Mechanochromic Liquid Crystals and their Polymer Blends

The long term goal of this research is to understand the influence of structure and properties of polymer blends with cholesteric liquid crystals (LC) to facilitate the design of adaptive/responsive optical color change to mechanical strain. Cholesterol ester liquid crystals form into periodic helical structures and give these liquid crystals their unique responsive optical properties. The color of the light reflected depends on the pitch length of the liquid crystal and can be influenced by external stimuli like mechanical deformation or temperature. Temperature-dependent properties depend upon a transition between the smectic and twisted nematic mesophases. If the smectic phase exists, temperature dependent reflecting properties are observed. Temperature-insensitive formulations can be achieved by ensuring the absence of the smectic mesophase transition. Such mixtures reflect a single color below its clearing point. Practically, this has been achieved using mixtures of components that have a smectic A mesophase (e.g. cholesteryl perlagonate and/or cholesteryl oleyl carbonate) with a component that does not form a smectic A mesophase (e.g. cholesteryl chloride). Such formulations are expected to change color in response to mechanical shear while remaining relatively unaffected by temperature changes. To achieve these mechanochromic LC formulations, we are investigating ternary mixtures of cholesteryl benzoate (CC), cholesteryl pelargonate (CP), and cholesteryl oleyl carbonate (COC). Through investigations of the formulation space using a ternary diagram, the broadest range of colors (reflected wavelengths) can be achieved by varying the COC concentration at a 1:1 ratio of CP:CC. Formulations that appear blue, green, and red were selected and mechanochromic behavior was demonstrated when applying a shear stress using a doctor blade with an estimated strain rate of 180 s^-1. However, the fast relaxation times associated with the liquid nature of the LC formulations meant a rapid return to the original color. To both lengthen the relaxation times and incorporate the formulations into practical structural materials, various methods were employed to combine LC formulations with an aromatic polyether-based thermoplastic polyurethane (TPU). Long term stability of separate layers of LC and TPU proved physically questionable. Initial efforts to combine LC formulations with TPU in tetrahydrofuran showed good compatibility in cast films and wet-spinning of fibers. A series of TPUs with varying physical properties is currently being investigated. Characterization of the mechanochromic response of these blends and the relationship of the TPU properties to the LC structure will be presented.