Liquid crystal is an ordered fluid with crystal arrangement, which is formed when the crystals of some substances lose their solid rigidity in molten or solution state. There are small molecular liquid crystals and high molecular liquid crystals. Polymer liquid crystal, also called liquid crystal polymer (LCP), is a new type of polymer material. Compared with other organic polymer materials, liquid crystal polymers have unique molecular structure and thermal behavior. ?
The molecules of LCP are usually rigid structures, and the length and diameter of molecular chains are relatively large, which looks like rods. The main structure part is called liquid crystal cell, which is the basis of LCP with liquid crystal characteristics. In addition, a flexible small alkane chain is attached to the structure to promote the liquid crystal flow, and the polar groups on the molecules ensure the orderly arrangement of the molecules in the liquid state. After being melted by heat or dissolved by solvent, it can form a transitional intermediate state-liquid crystal state. In the liquid crystal state, the molecular arrangement is one-dimensional or two-dimensional, that is, the molecular arrangement shows disorder in position, but it still has a certain degree of order in molecular orientation and shows good anisotropy. ?
According to the different molecular arrangement and order of liquid crystal polymers, LCP can be roughly divided into three different structural types: (1) nematic liquid crystal, in which the rigid parts of liquid crystal molecules are arranged in parallel, while the centers of gravity are arranged in disorder, maintaining one-dimensional order, and the liquid crystal molecules can move along its long axis without affecting the crystal phase structure, which is easy to flow and orient; (2) smectic liquid crystal is the closest to the crystal structure of all liquid crystals. Due to the strong interaction between polar functional groups, the rigid parts of the molecules are arranged in parallel, forming a layered structure perpendicular to the long axis of the molecules, which has two-dimensional order. This liquid crystal is viscous in all directions; (3) Cholesteric liquid crystal, named after many cholesteric derivatives in this liquid crystal substance, is characterized by parallel arrangement into a layered structure through the interaction of end groups, and its long axis is parallel to the plane. Between two adjacent layers, due to the action of optically active groups protruding out of the plane, the long axis orientation of molecules rotates regularly at a certain angle, forming a helicoid structure. This kind of liquid crystal can disperse the reflected white light and present a rainbow-like color. According to the way of forming liquid crystal, liquid crystal polymers can be divided into thermotropic liquid crystal polymers (TLCP) and lyotropic liquid crystal polymer (LLCP). There are also cases where LCP is bonded to the main chain, side chain or main side chain of macromolecules according to liquid crystal units. LCP can be divided into three types: main chain type, side chain type or composite type. In addition, the polymer network containing liquid crystal units is called cross-linked liquid crystal polymer. ?
Lyotropic liquid crystal polymer LLCP is soluble in solvents, and only at a certain concentration can it show liquid crystal properties. Its main chain is mostly aromatic polyamide, which is usually difficult to melt and can only be dissolved in some special solvents to prepare high-strength high-modulus high-temperature resistant high-performance fibers or coatings. ?
Thermotropic liquid crystal polymers (TLCP) are mostly aromatic polyester-based backbone polymers (or * * * polymers). In recent years, non-polyester TLCP products such as polyamide, polycarbonate, polyurethane, polyether and polyimide have appeared. This kind of material has excellent heat resistance, flame retardancy and drug resistance, can be extruded or injection molded, and has good fluidity, small linear expansion coefficient, low molding shrinkage and good molding processability. The product has high strength, high modulus, self-reinforcing performance, outstanding heat resistance, excellent alternating resistance to cold and hot, excellent corrosion resistance, flame retardancy and electrical properties. ?
Because of its excellent performance, TLCP has been widely used in electronic and electrical industry, communication, automobile industry, chemical industry, pharmaceutical industry and medical equipment, such as manufacturing connectors, integrated circuit sockets, printed circuit boards, electronic component packaging materials, chemical device parts, pump and valve parts, packing of distillation columns, automobile engine parts and parts in contact with fuel, optical fiber coating materials, aircraft parts and so on. Thermotropic liquid crystal polymers can also be mixed with thermoplastic resins. Thermotropic liquid crystal polymers can be used to modify thermoplastic resins to improve their properties. On the other hand, mixing with engineering plastics can reduce the cost of LCP.