Liquid Crystal (LC) Materials

Generally, a matter has three states: gas, liquid and solid. However, liquid crystal (LC) materials are the matter in a state that has characteristics between those of conventional liquids and those of solid crystals. As we know, in 1888, liquid crystal (LC) material was discovered for the first time by Friedrich Reinitzer. From then on, this special material had attracted more and more focus. In fact, people find that liquid crystal has such a structure which is partially ordered and anisotropic.

Figure 1 Schematic diagram of the position of liquid crystal in matter states.

Applications:

• Liquid crystal display field: Because of the advantages of low radiation, light weight and low energy consumption, liquid crystal display (LCD) has been well known. And now, it has been basic materials of display screens of mobile phones, televisions, laptops and computers.
• Biological field: There are a lot of liquid crystal substances in human body, and liposome is one of them. By using the liquid crystal properties, drugs can be wrapped in the hydrophobic area of liposome, and molecular recognition substances can be installed on the surface of liposome, so as to achieve the purpose of targeted drug delivery. The relationship between lyotropic liquid crystals and life sciences opens a door for further study of life sciences.

Figure 2 LCD used as nanocarriers in biological field.

• Organic solar cell materials: Discotic liquid crystal is famous for its unique structure and properties. The value of this type material is that it can reduce the cost of solar cells and can improve photoelectric conversion efficiency.
• Organic optoelectronic materials: Discotic liquid crystals often have electron donors and electron receptors. This kind of structure can assemble into columnar phase structure, which has the advantage of low fluidity of charge carrier, and this is expected to be a new kind of organic semiconductor material.

Figure 3 An example of liquid crystal material applied as organic optoelectronic material.

• High strength and high modulus materials: The molecular chain orientation is easy to occur under the action of external field force when the polymer contains the liquid crystal element in the molecular backbone or side chain. High strength and high modulus materials can be prepared by using liquid crystal.

Classification:

According to the formation conditions of liquid crystals, liquid crystals can be divided into lyotropic liquid crystals and thermotropic liquid crystals.

• Lyotropic liquid crystals: Lyotropic liquid crystals are formed by placing certain organic matter in a solvent that destroys the crystal lattice. Lyotropic liquid crystals are widely found in nature and organisms.
• Thermotropic liquid crystals: Thermotropic liquid crystal is a liquid crystal phase because of temperature change. This type of material is crystalline at low temperature, but liquid at high temperatures. Based on molecular arrangement, thermotropic liquid crystal can be classified into three groups: smectic, nematic and cholesteric. Smectic liquid crystal has high viscosity and the molecules are not easy to rotate. Therefore, the response speed of smectic liquid crystal is slow. Nematic liquid crystal molecules are easy to move freely along the long axis direction, so the viscosity is small, and the fluidity is good. Nematic liquid crystal molecules are relatively free of arrangement and movement, and are quite sensitive to external effects, so they are widely used in many fields. Most of cholesteric liquid crystals are cholesteryl derivatives and exhibit certain colored liquid crystal phase with phase transition. Cholesteric liquid crystals are widely used in the display field.

Figure 4 Different phases of thermotropic liquid crystals.

References:

1. Reinitzer F, Zur Kenernis des Cholesterins [J], Monathsh Chem, 1988,9:421-441.
2. Castellano, Joseph A. LIQUID CRYSTAL DISPLAY APPLICATIONS: Past, Present & Future[J]. Liquid Crystals Today, 1991, 1(1):4-6.
3. Salim M, Minamikawa H, Sugimura A, et al. Amphiphilic designer nano-carriers for controlled release: from drug delivery to diagnostics[J]. Med.chem.commun, 2014, 5(11):1602-1618.
4. Iino H, Usui T, Hanna J, et al. Liquid crystals for organic thin-film transistors[J]. Nature Communications, 2015, 6(1): 6828-6828.