclusion: In comparison to the effects of temperature-induced loss of molecular-order, addition of non-reactive, plasticizing TPE maintain low shrinkage of the liquid-crystal monomer systems by disrupting alignment among the mesogenic groups during polymerization. NIDCR grant P01 DE11688