Broad-Band Dielectric Spectroscopy of Liquid Crystals Confined in Random and Cylindrical Pores

1996 ◽  
Vol 464 ◽  
Author(s):  
G.P. Sinha ◽  
F.M. Aliev
Keyword(s):  
Liquid Crystals ◽  
Dielectric Spectroscopy ◽  
Dynamic Properties ◽  
Nematic Liquid Crystals ◽  
Broad Band ◽  
Low Frequency ◽  
Molecular Rotation ◽  
Retardation Factor ◽  
Short Axis ◽  
Cylindrical Pores

ABSTRACTUsing dielectric spectroscopy in the frequency range 0.1 Hz-1.5 GHz, we investigated the influence of confinement on the dynamic properties of polar nematic liquid crystals (LC) dispersed in porous matrices with randomly oriented, interconnected pores as well as in parallel cylindrical pores with different, pores sizes. The confinement has a strong influence on the dielectric properties of LC which resulted in the appearance of a low frequency relaxational process (f ≤ 10 KHz) absent in bulk and a strong modification of modes due to the molecular rotation around short axis and librational motion. The differences between bulk and confined behavior are: (a) - the dielectrically active modes in confined LC are not frozen even at temperatures about 20 degrees below the bulk crystallization temperature; (b) - in the temperature range corresponding to the anisotropie phase in pores, lnτ, where τ is the relaxation time corresponding to the molecular rotation around short axis, is not a linear function of 1/T and there is an evidence for smectic type order formation at sufficiently low T; (c) - the retardation factor g = τ/τis is ≃ 1.5, where as the typical value of g in bulk nematic liquid crystals is ≃ 4; (d) - smooth and small changes in T at phase transition in pores suggest that the “isotropie” phase of LC in pores is not bulk like isotropie phase with complete disorder in molecular orientations, and some degree of orientation order still persists.

Dielectric Relaxation in Filled Nematic Liquid Crystals

1999 ◽  
Vol 559 ◽  
Author(s):  
G.P. Sinha ◽  
M. Kreuzer ◽  
F.M. Aliev
Keyword(s):  
Liquid Crystal ◽  
Liquid Crystals ◽  
Nematic Liquid Crystals ◽  
Broad Band ◽  
Low Frequency ◽  
Slow Process ◽  
Hydrophobic Particles ◽  
Total Polarization ◽  
Tumbling Motion ◽  
Frequency Relaxation

ABSTRACTNematic liquid crystals filled with Aerosil particles are new heterogeneous materials important for different optoelectronic applications. These materials are suspensions of small silica particles, about 10-17 nm in diameter, dispersed in nematic liquid crystals. The particles are known to form a network structure dividing liquid crystal into domains with linear size approximately 250 nm. We used both hydrophilic and hydrophobic particles, filling them with the nematic liquid crystal-5CB.Broad band dielectric spectroscopy (1 mHz - 1.5 GHz) was applied for the investigation of these materials. Two bulk-like modes due to the rotation of molecules around the short axis and the tumbling motion were observed in filled 5CB. Additionally, a low frequency relaxation process and the dispersion of dielectric permittivity due to conductivity were also observed. The modification of the surface of the particles has stronger influence on the slow process and is less important for the molecular modes. The contribution of the slow process for the hydrophilic sample to the total polarization is greater than for the hydrophobic sample. In addition, the corresponding characteristic frequencies are lower for the case of hydrophilic samples. These facts suggest that the low frequency relaxation is an Aerosil particle-liquid crystal interface related phenomena and the origin of this process maybe explained on the basis of surface induced polarization.

Influence of Confinement on Molecular Reorientational Dynamics of Liquid Crystals: Broadband Dielectric Spectroscopy Investigations

2001 ◽  
Vol 699 ◽  
Author(s):  
Fouad M. Aliev
Keyword(s):  
Liquid Crystals ◽  
Dielectric Spectroscopy ◽  
Orientational Order ◽  
Relaxation Times ◽  
Thin Layers ◽  
Short Axis ◽  
Cylindrical Pore ◽  
Broadband Dielectric Spectroscopy ◽  
Librational Mode ◽  
Cylindrical Pores

AbstractBroadband dielectric spectroscopy has been applied for investigations of the dynamic behavior of liquid crystals (LCs) confined in porous matrices with random pores as well as in parallel cylindrical pores. We observed deep supercooling of LC in random pores. The relaxation times of the process due to the molecular rotation in deeply supercooled state are slower than at the temperatures corresponding to nematic phase by a factor of 106. This slowing down is accompanied by anomalous broadening of the dielectric spectra. For LC confined in cylindrical pores with homeotropic orientation on the pore walls we have investigated the relaxation of the librational mode. The dynamics of this mode is different from the behavior observed in investigations of relaxation due to reorientation of molecules around their short axis. The interpretation of the temperature dependencies of relaxation times of the librational mode needs the involvement of the temperature dependence of orientational order parameter. The investigations of the relaxation in thin LC layers formed on cylindrical pore walls show that the process due to rotation of molecules around their short axis (with single relaxation time for bulk LC) is the process with a distribution of relaxation times in thin layers and this process broadens with decreasing thickness of the layers.

TiO2 nanorod arrays induced broad-band reflection in chiral nematic liquid crystals with photo-polymerization network

2018 ◽  
Vol 46 (2) ◽  
pp. 210-218 ◽  
Author(s):  
Yang Liu ◽  
Dong Wang ◽  
Hong Gao ◽  
Jing-jing Zhang ◽  
Yuan Tao ◽  
...  
Keyword(s):  
Liquid Crystals ◽  
Nematic Liquid Crystals ◽  
Broad Band ◽  
Nanorod Arrays ◽  
Tio2 Nanorod ◽  
Photo Polymerization ◽  
Chiral Nematic ◽  
Tio2 Nanorod Arrays

The Influence of Boundary Conditions and Surface Layer Thickness on Dielectric Relaxation of Liquid Crystals Confined in Cylindrical Pores

2000 ◽  
Vol 651 ◽  
Author(s):  
Z. Nazario ◽  
G. P. Sinha ◽  
F.M. Aliev
Keyword(s):  
Liquid Crystal ◽  
Boundary Conditions ◽  
Temperature Dependence ◽  
Layer Thickness ◽  
Dielectric Spectroscopy ◽  
Relaxation Times ◽  
Thin Layers ◽  
Short Axis ◽  
Librational Mode ◽  
Cylindrical Pores

AbstractDielectric spectroscopy was applied to investigate the dynamic properties of liquid crystal octylcyanobiphenyl (8CB) confined in 2000 Å cylindrical pores of Anopore membranes with homeotropic and axial (planar) boundary conditions on the pore walls. Homeotropic boundary conditions allow the investigation of the librational mode in 8CB by dielectric spectroscopy. We found that the dynamics of the librational mode is totally different from the behavior observed in investigations of relaxation due to reorientation of molecules around their short axis. The interpretation of the temperature dependence of relaxation times and of the dielectric strength of the librational mode needs the involvement of the temperature dependence of orientational order parameter. For samples with axial boundary conditions, layers of LCs with different thickness were obtained on the pore walls as a result of controlled impregnation of porous matrices with 8CB from solutions of different liquid crystal concentration. The process due to rotation of molecules around their short axis with single relaxation time observed for bulk 8CB is replaced by a process with a distribution of relaxation times in thin layers. This relaxation process broadens with decreasing layer thickness.

EPR Study of the Temperature Dependence of Molecular Rotation in Nematic Liquid Crystals

1971 ◽  
Vol 12 (4) ◽  
pp. 335-344 ◽  
Author(s):  
C. F. Schwerdtfeger ◽  
M. Marušič ◽  
A. Mackay ◽  
R. Y. Dong
Keyword(s):  
Liquid Crystals ◽  
Temperature Dependence ◽  
Nematic Liquid Crystals ◽  
Molecular Rotation

scholarly journals A comparative study of nematic liquid crystals doped with harvested and non-harvested ferroelectric nanoparticles: phase transitions and dielectric properties

RSC Advances ◽  
2017 ◽  
Vol 7 (56) ◽  
pp. 35438-35444 ◽  
Author(s):  
Y. Lin ◽  
A. Daoudi ◽  
F. Dubois ◽  
J.-F. Blach ◽  
J.-F. Henninot ◽  
...  
Keyword(s):  
Phase Transitions ◽  
Liquid Crystal ◽  
Liquid Crystals ◽  
Dielectric Properties ◽  
Comparative Study ◽  
Dielectric Spectroscopy ◽  
Nematic Liquid Crystals ◽  
Broadband Dielectric Spectroscopy

Broadband dielectric spectroscopy is used to determine the dielectric properties and phase transitions of the 4-n-octyl-4′-cyanobiphenyl liquid crystal (8CB) doped with harvested and non-harvested ferroelectric nanoparticles.

Raman spectra and dynamic properties of molecules of nematic liquid crystals

1982 ◽  
Vol 37 (3) ◽  
pp. 1037-1042
Author(s):  
S. E. Yakovenko ◽  
V. I. Naumenko ◽  
A. A. Min'ko ◽  
N. M. Ksenofontova
Keyword(s):  
Liquid Crystals ◽  
Raman Spectra ◽  
Dynamic Properties ◽  
Nematic Liquid Crystals
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