Aliphatic polyimides as alignment layers for surface stabilized ferroelectric smectic C liquid crystal cells

Eight aliphatic polyimides have been synthesized and tested as alignment layers for surface stabilized ferroelectic liquid crystals with a number of room temperature ferroelectric mixtures. The cone angles are used as a quantitative measure of the bistability obtained in these cells. The effect of the structure of the polyimides on the cone angles obtained is discussed. It is shown that the structure of the polymer alignment layer strongly influences the cone angle found between the relaxed states in a surface stabilized ferroelectric liquid crystal cell. Highly crystalline polymers give a larger cone angle than less crystalline homologues.     

Stationary states of the surface stabilized ferroelectric liquid crystal layers in electric field

Stationary states of surface stabilized ferroelectric liquid crystal layers in an electric field are analysed by use of the Taylor expansion method based on catastrophe theory. Two kinds of director distribution within the flat smectic layers are taken into account: the uniform and the presplayed one. The butterfly catastrophe describes the properties of the cells correctly. The results have a qualitative character. Two categories of transitions can be predicted: switching between stable states characterized by opposite uniform orientations of the polarization vectors, and deformation of the director field which relaxes after removing the field. The threshold field strengths are found and the role of the system parameters is investigated.     

Computer simulations of interactions between liquid crystal molecules and polymer surfaces II. Alignment of smectic C-forming mesogens

Computer simulations have been performed to examine the behaviour of the liquid crystalline molecule 4-n-heptyl-2-fluoro-phenyl 4-n-octyloxybiphenyl-4-carboxylate (MBF) when in contact with crystalline polymer surfaces. The simulations form part of a study of the alignment interactions that are found in liquid crystal displays. MBF forms several smectic phases including a chiral smectic C* phase when suitably doped. In this paper we examine the way that layers of MBF molecules interact with the structure of the crystalline polymer surface, with the aim of understanding how molecular level interactions give rise to macroscopic phenomena such as the cone angle in ferroelectric liquid crystal devices. Molecular dynamics simulations consisting of a fixed crystalline polymer surface in contact with either a single MBF molecule or up to two layers of them (48 molecules) have been performed. A variety of simple polymer surfaces have been examined and the simulations show that the cone angle is highly dependent on the geometry of both the liquid crystal molecule and the polymer substrate. For molecules of MBF on polyethylene substrates, a cone angle of 20 is predicted, in line with experimental findings.     

Naphthyl benzoates versus phenyl benzoates A molecular structure-liquid crystal and ferroelectric behaviour relationship study

A comparative study of the liquid crystal and ferroelectric properties of several achiral, racemic and non-racemic 2,6-naphthyl benzoates and their 1,4-phenyl benzoate analogues is reported. In terms of pure compounds, both central core structures give rise to similar mesophase sequences; however, the naphthalene derivatives exhibit broader and more thermally stable liquid crystalline states. On the other hand, both central core units give rise to equivalent electric and optoelectronic properties when the compounds are used as chiral dopants in ferroelectric liquid crystal mixtures.     

Topological ferroelectric bistability in a polarization-modulated orthogonal smectic liquid crystal

We report a bent-core liquid crystal (LC) compound exhibiting two fluid smectic phases in which two-dimensional, polar, orthorhombic layers order into three-dimensional ferroelectric states. The lower-temperature phase has a uniform polarization field which responds in an analog fashion to applied electric field. The higher-temperature phase is a new smectic state with periodic undulation of the polarization, structurally modulated layers, and a bistable response to applied electric field which originates in the periodically splay-modulated bulk of the LC rather than by surface stabilization at the cell boundaries.     

Electromechanical effects in ferroelectric liquid crystalline polymers

Electric fields can induce mechanical vibrations in planar aligned sandwich cells of ferroelectric liquid crystals. Measurements on a polysiloxane and a polyacrylate side chain polymer proved that the electromechanical effect also exists in ferroelectric liquid crystalline polymers. The main characteristics of the electromechanical responses of these polymers are described and compared to the response of low molecular weight ferroelectric liquid crystals. According to the proposed interpretation, the vibrations in the direction parallel to both the smectic layers and cell substrates are due to coupling between the director rotation and the flow, while the resonances in the vibrations normal to the plates are connected to layer deformations.     

Ferroelectric Ionic Molecular Crystals with Significant Plasticity and a Low Melting Point: High Performance in Hot-Pressed Polycrystalline Plates and Melt-Grown Crystalline Sheets

Among ferroelectric crystals based on small molecules, plastic/ferroelectric crystals are currently receiving particular attention because they can be used as bulk polycrystals. Herein, we show that an ionic molecular ferroelectric crystal, guanidinium tetrafluoroborate, exhibits significant malleability and multiaxial ferroelectricity despite the absence of a plastic crystal phase. Powder samples of this crystal can be processed into transparent bulk crystalline plates either by press-forming or by melt-growing. The plates show high ferroelectric performance and related properties, demonstrating the largest hitherto reported spontaneous polarization for bulk polycrystals of small-molecule-based ferroelectrics. Owing to the ready availability of large-scale materials and processability into various bulk crystalline forms, this ferroelectric crystal represents a highly promising functional material that will boost research on diverse applications as bulk crystals.     

旋光性聚合物的铁电性能

旋光性高分子在结构上具有一定的空间特异性,因而可以有多种应用。根据铁电材料所必需具备的条件,经过特定的分子结构设计,可以使之具备铁电性能,据此,人们已合成了许多不同类型的铁电聚合物。本文按照铁电聚合物所处的聚集态不同,分别综述了具有铁电特性的晶态和液晶态旋光性聚合物的种类、结构特点及其铁电性能,分析讨论了影响液晶态旋光性聚合物的自发极化率和响应时间的因素。     

New ferroelectric and antiferroelectric liquid crystals studied by complementary methods

Four new synthesised liquid crystalline compounds belonging to the homologous series of fluorinated biphenyl benzoate esters have been studied to compare their dielectric and electrooptic properties. Three of the studied compounds exhibited ferro- and antiferroelectric phases while one of them exhibited only one liquid crystalline phase – ferroelectric SmC*. No paraelectric phase was detected and straight transition between isotropic liquid and ferroelectric phases was observed for all studied compounds. Tilt angle for all of the studied compounds was equal to ca. 45? in the liquid crystalline phases, except temperature range close to the isotropic liquid–ferroelectric smectic phase transition. Temperature dependences of helical pitch, spontaneous polarisation and switching time have been determined. Based on XRD results, temperature dependence of the layer thickness has also been found. Only one relaxation process has been revealed in the ferroelectric as well as antiferroelectric phases, even the bias field up to 8 V/µm was applied. The dielectric and electrooptic data are discussed based on the mean-field theory predictions.     

The influence of layer curvature on switching in ferroelectric B2 phases of liquid crystals having bent-shape molecules

In the B₂ phase of liquid crystalline compounds with bent-shape molecules ferroelectric switching can occur either by molecular rotation on the cone or by rotation of the molecule about its long axis (so-called chirality flipping), or by both mechanisms simultaneously. When the smectic layers of the B₂ phase are non-deformed and parallel the rotation of molecules under an external electric field occurs readily on the surface of the cone, while rotation around the long molecular axis is hindered by an energy barrier. Imposed deformation of smectic layers leads to interaction between local layer curvatures and molecular orientation, which results in the energy barrier hindering the molecular rotation by a cone. For appropriate constants describing this interaction the energy barrier can be so high that chirality flipping becomes the principal switching mode. An increase in the electric field can eliminate layer curvature, and therefore the energy barrier, so that switching with molecular rotation on the cone becomes possible. In the present contribution these mechanisms of switching are discussed and the influence of layer curvature on the switching mode is demonstrated.     

Phase separations in liquid crystal-colloid mixtures

We present a mean-field theory to describe phase separations in mixtures of a nematic liquid crystal and a colloidal particle. The theory takes into account an orientational ordering of liquid crystals and a crystalline ordering of colloidal particles. We calculate phase diagrams on the temperature-concentration plane, depending on interactions between a liquid crystal and a colloidal surface and a coupling between nematic and crystalline ordering. We find various phase separation processes, such as a nematic-crystal phase separation and nematic-isotropic-crystal triple point. Inside binodal curves, we find new unstable and metastable regions which are important in phase ordering dynamics. We also find a stable nematic-crystalline (NC) phase, where colloidal particles dispersed in a nematic phase can form a crystalline structure. The coexistence between two NC phases with different concentrations can be appear though the coupling between nematic and crystalline ordering.     

Theoretical studies of the influence of backflow on the dynamical behaviour of a Fredericks transition of a ferroelectric smectic C* liquid crystal in the bookshelf geometry

The elastic-hydrodynamic theory for the ferroelectric smectic C* phase is reviewed and the governing equations for the dynamical behaviour of a surface stabilized ferroelectric liquid crystalline cell are written down, taking the possibility of macroscopic mass flow in the system into account. The influence of backflow effects on the dynamical cell behaviour and a control parameter, determining whether backflow effects will be of importance or not, is derived. It is shown that when backflow effects are pronounced, the response time of the switching can be considerably decreased. Also, the shape of the c-director and velocity profiles across the cell are shown to be strongly dependent on the presence of backflow.     

Structure of surface-stabilized antiferroelectric liquid crystals cells

A comprehensive expression describing the director profile and energy parameters of surface-stabilized antiferroelectric liquid crystals has been derived. The expression is based on a previously developed ferroelectric liquid crystal model in which two contiguous smectic layers have been simultaneously computed. The antiferroelectric behaviour is mathematically described as an additional energy term relating these layers to each other through a coupling constant. The new energy term gives significant differences in the material behaviour, as compared with the ferroelectric phase. Electrically induced antiferroelectric-ferroelectric phase transitions are predicted as well. The effect of every term on the general expression has been analyzed. Comparisons with ferroelectric results are also included.     

Alpha sub-phase in a new ferroelectric fluorinated compound

Dielectric and DSC methods were used to study a new fluorinated liquid crystalline compound exhibiting ferroelectric and paraelectric phases as well as an intermediate alpha sub-phase. Two dielectric relaxation processes were revealed in the SmC* phase: a typical Goldstone mode over the whole temperature range and a soft mode in the pre-transition region on both sides of the SmC*–SmA* transition. From the temperature dependencies of the dielectric increments and critical frequencies for the dielectric relaxation processes observed in all the liquid crystalline phases, as well as from texture observations, it was shown that there is a SmC*_α sub-phase between the ferroelectric SmC* and paraelectric SmA* phases.     

Ferroelectric side group liquid crystalline polysiloxanes containing a chiral sulphinate derivative

A series of chiral liquid crystalline homo- and co-polysiloxanes is reported, in which the chirality is introduced via an arenesulphinate moiety. All polysiloxanes were prepared by using three polymer-analogous reactions and exhibit the ferroelectric SmC* phase. In the series of copolymers, a high heterogeneity in the ratio of mesogenic pendants is observed and explains the existence of large biphasic regions at the phase transitions. A complete characterization of the ferroelectric properties was performed for the copolysiloxanes, and revealed high values of the spontaneous polarization.     

Ferroelectric side group liquid crystalline polysiloxanes containing a chiral sulphinate derivative

A series of chiral liquid crystalline homo- and co-polysiloxanes is reported, in which the chirality is introduced via an arenesulphinate moiety. All polysiloxanes were prepared by using three polymer-analogous reactions and exhibit the ferroelectric SmC* phase. In the series of copolymers, a high heterogeneity in the ratio of mesogenic pendants is observed and explains the existence of large biphasic regions at the phase transitions. A complete characterization of the ferroelectric properties was performed for the copolysiloxanes, and revealed high values of the spontaneous polarization.     

Ferroelectric switching and electrochemistry of pyrrole substituted trialkylbenzene‐1,3,5‐tricarboxamides

We explore a new approach to organic ferroelectric diodes using a benzene‐tricarboxamide (BTA) core connected with C10 alkyl chains to pyrrole groups, which can be polymerized to provide a semiconducting ferroelectric material. The compound possesses a columnar hexagonal liquid crystalline (LC) phase and exhibits ferroelectric switching. At low switching frequencies, an additional process occurs, which leads to a high hysteretic charge density of up to ～1000 mC/m². Based on its slow rate, the formation of gas bubbles, and the emergence of characteristic polypyrrole absorption bands in the UV–Vis–NIR, the additional process is identified as the oxidative polymerization of pyrrole groups, enabled by the presence of amide groups. Polymerization of the pyrrole groups, which is essential to obtain semiconductivity, is limited to thin layers at the electrodes, amounting to ～17 nm after cycling for 21 h. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 673–683     

Rotational isomerism in 1,3-dimercaptopropane

It is well known that the molecules of 1,2-dihaloethanes [1], 1-halopropanes [2] and 1,2-dihalopropanes [3] exist in more than one rotational isomeric form trans (T) and gauche (G) in the liquid state and only in the trans (T) form in the crystalline state at low temperature. In the case of 1,3-dihalopropanes [4], however, the molecules exist in dynamic equilibrium of three types of rotameric forms, TT, TG and GG in the liquid phase, while GG is the only stable form in the crystalline phase. The number of rotational conformers present in 1,2-ethanedithiol [5], 1-propanethiol [6] and 1,2-propanedithiol [7] and their stabilities in the liquid and crystalline solid phases are observed to be similar to those of the corresponding haloalkanes [1–3]. Since such studies with 1,3-dimercaptopropane have not yet been made, its infrared spectrum in the liquid state and its Raman spectra in the liquid and solid states have been investigated. The present communication reports the results and their analyses with reference to the possibility of rotational isomerism in the molecule.     

Tolane oligomers: Model thermotropic liquid crystals

The liquid crystalline phases of several rigid-rod, non-polar tolane oligomers are characterized by differential scanning calorimetry and transmitted polarized light microscopy. We determine that a stable nematic phase can be formed at ambient pressure if the molecular axial ratio (length-to-width ratio) is greater than 4.5. A smectic phase forms in addition to the nematic phase if the axial ratio exceeds 6.1. Symmetrical fluorination of the terminal phenyl groups reveals that the liquid crystalline phase behaviour of these rigid rods is highly sensitive to perturbations of the charge distribution along the molecules. Nematic tolane oligomers can exhibit high stregth disclinations (s = ±3/2 and ±2) in their schlieren textures, and we discuss conditions that promote the stability of these defects.