Comportement sous pression de smectiques a bicouches

Liébert, L.; Daniels, W. B.

doi:10.1051/jphyslet:019770038016033300

Cited by 32 publications

(5 citation statements)

References 9 publications

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“…Table 1 includes for comparison the coefficients reported by Shashidhar and Venkatesh [9], and there is fair agreement except in the case of 7 CB. Both sets of data for 8 CB are reasonably consistent with the phase diagram for this substance reported by Liebert and Daniels [10].…”

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confidence: 89%

Nuclear magnetic resonance studies of liquid crystals under pressure

Wallis

Roy

1980

J. Phys. France

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The line width (ΔH) of the proton resonance has been measured as a function of temperature and pressure for four homologues of the alkyl cyanobiphenyl series (5, 6, 7 and 8 CB) and also for methoxy-cyanobiphenyl, p-azoxyanisole (PAA) and quinquephenyl. Values of the nematic order parameter (S2 = < P 2(cos θ)>) may be obtained from the results by dividing them by a scaling factor (ΔH/S2), which can in some cases be deduced theoretically and in other cases obtained by reference to magnetic susceptibility measurements at atmospheric pressure. Slight changes which we have observed in the shape of the resonance line as opposed to its width suggest that the mean conformation of the molecule is liable to vary with temperature and pressure in cases where it has a flexible end chain, though not otherwise. In principle the scaling factor (ΔH/S 2) may also vary with temperature and pressure in such cases, but the variation appears to be rather slight in practice. Our results for 5 CB and PAA are in general agreement with those reported by Horn and Faber and McColl respectively, who also used elevated pressures. The magnitude of S 2 at the nematic-isotropic transition (S2c) decreases on compression for 5 CB, 7 CB and 8 CB, but for 6 CB and the other substances we have investigated it appears to remain constant

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confidence: 89%

Nuclear magnetic resonance studies of liquid crystals under pressure

Wallis

Roy

1980

J. Phys. France

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“…36 A small change in the enthalpy at the A-N′ transition of 8CB has also been reported in many DSC studies. [22][23][24][25][26][27][28][29][30][31][32][33]35,[37][38][39][40][41][42][43][44][45][46][47][48][49][50] Because, for very weak transitions involving very small enthapy changes, DSC does not distinguish unambiguously between a latent heat anomaly and a specific heat anomaly, the detection, via careful density measurements, of a discontinuous volume change across the A-N′ transition is all the more important. These findings, together with evidence from high-pressure 13 and diamagnetic susceptibility 14 measurements and the existence of an A-N′ two-phase region for impure 8CB samples, 18,30,47 point to a weak first-order transition.…”

Section: Discussionmentioning

confidence: 99%

“…The picture from the de Gennes approach is consistent with mean-field molecular approaches 4-6 which predict a tricritical point 7,8 (a point at which a transition changes from first- to second-order) at T AN ‘ / T NI close to 0.89. Contradictory evidence on whether the A−N‘ transition is very weakly first-order, essentially second-order, or clearly second-order is encountered in experimental studies on p-cyanobenzylidene-p‘- n -octyloxyaniline (CBOOA), − p- n -octyloxy-p‘-cyanobiphenyl (8OCB), − and p- n -octyl-p‘-cyanobiphenyl (8CB), − three bilayer smectics. , On the other hand, experimental studies , on the hextyl, heptyl, and octyl members of the p,p‘-di- n -alkylazoxybenzene homologous series of liquid crystal gave results that are in agreement with the predictions of the molecular field theory as modified by Lee et al, while those of Brisbin et al and Le Grange and Mochel 20 on several liquid crystals gave results that agree with the prediction made by Lubensky and Chen 3 .…”

Section: Introductionmentioning

confidence: 99%

On the Nature of the Smectic A-to-Nematic Phase Transition of 8CB

Oweimreen

2001

J. Phys. Chem. B

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Density as a function of temperature was measured in the smectic A and nematic phases in the immediate vicinity of the smectic A-to-nematic transition temperature (T AN′ ) of p-n-octyl-p′-cyanobiphenyl (8CB). The very small volume change obtained at the smectic A-to-nematic (A-N′) phase transition of "pure" 8CB confirms that it is weakly first-order. There is also evidence that the A-N′ phase transition of pure 8CB takes place in two steps; the first is a very weak first-order transition, involving an extremely small volume change, to a phase of "intermediate" order which extends over about 0.07 K and then undergoes a second transition, which may or may not be second-order, to the nematic phase.

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“…In achiral liquid crystals, the nematic (N)-partial bilayer smectic A ðS A d Þ-reentrant nematic (N R )-monolayer smectic A ðS A 1 Þ phase sequence was first reported by Cladis [18] in a mixture of {p-[p-hexyloxybenzylidene-amino] benzonitrile} (HBAB) and [N-p-cyanobenzylidene-pn-octyloxyaniline] (CBOOA). It has also been observed [19][20][21] that the reentrance can be driven by increasing pressure in pure compounds and their mixtures. X-ray and microscopy studies showed [22,23] that the reentrant nematic phase is similar to the classical nematic phase but may coexist with crystalline fluctuations.…”

Section: Introductionmentioning

confidence: 90%

Pressure-driven reentrant phenomena in liquid crystals: the role of inverse layer spacing

Singh

Srivastava

2010

Liquid Crystals

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The occurrence of pressure-driven reentrant phenomena observed in high-pressure experimental studies in some achiral mesogenic materials has been explained using a thermodynamic model based on Landau-de Gennes theory. In this approach, the free-energy is expanded in terms of nematic, smectic A order parameters and the couplings (cubic and biquadratic) between them. The basic theme here is that the 'inverse layer spacing', which mimics an order parameter, becomes coupled to the nematic and smectic A order parameters. Secondly, in addition to the order parameter couplings the N-S A metastable temperature (which appears due to elimination of inverse layer spacing from the free-energy density expression) becomes pressure dependent. The occurrence of a pressure-driven reentrant nematic phase is explained in terms of these three pressure-dependent parameters. They all show smooth but rapid variation at the critical pressure beyond which nematic reentrance appears.

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Comportement sous pression de smectiques a bicouches

Cited by 32 publications

References 9 publications

Nuclear magnetic resonance studies of liquid crystals under pressure

Nuclear magnetic resonance studies of liquid crystals under pressure

On the Nature of the Smectic A-to-Nematic Phase Transition of 8CB

Pressure-driven reentrant phenomena in liquid crystals: the role of inverse layer spacing

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