X-ray and dielectric measurements of the smectic-A–hexatic-Btransition in bulk and confined geometries

Abstract

We report x-ray and dielectric measurements on a liquid crystal exhibiting a smectic- A -hexatic- B (Sm- A -Hex- B ) transition in bulk and confined geometries. The confined geometry experiments have been performed by filling the compound, n -butyl 4(') - n -hexyloxybiphenyl-4-carboxylate, into Anopore membranes with 20 nm pore size. The surprising result seen in the x-ray studies on both the bulk and the Anopore samples is the presence of unusually strong second harmonic reflection arising from the smectic layers in both the Sm- A and Hex- B phases. In fact, the third harmonic reflection is also observed in the entire temperature range of the Hex- B phase and close to the transition in the Sm- A phase of the bulk sample, with the ratio of I(2qo) / I(qo), and I(3qo) / I(qo) being approximately 1/40 and 1/470, respectively (here I(qo), I(2qo), and I(3qo) correspond to the peak intensities of the first, second, and third harmonic reflections). These features point to the fact that the layering is much better defined in this system, unlike the usual description of pure sinusoidal mass density wave with a typical value of 10(-4) for I(2qo) / I(qo). Another notable feature observed is that whereas the extent of positional correlations of the molecules within the smectic layer, is comparable in the Sm- A phase of the bulk and Anopore samples, the correlations are reduced in the Hex- B phase. In the bulk as well as the Anopore case, the critical behavior of the positional correlations is found to be well described by a power-law expression with the exponent alpha being slightly larger for the Anopore sample. It is also seen that the weak first order nature of the transition seen in the bulk, gets weakened further, perhaps becoming second order. Despite such significant differences between the bulk and Anopore samples, the dynamics associated with the director fluctuations is only slightly influenced by confinement.

Electro-optical Characteristics of Ferroelectric Liquid Crystal Embedded in Photonic Crystal Fiber

“…Confinement effects have been found in studies that utilized various experimental methods. Porous glasses, [3][4][5] aerosol dispersion, 6) millipore cellulose membranes, 7,8) silica gels, 9) and anopore membrane filters [10][11][12][13][14][15] have been used as the porous medium. Among these, an anopore membrane has a cylindrical microstructure.…”

“…Among these, an anopore membrane has a cylindrical microstructure. The characteristics of LCs in an anopore membrane has been investigated by X-ray diffraction (XRD), [8][9][10][11][12] differential scanning calorimetry (DSC), 5,7) nuclear magnetic resonance (NMR) spectroscopy, 4,16) dielectric spectroscopy, 8,9,[12][13][14][15][16] selective reflection, 17) and infrared spectroscopy. 3) Results of XRD experiments using anopore membranes having 200 nm pores revealed that the tilted phase of FLCs remains.…”

Unusual Dielectric and Electrical Switching Behavior in the de Vries SmecticAPhase of Two Organosiloxane Derivatives

“…Closer to the transition to the Sm C Ã phase, the electroclinic response shows a rapid increase with field, and moves towards a limiting value at high fields, resulting in ''Langevin behavior'' in chiral de Vries materials. Despite many reports on the various aspects of the de Vries SmA phase including a transition from the normal SmA to de Vries phase, and also between two variants of de Vries phases, the structural aspects of this phase are not yet completely understood [3][4][5][6][7][8][9][10]. In this Letter we present a novel finding of an antiferroelectric (AF)-like switching and dielectric behavior of this phase and suggest a model to explain it, which should be common especially to systems having structurally incompatible termini of the molecule, such as siloxane and hydrocarbon chains.…”