Abstract
Reversible colour change of leuco dye-based composites is in general closely related to their phase change, thus the two phenomena should occur at around the same temperature and should be influenced similarly. However, spatial confinement of the analysed sample affects the change in colour differently compared to its phase transition and the most pronounced effects can be observed during cooling. The bulk composite is coloured while still liquid and the colour hysteresis does not exhibit a loop. In an open-porous medium the colouration coincides well with the crystallization and the colour hysteresis widens to about 4 °C. Microencapsulated composite exhibits two crystallization processes, one of them taking place at the bulk crystallization temperature and the other one at about 20 °C lower. Under such conditions the composite is coloured just before the onset of the second crystallization, i.e. about 15 °C below crystallization in the bulk, and the corresponding colour hysteresis widens to 18 °C. The two crystallization forms are thermally independent and have the same crystalline structure. These effects should be taken into account when designing future applications where the phase-changing materials are implemented.
Highlights
Organic thermochromic (TC) materials are applied in a wide range of commercial products such as in colour-changing textiles[1,2,3,4], for temperature control of a foldable paper or textile[5] and even for colour-changing wood veneers[6]
In the bulk state (C6), the composite discolours before the melting is completed but it re-colours well above the freezing point. This confirms that colouration/discolouration of a TC composite may not follow the phase changes[15], a coloured liquid is formed before solidification and colourless solid before melting
The corresponding colour hysteresis possesses an unusual shape where the cooling and heating curves intersect. This is in agreement with our previous research - TC composites have a very narrow colour hysteresis and in some cases the two curves may cross each other[41]
Summary
Organic thermochromic (TC) materials are applied in a wide range of commercial products such as in colour-changing textiles[1,2,3,4], for temperature control of a foldable paper or textile[5] and even for colour-changing wood veneers[6]. A leuco dye-based TC composite can solidify without changing its colour, and the colourless solid state can exist within a reasonably wide temperature region[15] Such a state has not yet been studied in more detail. The phase changes of leuco dye-based TC composites are controlled by the applied co-solvent, which can be a long-chain alkyl alcohol, ester, ketone, ether, or acid[1,2]. Such PCMs were extensively investigated to determine the relationship between their structure and energy storage properties in eutectic mixtures for the solid-liquid transition[20,21,22,23,24]. Mini-emulsions of immiscible polymer blends with a broad size distribution of droplets were shown to have multiple crystallization peaks; a broader size distribution leads to a larger number of peaks[38]
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