Helix unwinding process in the chiral smectic C phase of MHPOBC as observed by conoscopy

Suwa, Shunichi; Takanishi, Yoichi; Hoshi, Hajime; Ishikawa, Ken; Takezoe, Hideo

doi:10.1080/0267829031000091147

Cited by 14 publications

(10 citation statements)

References 13 publications

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“…This results in a large positive biaxiality and a small apparent tilt angle as previously explained [9]. Later, Suwa and coworkers [7,8] estimated the director distribution in the deformed helical structure of a ferroelectric liquid crystal by using the 4 ϫ 4 matrix simulation, where a more precise quantitative estimation was tried for the first time. Though their method of using the 4 ϫ 4 matrix simulation is quite interesting, it showed apparent limitations.…”

Section: Estimation Of the Molecular Distribution Using The Conoscopimentioning

confidence: 97%

“…Since Gorecka et al [3] measured the biaxiality and the apparent tilt angle as a function of the applied field using conoscopy for a chiral smectic liquid crystal in its various phases, such as SmC * , SmC A * , and SmC ␥ * phases, this method adopting more or less the same procedure has frequently been used by a number of authors [2][3][4][5][6][7][8]. In those papers, the cells were considered as single-biaxial plates.…”

Section: B Single-biaxial-plate Approximationmentioning

confidence: 99%

“…To date, it has been accepted [8] that the use of a shorter wavelength is a more accurate conoscopic method in liquid crystals because it has the advantage of showing up more rings in the image, and this may thus reduce the errors arising from misassignment of the optic axes. However, the use of a longer wavelength leads to lower errors induced by the average-refractive-index approximation and also enlarges the observable range of the pitch of the materials that can be investigated using this technique.…”

Section: B Single-biaxial-plate Approximationmentioning

confidence: 99%

“…As stated above, conoscopy has been used for various scientific and technological applications including those for determining the refractive index ellipsoid [2][3][4][5][6], in estimating the molecular distribution [5][6][7][8], and in identifying the liquid crystalline phases [3,4,9]. Smectic C liquid crystal phases such as SmC * , SmC A * , SmC ␥ * , and AF * phases have a layered structure; the thickness of a layer is of the order of a few nanometers.…”

Section: Introductionmentioning

confidence: 99%

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Conoscopy of chiral smectic liquid crystal cells

2008

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The conoscopic method for investigating the optical properties of a liquid crystal cell is studied with the aim of determining the effects of the approximations used in the calculation on the results. We confirm that the chiral liquid crystal cell forming a helical structure can be regarded as a single biaxial plate for analyzing the conoscopic image only if the helical pitch is less than several multiples of the wavelength of light. This approximation implies that the square of the refractive index along a direction is averaged over all the layers. An incorrectly chosen value for one of the principal refractive indices to be used in the analysis of the conoscopic data can lead to an incorrect conclusion, especially for the case when the wavelength dispersion of the refractive index is neglected. A thicker cell and a longer wavelength of the incident light can minimize these limitations of the conoscopic method. We propose a novel simulation method to find the molecular distribution in a liquid crystal cell based on the average-refractive-index approximation and the conoscopic data. This is shown to be a fast, more efficient, and useful method for estimating the director distributions.

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Section: Estimation Of the Molecular Distribution Using The Conoscopimentioning

confidence: 97%

Section: B Single-biaxial-plate Approximationmentioning

confidence: 99%

Section: B Single-biaxial-plate Approximationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Conoscopy of chiral smectic liquid crystal cells

2008

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“…Mixture ͑b͒ shows an intermediate trend. As Suwa et al ͓13,14͔ correctly pointed out, the positive biaxiality cannot be explained by the conventional model, details of which will be described in the next section. Racemization affects only the spontaneous polarization and the pitch, but not the tilt angle or the quadrupolar order parameter ͓8͔.…”

Section: Conoscopic and Microscopic Experimentsmentioning

confidence: 99%

Interlayer interactions and the dependence of biaxiality of the chiral smectic-C*phase on electric field in the helical unwinding process

2007

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We investigate the unwinding process of pure and racemized mixtures of ͑1-methylheptyloxycarbonyl͒ phenyl-4Ј-carboctyloxy-biphenyl-4-carboxylate ͑MHPOOCBC͒ in homeotropic cells by studying ͑i͒ the optical texture under crossed polarizers and ͑ii͒ the tilted conoscopy as a function of the electric field for different temperatures. The tilted conoscopy yields biaxiality and the tilt angle for values of the latter up to 45°. We find that the unwinding process depends on the optical purity of the sample and the temperature. A cell with pure MHPOOCBC surprisingly exhibits a large positive biaxiality in the initial stages of the unwinding process. The magnitude of the positive biaxiality decreases with a decrease in the enantiomeric excess ͑EE͒ of the compound and eventually it becomes negative for a material having a low EE value. The negative biaxiality observed in a mixture with a low EE value is normal and can be explained by a double sine-Gordon equation proposed by Meyer. Microscopic observations show that the special unwinding behavior ͑positive biaxiality͒ is related to the helical fractures, that is, to the existence of discretely coiled helical structures. We show that these helical fractures arise from the existence of a narrow energy well in the synclinic position. This implies that the elastic constant near the synclinic position is significantly larger than in other positions. By adopting a semiempirical equation for the interlayer interactions which replaces the conventional equation, we successfully simulate the various unwinding behaviors. We reasonably conclude that the special interlayer interaction is due to the steric interactions between the two adjacent surfaces formed by somewhat tilted terminal parts of the molecules.

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