摘要:

The liquid crystal (LC) alignment capability in 4-n-pentyl-4′-cyanobiphenyl (5CB) on polyimide (PI) Langmuir-Blodgett (LB) surface with alkyl chain lengths was investigated. The pretilt angle of 5CB increased by unidirectional rubbing treatment on PI-LB surface. The induced optical retardation on PI-LB surface increased by the rubbing. The extrapolation length deof 5CB for rubbed PI-LB surfaces with even-number is small, compared with odd-number near the clearing temperature Tc. The extrapolation length deof 5CB with odd-number increases gradually as the temperature increases but it tends to diverge near the Tc. Therefore the polar anchoring strength with even-number is strong because of relatively high surface ordering caused by more crystalline surfaces. The anchoring energy of 5CB with alkyl chain length of 7 carbons is approximately 1 × 10−3(J/m2); it indicates the strong anchoring strength. In consequence, we suggest that the LC aligng capability in NLC on PI-LB surface is strongly related to the characteristics of the polymer.

ISSN:1058-725X    

DOI:10.1080/10587250008031028

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

1H and13C Fourier transform-nuclear magnetic resonance (FT-NMR) and mass spectrometric (MS) techniques were used to prove the structures of precursors used in the sol-gel synthesis of the high-frequency dielectric Ba(Mg1/3Ta2/3)O3. Various metal methoxyethoxides (MEO) of Ba, Ta, Mg-Ta and Ba-Mg-Ta systems were prepared in 2-methoxyethanol (MeOEtOH). The tantalum and the magnesium-tantalum precursors were determined to be Ta2(MEO)10and MgTa2(MEO)12, respectively. Similar spectroscopic studies for “Ba3MgTa2(MEO)15” indicated that the Ba-Mg-Ta alkoxide precursor has a polymer structure of (Ba, Mg, Ta)(MEO) complex and contains the bridging alkoxy groups between Ta and the alkaline earth metals.

ISSN:1058-725X    

DOI:10.1080/10587250008031029

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Systematic studies on the sorption and diffusion behaviors of gases in low-molar-mass liquid crystalline substances (LCs) have been performed. The experiments were carried out in different gas atmospheres over temperature ranges which covered the phase transitions of the LCs under investigation, and at pressures of up to 15 bar. The equilibrium concentration and the diffusion coefficient of the sorbed gases were found to be strongly dependent on the LC phase state, gas type and gas pressure and temperature. Furthermore, the influence of gases on the phase transition temperatures of LCs and the reversibility of gas sorption have been studied.

ISSN:1058-725X    

DOI:10.1080/10587250008031030

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Polymers with cyanobiphenyl groups (PCBAn) in their side-chains with different spacer lengths (n=2, 6, 11) and different molecular weights (Mw), were prepared. The phase behavior of the polymers and that of liquid-crystalline mixtures (LCMn) consisting of PCBAn and low-molecular weight liquid crystal, were observed. LCM2 and LCM6 showed liquid-crystalline property of nematic phase, while LCM11 showed liquid-crystalline property of smectic phase. The electrorheological properties of LCMn were investigated. The viscosity of LCMn increased upon application of a DC electric field, and the degree of increase was characteristic to each n value. LCM6 of different Mw showed nearly the same electrorheological behavior except for the LCM6 of lowest Mw, which displayed a large increase in viscosity upon application of the electric field.

ISSN:1058-725X    

DOI:10.1080/10587250008031031

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The synthesis and the phase behavior of stilbazoles and corresponding N-oxides containing different positions of heterocyclic N atoms, i.e. trans-2′-, 3′-, 4′- stilbazoles and their N-oxides, are reported. Their phase transition temperatures and the mesogenic behavior were investigated through the DSC, polarizing optical microscope and X-ray diffraction (XRD) measurements. Increasing dipole moments by oxidation of stilbazoles to analogous N-oxides induces the SAphase with an interdigitated bilayer packing. Both 2′-stilbazoles and analogous N-oxides do not possess mesogenic phases, which may be due that π electron polarizabilities are reduced by the molecular twist originated from dipoles along the ortho-direction. Besides, the positonal effects of the N-oxides on mesogenic properties are more distinct than those of analogous stilbazoles, and new mesogenic phases have been generated by tuning the position of the N-oxide function in the charged liquid crystals. Consequently, unique mesogenic properties may be introduced through adjusting the dipolar direction and strength of the heterocyclic atoms in the molecules.

ISSN:1058-725X    

DOI:10.1080/10587250008031032

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The crystal structure of 1:1 inclusion complex of hexyltrimethylammonium bromide (6TAB) and rac-1, 1′-bi-2-naphthol (BNP) was analysed by the X-ray diffraction method. It was shown that the bromide anion participates in a hydrogen bond with one of the OH groups of BNP (Ol···Br, 3.22(1)Å). The bromide anion also plays a role as the hydrogen acceptor of the OH group of the water molecule (OW···Br, 3.21(3)Å). A strong hydrogen bond was found between the other OH group of BNP and oxygen of the water molecule (O2···OW, 2.63(3)Å). The short C-H···C distances between hydrogen atoms of the alkyl chain together with the methyl groups and carbon atoms of the aromatic rings suggested C-H···π interactions. The atomic distances shorter than the sum of the van der Waals radius (2.97Å) were C10···H6 2.85Å, C14···H9 2.83Å, and C16···H11 2.88Å. The X-ray powder diffraction studies on the complex suggested that the complex crystals can be obtained by crystallization from the solution and also by mixing powdered samples in a mortar.

ISSN:1058-725X    

DOI:10.1080/10587250008031033

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In this paper we report the measurements of the thermally induced depolarization currents in a metallorganic liquid crystal. The depolarization current dependence on temperature has been interpreted in term of an internal electric field set up during the cooling down of the sample under a d.c. external applied voltage. The transition temperature shift verifies a Clausius-Clapeyron type equation also giving evidence of the saturation of the polarization. By analyzing the experimental data, we can hypothesize that there is a double order in the system: the nematic order, concerning the director a which behaves quite normally, and an electric order, concerning the electric dipoles. Whereas the first type of order induces a phase in the system (the nematic phase) it is doubtful that the second ones induces a real phase in the system.

ISSN:1058-725X    

DOI:10.1080/10587250008031034

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Novel binary, ternary and four component mixtures of (S)-4-(2-chloro-3-methylbutanoyloxy)phenyl-4-[(n-alkoxy)benzoates] and 4-n-alkoxyphenyl 4-n-alkoxybenzoates were prepared exhibiting SmC* phase in a wide range near to room temperature. Phase diagrams and physical properties e. g. electro-optical response, spontaneous polarization and switching time have been determined. The appearance and disappearance of different mesophases has been studied.

ISSN:1058-725X    

DOI:10.1080/10587250008031035

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Due to the existence of a macroscopic helicoidal structure, cholesteric liquid crystals exhibit many remarkable properties like selective light reflections. Bragg reflections occur when the helix pitch is of the order of the wavelength of the incident light propagating along the helix axis. The wavelength bandwidth is due to the medium birefringence and is typically limited to 50 nm, which is insufficient for some applications (full-colors displays, for example). The purpose of this paper is to show that, by favoring a pitch gradient in the helix during a novel experimental process, it is possible to get a cholesteric glassy material whose spectral characteristics evolve from a selective to a broad-band filter. Light reflection occurs on several hundreds of nanometers and several intermediary optical states are available during a thermal treatment. From a reciprocal viewpoint, the inverse transition, from a broad- to a narrow-band filter, is also demonstrated. Such a process is driven by one parameter: the annealing temperature. Then, the optical properties are permanently stored by quenching the viscous material to a glass at room temperature. The two steps, pitch gradient establishment and film hardening, are independently controlled. Different possible stages occuring during the thermal history of the sample are discussed. The very glassy state of these reflectors would give new opportunities for optical data storage.

ISSN:1058-725X    

DOI:10.1080/10587250008031036

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Polarized Fourier transform infrared (FTIR) and optical microscopy with crossed polarizer were used to investigate the thermal stability of the alignment layers formed by rubbing and polarized UV (PUV) irradiation on the polyimide (PI) films. The thermal stability of the PUV irradiated PI alignment layer is lower than that of the rubbed PI layer due to the fragmentation reaction of the PI by PUV. We also investigated the alignment of the liquid crystal by rubbing or PUV irradiation. Liquid crystal (LC) molecules align parallel to the predominant orientation of the PI and perpendicular to the PUV irradiation direction.

ISSN:1058-725X    

DOI:10.1080/10587250008031037

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor