Smectic Systems Research Articles

LC-elastomers: structure -property relations and concepts to improve applicability

ABSTRACTThis review deals with liquid crystalline elastomers (LCEs) and focuses thereby on three aspects. At first, (i) structure–property relations to optimise the magnitude of achievable actuation are discussed and then different exemptions and their origin. They apply especially to smectic systems with a strong segregation between mesogenic core and the interlayer region as they are found, e.g., in diluted smectic LC-polysiloxanes. In the next chapter, concepts to make LCEs better applicable as active elements in micromachines are discussed. To allow actuation in a device, which stays – as a whole – at constant temperature, the incorporation of ohmic heaters (electrical actuation) or suitable chromophores (photochemical actuation) is discussed. Finally, a system is presented which can be moved around in the decimetre range by magnetic fields and actuated locally (micrometre range) by a thermally or photochemically induced phase transition.

Nematic and smectic bithiophenes for UV sensing mechanism: Comparative calculations on different homologues

Four liquid crystalline molecules in the homologus series of 5,5′-Bis-(alkylpyridinyl)-2,2′-bithiophenes (nPBTP with n = 7, 8, 10, 12) have been investigated for UV sensing mechanism. Optimization of these compounds has been carried out through hybrid functional B3LYP along with 6-31 + G (d) basis set based on the crystallographic parameters as input. The UV absorption spectra of the molecules have been calculated and the configuration interaction singles (CIS) approach has been employed to study the electronic transitions features along with excited states calculation with the help of semiempirical Hamiltonian ZINDO. The INDO/S + CI method has been implemented for the same for comparison of the results. Various spectral parameters associated have been reported. The HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energies estimations, and the transition energies, the electron excitation nature concerning on the structural variations in the molecules are clarified. The results obtained suggest model UV sensing mechanism for full colour display applications. A comparison has been made between nematic and smectic systems.

Highly polarized absorption and emission from polymer-stabilized smectic guest-host systems

ABSTRACTThe optical properties of luminescent molecules dissolved in liquid crystals have led to their proposed use in luminescent thin-film polarizers. These molecules are typically required to fulfil a range of criteria related to their absorption and emission properties, degree of alignment and stability; however, concurrently satisfying these requirements has proven a barrier in their practical use. We obtained highly polarized absorption and emission luminescent thin-film polarizer using reactive liquid crystalline monomers and highly dichroic luminescent dyes, both of which have polymerizable end groups. In situ photopolymerization of the liquid crystalline mixtures in the highly ordered SmB phase resulted in the formation of cross-linked polymeric networks in which the anisotropic absorption and emission of the film were fixed. The as-obtained product exhibited a high dichroic ratio (DR = 30) with a large fluorescence quantum yield (ϕF = 0.77). The device is both cheap and easy to fabricate and has the potential to be used in practical electro-optic applications.

Systems with competing interlayer interactions and modulations in one direction: finding their structures.

Complex structures in polar smectic systems can be studied within framework of discrete phenomenological models. Considered interactions are usually described by nonlinear trigonometric functions that do not allow for a straightforward search for solutions. The review of three methods reported in the literature are presented and their appropriateness, advantages and disadvantages are discussed. Examples are given as an illustration for each method.

Effect of intralayer inhomogeneity on helical superstructures of liquid crystals

A mechanism to form distorted helical structures in ferroelectric liquid crystals is presented. It is shown that the deformation of helical interlayer structures of thin smectic systems can be considered as a direct consequence of the surface-induced spatial inhomogeneity in the distribution of the azimuthal molecular orientation within smectic layers. The intralayer azimuthal nonuniformity occurring in helical phases is argued to generate a local depolarizing electric field of the strength varying not only in smectic layers but also between the layers. The resulting modulation of the depolarization level in the direction of helical axes is shown to lead to a distortion of helices. Using a simple model, including the depolarization interaction, it is demonstrated that the degree of the helix deformation strongly varies as parameters of the model are changed. It is also shown that strong deformations of extremely short helices, found for appropriate values of the parameters, reflect nonuniformity of helicoidal superstructures of respective smectic subphases in real liquid crystalline systems.

Unsymmetric main-chain liquid crystal elastomers with tunable phase behaviour: elastic response

In this paper are reported the results of an investigation of the mechanical and thermoelastic properties of a new series of polysiloxane-based MC-LCEs containing different unsymmetric mesogenic units, siloxane spacers and cross-linkers. The structure of the networks has been varied systematically in order to study their properties. The mechanical properties of polydomain and monodomain elastomers were characterised by stress and strain measurements. The mechanical behaviour of the elastomer networks can be divided into two groups according to the phases they presented. Elastomers that contained flexible cross-linker, C1, and showed a SmC phase, demonstrated a non-linear elastic response when stress was applied in the axis of the domain director, in contrast to earlier experiments. In the axis normal to the director, at low strains, the response was linear and the Young's modulus was ten times lower compared to that in the axis of the director, demonstrating the marked anisotropic nature of these systems. The elastomers containing cross-linker C2 were nematic and showed lower Young's moduli compared to the smectic systems. The thermoelastic response of these systems showed spontaneous elongations up to 88% of strain when the samples were cooled from the isotropic to the liquid crystal phase, supporting stresses of 120 kPa. The properties are discussed in terms of the chemical structure of the networks in order to establish a correlation.

Relaxational dynamics of smectic phases on a curved substrate

We study the dynamics of pattern formation of two-dimensional smectic systems constrained to lie on a substrate with sinusoidal topography. We observe a coupling between defects and geometry that induces the preferential location of positive (negative) defects onto regions with positive (negative) Gaussian curvature. For the curvatures studied here we observe unbinding and self-organization of disclination pairs. The local orientation of the pattern and the location of topological defects can be accurately controlled with the curvature of the underlying substrate. Thus, long-range interactions arising from the geometry of the substrate lead to ordered patterns with potential applications to nanotechnology.

Main-chain smectic liquid-crystalline polymers as randomly disordered systems

We report a high-resolution X-ray lineshape study of main-chain smectic polymers. The results indicate that the layer ordering differs fundamentally from the algebraic decay typical for other smectic liquid-crystalline systems. The lineshapes are best described by broad squared Lorentzians indicating some form of short-range correlations. However, several higher harmonics are observed, which excludes simple liquid-like short-range order. This behaviour is tentatively attributed to a random field of defects associated with entangled hairpins in the main-chain polymer structure.

Resonant X‐ray Scattering: A Tool for Structure Elucidation in Liquid Crystals

The use of resonant X-ray scattering to determine structures in liquid crystal systems is surveyed. This powerful experimental technique utilises "forbidden reflections" to determine the subtle differences in interlayer orientation that differentiate several smectic systems. The technique relies on the materials containing an atom to which the X-ray energy can be tuned, usually sulphur or selenium. Experiments are often carried out on free-standing films that provide a highly monodomain structure that allows high-resolution measurements to be made, and, hence, structural details to be determined. Alternatively, resonant scattering has been demonstrated for materials contained in glass devices that permit the application of electric fields to the system, in a manner analogous to that used in liquid crystal devices. The resonant scattering technique provides unequivocal descriptions of the packing in smectic systems, and the way in which the packing is distorted in electric fields. This Minireview describes the principles behind resonant X-ray scattering, its application to liquid crystals and some of the potential for the future.

Rheology of a dispersion of low-molar-mass liquid crystal droplets in polydimethylsiloxane

We describe the linear viscoelastic response of dispersions of droplets of two biphenylcarbonitriles that exhibit both isotropic and liquid crystalline phases, 4′-pentyl-4-biphenylcarbonitrile (5CB) and 4′-octyl-4-biphenylcarbonitrile (8CB), in polydimethylsiloxane (PDMS). The Palierne emulsion model agrees well with the storage modulus data for 8CB in both isotropic and nematic phases, but the model overpredicts the magnitude of the interfacial relaxation in the 5CB dispersion, where the volume fraction of the dispersed phase required to fit the data is substantially smaller than the actual value. Similarly, the interfacial tension of 5CB against PDMS deduced from the Palierne fit shows much less temperature dependence than values measured with pendant drop tensiometry. The 8CB droplets are small, with a narrow size distribution, while the 5CB droplets are considerably larger and have a high degree of polydispersity. The basic mechanics of the Palierne theory appear to be applicable to nematic liquids, at least under conditions where the entropic elasticity is weak relative to the interfacial stress; the data suggest, however, that the theory is not applicable to smectic systems, and it needs to be used with considerable care with large droplets accompanied by a large degree of polydispersity.

Soft Material Characterization of the Lamellar Properties of Starch: Smectic Side-Chain Liquid-Crystalline Polymeric Approach

We compare and contrast the soft lamellar material properties of starch with those of a canonical multilayered microemulsion via the modeling of experimentally observed small-angle X-ray scattering (SAXS) patterns. Consequently, and by using a well-known lamellar microemulsion system as a valuable point of reference, model analysis of the SAXS data enables us to quantitatively characterize the lamellar structure of starch for the first time. We argue that this work provides strong evidence for viewing the lamellar structure of starch as a smectic side-chain liquid-crystalline polymer. Pursuing this approach, we are able to shed light on many of the interesting lamellar features of starch found in this work, over and above those found in more familiar smectic systems. We also address, and draw attention to, the possible use of valuable lessons gained from this work in the design and synthesis of novel soft lamellar materials.

Pseudo-Casimir force in chiral smectic liquid crystals.

We present a theoretical study of the pseudo-Casimir force in two chiral smectic systems: a homeotropic cell and a free-standing film. We consider the interaction induced by the fluctuations of orientational order. We demonstrate how the character of the force depends on the type of fluctuation modes and on boundary conditions. We focus on the temperature dependence of the force, which is marked by the vicinity of the smectic-A*-->smectic-C* phase transition. We find that at this transition the force diverges if the system is frustrated; otherwise it remains finite. We expose the analogy between the force in these smectic systems and in previously studied nematic systems, thus demonstrating the universality of the pseudo-Casimir interaction.

Smectic phases of semiflexible manifolds: constant-pressure ensemble.

We pursue the constant-pressure ensemble approach to elucidate the statistical mechanics of the smectic phases of semiflexible manifolds, such as two-dimensional smectic phases of long semiflexible polymers and three-dimensional lamellar fluid membrane phases. We use this approach to consider in detail sterically stabilized phases of semiflexible polymers in two-dimensional (2D) smectic systems. For these 2D systems, we obtain the universal constants characterizing the entropic repulsion between semiflexible polymers, such as those in the osmotic pressure P=alpha(k(B)T)(4/3)/kappa(1/3)(a-a(min))(5/3) with alpha found here to be congruent with 0.432 (here, a is the smectic phase period, and a(min) and kappa are the polymer cross-sectional diameter and bending rigidity constant, respectively). We address, by numerical simulations and analytic arguments, finite stacks of N semiflexible manifolds, and discuss in detail the practically interesting thermodynamic limit N--> infinity. We show that the thermodynamic limit is quickly approached within the constant-pressure ensemble: Already from numerical simulations involving just few semiflexible polymers under constant isotropic pressure, one can obtain the infinite 2D smectic equation of state within a few percent accuracy. We use our results to discuss the competition of electrostatic and entropic effects in quasi-2D smectic phases of DNA-cationic-lipid complexes. We use our quantitative results to discuss in detail the elasticity, topological defects, anomalous elasticity, and the effects of externally applied tension in sterically stabilized 2D smectic phases of long semiflexible polymers.

Chiral polymer hexatics: a new twist on DNA.

Theories of the " N+6" hexagonally ordered phase of DNA are generally based on treatments of hexatic order in smectic systems. Thus N+6 phases should be analogous to superconductors and should expel twist of the nematic director, in an analogy of the Meissner effect. However, in N+6 systems there is no smectic order, hence a radically different mathematical description is required. Here I present the appropriate theory and show that, as N+6 phases are not analogous to superconductors, they can exhibit twist in any direction, without forming any topological defects.

The Curvature Walls in Lyotropic Lamellar Phases

We have studied the textures of lamellar phases of lyotropic systems in the sponge-lamellar domain of coexistence. The typical defects of the smectic systems (focal conics domains or “FCDs”) are present, but our observations show also the existence of surfaces on which the director is discontinuous at optical scales. We have interpreted those surfaces as curvature walls. After having given their main geometric and energetic properties, we examine different cases (static and dynamic) which illustrate the importance of curvature walls in the formation of the lamellar textures.

PBa15 誘起スメクチック(pseudo側鎖型液晶共重合体/低分子液晶/カイラルドーパント/色素)四元複合系の熱相転移に基づくレーザー熱書き込み特性

PBa15 誘起スメクチック(pseudo側鎖型液晶共重合体/低分子液晶/カイラルドーパント/色素)四元複合系の熱相転移に基づくレーザー熱書き込み特性

Spacer Length Dependence of Mesomorphic Characteristics and Bistable Electro-Optical Effects for Smectic (Liquid Crystalline Copolymer/Liquid Crystals) Composite Systems

Mesomorphic characteristics and bistable electro-optical switching effects of smectic composite systems composed of side chain liquid crystalline copolymer (LCcoP) with a weak polar methoxy terminal group in a side chain and low molecular weight nematic liquid crystals (nematic LCs) with each strong polar cyano end group have been investigated. The spacer length of the mesogenic side chain in LCcoP has strongly affected the mesomorphic characteristics and the electro-optical effects of the (LCcoP/LCs) composite systems. The response speed of the binary composite systems became faster with a decrease of the spacer length. The binary composite systems showed reversible and bistable electro-optical switching characteristics with response time of ∼100 ms, that is, a light scattering state and a transparent one upon the application of ac electric fields with low frequency (up to - Hz) and high one (10–105 Hz), respectively. However, a light scattering state was also observed due to the sign reversal of dielectric anisotropy upon the application of an ac electric field above ∼105 Hz.

The Electroclinic Response in a Network-Stabilized Cholesteric Phase

The electroclinic effect is a very fast response of the optic axis in chiral orthogonal smectics (like SmA*) on the application of an electric field. In smectic systems it represents a soft mode which takes on a high amplitude near the A*→C* transition but otherwise is limited to a tilt of typically 2–5°. This response can also be found in chiral nematics, although its origin here is still obscure and so small–typically 0.1° or less–that it has been of academic interest only. In two cases investigated of a cholesteric phase which has a direct transition to SmC*, the electroclinic effect could be enhanced to the same level as in SmA* materials by photoreticulation without slowing down the response speed. Remarkably, the temperatures at which this high amplitude/high speed response is measured are actually lower than in the corresponding measurements on the basic mixtures.

Matter diffusion in hexagonal columnar phases

Matter diffusion experiments are carried out in hexagonal columnar phases of mesogenic disk-like and polycatenar compounds. The diffusion characteristics of two dyes of different sizes diffused into these mesophases are determined by measuring the diffusion coefficients along the columns and perpendicularly to them. The nature of diffusion versus the structure of these mesophases in discussed in comparison with previous results obtained in nematic and smectic systems.

2-8b (側鎖型液晶共重合体/低分子液晶) 誘起スメクチック複合系の液晶特性と双安定性光スイッチングメモリー特性に及ぼす側鎖スペーサ長の影響

2-8b (側鎖型液晶共重合体/低分子液晶) 誘起スメクチック複合系の液晶特性と双安定性光スイッチングメモリー特性に及ぼす側鎖スペーサ長の影響