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with people's gradual understanding of liquid crystals, it is found that liquid crystal substances are basically organic compounds, and one in every 200 existing organic compounds has a liquid crystal phase. From the composition and physical conditions of 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 some organic matters and destroying the crystal lattice due to heating is called thermotropic liquid crystal, which is the liquid crystal phase caused by temperature change as mentioned earlier. Put some organic matter in a certain solvent, the liquid crystal formed by the solvent destroying the crystal lattice is called lyotropic liquid crystal, which is the liquid crystal phase due to the change of solution concentration, the most common ones are soapy water. At present, the liquid crystal materials used for display are basically thermotropic liquid crystals, while there are a large number of lyotropic liquid crystals in biological systems. At present, there are nearly 10,000 kinds of liquid crystal substances found. The molecules that make up the liquid crystal substance are generally slender rods or flat sheets, and form special arrangements in each liquid crystal phase.
liquid crystals formed by rod-shaped molecules have three major types of liquid crystal phases: subcrystalline phase ( Smecticliquidcrystals), Nematic phase ( Nematicliquidcrystals)And cholesteric phase ( Cholestericliquidcrystals). Smectic comes from Greek and means soap. Because this type of liquid crystal shows a unique polarizing microscope image in a concentrated soap aqueous solution, it is named soap phase. The molecules are arranged in layers in the same direction and are relatively close to the crystal, so they are translated into the near crystal phase. Nematic is also from Greek, which means filamentous. Because this thin layer of liquid crystal presents filamentous texture when observed under a polarizing microscope, it is called filamentous phase. The molecular position is messy, but the direction is roughly the same, so the translation is oriented. Cholesteric liquid crystal is called cholesteric phase because it was first found in cholesterol substances.
The near crystalline liquid crystal is composed of rod-shaped or strip-shaped molecules, the molecules are arranged in layers, and the long axes of the molecules in the layer are parallel to each other, its direction can be perpendicular to the level or inclined to the level. Because the molecules are arranged neatly, their regularity is close to the crystal and has two-dimensional order. The position of the molecular centroid is disordered in the layer and can be translated freely, thus having fluidity, but the viscosity coefficient is very large. Molecules can slide back and forth, left and right, but cannot move between the upper and lower layers. Because of its high order, the near crystalline phase often appears in the lower temperature range.
the rod-like molecules of nematic liquid crystals still maintain an arrangement parallel to the direction of the molecular axis, but there is no layered structure in the near crystalline liquid crystal. The center of gravity of molecules in nematic phase is chaotic, but molecules (Rod)The pointing vector n of is generally the same, as shown in Figure 1-1-Shown in 8. In the figure, a completely symmetrical rod is deliberately used to represent molecules, I . e. the rod is not a tip, a circle, and there is no n and-N distinction. This equivalence is nematic liquid crystal and other liquid crystals (Such as near crystal phase)A basic feature. However, the ordered arrangement of the direction vectors of nematic phase molecules makes the optical and electrical properties of nematic phase substances, I . e. refractive index and dielectric constant, it varies along and perpendicular to this orderly arrangement. It is precisely because nematic liquid crystals show positive birefringence in optics, uniaxial and electrical dielectric constant anisotropy that it is possible to use electricity to control optical properties or liquid crystal displays.
In addition, compared with the near crystal phase liquid crystal, nematic liquid crystal has small viscosity and rich fluidity. The main reason for this fluidity is that each molecule of 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 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 ultra-twisted nematic liquid crystal displays, are nematic liquid crystal materials. Cholesterol is lipified or replaced by halogen and presents a liquid crystal phase, which is called cholesteric liquid crystal. This kind of liquid crystal molecules are flat in shape and arranged in layers, and the molecules in the layers are parallel to each other. The long axis direction of molecules in different layers changes slightly and is arranged into a spiral structure along the normal direction of the layers.
when different molecular long axis arrangements undergo 360 changes along the snail direction, back to the initial orientation, this periodic interlayer distance is called the pitch of cholesteric liquid crystal (P). Cholesteric phase is actually a distorted state of nematic phase.
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