BOĞAZİÇİ UNIVERSITY ME 210 MATERIAL SCIENCE LIQUID CRYSTALS BY AHMET GÖKHAN YALÇIN Submitted to: Ercan BALIKÇI FALL 2005
A NEW PHASE OF MATTER: LIQUID CRYSTALS ABSTRACT: Liquid crystals have different physical and chemical properties than those of solids and liquids, and these properties contribute to their applicability in many areas such as biotechnology and nanotechnology. In this paper, I talk about the properties, structures, and applications of liquid crystals. Firstly, I talk about the differences between the solid phase, liquid phase, and the liquid crystal phase. Then, the two basic types of liquid crystals are mentioned, which are lyotropic and thermotropic. After that, they are classified into four phases according to their molecular order: nematic, smectic, cholesteric, and columnar. Then, I advert to polymorphism in liquid crystals. Finally, various applications, advantages of these applications, and other uses of liquid crystals are mentioned. INTRODUCTION: I have chosen the liquid crystals to study because it has increasing significance in industry and science and there are many applications of them; for example, stress testing of materials, holography, and the visualization of radio frequency waves. Moreover, it will become more important as the field of nanotechnology advances. The study of the liquid crystals started in 1888 when an Austrian botanist named Friedrich Reinitzer observed the fact that cholesteryl benzoate had two distinct melting points [1]. Reinitzer increased the temperature of the solid state and then observed the change of the solid into a hazy liquid. When he increased the temperature further the material changed its phase into a transparent liquid. From this experiment we understand that cholesteryl benzoate has another phase between solids and liquids that has the properties between these two phases. So, a new phase was discovered. I have discussed the basic properties of these phases below. Firstly, we have to define liquid crystals. Before defining the liquid crystal, I want to explain the three common states of matter. These are solid, liquid, and gas, in which molecules/atoms have different degree of order. In the solid (crystalline) state molecules have a rigid arrangement and they generally stay in a fixed orientation and position, therefore there must be large forces to maintain the molecules in this order so it is difficult to deform solids. On the other hand, there are no fixed orientations in liquid state and also molecules in liquid state are more mobile. Because liquids have less order than solids, it is easier to deform liquids when compared to solids. In the gas state, the mobility of the molecules increases thus intermolecular forces are overcome, which leads to less order in the gas state than in the liquid state. Let s think about the phase transitions between these three different states. The melting of ice at zero degree centigrade to form liquid water is an example of a single transition from solid to isotropic liquid phase. However, many materials exhibit more than a single transition when passing from solid to liquid, which proves the presence of one or more intermediate phases. The molecular ordering of these intermediate phases, known as mesophases, is in between that of solids and liquids [2]. There are two basic types of mesophases. First there are those which are characterized by rigid rotational disorder and they have 3-dimensional crystal lattice (i.e., disordered crystal mesophases), and second, there are those which have rotational order and they have no lattice (i.e., ordered fluid mesophases) [2]. Disordered crystal mesophases are known as plastic crystals. In this mesophase, molecules are rotationally disordered but translationally well ordered. Ordered fluid mesophases are generally called liquid crystals, which have an order which is intermediate between that of a
liquid and that of a crystal. Figure 1 below shows the average alignment of the molecules for each phase. Figure 1. Average alignment of phases [1]. THEORY: The liquid crystalline state s peculiar property is the molecules (mesogens) tendency to point along a common axis, called the director [1]. In contrast, we cannot observe any intrinsic order in the liquid phase. The amount of the order in a liquid crystal is measured by the order parameter (O), as given by the equation below [3]. O= (3cos²θ-1)/2. Figure 2. Order vs. temperature for a typical liquid crystal [3]. θ is the angle between the long axis of the each molecule and the director. For an isotropic liquid, the order parameter is zero, as we expected; in contrast to isotropic liquids, order parameter is one in a perfect crystal. This order parameter is dependent on the temperature of the material. Figure 2 represents the relationship between temperature and order parameter. In this figure T is the c temperature of transition between the liquid crystal and liquid states. Now we can differentiate the liquid crystal mesophases into two groups that is thermotropic and lyotropic. This classification is due to mechanisms which urge their self-organization. Lyotropic Liquid Crystals: The anisotropic solution mesophases for high solute concentration which are formed by the solutions of rod-like entities in an isotropic solvent are called lyotropic liquid crystals [2]. Amphiphilic compounds are substances which lyotropic liquid crystal molecules belong
to. Amphiphilic compounds are characterized by two groups, which have different solubility, having in the same molecule. First is hydrophilic group which is insoluble in hydrocarbons and water soluble, the second is lipophilic group which is, in contrast to hydrophilic group, water insoluble and soluble in hydrocarbons [4]. DNA and certain viruses form lyotropic mesophases when they dissolve in an appropriate solvent. Lyotropic liquid crystals are important for biological systems, e.g. membranes. Thermotropic Liquid Crystals: Thermotropic transactions occur in most liquid crystals, and they are defined by the fact that the transitions to the liquid crystalline state are induced thermally. [1]. Namely, we can reach at the liquid crystalline state by lowering the temperature of the liquid or rising the temperature of the solid. Thermotropic liquid crystals can be classified in to two groups as enantiotropic and monotropic. In enantiotropic liquid crystals, we can reach liquid crystalline state either lowering the temperature of the liquid or rising the temperature of the solid, but in monotropic we can obtain liquid crystals either increase in the solid s temperature or decrease in the liquid s temperature, but not both [1]. Liquid Crystal Phases: Many types of the liquid crystal states exist because of the amount of the order in the material. We can classify the four major liquid crystal phases, nematic, smectic, cholesteric, and columnar. However, we cannot say that all of these phases are either thermotropic or lyotropic because these phases are just the classification of the liquid crystals in terms of the molecular order. 1.Nematic Phases: Figure 3. Alignment of the molecules and the director in nematics [6]. In nematic liquid crystals the molecules have no positional order, but they tend to point in the same direction, along the director. As seen in liquids, the molecules in nematic phase float around too, however they are ordered in their orientation, so we can say that nematic phases are close to liquid phase [5]. From Figure 3, we can see the average alignment of the molecules and the director in nematics. The arrangementof the nematics can be controlled with applied electric fields. For example, in liquid crystal displays (LCD), nematics are used commonly, with the devices having the twisted nematic geometry [6]. 2.Smectic Phases: In contrast to nematic, smectic phase is close to solid phase. In smectics, arrangement of the liquid crystals is layered, in which the molecules move freely but they cannot move freely between the layers [5]. Still, molecules have orientation in the same direction. We can classify the smectics into three types, smectic A, C, and B. These phases have one common property that is they have layered structure, so they have partial translational ordering which makes smectics more viscous than other phases.
a. Smectic A mesophase: The director, namely the molecules orientation, is perpendicular to the smectic plane in smectic-a mesophase; Figure 4a. The layers are fluid, and there is a high probability of the inter-layer diffusion when compared to smectic-c. b. Smectic C mesophase: In smectic-c phase the director is at a constant tilt angle with respect to the layer normal; Figure 4b. As in smectic-a, phase layers are fluid but there is a lower probability of the inter-layer diffusion. Figure 4. Liquid crystalline mesophases [7]. c. Smectic B mesophase: Smectic-B phase also orients with direction perpendicular to the smectic plane, but the order of the molecules is a system of hexagons within the layer [1]. In contrast to smectic A and C phases, layers are not fluid any more. However the mechanical properties of smectic-b phases are quite different from the solids, so smectic-b phase may be a plastic crystal, indeed. 3.Cholesteric Phases: The cholesteric (or chiral nematic) liquid crystal phase is composed of nematic molecules which contains a chiral center that produces intermolecular forces which provides orientation between molecules at a small angle. This situation results in a structure that can be seen as a stack of a very thin 2-D nematic-like layers in which the director in layers bent with respect to adjacent layers [1]; Figure 4c. 4.Columnar Phases: They differ from other types of liquid crystals in terms of their shape that is like disk instead of long rod; Figure 4d. This mesophase is characterized by the pile of the columns of molecules. The arrangement of the columns and molecules in these columns results in new mesophases [1]. Polymorphism in Liquid Crystals: Most of the materials observe more than one mesophase when passing between the solid and isotropic liquid phases. We call such materials as polymorphous. 1) If material has nematic and smectic trimorphous phases, the order of stability with increasing temperature will be, solid smectic B smectic C smectic A nematic isotropic. 2) If material has both cholesteric and smectic mesophases, the order will be, solid smectic A cholesteric isotropic.
Applications of Liquid Crystals: Liquid crystal Thermometers: Liquid crystal thermometers depict temperatures as colors and they are used to follow the changes in temperature that are caused by heat flows. They may be used to indicate that heat can flow by conduction, convection, and radiation [8]. The molecules in the liquid crystal thermometer are made of cholesterol. Cholesteric liquid crystals reflect light whose wavelength is equal to the pitch and the color reflected is dependent upon temperature because the pitch is dependent upon temperature. In this material, liquid crystals make it possible to measure the temperature accurately just by observing the color of the temperature [8]. Liquid crystals can be adjusted by the producer to change colors over different temperature ranges, but 30 ºC-35 ºC seems to be a common range. Liquid Crystal Displays: A liquid crystal display (LCD) is a thin, flat display device made up of monochrome pixels lined up in front of a reflector. Each of the pixels (picture element) is formed of a column of liquid crystal molecules that are hanged up between two transparent electrodes and two polarizing filters. In order to enable the passing of light from one filter through the other the liquid crystal twines the polarization of the light [9]. Figure 5. Structure of liquid crystal display [10]. Figure 5 shows the structure of the LCD. Liquid crystal molecules have electric charges on them. When applying electrical charges to electrodes over each pixel, molecules are twisted by electrostatic forces. This changes the twist of light, and enables the passing of the different degrees of light to pass (or not to pass) through these polarizing filters. Liquid crystal molecules are in a relaxed state before applying an electrical charge. When a charge applied to these molecules, they align themselves in a twisted structure. When an electrical charge is applied to electrodes, molecules align parallel to the electric field, so they limit the rotation of the light. If liquid crystals are not twisted, there will be a light polarization perpendicular to the second filter and then light will be completely blocked. Therefore the appearance of the pixel will be aphotic, which means lacking of light. Light can pass in varying amounts by controlling the twist of the liquid crystals in each pixel [9]. Optical Imaging: An application, which is only now being explored, is optical imaging and recording. In this technology there is a liquid crystal cell between the layers of photoconductor. When we apply a light to the photoconductor, it results in an increase in the material s conductivity. This leads to an electric field to arise in the liquid crystal fitting to the light intensity. The electric movement may be conveyed by an electrode that enables recording of the image. Optical imaging is still being developed and it is a promising area of the liquid crystal research [1]. Liquid crystals have also many other uses. They are used in medical applications where, for example temporary pressure conveyed by a walking foot on the ground is measured [1]. In addition they are used for nondestructive mechanical testing of materials under stress. This technique is also for the visualization of RF (radio frequency) waves in waveguides. [1]. As new types of liquid crystals are researched, these materials are acquiring greater significance in scientific and industrial applications.
DISCUSSION: We can observe the effects of the liquid crystal technology in many areas of science, engineering and also in device technology. Applications are still being discovered and they will continue to produce solutions to many different problems in different branches. For instance if we look at the basic differences between LCD and cathode ray tube we see that liquid crystal displays take up less space and consume less power than traditional cathode-ray tube monitors. However the color quality of them cannot compare that of cathode-ray tubes and they are expensive when compared to cathode-ray tubes, so liquid crystal displays are still a luxury for most. We observe that some companies have already beheld the importance of the liquid crystals so they are investing much more money to this field. The Japanese producers are dominant in the LCD market nowadays. They control 95% of the markets with a variety of products [11]. Other rivals are Korea and the US. What is more, the three Japanese giants, Hitachi, Toshiba, and Matsushita invest $1 billion into the new company that will begin operating in 2006 and produce 32- inch panels a year in 2008 and produce also large liquid crystals for flat screen televisions [12]. Because liquid crystal thermometers are inexpensive and they are easy to read the temperature values it will be one of the most useful applications of the liquid crystals. In addition to these features it s being non-toxic and the accuracy of its values will make liquid crystals more applicable in the future. Optical imaging, which is one of the most important applications of the liquid crystals, will contribute to early detection of various cancers. Therefore, it will be a significant application in the medical science, so the number of the optical imaging laboratories must be increased in order to perform more research about this topic. CONCLUSION: I have discussed the properties and applications of the liquid crystals. I have stated that liquid crystals are one of the phases of materials that have common characteristics between liquid phase and solid phase. Namely, they have the degree of the order that is smaller than solids and higher than liquids. In addition, we have seen that there are many types of liquid crystals according to their molecular order. And, these types result in the practicability of them into the many branches of science and industry. Also, I have mentioned to the polymorphism in liquid crystals, and I stated that the polymorphous liquid crystals with different type of liquid crystals will have different order of stability with increasing temperatures. After these properties of liquid crystals, I have discussed about the applications of liquid crystals, such as LCD, liquid crystal thermometer, and optical imaging. There are still unsolved problems about LCDs but this area will be the one of the most significant areas in the future because statistics show that computer monitors will be overtaken by liquid crystal displays in the near future. I have also discussed about liquid crystal thermometers which are used to follow the temperature changes because of heat flows. In addition to these applications, I have talked about optical imaging and the structure of the models for this technology and stated that liquid crystal which is between the layers of photoconductor is affected by the application of a light to the photoconductor.
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