Cholesteric temperature indicator

The cholesteric temperature indicator is usually sprayed or painted on in a 10% solution. The indicator solvent must have completely evaporated before reproducible results can be expected. 

High viscosity is the main drawback for cholesteric liquid crystalline polymers in applications. However, cholesteric polymers have their advantages. They may exhibit the memory effect. These polymers make processing and handling easily. Therefore, cholesteric liquid crystalline polymers have become the new materials for optical filters, temperature indicators, etc. The Merck product with the trade name Transmax is actually associated with the cholesteric liquid crystalline polymers. 

With the help of cholesteric liquid crystals, intensity distributions of microwave fields can be rendered visible [12]. A suitable detector (Fig. 2) consists of a thin resistive metal film with a surface resistance of 200 12/a, to 400 2/d coated with a 10 jLtm to 20 jum layer of a cholesteric liquid crystal. A polyester film deposited on both sides protects the device. When this foil is introduced into a microwave field of sufficient power density, absorption in the metal film produces a temperature profile proportional to the field intensity distribution. The liquid crystal temperature indicator then makes this profile visible. 

The CD reflection spectra are quite sharp at all temperatures, and the reflection wavelength, corresponding to the optical pitch of the TChLC phase, increased progressively with temperature from 500 nm at 70 °C to 1,000 nm at 140 °C. It was considered that the positive sign of the CD reflection band indicated M screw sense helicity of the cholesteric phase. Very recently, a smectic A-cholesteric phase transition was also observed for PDMBS.348... 

After curing a nearly constant double refraction of A = 0.1 is observed in the interval of the measured temperatures. This indicates the preservation of the liquid crystalline order when the temperature is increased From the relation between double refraction and bandwidth of the cholesteric reflection, the bandwidth of a green reflecting LC siloxane may be calculated [2] ... 

The reflective wavelength of cholesteric liquid crystals varies according to temperature. Such an effect has been made useful in thermography. It has been applied in the diagnosis of cancers by displaying the skin temperature distribution. It has also been applied to test faults in integrated circuits. The applications also include thermometers and temperature warning indicators and non-destructive detection. 

Mixtures dependence of pitch on composition We have seen in 4.1.6 that a mixture of right- and left-handed cholesterics adopts a helical structure whose pitch is sensitive to temperature and composition. This result was first described by Friedel. For a given composition, there is an inversion of the rotatory power as the temperature is varied, indicating a change of handedness of the helix. The inverse pitch exhibits a linear dependence on temperature, passing through zero at the nematic point where there is an exact compensation of the right- and left-handed forms (fig. 4.1.16). 

Helix-sense-selective polymerization has been achieved for several other optically active isocyanide monomers. In the polymerization of l-105 with NiCl2, helix sense was controlled by solvent and temperature. The spectra of poly-105 obtained in nonpolar solvents such as CCI4 and toluene were almost mirror images of those of poly-105 obtained in polar THF, indicating that the polymers have opposite helical senses, that is, the polymers are diastereomeric helices. The poly-105s with opposite helical senses showed cholesteric LC phases with opposite twist senses. 

The pitch of a cholesteric may vary quite rapidly with temperature, particularly if the phase diagram of the material contains a cholesteric to smectic A transition just below the temperature range of interest since the pitch in this case diverges to infinity as the smectic A phase is approached. Thus, the color of the selective reflection can be used a s a sensitive indicator of temperature. Material for this purpose is incorporated into plastic films with a black backing to absorb the unreflected polarization, or it is encapsulated to apply as a paint and used for a variety of medical, engineering, and consumer applications. 

---