Colour-changing materials

The optical spectra of other colour-changing materials exhibiting the alexandrite effect , including chromium- and vanadium-bearing garnets and... 

Most colour-changing materials are sensitive to external influences such as sunlight, high temperatures, washing or ironing. Moreover, they often have a low fatigue resistance, ie, a short lifetime. For now, these drawbacks limit their applications on textile substrates. Ageing studies and optimization of their lifetime are very important perspectives that will allow the development of their use in many different applications. 

In effect the chemist, and chemistry teacher, explains the observed chemical behaviour of matter (substances) - colour changes, precipitation from solution, characteristic flame colours, etc. - in terms of the very differenthQ miom of the quanticles that are considered to form the materials at the sub-microscopic level. Much of this involves the reconfiguration of systems of negative electrons and positively charged atomic cores (or kernels ) due to electrical interactions constrained by the allowed quantum states. 

Electrochromism can be defined as a colour change induced in a material by an applied electric field or current. Some ions in solid compounds can be reduced or oxidised (redox) electrochromically with a consequent change in colour. WO3 and M0O3 solid films have been extensively used for this purpose. The electrochromic reaction is expressed by... 

Whilst this definition is academically accurate for many inorganic and organic materials, the label thermochromic has also been applied to important technical areas that involve other external influences as well as heat in the observed colour change, e.g. thermochromic pigments. 

One alternative approach is to use photoisomerisable chiral compounds where the E and Z isomers have different helical twisting powers, e.g. menthone derivatives. By incorporating co-polymers, prepared from menthone containing monomers and cyano esters (5.5), as dopants into nematic LC mixtures materials, e.g. a mixture of cyanobiphenyls and cyanoterphenyls (E7 available from Merck), colour change can be effected by irradiating with UV light (365 nm). The colour obtained is dependent... 

These materials can store information on a molecular scale. The main requirement here is a molecular transformation associated with a colour change that is truly reversible. A typical reaction is the dimerization of the diene (Guru Row et al., 1983)... 

To distinguish between colour changes caused by ageing of the paint rather than by the rubber a blank test assembly may be used. A blank is an assembly prepared and tested in the same manner as the samples under test but the rubber is replaced by an inert material such as aluminium. 

Kaneto et al.523) have made measurements on the diffusion of lead perchlorate in polythiophene by following the colour change. They found a diffusion coefficient which varied from 10-1° to I0-12 cm2 s 1, depending upon the applied potential. The complexities introduced by morphological heterogeneity, counter-ion motion and solvent effects mean that further studies will be required to determine the relative importance of factors affecting diffusion in these materials. 

Notes. (1) Remove about 0.25 ml of the hydrolysate, cool it and basify it with 5 m sodium hydroxide solution. To a portion add a few drops of very dilute copper(u) sulphate solution, and note the absence of any colour change. As a control, prepare a specimen of biuret (NH2-CO-NH-CO-NH2) by heating about lOmg of urea just above its melting point for about 2 minutes. Add a little basified hydrolysate warm to dissolve, cool and add a trace of copper(u) sulphate. A deep pink colour superimposed upon the pale brown colour of the hydrolysate should be observed. If the hydrolysate gives a similar colour originally, it contains peptide material and hydrolysis should be continued until the biuret test is negative. 

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