| The effects of molecular order on physical and mechanical properties of photo-polymerizable liquid crystal acrylate (methacrylate) monomer systems were investigated in this study under two specific situations, one in which the molecular order was varied as a function of temperature, and another in which the molecular order was varied as a function of the proportion of a non liquid crystal component, triphenylethylene.;2-tert-butyl-1,4-phenylene bis(4-(6-(acryloyloxy)hexyloxy)benzoate) was polymerized within a temperature range between room temperature to 90°C. Over this range the molecular order decreases in the nematic phase and is transformed to the isotropic phase at approximate 45°C. High molecular order in the nematic phase was found to enhance polymerization rate, increase degree of conversion, and decrease polymerization shrinkage. Also, molecular order in the monomer has a significant influence on the formation of polymer network structure, and consequently affects mechanical properties. Isotropic monomer tends to form a network with both higher chemical crosslinks and physical entanglements, yielding a polymer with increased mechanical strength and high Tg, while nematic monomer maintains a high percentage of liquid crystalline regions during formation of the network, yielding a polymer with high mechanical modulus and toughness. In addition, during polymerization, the mesogenic groups align to form ordered liquid crystal domains even when polymerization occurs at a temperature (90°C) well above the nematic-to-isotropic transition temperature. A gain in molecular order in samples polymerized above the nematic-to-isotropic transition accounts for the desirable enhancement of polymerization rate and degree of conversion, and also for the unacceptably high polymerization shrinkage.;When molecular order of the nematic monomers was reduced by addition of triphenylethylene, similar results, such as decreased polymerization rate and increased polymerization shrinkage, were obtained. However, in comparing the effects of temperature and triphenylethylene additive, there is a significant difference in formation of polymers, causing substantially different results in performance properties. When polymerized from isotropic states, monomers tend to form an anisotropic network if the molecular order is annihilated by increasing temperature, while monomers tend to maintain the isotropic phase if molecular order is annihilated by addition of triphenylethylene. The first consequence of the difference is that shrinkage increases much less as molecular order decreases in liquid crystal monomer admixtures. The second consequence is to form a more homogeneous and thermodynamically equilibrated network. In addition, the inherent plasticizing property of triphenylethylene was found to significantly affect the performance of mixtures, such as suppressing Tg and impairing mechanical properties of the polymer, when triphenylethylene exceeded 2% in this study. |
