Types of LCD liquid crystals

With the gradual understanding of liquid crystals, it has been discovered that mobile LCDscreen substances are basically organic compounds, and one of every 200 existing organic compounds has a liquid crystal phase. From the perspective of the composition and the physical conditions for the appearance of the liquid crystal phase, liquid crystals can be divided into two categories: thermotropic liquid crystals and lyotropic liquid crystals. The liquid crystal formed by heating and dissolving certain organic substances and destroying the crystalline lattice due to heating is called thermotropic liquid crystal, which is the liquid crystal phase that appears due to temperature changes as mentioned above. Putting some organics in a certain solvent, the liquid crystal formed by the solvent destroying the crystal lattice is called lyotropic liquid crystal. It is the liquid crystal phase that appears due to the change of the solution concentration. The most common ones are soapy water and so on. The liquid crystal materials currently used for display are basically thermotropic liquid crystals, but there are a large number of lyotropic liquid crystals in biological systems. Nearly 10,000 kinds of liquid crystal substances have been discovered so far. The molecules that make up the liquid crystal substance are generally in the shape of elongated rods or flat plates, and form a special arrangement in each liquid crystal phase. Liquid crystals formed by rod-shaped molecules have three types of liquid crystal phases: smectic liquid crystals, nematic liquid crystals and cholestic liquid crystals. Smectic comes from the Greek word and means soapy. Because this type of liquid crystal displays a unique polarized microscope image in a thick soap solution, it is named soap phase. The molecules are arranged in layers, have the same direction, and are relatively close to crystals, so it is translated as smectic phase. Nematic is also derived from the Greek word, which means filamentous. When the thin layer of this mobile LCDscreen is observed under a polarizing microscope, it exhibits a filamentous texture, so it is called silk phase. The molecular position is messy, but the direction is roughly the same, so it is translated as nematic. Cholesteric liquid crystals are called cholesteric phases because they were first discovered from cholesteric substances. Smectic phase liquid crystals are composed of rod-shaped or strip-shaped molecules. The molecules are arranged in layers. The long axes of the molecules in the layer are parallel to each other, and the direction can be perpendicular to the plane or arranged obliquely to the plane. Because the molecules are arranged neatly, their regularity is close to that of crystals, and it has two-dimensional order. The position of the molecular center of mass is disordered in the layer and can be translated freely, so that it has fluidity, but the viscosity coefficient is very large. The molecule can slide back and forth, left and right, but cannot move between the upper and lower layers. Because of its high degree of order, the smectic phase often appears in the lower temperature range. Smectic phase liquid crystal molecule arrangement The rod-shaped molecules of nematic liquid crystal also maintain the arrangement state parallel to the direction of the molecular axis, but there is no layered structure of the smectic liquid crystal. The center of gravity of the molecules in the nematic phase is chaotic and disorderly, but the directors n of the molecules (rods) are roughly the same, as shown in Figure 1-1-8. In the figure, a completely symmetrical rod is deliberately used to represent the molecule, that is, the rod is not pointed at one end but round at the other end, and there is no distinction between n and -n. This equivalence is a basic characteristic of nematic liquid crystals and other liquid crystals (such as smectic phases). The orderly arrangement of the directors of the nematic molecules makes the optical and electrical properties of the nematic material, namely the refractive index and the dielectric constant, different along and perpendicular to the direction of this orderly arrangement. It is precisely because the nematic liquid crystals optically display positive birefringence uniaxiality and electrical dielectric constant anisotropy, it is possible to use electricity to control optical performance or liquid crystal display. Nematic liquid crystal molecule arrangement In addition, compared with smectic liquid crystal, nematic liquid crystal has a low viscosity and is rich in fluidity. The reason for this fluidity is mainly due to the fact that each molecule of the nematic liquid crystal is easy to move freely along the long axis direction. In fact, the viscosity coefficient of many nematic liquid crystals is only several times that of water. The arrangement and movement of nematic liquid crystal molecules are relatively free, and they are quite sensitive to external effects, so they are widely used. At present, liquid crystal materials used in liquid crystal displays, such as twisted nematic liquid crystal displays and super twisted nematic liquid crystal displays, are all nematic liquid crystal materials. After being esterified or substituted by halogen, cholesterol presents a liquid crystal phase, which is called cholesteric liquid crystal. This type of liquid crystal molecules are in a flat shape, arranged in layers, and the molecules in the layer are parallel to each other. The direction of the long axis of the molecules of different layers changes slightly, and they are arranged in a spiral structure along the normal direction of the layer. molecule director When the long axis arrangement of different molecules undergoes a 360° change along the spiral direction, they return to the original orientation. This periodic interlayer distance is called the pitch (P) of the cholesteric liquid crystal. Cholesteric phase is actually a distortion state of nematic phase.
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