The aim of the paper was to find a direct connection between dynamic colour changes, phase changes and chemical interactions in model three-component leuco dye based thermochromic systems. The model systems, containing crystal violet lactone as a colour former, bisphenol A as a developer and 1tetradecanol as a co-solvent, were analysed by DSC and FTIR spectroscopy and the results were related to the characteristics of the dynamic colour change. The ternary thermochromic systems were also compared with binary mixtures of the co-solvent with the developer and colour former, respectively. The temperatures characterizing the dynamic colour change at decolouration limits were directly related to the solid-liquid transition on heating and liquid-solid transition on cooling, regardless the concentration of bisphenol A. In ternary thermochromic systems, an indistinctive phase transition at the temperatures below the solid-solid (crystal-rotator) transition was observed. The straight connection between the phase transitions and temperatures characterizing the dynamic colour change at colouration limits was not proved. The colour contrast of thermochromic systems was found to be directly related to the ratio of integrated intensity of lactone ring opened (solid) and lactone ring closed (liquid) carbonyl vibration characterized by infrared spectroscopy.

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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 cosolvent. 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.

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