Formulation and Applications of Thermochromic Mixtures

Certain chiral nematic liquid crystals (or mixtures) are found to exhibit thermo-chromism, that is, temperature dependent selective reflection here the pitch length of such materials or mixtures is often found to have an inverse relationship with temperature and can depend on the nature of the material s mesomorphism. Thermochrom-ism is found to be at its most spectacular in materials which display chiral nematic phases and underlying smectic phases this is exemplified for a material with an I-N -SmA sequence (on cooling) and is shown schematically in Fig. 2. 

As an aside, it may be of interest to the reader that although selective reflection is usually a phenonemon associated with cal-amitic liquid crystalline systems, various multiyne materials have recently been demonstrated to show selective reflection as well as a helix inversion in chiral discotic nematic phases [15, 16]. 

Rarely, if ever, does one particular chiral nematic compound display ideal behaviour, which would make it suitable for use in a given application, and normally carefully formulated mixtures of a variety of meso-genic and non-mesogenic components, which may have very different thermodynamic properties, are used. Formulation of mixtures is frequently a complex trade-off of one property over another and is usually achieved by application of the Schroder-van Laar equation [17-20], which allows the thermodynamic characteristics of a par- 

As may now be more clearly appreciated, the potential uses of chiral nematic liquid crystals depend on either the twisting pow- 

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