High Pressure DTA Study on the Phase Behaviours in Some Selected Plastic and Liquid Crystals

Hartmann, Martina; Jenau, M.; Würflinger, A.; Godlewska, M.; Urban, S.

doi:10.1524/zpch.1992.177.part_2.195

Cited by 21 publications

(18 citation statements)

References 26 publications

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“…These transitions are also observed in DTA measurements 21 and Figure 2 compares the two. Only the transition temperatures for the isotropic-nematic phase transition agree well.…”

Section: Resultssupporting

confidence: 60%

Interaction Potential in Nematogenic 6CHBT

et al. 2008

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Pressure-volume-temperature (PVT) measurements were carried out on the nematic liquid crystal 4(trans-4′-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT). In combination with previous dielectric relaxation measurements at elevated pressure and new measurements extending to GHz frequencies, the characteristics of the anisotropic interaction potential were determined. The thermodynamic potential parameter, Γ, which measures the variation of the interaction energy with volume, equals 5.03 ( 0.06, with a barrier height equal to ∼7 kJ/mol. Thus, there is a low potential barrier to reorientations of the 6CHBT molecule about its short axis; however, the retarding potential is strongly volume-dependent. The longitudinal reorientational times determined for various thermodynamic conditions superpose using a scaling exponent equal to Γ within the experimental error. It then follows, as found recently for other liquid crystals, that this relaxation time must be constant along the pressure-dependent clearing line. Thus, the control parameter (e.g., Gibbs free energy) governing the competition between the anisotropic energy and the entropy must also govern the rotational dynamics in the ordered phase of this liquid crystal.

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“…These transitions are also observed in DTA measurements 21 and Figure 2 compares the two. Only the transition temperatures for the isotropic-nematic phase transition agree well.…”

Section: Resultssupporting

confidence: 60%

Interaction Potential in Nematogenic 6CHBT

et al. 2008

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“…4, where also previous data obtained with DTA [11] and high-pressure optical measurements [12] have been included. The crystal -smectic A and smectic A -nematic phase boundaries converge, resulting in a triple point at p = 300 MPa,…”

Section: Resultsmentioning

confidence: 99%

PVT Measurements on 4′-n-Octyl-Biphenyl-4-Carbonitrile (8CB) up to 300 MPa

Sandmann

Würflinger

1998

Zeitschrift Für Naturforschung A

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P, V m , Tdata have been measured for the smectic, nematic and isotropic phases of 4'-n-octyl-biphenyl-4-carbonitrile (8CB) in the temperature range 300-370 K and pressures up to 300 MPa. At atmospheric pressure all phase transitions appear to be of first order due to a discontinuity in the density. The volume change at the smectic A -nematic transition is only a tenth of the volume change at the clearing temperature. At moderate pressures below 80 MPa the S A -N transition could be detected as a discontinuity in the period of oscillation in measurements with a high-pressure vibrating tube densimeter. At higher pressures the discontinuity seems to die away, possibly indicating a change from first order to second order transition. From the volume changes and the slopes of the transition lines we calculate the enthalpy changes at the phase transition. The p, V m , T data enable us to calculate the volume part of the entropy and the molecular field parameter y=9ln 7 N i/3ln V NI .

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“…It seems worth to note that the sum of the transition enthalpies from the solid to the isotropic phase is nearly the same in both 4TCB and 5CB. For 6CHBT, a bit longer molecule [3] with the same CNS group as 4TCB (but with the hexane ring instead of benzene ring), the respective values are different, but the tendency observed is the same as for5CB, i.e. AH= 26.8 kJ/mol at melting is much larger than AH= 1.6 kJ/mol at the clearing point.…”

Section: Butyl-thiocyanobiphenyl (4tcb)mentioning

confidence: 93%

“…Contrary to that for liquid crystalline materials possessing the nematic phase, the enthalpy changes are much larger for the solid -nematic transition than for the nematic -isotropic transition. For example for 5CB [3,9] A//(Cr-N)= 13.4 kJ/mol and A//(N-I) = 0.33 kJ/mol. It seems worth to note that the sum of the transition enthalpies from the solid to the isotropic phase is nearly the same in both 4TCB and 5CB.…”

Section: Butyl-thiocyanobiphenyl (4tcb)mentioning

confidence: 99%

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High-pressure DTA Studies of the Phase Behaviors of 4-n -butyl-thiocyanobiphenyl (4TCB) and 4-w-pentyl-4’-n-phenyl-cyanocyclohexane (5HCP)

Massalska-Arodź

Würflinger

Büsinga

1999

Zeitschrift Für Naturforschung A

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DTA measurements of 4-/i-butyl-thiocyanobiphenyl (4TCB) and/?-cyano-/?'-pentylphenyl-cyclohexane (5HCP) have been performed in the temperature range 220 K-390 K and pressures up to 400 MPa. For 4TCP a transition from a crystalline to a liquid crystal phase (probably smectic E) could be detected at higher pressures > 90 MPa. The pressure dependence of the transition temperature has been established. At pressures lower than 88.7 MPa no transition of SmE into a crystal or into a glass has been found. For 5HCP only the melting curve was observed, in contrast to 5PCH, which displays a liquid crystalline nematic phase.

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High Pressure DTA Study on the Phase Behaviours in Some Selected Plastic and Liquid Crystals

Cited by 21 publications

References 26 publications

Interaction Potential in Nematogenic 6CHBT

Interaction Potential in Nematogenic 6CHBT

PVT Measurements on 4′-n-Octyl-Biphenyl-4-Carbonitrile (8CB) up to 300 MPa

High-pressure DTA Studies of the Phase Behaviors of 4-n -butyl-thiocyanobiphenyl (4TCB) and 4-w-pentyl-4’-n-phenyl-cyanocyclohexane (5HCP)

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