Liquid crystals are a general term for ordered fluids formed by certain substances in a molten state or in a solution state. The discovery of liquid crystals can be traced back to 1888. The Austrian botanist FReinitzer found that heating crystals of Chol2esteryl Benzoate (C6H5CO2C27H45, referred to as CB) to 145.5e melts into a turbid liquid, and 145.5e is the melting point of the substance. Continue to heat up to 178.5e, the turbid liquid will suddenly turn into a clear liquid, and this process from turbid to clear is reversible . OLehmann systematically studies that the mechanical properties of some materials are similar to isotropic liquids in a certain temperature range ; however, their optical properties are similar to those of crystals and are anisotropic. Therefore, these phases between the liquid and the crystal are referred to as liquid crystal phases.
In the 1930s, people have been studying LCD for a long time, but due to low productivity, they have not been applied. Until the mid-1960s, due to the rapid development of new technologies such as microelectronics, aerospace, laser, microwave, ultrasonic, holographic, nuclear magnetic resonance and gas-liquid chromatography, it was required to use some media that respond sensitively to low-energy excitation to make liquid crystals. Have a place to use. After R. Williams published the electro-optical effect of liquid crystal in 1967, liquid crystal applications have received much attention and research has spread throughout various fields. The research of liquid crystal in the late 1960s also expanded into the field of polymers. The development of low molecular liquid crystals has been nearly a hundred years old and has gained in-depth research and extensive application. The development of polymer liquid crystals has been around for a long time. The polymer liquid crystal that scientists first observed was the structural unit of the nervous tissue, which consists of phospholipids and alcohols. In 1937, Bawden and Pirie discovered liquid crystal behavior in a suspension of tobacco mosaic virus. In the 1950s, Elliott and Robinson and others discovered that synthetic peptides also have liquid crystal properties. In the 1970s, high-strength and high-modulus aromatic polyamide fibers were introduced, and the application of liquid crystal spinning technology marked the beginning of a new stage in the study of polymer liquid crystals . The emergence of heat-induced polystyrene liquid crystal in the mid-1970s opened up a new field of polymer liquid crystal. Many rigid and semi-rigid chain polymers have been discovered, and some flexible chain polymers and many biopolymers have liquid crystal behavior. It has been found that macromolecules polymerized from monomers of rigid rod-like straight structures have similar anisotropy to low molecular liquid crystals. This macromolecule exhibits a high degree of order, and its melt or solution can be somehow Spontaneously oriented.
In 1968, DuPont Company of the United States first began to study the liquid crystal polymer poly(p-phenylene terephthalate) and used it to spin in a liquid crystal state at a certain temperature in the solution. The research was successful in 1971, named B. Fiber, which was put into industrial production in February 1972, and also published PRD-49 new fiber (polyparaxylamine). In 1973, the B-fiber and PRD-9 fiber trade names Kevlar and Kevlar-49 (referred to as aramid 1414 and aramid 14 in China, respectively) Kevlar liquid fiber opened the application history of polymer liquid crystal materials, and subsequently, In the field of polymer science, the development and development of various liquid crystal polymers began to flourish. At present, a wide variety of liquid crystal polymers have been successfully developed in the laboratory, and some of them have achieved industrial production, such as the industrialization of thermoplastic injection molded liquid crystal polymers, produced by Dartco Manufacturing Co., Ltd. under the trade name Xydar. A thermoplastic that is resistant to high temperatures and has other excellent properties .
(1) High tensile strength and high modulus in the orientation direction
Most commercial liquid crystal polymer products have this property. Compared with the flexible chain polymer, the liquid crystal polymer with a mesogenic element in the main chain or side chain of the molecule has the most prominent feature of molecular chain orientation in the external force field. Therefore, even without the addition of the reinforcing material, it can reach or exceed the mechanical strength of the ordinary engineering material enhanced by more than ten percent of the glass fiber, exhibiting the characteristics of high strength and high modulus. For example, Kevlar's specific strength and specific modulus are ten times that of steel .
(2) outstanding heat resistance
Since the mesogenic unit of the liquid crystal polymer is mostly composed of an aromatic ring, its heat resistance is relatively prominent. For example, Xydar has a melting point of 421 ° C, a decomposition temperature of 560 ° C in air, and a heat distortion temperature of 350 ° C, which is significantly higher than most plastics.
(3) Very low coefficient of thermal expansion
Due to the high orientation order, the liquid crystal polymer has a coefficient of expansion in the flow direction that is one order of magnitude lower than that of ordinary engineering plastics, reaching the level of general metals and even negative values.
(4) Excellent flame retardancy
The liquid crystal polymer molecular chain is composed of a large number of aromatic rings, and it is particularly difficult to burn except for the fiber containing a hydrazide bond, and carbonization after combustion indicates that the limit of oxygen resistance (LOI) of the polymer is relatively high.