Cyanobiphenyl Liquid Crystals Research Articles

Coupling between molecular vibrations and liquid crystalline order parameters.

Specific Raman active modes in two prototype cyanobiphenyl liquid crystals are shown to display a temperature dependent softening proportional to either the nematic or smectic order parameters, while other vibrations (like the C identical withN stretch mode) remain unaltered. This selective coupling between intramolecular vibrations and the liquid crystalline order is related to the intrinsic symmetry of the modes. The method provides a simple, microscopic, noninvasive optical technique with which the liquid crystalline order parameters can be qualitatively mapped out.

Photoinduced Disordering Dynamics of a Cyanobiphenyl Liquid Crystal Containing Quinizarin

Photoinduced changes in a liquid crystal (LC) alignment were measured for 4-octyloxy-4′-cyanobiphenyl containing quinizarin. Kinetic analyses were carried out for both photoinduced response and thermal relaxation of the LC alignment in the temperature range from 50°C to 78°C. Thermal relaxation curves of birefringence after termination of Ar+ laser irradiation show only one component in the smectic A phase, and two components in the nematic phase, revealing the microscopic environment of dye molecules in LC solvents. This change does not exactly correspond to the macroscopic phase transition from smectic to nematic, which suggests interaction between LC mesogens and dye molecules due to photoexcitation. An unrecoverable portion of birefringence for thermal ordering/relaxation processes after termination of photoirradiation was observed in the smectic phase measured in the course of heating process.

Robust liquid crystal droplets

The preparation and properties of cyanobiphenyl liquid crystal droplets encapsulated by the polymerizable lecithin 1,2-bis(10,12-tricosadiynoyl)-sn-glyero-3-phosphocholine (DC8,9PC) are described. Under a wide variety of preparation conditions the droplets obtain a diameter of approximately 10 mum. These droplets are stable for periods of over one year at room temperature. Furthermore, they are stable upon temperature cycling between the nematic and isotropic phases and between the smectic A to nematic to isotropic phase transitions.

Effect of encapsulation in MCM-41 type molecular sieves on vibrational spectra of liquid crystalline state

The paper deals with the investigation of IR absorption spectra of a pure 4- n-pentyl-4′-cyanobiphenyl (5CB) liquid crystal in nematic liquid crystalline (LC) state, pure aluminosilicate molecular sieves of MCM-41 type (with channels about 40 Å in diameter) and their heterogeneous binary systems. It was shown that 5CB molecules are able to interact with active centers in the MCM-41 channels forming sufficiently strong hydrogen bonds of the —CN⋯H—O— type. This conclusion was confirmed by the data obtained from DSC measurements. Relative amount of the 5CB molecules interacting with the channel walls and that of the molecules retaining the LC state when encapsulated in molecular sieves MCM-41 and CuMCM-41 were estimated.

Applicability of 8OCB for temperature calibration of temperature modulated calorimeters

Calibration of temperature modulated calorimeters (TMC) is essential because it is often not possible to calculate heat capacity and sample temperature directly from the quantities measured. The question arises whether temperature calibration used in conventional calorimetry is also valid for TMC’s. To prove this, a well defined transition is needed which does not disturb the temperature profile within the sample during modulation. The response of the whole system (calorimeter, sample, transition) must be linear. Some low energy liquid crystal transitions behave in this way, under certain conditions, and can be used for temperature calibration in TMC’s. The procedure is demonstrated for the nematic to smectic-A transition in a cyanobiphenyl liquid crystal (8OCB). The applicability of 8OCB for temperature calibration is shown for temperature modulated DSC, AC calorimeter, 3 ω method and photo-acoustic method.

Fluorescence study on a physical gel of a cyanobiphenyl liquid crystal

Fluorescence study on a physical gel of a cyanobiphenyl liquid crystal

Disorder effects in low concentration dispersions of small quartz spheres in cyanobiphenyl liquid crystals

A photopyroelectric study of the specific heat and the thermal conductivity of small quartz sphere (aerosil) dispersions in heptylcyanobiphenyl (7CB) and octylcyanobiphenyl (8CB) liquid crystals with a concentration rho(s)=0.005 g cm(-3) is reported. The thermal conductivity data show that, even at this low concentration, the elastic strain introduced in the liquid crystal by the aerosil particle is not completely annealed at the nematic-isotropic (NI) phase transition. In 8CB, annealing has been found at the smectic-nematic phase transition, while in the case of 7CB it occurs in the nematic phase also. Moreover, the depression of the NI transition temperature has been found to be much smaller than the one predicted by a random field model in which quenched disorder simply affects the average order of the sample and no effects of the elastic strain are considered.

Anchoring characteristics and interfacial interactions in a polymer dispersed liquid crystal: a molecular dynamics study

Molecular dynamics (MD) simulations were carried out to gain an understanding of the interfacial interactions and the phase separation process in a polymer dispersed liquid crystalline (PDLC) system. The most important components of the PDLC system of interest are: E7 (an eutectic mixture of cyanobiphenyl liquid crystals), a crosslinked polyacrylate, and octanoic acid (OA). The miscibility of the different components was investigated and the anchoring characteristics of 4- n-pentyl-4′-cyanobiphenyl (5CB) molecules on the penta-acrylate polymer surface were analyzed. The effects of introducing a surfactant on the miscibility and the anchoring strengths were also studied. Bilayers of polymer and liquid crystal (LC) of different alignments, show that an amorphous 5CB structure on the polymer surface was energetically most favorable, whereas homeotropic alignment had the next higher energy and planar alignment was the least favorable. Calculated solubility parameters indicate that prior to polymerization, the prepolymer and LC are miscible and OA is equally immiscible with both of them. Upon polymerization, the polymer is no longer miscible with the LC; phase separation occurs and OA acts as a surfactant forming a layer between the polymer and the LC. Anchoring energies calculated from the interfacial tensions indicate that the LC and polymer interface is strong. On addition of the surfactant, the anchoring energy at the interface of the LC and the surfactant becomes weaker. This decrease in anchoring strength may be one of the major factors responsible for a reduction in droplet size and also a lowering of critical field for switching, both of which are observed experimentally in volume holograms made of these PDLC materials.

Organization of cyanobiphenyl liquid crystal molecules in prewetting films spreading on silicon wafers.

We observe prewetting films of 8CB (4'-n-octyl-4-cyanobiphenyl) spreading at room temperature on silicon wafers by ellipsometry and x-ray reflectivity. Ellipsometry indicates the formation of a nondense monolayer spreading in front of a 45-A-thick film. X-ray reflectivity, performed using a ribbon geometry for the liquid crystal (LC) reservoir, allows us to determine the organization of the 8CB molecules in the homogenous film. It consists of a trilayer stacking with a smecticlike bilayer standing above a polar monolayer with tilted molecules. We show that the thickness of the bilayer is equal to the smectic periodicity in the bulk material and that the tilt angle of the molecules in contact with the solid surface is close to 60 degrees, in good agreement with second-harmonic generation studies reported by other groups. Such organization can be precisely determined using x-ray reflectivity because it induces a modulation of the electron density along the normal to the surface. Furthermore, a study of the ellispometric profile of a drop heated in the nematic phase, where we observe a complete spreading of the LC, shows the complex structuration of the LC close to the solid interface. In particular, the spreading behavior of the trilayer compared to the subsequent smecticlike bilayers indicates the existence of specific interaction between the trilayer and silicon wafer.

Systematic Study of Odd-Even Effect in Molecular Alignments of Cyanobiphenyl Liquid Crystals with Scanning Tunneling Microscope. I. Experimental Observation

Systematic observations are carried out for molecular alignments of monolayers of homologous series in cyanobiphenyle (CB) liquid crystals such as 6CB to 12CB on highly oriented pyrolytic graphite (HOPG) substrates using a scanning tunneling microscope. For 6CB the molecular alignments are also observed on molybdenum disulfide substrates (MoS 2 ) as well as HOPG. In 6CB monolayers, the characteristic structures indicating both properties of a single and double row previously reported are observed. In 7CB, 9CB and 11CB monolayers only the single row property is observed. The monolayer alignments of 8CB, 10CB and 12CB show on the other hand double row characteristics. Thus all the observed aspects clearly indicate the odd-even effect of carbon numbers in an alkyl chain. The morphology of molecular alignments due to the odd-even effect in monolayers of cyanobiphenyl liquid crystals on HOPG are systematically determined. Based on the experimental observation a qualitative model on the molecular alignments of ...

Molecular Alignment in Monolayers of Mixtures of Cyanobiphenyl Liquid Crystals on MoS2Substrate

Systematic observations are carried out on the molecular alignment in monolayers of mixtures of n -cyanobiphenyl ( n CB) liquid crystals such as n =7CB to 12CB on a molybdenum disulfide substrate (MoS 2 ) using a scanning tunneling microscope. Several different types of specific alignments of the mixtures are observed depending on the mixture rate and the combination of n CB. There are also mixture combinations in which no specific alignment is observed.

Polarized ATR-IR Studies of Surface-Field-Induced Anchoring of Alkyl Cyanobiphenyls

We report initial studies of the use of polarized ATR-IR spectroscopy to obtain information on molecular alignment, on the mesoscopic scale, in the interfacial region of alkyl cyanobiphenyl liquid crystals in contact with self-assembled monolayers (SAMs). Specific mesoscopic alignment was induced by variation of the surface field, via the ω-functional group of self-assembled monolayers of alkanethiol derivatives. Our experimental approach involved the deposition of a thin gold layer onto a ZnSe prism. The presence of this layer strongly attenuates the s-polarized field leading to an “effective surface selection rule”, which can be used to determine the average orientation of the liquid crystal molecules within the attenuation depth of the evanescent field. A frequency shift of the C⋮N stretching band was seen in the case of homeotropic anchoring.

The gallophosphate molecular sieve cloverite as a host for liquid crystals

The gallophosphate cloverite has been investigated because of its applicability to act as a host for larger molecules. 4- n-pentyl-4′-cyanobiphenyl (5CB) and 4- n-octyl-4′-cyanobiphenyl (8CB) liquid crystals were used as guest molecules. The results show that the adsorption of liquid crystals from the gas phase proceeds without destruction of the cloverite structure. The typical phase properties of the liquid crystals remain unchanged after adsorption on cloverite: however, they appear with lower intensity in the DSC curve. A new smectic phase seems to be formed inside the cloverite pore system. FTIR measurements show the interaction of 5CB with the Lewis and Brønsted acid sites of cloverite. Desorption of liquid crystals out of the pore system proceeds slowly under liberation of the POH groups of the lattice. In summary, the results show that the gallophosphate cloverite is a suitable host for large molecules.

Temperature calibration of temperature-modulated differential scanning calorimeters

Calibration of differential scanning calorimeters (DSC) is essential because it is not possible to calculate heat-flow rate and sample temperature directly from the quantities measured. The question arises whether the calibration used in the conventional DSC mode is also valid for temperature-modulated DSC (TMDSC). To prove this, a well-defined transition is needed which does not disturb the temperature profile within the sample during the modulation, the response of the whole system (calorimeter, sample, transition) must be linear. Some low-energy liquid-crystal transitions behave in this way under certain conditions and can be used for temperature calibration in TMDSC. The procedure is demonstrated for a smectic-A—nematic transition in a cyanobiphenyl liquid crystal (8OCB). Limitations of the method are discussed — in particular, rate effects. The TMDSC measurements also give some information about the order of phase transitions (8OCB: SN — second-order; NI — first-order).

Mixture Alignments on Graphite Substrates in Cyanobiphenyl Liquid Crystals Observed using Scanning Tunneling Microscopy

New alignments in three types of mixtures (9CB-8CB, 10CB-9CB and 10CB-8CB, where CB means 4'-n-alkyl-4-cyanobiphenyl) are observed. In the mixture of 9CB and 8CB, there is only one specific alignment. For the mixtures 10CB-9CB and 10CB-8CB, there are three and two specific alignments respectively. The feature that is common to all these mixtures is an anti-parallel alignment in a row of neighboring heterogeneous molecules. These characteristic mixture-alignments suggest a variety of molecular interactions in the mixtures.

Low-frequency dielectric dispersion properties of p-pentyl-p?-cyano-biphenyl

The behavior of impurity ions in the p-pentyl-p′-cyano-biphenyl liquid crystal cell were investigated by evaluating the low-frequency dielectric dispersion characteristic of the liquid crystal cell. The dependence of the behavior of the impurity ions on the presence of polyimide LB film was also investigated. It was found that in a liquid crystal cell without an LB layer, an electric double layer was formed, and that in a cell with an LB layer, interfacial polarization was present between the liquid crystal layer and LB layer. It was also found that in a liquid crystal cell with ultrathin LB films, the characteristics of the cell strongly depended on the number of LB layers. © 1997 Scripta Technica, Inc. Electron Comm Jpn Pt 2, 80(5): 40–46, 1997

Nonmesomorphic Solute−Mesomorphic Solvent Interactions Studies. 3. A Unified Approach To Probe Liquid Crystalline Order by Electron Spin Resonance Spectroscopy

This paper reports values for the A and g magnetic tensor components for PD-Tempone in the rigid limit of the n-pentyl (5CB), n-hexyl (6CB), n-heptyl (7CB), and n-octyl (8CB) cyanobiphenyl liquid crystals and for Tempo-Palmitate in the rigid limit of 5CB and 6CB. A unified approach based on introducing a factor into the “distortion theory” expression for obtaining probe order parameters Sii is successfully tested. The sign and value of this factor give the probability of alignment of the ith axis of the probe along the applied magnetic field.

Chemical modification of the polymer surface and consequences on electrooptical properties of liquid crystal—polymer composites

In order to improve our understanding of the mechanisms involved in the electrooptical effects of liquid crystal—polymer composites, and to identify the relevant parameters of these complex systems, we have investigated the influence of the chemical nature of the polymer surface on the anchoring of the liquid crystal. Our method consists in removing the liquid crystal from the composite by dissolution followed by supercritical drying of the solvent. The surface of the polymer is then modified by different reagents and the liquid crystal is reintroduced to the cell. Comparison of the results shows that for a cyanobiphenyl liquid crystal, fluorinated or hydroxylated chemical moieties on the surface of the polymer induce a stronger anchoring than alkyl moieties. This phenomenon seems to be due to the enhanced chemical compatibility between the liquid crystal and the surface of the polymer in the case of alkyl moieties. Finally, the study has shown that a reduction of anchoring of the liquid crystal is responsible for a decrease in driving voltage, rise time and hysteresis and an increase in decay time and off-state transmission.

Studying topography and sub-surface structure of 8CB liquid crystal films with shear-force microscopy

Scanning shear-force microscopy, which is usually employed to track scanning near-field optical microscopy probes across samples, was used to observe the sub-surface smectic layer structure through the thickness of a film of the alkyl cyanobiphenyl liquid crystal 8CB on a highly oriented pyrolytic graphite substrate from force-distance measurements. With the noncontact imaging shear-force microscopy supplies, it was possible to follow, in the topography, the movement of a monolayer of the 8CB molecules. This has enabled the diffusion of molecules in a precursor film of 8CB to be observed without significantly disrupting the free liquid surface.

Molecular orientation of a cyanobiphenyl liquid crystal in freely suspended films and at the air-water interface

Langmuir films and freely suspended (FS) films of 4'-n-octyl-4-cyanobiphenyl (8CB) in the smectic A phase have been studied by Fourier transform infrared spectroscopy. Examination of FS film transmission spectra gives a good estimation of molecular group orientation in the uniaxial film. At the air-water interface 8CB shows a first-order phase transition from a monolayer to a trilayer when the surface pressure increases. We have studied such films using polarisation modulated infrared reflection absorption spectroscopy. Specific surface selection rules on dielectrics allow an estimation of the orientation of the cyanobiphenyl core in the monolayer and in the trilayer and comparison of molecular organisations in the two types of films.