Fast molecular reorientations in liquid crystals

Lalanne, J. R.; Destrade, C.; Nguyen, H. T.; Marcerou, J. P.

doi:10.1103/physreva.44.6632

Cited by 26 publications

(14 citation statements)

References 44 publications

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“…Our results support the dielectric spectroscopy experiments reported by Vallerien and Kremer et al [38,39], as well as the transient Kerr experiments reported by O'Brien et al [34]. The slowing down of the rotation around the long axis reported by Lalanne et al [36,40], was not detected by our C-13 NMR study. C-13 TIP measurements with MAS were performed for nonoriented (9-MONBIC and (S, R)-MONBIC in the smectic phases.…”

Section: Fm(t)fm(t) = Cfjk"'(t)g~~'(t)h~~'(t) (9)supporting

confidence: 82%

“…Therefore, we can assume TI = w in the spectral densities. Lalanne et al have investigated fast individual molecular reorientations in the SA phase in the vicinity of the S A to Sz transition by means of the degenerate four-waves-mixing technique [36]. The relaxation time of the motion in the SA phase of ZLI3488 is estimated to be about 30ps.…”

Section: Fm(t)fm(t) = Cfjk"'(t)g~~'(t)h~~'(t) (9)mentioning

confidence: 99%

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Microscopic organization of molecules in smectic A and chiral (racemic) smectic C phases: Dynamic molecular deformation effect on the S_Ato S_C∗ (S_C) transition

Yoshizawa¹,

Kikuzaki

Fukumasa³

1995

Liquid Crystals

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We investigated the correlation between orientational order and microscopic organization of the molecules in smectic A and chiral (racemic) smectic C phases by means of solid-state C-13 NMR, powder X-ray diffraction, and electrooptical measurements. The compounds under investigation are 4-((S)-2-methyloctanoyl)phenyl 4'-nonylbiphenyl-4-carhoxylate ((5')-MONBIC) and its corresponding racemic compound ((S, R)-MONBIC). Static C-13 NMR indicates that: (1) the orientational angle of the tail with respect to the magnetic field decreases slightly both in the SA and S; phases as decreasing temperature, and ( 2 ) the angle of the core with respect to the field decreases in the SA phase but increases in the S$ phase as decreasing temperature. Analysis of C-13 TI reveals that the dynamic molecular deformation for the core part can occur near the transition. We discuss the dynamic molecular deformation in comparison with the reorientation of the director at the SA to S; transition. Based on the experimental results, we propose the structural model in which describes the microscopic organization of the molecules in the mesophases.

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Section: Fm(t)fm(t) = Cfjk"'(t)g~~'(t)h~~'(t) (9)supporting

confidence: 82%

Section: Fm(t)fm(t) = Cfjk"'(t)g~~'(t)h~~'(t) (9)mentioning

confidence: 99%

Microscopic organization of molecules in smectic A and chiral (racemic) smectic C phases: Dynamic molecular deformation effect on the S_Ato S_C∗ (S_C) transition

Yoshizawa¹,

Kikuzaki

Fukumasa³

1995

Liquid Crystals

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“…Using a degenerate four-wave mixing technique, Lalanne et al presented contradicting results [8,9]: While for an achiral compound and a racemic mixture the individual reorientation time around the molecular long axis was found to be unchanged down to the SA-Sz transition, it increased by a factor of about 3 in the chiral compound. But these experiments could not be confirmed in transient optical Kerr measurements by O'Brien et al [lo].…”

Section: Low Molecular Weight Flcmentioning

confidence: 88%

Collective and molecular dynamics in low molar mass and polymeric ferroelectric liquid crystals

1993

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By use of a novel sample preparation technique, it is possible to measure with one sample cell the collective and molecular dynamics (lo-' Hz-10' Hz) of macroscopically (in bookshelf geometry) oriented ferroelectric liquid crystals. Below lo6 Hz, two ferroelectric modes, Goldstone-and soft-mode, are observed, being assigned to fluctuations of the phase and amplitude of the helical superstructure respectively. Between lo6 and lo' Hz, one dielectric loss process exists, the 8-relaxation which originates from the librational motion (hindered rotation) of the mesogen around its molecular long axis. This process does not split or broaden at the non-ferroelectric-ferroelectric phase transition and it has an Arrhenius type temperature dependence. In comparing a racemic mixture with a chiral sample, it performs a similar frequency and temperature dependence. The experimental findings for the 8-relaxation are in qualitative contrast to the predictions of the (generalized) Landau expansion of the free energy at the non-ferroelectricferroelectric phase transition. The experiment also leads to a modified understanding for the molecular origin of ferroelectricity in FLCs. IntroductionThe collective and molecular dynamics [l, 21 in ferroelectric liquid crystals (FLCs) is characterized by two dynamic regimes: Below lo6 Hz, the ferroelectric modes are observed, the Goldstone-and the soft-mode which are assigned to fluctuations of the phase and the amplitude of the helical superstructure, respectively. These processes give rise to huge dielectric losses which determine their collective character. In the frequency range from lo6 Hz to lo9 Hz, /3-relaxation C2-41 is found which is assigned to the libration (hindered rotation) of the mesogen around its molecular long axis. As a local (non-collective) relaxation process it has, compared to the ferroelectric modes, a much weaker dielectric strength. It obeys an Arrhenius-type temperature dependence and is nearly unaffected by the non-ferroelectric-ferroelectric phase transition.The (generalized) Landau expansion of the free energy at the non-ferroelectricferroelectric phase transition predicts well the collective dynamics (< lo6 Hz) in FLCs [5, 61. In the frequency regime from lo6 Hz to lo9 Hz, two polarization modes are predicted in the (ferroelectric) S z phase which are degenerate in the (non-ferroelectric) SA phase. These processes are supposed to change their temperature dependence at the SA-S,* phase transition, resulting in a non-Arrhenius temperature behaviour for which the relaxation rate increases with decreasing temperature.

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“…this can contribute to clarifying the existing controversy regarding the possible existence of a slowing down of this rotation at the SA-SE transition, as claimed by Lalanne et al [12].…”

Section: Introductionmentioning

confidence: 86%

Molecular dynamics study of the ferroelectric liquid crystal CI IPNOC by proton spin-lattice relaxation

Ferraz¹,

Figueirinhas²,

Sebastião³

et al. 1993

Liquid Crystals

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Proton spin-lattice relaxation studies were carried out in the S, and S z phases of the liquid crystal C1 IPNOC using both conventional and fast field cycling NMR techniques. Ti dispersion curves were obtained at two different temperatures for each mesophase covering frequencies from lo2 to 3 x 10' Hz. In both mesophases the TI data can be described assuming the presence of three different relaxation mechanisms, namely local molecular rotations, molecular self-diffusion and collective motions. The self-diffusion constant D, was evaluated for several temperatures and the activation energy associated with the diffusion process was obtained. The expected contribution of the soft-mode for the spin-lattice relaxation could not be separated from the contribution of other collective motions. The correlation times associated with the rotations around the molecular long axis and with the fluctuations of this axis were evaluated for both the S, and the S z phases.

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Fast molecular reorientations in liquid crystals

Cited by 26 publications

References 44 publications

Microscopic organization of molecules in smectic A and chiral (racemic) smectic C phases: Dynamic molecular deformation effect on the S_Ato S_C∗ (S_C) transition

Microscopic organization of molecules in smectic A and chiral (racemic) smectic C phases: Dynamic molecular deformation effect on the S_Ato S_C∗ (S_C) transition

Collective and molecular dynamics in low molar mass and polymeric ferroelectric liquid crystals

Molecular dynamics study of the ferroelectric liquid crystal CI IPNOC by proton spin-lattice relaxation

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Fast molecular reorientations in liquid crystals

Cited by 26 publications

References 44 publications

Microscopic organization of molecules in smectic A and chiral (racemic) smectic C phases: Dynamic molecular deformation effect on the SAto SC∗ (SC) transition

Microscopic organization of molecules in smectic A and chiral (racemic) smectic C phases: Dynamic molecular deformation effect on the SAto SC∗ (SC) transition

Collective and molecular dynamics in low molar mass and polymeric ferroelectric liquid crystals

Molecular dynamics study of the ferroelectric liquid crystal CI IPNOC by proton spin-lattice relaxation

Contact Info

Product

Resources

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Microscopic organization of molecules in smectic A and chiral (racemic) smectic C phases: Dynamic molecular deformation effect on the S_Ato S_C∗ (S_C) transition

Microscopic organization of molecules in smectic A and chiral (racemic) smectic C phases: Dynamic molecular deformation effect on the S_Ato S_C∗ (S_C) transition