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Journal ArticleDOI

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.

11 Sep 2002-Physical Review E (American Physical Society)-Vol. 66, Iss: 3, pp 031703-031703
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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Journal ArticleDOI
TL;DR: In this article, the authors investigated the origin of the unusally high tilt angle between the optic axis and the smectic layer normal in the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements.
Abstract: We have investigated the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements in order to assess the origin of the unusally high tilt angle between the optic axis and the smectic layer normal in this material. The optical birefringence increases strongly below the transition to the tilted phases, showing that the onset of tilt is coupled with a considerable increase in orientational order. The layer spacing in the smectic A* (SmA*) phase is notably smaller than the extended length of the molecules constituting the mixture, and the shrinkage in smectic C* (SmC*) and smectic Ca* (SmCa*) is much less than the optical tilt angle would predict. These observations indicate that the tilting transition in W107 to a large extent follows the asymmetric de Vries diffuse cone model. The molecules are on average considerably tilted with respect to the layer normal already in the SmA* phase but the tilting directions are there randomly distributed, giving the ...

43 citations

Journal ArticleDOI
TL;DR: In this article, a homologous series of liquid-crystalline compounds with an azo-group in the molecular core have been synthesized and a tendency for the layer shrinkage at the SmA-SmC* phase transition to decrease with decreasing alkyl chain length was observed, with one homologue exhibiting almost no change in layer spacing.
Abstract: A homologous series of liquid-crystalline compounds with an azo-group in the molecular core have been synthesized. A tendency for the layer shrinkage at the SmA–SmC* phase transition to decrease with decreasing alkyl chain length (n) was observed, with one homologue exhibiting almost no change in layer spacing. All studied compounds exhibited the photoferroelectric effect, namely a lowering of the spontaneous polarization and tilt angle in the SmC* phase under illumination by visible light. This effect is stronger for homologues with lower n and the time constant of this process, connected with the trans–cisisomerization of the azo group, follows the Arrhenius law. In addition to these effects, a lowering of the phase-transition temperature to the ferroelectric phase by a few Kelvin also occurs. Under sufficiently high light intensity, the ferroelectric SmC* phase completely disappears. After the light is switched off, the spontaneous quantities are restored to their original values.

37 citations

Journal ArticleDOI
TL;DR: In this paper, the 2-phenylpyrimidine core was replaced with a chemically inert carbosilane end-group to improve de Vries-like properties of mesogens.
Abstract: Smectic liquid crystals with ‘de Vries-like’ properties are characterized by a maximum layer contraction of ≤1% upon transition from the non-tilted SmA phase to the tilted SmC phase. To expand the current library of ‘de Vries-like’ liquid crystals, we have developed a rational design strategy based on a concept of frustration between two structural elements, one promoting the formation of a SmA phase (chloro-terminated side-chain) and another promoting the formation of a SmC phase (siloxane-terminated side-chain). In this paper, we show that one can tune this apparent frustration—and further improve de Vries-like properties—by substituting the 2-phenylpyrimidine core in our first-generation siloxane-terminated mesogens with one of three cores known to be stronger SmC-promoting elements: 6-phenylpyridazine, 2-phenylpyridine and 2-phenylthiadiazole. We also address a fundamental design flaw of siloxane-terminated mesogens, i.e., the hydrolytic instability of siloxane oligomers, by substituting the siloxane end-group with a chemically inert carbosilane end-group. As a result of this study, we found a carbosilane-terminated 2-phenylthiadiazole mesogen that forms a SmC phase at room temperature with de Vries-like properties that are comparable to those of bona fide de Vries-like liquid crystals.

37 citations

Journal ArticleDOI
TL;DR: Landau theory shows that de Vries behavior occurs in models with unusually small orientational order and is preceded by a first order I-A transition, and shows that 1st order A-C transitions are also possible.
Abstract: We show that Landau theory for the isotropic (I), nematic (N), smectic-A, and smectic-C phases generically, but not ubiquitously, implies "de Vries" behavior: i.e., a continuous A-C transition can occur with little layer contraction while the birefringence increases significantly once the system moves into the C phase. Our theory shows that 1st order A-C transitions are also possible. These transitions can be de Vries-like, but in general need not be. Generally, de Vries behavior occurs in models with unusually small orientational order and is preceded by a first order I-A transition. These results correspond well with experimental work to date.

37 citations

Journal ArticleDOI
TL;DR: In this article, the de Vries model of SmA* phases as well as the dimers creation process were used to analyze the optical tilt Θo angle in comparison with a tilt angle obtained from X-ray measurements.
Abstract: The compounds from the fluorinated nF6B (n = 1,2,3,4,5,6,7) homologous series were investigated by means of optical, electrooptical, refractometrical, dielectric, densytometric, X-ray and IR spectroscopy methods. For some compounds being a subject of investigations a computer simulation of molecular properties were done. In our opinion high value of optical tilt Θo angle in comparison with a tilt angle obtained from X-ray measurements can be interpreted in terms of de Vries model of SmA* phases as well as of dimers creation process.

35 citations