Influence of developer on structural, optical and thermal properties of a benzofluoran-based thermochromic composite

Thermochromic composites of benzofluoran dye, bisphenol A developer and octadecanol solvent were investigated to analyse how different molar ratios of developer influences the structural, optical and thermal properties. Increasing the amount of developer gives stronger absorption in the visible region and larger colour contrast between coloured and discoloured states. The colour clearing temperature of all studied composites is well below their melting point and the phase transitions are influenced only little by the content of the developer. The crystalline structure is fully controlled by the solvent; it changes from the gamma phase to the liquid at heating and returns through the rotator phase into the gamma phase at cooling. Crystallinity of the composites diminishes with the content of the developer, causing increased scattering of light. The dye: developer molar ratio equal to 1:3 was determined to be large enough to open lactone ring in all dye molecules. Colour hysteresis of such composites shrinks to practically single sigmoidal curve whereas the colour contrast still increases with higher amount of the developer. This effect could be attributed to developer - solvent interactions in a composite with lower crystallinity. (C). 2014 Elsevier Ltd. All rights reserved.