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1.
Stanis aw Urban Maria Massalska-Arod
Albert Wü rflinger Roman D browski 《Liquid crystals》2003,30(3):313-318
The pressure-temperature ( p - T ) phase diagrams for four smectogenic members of the 4'-alkyl-4-cyanobiphenyl homologous series ( n CB, n =9, 10, 11 and 12) over the temperature range 320-410 K and pressure range 0.1-300 MPa (3 kbar) were constructed using DTA. At 1 atm 9CB exhibits nematic and smectic A d phases, while the other members show only the smectic A d phase. However, at elevated pressures the clearing line splits in the case of 10CB and 11CB which indicates the induction of a nematic phase. It was found that the triple point, where the isotropic, nematic and smectic phases coexist, is strongly shifted to higher pressures with increasing chain length. This was interpreted as being caused by a loss of the rod-like shape of the molecules containing longer alkyl tails which explore a range of conformations. The slope of the clearing line, d T /d p , depends strongly on the length of the alkyl chain for the n CB series, but does not show a step-wise change between the nematogenic and smectogenic members. 相似文献
2.
The pressure-temperature phase diagram of n-octyl-isothiocyanato-biphenyl (8BT) in the pressure range up to 250 MPa (2.5 kbar) and the temperature range 250-400 K was established with the aid of DTA. At 1 atm the substance exhibits exclusively CrE polymorphism. At pressures above 190 MPa, the clearing line splits showing an additional phase which is not yet identified. Dielectric relaxation measurements on the CrE phase of 8BT were performed in the pressure range 0.1-120 MPa and the temperature range 304-345 K. A Debye-type relaxation process was observed in the frequency range 100 Hz-1 MHz. The longitudinal relaxation time τ, characterizing the molecular reorientations around the short axis, was analysed with respect to the pressure and temperature, yielding the activation volume, Δ# V = RT(? ln τ/?p)T, and activation enthalpy, Δ# H = R(? ln τ/? T-1)p, respectively. The results are compared with analogous data obtained recently for similar compounds having other liquid crystalline phases (N, SmA). 相似文献
3.
The phase behaviour of solid cyclopentanone, cyclopentanol, and cyclohexanone was investigated from 80 K to the melting temperature and up to 3 Kbar, using a low-temperature high-pressure differential thermal analysis apparatus. The melting temperature of cyclopentanone rises from 221.7 K at atmospheric pressure to 278 K at 2900 bar. No solid solid transition was observed. The melting temperature of cyclohexanone rises from 242.4 K at atmospheric pressure to 312 K at 2840 bar. Its well-known solid solid transition at atmospheric pressure (220.3 K) splits into two different solid solid transitions at elevated pressures. The melting temperature of cyclopentanol rises from 256 K at atmospheric pressure to 328 K at 2600 bar. Cyclopentanol exhibits two well-known solid solid transitions (236.4 K and 202.5 K at 1 atm), but an additional metastable form has been observed in the present work. the transition temperature being 195 K at 1 atm. Volume changes accompanying the phase transitions have been calculated using the Clausius Clapeyron equation. 相似文献
4.
PVT data have been determined for cyclohexane (194–338 K), cyclohexanone (214–303 K), and Cyclopentanol (204–325 K) up to 3000 bars, as well as the volume changes accompanying the phase transitions. The volume changes allow the calculation of the corresponding enthalpy and entropy changes on using the Clausius-Clapeyron equation. The volume changes of melting decrease considerably with increasing pressure, whereas those due to solid-solid transitions exhibit much less of a pressure dependence. The PVT data of the liquid phases are described with the Tait equation. 相似文献
5.
Dielectric studies of 4-n-hexyl-4'-cyanobiphenyl (6CB) were performed in the pressure range 0·1-150 MPa, the frequency range 1 kHz-13 MHz and the temperature range 290-327 K. The temperature and pressure dependencies of the static permittivity ε0| and the relaxation time τ| are analysed and compared with analogous data obtained recently for 5CB and 6CHBT (parts I-III of this series). It was shown that in the nematic phases of both 4-n-alkyl-4'-cyanobiphenyls the dipole-dipole correlations are broken by relatively low pressure. 相似文献
6.
R. Landau A. Würflinger und G. M. Schneider 《Fresenius' Journal of Analytical Chemistry》1977,286(3-4):216-218
Zusammenfassung Eine Methode zur Bestimmung kleiner Wassergehalte in Methanol (0,02–1 Mol-%) mit Differentialthermoanalyse (DTA) wird beschrieben. Die Wirksamkeit verschiedener Trocknungsverfahren wird untersucht. Das Verfahren wird im Vergleich mit anderen Wasserbestimmungsmethoden in organischen Flüssigkeiten diskutiert.Die Deutsche Forschungsgemeinschaft und der Fonds der Chemischen Industrie unterstützten die Untersuchungen durch Sachmittel. 相似文献
7.
For the first time pressure-volume-temperature (PVT) measurements for the crystal-like smectic E phase have been performed. The phase diagram of 4′-n -octyl-4-isothiocyanatobiphenyl 8BT has been recently established using differential thermal analysis up to 250 MPa. 8BT exhibits a splitting of the clearing line above 170 MPa. PVT data have been measured in the same pressure range for temperatures between 313 and 393 K. Volume and enthalpy changes accompanying the clearing line of 8BT are also presented. The configurational part of the entropy change at the CrE-I transition of 8BT amounts to ≈ 60%. Using the PVT data and recently published dielectric relaxation results, the isochoric activation energy was calculated (giving ≈ 50% of the activation enthalpy); this is compared with analogous results for other liquid crystals. 相似文献
8.
Abstract The results of high pressure dielectric studies of 4-n-pentyl-4′-cyanobiphenyl (5CB) are analysed in terms of theories of the nematic state. The retardation factor g∥ = τ∥/τ0 and the effective, single-particle potential of mean torque were calculated at the nematic–isotropic transition temperature T NI and along the isothermal, isobaric and isochoric paths within the nematic phase of 5CB. The potential of mean torque is compared with the order parameter known for the same conditions. The values of parameter γ relating the potential to the volume is discussed. 相似文献
9.
Abstract Dielectric studies of 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzene (6CHBT) were performed in the pressure range 0·1–150 MPa, the frequency range 1 kHz–13 MHz and the temperature range 295–325 K. The temperature and pressure dependencies of the static permittivity ?0∥ and of the relaxation time τ∥ are analysed and compared with the analogous data obtained recently for 4-n-pentyl-4′-cyanobiphenyl (5CB) (Parts I and II of this series). Marked differences in the dielectric properties of the nematic phases of the two substances are observed. They are interpreted as a result of varying degrees of molecular association in particular compounds. It is concluded that in the nematic phase of 6CHBT dipole–dipole correlations do not exist or are very weak, whereas for 5CB they are easily broken by a relatively low pressure. 相似文献
10.
Stanisław Urban Albert Würflinger Bo Gestblom Roman Dąbrowski Jan Przedmojski 《Liquid crystals》2013,40(3):305-311
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 Ad 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. 相似文献