Molecular Tilt Angle Research Articles

X-RAY INVESTIGATION ON TEMPERATURE DEPENDENCE OF THE TILT ANGLE IN FERROELECTRIC LIQUID CRYSTALS

The temperature dependence of the molecular tilt angle in a series of ferroelectric liquid crystals is deduced from X-ray diffraction measurement. The critical exponent β has been estimated from the experimental data. The non-classical value of β for the compounds lies in the range 0.36–0.41.

Relationship between structures and polymerizabilitues of ω-tricosenoic acid thin films

Deposition of ω-tricosenoic acid films was tried by the Langmuir-Blodgett (LB) and vacuum deposition (VD) method, and structures and polmerizabilities of the films were investigated for applications to thin resist film. It was shown that the LB and VD films had predominant molecular orientation tilted to the substrate normal, and that the LB films had larger molecular tilt angles than those of the VD films. Moreover, it was proved that the polymerizability of ω-tricosenoic acid film was dependent on the molecular tilt angle.

A Soliton Model for the Chevron Layer Structure in SmC* Liquid Crystals

Abstract In this work, a simple explanation is given for the chevron layer structure which has been observed in SmC* phase. A phenomenological elastic free energy concerned with layer distortion, c−director deformation, and their coupling is proposed to discuss the layer structure of them. It will be shown that there exists a soliton solution corresponding to the chevron structure. The transition layer thickness of the chevron is theoretically estimated as the penetration length (∼molecular length)/molecular tilt angle. In similar to the kinked layer structure, the c-director rotation angle is also found to show a soliton-like behaviour within the transition region. In addition, the existence of the selective pre-tilt at the bounding surface is explained in the relation with the chevron structure.

Electric-field-induced layer reorientation in ferroelectric liquid crystals.

We show that a sufficiently large ac electric field reorients the smectic layers irreversibly in chiral smectic-C liquid crystals. Using uniformly aligned samples, prepared by unidirectional buffing, it is demonstrated that at a given temperature the layers tilt away from the buffing axis, b^, by an angle equal to the molecular tilt angle of the smectic-C phase. In this new configuration the molecules lie along b^ and the layers are tilted. It is also found that a small dc bias field can control the direction of the tilted layers with respect to b^. Furthermore, microscopic observations suggest that the chevron layers, common in the ferroelectric smectic-C phase, are absent after the electic field treatment which eliminates zig-zag-type defects.

Temperature Dependence of Molecular Tilt Angle and Rotatory Power in a Ferroelectric Liquid Crystal with Polarization Sign Reversal

Abstract The molecular tilt angle of the ferroelectric liquid crystal IS-2424 monotonously increases in the smectic C* phase with decreasing temperature from zero at the smectic A-smectic C* transition to about θ = 15°. It can be described by a critical exponent β = 0.26 ± 0.02. The temperature dependence of the tilt angle is definitely not proportional to the temperature dependence of the spontaneous polarization which shows a sign reversal.

Relationships between Mesophase Behavior and Crystal Structures of Chiral Smectogenic 4-[(S)-2-Methylbutyl]phenyl 4′-Alkoxybiphenyl-4-carboxylates

Abstract Interlayer distances of Sm*C and Sm A phases of the title series were measured as functions of temperature by X-ray diffraction. With increase in temperature, molecular tilt angles in Sm*C decrease from 22–25 to 0°. Crystal structures of the heptyl (1) and octyl (2) homologues have been determined by X-ray analysis. (1): 4-[(S)-2-Methylbutyl]phenyl 4′-heptyloxybiphenyl-4-carboxylate, monoclinic, P21, a=26.929(4), b=5.4249(5), c=9.2607(9) Å, β=96.78(1)°, Z=2. (2): 4-[(S)-2-Methylbutyl]phenyl 4′-octyloxybiphenyl-4-carboxylate, orthorhombic, P212121, a=9.249(1), b=56.259(9), c=5.412(1) Å, Z=4. Final R were 0.076 and 0.121, respectively. Both crystals have distinct layer structures similar to those of Sm*C. In the former crystal, the molecules tilt to one direction uniformly throughout layers, while in the latter, they take two opposite directions alternately from layer to layer. However, structures within a layer are very similar to each other. Each molecule has a coplanar biphenyl moiety and a twisted paraffin chain. Mesophase behavior was interpreted very well by the molecular interaction revealed in the crystal structures.

Crystal structures of chiral smectogens

Abstract Crystal structures of three chiral smectogens, 4-pentyloxyphenyl and 4-heptyloxyphenyl (S)-4′-(2″-methylbutyl)biphenyl-4-carboxylates, which have SmC adjacent to cholesterics, and (S)—4—(2′—methylbutyl)phenyl 4′-heptyloxy—biphenyl-4-carboxylate, which has SmC adjacent to Sm A, were determined by X-ray analyses. All the crystals have layer structures. Paraffinic chains aggregate regularly with extended conformations in the two former crystals, while they take a twisted conformation in the third crystal. Molecular tilt angles in the layers are closely related to those found in the SmC phases.

High resolution electron microscopy of interfaces in chlorinated phthalocyanine molecular crystals

SUMMARYMolecular images of chlorinated copper phthalocyanine have shown the arrangement of molecules at the interface between two crystals. Mismatch does not cause distortion or recrystallization for low index junctions but for more irregular interfaces strain causes alteration of the molecular tilt angle and accommodation of the mismatch by the formation of edge dislocations whose Burger's vectors are parallel to the interface.

Measures of the Molecular Tilt Angle and Optical Anisotropy n Ferroelectric Liquid Crystals

Abstract A pulse electr-optical technique is described which allows the molecular tilt angle and optical anisotropy of ferroelectric liquid crystals to be measured. The technique provides high accuracy and reproducibility of the measurement and is insensitive to the non-homogeneity of a liquid crystalline layer. The temperature behaviour of the tilt angle and the optical anisotropy is studied for two “classical” substances (DOBAMBC and HOBACLPC) and several ferroelectric mixtures.

The Dielectric Anomaly Near the Transition from the Smectic A* to Smectic C* Phase and Visco-Elastic Properties of Ferroelectric Liquid Crystals

Abstract A pyro-electric technique is developed which allows the measurement of the dielectric response near the A*-C* phase transition in ferroelectric liquid crystals. The temperature dependence of the elastic modulus K θ(T) corresponding to the molecular tilt in smectic layers is calculated from the experimental data. Direct pyro-electric measurements of the relaxation time for the spontaneous polarization P s and the data on K θ(T) allow us to calculate the temperature behaviour of the twist-viscosity γ1(T) for the smectic C* phase. The curves γ1(T) are compared for the smectic C* and the nematic phase of the same compound and the dependence of the twist viscosity on the molecular tilt angle in the C* phase is investigated. The results of the dielectic measurements are discussed in terms of the mean-field approximation.

Structures of the Chiral Smectic C Phase with Selective Reflection in the Wavelength Range of Visible Light. I. Interlayer Distances and Helical Pitches of the (+)-4-n-Alkyloxyphenyl 4-(2”-Methylbutyl)-biphenyl 4′-Carboxylates Series

Abstract Interlayer distances and helical pitches were studied as functions of temperature by X-ray diffraction and transmission spectral measurements for chiral smectic C phases of (+)-4-n-alkyloxyphenyl 4-(2′-methylbutyl)biphenyl 4′-carboxylates, which had selective reflection in the wavelength range of visible light. The relationship between the interlayer distances and the length of the paraffinic chain gives some information about the structure within the layers: the molecular tilt angle (with respect to the normal to the layer plane) is 45–48° and the paraffinic chains form double layers. Interlayer structures and interactions are discussed, in the light of the interlayer distances and the helical pitches.

On mechanisms of dipolar ordering in ferroelectric liquid crystals

The temperature dependences of a “slow” and “fast” modes of the spontaneous polarization were measured for some ferroelectric liquid crystalline mixture composed of chiral and achiral components. The “fast” polarization which is proportional to θ2 (θ is the molecular tilt angle in the smectic C phase) is shown to be due to the long-range interactions of permanent and induced dipoles. The behaviour of the spontaneous polarization as a function of the dipole moments of a chiral component and susceptibility of an achiral matrix is discussed.

13C NMR in ferroelectric smectic liquid crystals

Abstract The temperature dependence of the 13C NMR chemical shifts has been measured in the isotropic, the smectic A and the smectic C* phases of chiral DOBAMBC (p-decyloxybenzylidene p′-amino 2-methyl butyl cinnamate) and HOBACPC (hexyloxybenzylidene p′-amino 2-chloro α propyl cinnamate). The measurements allowed for a microscopic determination of the temperature dependence of the molecular tilt angle and the dipolar ordering of the C=O groups-i.e. the in-plane spontaneous polarization-without destroying the helical structure.

The Implications of the Diffuse-Cone Model for SmecticAandCPhases andA-CPhase Transitions

Abstract The diffuse-cone model, developed on the basis of the existence of orientational disorder in smectic phases, is valid for all smectic A and C phases, and also, with appropriate modifications, for all other smectic phases. The fundamental angular parameter is not the optical tilt angle τ, but the preferred molecular tilt angle θm. This explains why τ does not follow the (TA-C-T)0.35 dependency predicted by De Gennes whereas θm does. The diffuse-cone model also explains why the tilt angle obtained from measurements of the smectic layer thickness will in general not be equal to τ, and thus invalidates arguments based on such differences which were used to support a particular structural model.

Why the Molecules are Tilted in all Smectic A Phases, and How the Layer Thickness Can be Used to Measure Orientational Disorder*

Abstract Even though the average molecular direction in smectic A phases is perpendicular to the smectic planes, most molecules in these phases have rather large tilt angles because the orientational order parameter S in all smectic A phases is significantly less than unity. This molecular tilt inherent to all A phases accounts for the observed differences between layer thickness d and molecular length l of four quite different compounds, without any need for assuming additional tilt, interdigitation, or kinking of chains. From the differences between d and l , S values from 0.78 to 0.84 are obtained, which yield average molecular tilt angles from 20° to 17°. It is predicted that all smectic A phases with monomolecular layers will follow the same pattern. The concept of “inherent tilt” should apply to all orthogonal smectic phases, thermotropic as well as lytropic.

Molecular tilt in the smectic C phase : a zigzag model

We have collected data concerning the molecular tilt angle of 4 smectic C compounds, presenting a second order transition to a smectic A phase. We have completed these data by direct measurements, using the conoscopic technique, for the tilt angle of the optical axis, and the X-ray technique, to follow the varying thickness of smectic layers. The comparison of X-rays and optical data shows a discrepancy which becomes very large for the molecules which possess long aliphatic end chains. Resuming an idea from Guillon and Skoulios, we propose a molecular model for the C phase derived from lyotropic systems: the rigid central core, optically anisotropic, imposes the tilt of the optical axis. The melted end chains are on the average closer to the normal to the layers. The rigidity parameter, ratio of the rigid core length to total length of the molecule, will be important to describe the mechanical properties of smectics.