Pitch Behavior of the Cholesteric Helix at the Phase Transition Point Cholesteric/Smectic A up to 2500 Bars. Study of Some Cholesteryl n-Alkanoates

Experimental evidence is given, for the first time, for a finite pitch of the cholesteric helix at the phase transition point cholesteric/smectic A (Ch/S_A). This finite pitch changes into an infinite one at a given point on the phase equilibrium line Ch/S_A. Therefore this point is called the pitch infinity point (PIP).

With increasing n-alkyl chain length of the investigated cholesteryl n-alkanoates the pressure coordinate of the PIP is shifted to higher values up to a chain length of 13 C atoms. For the alkanoates with 13,15 and 17 C atoms the same PIP pressure of about 1000 bars was determined.

The Ch/S_A phase equilibrium line of the cholesteryl esters could at best be followed up to 2600 bars. The respective transition enthalpies were measured at normal pressure and the corresponding changes in volume calculated by means of the Clausius-Clapeyron equation.     

Nano Molecular Self Assembly in the Formation of Induced Mesomorphism in Mixture of ChCl,DTAC, and EG

The multi-component system of cholesteryl chloride (ChCl), dodecyl trimethylammonium chloride (DTAC), and ethylene glycol (EG) exhibits very interesting liquid crystalline mesophases like cholesteric and SmA, SmC, and SmB phases sequentially when the specimen is cooled from its isotropic phase. These phases have been characterized by employing optical and X-ray studies. Pitch of the cholesteric phase has been calculated and discussed. Variation of pitch from the cholesteric phase to smectic phase is smooth and continuous. The temperature variation of optical anisotropy and electrical-conductivity has also been discussed. It has been found that wherever there is a phase transition, the value of electrical susceptibility changes appreciably.     

On the influence of the pre-ageing temperature on the reversion behaviour of an Al-4.5 at.% Zn-2.0 at.% Mg alloy

The reversion behaviour of an Al-Zn(4.5)-Mg(2.0) alloy was investigated by SAXS in dependence of the pre-ageing temperature, T_pre, (ranging between 60°C and 100°C) and the reversion temperature, T_rev, (120°C till 200°C) starting with precipitates having a radius of (1.2 ± 0.1) nm and (1.5 ± 0.1) nm, respectively. During the reversion treatment applied up to T_rev = 160°C three stages could be distinctly distinguished, namely the dissolution of unstable zones, the growth of the stable zones on the expense of the dissolved one, and at last the coarsening of the precipitates by the OSTWALD-ripening process, where the structure changes become independent of the pre-history (start radius). The change of T_pre from 60°C to 100°C does not influence the structure changes going on at T_rev, that means between 60°C and 100°C the same type of G.P. zones grows.     

Crosslinked Cholesteric Network from the Acrylic Acid Ester of (Hydroxypropyl)Cellulose

The acrylic acid ester of (hydroxypropyl)cellulose was prepared from (hydroxypropyl)cellulose and acryloyl chloride. The resultant polymer, with 2.2 ester groups per anhydroglucose unit, formed a thermotropic cholesteric mesophase with visible reflection hands at temperatures between ambient and 60°C. By exposing a thin layer of the mesophase to UV light, the mesophase structure was stabilized to give a crosslinked cholesteric film.     

Synthesis and Nonlinear Optical Properties of Novel Polyurethane Containing Cyanovinylnitroresorcinoxy Group

Novel X-type polyurethane 5 containing 4-(2′-carbomethoxy-2′-cyano)vinyl-6-nitroresorcinoxy groups as nonlinear optical (NLO) chromophores, which constitute part of the polymer backbone, was prepared and characterized. Polyurethane 5 is soluble in common organic solvents such as acetone and N,N-dimethylformamide. It shows thermal stabilities up to 260°C from thermogravimetric analysis with glass transition temperature obtained from differential scanning calorimetry near 108°C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer film at 1064 nm fundamental wavelength is 9.83 × 10^?9 esu. Polymer 5 exhibits a high thermal stability even at 2°C higher than T_g, and no significant SHG decay is observed below 110°C, which is acceptable for nonlinear optical device applications.     

Some Peculiarities of Cholesteric-to-nematic Transition in Cholesteric Liquid Crystal Mixtures

The cholesteric-to-nematic (CN) transition in a binar compensated mixture consisting of 59% cholesteryl chloride and 41% cholesteryl pelargonate has been investigated. The peculiarities of temperature and field dependences of the CN-transition were determined by a diffraction method for a case when the sample thickness is greater than the cholesteric pitch. It is shown that the electric field induced cholesteric-to-nematic transition may occur through the distortions periodic in the full pitch of the cholesteric structure.     

Density Measurements in cholesteryl Myristate

Abstract

The densities of cholesteryl myristate were measured in the smectic, cholesteric, blue and isotropic phase. At the transitions smectic/cholesteric and bluelisotropic within the temperature interval of 0.01 to 0.02°K discontinuities in the density curves occured. which indicate phase transitions of first order. At the transition cholesteric/blue only a change in the expansion coefficients was observed. which points to a phase transition of second order.

Die Dichten von Cholesterylmyristinat wurden in der smektischen, cholesterinischen, blauen und isotropen Phase gemessen. An den Umwandlungen smektisch/cholesterinisch und blau/isotrop treten innerhalb eines Temperdturintervah von 0,O1 bis 0.02°K Diskontinuitäten in den Dichtekurven auf. welche Phasenumwandlungen 1. Ordnung anzeigen. An der Umwandlung cholesterinisch/blau ist nur eine änderung in den Ausdehnungskoeffizienten vorhanden, welche auf eine Phasenumwandlung 2. Ordnung hinweist.     

New chiral imines based on S-α-phenylethylamine and S-α-benzylethylamine in induced cholesteric and smectic mesophases: II. Behavior of chiral dopants in ester liquid-crystal systems

The temperature dependences of the pitch of induced helical structures in the ester liquid-crystal systems containing hydroxyphenyl benzoate derivatives and chiral N-aroyloxybenzylidene-S-α-phenylethylamines or N-aroyloxybenzylidene-S-α-benzylethylamines have been measured. The twisting power of chiral dopants is calculated and the temperature dependence of the helical pitch in the induced cholesteric (N ^*) and smectic C ^* mesophases is quantitatively characterized. The parameters A = dT _is/dC that characterize the effect of dopants on the thermal stability of the N ^* phases are determined from the concentration dependences of the cholesteric-isotropic transition temperature T _ts for the n-hexyloxyphenyl-n-butylbenzoate-chiral dopant systems. The results obtained are compared with the data for the 4-n-pentyl-4′-cyanobiphenyl (5CB)-based cholesteric systems induced by the chiral dopants under consideration.     

Cholesteric Pitch and Blue Phases in a Chiral-Racemic Mixture

The cholesteric and “blue” phases (BP) of a mixture of the chiral and racemic forms of the same compound were studied as a function of composition and temperature. It was found that the clearing point and the cholesteric-S_A transition temperature was independent of composition. The pitch of the cholesteric at a given temperature was inversely proportional to the fraction of chiral material, as was the lattice parameter of BPII. The lattice parameter of BPI did not follow the above rule. No second-order BPI-BPIIb transition was seen.

It is concluded that this mixture is thermodynamically ideal^(6,7) in that the intermolecular interactions between two molecules are independent of their relative chirality. Thus, this system comes close to the theorist's ideal of one in which only the chirality is varied.     

Passive Broadband Reflector: Elaboration and Spectral Properties

Abstract

We present passive broadening obtained by assembling two layers of cholesteric photo-polymerisable monomers having different pitches at the same temperature. A more or less large diffusion occurs between the two layers, which is dependent of various parameters such as degree of cross-linking of the two layers, temperature dependence, time evolution at a given temperature, thickness of the layers… The presented work consider the spectral properties of the sandwich structure and its correlation with the diffusion profile at the interface between the two layers. The band shape of the reflected and transmitted light were studied. Then, the reflected intensity shows a widening of the reflection associated with the existence of a pitch gradient inside the sample, which can be blocked by photo-polymerisation. At short time, the two peaks corresponding to each liquid crystal polymer are still present but the diffusion occurs at the interface. After a longer time, the broadening band still decreases and a single large peak is observed.     

Thermal conductivity and refractive indices of Nd:GdVO4 crystals

The thermal conductivities of Nd:YAG, M(Y,Gd)VO₄ crystals were measured at 298 K. The value of Nd:GdVO₄ crystal along <001> direction was 11.4 W/mK, which was higher than that of YAG crystal measured to be 10.7 W/mK. The principal refractive indices of Nd:GdVO₄ crystal in the temperature range from 20 °C to 170 °C were determined by auto‐collimation method. Based on the measured values of refractive indices, the Sellmeier equation and expression of temperature dependence of refractive indices have been obtained. The measured results show that the birefringence Δn is 0.22007 at 20 °C and temperature coefficient of birefringence is 4.33 × 10⁻⁶/°C for 1.064 μm. These results prove that the GdVO₄ crystal is a laser crystal with excellent thermal and birefringence properties. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and Properties of Tetragonal Tungsten Bronze Type Potassium Lithium Niobate Single Crystals

Tetragonal tungsten bronze type potassium lithium niobate single crystals with the Nb/Li ratio larger than 3 are grown by the flux pulling method in our laboratory, and it dose not crack when cooling through the paraelectric/ferroelectric phase transition. Crystal growth is studied in two orientations with growth along [100] and [001], and the latter's quality is superior to that of the former. The lattice constants are a = 1.2575 nm and c = 0.3997 nm at 298 K. A Curie temperature is about 480 ± 3°C.     

Synthesis and Properties of Novel Nonlinear Optical Polyester Containing Cyanovinylnitroresorcinoxy Group

Novel X-type polyester 5 containing 4-(2’-carbomethoxy-2’-cyano)vinyl-6-nitroresorcinoxy group as nonlinear optical (NLO) chromophore, which constitutes parts of the polymer backbone, was prepared and characterized. Polyester 5 is soluble in common organic solvents such as N,N-dimethylformamide and acetone. Polyester 5 shows a thermal stability up to 280°C from thermogravimetric analysis with glass-transition temperature obtained from differential scanning calorimetry of near 116°C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer film at the 1064 nm fundamental wavelength is 4.25 × 10^?9 esu. The dipole alignment exhibits a thermal stability even at 4°C higher than glass-transition temperature (T_g), and no significant SHG decay is observed below 120°C due to the partial main-chain character of polymer structure, which is acceptable for NLO device applications.     

Study on the ferroelectric phase transition in potassium lithium niobate (KLN)

The ferroelectric phase of potassium lithium niobate K₃Li_2–xNb_5+xO_15+2x (KLN) in the range of 0.15 < x < 0.5 is a very promising material for the second harmonic generation (SHG) in the blue visible region (∼410 nm). The ferroelectric phase transition was shown to occur between 400 and 500°C depending on the composition of the KLN phase. In this study several analysis techniques were used to investigate the phase transition on ferroelectric (x = 0.3) KLN samples. The temperature‐dependent measurements of the relative dielectric constant ε₃₃ provided a phase transition temperature of about 470°C. In our DTA experiments, a small but reproducible thermal effect at the phase transition in KLN was indicated. The temperature‐dependent birefringence measurement technique, applied the first time on KLN, shows a second order behaviour at a temperature of 467 °C. However, this phase transition is accompanied by a small thermal effect. The DSC analysis for the other KLN composition (x = 0.5) provided a phase transition temperature of 514 °C. The appearance of a phase transition in the paraelectric KLN phase (Nb content higher than 55 mol%) was also studied. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Resistivity Measurements on Aligned Amphiphilic Liquid Crystalline States

We measured the resistivity of a ternary liquid mixture consisting of decylammoniumchloride, ammoniumchloride and water (45:5:50 wt%). The mixture forms a nematic phase between 41°C and 61°C; below 41°C a neat soap, and above 61°C an isotropic micellar solution. The resistivity decreases with increasing temperature. In the nematic and in the neat soap phases the resistivity is anisotropic, and it is higher for currents parallel to the director. The anisotropy increases strongly with decreasing temperature, due to a much stronger temperature dependence of the resistivity parallel to the director. The changes accompanying the phase transitions are small. The surprisingly small change of the resistivity parallel to the director at the nematic to neat soap transition indicates that the lamellae in the neat soap contain a large number of defects.     

Synthesis and Nonlinear Optical Properties of Novel Polyester Containing Nitrophenylazonitroresorcinoxy Group

A new X-type polyester (4) containing nitrophenylazonitroresorcinoxy groups as NLO chromophores, which are components of the polymer backbone was prepared. Polyester 4 is soluble in common organic solvents such as N,N-dimethylformamide and dimethylsulfoxide. It shows a thermal stability up to 280°C from thermogravimetric analysis with glass-transition temperature (T_g) obtained from differential scanning calorimetry near 120°C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer films at the 1064 nm fundamental wavelength is around 5.08 × 10^?9 esu. The dipole alignment exhibits a thermal stability even at 5°C higher than T_g, and no SHG decay was observed below 125°C due to the partial main-chain character of polymer structure, which is acceptable for NLO device applications.     

High Pressure Investigations of the Phase Transitions in Cholesteryl Pentanoate and Cholesteryl Hexanoate

At high pressures too cholesteryl n-penta-noate (CH-5) and cholesteryl n-hexanoate (CH-6) exhibit only a cholesteric liquid crystalline phase. Our light reflection measurements up t o 2600 bars yield no indication of a pressure induced smectic phase which was suggested by Tikhomirova and Ginzberg' for CH-5 at about 850 bars and 120°C. The Obtained pressure (P_t)-temperature (T_t) phase diagrams show the expected widening of the cholesteric range with P_t and T_t. In the case of CH-5 the marked crystallization curve besides a melting curve could be explained by the transformation of the cholesteric into a metastable crystalline solid phase.     

Square-Planar trans-Bis-(1-p-n-octylphenylbutane-1,3-dionato) copper (II), a New Compound Exhibiting Three Kinds of ‘Double Melting’ Behaviour

The new compound, bis-(1-p-n-octylphenylbutane-1,3-dionato) copper(II) has four polymorphs, C¹ (m.p. 96°C), all having the same squareplanar trans structure. C¹, C^a, and C^m exhibit ‘double melting’ behaviour, i.e., they melt at their was observed only on very rapid heating, while that of C^a and C^m only on slow heating. Slow solid-solid phase transition from C¹ to C^h was observed at ca 50°C. Each polymorph could be obtained as a stable form at room temperature and gave a quite different X-ray diffraction powder pattern. The interrelationships of the four forms have been clarified by differential scanning calorimetric and micriscopic measurements. When the melt of the complex is rapidly cooled, it converts into a glassy state, which is stable at room temperature. The glass transition temperature T₈ and the crystallizing temperature T_e are very close to each other at 52.5°C and 58.0°C, respectively.     

Thermal behaviour of beta-uranophane

Beta-uranophane from Perus, SP, Brazil- has been heated progressively up to 1.350°C, and its thermal behaviour has been investigated thoroughly by several methods: thermogravimetry, differential thermal analysis, optical measurements (variation of refraction indices), infra-red spectroscopy, and X-ray diffraction (powder photographs at room temperature of previously heated material, and powder photographs at high temperatures in special cameras). – The mineral seems to be stable up to a temperature of 200°C, confirmed by the several experimental procedures. The differential thermal analysis gives a sharper point of transformation at 165°C, which should be ascribed to water evaporation (zeolitic water). At this temperature the beta-uranophane should change to another isostructural compound, containing a small amount of crystallization water. Alternatively, crystal structure of the mineral could be transformed into a random layer structure, due probably to loss of the structural water. This kind of crystal disorder induces, as a rule, in the X-ray powder diagrams, absences of the hkl reflections, leaving merely the prismatic (hk0 and 0kl) and the pinacoidal reflections (h00, 0k0, 00l, and h0l). This could explain the small number of observed lines in the powder photographs of the material heated above 200°C. A new phase transformation is reported in the temperature interval of 700 to 800°C. This transformation should be related to the beta-uranophane chemical breakdown into two or more new crystalline phases. The chemical composition of such products is discussed in the paper. As the temperature rises to a higher level, a phase transformation takes place between 1000 and 1200°C and yet another one in the range of 1200 to 1350°C. The latter transition has been detected with the high temperature camera, and could be ascribed to the formation of an alloy between the platinum holder and the breakdown products of beta-uranophane. Infra-red espectroscopy has been proved inadequate for the study of the phase transformations of beta-uranophane. However, some conclusions could be reached in connection with the water content at several temperature levels.     

Changes in adsorption and optical properties of liquid crystal langmuir films at structural phase transition

The adsorption isotherms of water molecules, absorption spectra, and spectra of diffuse scattering and polarization of reflected light are studied for ultrathin Langmuir films prepared based on liquid crystals. A structural phase transition near 70°C is detected. Some specific features of the reflection spectra at the phase transition temperature are found. Suggestions are made regarding the nature of the phase transition.