Thermal stability in hydrogen bond liquid crystals

Posted by: Dr V. N. Vijayakumar

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Thermal stability in hydrogen bond liquid crystals

Liquid crystals (LC) have intermediate phases which share the characteristics of both liquid and solid. The color changes in the LC phase with temperature have been a fascinating observation in recent years.  Thermochromic hydrogen bond liquid crystals (THBLC) and their stretchy mesomorphic effect are more mesmerizing to bring advanced technology in the thermochromic sensors in our day-to-day life. Specifically, optically dynamic LC thermal sensors, act as flexible display appliances, printing and drying technology. The thermochromic effect is the added advantage of soft material, which demonstrate various color at different temperatures.

Thermotropic LCs

The stability of the specified order in LC is comparable while changing the refractive index of the medium with temperature. So, the soft nature of the specific medium relatively depends on the internal alignment of molecules and the structure of the material. The dielectric and anisotropic properties of the medium give rise to the electro-optic effect of the same.    Thermotropic phases occur in a certain temperature range based on their optical activity. The material will enter a typical isotropic liquid phase if the temperature rises too much because thermal motion will ruin the LC phase’s fine cooperative ordering. At very low temperatures most of the LCs move toward conventional crystal.


Thermochromic effect in HBLC

HBLC spectacles have various colors due to the change in wavelength of incident light via LC molecules. The change in the refractive index of the medium intern changes the incident light’s wavelength. As a result, the various colors without textural changes in LCs are observed, which is called as thermochromic effect. Most probably, ionic liquid crystals possess thermochromic phenomenon. There are few literature reports available on surface thermography and thermochromic LC calibrations.


Added thermal stability in HBLC

Thermotropic HBLCs can alter their liquid crystal properties while varying temperature, this dynamic behavior with extended thermal stability in the mesophase range plays a pivotal role in thermochromic sensor applications. The extensible growth of high thermal stability HBLCs is the most preferable material for opt-electronic device applications, such as digital thermal sensors, optical modulators, optical shutters, etc, Intermolecular hydrogen bonds between the molecules induce the thermochromic effect due to the modification of internal structure without changing its physical properties.  In addition, the increasing chain length of the HBLC complex enhances the thermal stability of a particular LC phase. Therefore, the enriched thermal stability in nematic or smectic LC phases added its potential applications in diverse fields.


Wide thermal range thermochromic HBLC sensors

Specifically, Smectic A (focal-conic defect) phase HBLC possesses high-range thermal sensors because of its fascinating color change response with respect to temperature. Similarly, the threaded nematic phase without any positional order also responds to the temperature by exhibiting red, blue, and green (RBG) colors within the same phase. This type of thermochromic color variation is more predominant in H-bond LC complexes. Due to its inherent properties HBLCs with extended mesophase width are extensively used in thermal sensors. These HBLC sensors are highly sensitive when compared with others. Because the soft molecules of thermotropic HBLC are easily oriented with respect to the external temperature. In addition to this, the exact color variation clearly indicates better performance when compared with other conventional type sensors. Another interesting observation is the reversibility of the same origination color while applying and withdrawing temperature on the THBLCs.

THBLCs are appealing options for a range of sensing applications since they can be tuned or modulated optically, and the output can be seen with the naked eye. The higher temperature is clearly observed from the shifting of wavelength towards the maximum in the visible region of red and the lower wavelength is identified by blue in color. Thus, the highly stable and more precise temperature is monitored using HBLC materials with very low operating temperatures.

The HBLC complex is filled in a fiber probe at room temperature. The total internal reflection of light varies with temperature under the isolated environment, which can achieve high sensitivity temperature measurement. It has low cost and high precision compared with conventional ones. So, HBLCs color variable thermochromic sensors are potent candidates for temperature measurement.


Applications of HBLC as a sensor in different fields


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