Ferroelectric liquid crystalline polarization

The photoinduced flop of the polarization of ferroelectric liquid crystals showing a quick response was achieved by dissolving azobenzene moieties in the liquid crystals.114,11 Photocontrol of the switching behavior of ferroelectric liquid-crystalline polymers (33, Figure 14) was induced by admixing azobenzene derivatives.116... 

For the small molecular mass ferroelectric liquid crystal when reversing the polarity of the applied electric voltage the molecules rotate locally while their molecular mass centers don t necessarily move accordingly. But for side chain ferroelectric liquid crystalline polymers, as one of the side group ends is confined to backbone, shown in Figure 6.43, the polarity reversion must be accompanied by the movement of their mass centers, which causes a backflow in order to re-distribute the mass centers. Moreover, the side groups may collide with each other. The effect results in the displacement of the backbone. The above effects increase the difficulty of re-orientation and hence increase the viscosity. 

Endo et al. (1992) measured the optical transmission and the polarity-reverse current during the polarity reversion of a side chain ferroelectric liquid crystalline polymer. It was found that both parameters reached peak values at the same time. It was concluded that the rigid core of the side groups responsible for birefringence moves simultaneously with the dipole moment reversion and the latter contributes to the polarity reversion current. The FTIR experiment suggested that the backbone moves when the polarity is reversed. 

If some of the side groups are substituted by dyes or fluorescent molecules, the display may work without polarizers. It is expected that large screen ferroelectric liquid crystalline polymer displays will come out in the near future. 

Ferroelectric liquid crystals (FLC) are of great interest due to their fast electro-optical response which is about 1,000 times faster than conventional twisted nematic cells [131]. The geometry used is called a surface stabilized FLC cell which utilizes a very thin gap (=2 pm) to unwind the FLC supramolecular pitch (=1-2 pm) since the bulk FLC materials do not show macroscopic polarization. This very thin gap, however, leads to difficulties in manufacturing large panels and very poor shock resistance. Researchers have proposed the concept of microphase stabilized FLC [79,109, 130] using FLC-coil diblock copolymers for electro-optical applications as shown in Fig. 15. This concept takes advantage of ferroelectric liquid crystallinity and block copolymer microphase separation since the block... 

In addition to investigating ferroelectric liquid crystalline conjugated polymers with dynamic switching functionahties under an electric field, our group has also developed photoresponsive liquid crystalline conjugated polymers with dynamic switching of linearly and circularly polarized luminescence. This was accomplished through the use of a photoisomerizable moiety in the polymer side chain. 

Kapitza, H., Zentel. R., Twieg, R. J., Nguyen, C., Vallerien, S. U., Kremer, F. and Willson, C. G Ferroelectric liquid-crystalline polysiloxanes with high spontaneous polarization and possible applications in nonlinear optics, Adv. Mater., 2, 539-543 (1990). 

Application of the concepts obtained in the study of PVDF and its copolymers has been made for other types of novel polymers with similar polar chemical structure. For example, poly(vinylidene cyanide) (PVDCN) and its copolymer with vinyl n-fatty acid ester have ciMmical structures close to that of PVDF F atoms arc displaced by CN groups. These polymers are also attracting attention because of their characteristic electric properties. Odd-numbered members of nylon (nylon 7, nylon 11, etc.) and ferroelectric liquid-crystalline polymers (FLCP) are also the candidates for ferroelectric polymers. These substances are still new. and their structural study has not yet been developed extensively compared with the study of PVDF and its copolymers. 

In addition, some liquid-crystalline elastomers are ferroelectric (possess spontaneous electric polarization) [196,197], or piezoelectric (become electrically... 

The experimental observations could be consistently explained if the general tilt structure (SmCo) inside the layers is assumed. For most bent-core smectics the polar vector is perpendicular to the tilt plane, defined by the layer normal and averaged long axis direction, just as polarization in the ferroelectric rod-like liquid crystalline systems. However, since in the bent-core liquid crystals the polar order is decoupled from the tilt order, the polar director can in general have any direction in space thus it can also have a non-zero component along the layer normal. This can be achieved by a combination of tilting (rotation around the polar director) and leaning (rotation around the direction perpendicular to the polar director) of... 

Nakata M, Link DR, Takanishi Y, Takahasi Y, Thisayukta J, Niwano H, Coleman DA, Watanabe J, Iida A, Clark NA, Takezoe H (2005) Electric-field-induced transition between the polarization-modulated and ferroelectric smectic-CgPf liquid crystalline states studied using microbeam X-ray diffraction. Phys Rev E 71 011705... 

Shibaev et al. (1984) first synthesized a ferroelectric side chain polymeric liquid crystal. In the following years a lot of liquid crystalline polymers of such kind were synthesized. In early research studies techniques used to understand polymers and whether they showed the liquid crystal phase were limited so that the conclusion was ambiguous. It was only in 1988 when Uchida et al. (1988) measured the spontaneous polarization and the tilt angle that people became convinced that this side chain polymer in the literature (Shibaev et al., 1984) is indeed a ferroelectric liquid crystal. 

It has been found experimentally that the spontaneous polarization Ps of side chain liquid crystalline polymers is more or less the same as that of small molecular mass liquid crystals, i.e., so far as the chemical formula of the side group of a side chain liquid crystalline polymer is the same as that of a small molecular mass liquid crystal. This phenomenon illustrates that either the side groups in the side chain liquid crystalline polymer or the small molecular mass ferroelectric liquid crystal in their Sc phase are aligned in same way. The dependence of Ps on temperature for the three small molecular mass ferroelectric liquid crystals LI, L2 and L3 and their polymer counterparts PI, P2 and P3 are depicted in Figure 6.40. 

None of the complexes with three and four chiral chains was liquid crystalline. In addition, the number and position of these chains drastically influenced the ferroelectric properties, particularly spontaneous polarization, as well as the... 

The molecule is a liquid crystalline polymer with chiral smectic C phase forming parts attached as side chains. The field required to switch the direction of polarization of the polymer is very low (0.3 MVm ). There is a lot of interest in liquid crystalline ferroelectric polymers, because of their possible use for fast-switching electro-optical devices. More information about ferroelectric liquid crystals can be found in references [36,37]. 

In the proper ferroelectrics, the spontaneous polarisation appears as a result of the polarisation catastrophe or, in other words, due to electric dipole-dipole interactions. There are also improper ferroelectrics, in particular, liquid crystalline ones, in which a structural transition into a polar phase occurs due to other interactions and, consequently, appears as a secondary phenomenon. We shall discuss this case later. For simplicity, the square of spontaneous polarisation vector can be taken as a scalar order parameter for the transition from the higher symmetry paraelectric phase to the lower symmetry ferroelectric phase. Therefore, in the absence of an external field, we can expand the free energy density in a series over P (T) and this expansion for ferroelectrics is called Landau-Ginzburg expansion ... 

Dantlgraber, G. Eremin, A. Diele, S. Hauser, A. Kresse, H. Pelzl, G. Tschierske, C. Chirality and macroscopic polar order in a ferroelectric smectic liquid-crystalline phase formed by achiral polyphilic bent-core molecules. Angew. Chem. Int. Ed. 2002, 41, 2408-2412. 

The investigated surfactant/solvent mixtures of the diol C50 and water or for-mamide, respectively, are the first lyotropic systems to form a lamellar, fluid and tilted liquid crystalline phase which contains chiral surfactant molecules. The main issue of the present chapter is thus to demonstrate whether or not the lyotropic SmC analog phase exhibits similar chirality effects as known from its thermotropic counterpart. The most outstanding manifestations of chirality in the thermotropic SmC phase are helicity, due to a chirality-induced precession of the director, and ferroelectricity, due to its polar C2-point group symmetry. Thus, the focus of this chapter is on the detection and analysis of those two macroscopic chirality effects. 

The plethora of liquid crystal structures and phases is categorized into two main classes thermotropic and lyotropic liquid crystals. While thermotropic liquid crystals are formed by, e.g., rod- or disc-shaped molecules in a certain temperature range, lyotropic liquid crystals are liquid crystalline solutions, built up by, e.g., aggregates of amphiphilic molecules in a certain concentration range. Many liquid crystal phases are found in thermotropic as well as in lyotropic systems. In some cases, however, the lyotropic analog of a thermotropic phase has never been observed. The probably most interesting of these missing link cases is the thermotropic chiral smectic C (SmC ) phase, which has become famous as the only spontaneously polarized, ferroelectric fluid in nature. 

Keller, P., Shao, R., Walba, D. M., and Brunei, M., The first high polarization ferroelectric main chain liquid crystalline polymers. Liq, Cryst., 18, 915-918 (1995). 

Ferroelectric materials are a subclass of pyro- and piezoelectric materials (Fig. 1) (see Piezoelectric Polymers). They are very rarely foimd in crystalline organic or polymeric materials because ferroelectric hysteresis requires enough molecular mobility to reorient molecular dipoles in space. So semicrystalline poly(vinylidene fluoride) (PVDF) is nearly the only known compoimd (1). On the contrary, ferroelectric behavior is very often observed in chiral liquid crystalline materials, both low molar mass and poljuneric. For an overview of ferroelectric liquid crystals, see Reference 2. Tilted smectic liquid crystals that are made from chiral molecules lack the symmetry plane perpendicular to the smectic layer structure (Fig. 2). Therefore, they develop a spontaneous electric polarization, which is oriented perpendicular to the layer normal and perpendicular to the tilt direction. Because of the liquid-like structure inside the smectic layers, the direction of the tilt and thns the polar axis can be easily switched in external electric fields (see Figs. 2 and 3). 

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