Thermodynamical properties of glass forming systems: A Nuclear Magnetic Resonance analysis

We present a Nuclear Magnetic Resonance (NMR) study of the thermal evolution of the magnetic properties of three different glass formers: glycerol, o-terphenyl and salol. In particular, we analyze how the response of these liquids to the applied magnetic field changes with temperature. We focus on the total magnetization and on the chemical shift of each protonated group. By means of these quantities we account that the dynamics of the glass forming materials, on decreasing the temperature, is dominated by the onset of well defined local inhomogeneities due to precise microscopic cooperative processes. Just these “dynamical heterogeneities” and their energetic topology determine the dynamic crossover from fragile (super Arrhenius) to strong (pure Arrhenius) glass forming behavior. The specific heat changes evaluable from the measured NMR chemical shift associate this phenomenon, and all the related ones, to local configurational changes.     

The Effect of Rhodamine 6G on Phase Transitions in Cholesteryl Ester Mixtures

Compensated cholesteric mixtures were the subject of a whole range of studies.^1-6 Due to their characteristic properties (the existence of a temperature at which compensation occurs and the dependence of the critical field on temperature), the orientation of guest molecules by compensated cholesteric hosts⁷ shows some particular features in comparison to the orienting mechanisms acting within nematic liquid crystals^8-15 or within chiral nematic mixture.^16,17 When the colour of sample varies due to the influence of the electric field and temperature the guest dye contributes to it as well the compensated cholesteric host.

In this paper we report the modification of compensated cholesteric mixture properties due to the introduction of Rhodamine 6G as guest.     

Phase Transitions in Polypeptide Liquid Crystals: A Reentrant Isotropic Phase

Optical and C-13 NMR examinations of concentrated solutions of poly-γ-benzyl-L-glutamate (PBLG) in a mixed solvent system containing a denaturant acid, show that this polymer liquid crystal has both a low and high temperature isotropic phase. The latter is due to thermal disruption of long range orientational order of the elongated macromolecules. The former reentrant isotropic phase is a result of an intramolecular helix to random coil transition, which leads to a macromolecular conformation inconsistent with liquid crystallinity.     

A Study on the First Order Phase Transitions in Hydrocarbon Crystals

A polarization hot stage microscopy study was made on the phase transitions in n-octacosane and n-hexatriacontane hydrocarbon crystals. Advancement rates of transition fronts of high temperature phase in low temperature phase were measured. Transition in the interior layers was detected. In order to explain the observations, we propose a model, which involves intrachain defects in the extended chain configuration. This model is an advanced stage of the model proposed by STROBL, basing on the SAXS results.     

Probing Orientation Phenomena in Strained Rubbers with Deuterium Nuclear Magnetic Resonance

The orientational order generated in rubbers by uniaxial stress is probed with deuterium NMR. Deuterated swelling agents or labelled polymeric chain segments exhibit a residual quadrupolar interaction when the elastomer is under constraint. This induced anisotropy has a distinctive dependence on the network extension ratio and the cross-linking density of the material.     

Deformation of Nematic Liquid Crystals in Electric and Magnetic Fields as seen by Proton Nuclear Magnetic Resonance

We have calculated the deformation of the director field within a nematic liquid-crystal cell when both extenal electric and magnetic fields are present. It can be proved experimentally that the line shape of proton nuclear magnetic resonance is strongly affected by the orientational distribution of the director.

The analysis of the NMR-line shape of LC-sandwich cells in this manner can yield various parameters relating to the liquid crystalline material.     

High Pressure Study of Phase Transitions in DMPC-Water System

We have carried out a detailed pressure study of the three phase transitions observed in hydrated dimyristoyl phosphatidylcholine (DMPC) containing 95% by weight of water. The P-T diagram shows a Gel III-Gel II-Gel I triple point at 3.5 kbar, 41[ddot]C. It is found that the Gel III phase can be obtained by pressure-annealing the sample for about 12 hours at room temperature.     

The Yttrium,Lanthanum, and Lanthanide Trifluorides: Internal Periodicity of Phase Transitions

Crystallography Reports - Internal periodicity of phase transitions (fusion and polymorphism) has been revealed in trifluorides of Y, La, and 14 lanthanides. This periodicity is determined by the...     

Concentration Magnetic Phase Transitions in the (La1−xCaxMnO3 System

Comprehensive investigations into crystal structures, electrical and magnetic properties of the (La_1−xCa_x)MnO₃ (x = 0 ÷ 0.6) solid solutions are performed. Two concentration magnetic phase transitions are found: antiferromagnetic-ferromagnetic with x = 0.14 and ferromagnetic (T_c = 150 K) - ferromagnetic (T_c = 260 K) at f = 0.2. It is shown that the first of them is due to the crystal structure O′ O orthorhombic transition, the second - to the increase of ferromagnetic interaction at the expense of additional interaction through conduction electrons. A diagram of the magnetic states for (La_1−xCa_x)MnO₃ at x = 0 / 0.6 is given.     

Thermal Studies on the Phase Transitions of p-n-Dodecyloxy and p-n-Tetradecyloxy Benzoic Acids.: (Part III)

Thermal properties of p-n-dodecyloxy and p-n-tetradecyloxy benzoic acids have been investigated using a differential scanning calorimeter. Heat capacities of thermally stable and metastable crystals of these acids were estimated and the effects of the paraffinic chain attached to the benzoic acid on the formation of a metastable crystalline state are discussed. It is suggested that the disorder of hydrogen bond configuration in associating carboxylic acids in pairs brings about the formation of a metastable crystal state for the alkoxy benzoic acids in general. Under the same conditions the mobile paraffinic chains control the modification of the hydrogen bond during the course of crystallization.     

Semifluorinated Methacrylate Random Copolymers: Phase Transitions and Molecular Dynamics

Random copolymers of methyl methacrylate (MMA) and sermifluorinated methacrylate (sfMA), with constant side chain length (H₁₀F₁₀), as comonomers and various sfMA molar contents were studied by Dielectric Relaxation Spectroscopy (DRS) technique with respect to their phase transitions and molecular dynamics. DRS technique was proven a suitable technique for the detection of the phase transitions that take place in the systems under investigation, as it follows from the comparison with Differential Scanning Calorimetry (DSC) technique, which is traditionally used. Regarding molecular mobility, molecular motions of both the main chain and the sf side chains were followed, while different dynamics was recorded depending on the structure of the copolymers.     

NMR Study of the Phase Transitions in Some Derivatives of Cholesterol and B-Nor Cholesterol

In this paper we analyze by broad line NMR spectroscopy method, the influence of modifying the cholesterol skeleton upon mesomorphic properties and characteristics (phase transition points, mesomorphic ranges, etc.) in some derivatives of cholesterol. From our studies regarding the phase transition points by NMR spectroscopy method, we can assert that derivatives of the B-nor cholesterol have the same behavior as derivatives of the cholesterol.     

Conformational Mobility and Phase Transitions in Alkyloxybenzoate of Cholesterol

Abstract

The structural particularities of polymorphic modifications and cholesterol liquid crystal phase have been investigated by theoretical and experimental IR spectroscopy methods. The spectra were recorded in the interval of temperatures 293–523 K within the range of 600–3600 cm^?1. The frequencies of normal modes and intensities in the IR spectra have been calculated for the alkyloxybenzoate of cholesterol conformers, which may be realized under certain temperatures. Comparison of results of theoretical modelling and recorded spectra confirmed suggestion about conformational mobility basically defined with changing of steroid kennel orientation comparatively stayed part of molecule in the different phase state.     

Deuteron Magnetic Resonance Study of Order and Dynamics in Liquid Crystal Polymers

Thermotropic nematic polyesters, specifically deuterated at different positions of the polymer chain are studied by multiple pulse dynamic NMR. Analysis of the NMR experiments is achieved, employing a comprehensive model, based on the stochastic Liouville equation. Computer simulations provide the orientational distributions and conformations of the polymer chains and the correlation times of the various motions. In the anisotropic melt the correlation times for chain reorientation and trans-gauche-isomerization are in the range of 0.1–10 ns. Decreasing the temperature of the solid polymer first freezes the intermolecular motions. Thus, below the glass transition only intramolecular motions such as trans-gauche isomerization and ring flips can be detected. The chain order parameter of the nematic melt is S(ZZ) = 0.85. In addition, the chains adopt a highly extended conformation, evidenced by a trans population of n(t) = 0.8 throughout the entire spacer. This microorder is retained, when the polymer is cooled below the melting point and glass transition, respectively.     

Phase Diagram Involving the Thermal Stability of TGB and Induced Smectic Phases in Binary Mixture of Thermotropic Liquid Crystals

We report the results of our studies on the optical and thermal properties of the mixture of terephthalidene-bis-4, n-alkylaniline and cholesteryl nonanoate, which exhibits very interesting liquid crystalline mesophases such as cholesteric, twisted grain boundary, smectic-A, smectic-C^*, smectic-C, smectic-I, and smectic-G, sequentially when the specimen is cooled from its isotropic phase. These phases have been characterized by using microscopic techniques. The temperature variations of optical anisotropy, electrical conductivity, ultrasonic velocity, and molar and adiabatic compressibility have also been discussed.     

Phase Diagram and Dielectric Studies in Hydrogen-Bonded Liquid Crystal System

The thermal and dielectric behaviors of mixtures were investigated for systems of p-n-hexyloxybenzoic acid (6OBA) and p-n-octyloxybenzoic acid (8OBA) by differential scanning calorimetry, polarized optical microscopy, and dielectric measurements. The T-X phase diagram was obtained for this system. Eutectic composition was calculated and received experimentally. Enthalpies of mixing were calculated to describe phase transitions physically. All mixtures show enantiotropic smectic and nematic phases. Dielectric permittivity has higher value for the mixture with fractional proportion of 30 to 70% mol for 8OBA than that for initial acid. It is assumed that co-crystallization of initial compound occurs when the ratio of components is close to equimolar ratio.     

立方相KTa0.67Nb0.33O3晶体的系列相变研究

采用拉曼散射技术研究了立方相KTa0.67Nb0.33O3晶体的系列相变,从声子角度分析了各相变特点.降温过程中立方-四方和四方-正交相变分别发生在-29℃和-95℃左右,正交-三方相变可能发生在-150℃左右.最后简单讨论了晶体的相变机制和居里点与晶体组分的关系,并比较分析了居里点的实验值与理论值的差异.     

Undoped Trans - (CH)X : Magnetic Resonance and Structure

Arguments are given that soliton conditions might be best realized at the surfaces of the fibrils constituting the Shirakawa polyacetylene. The implications of this hypothesis on the interpretation of magnetic resonance and other experiments are discussed.     

Resistive and Magnetic Susceptibility Transitions in Superconducting (TMTSF)2C104

We report measurements of the ac magnetic susceptibility and dc resistive superconducting transitions in the organic superconductor (TMTSF)₂C10₄. Inductive measurements show complete diamagnetic shielding below a broad transition and initial flux penetration at very low fields [H_c1(0) < 1 Oe]. The resistive transition is also broad, but occurs at a significantly higher temperature than the inductive transition, T_c = 1.0 K and 0.65 K respectively. Resistance measurements also show evidence of a phase transition in the vicinity of 24 K. Magnetic field induced transitions, measured both inductively and resistively, show marked anistropy both in magnitude and in breadth of the transition. Results suggest that (TMTSF)₂C10₄ is a quasi ID or 2D superconductor at high temperatures and high magnetic fields and an anisotropic bulk superconductor at low temperatures and fields. Associated thermoelectric power measurements suggest that spin density waves coexist with the superconducting state.     

The Studies of Ferroelastic Phase Transitions and Domain Walls in KMgCl3 Crystal

The ferroelastic phase transitions of KMgCl₃ crystal grown by the Czochralski method were investigated by direct observation using a polarizing microscope with varying temperature. The results showed that the KMgCl₃ crystal undergoes two pure ferroelastic phase transitions from cubic prototypic phase to tetragonal, and to orthorhombic structure at about 312 °C and 224 °C, respectively, and can be classified into m3mFmmm(p) species. The permissible domain walls of this species were analyzed by both the strain tensor and group theoretical method.