Ferroelectric smectics

Rieker T P, Clark N A, Smith G S, Parmar D S, Sirota E B and Safinya C R 1987 Chevron local layer structure in surface-stabilized ferroelectric smectic-C cells Phys.Rev.Lett 59 2658-61... 

When suitably doped, MBF can form a surface-stabilised-ferroelectric smectic-C (SSFLC) structure. Using simple assumptions regarding core orientations, Binger and Hanna are able to place an upper limit on the SSFLC cone angle for MBF of 30°. 

From this discussion the clear similarity between the SmAPA and SmCsPA structures is easily seen. In addition, the suggestion of Brand et al.29 that a bilayer smectic with all anticlinic layer interfaces (the SmAPF) would produce an achiral ferroelectric smectic follows directly. The unanticipated tilt of the director in the tilt plane, leading to a chiral layer structure, seems to be a general response of the bent-core mesogens to the spontaneous nonpolar symmetry breaking occurring in these rigid dimer structures. 

Niori, T. Sekine, T. Watanabe, J. Furukawa, T. Takezoe, H. Distinct Ferroelectric Smectic Liquid Crystals Consisting of Achiral Molecules with Banana Shape, Abstracts of the 16th International Liquid Crystal Conference, Kent State University, Kent, OH, 1996, p. 126. 

Niori T, Sekine T, Watanabe J, Takezoe H (1996) Distinct ferroelectric smectic liquid crystals consisting of banana shaped achiral molecules. J Mater Chem 6 1231-1233... 

Watanabe J, Niori T, Sekine T, Takezoe H (1998) Fmstrated structure induced on ferroelectric smectic phases in banana-shaped molecular system. Jpn J Appl Lett 37 L139-L142... 

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... 

A vast array of covalent molecules have been synthesised over the years in the search for LCs that show the useful cholesteric and ferroelectric smectic C phases, often on a trial and error basis ignoring the interactions between the molecules. The idea that one could think of the interactions between the molecules as a kind of molecular recognition came from the careful analysis of the conformations of molecules in the layers [77,78]. The arguments are based on the symmetry limitations of the angle formed by the alkyl chain and the phenyl benzoate moiety in the molecules that were the subject of this study. A molecular recognition site within the phase was used as the basis for these speculations , which have actually proved rather successful. The actual interactions between molecules are usually weak, but the formation of layers of aromatic and aliphatic units in these mesophases gives rise to their unique properties. 

Further studies by Nishiyama et al. [34-45] showed that when taken in isolation, only one of the aromatic units within a supermolecular system has a propensity to exhibit liquid crystal phases, then the supermolecular material itself could be mesomorphic, see Fig. 5. For example, for the top molecular structure, 5 [45], in Fig. 5, only the biphenyl unit at the center of the structure supports mesophase formation, whereas the benzoate units are too isolated from the biphenyl moiety in order to affect mesomorphic behavior. The second material, 6 [45] has terminal phenyl units, which are only connected by aliphatic chains to the benzoate units. Thus in this case, the material has four aromatic units out of six which are not in positions that can enhance mesophase formation. However, the second material has similar transition temperatures and phase sequences to the first, i.e., both materials exhibit an unidentified smectic phase and a synclinic ferroelectric smectic C phase. If the third material, 7 [38], is examined, it can be seen that the mesogenic unit at the center of the supermolecule is an azobenzene unit which is more strongly supportive of mesophase behavior than the simple biphenyl moiety. Thus the clearing point is higher for this material in comparison to the other two. The attachment of the terminal phenyl unit is by a methylene spacer of odd parity, and as a consequence the smectic C phase has an anticlinic structure rather than synclinic. 

Once the helical structure of the Sc phase is unwound, ferroelectricity is displayed (see Chapter 6 for the details). In recent years, many experimental studies have revealed that some liquid crystal compounds show new types of smectic phases with complex tilt and dipole order, such as the anti-ferroelectric smectic C phase, Sca phase, and the ferrielectric smectic C phase, Sc7 phase. For instance, in the Sca phase, the spontaneous polarization Ps is opposite for successive layers. It was found experimentally that the chiral So phase is in fact similar to the anti-ferroelectric Sca phase. 

Similarly to the molecular engineering of calamitic molecules to produce ferroelectric smectic C phases [129], disk-like molecules with chiral peripheral chains tilted with respect to the columnar axis were predicted to lead to ferroelectric columnar mesophases [130]. Indeed, as it is the case with all flat disk-shaped mesogenic molecules, the tilt is mainly associated with the flat rigid aromatic cores of the molecules, the side-chains being in a disordered state around the columnar core. Thus, the nearest part of the chains from the cores makes an angle with the plane of the tilted aromatic part of the molecules. If the chiral centre and the dipole moment are located close to the core, then each column possesses a non-zero time averaged dipole moment, and therefore a spontaneous polarization. For reasons of symmetry, this polarization must be, on average, perpendicular to both the columnar axis and to the tilt direction in other words, the polarization is parallel to the axis about which the disk-shaped molecules rotate when they tilt as shown in Fig. 29. 

Initially the octyl to dodecyl compounds were prepared and these were found to exhibit relatively normal behavior, i.e. smectic A phases were found for the lower homologues with smectic and ferroelectric smectic C phases occurring for the higher members. However, when the tetradecyl homologue was examined in the polarizing transmitted light microscope, an iridescent helical mesophase was observed which upon cooling underwent a further phase transition to a ferroelectric smectic phase. In addition, this compound was also found to exhibit antiferroelectric and ferrielectric phases. 

As this compound was one of the higher homologues in the series, and because we knew that the earlier homologues did not exhibit a chiral nematic phase, it was clear that the new phase also could not be a chiral nematic phase. In addition, it was clear from the formation of the defect structures seen in the microscope that the phase first formed from the isotropic liquid possessed a helix, see Plate 1, which had its heli-axis at right angles to the heli-axis in the lower temperature chiral ferroelectric smectic phase. This simple observation meant that if the phase was a lamellar smectic phase then the helix would have to be formed, inconceivably, in a direction parallel to the layers. Synthesis of the achiral variant confirmed that the phase formed first on cooling from the isotropic liquid was indeed a smectic A phase, and thus we immediately knew that we had found a smectic A phase where the helical macro structure formed in the planes of the layers, and thus the helix must... 

As noted earlier form the work of Freidzon [41], side chain liquid crystal polymers derived from cholesterol can also apparently exhibit TGB phases. However, work on non-steroid systems also reveals that TGB phases can also be formed in typical chiral polymers. For example the polymethacrylate, structure 16, exhibits chiral nematic, TGBA , smectic A , and ferroelectric smectic phases. 

The interest in chiral dimers was stimulated to a large extent by the expectation that placing the chiral centre in the spacer, at least for even dimers, should increase its orientational order compared to that for a chiral centre located in a terminal chain and this in turn should result in an enhancement of the form chirality of the phase. This suggestion has still to be extensively investigated but the limited data available indicates that dimers having chiral spacers actually exhibit ferroelectric smectic C phases with low values of spontaneous polarisation [140]. 

Fig. 5.10.1. (a) Helicoidal structure of the ferroelectric smectic C phase, (b) a poled sample with the helix unwound by an electric field applied normal to the... 

By virtue of their symmetry, ferroelectric smectics are piezoelectric. Polarization can be induced by mechanical shear and, conversely, an electric field can produce shear flow. They also possess pyroelectric properties. 

Figure 33. A polyphilic mesogen consisting of three different parts a fluorinated alkyl chain, a common alkyl, and an aromatic part. The terminal CH2-CF3 group enables a smooth junction to the next layer. The phase sequence observed for this material is Cr 95°C (SmX 92 °C) SmA 113 °C Iso Cr=crystalline, SmX= a ferroelectric smectic phase [162].

Perhaps one of the most important applications of chiral induction is in the area of liquid crystals. Upon addition of a wide range of appropriate chiral compounds, the achiral nematic, smectic C, and discotic phases are converted into the chiral cholesteric (or twisted nematic), the ferroelectric smectic C and the chiral discotic phases. As a first example, we take the induction of chirality in the columns of aromatic chromophores present in some liquid-crystalline polymers. " The polymers, achiral polyesters incorporating triphenylene moieties, display discotic mesophases, which upon doping with chiral electron acceptors based on tetranitro-9-fluorene, form chiral discotic phases in which the chirality is determined by the dopant. These conclusions were reached on the basis of CD spectra in which strong Cotton effects were observed. Interestingly, the chiral dopants were unable to dramatically influence the chiral winding of triphenylene polymers that already incorporated ste-reogenic centers. 

Freely-suspended Films of Polymeric Liquid Crystals. The stabilization of freely-suspended films by using polymeric liquid crystals is obviously interesting and has been attempted previously. Unfortunately it seems to be extremely difficult to polymerize films of liquid crystalline monomers as these films were reported to always break during polymerization. It seems to be equally difficult to fabricate FS-films of polymeric liquid crystals in their smectic A and smectic C phases, most likely due to their enhanced viscosities. However, if one heats slightly into the isotropic phase it is possible to spread a film across an aperture which thins out to form a truly freely-suspended liquid crystal film after cooling into the smectic phases (57). Films of this type are homeotropic in the smectic A phase and show birefringence when cooled to the ferroelectric smectic C ... 

The concept of defects came about from crystallography. Defects are dismptions of ideal crystal lattice such as vacancies (point defects) or dislocations (linear defects). In numerous liquid crystalline phases, there is variety of defects and many of them are not observed in the solid crystals. A study of defects in liquid crystals is very important from both the academic and practical points of view [7,8]. Defects in liquid crystals are very useful for (i) identification of different phases by microscopic observation of the characteristic defects (ii) study of the elastic properties by observation of defect interactions (iii) understanding of the three-dimensional periodic structures (e.g., the blue phase in cholesterics) using a new concept of lattices of defects (iv) modelling of fundamental physical phenomena such as magnetic monopoles, interaction of quarks, etc. In the optical technology, defects usually play the detrimental role examples are defect walls in the twist nematic cells, shock instability in ferroelectric smectics, Grandjean disclinations in cholesteric cells used in dye microlasers, etc. However, more recently, defect structures find their applications in three-dimensional photonic crystals (e.g. blue phases), the bistable displays and smart memory cards. 

Bminsma, R., Prost, J. Fluctuation forces and the Devil s staircase in ferroelectric smectic C s. J Phys. 11 France 4, 1209-1219 (1994)... 

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. 

Brehmer et al. [114] were the first to publish if, measurements for ferroelectric LC elastomer (Fig. 27). The plot was later widely reproduced [15,83,110]. It is difficult, however, to understand the absence of any pronounced changes in the if, value at the phase transition from the ferroelectric smectic C phase to the non-ferroelectric smectic A phase. On the other hand, we should note that Finkelmann and Eckert [116] recently reported for ferroelectric LC elastomers if,(7 ) curves similar to those shown in Fig. 24 for the coefficient if,. 

An idea proposed for periodic noncentrosymmetric layers in order to obtain longitudinal ferroelectric smectics was to use a structural sequence of a- —y-ci... 

Temperature phase synchronism is obtained in optical second-harmonic generation in a ferroelectric smectic C whose helicoid is untwisted by a constant electric field. The nonlinear second-harmonic susceptibility due to spontaneous polar ordering of the molecules is estimated. 

The ferroelectric smectics C and H are the only liquid-crystal phases with a polar structure that should make possible in them second-harmonic generation (SHG) via the quadratic nonlinearity Up to now, only nonsynchron-ous SHG was reported in a ferroelectric liquid crystal (LC). - We present here the results of an investigation of phase-synchronous SHG in the ferroelectric phase C with untwisted helicoid. The investigated substance was /7-decy-loxybenzylidene-/7-amino-2-methylbutylcinnamate (DO-MAMBC). 

The sign of the spontaneous polarization in ferroelectric smectics depends on both the sense of chirality and direction of the electrical dipole with respect to a molecular skeleton. The magnitude of the polarization is a function of the coupling between a chiral center and the transverse electrical dipole. The larger the dipole moment, and the closer its location to a chiral center, the higher polarization is observed [75]. 

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