Termotrop ve liotrop sıvı kristallerde faz geçişlerinin incelenmesi
Başlık çevirisi mevcut değil.
- Tez No: 55721
- Danışmanlar: PROF.DR. HAMİT YURTSEVEN
- Tez Türü: Yüksek Lisans
- Konular: Fizik ve Fizik Mühendisliği, Physics and Physics Engineering
- Anahtar Kelimeler: Faz geçişleri, Sıvı kristaller, Phase transitions, Liquid crystals
- Yıl: 1996
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Termotrop ve liotrop sıvı kristallerin metal oksit ince film kaplı yüzeyler üzerindeki yönelimleri deneysel olarak gözlemlendi. Yönelimlerin kalitesi, politermik mikroskobi yöntemi yardımıyla çiftkırılma ve yollar farkı ölçülerek belirlendi. Düz ve metal oksit ince film kaplı camların liotrop ve termotrop sıvı kristallerin faz geçişi sıcaklıklarına, faz geçişi aralığına etkileri, polarizasyon mikroskobu ve özel olarak yaptığımız kapilyar sıcaklık kaması yardımıyla termoçiftten 10"3 °C duyarlılıkla ölçüldü. Yapılan ölçüm ve hesaplamalar sonucunda ince film kaplı kapilyardaki faz geçiş sıcaklığı, kaplanmamış camdakinden daha düşük olup, faz geçişi sıcaklık aralığının arttığı bulundu. X - adım aletiyle ince film kaplı yüzeylerin taramasından elde edilen sinüssel yüzeyin dalga genliği ve dalga vektörü kullanılarak yapılan termotrop nematiklerin bu tür yüzeylerdeki enerji farkı hesaplamalarından elde edilen sonuçların sıvı kristal yönelimine, faz geçişi sıcaklıklarına ve faz geçiş aralıklarına etkilerinin açıklaması yapıldı. Ayrıca paramorf sıvı kristallerin düz camlardaki faz geçiş sıcaklıkları ve faz geçişi aralıkları ölçülerek, fiziksel açıklaması yapıldı. Paramorf sıvı kristallerin faz geçişlerinde yapı değişimleri genel olarak gözlendi ve faz geçişi sıcaklıkları ve faz geçişi aralıkları kesin olarak belirlendi. SmA- SmC* faz geçişi için serbest enerji fonksiyonu yazılarak, faz geçişi civarında, yapının karakteristik fiziksel büyüklükleri olan polarizasyon (P) nin, düzen parametresi (9) ile olan bağımlılığı matematiksel olarak elde edildi. Sonuçlar deneysel verilerle karşılaştırılarak yorumlandı.
Özet (Çeviri)
SUMMARY.-RESEARCH OF PHASE TRANSITION ON TERMOTROPİC AND LYOTROPİC ÜQUİD CRYSTALS Liquid crystals have experienced an explosive growth in the last years. This has increased considerably because successful applications of liquid crystals have been performed, particularly in the area of electraoptical displays. A direct outcome of all this activity in research and in commerce has caused the subject of liquid crystals to become important in the audience of scientists from a range of disciplines. Liquid crystals can roughly be divided into two areas, liotropic liquid crystals and thermotropik liquid crystals. Liotropic liquid crystals are formed from compounds with amphiphile and solvents (commenly water). Between the pure amphiphile and its isotropic solution in an excess of the solvent, there exist at intermediate concentration the structured phase, which consists of ordered arragements of micelles composed of amphiphile and solvent, which exhibits anisotropik properties characteristic of liquid crystals phases. Thermotropic liquid crystals which are formed from compounds whose molecules are mainly either rod-shaped or disc-shaped, either by heating the crystalline solid or by cooling the isotropic liquid, by thermal effects. Several metarials exist in a liquid crystal phase at room temperature, and many other cases of this type can be produced by using mixtures of conpounds. All components of the mixture need not necessarily be individually capable of forming liquid crystal phases or mesophases, need not be mesogens. In recent years many group, have investigated the oriantation of thermotropic liquid crystals on solid, because of its essential importance for manufacturing of display devices. Today it is well known that the orientation of thermotropic nematics can be determined by thin films of amphophilic or hydrophobic substances on the substrate surface. Mono or multilayers of such materials often lead to a homeotropic orientation. Not only the coating, but also the surface topology is able to cause a special liquid crystal orientation. Under the influence of parallel grooves on substrate surface, thermotropic nematics may be orianted uniformly to the grooves in order to minimize the elastic deformation energy of their director field. In lyotropic nematic systems aggregates of amphiphilic molecules take a part of the molecules of thermotropic nematics and are oriented in particular direction under the influence of the neighboring solid surface, or of an electric or magnetic field. VIThe orientating mechanisms that govern the interaction between a liquid crystal molecule and a solid substrate are important topic in the physics of interfacial phenomena. This mechanism can be classified into two categories, short-range substrate- fluid anisotropic interactions generally chemical in nature, and long-range fluid-fluid anisotropic interactions resulting from liquid crystalline properties. Several theories based on Landau - de Gennes formalism, Maier-Saupe and molecular aproaches have been developed to describe orientational ordering near a solid surface. It is well known that the directors of many nematic liquid crystals tend to assume an orientation parallel to the direction in which an adjacent solid surface has previously been rubbed. This mechanism for such augment is based primarily on geometrical factors rather than detailed molecular forces. A simple explanation of orientation parallel to the rubbing direction can be made on the basis of the additional elastic energy that would occur in a nematic liquid crystal due to distortion near a grand like wavy surface if the liquid crystal molecules were forced to lie aganist the surface with directors lying across, rather than paralel to, the grooves and ridges. If the surface is rough in both dimensions, such energy considerations would explain a tendency for the molecules to line up normal to the surface even when they would lie flat on a perfectly smooth surface of the same material at the same tempareture. Let us consider the effect of a gratinglike deformation of surface shape caused by rubbing. Suppose that rubbing the surface scribes small grooves or deposits small threads on the surface giving a shape that can be approximately described by a sinisoidal wave z=ASin q x (1) For simplicity, suppose that the bend and splay elastic constant Kn and K33 are alike. Let q>(x,z) be the azimuth of molecules if they lie in the x,z plane normal to the grooves and ridges on the surface. The minimum energy for such a material with directors oriented in this plane would occur if the azimuth (p obeys the equation £?^?=o <2> ok2 âz1 A solution to this equation that would match a gratinglike boundry is <p(x,z)= AqCos(qx)eq: (3) Notice that the surface defined by eq (1) coincides quite closely with the directors in eq (3) if the dimensionless product Aq is not large. The energy density due to elastic strain in this simplified nematic liquid crystal model is U.-^ âc) \dz) ?f{Aqf q2 exp{-2qz) (4) VIIand the total energy per unit area, which is largely confined within a distance (2q)“1 of the surface, is -Jc 1 p = )U(z)dz = -Ku{Aq)2q (5) o 4 If the molecules lie parallel to the ridges, there is no strain energy near the surface. U and p represent extra elastic energy that can be removed by reorientation of the molecule to a position parallel to the ridges. Thin films effect the orientational properties of termotrop and liotrop nematics. Nematic liquid crystal molecules lie parallel or perpendicular to the surface which coated with thin films because of the deformation energy influence. In this work texture transformations in the region of phase transition to the isotropical state of thermotrop and liotrop nematics, were studied by capillary temperature wedge method. Phase transition temperature and the width of phase transitions have been determined with a good accuracy. The values of biefrigence, the path difference and the sign of biefrigence of oriented planar texture were estimated. For these investigations an original system with cappillary temperature wedge was made. In this system the temperature gradient of the sample by means of symmetrical heat transfer was used. Capillary temperature wedge systems were composed of plane capillary, the temperature control and registration components. In plane capillary system three Cu-Cn thermocouples with equal distance were placed in contact with sample. In polarization microscope the location of phase front in gradient field of 1 °C/mm was estimated with the accuracy of±10”30C Thermotrop and liotrop nematics were used as objects of investigations. Thermotrop nematics were chosen as an eutectical mixture of methoxybenzilidene buthylanaline (MBBA) and ethoxybenzilidene buthyl analine (EBBA). Liotrop nematic was nematic-calamatic Nc with rod like micelle agregats as the structural units. This nematic was prepared on the base of zwitterionic amphiphile tetradecyl dimethyl amine oxide. The twice distilled water was used as solvent. The components of liotrop system have been weighted by means of precision balance with the accuracy of-10“3 g. The temperature gradient was settled along cappillary. A clear boundary of the phase transition with sufficently narrow heterophase region arised. The phase front was shifted strictly across capillary axis with temperature changing along this axis. Thermotrop nematic-izotrop liquid phase transition was equal to 52.328 ° C with the width of phase front 0.28 1° C. The phase transition was 47.450 ° C and width of phase transition was 0.497 ° C in the capillary which was coated with Zr02. This measurement was taken at 46.975 ° C and 1. 121 ° C in the capillary which was coated with AI2O3. - viiiAfter filling up the plane cappillary with the nematic calamatic, a striped texture with faint biefrigence was observed at once. Along the cappillary forces the orientation of these biefrigence strips took place. The original texture was instable during 35-40 min, then it transformed to typical biefrigent nematic texture. The observation of phase transition in the nematic calamatic would be optained in such sample by means of capillary temperature gradien. There was no clear Nc- isotrop liquid phase transition. In this case the value of biefrigence was 0.03922 at 72.527 nm path difference. The investigations by means of X plate and Berrek compensator have showed that this planar texture possessed the negative biefrigence. The phase transition to isotropic state was realised at 48.871 ° C and the width of phase transition was 2.6 1 0 ° C. Phase transition was 47.478 ° C and the width of the phase transition region was determined for the samples coated with Zr02 and they were at 34.230 ° C, 2.090 0 C, respectively, for the samples coated with Ta20s. Paramorph liquid crystals which consists of a few mezophases are very important for explaining the interaction between mezophases which effect the width of the front of the phase transition. We have investigated two paramorphs, namely 4 butuloksiphenyl ether of noniloksibenzoyl (H-134) and ether of complex acid base on the schiff (H-126) by polarization microscope and capillary temperature wedge. We find the phase transitions and widths of the front of the phase transition temperatures with 10'3 ° C accuracy. The physical properties of liquid crystals are strongly influenced by the presence of chiral molecules. Besides the helical structure which is induced in the nematic and some tilted smectic phases, the presence of chiral molecules leads to a coupling between tilt angle and polarization in smectic liquid crystals. Therefore a spontaneous polarization is shown by smectic phases of chiral molecules which exhibit a tilt angle as an intrinsic structural property, whereas in not- tilted smectic phases of chiral molecules a coupling between induced tilt angle and induced polarization exist, which is to be observed as the electroclinic effect. Ch Bahr, G. Heppke and B. Sabaschus studied have changes in the polarization at the SmC*-SmA phase transition region for 4-(3-methyl-2- chlorobutanoyloxy)- 4-heptyloxybiphenyl. They have investigated the physical properties of the chiral liquid crystal at the phase transition region. Near the phase transition, the Landau free energy is g- go+^(T~To)0z +^be4 +^c06 +-^-P2 -6P0 (6) 6 so 2 0/ 4 6 2^0e0 for the P9 coupling. For the free energy using the minimization we find that - = 0 dP IXP = Gxo6ö9 (7) From the second minimization condition - = 0 and using 60 »_», «a «P/y=3/16 (8) which is condition for the first order transition (see the Appendix), we find the phase transition temperature as 3A2 A (?Xoeo I6ac a /,= 7ö+t^:+-t^- W and the order parameter as,=1__IA_(10) Fitting the equation for the order parameter and the polarization to the experimantal data, we find the best fitted parameters. Since the fitting for the PQ coupling is no good, we try to fit the experimental data using the P^2 coupling in the free energy. For this coupling the Landau free energy is 8 = 8,+l<T~rc)Û2 + ^b0A+~c06 +^77”'>2 -OP20Z +\eP4 (11) and from the minimization condition - = 0, we find the polarization cP 4\ 1 > 2G0-XoeoJ (12) From the second minimization- = 0, we can find the phase transition temperature as 60 W_J&± 3G4 _ 2G \6ac lace ace1 Xoeoae t* = 7o +77r:-^r+~r- - O3) and the order parameter equation becomes ^4.. 4G\ b1 2Ga*16G4* 12G 0 = \Î-5(14) u 2cTherefore the free energy with the P292 coupling give, a good explanation for the experiinantal data. XI
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