Organic thermochromic composites

Very much like the photochromic systems described previously in this chapter, organic thermochromic composites are prone to degradation caused by various external stimuli. To evaluate the extent of the damage and to determine the conditions under which the microcapsules may be used without the fear of lost functionality or complete loss of the colour-changing phenomenon, the following investigation was done. 

Microcapsules, i.e. organic thermochromic composites, were found to be completely destroyed after only one washing cycle according to the ISO 105C01 standard (Figure 2.20). 

The higher concentrations of the same adhesive in the case of the green, organic thermochromic composites caused the shifting of the first reflectance minimum, i.e. their colouristic parameters are bathochromically shifted compared to prints obtained by formulations using lower concentration of the adhesive. 

Figure 1.19 Schematic of a composite organic thermochromic pigment.

Problems associated with fastness properties of thermochromic composites remain to limit a wide range of apphcation possibUities, and at the forefront of the most important issues to be dealt with is poor fastness to UV irradiation, which excludes long-term exposure to daylight and direct sunlight, thus limiting the products for indoor use only. It was also found that temperature directly influences the properties of organic thermochromic pigments, and therefore they should be shielded from undesirable sources of heat. 

Furthermore, exploitation of fundamental coordination chemistry for solid state synthesis affords a variety of metal ions with preferred coordination geometries and numerous ligand architectures for the construction of metal-organic composite materials. Such hybrid inorganic-organic materials exhibit useful physical properties with applications to optics," magnetism," transport," thermochromism," and conductance. ... 

A new free-standing silica-based composite film that exhibits reversible thermochromic reflection, induced by a L.C. guest in the pores of iridescent mesoporous films, has been described. The authors have demonstrated that selective reflection from the novel mesoporous organosilica material with chiral nematic organization can be reversibly switched by thermal cycling of the 8CB guest between its isotropic and L.C. states, which was proven by SS NMR experiments. ... 

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