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1.
Langmuir-Blodgett films and cast films of a ferroelectric liquid crystal of sec-butyl-6-(4-(nonyloxy)benzoyloxy)-2-naphthoate have been fabricated. Their thermal behavior was investigated using infrared spectroscopy at elevated temperature combined with principal component analysis. The result shows a new phase transition from smectic A to nematic phase, compared to the phase sequence obtained by polarizing optical microscopy. Another solid transition of different isomeric crystals was also found, which was confirmed by calorimetric measurement.  相似文献   

2.
The spontaneous polarization and electro-optic response of ferroelectric liquid crystals (FLCs) were investigated in a cell fabricated with a polyimide alignment layer coated by the Langmuir-Blodgett method. The surface properties of the cured polyimide layers were monitored by contact angle measurement, and by FTIR spectroscopy and AFM for the orientation and surface roughness, respectively. The apparent spontaneous polarization of an FLC determined in a practical sandwich-cell depended on various conditions such as cell thickness, cooling rate from the smectic A to chiral smectic C phase, and deposition pressure. Electro-optic response and decay times of FLCs were also measured. Furthermore, the ions in the FLC mixture reduced the magnitude of the effective electric field, but had no effect at high frequency.  相似文献   

3.
Side-on liquid crystal polyacrylate in Langmuir-Blodgett films   总被引:1,自引:0,他引:1  
A side-on fixed liquid crystal polyacrylate has been investigated in Langmuir and Langmuir-Blodgett (LB) films. High in-plane orientation of the mesogenic groups has been observed within the LB multilayers, showing the ability of the LB technique to align a liquid crystal. The analysis of the in-plane order versus the dipping speed suggests some self-aggregation of the polymer in the monolayer. Within these aggregates, the molecular orientation (in-plane and out-plane) has been deduced from infrared dichroism experiments; the mesogenic group lies mainly parallel to the dipping direction and relatively flat on the substrate. This alignment of the polymer is however partially lost with time, leading to materials with less in-plane anisotropy.  相似文献   

4.
《Liquid crystals》1998,24(6):793-798
The influence of the surface treatment on the anisotropic part of the surface tension of a lyotropic discotic micellar nematic liquid crystal doped with ferrofluid is analysed. The treatment involves Langmuir-Blodgett (LB) films deposited on the glass substrates. Textural observations and phase-shift measurements of the transmitted light were carried out. A phenomenological model which allows calculation of the director average orientation in the bulk and at the surfaces is used for interpreting the experimental data. Our results show that the average director alignment depends on the concentration of the magnetic grains of ferrofluid at the glass surfaces. On the contrary, it is practically independent of the nature of the molecule deposited on the glass substrate. According to the model, the flat glass surface, independently of the treatment or the absence of any treatment, stabilizes onto itself a lamellar layer (or bilayer) formed by the amphiphilic molecules of the liquid crystal. This layer screens the effect of the LB film (or of the pure untreated substrate) on the alignment of the lyotropic nematic mesophase.  相似文献   

5.
The alignment of a nematic liquid crystal (5CB) induced by several substituted calixarene-based films is reported. Calixarene molecules consisting of four or six moieties and different substituents (acyl and azobenzene groups) were synthesized. Films of such molecules were deposited using a Langmuir-Blodgett technique onto glass plates and were characterized using atomic force microscopy. These treated plates were used to prepare liquid crystal cells, and the overall alignment studied. In the case of photosensitive molecules, photoinduced reorientation experiments were undertaken and are reported. It is shown that it is not important to have a large number of interacting sites on the surface to induce a reorientation of the liquid crystal.  相似文献   

6.
The alignment of a nematic liquid crystal (5CB) induced by several substituted calixarene-based films is reported. Calixarene molecules consisting of four or six moieties and different substituents (acyl and azobenzene groups) were synthesized. Films of such molecules were deposited using a Langmuir-Blodgett technique onto glass plates and were characterized using atomic force microscopy. These treated plates were used to prepare liquid crystal cells, and the overall alignment studied. In the case of photosensitive molecules, photoinduced reorientation experiments were undertaken and are reported. It is shown that it is not important to have a large number of interacting sites on the surface to induce a reorientation of the liquid crystal.  相似文献   

7.
X-ray reflectivity studies on Langmuir-Blodgett multilayer films of side-chain liquid crystal polymers are reported. The films have a high degree of lamellar order. The layer periodicity is independent of the type of monolayer deposition, implying a reorientation of the side group mesogens following the deposition process. X-ray reflectivity from thin films displays subsidiary maxima permitting a quantitative measure of the change in side-chain density between multilayer and monolayer. A unit cell density profile is calculated for thick films assuming a symmetric unit cell.  相似文献   

8.
The first single crystal structure of a Group VA halide salt with three equivalent long n-alkyl chains, benzyltrioctadecylammonium bromide (BzN18Br), is reported. It consists of alternating interdigitated and non-interdigitated regions of alkyl chains separated by ionic planes. Two chains per molecule are paired and extend to one side in a non-interdigitated region. The third chain is on the opposite side of the ionic plane and pairs intermolecularly to form an adjacent, interdigitated region. The thickness of two nearly extended molecules defines the bilayer unit-two ionic planes flanked by a region with intramolecularly paired chains and separated by an interdigitated chain region. Powder X-ray diffraction and optical microscopy data of liquid crystalline BzN18Br are consistent with an enantiotropic smectic A2 (SmA2) phase: the three n-alkyl chains of each molecule are projected from one side of an ionic plane, and head groups of neighbouring molecules are oriented head-to-head, in a non-interdigitated bilayer assembly. The structure of BzN18Br fills an important gap in our knowledge about the crystal packing of ammonium and phosphonium salts with one-four equivalent long n-alkyl chains. A comparison of their packing arrangements is made and the transitional nature of the BzN18Br structures is demonstrated. Although salts with one, two, or three long n-alkyl chains form SmA2 phases, each is distinctive in its molecular packing. A large molecular reorganization accompanies the crystal-to-liquid crystal transition of BzN18Br.  相似文献   

9.
A polymer film of polyvinylbutyral with dispersed droplets of a ferroelectric liquid crystalline mixture (FLC309c) has been prepared and characterized. The collective processes have been studied by dielectric relaxation spectroscopy in the frequency range 10 Hz to 13 MHz. In comparison with the FLC309c mixture, the polymer dispersed ferroelectric liquid crystal (PDFLC309c) based on FLC309c exhibits a Goldstone-like mode relaxation with a much higher relaxation frequency, but a smaller dielectric strength than the Goldstone mode observed for the FLC309c mixture. The spontaneous polarization of PDFLC309c decreases by nearly one order of magnitude in comparison with FLC309c, while the tilt angle decreases by 20%. Considering these results, we believe that a non-switching region exists near the polymer boundaries and that significant deformations of the helical structure occur due to stronger anchoring.  相似文献   

10.
Measurements of the complex dielectric permittivity in the frequency range 30 Hz-13MHz have been made for the chiral smectic C and smectic A phases of the mixture ZLI-3654. In the S*C phase a large contribution to the electric permittivity coming from the Goldstone mode was found. For the pretransitional region S*C-SA, only the soft mode has been detected. It is found that the molecular relaxation originating from the reorientation around the short axis is well separated from the soft mode even in the pretransitional region. The influence of different relaxation modes on the dielectric anisotropy is presented.  相似文献   

11.
《Liquid crystals》1999,26(11):1587-1590
Dielectric spectroscopy is a very useful method for investigation of the structure and dynamics of liquid crystals. However, with few exceptions, most investigations have been only in the linear regime. In this note we present a simple method for extraction of the non-linear contributions to the dielectric constant and as an example give the results obtained for a ferroelectric liquid crystal.  相似文献   

12.
A study of dichroic dye-liquid crystal mixtures (guest-host systems) in monolayers formed at a gas-liquid interface (Langmuir films) and at a solid surface (Langmuir-Blodgett films) has been made. As a host 4- n -octyl-4'-cyanobiphenyl (8CB) or 4- n -pentyl-4"-cyano- p -terphenyl (5CT) were chosen, while three dichroic azo dyes with various molecular structures were used as guest species. The dyes were added to the liquid crystal matrices at a concentration corresponding to the whole range of molar fractions and the surface pressure-mean molecular area isotherms for Langmuir films were recorded. On the basis of the isotherms, conclusions about the molecular organization and the miscibility of the components in the ultrathin films were drawn. The Langmuir films were transferred onto the quartz plates at surface pressures below the collapse point. The polarized absorption spectra of the Langmuir-Blodgett films were recorded and information about the alignment and intermolecular interactions in the mixtures of the non-amphiphilic dichroic dyes and the liquid crystals with strongly polar terminal groups were obtained.  相似文献   

13.
A study of dichroic dye-liquid crystal mixtures (guest-host systems) in monolayers formed at a gas-liquid interface (Langmuir films) and at a solid surface (Langmuir-Blodgett films) has been made. As a host 4- n -octyl-4′-cyanobiphenyl (8CB) or 4- n -pentyl-4″-cyano- p -terphenyl (5CT) were chosen, while three dichroic azo dyes with various molecular structures were used as guest species. The dyes were added to the liquid crystal matrices at a concentration corresponding to the whole range of molar fractions and the surface pressure-mean molecular area isotherms for Langmuir films were recorded. On the basis of the isotherms, conclusions about the molecular organization and the miscibility of the components in the ultrathin films were drawn. The Langmuir films were transferred onto the quartz plates at surface pressures below the collapse point. The polarized absorption spectra of the Langmuir-Blodgett films were recorded and information about the alignment and intermolecular interactions in the mixtures of the non-amphiphilic dichroic dyes and the liquid crystals with strongly polar terminal groups were obtained.  相似文献   

14.
The homogeneous alignment of a liquid crystal material, 4'-n-octyl-4-cyanobiphenyl (8CB), was achieved by polyimide Langmuir-Blodgett (LB) films. Scanning electron microscopy and scanning tunnelling microscopy measurements show that the alignment of the polyimide LB films with a grooveless surface occurs due to the orientation of the polyimide chains. We directly observe 8CB monolayers on oriented polyimide LB films. We find that the monolayers form a two dimensional crystalline structure and the 8CB molecules are always aligned along the chain direction of the polyimide. The results show that the alignment of 8CB molecules arises by an epitaxial growth on the oriented polyimide LB films.  相似文献   

15.
Abstract

The homogeneous alignment of a liquid crystal material, 4′-n-octyl-4-cyanobiphenyl (8CB), was achieved by polyimide Langmuir-Blodgett (LB) films. Scanning electron microscopy and scanning tunnelling microscopy measurements show that the alignment of the polyimide LB films with a grooveless surface occurs due to the orientation of the polyimide chains. We directly observe 8CB monolayers on oriented polyimide LB films. We find that the monolayers form a two dimensional crystalline structure and the 8CB molecules are always aligned along the chain direction of the polyimide. The results show that the alignment of 8CB molecules arises by an epitaxial growth on the oriented polyimide LB films.  相似文献   

16.
One cause of the hysteresis in the voltage-transmittance curve is polarization in the alignment layers brought about by the spontaneous polarization of FLC molecules. The hysteresis was reduced by increasing the conductivity of the alignment layers. The structural changes on switching were examined by measuring time-resolved shifts of FT-IR peaks of selected groups. The shifts of selected groups in the IR peak can be attributed to changes in the local dielectric constants of the ambient environment during switching by molecular orbital calculations. The orientation on switching in the SmC* phase was found to be similar to that in the N* phase.  相似文献   

17.
We have measured the d.c. electric field dependence of the birefringence and conoscopic images for the smectic C* phase of a partially racemized ferroelectric chiral smectic liquid crystal CE-8. The experiments were performed using 50mu m thick homeotropic cells with lateral electrodes which created a d.c. electric field parallel to the smectic layers. The observed field induced birefringence shows a characteristic step-like behaviour which is due to the stepby-step unwinding of the helical structure in a sample with finite dimensions along the helical axis. In conoscopic observations we observe that these steps are associated with moving disclination lines that traverse the sample in the direction of the smectic layers. The observed electric field dependence of the birefringence is discussed in terms of the soliton-like unwinding of helical smectic structures and compared with the predictions of the Landau theory. A qualitatively good agreement is obtained.  相似文献   

18.
《Liquid crystals》1997,23(5):659-666
We synthesized novel fluorine-substituted chiral compounds having a fluorine atom at an asymmetric carbon and a difluorinated biphenyl ring as a core of a liquid crystal molecule by means of an original method. The ferroelectric mixtures were obtained by mixing the chiral compounds with the non-chiral liquid crystal mixture with a wide SmC temperature range between 29 C (crystallized) and 78 C. They show a large P s greater than 10 nC cm -2 at 5 wt% of the chiral compound. Two types of cell, called 'parallel' and 'antiparallel', were fabricated according to the relative direction of the rubbing direction on the substrates. The parallel cells filled with the FLC mixtures show the usual bistable SSFLC (surface-stabilized ferroelectric liquid crystal). The fast response time of 60mus (22V pp pulse width 250mus, at room temperature) was obtained. The apparent cone angle was 45.2 in the switching state and 40.5 in the memory state. On the other hand, the antiparallel cells show an unusual monostable behaviour, i.e. the director falls back to the original configuration when the applied voltage is switched off. The surface-stabilized monostable cells show very attractive characteristics for application for a TFT-active matrix LCD; a high contrast of 81 :1, a fast response time (of the order of 1ms), and an analogue-grey scale with excellent linearity within the low voltage range below 4 V.  相似文献   

19.
Langmuir (L) and Langmuir-Blodgett (LB) films of 3,4,9,10-tetra(heptyloxycarbonyl)perylene and its binary mixtures with 4-octyl-4'-cyanobiphenyl (8CB) and 4-pentyl-4'-cyano-p-terphenyl (5CT) have been studied. On the basis of the surface pressure-mean molecular area isotherms of L films, the alignment of the molecules on the air-water interface has been estimated. The L films were transferred onto quartz plates at surface pressures below the collapse point. The absorption and fluorescence spectra of LB films, obtained using unpolarized and linearly polarized light, were recorded. The results obtained have led to conclusions on the arrangement of the dye and liquid crystal molecules on the air-solid substrate interface. The fluorescence spectra revealed the formation of excited dimers in LB films.  相似文献   

20.
Abstract

The excitation of optical modes is used to study the optical tensor configuration in a thin ferroelectric liquid crystal layer, cooled from the initially homeotropically aligned nematic phase. By monitoring the angular dependent reflectivity for plane polarized radiation coupled into the guided modes in the smectic C* layer and subsequently fitting the recorded data to predictions from multilayer optics theory, the optical tensor configuration in the layer is fully evaluated. Iteratively modelling the full tilt/twist profile in the cell, progressively converging the predicted reflectivity to experimental data, gives a complete and very well specified picture of the optical tensor throughout the cell. By studying the cell at various temperatures, the temperature dependence of the tilt of the major axis of the optic tensor (which may be related to the cone angle if the smectic layers are parallel to the cell surface) has been established. The temperature dependent optical dielectric constants have also been obtained.  相似文献   

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