Effect of Molecular Order on Liquid Crystal Monomer Performance Properties

Previously our group developed a class of liquid crystal monomers having markedly lower polymerization shrinkage than those currently available. Recent work also showed that an ordered liquid crystalline phase would affect mechanical properties of the polymer product.

Objectives: The objective was to understand the effects of molecular order of the nematic (n) liquid crystalline phase on cure shrinkage, polymerization kinetics, network structure and mechanical performance.

Methods: A selected liquid crystal monomer 2-(t-butyl)-1, 4-bis-(4-(6-acryloyloxyhexyl-1-oxy) benzoyloxy) benzene was synthesized and purified. Changes in degree of conversion (DC) with exposure time at different temperatures were determined using a Bruker reflectance FTIR with a heated stage. Monomers were cured at room temperature (RT), 40°C, 50°C and 60°C. Transverse strength and modulus were measured in 3-point bending with an MTS-Instron universal testing machine. Glass transition temperature was measured with a Perkin Elmer DMA-7, nematic-isotropic (n→i) transitions with a Perkin Elmer Pyrus DSC, and cure shrinkage by density gradient column.

Results: This monomer has a n→i transition at about 45°C. The initial photo-polymerization rate didn't decrease significantly until the cure temperature was below 30°C, implying that the promoting effects of molecular order and cure temperature compete. The maximum values of elastic modulus and strength appeared in specimens cured at RT and 60°C, implying that both liquid crystal polymer domains and high-efficiency crosslinking contribute to improved mechanical performance. Volumetric shrinkage was greatly decreased when polymerization occurred from the nematic liquid crystalline phase, e.g., cure shrinkage at RT is 2.21% compared to 6.35% at 60°C.

Conclusions: An ordered monomer system can greatly contribute to reducing cure shrinkage. Varying the monomer molecular order strongly affects the structure of the formed polymer network, and hence the resulting mechanical properties.

Funded by NIDCR grant P01 DE11688