DCS Study of Smectic Phase Transitions

A previously described technique for measuring the order of phase transitions^l-3 using DSC has been applied to the liquid crystalline transitions found in TBBA. All of the values obtained were in agreement with those that have been measured by other techniques, and also with what would be anticipated from the nature of the transition. This study has shown that the technique is applicable to smectic-smectic transitions in liquid crystals.     

Successive Phase Transitions in Lipid Membranes

On the basis of a statistical model previously proposed by the present authors, a new sort of phase transition recently observed in the fluid-like phase of DPL bilayer and mono-layer above the main transition temperature is investigated as successive orientational transitions accompanied with conformational changes. The effects of lateral pressure on the transition temperatures and latent entropies observed in DPL monolayer are also explained consistently. This type of phase transition is expected to occur in DML, DSL and DBL as well, although it has not been observed yet in the case of these molecules.     

Phase Transitions of A Quasi-One-Dimensional Fermi Gas

Phase transitions of a quasi-one-dimensional Fermi gas are studied by the use of the renormalization group method for the fermion system without the bosonization. Intrachain backward and forward scatterings, pair electron hopping between different chains and exchange scattering of electrons in different chains are taken account of. Pair electron hopping gives rise to the singlet and triplet superconductor phase transitions. Electron exchange scattering gives rise to the spin density wave and charge density wave transitions. Calculation is made by the use of the one-dimensional dispersion. Following Menyhárd's method (Ref. 1) for the model with interchain backward scattering only, the change of the fluctuation from the one-dimensional region to the three-dimensional region is described. The transition temperatures are given by the bare couplings at high temperatures.     

Sequence of Phase Transitions in Solid Mbba

Combined neutron diffraction and Raman scattering measurements were carried out on a series of different phases of the same MBBA sample. The change in the diffraction patterns and in the optical spectra are used to indicate the transitions between the different phases investigated in the whole temperature range of interest. Besides the isotropic and nematic liquid crystalline phases, 7 different solid structures were found and a schematic phase diagram was constructed to show the thermal procedur for producing these phases.     

Transitions de Phase en Series Mesogenes:

The polymorphism of the di(4-alkylphenyl) and di(4-alkoxyphenyl) tetrathiafulvalene (TTF) is investigated by optical, crystallographic and calorimetric methods. All the studied derivatives are mesomorphic. The first mesophase is always a smectic G phase; the second one is a nematic N phase for short chains and a smectic C phase for long chains.

The entropies corresponding to the passage high-temperature crystalline phase C₁ → S_G and to the “melting” (C₁ → mesophases → isotropic phase I) increase with the length of the chain and with the presence of the oxygen atom in the radical.     

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.     

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.     

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.     

Phase Transitions of Urea Adducts with n-Paraffins under High Pressure

The temperatures of the phase transition between orthorhombic and hexagonal structures of the urea adducts with n-paraffins, n-C_nH_2n+2 (n = 14–44), have been measured by differential thermal analysis for the pressure range up to 250MPa. The transition temperatures increase both with pressure and with the number of carbon atoms of the guest paraffins. The increasing rate of the transition temperatures with pressure is about half of that of pure paraffin crystals. The pressure dependences of the lattice constants have also been measured by X-ray diffraction. The specific volume changes discontinuously at the transition pressure; the result suggests that the phase transition of the urea adducts with n-paraffins is a first-order one. Furthermore, the specific volume change at the transition pressure decreases with the number of carbon atoms of the guest paraffins; 3.4 × 10-⁶ m³/Kg for n = 16 and 2.1 × 10-⁶ m³/Kg for n = 44.     

Metastability in Potassium Palmitate/D2O Phase Transitions

A perdeuterated potassium palmitate/deuterium oxide mixture (66/34% by weight respectively) weas examined using time resolved x-ray diffraction. The dynamic phase initial diagram indicated that upon heating, the crystalline bilayer phase undergoes a transition to a liquid-crystalline phase at approximately 75[ddot]C via two metastable states. One of the metastable states is a disordered cyrstalline bilayer and the other is a liquid crystalline bilayer. Upon subsequent cooling, the liquid-crystalline bilayer transforms directly to a getl bilayer phase.     

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.     

Structural Phase Transitions in Zinc(II) Perchlorate Hexahydrate

Phase transitions in zinc perchlorate hexahydrate are studied by X-ray powder diffraction method. Structural changes in the vicinity of transitions detected at 364.5 K and 256.3 K are characterized. At the former the orthorhombic Pmn2₁ phase transforms into the Pmna one (a = 7.812(1), b = 13.518(3) and c = 5.279(1) Å at 348.6 K). The latter involves changes of the orthorhombic unit cell (a = 7.7922(9), b = 13.537(2) and c = 5.0588(6) Å at 251.0 K). Temperature dependence of lattice parameters in the range of 180 to 363.3 K are presented.     

立方晶系晶体本征铁弹相变序参量的选取

在本征铁弹相变的软模理论的基础上,依据弹性本构关系和居里原理,给出了立方晶系晶体所有可能的自发应变的种类,研究了立方晶系晶体本征铁弹相变序参量的选取问题.以Th群和Oh群为例,阐明了不同类的序参量导致晶体对称性变化相同的的原因,提出了该情况下序参量选取的“就少不就多”的原则.得到了发生本征铁弹相变时所有可能的序参量及晶体的对称性的变化.     

Landau Theory of Transitions from the Drd Phase of Columnar Liquid Crystals

Possible symmetry patterns of disordered columnar phases associated with the continuous symmetry breaking of the D_rd (pgg) phase are given. The results may be used to predict new mesophases which are as yet undiscovered and should be of aid in identifying unconfirmed phases. The universality classes of the possible transitions are also given.     

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.     

Phase Transitions in Chlorobenzene-Cis-Decalin Mixtures: A Nuclear Magnetic Resonance,Thermal Analysis and Phase Diagram Study

Using nuclear magnetic resonance and differential scanning calorimetry we have been able to observe the molecular rotation properties of chlorobenzene-cis-decalin mixtures in their glassy, amorphous and crystalline phases. The results indicate: that in “rapidly” cooled samples the behavior of the host dominates the properties of the mixtures; that reorientation of the guest molecule is less restricted in the amorphous phase than in the glassy phase; that when the material crystallizes from the amorphous phase on warming, reorientation again becomes severely restricted. The temperatures at which these phenomena occur agree with the phase diagram that has been determined for these materials. Similar experiments on t butyl chloride-cis-decalin mixtures support the above conclusions. These conclusions are in agreement with the previous dielectric studies.

Our magnetic resonance and thermal analysis experiments support the argument that the behavior of the CD host dominates the behavior of guest polar molecules in rapidly cooled mixtures. At the lowest temperature (110 K) CD forms a glassy phase unaccompanied by any heat of crystallization. On warming this glassy phase transforms near 150 K to an amorphous phase where considerable motion of both host and guest molecules occurs. Above 160 K the material crystallizes and reorientation is once again severely restricted up to the eutectic melting of 210 K. There appears to be no time dependence to these transformations and, as long as one stays below the crystallization temperature, the glassy-amorphous transition is reversible.     

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.     

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.     

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.     

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.