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Chemistry and Applications of Leuco Dyes

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  • Chemistry and Applications of Leuco Dyes (Topics in Applied Chemistry).
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Responsibility edited by Ramaiah Muthyala. Physical description 1 online resource xviii, pages : illustrations. Series Topics in applied chemistry. Online Available online. SpringerLink Full view.

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Relation between colour- and phase changes of a leuco dye-based thermochromic composite

Sign up to take part. A Nature Research Journal. 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.

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Under such conditions the composite is coloured just before the onset of the second crystallization, i. 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.

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 these applications the TC material is used in a spatially confined form.

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During the last decades, the leuco dye-based TC materials have received substantial attention from both researchers and industry. In these materials, the temperature-induced colour change occurs due to molecular rearrangement of colour formers leuco dyes in the presence of a developer and this process is controlled by a phase change of the co-solvent. The molecular mechanisms were thoroughly studied in fully coloured as well as in totally decoloured states, i. However, 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 Such a state has not yet been studied in more detail.

The colour change as a function of temperature produces a hysteresis which is very narrow for TC composites and much wider for printing inks where these are microencapsulated to protect the phase-changing material PCM from its surrounding 16 , 17 , 18 , The shape of the colour hysteresis provides the first evidence of changes in the TC composite, such as poor fastness against light and high temperatures To the best of our knowledge, the relationship between the phase- and colour changes has not yet been analysed. 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 , PCMs are commonly used in a microencapsulated form 1 , 25 , 26 , 27 , 28 , 29 , 30 but this can change their thermal properties when measured in the bulk In many cases, supercooling lowers crystallization temperature, depending on the type of polymer shell and the size of microcapsules 32 , 33 , 34 , A greatly reduced number of nucleation sites induces an independent crystallization within isolated parts of the material, known as homogeneous nucleation.

Different nucleation efficiency in microcapsules of various sizes may result in multiple crystallization peaks 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 Large effects were also observed in porous media.

Crystallization of a fluid in a nanometer-sized porous material resulted in a much lower freezing temperature, i. The most important research of a TC composite in spatially confined conditions was done for its application as the active core in fibres made by melt coaxial electrospinning. It was shown that the thermal and colouration properties of the active core were only slightly affected in fibres with a diameter of 0.

It was also concluded that the phase transition temperature might not be the only parameter governing the colour transition temperature 4. The above results were a strong motivation for our research. If the colour of leuco dye-based TC composites follows their phase transition temperatures, their colour and phase changes should be equally affected by a spatial micro-arrangement.

To analyse this assumption, the same TC composite was studied both in the bulk and in a confined form. Because TC composites can only seldom be applied in the bulk, the obtained results should be important for a design of the functional properties of the final application. The shape of the hysteresis was described by the loop width and four characteristic temperatures, describing the onset T 1 and T 3 and termination T 2 and T 4 of the TC effect: discolouration at heating T 1 and T 2 and colouration at cooling T 3 and T 4 , respectively Fig.

S1 , Supplementary information. The temperature-dependent colour and thermal properties of the three forms of the TC composite are shown in Fig.

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Thermal DSC, black curve and colorimetric properties blue curve with circles of the a TC composite in the bulk, b inside the chromatographic paper and c in microcapsules. Pictures of the corresponding holders are included. The bulk composite is shown in the fully coloured and totally discoloured states a. In the bulk state C6 , the composite discolours before the melting is completed transition A but it re-colours well above the freezing point transition B. 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 also cross each other Moreover, discolouration of the sample at heating and re-colouration at cooling coincide well with the temperatures where melting A and crystallization B processes are completed.

Re-colouration of the sample occurs at considerably lower temperatures than in the other two forms and completes just before the onset of the transition C. A stronger influence of the spatial conditions of the TC composite on its colouration and phase changes was recorded at cooling, the most pronounced in the microencapsulated form.


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A direct comparison of the thermal and colouration properties of the analysed TC composite is shown in Fig. S2 Supplementary information. A slow DSC experiment Fig. The results were compared with those for the ML co-solvent in the same three forms Fig.

The heat fluxes were normalized according to the melting peak transition A. DSC thermograms of the TC composite and of the applied co-solvent show the same features with systematically larger transition enthalpies with the latter, thus these transitions are driven by the co-solvent. S3 , Supplementary information. Thus, the spatial separation of the PCM is not critical and transition B can be regarded as having a bulk nature.

The size effect of the microcapsules is discussed later in the article. To understand the measured crystallisation properties, two questions have to be answered — first, are the transitions B and C thermally related and, second, if the two transitions lead to the same crystal structure i. The first question was addressed by combining static and dynamic DSC measurements and the second one by performing powder XRD measurements.

Moreover, almost an identical curve was produced upon heating compare blue and red curves in Fig. Similar results were obtained for the TC composite and for the co-solvent. It is therefore safe to conclude that the transitions B and C are thermally independent. We can conclude that the presence of the dye does not affect the crystallization to the extent that another polymorph of ML or co-crystal would crystallize. It has to be noted that no crystal structure data for ML are available so that structure-related discussion of the powder patterns is neither possible nor needed for the purpose of this work.

The microencapsulated samples have a broad size distribution, ranging from 1.