Smectic Layer Structure Research Articles

Dipolar interactions, molecular flexibility, and flexoelectricity in bent-core liquid crystals

The effects of dipolar interactions and molecular flexibility on the structure and phase behavior of bent-core molecular fluids are studied using Monte Carlo computer simulations. Some calculations of flexoelectric coefficients are also reported. The rigid cores of the model molecules consist of either five or seven soft spheres arranged in a "V" shape with external bend angle gamma. With purely repulsive sphere-sphere interactions and gamma = 0 degrees (linear molecules) the seven-sphere model exhibits isotropic, uniaxial nematic, and untilted and tilted smectic phases. With gamma > or = 20 degrees the untilted smectic phases disappear, while the system with gamma > or = 40 degrees shows a direct tilted smectic-isotropic fluid transition. The addition of electrostatic interactions between transverse dipole moments on the apical spheres is generally seen to reduce the degree of molecular inclination in tilted phases, and destabilizes the nematic and untilted smectic phases of linear molecules. The effects of adding three-segment flexible tails to the ends of five-sphere bent-core molecules are examined using configurational-bias Monte Carlo simulations. Only isotropic and smectic phases are observed. On the one hand, molecular flexibility gives rise to pronounced fluctuations in the smectic-layer structure, bringing the simulated system in better correspondence with real materials; on the other hand, the smectic phase shows almost no tilt. Lastly, the flexoelectric coefficients of various nematic phases--with and without attractive sphere-sphere interactions--are presented. The results are encouraging, but a large computational effort is required to evaluate the appropriate fluctuation relations reliably.

Time resolved studies of the switching behaviour of smectic liquid crystals

The field-induced alignment of a smectic A phase is a complicated process involving director rotation via the interaction with the field and the layer rotation via the molecular interactions. We have investigated the alignment of the smectic A phase of 4-octyl-4′-cyanobiphenyl (8CB). The experiment consists of creating a monodomain sample by cooling the sample in a magnetic field of 7T. The sample is then turned quickly about an axis parallel to the X-ray beam, but orthogonal to the field. The alignment behaviour is temperature dependent and changes as the rotation angle exceeds 45°. Numerical simulations based on the technique of dissipative particle dynamics were used to gain insights into the experiments. We find that following a rotation, part of the sample disorders and the rest rotates as a collection of domains, in which the integrity of the smectic layer structure is conserved.

Smectic Layer Structures in Complex Geometries—Modelling Complex Layer Structures in Smectic Liquid Crystals

In order to understand the operation of liquid crystal devices based on tilted smectic materials it is highly important to also understand the formation of the smectic liquid crystal layer structures. However, in many situations this is not easy to do. It is well established that in devices filled with materials that have an upper lying SmA phase a “chevron” structure forms when cooled from the SmA to the tilted smectic phase due to the layer thinning combined with layer packing constraints. But it is less well understood how the structure is influenced by the application of an electric field. Experimental work has been done in both ferroelectric and anti-ferroelectric phases which shows a transition from chevron towards bookshelf structure. In this paper we present some of our recent theoretical work in the area of smectic layer structures where “melting” of the smectic layer ordering is allowed.

Molecular Design Controlling Smectic Stability and Clinicity

ABSTRACT Both peripheral ends of some straight-shaped mesogenic compounds, possessing a biphenyl or azobenzene as a central rigid core, have been systematically modified, and the effect on the liquid-crystalline properties investigated. A fluorinated moiety introduced into the peripheral alkyl chains stabilized the smectic phase, whereas that into the terminally positioned phenyl rings resulted in the appearance of the nematic phase. Introduction of phenyl rings into the middle of the flexible chains produced a well-defined smectic layered structure. Further introduction of another phenyl ring at each terminal end was found to show a significant effect on the determination of the clinicity.

主鎖型液晶性高分子のガラス状態―スメクチック液晶ガラスと液体ガラス

Liquid crystal (LC) glass is the solid resulting from cooling the thermotropic LC without crystallization. It is analogous to the amorphous glass, namely isotropic liquid glass, in showing a jump in the specific heat and a sudden decrease in the elastic constant at a temperature on heating but different in possessing the structure identical to the preceding liquid crystal, in other words, long-range order in the molecule arrangement We have found difference in dynamics between these two glassy states with using thermotropic main chain smectic LC polyesters, poly(melhylene 4,4'-bibenzoate) (BB-n) whose solid state is proved to be composed of crystal and smectic LC glass. The glass transition of the smectic LC glass detected as an a-relaxation by a dynamic mechanical method shows considerable mechanical anisotropy such that the tan δ peak measured in a direction parallel to the layer is larger than that in a perpendicular direction. This anisotropy can be explained according to the smectic layer structure. Trends in α-relaxation temperatures (T α ) for BB-n polyesters, poly(alkylene 2,6-naphthalene dicarboxylate)s (N-n) and BB-6-co-N-6 copolymer system show that the glass transition temperature of the LC glass is lower than that of the isotropic liquid glass. The difference in the glass transition temperature was found also for the smectic LC glass and the isotropic liquid glass which were prepared by cooling the isotropic liquid of a poly(1-methyl propylene 4,4'-bibenzoate) (BB-3(1-Me)) at a rate of 1°C min -1 and 100°C min -1 respectively. On heating, the smectic LC glass showed a glass transition (T g LC ) of 81 °C, while the isotropic liquid glass showed a glass transition (T g iso ) of 93 °C and transformed to the smectic LC at 110 °C. The corresponding heat capacity increases were 50.1 J mol -1 K -1 and 78.5 J mol -1 K -1 for the smectic LC glass and the isotropic liquid glass, respectively. The enthalpy-temperature relationship in all the states was estimated by integration of the heat capacity measured by the temperature-modulated DSC method. Temperature dependences of the enthalpy relaxation time of the two glasses obey Vogel-Fulcher-Tamman (VFT) relationship, and clearly show a single ideal glass temperature T 0 LC (= 342 K) for the smectic LC, but two ideal glass temperatures To,' (= 343 K) and T 0H iso (= 353 K) for the isotropic liquid. T 0L iso is 10 K lower than T 0H iso , and corresponds to T 0 LC . According to a mode coupling theory of the glass transition of hard ellipsoid molecule liquid, the glass transition of the isotropic liquid at T 0L iso is associated to the freezing of translation molecular motions, while the transition at T 0H iso to the freezing of molecular orientation fluctuation. The correspondence of T 0 LC to T 0L iso suggests that the glass transition of the smectic LC is dominated by the freezing of translation molecular motions in the smectic layer direction.

Self-Restoration by Smectic Layer Structures of Monostable Ferroelectric Liquid Crystal in Flexible Devices

In this paper, we discuss a self-restoration phenomenon affecting smectic layer deformation and molecular alignment in monostable ferroelectric liquid crystals used for flexible displays. First, the mechanical stability of tilted `bookshelf' structures of smectic layers anchored on substrates using alignment layers was examined by precisely shearing two substrates. The microscopic texture of a monostable ferroelectric liquid crystal showed tolerance to shearing and the self-restoration was thought to be due to a smectic layer reconnection phenomenon, whereas a conventional bistable ferroelectric liquid crystal film generated alignment defects due to the tilting of the stable molecular direction. We then fabricated a flexible monostable device containing fine polymer fiber networks, where anchoring of molecularly aligned polymer fibers led to the observation of monostable liquid crystal switching. A uniform liquid crystal alignment was maintained in a 100 mm×100 mm device even after bending more than 10000 times at a minimum radius of curvature of 20 mm, due to the enhanced shearing tolerance of the smectic layer structure.

Magnetically complexed collagen nanocomposites

Rare-earth ions with unfilled 4f shells, Gd3+ (J = ) and Dy3+ (J = ) are bound to glutamic acid residues at the ends of long linear oligopeptides derived from those in naturally occurring collagen proteins. The peptide, which folds into a triple helix about 10 nm long, consists of a sequence 36 amino acids long. The rare-earth–collagen complex self-assembles into layers through a smectic A liquid crystalline phase. The smectic layered structure is retained on drying, and the acidic peptide end blocks localize the rare-earth ions in sheets approximately 1–2 nm thick separated by the collagen-like peptide of the complex. X-ray crystallographic, scanning electron microscopy and optical studies demonstrate the liquid crystallinity of the peptides and the incorporation of the rare-earth ions into sheets between the smectic layers. Magnetic susceptibility (2 K ≤ T ≤ 300 K) and low-temperature isothermal magnetization (0 T ≤ H ≤ 5 T) show the rare-earth–collagen complexes are paramagnetic in the rare-earth concentration range studied (not more than 0.3–0.4 Gd or Dy ion per molecule). This technique of bonding rare-earth ions to oligopeptides, and using the oligopeptide mesophase behaviour to organize attached ions, can be used to tether other rare-earth and/or transition-metal ions, creating layered magnetic structures. The interlayer spacing is dictated solely by the intervening oligopeptide length.

Electric-Field-Induced Layer Rearrangement in the Smectic A Phase of Nonsymmetric Dimeric Liquid Crystal

A field-induced smectic layer rearrangement has been investigated in the smectic A phase of non-symmetric dimeric liquid crystals possessing cyanobiphenyl and phenylpyrimidine groups. The dimerization of mesogens likely lowers the stability of smectic layer structures, and smectic layers are rearranged by applying a low electric field in both planarly and homeotropically aligned cell geometries.

High tilted antiferroelectric liquid crystalline materials

Phase transitions, smectic layer structure, helical pitch and electrooptical properties of recently prepared high tilted antiferroelectric compounds are characterized. The compounds enable to prepare broad temperature range (−20 to 100 °C) orthoconic antiferroelectric mixtures, having 45° tilt independent of temperature in broad temperature range, and exhibiting excellent contrast and grey level scale.

High tilted antiferroelectric liquid crystalline materials

Phase transitions, smectic layer structure, helical pitch and electrooptical properties of recently prepared high tilted antiferroelectric compounds are characterized. The compounds enable to prepare broad temperature range (−20 to 100 °C) orthoconic antiferroelectric mixtures, having 45° tilt independent of temperature in broad temperature range, and exhibiting excellent contrast and grey level scale.

Thermotropic Liquid Crystals of Main‐Chain Polyesters having a Mesogenic 4,4′‐Biphenyldicarboxylate Unit, 14

AbstractSummary: Anisotropy in molecular motions of a glassy smectic CA (SmCA) liquid crystal of a main‐chain BB‐5(3‐Me) polyester was examined by dynamic mechanical analysis (DMA) for fibrous monodomain samples with two distinct orientations of smectic layers perpendicular and parallel to the fiber axis. The α‐process attributed to micro‐Brownian motion of the polymer on the glass transition shows clear anisotropy explainable by the nature of the smectic layer structure. Time‐temperature superposition is applicable to the α‐process, so that the frequency of the micro‐Brownian motion in the SmCA phase can be estimated despite the narrow frequency range in DMA. The results suggest that the decoupling of the molecular motions in two characteristic directions are parallel and perpendicular to the layer. The amplitude of the micro‐Brownian motion in the layer normal direction is 3 times smaller than that in the layer direction, and the frequency in the layer normal direction is 2.5 times higher than that in the layer direction.Illustration of molecular packing structures: fiber A (left side) and fiber B (right side) with the orientations of polymers parallel and perpendicular to the fiber, respectively.imageIllustration of molecular packing structures: fiber A (left side) and fiber B (right side) with the orientations of polymers parallel and perpendicular to the fiber, respectively.

129Xe Nuclear Shielding and Diffusion in the A and C* Phases of a Chiral Smectogen

The 1 2 9 Xe NMR shielding and diffusion of xenon dissolved in the chiral liquid-crystal 1-methylheptyl 4'-(4-n-decyloxybenzoyloxy)biphenyl-4-carboxylate (10B 1M7) were studied over the temperature range covering the isotropic (I), smectic A (SmA), and ferroelectric, ferrielectric, and antiferroelectric smectic C*(SmC*) phases. The 1 2 9 Xe shielding reveals clearly the I-SmA and SmA-SmC*(ferroelectric) phase transitions. The SmC* sub-phase transitions can also be detected by smaller but distinguishable shifts of the shielding accompanied by line width changes. A theoretical model developed earlier is applied to the shielding data, confirming a negative anisotropic contribution to the shielding as the SmA phase forms and allowing the evaluation of the tilt angle in the SmC*. Diffusion experiments were mostly performed in the direction parallel to the layer normal, i.e., along the external magnetic field, D∥, in the smectic phases but a few experiments were carried out in the in-plane direction, i.e., perpendicular to the external magnetic field, D⊥, as well. These experiments indicate large anisotropy, D⊥/D∥, of the 1 2 9 Xe diffusion tensor that increases as the temperature decreases. Application of the Arrhenius equation to the temperature-dependence of D∥ reveals different activation energies for each studied phase. This can be correlated to changes in the smectic layer structure that occur between the different smectic phases.

Memory effect of ferroelectric liquid crystal stabilized by polymer fibers

AbstractWe investigated the bistable state of the molecular orientation of ferroelectric liquid crystal that is formed between micro‐polymer fibers, and demonstrated the occurrence of a memory effect when the smectic layer structure is corrected. In this study, we introduced high‐pretilt alignment layer and formed polymer fibers in the liquid crystal by using the photopolymerization phase separation method in order to make the creasing and bending of the layer in the direction of thickness unidirectional. In addition, we treated the composite film of the liquid crystal and the polymer fiber by applying an AC voltage in order to hold the cone axis of the liquid crystal molecule in a rubbing direction during the gradual cooling process in which the liquid crystal molecule tilt (phase transition from the smectic A phase to the smectic C* phase). The results of trial production confirmed an alleviation of the bending of the smectic layer in the horizontal plane and the occurrence of a memory effect, resulting in a minute distribution of bistable switching domains and a memory function involving area‐wise grayscale. © 2004 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 87(4): 16–24, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.10082

Small-Angle X-Ray Analysis of Smectic a Cholesteric Liquid Crystal Phase Transition in Rigid-Rod Helical Polysilane

The smectic A Cholesteric liquid crystal phase transition of rigid-rod helical polysilane was studied by means of synchrotron radiation small-angle X-ray scattering (SR-SAXS). The information on the smectic trans-layer electron density distribution was clarified by fitting the model of electron density distribution to the diffraction data. Each of the layers was approximated by the boxlike function to introduce the molecular form factor of the polymer. The distribution of the molecular center of gravity within the layers due to the thermal motion was taken into consideration by convolution with the Gaussian function. The elucidated smectic layer structure was not like the conventional structure with their ends packed into the interlayer space. The layers with bumpy surfaces interdigitate in a key-and-keyhole manner to pack the molecules with different lengths into layers due to their finite molecular weight distribution.

P‐101: Eliminating Layer Undulation Textures in SSFLC Devices

AbstractTemperature dependence of the smectic layer structures was studied by high resolution X‐ray diffraction analysis. The investigation was based on two different alignment SSFLC cells. The effect of layer spacing changes in the SmA phase on defects seen in the SmC phase of the devices is investigated. It is shown that when considering a defect free SmC* texture, the layer spacing changes in the SmA phase are more significant than changes in the SmC phase. An explanation is also proposed.

Achiral non-branched rod-shaped liquid crystals showing anticlinic smectic phase

Both peripheral ends of some straight-shaped mesogenic compounds, possessing a biphenyl or azobenzene moiety as the central rigid core, were systematically modified, and the effect on the liquid-crystalline properties investigated. Introduction of phenyl rings into the middle of the flexible chains was found to produce a well-defined smectic layered structure. Further introduction of another phenyl ring at each terminal end of the peripheral chain had a significant effect on the determination of the molecular tilt direction in the smectic phase, i.e., synclinic or anticlinic. Thus, novel molecular architecture for stabilizing the anticlinic smectic phase was introduced and the origin of the anticlinic ordering discussed.

Liquid crystal trimers showing stable anticlinic structures in smectic C and I phases

A homogeneous series of chiral trimeric compounds, possessing three biphenyl mesogenic cores connected by flexible alkyl spacers, were prepared and the liquid-crystalline properties investigated. The trimers were designed to have chiral moieties at both peripheral ends of the molecular structure and the properties were compared with those of the related dimeric and monomeric systems. Some of the optically active trimers showed stable antiferroelectric smectic C and I phases. The racemic mixture of optical isomers also showed anticlinic versions of the smectic C and I phases, however, synclinic structures were not observed. The optical isomers and the racemic mixture showed quite similar transition temperatures, suggesting that the chirality-induced properties have little effect on the stability of the anticlinic structures. The trimers showed a third-order X-ray diffraction peak indicating the formation of a higher order smectic layer structure, which is considered to stabilize the anticlinic molecular ordering.

Smectic layer structure of ferroelectric liquid crystal formed between fine polymer fibres

This paper describes the alignment of ferroelectric liquid crystal (FLC) structures formed between aligned polymer fibres, where the FLC smectic layers are determined by polarising microscopy and X-ray diffraction. The FLC/polymer composite films were formed from a nematic phase FLC/monomer solution using a photopolymerisation-induced phase separation method. It was found that bending of the FLC smectic layers was induced in both the film plane and the cross-sectional plane at the phase transition from smectic A to chiral smectic C of the FLC material. The light transmittance properties of the composite film between crossed polarizers was analysed by light propagation simulation in several optical anisotropic media, based on the evaluated smectic layer model.

Smectic Layer Deformation of Ferroelectric Liquid Crystal Sandwiched between Polymer Walls with Anchoring Effects

We studied smectic layer structures of ferroelectric liquid crystal (FLC) formed in elongated small spaces surrounded by molecule-aligned polymer walls and rubbed polyimide alignment layers. The polymer walls, which are parallel to the rubbing direction and vertical to the alignment layers, were formed by the photopolymerization of an aligned monomer under patterned ultraviolet light irradiation. From the observation of the alignment textures of the FLC between the polymer walls with a polarizing microscope, it was found that the smectic layer structure was changed from vertical plane bending alignment (chevron structure), as observed with a conventional surface-stabilized FLC, into horizontal plane bending, as the interval between the polymer walls decreased. It is thought that the smectic layer structure is governed by the competition between the anchoring effects of the alignment polyimide layers and the molecule-aligned polymer walls.

Self-Assembly and Viscoelastic Properties of Semifluorinated Polyesters

The structure and dynamics of polyesters composed from a rigid aromatic backbone and oxyalkylperfluoroalkyl side chains have been studied using differential scanning calorimetry, small- and wide-angle X-ray scattering, pressure-volume-temperature measurements and rheology. The system was found to undergo three phase transitions. Starting from lower temperatures they reflect the melting of the fluorinated side chains, the unfreezing of the backbone mobility and the melting of the smectic layered structure. In addition, rheology has shown that these compounds are thermorheologically complex at all temperatures investigated and exhibit non-Newtonian low-frequency response.