The Dynamic Properites of Confined Antiferroelectric Liquid Crystal Investigated By Photon Correlation Spectroscopy
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of local spontaneous polarization is a determining factor for ferrielectricity. The Ising model [6], adapted to liquid crystals [4], takes into account the long-range electrostatic interaction between all smectic layers (spins) and predicts the existence of an infinite number of ferrielectric phases [4]. According to the Ising model, the ferrielectric phases can be characterized by the parameter qT=F/(A+F), which denotes the fraction of ferroelectric ordering (F) which appears together with antiferroelectric ordering (A) in the periodic structure. Following this definition, the SmCA is the antiferroelectric phase with qT =0, and the SmC* is ferroelectric with qT =1. However, practically, only a few ferrielectric phases have been observed: the ferrielectric phase SmCy (qT =1/3), antiferroelectric phase AF (qT=l/ 2 ), and some other ferrielectric subphases (FiLC, SmC0) that cannot be described by the qT parameter. The effect of the surface on the appearance of ferrielectric phases has been reported in several publications [7,8]. Substrates may cause a preferable orientation of molecular tilt, and also affect the delicate energy balance among ferrielectric phases. Therefore in a sufficiently thin cell the ferrielectric phases can be suppressed or induced by the surface [7,8]. In cylindrical pores the molecular tilt directions are equivalent with respect to azimuthal angle if planar boundary conditions are assumed. Therefore we expect that in cylindrical pores the effect of the surface on the appearance of ferrielectric will be negligibly small. Dynamic light scattering provides information about relaxation processes in the sample, which are different in different ferrielectric/antiferroelectric phases. The relaxation processes in 27 Mat. Res. Soc. Symp. Proc. Vol. 559 ©1999 Materials Research Society
bulk ferrielectric/antiferroelectric phases have been studied theoretically [9,10,11] and experimentally by dielectric [7,12-14] and electrooptic [10,12] spectroscopy and dynamic light scattering [11,14]. A total of four relaxation processes have been observed: two molecular processes due to the reorientation around the short and long axes, and two collective processes, viz., an optic-like phason [11] (or anti-phase mode [13]) and acoustic-like phason [II] (or inphase mode [13]). In this paper we report on the influence of confinement on the appearance of antiferroelectric/ferrielectric phases in AFLCs. EXPERIMENT We have performed investigations in both bulk and AFLC confined in cylindrical pores (Anopore membranes) of two different diameters: 200 A and 2000A. The AFLC used in the experiments was (R) - 1 - methylheptyl - 4 -(4" n- dodecyloxybiphenyl-4'carbonoyloxy) - 3 fluorobenzoate. (AS-573, Hull, UK). The phase transitions sequence of the bulk sample (0.5°C) has been determined by the macroscopic polarization measurements and dielectric spectroscopy [7]: SmCA 78 TC SmCj 83 TC AF 85 TC FiLC 91 °C SmC* 93 °C SmA 106 TC Is. In our experiments the bulk liquid crystal was placed between two untreated glass plates having
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