Thermal chiral nematics

The mesogenic behavior of 43 emphasizes the role played by the oc-tasilsesquioxane core when comparing with the related side-chain liquid-crystalline polysiloxanes [95,96]. It is remarkable that the thermal stability of the chiral nematic phase is similar in both the polymer and the dendrimer, suggesting that the cubic core does not perturb significantly the associations between the mesogens necessary to support the chiral nematic phase. 

These two ring azomethines gave chiral nematic phases which had relatively low thermal stabilities, but comparatively short phase ranges as demonstrated by compounds 28 and 29. 

The introduction of a third ring to give materials such as compound 30, led to markedly higher thermal stabilities. This compound is reported as having an optical rotation of 25 000 degrees mm in its chiral nematic phase, further highlighting the extraordinary optical properties associated with the chiral nematic mesophase. 

In the important physical properties we have discussed so far, the molecular structure contributes to the bulk optical, electrical, and thermal properties of the chiral nemat-... 

Biomedical thermography [222] is used extensively as a thermal mapping technique to indicate a wide range of subcutaneous medical disorders. The first use of chiral nematics to indicate skin temperature was over 30 years ago [223]. Such devices are normally constrained to indicate temperature in the 30-33°C temperature range, over which the whole color play can be exhibited. Such films have been used to indicate breast cancer [224], for placental location [225], to identify vascular disorders [226], and for skin grafting [227]. The use of chiral nematics in such areas presents an inexpensive rapid screening technique which is only indicative, since local patient and environment conditions, e.g., room and patient temperatures and internal film pressure, may lead to some false readings. This is discussed in more detail in [97]. 

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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