DTA,X-ray and dielectric relaxation studies of 14CB at atmospheric and elevated pressures

The pressure-temperature phase diagram of 4′-tetradecyl-4-cyanobiphenyl (14CB) up to 220 MPa (2.2 kbar) and between 320–400 K was established using DTA. The temperature range of the smectic A (SmA) phase slightly increases with pressure. The layer spacing d at 1 atm was determined as a function of temperature using X-ray diffraction. It was related to the molecular length l by the ratio d/l ? 1.4. The dielectric relaxation measurements in the isotropic and smectic A_d phases of 14CB at 1 atm were performed in the frequency range 10 kHz-3 GHz. Contributions from both principal rotational motions, i.e. around the short and long molecular axes, were separated. The relaxation measurements under high pressure in the SmA phase covered the low frequency process. The longitudinal relaxation time τ₁, characterizing the molecular reorientations around the short axis, was analysed with respect to the pressure and temperature dependences, giving activation volumes, Δ^# V = RT (? ln τ₁ / ?p)_T, and activation enthalpies, Δ^# H = RT(? ln τ₁ / ?T ^-1)_p, respectively. Surprisingly, all the activation quantities characterizing the rotational motions of 14CB molecules under different conditions are nearly the same as those determined recently for the much shorter homologue, 8CB. This indicates that the 14CB molecule is in fact relatively short due to conformational motions of the alkyl tail.     

Volumetric,dielectric, calorimetric and X-ray studies of smectogenic 10PBO8 at atmospheric and elevated pressures

The synthesis and pressure–volume–temperature (PVT), differential thermal analysis (DTA), dielectric and X-ray diffraction data of 2-(4-octylcarbonyloxyphenyl)-5-decylpyrimidine (10PBO8) are presented. The substance exhibits two crystalline and smectic C (SmC) phases on heating and a SmC–monotropic crystalline smectic B (SmB_cr) SmB_cr–crystal sequence of phase transitions on cooling. Above ca. 15 MPa, the SmB_cr phase becomes enantiotropic (reversible polymorphism). The phase behaviour and molecular dynamics in the liquid crystalline phases are analysed and discussed, with the conformational component of the total entropy for the SmC–isotropic liquid transition estimated. We also calculate from the PVT results the potential parameter characterising the steepness of the interaction potential.     

Characteristics of methanation of carbon monoxide on a nickel-aluminum-calcium catalyst at atmospheric and elevated pressures

Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 2, pp. 254–255, March–April, 1988.     

Crystallization of polyethylene at elevated pressures

The crystallization from the melt of three sharp polyethylene fractions has been studied at 5 kbar. It has been shown that the thickness of so-called extended-chain lamellae is a function of time, temperature, and molecular weight. There is by no means just the fully extended molecular configuration present. Crystallization is qualitatively similar to that of chain-folded crystals at 1 bar, giving an optimum lamellar thickness which increases with time and decreasing supercooling. Fractional crystallization is widespread and is a major cause of disparate lamellar thickness. Isothermal thickening of lamellae during crystallization has been established directly. Morphological detail suggests further that layers can increase their thickness tenfold over their initial size.     

Anomalous narrowing of the structural relaxation dispersion of tris(dimethylsiloxy)phenylsilane at elevated pressures

Broadband dielectric relaxation measurements of tris(dimethylsiloxy)phenylsilane were made at ambient pressure and at elevated pressures. The data show an anomalous behavior not previously seen in any other glass-formers; namely, the structural alpha-relaxation loss peak narrows with increasing pressure and temperature at constant peak frequency. Interpreted by the coupling model, the effect is due to reduction of intermolecular coupling at elevated pressures. This interpretation has support from the observed decrease of the separation between the alpha-relaxation and the Johari-Goldstein secondary relaxation, as well as the smaller steepness or "fragility" index m of the data obtained at 1.7 GPa than at ambient pressure.     

Modeling of a free-burning,high-intensity arc at elevated pressures

A previously established model has been applied to a free-burning high-intensity argon arc at elevated pressures for simulating the situations experienced, for example, in plasma processing or in underwater welding. With the calculated thermodyanmic and transport properties and appropriate boundary conditions, solutions of the entire arc are obtained with exception of the electrode sheath regions. The results show that the arc contracts as the pressure increases. As a consequence of this contraction, the current density, the enthalpy flux, and the voltage drop increase also while the velocity of the induced cathode jet decreases.     

NMR studies of carbon dioxide and methane self-diffusion in ZIF-8 at elevated gas pressures

Self-diffusion measurements with methane and carbon dioxide adsorbed in the Zeolitic Imidazolate Framework-8 (ZIF-8) were performed by ¹H and ¹³C pulsed field gradient nuclear magnetic resonance (PFG NMR). The experiments were conducted at 298 K and variable pressures of 7 to 15 bar in the gas phase above the ZIF-8 bed. Via known adsorption isotherms these pressures were converted to loadings of the adsorbed molecules. The self-diffusion coefficients of carbon dioxide measured by PFG NMR are found to be independent of loading. They are in good agreement with results from molecular dynamic (MD) simulations and resume the trend previously found by IR microscopy at lower loadings. Methane diffuses in ZIF-8 only slightly slower than carbon dioxide. Its experimentally obtained self-diffusion coefficients are about a factor of two smaller than the corresponding values determined by MD simulations using flexible frameworks.     

Vibrational spectral studies of solutions at elevated temperatures and pressures. IX. Acetic acid

Raman spectra of glacial acetic acid from 350 to 3700 cm^–1 have been measured at temperatures up to 275°C and at a pressure of 9 MPa. Raman spectra of aqueous solutions of acetic acid from 3.9 to 16 molar have been measured up to 200°C at a pressure of 7 MPa. The spectral region 800 to 1850 cm^–1 for both glacial acetic acid and its aqueous solutions have been studied in detail since this region is significantly affected by variations in temperature and concentration. An interpretation of the bands in this spectral region was made with the aid of factor analysis, difference spectroscopy, band resolution techniques and the existing extensive literature. The results suggest that the major equilibrium in glacial acetic acid is between cyclic and linear dimers; however, in aqueous solutions in the concentration range studied, mono- and di-hydrated dimers and cyclic dimers are the predominant species.     

Isothermal and nonisothermal dielectric relaxation studies on polycarbonate

Dielectric relaxation measurements at very low frequency were carried out on polycarbonate. Two prominent peaks were resolved from the broad β-relaxation peak with activation energies of 0.28 and 0.52 eV, respectively. Thermally stimulated discharge current was also studied and compared to the results obtained from the dielectric relaxation measurements.     

Light scattering and dielectric relaxation studies of glass forming liquids

Extended Abstract: Glass forming organic liquids and polymers exhibit long range density fluctuations with correlation length ξ in the range of 10–300 nm at temperatures above T_g (1 - 6). This follows from dynamic and static light scattering experiments revealing some unexpected features, which cannot be explained on the basis of conventional liquid state theories: (i) In static light scattering the intensity I(q → 0) is no longer proportional to the isothermal compressibility, (ii) This excess scattering I_exc shows a strong q-dependence (q = (4π/Λ.)sin(θ/2)) corresponding to a correlation length ξ in the above mentioned range, (iii) The Landau-Placzek ratio I_Rayleigh/2I_Brillouin is much too high compared with the results of light scattering theories, (iv) In photon correlation spectroscopy a new ultraslow hydrodynamic mode (Γ ˜ q²) is detected with relaxation rates Γ about 10⁻⁶ to 10⁻⁹ lower than those of the α-process at a given temperature. In order to explain these observations, a two-state fluid model is proposed, which starts from the coexistence of “liquid-like” and “aperiodic solid-like” regions within the liquids. Such ideas have been discussed many times before, so for example A.R. Ubbelohde (7) speculates about “anticrystalline” clusters in liquids. Molecular dynamics simulations of atomic liquids showed that long range orientational fluctuations appear upon supercooling (8). A preferred icosahedral ordering is observed (9) and the number of icosahedral clusters increases with decreasing temperature (10). In connection with the interpretation of the dynamics of supercooled liquids different “two-state” models have been proposed (11 - 15). For the explanation of the light scattering results we propose that the molecules in the different dynamic states (“liquid” or “solid”) aggregate during annealing of the liquid at temperatures above T_g. Experiments showed that the equilibration times can be rather long (3 - 5), but nevertheless the liquids exhibiting long range density fluctuations are in the state of lowest free energy. We claim that our observations are the first experimental proof of the existence of such different dynamic states, which have been discussed many times before. The extended secondary clusters can also be detected by ultra small angle X-ray scattering.     

Study of a thermotropic liquid-crystalline polyester at elevated pressures

A thermotropic liquid-crystalline copolyester of 20% hydroxybenzoic acid, 40% isophthalic acid, and 40% hydroquinone polymer was studied at elevated pressures. The characterization techniques at elevated pressures (0–1000 bar) included high-pressure differential thermal analysis and dilatometry; at atmospheric pressure, differential scanning calorimetry, thermal optical analysis, and x-ray analysis were employed. The mechanical properties of the solid specimens prepared at different pressures were studied by compression and dynamic rotation mechanical testing techniques. High-pressure induced a new crystal habit in the solid state and a new mesophase in the melt. These transitions are summarized in a proposed phase diagram. Mechanical tests on the material produced at elevated pressure indicate the possibility of improved properties, implying that the pressure-dependent morphological changes in thermotropic copolyesters could be of practical significance. The finding of a pressure-induced mesophase also confirmed the possibility of extending the range of polymers which might exhibit liquid crystallinity via the application of pressure.     

Moisture–absorption,dielectric relaxation,and thermal conductivity studies of polymer composites

In the search for new packaging materials for the electrical/electronics industry, three types of polymer composites have been studied. Silicone/boron nitride powders, polyurethane/alumina powders, and polyurethane/carbon fibers have all been synthesized to study the moisture–absorption kinetics, thermal conductivities, and the dielectric loss spectra under various levels of humidity. The water uptake data indicate that water molecules are absorbed not only by the polymer matrix, but also by the interfaces introduced by the fillers. For all materials, the dielectric relaxation spectroscopy shows the presence of a peak in the 175–200 K range, which is largely due to absorbed water. The silicone/boron nitride samples absorbed the least amount of moisture. Incorporating this result with the thermal conductivity data of the three types of polymer composites, it is concluded that silicone polymers embedded with boron nitride can best serve as the coating for the electronic devices that require heat dissipation and moisture resistance, in addition to electrical insulation. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2259–2265, 1998     

Vapor-liquid equilibrium for phenanthrene-toluene mixtures at elevated temperatures and pressures

Vapor and liquid equilibrium compositions are reported for the phenanthrene-toluene system at 593, 633, and 675 K and at pressures ranging from 7.3 bar to the binary mixture critical pressure at each temperature. Mixture critical pressures were obtained by visual observation of critical opalescence within a view cell. A flow apparatus for measuring VLE and LLE that is capable of operation at pressures up to 350 bar and temperatures to 675 K is also described. The apparatus can be used with materials such as phenanthrene and petroleum pitches which are solids at room temperature.     

Stabilization and growth of non-native nanocrystals at low and atmospheric pressures

The stabilization and growth of nanocrystals in "non-native" structures is explored via density functional calculations. Non-native and "native" bulk structures differ in their discrete translational symmetry. Computations suggest that the lower surface energy of the non-native structures always facilitates their stabilization in the early stages of crystal growth. In the compound semiconductors considered here, the transition pathways between non-native and native structures involve planar or near-planar depolarized layers and the growth conditions have significant effects on the stabilization and growth of non-native structures. The findings of this study help in identifying heuristics for the synthesis of non-native nanocrystals.     

Broadband dielectric relaxation study of 6CB and 6CB-aerosil dispersions in the nematic and isotropic phases

Experimental studies of dielectric relaxation in the nematic and isotropic phases of 6CB (4-hexyl-4′-cyanobiphenyl) and two mixtures of 6CB perturbed by 160 Å hydrophobic silica aerosil are presented. The measurements have been made from 1 MHz to 1 GHz in the temperature range from 19 to 40°C. For bulk 6CB, the dynamic aspect in the isotropic phase as well as the principal dielectric permittivities ε ∥ * (ω) and ε ┴ * (ω) in the nematic phase have been fully explored. For the mixtures, measurements on the isotropic phase and also on homeotropically aligned samples in the nematic phase have been made, and these results are systematically compared with those for the bulk. The possible molecular reorientational movements corresponding to the different absorption domains in the dielectric spectrum are discussed and compared with the previously proposed interpretations.     

Vapor pressures and enthalpies of vaporization of thianthrene,acridine, and 9-methylanthracene at elevated temperatures

Vapor pressures (0.13 to 197.99 kPa) were measured of three polynuclear compounds, one containing a sulfur group, the second containing pyridinic nitrogen, and the third a methyl group. The temperature ranges of the measurements were as follows: acridine, 423.80 to 621.16 K; thianthrene, 430.79 to 593.01 K; and 9-methylanthracene, 423.76 to 587.64 K. The measurements were performed in a high-temperature static apparatus. For each compound Chebyshev polynomials have been used to fit the experimental vapor pressures. The results have been further processed to evaluate the enthalpies of vaporization.     

The sublimation of some metal fluoride complexes at elevated pressures

The sublimation of NONbF₆, NOTaF₆, NOBF₄, NOGeF₅, NOPF₆ and NOSbF₆ was measured by DSC at various pressures up to 50 atm. The relationship between temperature and equilibrium vapor pressure was obtained for each compound by the Clapeyron—Clausius plot. At the same time, the apparent molecular weights of the compounds could be estimated by using the enthalpies which were reported previously.     

Solubility of carbon dioxide in cellulose acetate at elevated pressures

The solubility of carbon dioxide in symmetric (dense) cellulose 2.4-acetate has been measured at temperatures from 0 to 70°C and pressures up to 45 atm. The polymer samples were prepared by slowly drying asymmetric reverse osmosis membranes. The solubility isotherms can be described satisfactorily up to 60°C by the “dual-sorption” model for glassy polymers. The model cannot represent the experimental data above 60°C, possibly because of a second-order transition in the polymer between 60 and 70°C. An analysis of the dual-sorption parameters and of the heats of solution and “hole filling” suggests that the polymer samples contained a relatively large volume of microcavities. Gas solution appears to occur predominantly in microcavities, a large fraction of the penetrant moleculers being immobilized or partially immobilized. The solubilities obtained in this work are compared with similar data computed from time-lag measurements of other investigators, and the validity of the dual-sorption model is examined for the present case.     

Energetics of stress relaxation in glassy polymers at high pressures

Kinetics of relaxation of the shear stress in glassy poly(methyl methacrylate) were studied in isobaric, isothermal and isochoric conditions. In isochoric conditions the relaxation rate depends only on temperature and polymer volume is the main parameter for the rate of stress relaxation. Isobaric and isochoric activation energies and activation volume of the stress relaxation were determined. The results are shown to satisfy the thermodynamic relation of activation parameters of the relaxation process. The size of the area in which the elementary stress relaxation process occurs in glassy poly(methyl methacrylate) was evaluated.