Classifications of Liquid Crystals

Classifications of Liquid Crystals 1. Introduction 1. Introduction to Liquid Crystals 1.1 Historical background Liquid crystals are a unique state of matter, between solid (crystalline) and liquid (isotropic) phases some compounds form a distinct, different intermediate phase, sometimes referred to as the â€Å"fourth state of matter† or â€Å"mesophase†. These compounds display properties of both solid and liquid. Anisotropic intermolecular interactions of the molecules, or mesogens, within a liquid crystalline material mean that the molecules possess some orientational or positional order but with a lower degree of organisation compared with a crystalline solid. This means liquid crystals possess liquid-like flowing behaviour, but because of their positional order, such compounds are often more viscous. Liquid crystals are attributed to their sensitivity to various stimuli, such as temperature, electric and magnetic fields. This sensitivity, combined with the self-assembling behaviour of liquid crystals make them extremely interesting and fascinating for chemists and physicists . Friedrich Reinitzer, a botanist at the Karl-Ferdinands-Università ¤t was the first to discover and publish work on liquid crystals[1] in 1888. In this paper he described the melting phenomena of cholesterol benzoate, in which there were two melting points; the compound transforms into a cloudy fluid at 145.5  °C which became clear at 178.5  °C.[1] Reinitzer could not explain these observations, so he sought help from physicist Otto Lehmann, an expert in polarisation microscopy. Lehmann investigated the optical properties of the compound and found crystallites in the cloudy fluid, and observed this to be an intermediate phase (or â€Å"mesophase† – from Ancient Greek,ÃŽ ¼ÃƒÅ½Ã‚ ­ÃÆ'ÃŽ ¿Ãâ€š (mà ©sos) meaning â€Å"middle†) between the liquid and solid phase. This intermediate phase showed birefringence but also flowed like a liquid.[2] In 1904, Lehmann was the first to coin the term â€Å"Liquid crystal† for these types of compounds. Vorlà ¤nder, a German chemist, was another pioneer in liquid crystal research and he discovered that molecular shape was very important in the geometry of the mesophase.[3] In 1935 Vorlà ¤nder retired but due to his research most of the liquid crystals up this date had been synthesised by him. For a long time liquid crystals were merely scientific curiosity.[4] The unique properties of liquid crystals together with some molecules being sensitive to electric fields, began the research by Heilmeier[5] in 1968 which led to the discovery and widespread applications in liquid crystal displays (LCDs).[6][7][8][9][10][11] 1.2 Classifying Liquid Crystals Liquid crystals are classified in many ways,[12][13] molecules within the mesophases (mesogens) can be calamitic (rod-like), discotic (disc-like), amphiphilic, nonamphiphilic, metal containing, non-metal containing and low molecular weight or polymeric. Liquid crystals either show thermotropic behaviour or lyotropic behaviour. Thermotropic behaviour means the compounds are liquid crystalline within a defined temperature range, below this range compounds are crystalline and above it compounds are isotropic liquids (figure 1). Thermotropic liquid crystalline compounds also require no solvent. Lyotropic liquid crystals are dependent on solvents, where solvent concentration affects aggregation and liquid crystal behaviour. Figure 1: Rod-like (calamitic) molecules representing molecular arrangement of thermotropic liquid crystalline phase transitions. This work is focused on calamitic, thermotropic liquid crystals. Mesophases of such liquid crystals can be divided into categories which depend on the orientation and order. The main categories are nematic (N), smectic (Sm) and cholesteric (N*) (or chiral nematic) (figure 2). Figure 2: Molecular arrangements of different sub-phase allignments. 1.2.2 Nematic Phases The nematic phase is the simplest mesophase known; there is long range orientational order between the mesogens but no short range positional order. Mesogens are able to freely translate and rotate along the director field (n) but are randomly distributed in space. â€Å"Nematic† comes from the Ancient Greek word ÃŽ ½ÃŽ µÃŽ ¼ÃŽ ± (â€Å"nema† meaning thread) as when observed through a microscope thread-like structures are observed. 1.2.1 Smectic Phases Liquid crystals with smectic mesophases were originally discovered from amphiphilic molecules, the phase type coming from the Ancient Greek word ÏÆ'ÃŽ ¼ÃŽ µÃŽ ºÃâ€žÃŽ ¿ÃÆ' (â€Å"smektos† meaning soap-like). Today, the word smectic is used for liquid crystals in which the molecules occupy orientational order, but are also organised in layers[14]. These organised layers can slide relative to each other, which contributes to the liquid character of the liquid crystal phase. Many smectic phases have been discovered, each smectic phase differs in the orientation and position of the mesogens.[15] Smectic phases are distinguished by letters; SmA, SmB, SmC etc. with SmA and SmC being the most common. Smectic phases are distinguished via the molecular orientation within the layers (figure 2).[16][17] Smectic A (SmA) mesophases have molecules aligned along a director field (n) and are parallel to the layer normal and in smectic C (SmC) mesophases the molecules are tilted at a certai n angle away from the layer normal.[17] Compounds exhibiting smectic mesophases are sometimes referred to as two dimensional liquids, since there is no positional order within each layer.[14] 1.2.3 Cholesteric Phases The cholesteric phase is the nematic phase only mesogens change orientation in a helical manner with respect to the director field (n). The director field in the cholesteric phase changes direction and is perpendicular to the direction of the helix. The chirality comes from the molecules orienting themselves into a helix; each layer is non-superimposable. Within the cholesteric mesophase is the pitch, which is defined as the distance between one mesogenic layer and another which has rotated 360  ° to the director field (figure 3). Figure 3: Representation of the cholesteric mesophase with the cholesteric pitch. The first observed liquid crystals based on cholesterol derivatives showed this mesophase type, which is where the mesophase name â€Å"cholesteric† originates.[1][2] 1.2.4 Chiral Liquid Crystals There are 3 main types of chiral liquid crystals. The first type was found with cholesterol benzoate (the first liquid crystal discovery) in which the molecule contains one or more stereocentres. These form liquid crystals with helical order in the mesophase. The second type contains two molecules; a chiral guest molecule and an achiral nematic molecule. These molecules are mixed together to form chiral cholesteric structures. In the third type, mesogens layer themselves perpendicular to each other forming a helix or twist. These molecules do not need to be chiral, as the perpendicular stacking form helixes, giving this third type the name twisted nematic. Twisted nematic phases are found in liquid crystal displays and chiral liquid crystals overall are useful in the field of stereochemistry, optics and material science.[12] 1.3 Ionic Liquid Crystals Knight and Shaw were the first to discover liquid crystals which were ionic in 1938; the compounds had a pyridinium cationic core and flexible alkyl chains attached.[18] Ionic liquid crystals possess properties characteristic of liquid crystals (molecular order, mobility, anisotropy of physical properties) and ionic liquids (conductivity, thermal stability)[19] making them very interesting and useful for ion conduction (aligned ionic liquids[20][21]), electroluminescence (which can be used in organic light emitting diodes), manufacturing of displays[10], spatial light modulators[22], optical connectors and switches[23], molecular sensors and detectors[24][25] References [1] F. Reinitzer, Monatsh Chem 1888, 9, 421–441. [2] O. Lehmann, Z Phys Chem 1889, 4, 462–472. [3] D. Vorlà ¤nder, Z Phys Chem 1923, 105, 211–254. [4] T. J. Sluckin, D. A. Dunmur, H. Stegemeyer, Crytals That Flow Classic Papers from the History of Liquid Crystals, 2004. [5] G. H. Heilmeier, L. A. Zanoni, L. A. Barton, Appl Phys Lett 1968, 13, 46–47. [6] M. Schadt, H. Seiberle, A. Schuster, Nature 1996, 381, 212–215. [7] M. Schadt, W. Helfrich, Appl Phys Lett 1971, 18, 127–128. [8] M. Schadt, Jpn. J. Appl. Phys. 2009, 48, 03B001. [9] H. Kawamoto, Process IEEE 2002, 90, 460–500. [10] M. A. Karim, Electro-Optical Displays, CRC Press, 1992. [11] P. Yeh, C. Gu, Optics of Liquid Crystal Displays, John Wiley Sons, 2010. [12] I. Dierking, in Textures Liq. Cryst., Wiley-VCH Verlag GmbH Co. KGaA, 2003, pp. i–xi. [13] P. J. Collings, J. S. Patel, Handbook of Liquid Crystal Research, Oxford University Press, 1997. [14] A. Jakli, A. Saupe, One- and Two-Dimensional Fluids: Properties of Smectic, Lamellar and Columnar Liquid Crystals, CRC Press, 2006. [15] C. Bahr, in Chirality Liq. Cryst. (Eds.: H.-S. Kitzerow, C. Bahr), Springer New York, 2001, pp. 223–250. [16] M. Barà ³n, Pure Appl. Chem. 2001, 73, DOI 10.1351/pac200173050845. [17] B. Chen, X. B. Zeng, U. Baumeister, S. Diele, G. Ungar, C. Tschierske, Angew. Chem. Int. Ed. 2004, 43, 4621–4625. [18] G. A. Knight, B. D. Shaw, J. Chem. Soc. 1938, 682–683. [19] K. Binnemans, Chem. Rev. 2005, 105, 4148–4204. [20] T. Kato, Science 2002, 295, 2414–2418. [21] M. Yoshio, T. Kagata, K. Hoshino, T. Mukai, H. Ohno, T. Kato, J. Am. Chem. Soc. 2006, 128, 5570–5577. [22] B. Bahadur, Liquid Crystals — Applications and Uses: (Volume 3), WORLD SCIENTIFIC, 1992. [23] J. L. De Bougrenet de la Tocnaye, Liq. Cryst. 2004, 31, 241–269. [24] A. Hussain, A. S. Pina, A. C. A. Roque, Biosens. Bioelectron. 2009, 25, 1–8. [25] C. Ohm, M. Brehmer, R. Zentel, Adv. Mater. 2010, 22, 3366–3387.

Topshop SWOT Analysis

Topshop is a fashion store that has around 300 stores in the UK, and over 100 in international places. It was launched in 1964, and Topshop has become one of fashion’s largest success stories. They have captured the most fashionable styles every seasons, its bold and saucy approach to style has attracted fashion-conscious shoppers and industry insiders alike. Topshop’s triumph has been impressive, despite its humble start. Topshop started in the basement of Peter Robinson department store in 1964 and a few years later became an independent retailer. In 1994, the brand took over the entire 90,000 sq ft space at London Oxford Circus, when the iconic Topshop flagship was launched. It is still there today, and is said to be the world’s biggest fashion store on the high street, captivating over 200,000 shoppers each week. As an early pioneer of high street and designer collaborations, Topshop’s partnerships with brands such as Cella Birtwell and Kate Moss, who is the history’s most iconic fashion names, and they have further secured the brand’s reputation as a fashion leader. Topshop’s strength. Topshop’s strength is to access to target market with over 300 stores nationwide, and they are doing online shopping and shipping to over 100 countries now. Topshop also have a wide variety of merchandise, with thousands of looks per season, their looks per season comes along with creative design concepts and fresh ideas every year, these strongly says that the trend awareness of Topshop is very high. Topshop is also said to be the largest fashion store in UK, offering free personal style consultants, competing with the high-end department stores. They are in collaborations with designers and celebrities like Kate Moss and Cella Birtwell. Lastly, Topshop is part of arcadia, which is a group managing several successful brands. (177) Topshop’s weakness The weakness of Topshop is that few of the garments quality is not matching with the price, and that they have too many products and the staffs are not well educated enough about the merchandise, for example the material of the apparels. Most of the merchandise are made suited for only the younger demographic and few of the ad campaigns are not eye catching enough. The company is said to be global, however it has a presence only in few countries worldwide. Topshop’s opportunities Topshop’s opportunities are that they have the potential for further globalization of the brand and more than 60 cities with a potential for a Topshop has been identified. Topshop also have th potential to franchise brand and develop new products and services like more trained staff. Topshop also have the opportunities to become the largest fashion store in other cities than London. Topshop’s threats Topshop’s biggest threat is the constant new entries and competitors in their home market. There might be possible recession and people are less likely to spend money on trendy items Topshop have in store, given the current weaknesses of currency, Topshop is not the bargain it used to be. With constant new entrant, there will be price wars with their competitors. Topshop being a global retailer means that they are exposed to political problems in the countries that they operate in. With Topshop’s success, there is a potential for a brand that will copy the style that Topshop has for the past few years of trends. (377)

What You Should Do About Student Papers for Sale Starting in the Next Nine Minutes

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Jeffrey Dahmer An Strange Boy - 1646 Words

Chelsea Parreira Ms.Meredith English 1 CP Period 9 20 January 2015 Jeffrey Dahmer Jeffrey Dahmer was born on May 21, 1960 in Milwaukee Wisconsin to Joyce and Lionel Dahmer. Jeffrey Dahmer grew up a very joyful, outgoing, happy kid. Dahmer was like this until his brother was born. After the birth of his brother, he seemed lonely as if he was seeking love and attention. Around the age of 6, Dahmer had a double hernia operation. Joyce and Lionel Dahmer soon realized that their son had really changed. Dahmer was shy and kept to himself, almost as if he felt almost incomplete. Around the age of 15, Dahmer was a very strange boy. He started questioning a lot of things such as his sexuality, his sexual desires and his obsessive interest in†¦show more content†¦After Hicks was dead dahmer maturbated over the coupse of the body. Later dahmer would chop up the body and put it into an acid that would turn everything to a liquid solution aside from the bones which he would smash up and throw in the woods by his house. After this first murder dahmer realized it made him feel better. Dahmer didn t want people to leave him so when he would kill them and do the things he did with them it made him feel complete, as if there was a part missing and when he did this he felt like a whole again. If I d been thinking rationally I would have stopped. I wasn t thinking rationally because it just increased and increased. It was almost like I wanted to get to a point where it was out of my control and there was no return. I mean, I was very careful for years and years, you know. Very careful, very careful about making sure that nothing incriminating remained, but these last few months, they just went nuts†¦ It just seemed like it went into a frenzy this last month. Everything really came crashing down. The whole thing started falling down around my head†¦ That was the last week I was going to be in that apartment building. I was going to have to move out and find somewhere to put all my possessions. Should I get a chest and put what I wanted to keep in that, and get rid of the rest? Or should I put an end to this, try to stop this and find a better direction for my life? That s what