Optical and x-ray evidence of the "de Vries" Sm-A*-Sm-C* transition in a non-layer-shrinkage ferroelectric liquid crystal with very weak interlayer tilt correlation.
TL;DR: A non-layer-shrinkage fluorinated ferroelectric liquid crystal compound, 8422[2F3], has been characterized by means of optical, x-ray, and calorimetric methods and there is strong evidence that the second-order Sm-A*-Sm-C* transition is well described by the diffuse cone model of de Vries.
Abstract: A non-layer-shrinkage fluorinated ferroelectric liquid crystal compound, 8422[2F3], has been characterized by means of optical, x-ray, and calorimetric methods. The orientational distribution within macroscopic volumes, determined through wide-angle x-ray scattering and birefringence measurements, was found to be identical in the $\mathrm{Sm}\ensuremath{-}{A}^{*}$ and helical $\mathrm{Sm}\ensuremath{-}{C}^{*}$ phases. Together with the absence of layer shrinkage, this constitutes strong evidence that the second-order $\mathrm{Sm}\ensuremath{-}{A}^{*}--\mathrm{Sm}\ensuremath{-}{C}^{*}$ transition in this material is well described by the diffuse cone model of de Vries. The absolute values of the layer spacing show that the molecules aggregate to antiparallel pairs. The molecular interaction across the layer boundaries will then occur only between fluorine atoms, leading to unusually weak interlayer tilt direction correlation. This explains the experimental observations of a very easily disturbed $\mathrm{Sm}\ensuremath{-}{C}^{*}$ helix and a peculiar surface-stabilized texture. Tilt angle and birefringence values as a function of field and temperature have been evaluated in the $\mathrm{Sm}\ensuremath{-}{A}^{*}$ and $\mathrm{Sm}\ensuremath{-}{C}^{*}$ phases and the results corroborate the conclusions from the x-ray investigations.
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TL;DR: Results are discussed in terms of the packing parameters, which indicate that the phase behaviour of the thermotropic tyrosine-based ILCs shows analogies to those of lyotropic liquid crystals.
Abstract: Synthetic strategies were developed to prepare l-tyrosine-based ionic liquid crystals with structural variations at the carboxylic and phenolic OH groups as well as the amino functionality. Salt metathesis additionally led to counterion variation. The liquid-crystalline properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS, SAXS). The symmetrical ILC chlorides bearing the same alkyl chain at both the ester and ether but either an acyclic or cyclic guanidinium group displayed enantiotropic SmA2 mesophases with phase widths of 31–88 K irrespective of the head group. It was particularly the replacement of chloride in the acyclic guanidinium ILC by hexafluorophosphate that induced a phase change from SmA2 to Colr. This phase change was attributed to a higher curvature of the interface due to the larger anion, which increased the effective head group cross-sectional area of the amphiphilic ILC. The unsymmetrical acyclic guanidinium chlorides, bearing a constant C14 ester and variable alkyl chains on the phenolic position, formed enantiotropic SmA2 phases. The derivative with the largest difference in chain lengths, however, displayed a Colr phase, resulting from discoid aggregates of the cone-shaped guanidinium chloride. The results are discussed in terms of the packing parameters, which indicate that the phase behaviour of the thermotropic tyrosine-based ILCs shows analogies to those of lyotropic liquid crystals.
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TL;DR: In this paper, a calamitic hockey stick shaped mesogen was analyzed using X-ray diffraction, birefringence, density and static dielectric permittivity measurements.
Abstract: X-ray diffraction, birefringence, density and static dielectric permittivity measurements were performed on a calamitic hockey stick shaped mesogen. In addition to SmA phase, this compound exhibits two tilted mesophases – the SmC, synclinic and the SmCa, anticlinic phase. From x-ray diffraction and birefringence data, the orientational order parameters have been determined. The order parameters values drop at the SmC – SmCa phase boundary. Possible cause for the drop in the order parameter values has been discussed. Interestingly, it has been found that the dielectric anisotropy of this compound changes from positive to negative values in the SmCa phase.
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TL;DR: Ferroelectric smectic main-chain liquid crystalline elastomers were synthesized and subjected to simple shear deformation to achieve reorientation of the layer structure and the formation of a macroscopic spontaneous polarization as mentioned in this paper.
Abstract: Ferroelectric smectic main-chain liquid crystalline elastomers were synthesized and subjected to simple shear deformation to achieve reorientation of the layer structure and the formation of a macroscopic spontaneous polarization. The phase behavior and structural parameters of these elastomers are investigated and discussed. A shear-induced splitting of the X-ray scattering intensity distribution is found. A splitting of the sample in domains of two director orientations that couple differently to the shear deformation is proposed. The smectic layers also move at different affinities depending on their initial orientation. The smectic layer spacing was found to be nearly constant even though the smectic tilt angle changes significantly. The realignment of the smectic layers indicates a migration process, where depending on their original orientation the layer normals are leaving or entering the X-ray scattering plane. The spontaneous polarization was found to be linearly dependent on the dopant concentra...
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TL;DR: In this article, a thickness independent memory in a class of FLCs called the de Vries electroclinic liquid crystals was observed by dielectric spectroscopy and texture observation.
Abstract: Memory in ferroelectric liquid crystals (FLCs) is well known in the literature where thickness of the cell is less than the pitch value of the material. Here, we report a thickness independent memory in a class of FLCs called the de Vries electroclinic liquid crystals. Thickness independency of memory effect is observed by dielectric spectroscopy and texture observation. The memory observed in SmC* phase of de Vries material is entirely different from conventional FLCs. In the former case, it is the inherent property of the material but in the latter it is dependent on the cell geometry. In de Vries material, it is probably the randomization that is playing a major role.
18 citations
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TL;DR: The first H NMR investigation of a de Vries liquid crystal compound selectively labeled in the aromatic core, is reported, and the trend of the tilt angle of the deuterated moiety within the SmA and SmC* phases is monitored with an NMR study at high magnetic field.
Abstract: De Vries-type phase transitions in liquid crystalline (LC) smectogens have recently attracted the attention of the liquid crystal community due to their potential in the development of new ferroelectric (FLC) and antiferroelectric (AFLC) electrooptic systems. Indeed, one of the major problems to the application of ferroelectric smectic materials is related to the layer shrinkage at the transition from the not-tilted SmA phase to the polar SmC* phase. This shrinking normally produces zigzag defects due to the opposite distribution of chevron configurations, thus limiting the performance and quality of electrooptic devices. In the last decades, several compounds have been found to behave differently from standard ones. In particular, they do not show any layer shrinkage at the SmA–SmC* transition, and the layer spacing remains substantially constant within the ferroelectric phase. Several models have been proposed to explain such an unconventional feature. The first interpretation, still partially valid, is the one described by the crystallographer de Vries, known as the “diffuse cone model”. 5] He proposed that in these systems LC molecules are tilted, with respect to the layer normal (“l” in Scheme 1), also in the SmA phase, but that the azimuthal angle is randomly distributed within the smectic layer. The SmA–SmC* phase transition is thus seen as a disorder–order transition in the azimuthal directions of the molecular tilt. Several experimental works on different liquid crystals showing de Vries-type transitions, basically confirmed this hypothesis, even though this does not exclude other possible explanations, such as the presence of partially interdigitation among consecutive smectic layers as well as conformational changes at the SmA–SmC* phase transition. For this reason, the nature of the de Vries transition remains a challenging aspect for researchers active in the material science field. However, in the recent years, fundamental progresses have been done in the comprehension of the de Vries -type materials. In addition to layer shrinkage (less than 5%) at the SmA–SmC* transition, the SmA phases formed by de Vries LCs have an uncommonly large electroclinic effect, which is strongly connected to the presence of significant tilt of molecules. The application of external electric fields determines an increase of the induced tilt angle, often defined as “optical tilt” to distinguish it from the molecular tilt, whose temperature dependence is well described by the Landau mean field theory. Moreover, the large soft-mode absorption detected by dielectric measurements, and the high birefringence are additional characterizing features of de Vries SmA phases. To our best knowledge, NMR spectroscopy has never been applied to de Vries-type LC systems, despite its great potential in the determination of both local and molecular properties, namely orientational and conformational ones. Herein, the first H NMR investigation of a de Vries liquid crystal compound [the (S)-hexyl-lactate derivative abbreviated as 9HL] selectively labeled in the aromatic core (Scheme 1), is reported. We monitored the trend of the tilt angle of the deuterated moiety within the SmA and SmC* phases with an NMR study at high magnetic field. This approach, recently used to investigate standard ferroelectric LCs, 28] takes advantage of the ability of high magnetic fields (H) in unwinding the supramolecular heliScheme 1. Molecular structure of the 9HL-d2 sample under investigation. Optimized geometry is displayed with the orientation of the local director of the deuterated phenyl fragment (np) and the layer normal (l) to the SmA planes (p).
18 citations