Ferroelectricity molecular characteristics

Ferroelectric liquid crystals (FLC) have attracted attention because of their high speed response and memory effect (7-5). The characteristics of fast response and memory effect make them suitable in electro-optical device applications, such as display, light valve and memory devices. Ferroelectric side chain liquid crystalline polymers (FLCPs) exhibit desirable mechanical properties of polymers and electro-optical properties of low molecular weight FLC, which have been investigated extensively Corresponding author. 

For long-chain molecules there are different geometric possibilities for the orientation of molecular dipole vectors with respect to the backbone. Following the notation of Stockmayer (1967), polymers are classified as type A (with dipoles fixed parallel to the mainchain, e.g., ds-l,4-polyisoprene and polyethers), type B [with dipole moments rigidly attached perpendicular to the mainchain e.g., poly (vinyl acetate) and most synthetic polymers], or type C [with a more-or-less flexible polar sidechain e.g.,poly(n-alkyl methacrylate)s]. However, a polymer possessing only one type of dipole moment is an exceptional case. The timescale (speed) of each polarization (and subsequent relaxation) process will determine whether this process will be monitored by a particular dielectric technique. Characteristics and fundamental peculiarities of relaxations generally found in polymers are discussed hereafter. Note that cases where finite polarization is present even in the absence of an external field (e.g., the permanent polarization in ferroelectrics) are not considered. 

In general, the spontaneous pdarization in the Sc phase is about two orders of magnitude lower than one could expect if the dipoles were to add to frill strength. Depending on the molecular structure of the FLC, the magnitude of polarization varies typically between 2 nC/cm and 500 nC/cm In contrast to solid-state ferroelectrics, the existence of spontaneous polarization in the Sc phase is not the result of dipole-dipole interactions but is due to a secondary effect of the intermolecular forces of the optically active molecules, as is characteristic of improper ferroelectrics. 

In other words, molecular design and the synthesis were carried to express various characteristics for the required purposes that depend on the specific application of the ferroelectric liquid crystal. For example, ferroelectric liquid crystals with large spontaneous polarization, a large tilt angle with small temperature dependence, a specific viscosity, a small rotational viscosity of the molecules along their long axis, a chiral smectic C phase over a wide temperature range, a characteristic anisotropy of the dielectric constant, a suitable phase sequence, a suitable helical pitch, photochemical stability, and so on have been newly synthesized [3-5]. 

Fume H, Takahashi T, Kobayashi S, Yokoyama H (2002) Models of molecular alignment stmcture in polymer-stabilized ferroelectric liquid crystals. Jpn J Appl Phys 40(l) 7230-7233 Giepehnann F, Zugenmaier P (1995) Mean-field coefficients and the electroclinic effect of a ferroelectric hquid crystal. Phys Rev E 52(2) 1762-1772 Giepehnann F, Hermann A, ZugeranaiCT P (1997) Experimental determination of Landau-expansion coefficients in ferroelectric liquid crystals. Ferroelectrics 200 237-256 Guymon CA, Hoggan EN, Walba DM, Qark NA, Bowman CN (1995) Phase behaviour and electro-optic characteristics of a polymer stabilized ferroelectric liquid crystal. Liq Cryst 19 719-727... 

The switching memory effect is a reflection of the fact that the electric displacement, being the function of both the applied field and the material s properties, needs some finite time to adjust to the value of the electric field. The widely accepted model of the instantaneous relationship between the electric displacement and the electric field in the NLC is invalid when the characteristic times of the director dynamics are close to the relaxation times for molecular permanent dipoles. This time scale is typically in the submillisecond range which is of great interest for modem fast-switching devices. The electric displacement (as well as the dielectric torque density) becomes a function of the static dielectric properties of the NLC, the present and past electric field, and the present and past director. We discussed the recently proposed theory and experimental verification of the phenomenon [11]. The model in Ref [11] should be applicable to dynamic reorientation of other LC phases in the appropriate range of times/frequencies. In the case of ferroelectric LCs, the theory should be supplemented by the consideration of spontaneous electric polarization. A similar approach should be also... 

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