We study the orientation and order parameter of a liquid crystalline random side chain copolymer by 13C NMR. Evidence has previously been presented that this material forms a de Vries smectic A phase. The NMR data show that the molecular tilt angle in the smectic A phase is very small or zero and the smectic A–smectic C* transition is attributed predominantly to a change of the molecular tilt rather than azimuthal ordering. We discuss the NMR results in the context of earlier X‐ray and elastic characterizations of structurally similar materials. 相似文献
It is well known that, with respect to the director, nematic elastomers can be macroscopically aligned by uniaxial mechanical fields. Extending this method to a chiral smectic C elastomer, depending on the experimental set-up either smectic layer orientation or director orientation parallel to the stress axis occurs. In order to align the director and the smectic layers a biaxial mechanical field (e.g. shear field) consistent with the phase symmetry has to be used to achieve a macroscopically uniform orientation of the untwisted smectic C* structure. 相似文献
Summary: The orientation behavior of a smectic A elastomer is investigated by applying a biaxial mechanical field to the elastomer in a swollen state. The network is composed of a siloxane‐polymer backbone, a bi‐functional cross‐linker, and a monomer with a perfluorinated tail. In this work, biaxial deformation is successfully achieved to macroscopically orient the smectic A phase in a uniform, homeotropic fashion. We describe the orientation process in detail and discuss the microstructure of the smectic A phase organized in the monomer, the linear polymer, and the elastomer determined by using X‐ray diffraction data.
A photograph of the oriented smectic A elastomer E4F (above) and a schematic model of its homeotropic orientation (below). 相似文献
Abstract Detailed dielectric permittivity and relaxation investigations have been performed on compounds having different liquid-crystalline phases. At the smectic B–smectic A as well as the smectic G–smectic B transitions definite jumps were found in the dielectric relaxation times associated with rotation of the molecules around their short axis. For the interpretation of the large jumps in the relaxation times the change of the phonon spectra at the two dimensional crystal-two dimensional liquid phase transition was assumed. To verify this idea an inelastic neutron scattering study was performed. The measurements have proved the good orientation of the smectic A and smectic B phases. The values of the layer spacing, and the appearance of libron peaks for the smectic B phase at different momentum transfer were determined. 相似文献
Optical activity measurements in the isotropic phase of two antiferroelectric liquid crystal systems in which the chirality can be varied reveal unusual behaviour of the short range order. In one system the phase sequence as the chirality is increased is smectic A, smectic C*A, and smectic Q. In the other system the phase sequence is smectic C*, smectic C*A, and smectic Q as the chirality is increased. The short range order of the isotropic phase behaves similarly for these systems, showing mean field behaviour at low chirality and far above the phase transition, but deviating from this behaviour significantly as the chirality is increased and the phase transition is approached. These optical activity results indicate how different is the short range order in the isotropic phase for these antiferroelectric liquid crystal systems and demonstrates the crucial role played by chirality. Past theoretical work that includes smecticlike fluctuations in the calculation of short range order in the isotropic phase is capable of qualitatively explaining these results. 相似文献
A liquid‐crystalline (LC) phenylterthiophene derivative, which exhibited an ordered smectic phase at room temperature, was purified by vacuum sublimation under a flow of nitrogen. During the sublimation process, thin plates with sizes of 1 mm grew on the surface of the vacuum tube. The crystals exhibited the same X‐ray diffraction patterns as the ordered smectic phase of the LC state that was formed through a conventional recrystallization process by using organic solvents. Because of the removal of chemical impurities, the hole mobility in the ordered smectic phase of the vacuum‐grown thin plates increased to 1.2×10?1 cm2 V?1 s?1 at room temperature, whereas that of the LC precipitates was 7×10?2 cm2 V?1 s?1. The hole mobility in the ordered smectic phase of the vacuum‐sublimated sample was temperature‐independent between 400 and 220 K. The electric‐field dependence of the hole mobility was also very small within this temperature range. The temperature dependence of hole mobility was well‐described by the Hoesterey–Letson model. The hole‐transport characteristics indicate that band‐like conduction affected by the localized states, rather than a charge‐carrier‐hopping mechanism, is a valid mechanism for hole transport in an ordered smectic phase. 相似文献
The effect of crosslinking density on the phase behaviour of smectic liquid crystalline networks was studied; the results showed that crosslinking in their smectic phases can greatly enhance the stability of the liquid crystalline phase. The higher the crosslinking density, the higher the smectic-isotropic transition temperature. The mechanically-induced orientation was studied by polarized FTIR spectroscopy. The smectic liquid crystalline network could be oriented parallel to the mechanical field at higher draw ratio λ, while at lower λ the mesogenic groups are oriented perpendicular to the field for the networks with higher crosslinking density. The observed mechanically-induced orientation is interpreted by a proposed mechanism. 相似文献
We report a new possibility for liquid crystalline organic semiconductors. These materials exhibit smectic liquid crystalline phases, in which the molecules assume a smectic molecular order by self‐assembly. Because of the strong dispersion force among long alkyl chains, on cooling, smectic molecular order was retained at room temperature. A charge transport ability was also retained. The conductivity of a device having smectic liquid crystalline order is about 5×107 that of a device with no smectic order. The current?–?voltage characteristic of the device has a very sharp increase at low threshold voltage (5?V). A high carrier mobility of 1.8×10‐2 was observed in the smectic phase of one of the compounds studied (e). 相似文献
A homologous series of rod-like molecules with three phenyl rings in the core and terminal alkoxy chains are synthesised from mesogenic two-ring aldehyde by coupling with non-mesogenic 4-alkoxy anilines. The mesophase properties are evaluated with hot-stage optical polarising microscopy and differential scanning calorimetry, and accordingly, all the molecules exhibited enantiotropic nematic and smectic C phases along with monotropic low-temperature smectic phases. For a representative homologue, the existence of smectic C phase is further confirmed by noticing a sharp reflection at small angle region in powder X-ray diffraction which varies with change in temperature. The main focus of the investigation, however, is the clear demonstration of chemical shift assignment of static 13C NMR of a representative three-phenyl ring mesogen in smectic C phase. In this novel approach, the static 13C NMR spectral data of synthetic mesogenic intermediate namely two-ring aldehyde are utilised for the chemical shift assignment of three-ring mesogens. Further, the orientational order parameter of two-ring aldehyde in smectic A and three-ring mesogen in smectic C phase is carried out by measuring the 13C-1H dipolar couplings by 2D separated local field spectroscopy. 相似文献
Films of a symmetric liquid‐crystalline/isotropic block copolymer consisting of a smectic LC side‐chain polymer and polystyrene were prepared by solvent casting from solution and from the isotropic melt. By annealing the solvent‐cast film in the SA phase an oriented microphase‐separated film of lamellar morphology was obtained in which both the lamellae of the block copolymer and the smectic layers of the LC block were oriented parallel to the film surface. A lamellar morphology with perpendicular orientation of lamellae and smectic layers was generated by cooling the block copolymer from the melt. 相似文献
Electromechanical and electro-optical properties which result from an interplay of ferroelectricity and elasticity in slightly crosslinked ferroelectric liquid crystalline elastomers are studied in the vicinity of the smectic C-smectic A phase transition. In the chiral smectic C phase, the formation of the network during the crosslinking procedure stabilizes the polar orientation which is present. On heating the smectic C network into the chiral smectic A phase, an induced tilt is observed (mechanoclinic effect). The internal mechanical stress induced by the network formation appears to act like an external electric bias-voltage. The results are discussed in terms of a simple Landau model. 相似文献
Ferroelectricity and thus piezoelectricity are well known features of low molar mass chiral smectic C (Sc*) liquid crystals. We present a new technique to orientate macroscopically the director as well as the smectic layers of a Sc*-polymer network by applying a suitable mechanical field. The uniform orientation of the network film is checked by X-ray diffraction. According to the phase symmetry electromechanical measurements indicate the existence of a polar axis of the network and the direct as well as the inverse piezoelectric effect is demonstrated. 相似文献
Phase transition behavior of polyesters derived from 2,7-phenanthrene dicarbonic acid diethylester and alkanediols with even
methylene carbon number was investigated by the simultaneous DSC-XRD method. The smectic A phase was observed on cooling from
the molten state. The transition entropy from the isotropic state to the smectic A phase was about 9.0 J mol-1 K-1, which depended on the methylene carbon number. The linear expansion coefficients, based on the (001) spacing of the crystalline
phase at room temperature, were 1.3·10-4 K-1 (crystalline phase), 5.7·10-4 K-1 (crystallization region), 1.7·10-3 K-1 (smectic A phase) during cooling, and 1.5·10-4 K-1 (crystalline phase), and 1.0·10-3K-1(melting region) on heating.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
Unit cell parameters calculated from X-ray powder diffraction data are presented for the crystalline phase of a liquid crystal 4-butyloxyphenyl 4′-decyloxybenzoate: a = 23.098 (4) Å, b = 5.974 (6) Å, c = 12.357 (10) Å, β = 121.5283 (788)°, unit-cell volume V = 1453.56 Å3. Temperature dependent X-ray diffraction data confirmed the existence of smectic A and smectic C mesophases and a more ordered, tilted crystalline smectic phase. Possibility of existence of previously reported smectic B phase as well as another crystalline phase was refuted. 相似文献
The orientation of different segments of 4'-cyanophenyl 4-heptylbenzoate (7CPB) has been investigated using 13C NMR. The method of proton-encoded local field (PELF) spectroscopy in combination with off-magic-angle spinning (OMAS) of the sample was used. High resolution 2D spectra were obtained, from which the order parameters were calculated. Linear relations between the obtained order parameters and anisotropic chemical shifts determined by 1D 13C NMR were established and semi-empirical parameters were achieved. A 1:2 mixture of 7CPB and its chain-perfluorinated analogue (7PFCPB) showed interesting phase behaviour with the change of temperature. It was studied by the use of 13C NMR and polarizing optical microscopy. The order parameters of 7CPB in the smectic A phase of the mixture were calculated using the semi-empirical parameters obtained from the 2D NMR method. 相似文献
We study the orientation and order parameter of a liquid crystalline random side chain copolymer by 13C NMR. Evidence has previously been presented that this material forms a de Vries smectic A phase. The NMR data show that the molecular tilt angle in the smectic A phase is very small or zero and the smectic A-smectic C* transition is attributed predominantly to a change of the molecular tilt rather than azimuthal ordering. We discuss the NMR results in the context of earlier X-ray and elastic characterizations of structurally similar materials. 相似文献
Abstract Three acrylate side-chain polymers in which the mesogenic moieties are based on the 4-n-alkoxyphenyl-4′-(4″-methylhexyloxy) benzoates have been characterized by differential scanning calorimetry, optical microscopy and X-ray diffraction. For shorter flexible spacers (n = 2) both smectic A and C* phases are observed thus making this polymer interesting for the fabrication of electro-optical devices based on ferroelectric properties (a smectic A phase is required for alignment purposes). For longer flexible spacers, (n = 6, 11) only the smectic A phase remains. 相似文献