Polymer liquid crystal is a new polymer material with high strength and high modulus. In this paper, the synthesis, structure and properties of polymer liquid crystals and their applications in composite materials, fibers and liquid crystal display technology are reviewed. Liquid crystal is an ordered fluid formed by some small molecular organic compounds or some polymers in molten or liquid state. It has both crystal anisotropy and liquid fluidity, which is a transitional state. This intermediate state is called liquid crystal state, also called the fourth state or mesomorphic state of matter. The substance in this state is called liquid crystal [1]. Polymer liquid crystal material is a new type of special polymer material, which has been used in aviation, navigation and automobile industry together with fibers, composite materials and injection molded parts [2]. In this paper, the development history of polymer liquid crystal is briefly introduced, and the application of polymer liquid crystal in fiber, plastic, composite material, separation material, information material and biological material is expounded in detail.
Key words: polymer liquid crystal, characteristics, synthesis, research, progress and application.
order
Liquid crystal is an intermediate state between liquid state and crystal state, which has both the characteristics of liquid flowing easily and some characteristics of crystal. Isotropic liquid is transparent, but liquid crystal is often turbid, which is also a main feature that distinguishes liquid crystal from isotropic liquid. The turbidity of liquid crystal is caused by the light scattering caused by the fluctuation of liquid crystal molecular orientation, and the light scattering of liquid crystal is 654.38+00,000 times stronger than that of isotropic liquid [3]. Polymer liquid crystal is formed by bonding liquid crystal units with relatively small molecular weight, which can be rod-shaped; It can also be disc-shaped; Or more complex two-dimensional or even three-dimensional shapes; You can even have both; It can also be an amphiphilic molecule [4]. In a word, liquid crystal science has made many important developments, and its research fields cover physics, chemistry, electronics and biology, and it has developed into important disciplines such as liquid crystal chemistry, molecular physics, biological liquid crystal and liquid crystal molecular spectroscopy [5].
Characteristics of polymer liquid crystals
There are many similarities between polymer liquid crystal and low molecular liquid crystal, such as alignment orientation, thermal fluctuation and light scattering, electric field effect, thermal transition, parity, viscosity, electro-optical effect and so on. Polymer liquid crystals have unique properties:
(1) In electric and magnetic fields, the electric or magnetic field intensity required for polymer liquid crystal orientation is much greater than that of low molecular liquid, and thermotropic liquid has high thermal transition temperature and high viscosity.
(2) Parity, that is, TM, TN, △ s and △ h of mesomorphic state are abnormally low, even high, with different elastic intervals. Not only the main chain has parity effect, but also the side chain has parity effect.
(3) Rheological behavior of polymer liquid crystals The rheological behavior of polymer liquid crystals has great influence on the application of polymer materials. For example, viscosity is a function of temperature, and at a certain temperature, viscosity becomes smaller. Viscosity has a great influence on the shear layer. At low shear rate, the order of liquid deviating from Newtonian fluid decreases-the viscosity decreases with the increase of molecular standard.
(4) Liquid phase transition: At a certain concentration, the liquid crystal transition temperature increases with the increase of polymerization degree. In isotropic extruders, the phase transition temperature decreases with the decrease of polymer concentration. At a certain temperature, the greater the degree of polymerization, the lower the critical concentration of mesophase.
(5) Electro-optic effect of liquid products. The so-called electro-optic effect refers to the optical change of liquid crystal under the action of electric field, which is embodied as: the formation of phase domain, the electric field can cause nematic phase, and the liquid crystal produces William phase domain; Dynamic scattering, the ions in liquid crystal, under the action of alternating electric field, increase with the increase of voltage, and when it exceeds the elastic limit, it produces turbulence; When there are other anisotropic molecules in the guest-host interaction liquid crystal, the two molecules move in an interactive arrangement when an electric field is applied [6].
The dielectric properties of polymer liquid and the dielectric anisotropy of conductive liquid crystal are the main parameters that determine the behavior of liquid crystal molecules in electric field. The dielectric anisotropy of liquid crystals depends on the permanent dipole moment and molecular polarization contained in the molecular structure of liquid crystals. If the polarization along the molecular axis is greater than the polarization perpendicular to the molecular axis, a positive dielectric anisotropic liquid crystal will be obtained, and vice versa [7].
Processing of polymer liquid crystal
Lyotropic liquid crystal and thermotropic liquid crystal adopt completely different processing methods, but there are still some similarities in how to obtain fiber orientation structure. At present, lyotropic liquid crystal mainly adopts wet spinning and dry jet wet spinning. The difference between the two is that the former is to immerse the spinneret in the coagulation bath, while the latter is to spray the filaments into the air and then introduce them into the coagulation bath. However, the physical properties of fibers obtained by two different methods are obviously different. As shown in table 1, the reason is not clear at present, which may be related to the additional stretching effect obtained between the spinneret hole and the liquid bath.
From the table 1, it can be seen that even as-spun fibers have high strength and modulus, which is related to the high molecular orientation obtained by liquid crystal spinning [8].
Research progress and application of three-type polymer liquid crystals
3. 1. Cellulose liquid crystal [9- 10]
In 1976, D. G. Gary first reported that the molecular weight of hydroxypropyl cellulose, a derivative of cellulose liquid crystal, is 105, and its 2% ~ 5% aqueous solution can form a cholesteric liquid crystal solution with rainbow color, strong birefringence and optical rotation. Cellulose derivatives can form liquid crystal phase in various solvents such as water, acetic acid and acetone. Various textures of liquid crystal solution can be observed under polarizing microscope, such as disc texture, stripe texture, plane texture, pseudo-isotropic texture, fingerprint texture and so on. The existence of these textures is closely related to the external conditions such as the temperature and concentration of the solution. In addition, various dislocation structures can be observed. The synthesis of cholesteric liquid crystal polymer complex containing cellulose derivatives makes the application of electron microscope and atomic microscope in the study of fine structure of cholesteric liquid crystal, which makes the study of cholesteric liquid crystal structure reach a more microscopic level. Because the liquid crystal solution of cellulose can simulate a new polymer composite with high strength and high modulus, it is a good model compound for studying semi-rigid chain polymer liquid crystal phase. Therefore, more liquid crystalline cellulose products with better performance should be developed, such as high-strength and high-modulus fibers, high-performance cellulose liquid crystal composite materials, high-performance cellulose liquid crystal separation films, special optical materials and so on.
3.2. Chitin liquid crystal [1 1- 13]
Due to the existence of various forms of hydrogen bond groups in the molecule, it has a microcrystalline structure, and its melting point is higher than the decomposition temperature, so it cannot be melted or dissolved, and it is only soluble in a few special solvents such as methanesulfonic acid. Chitin has spiral or double helix structure, generally cholesteric, and also has bond rigidity and crystallinity. It can also be modified by chemical reaction to prepare N2 acetylated derivatives of chitin esters and chitin ethers. Because of the strong hydrogen bond between chitin molecules, it is easy to form tight molecular bundles between molecules, which has a good fiber-forming tendency. Chitin can be dissolved in a suitable solvent to make a solution with a certain concentration, viscosity and good stability, and has good spinnability. Chitin has biological activity, biocompatibility and biodegradability, and is nontoxic (LD50 16 g/kg body weight). But also can form films or fibers, so it can be widely used in medical materials. Recently, chitosan, a derivative of chitin, has been made into nonwoven artificial skin. The huge storage of chitin and the diversity of derivative methods make the study of chitin liquid crystal of great scientific value. Widely used in industry, agriculture, medicine, environmental protection and other fields, the research of chitin material is considered as the most promising polysaccharide research in 2 1 century.
3.3. Ferroelectric liquid crystal [14 ~ 16]
The molecules of ferroelectric liquid crystals are arranged in layers, stacked layer by layer, and the molecules in the layers are parallel to each other, but it is found that the opposite layers are inclined (the distance between layers is less than the length of molecules), and the layers form a spiral arrangement along the normal of the layers. Ferroelectric liquid crystal phase has a spontaneous polarization vector Ps perpendicular to the molecule and parallel to the layer, showing ferroelectricity (ferroelectricity refers to the characteristic that the polarization direction of liquid crystal molecules changes under the action of electric or magnetic fields). Ferroelectric liquid crystal has both display application and photoelectric characteristics, especially its nonlinear optical characteristics [NLO refers to the nonlinear polarization effect caused by the interaction between light waves and substance molecules when strong coherent light (such as laser) propagates in nonlinear media]. Nonlinear optical effect is one of the core problems in the transmission, processing and storage of optical signals by optoelectronic devices in modern communication systems. Ferroelectric liquid crystal organic nonlinear optical materials have excellent characteristics such as fast response speed, high laser damage threshold, low branch dielectric constant, low absorption coefficient, chemical and structural stability. Especially in the field of liquid crystal display materials, domestic enterprises that have formed mass production scale, such as Shijiazhuang Shilike Liquid Crystal Materials Co., Ltd. and Tsinghua Wangya Liquid Crystal Materials Co., Ltd., have developed or are developing chiral liquid crystal additives for mixed crystal materials of T N, ST N and TFT2 LCD, which have achieved good economic benefits and greatly promoted the development and progress of liquid crystal materials for liquid crystal display in China. For ferroelectric liquid crystal polymers, their application fields are mainly optical recording and storage materials, display materials, ferroelectric and piezoelectric materials, nonlinear optical materials, materials with separation function and photochromic materials.
3.4. Dish liquid crystal [17]
The typical structural feature of discotic liquid crystals is that discotic molecules are arranged in columns. At first, people studied the liquid crystal properties of various compounds with discoid symmetric molecular structure, and found many discoid liquid crystals with benzene ring as the core and composed of nonpolar molecules with good symmetry. Later, discotic liquid crystals composed of discotic or flat symmetric molecules centered on non-benzene rings and liquid crystals that can form discotic or flat symmetric combinations through intermolecular or intermolecular internal forces were discovered. In 1977, S.Chandrasekhar and others discovered for the first time that ester compounds of pyromellitic acid have the properties of discotic liquid crystals, which are different from traditional thermotropic liquid crystals in molecular structure, phase transition behavior and physical properties (discotic liquid crystals have high symmetry, so they show wide phase transition behavior, high enthalpy change and large refractive index). Dislike liquid crystals with electron donors and acceptors have low mobility of charge carriers due to small overlap between adjacent systems in the column, and are expected to become new organic semiconductor materials or organic photoconductor materials, with potential application prospects.
3.5. Halogenated liquid crystal [18]
Halogenated liquid crystals are liquid crystals containing F, Cl, Br and I atoms on the end groups, side groups and chiral central bridges of liquid crystal molecules. The introduction of halogen atoms and halogen-containing groups will affect the polarity and polarization of liquid crystal molecules because of their strong electron-withdrawing properties. According to its electronegativity, the position and quantity of liquid crystal system in the molecule, halogen atoms endow liquid crystals with different properties, such as terminal halogenated liquid crystals increase nematic stability in aromatic systems, and halogenated liquid crystals are widely used in multi-channel driven high-response mixed liquid crystals, which have the following properties: ① Due to the introduction of halogen atoms, the melting point is reduced, the smectic phase is suppressed or eliminated, and nematic mixed liquid crystals with wide nematic phase range can be prepared; ② Moderate electro-optical performance, high viscosity and good thermal, optical and chemical stability; ③ It has positive hydrophobic parameters and high voltage retention rate, and is suitable for the requirements of high-performance liquid crystal displays such as AMLCD and PDLLLCD; (4) Halogen atoms are introduced into ferroelectric and antiferroelectric liquid crystals to increase the spontaneous polarization value Ps3 or to produce Sc3 phase as the main liquid crystal, such as the monofluoro compound of terphenyl. The vigorous development and wide application of halogenated liquid crystals occurred after the mid-1980s, which is closely related to the development of various high-performance liquid crystal displays. Up to now, the most studied liquid crystals are fluorine-containing liquid crystals, followed by chlorine-containing liquid crystals, and bromination and iodination are mainly used as intermediates of liquid crystals. Halogen atoms and halogen-containing groups are introduced into different positions of different types of liquid crystal molecules, depending on the size of their electronegative groups and the different effects caused by the number on the polarization anisotropy, molecular packing compactness and steric hindrance of liquid crystal molecules, thus affecting a series of physical properties of liquid crystals, such as electricity, light, viscosity and phase behavior, which provides a wide choice for preparing various high-performance mixed liquid crystals.
3.6. Thermotropic polymer liquid crystal plastic [19-20]
Because both aromatic polyamides and aromatic heterocyclic liquid crystal polymers are lyotropic, that is, they cannot be processed by melt extrusion, so their application in the field of high-performance engineering plastics is limited. Thermotropic liquid crystal polymers represented by aromatic polyester liquid crystal polymers just make up for the shortage of lyotropic liquid crystal polymer. At present, the commercialized thermotropic liquid crystal polymer polyarylate can be roughly divided into three types: Type I represented by Xylar of Amoco Company and Ekonol of Sumitomo Company, Type II represented by Vectra of Hoechst2Celanese Company and Type III represented by Rodrun LC 5000 of Unitika Company. Type I belongs to biphenyl series, and its molecules and basic components are p-hydroxybenzoic acid (HBA), 4,4' biphenol (BP) and terephthalic acid (TPA) and isophthalic acid (IPA) in different proportions. Type ⅱ belongs to naphthalene series, and its main components are HBA and 6- hydroxy -2- naphthoic acid (HNA). Type ⅲ is a poly product of HBA and PET. Type I has the best heat resistance and is suitable for occasions requiring high temperature performance, but it is difficult to process; Type ⅲ has poor thermal performance; Type ⅱ has good comprehensive performance and moderate heat resistance. PET/ 60PHB*** polyester system was studied by China Hongding. Its structure and liquid crystal properties were analyzed by nuclear magnetic resonance and differential scanning calorimetry. The results show that the polymer PET/60PHB is a random polyester of PET and PHB, belonging to nematic thermotropic liquid crystal. The processing test shows that the polyester has excellent processing fluidity, and its mechanical properties, heat resistance and electrical insulation all reach or exceed the level of similar foreign products, in which the tensile strength exceeds 600 MPa and the thermal expansion coefficient is close to that of ceramics. These two unique properties show the unique and broad application prospect of this liquid crystal polyester as engineering plastics.
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Chitin and 10 derivatives are all liquid crystals, which have formed a large class of natural polymer liquid crystals. Moreover, due to the huge chitin reserves and various derivative ways, the study of chitin liquid crystal has important scientific value, but there is little in-depth basic research at present, especially in China. In addition to further studying the structural factors of chitin liquid crystal formation and the laws of liquid crystal structure, the following aspects deserve attention: ① the development of thermotropic chitin liquid crystal and the development of composite materials containing chitin liquid crystal; (2) The relationship between the structure of the derivative and the pitch of cholesteric phase was studied, and the controllable pitch range was that the material was used for thermochromic display. ③ Application of liquid crystal film in separation; ④ Liquid crystal behavior of chitin in biological tissues.
In a word, with the improvement of polymer liquid crystal theory, its application is increasingly extensive. People have not only developed a large number of polymer liquid crystal materials with high strength, high modulus, display and information storage functions, but also continuously explored their applications in other fields. Because of its rheology, anisotropy, good thermal stability, excellent dielectric, optical and mechanical properties, chemical resistance, low flammability and excellent dimensional stability, it is certain that as an interdisciplinary subject, polymer liquid crystal materials science will play an increasingly important role in high-performance structural materials, information recording materials, functional films and nonlinear optical materials [2 1].
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