Synthesis and Properties of Liquid Crystals for VAN

Synthesis and Properties of Liquid Crystals for VAN Synthesis and properties of liquid crystals for vertically aligned nematic (VAN) displays Introduction Liquid crystals where first observed in 1853 and 1855 by Rudolph Virchow and C. Mettenheimer respectively, both observed a flowing fluid like compound which was birefringent (brightly coloured) between cross-polarisers much like a crystalline solid, hence the compound was both liquid and crystal thus liquid crystal. [3, 4]. It was not until the late 1980’s that Liquid crystals and their truly fascinating fundamental properties began their tremendous success in commercial applications.1–5 Subsequently giving rise to the vertically aligned nematic (VAN) mode in the early 1990’s. The liquid crystalline phase can be best described as a hybrid of the two most common phases of matter, Liquids and Crystals. LC compounds diffuse about much like the molecules of a liquid giving them a fluid nature, combined with this they manage to maintain a small magnitude of orientational order and sometimes some positional order in a similar manner as a crystalline solid would. Hence, liquid crystals are anisotropic fluids. Properties of Liquid crystals and the nematic mesophase The nematic phase of calamitic (rod like) liquid crystals is the simplest liquid crystal phase. In this phase the molecules maintain a preferred orientatioanl direction as they diffuse throughout the sample. There is no positional order in the phase as depicted by figure 1.1. Synthesis of Liquid Crystals General Synthesis Generally, the most common liquid crystals are based on aromatic sub units due to their ease of synthesis and obtainability. The vast majority of LC building blocks are commercially accessible or fairly simple to synthesize via electrophilic substitutions such as Friedel-Crafts acylation, bromonation and nitration. For those functional groups that cannot be directly substituted interconverions usually take place with bromine often being the chosen leaving group (e.g., CO2H, NH2, CN and OH). Due to the individual nature of substituents their specific directing effect and a specific effect on the rate of reaction must be taken into consideration. By taking this into account reactions must be carried out in the appropriate order to arrive at the desired product. Figure 1 Electrohpilic Substiutions of Benzene A key advancement in synthesis arrived with the recognition that a wide range of intermediates could be efficiently prepared from alkyl-bromo-benzenes due to the ease of conversion of the bromo substituent into a previously inaccessible groups. From a range of synthetic methods described in scheme 1 a valuable number of carboxcylic acids and phenols can be prepared. This follows on to the synthesis of multi-aryl LC materials where esterification (see Scheme 2) is employed to couple multiple aryl units. Esterification commonly occurs in two processes firstly, the traditional method (Method A) of converting the carboxcylic acid into the acid chloride derivative with either thionyl chloride or oxalyl chloride. The acid chloride is then reacted with the phenol in the presence of a base to remove the hydrogen chloride as it is formed. The second and more recent method (Method B) involves an in-situ reaction which uses N,N-dicyclohexylcarbodiimide (DCC) to activate the acid towards nucleop hilic attack from the phenol and a proton transfer catalyst ( 4-(N,N-dimethylamino)pyridine ) (DMAP). Scheme 2 Esterification coupling reaction LC materials with multiaryl cores (e.g., biphenyls and terphenyls) are somewhat more difficult to produce due to the direct bond between aryl sections. However, the development of palladium-catalysed cross-coupling reactions has created a means in which to form the direct carbon-carbon bonds needed. There are a vast number of methods to facilitate the generation of these carbon-carbon bonds but by far the most prolific involves the use of aryl bromides (4) and arylboronic acids (5). Figure 3 Palladium catalysed cross-coupling Alternative to the use aryl bromides are the aryl iodides, there increased stability as a leaving group provide a reaction pathway with an increased rate of reaction. Chloro and triflate are also other viable leaving groups, where the triflate group is essential in the synthesis of alkenyl-substituted LCs. Perhaps the most important palladium-catalysed cross-coupling reaction is the selective coupling that can occur by using a bromo-fluoro-iodo-substituted system (see Scheme 4) Figure 4 Dicouplong reactions of Benzene derivatives As the iodo group is a better leaving group it can be coupled with an arylboronic acid, following purification a second coupling reaction can occur on the bromo site giving rise to the synthesis of LC materials with more than two aromatic core units. In order to control the mesomorphic and physical properties of LC lateral substitutions are often employed, the fluoro substituent is the most commonly used lateral unit, as it is electron withdrawing in nature it renders adjacent H atoms acidic and thus making them vulnerable to strong basic conditions. By taking advantage of this vulnerability the desired functional groups for example the boronic acids needed for cross-coupling reactions are far more easily obtained. The only consistent approach for introducing a fluoro substituent into an aromatic system is via the diazotisation and successive fluoronation of the chosen aromatic amine, which in turn generated from the reduction of the nitroarene generated from the nitration of the bas ic aryl unit. Nonetheless, a broad variety of simple fluoro-substituted materials can be easily acquired commercially and thus synthesis often begins with fluro substituents already present (see Scheme 5). Unfortunately this gives rise to complications when trying to introduce terminal alkyl chains to the fluorinated compounds. Accordingly, a different approach is required and thus bromo-fluoro-iodo-benzene units are needed for successful synthesis of fluoro-substituted LC materials. Scheme 4 shows some reactions of these units to synthesis some adavance LC materials. The finishing touches Liquid crystals for VAN mode displays must have one vital property in order to be considered for this application, negative dielectric anisotropy. Negative dielectric anisotropy can be introduced by creating a strong lateral dipole within the LC material this is done by introducing lateral groups with high electronegativity such as fluorine as explained previously in this section, lateral chloro substitutents have also been considered in order to create negative dielectric anisotropy as they create a greater dipole than fluorine. However, the greater size of the chloro substituent renders it of little use as this subsequently gives the material low liquid crystal phase stability and high viscosity making it useless in VAN mode displays. Figure 5 Subsitution reactions of difluroaryl compounds Vertically aligned nematic (VAN) liquid crystal displays About the VAN displays The vertically aligned nematic (VAN) mode first came into development in the early 1990’s, first generation LC materials were based on rod like molecular structures and managed to achieve fast switching times of around 25ms. Unfortunately, the early attempts to introduce displays of this kind failed. This was for two major reasons, a switching time of What makes up a VAN display? VAN devices are made up of two parallel glass plates separated by a small gap of 3-10Â µm containing the nematic liquid crystal phase, on the top piece of glass sit a thin film of material which polarises a light that passes through it. On the inside of the top piece of glass there is a indium oxide (ITO) layer which acts as a conductor, this layer is linked to a surfactant. The inner layer of the bottom piece of glass is also coated with the ITO layer and the surfactant. The surfactant enables the liquid crystal to be connected with the conductor thus enabling the flow of a current. The display can be designed to be either passive or active. When passive the display does not generate any light itself it instead uses ambient light from surroundings which is reflected by a mirror like surface below the bottom piece of glass. When designed to be active the display is built with a light source behind the display which passes directly through the display rather than being reflected Working principle of VAN displays The average molecular orientation (director orientation) without the electric field is perpendicular to the substrate of the display. With this homeotropic orientation and crossed polarizers, the VA mode is working in the so called normally black mode. For the incident light the liquid crystal in the off state behaves like an isotropic medium (the light sees only the ordinary refractive index). As a consequence very good black states can be achieved independent of the wavelength of the light and the operating temperature. Pixel and electrode design of VA displays allow for a high aperture ratio resulting in a high brightness of the display. These two points are the main reason for the good contrast of VA LCDs.. Since the directors are oriented homeotropically in the off state, they can be tilted randomly in any direction by the electric field. This leads to disclination lines between domains of equal orientation, thus deteriorating the optical performance. Figure 6 VA Mode working display As VAN displays use LC materials with negative dielectric anisotropy, application of a voltage to the ITO films cause the director to tilt away from the normal to the glass surfaces as show in figure 2. This introduces a birefringence because the index of refraction for light polarised parallel to the director is different from the index of refraction for light polarised perpendicular to the director. Some of the resultant elliptically polarised light (all of it if the retardation is 180) passes through the crossed polariser and the display appears bright. In fact, since the retardation depends on the magnitude of the voltage applied to the display, this type of display can be used to produce a range of intensities of light. This is called a grey scale. For VA you have perfect black in the off-state and if apply a voltage the VA materials moves into the parallel position and this is bright. Therefore, you get a better contrast ration in VA displays. The second advantage is the switch ing process. It’s intrinsically faster to move the molecules this way.

AT&T Case study Essay

1. Review AT&T’s past financial policies and financing choices. Were these appropriate for the nature of the business? AT&T Corp., one of the largest companies in the United States, has had a long and storied history. Initially, AT&T operated as a monopoly, but in 1982, Justice Department, broke up the company into individual companies. Prior to divesture (in 1981), AT&T was the largest private company in the world and despite many challenges, AT&T remained an archetypical â€Å"widow-and-orphans† stock for a long time. The term â€Å"widows and orphans† was used to describe stocks with a relatively high degree of safety and dividend income and numbers from exhibit 1speak for themselves. By 1982 the company increased all its key financial indicators. Revenues and operating earnings increased 12% and 6% respectively comparing to year 1981. In ten years, the company raised its revenues, net income, cash and assets more than 2 times. It`s worth to mention that AT&T was able to reduce its total outstanding debt by $ 725 mil and at year-end, the company`s debt ratio stood 42.3% down from 46.7% in 1980. In addition, AT&T neither cancelled nor lowered dividends, and only increased dividend per share by 10% annually. Aforementioned facts suggest that company`s financial policies and financing choices were appropriate for the nature of the business and that AT&T was one stable, reliable and profitable companies in the world by 1982. 2. In what fundamental ways will AT&T’s business change in the near future? Throughout most of the 20th century, AT&T held a monopoly on phone service in the United States. In 1982, through an agreement between AT&T and the U.S. Department of Justice, AT&T agreed to divest itself of its local telephone operations but retain some of its businesses. The principal provision of the antitrust settlement was that the corporation would be split into seven completely independent regional corporations. Each regional company would continue to provide local telephone and other telecommunication services in addition to ability to sell telephone equipment. The remaining or new AT&T would focus on long distance, R&D and manufacturing arms. 3. In view of AT&T’s changing strategic and economic environment, what debt policy would you recommend? What other financial policies are appropriate for the â€Å"new† AT&T? What are likely to be the consequences of these financial policies? The new â€Å"AT&T† was no more monopoly and the company`s management group faced new challenges. First of all, I think that when AT&T was monopoly, managers were averse to risk and led more financially conservative policy. After divestiture, it was clear that the external environment changed and effective actions needed to be taken to beat off intense competition. Moreover, there were several serious questions about future profitability of the new AT&T divisions (especially Western Electric) and it was not clear whether they would be able finance their operations in competitive markets. Taking into consideration above stated facts, at that phase I would recommend to shift gears from debt financing and switch toward equity financing. In addition to the change in financial policy, I would also recommend considering a new acquisition strategy to reach more diversified portfolio and to broaden the company’s scope in other areas.

The Impacts of the Residential School System on the...

‘I want to get rid of the Indian problem. That is my whole point. Our objective is to continue until there is not a single Indian in Canada that has not been absorbed into the body politic and there is no Indian question’ (Scott in Grant, 1996: 273). Canada is a vast territory widely recognised for the cultural and geographical diversity in nature. This alone poses a distinct challenge to understanding a unified conception of Aboriginal Geographies of Canada - particularly for understanding the Geographies of The Canadian Residential Schools System (RSS). The Canadian Residential School System was an early government led approach to Indigenous ‘development’. It was initially implemented to educate the ‘uncivilised’ Aboriginal populations of Canada as a way to assimilate Indigenous people into the colonial settler population. This created profound effects on Indigenous people in Canada, including the spaces and places so integral to their culture. In this essay I will discuss the impacts of the residential school system on the Aboriginal people of Canada with particular reference to how space and place are critical importance to mapping the geographies of the Canadian Residential School System. In conclus ion, I suggest that the impacts of the Canadian Residential school system have had direct impacts on not only physical space, but also the places that have intrinsic meaning to both the identity and culture of Aboriginal people in Canada. Fig 1. SelectedShow MoreRelatedTruth, Reconciliation, Healing : A Curriculum1478 Words   |  6 Pagespaper brings together the two perspectives of residential school abuse and the viewpoint of healing. Intergenerational effects have been created due to residential school and have devastating impacts on Aboriginal communities. This paper explores how a school curriculum would help spread awareness of the wrongs committed against Aboriginal communities and how we can set the path for healing. 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My Years at Troy High School Essay - 1128 Words

Four years of my life drained away at this place called Troy High School. After this year I will be walking away from Troy, worn and torn from the hours of anticipation for grades to be posted, the next weekend to arrive, the answer to whether that special someone will say yes to the next dance, or the unbearable wait for that painstaking bell to ring. Troy is a place of education, a very good one at that, with its Blue Ribbon Award and national recognition, you’d think I would be walking away with knowledge that will serve me well for decades to come, but no. I could have learned the same things I learned at Troy anywhere, it is the insight I picked up that will take me far. â€Å"What insight?† you ask. Its priceless wisdom really, it’s a†¦show more content†¦Shall we go through the categories of classes? 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