Combined colorimetric and thermal analyses of reversible thermochromic composites using crystal violet lactone as a colour former

In present study, the intention was to investigate how the colour change of thermochromic composites is connected with their phase transitions. Crystal violet lactone was used as a colour former and bisphenol A as a developer. Two co-solvents, 1-tetradecanol and 1-tetradecanoic acid, were selected due to the same length of the aliphatic chain and different functionality. Series of seven thermochromic systems varying in co-solvent's molar ratio were prepared for each type of the co-solvent. The colour change was measured by reflectance spectroscopy, and temperatures characterizing the dynamic colour change were computed from the cumulative colour difference function. The onset and endset temperatures of thermochromic system's phase transitions were obtained from DSC analyses. Connections between the fastest colour changes and the decolouration limits at raised temperatures to solid-solid, solid-liquid and liquid-solid transitions are demonstrated for both heating and cooling, respectively. Different behaviour of compared systems with alcoholic and acidic co-solvent is discussed as well.