Thermal and colour properties of leuco dye-based thermochromic composite with dodecanol solvent

Abstract

Leuco dye–developer–solvent composites with thermochromic properties were studied. The applied 1-dodecanol solvent shows β crystalline, α solid (rotator) and liquid phases. The inclusion of the crystal violet lactone dye increases transition temperatures of all phases and strengthens the α phase. The benzyl 4-hydroxybenzoate developer causes opposite effects and prevents the appearance of the α phase. These consequences combine in ternary composites, where lower transition temperatures were obtained as in the solvent, but the α phase is formed more likely as expected due to the presence of the developer. The hydration of the composite strengthens its crystalline network, but makes α phase more unstable. The thermal properties were related to the temperature-dependent colour. Composites with well-resolved α and β phases at cooling show a typical colour hysteresis. If only weak α phase or none at all was found in DSC curves, the colour hysteresis has an unusual shape with heating and cooling curves crossing each other. The hydration of the composite changes the colorimetric properties in accordance with the relation found for anhydrous composite. More conclusions and interrelations between the two properties require application of other analytical methods.