Transition chiral smectics

The first chiral combined lc polymers prepared for this purpose showed the desired cholesteric and chiral smectic C phases only at high temperatures (8) (the melting point was always above 100°C). By using lateral substituents (see Figure 3) it is possible however to suppress the melting temperature and to obtain polymers with a glass transition temperature of about room temperature, without losing the cholesteric and chiral smectic C phases (9). 

In a chiral smectic (Sc ) phase, the tilt angle is the same within a layer, but the tilt direction processes and traces a helical path through a stack of layers (Figure 43). It has been demonstrated that when such a helix is completely unwound, as in a surface stabilized ferroelectric liquid crystal cell, then changing the tilt of the molecules fi om +0 to —0 by alternating the direction of an applied field results in a substantial electro-optic effect, which has the features of veiy fast switching (%1 - lOps), high contrast and bistability [87]. The smectic A phase of chiral molecules may also exhibit an electro-optic effect, this arises due to molecular tilt fluctuations which transition is approached, which are combined with a... 

The texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

X-ray measurements on LC elastomers have shown [6-8] that the reversible transition between a chiral smectic C phase with and without a helical superstructure can be induced mechanically. The helix untwisted state corresponds in this case to a polar ferroelectric monodomain. The piezoelectricity arising from this deformation of the helical superstructure (which does not require a complete untwisting) has been demonstrated [9] for polymers cross-linked by polymerization of pendant acrylate groups (Figure 15). 

The decrease in order parameter in LCs caused by the photoisomerization of azobenzene molecules are also known to induce phase transition from chiral smectic phase to cholesteric phase. This type of transition is mainly observed in chiral mesogenic compounds. In 1999, My et al. conducted a systematic research on the phase transition behavior of a series of chiral azobenzene LCs [44]. A photo... 

C.C. Huang, S. Dumrongrattana, G. Nounesis, J.J. Stofko and P.A. Arimilli, Heat-capacity, tilt-angle, and polarization measurements near the smectic-A-chiral-smectic-C transition of one liquid-crystal compound, Phys. Rev. A 35(3), 1460-1463, (1987). doi 10.1103/PhysRevA.35.1460... 

Figure 5 Photomicrographs of liquid crystal textures seen in the polarizing microscope. (A) The Schlieren texture of a smectic I phase. (B) The focal conic texture of a chiral smectic C phase, which has a helical structure, forming at a transition from the liquid. The pitch of the helix shows up as parallel lines that are parallel to the molecule layers (each line corresponds to about a thousand molecular layers). The pitch lines reflect accurately the layer structure in the focal-conic domain. (A) Courtesy of JW Goodby, University of Hull, UK, with permission. (B) Reproduced with permission from Gordon and Breach, Switzerland.)...

This is a chiral smectic A with symmetry Dqo. Its properties are similar to those of the achiral SmA. However, close to the transition to the smectic C phase, the chiral smectic A phase shows interesting pretransitional phenomena in the dielectric and electrooptical effects (the so-caUed soft dielectric mode and electroclinic effect). They will be discussed in Chapter 13. 

The Goldstone mode in an achiral SmC tries to restore the symmetry of the smectic A phase Cooh —> Dooh by free rotation of the director along the conical surface with the smectic layer normal as a rotation axis. Thus, like chiral molecules convert a nematic into a cholesteric, they convert an achiral SmC into chiral SmC without any phase transition. In addition, mixing left (L)- and right (R)-handed additives results in a partial or complete compensation of the helical pitch both in cholesterics and chiral smectic C. For example, the L- and R- isomers of the same molecule taken in the equal amounts would give us a racemic mixture, that is achiral SmC without helicity and polarity. 

Due to low symmetry (C2) of the chiral smectic C phase, its theoretical description is very complicated. Even description of the achiral smectic C phase is not at all simple. In the chiral SmC phase two new aspects are very important, the spatially modulated (helical) structure and the presence of spontaneous polarisation. The strict theory of the SmA -SmC transition developed by Pikin [10] is based on consideration of the two-component order parameter, represented by the c-director whose projections ( 1, 2) = are combinations of the director compo-... 

Johno, M., Itoh, K., Lee, J., Ouchi, Y., Takezoe, H., Fukuda, A., Kitazume, T. Temporal and spatial behaviour of the field-induced transition between antiferroelectric and ferroelectric phases in chiral smectics. Jpn. J. Appl. Phys. 29, L107-L110 (1990)... 

The precursor 6 exhibits the enantiotropic nature of chiral nematic (N ), chiral smectic C (SmC ) and chiral smectic I (SmI ) phases. The shell-printed texture of the SmC phase and the rose-like texture of the SmI phase can be clearly seen in Figure 12.6. The thiophene monomers, M2 and M3, show enantiotropic N, SmA and SmC phases. The SmC phase is characteristic of ferroelectricity. The polymers show various mesophases. The phase transition temperatures are summarized in Table 12.4. PI shows an enantiotropic SmA phase. P2 shows enantiotropic SmA, SmC and SmB phases. The fan-shaped texture of the SmA phase and the striated fan-shaped texture of the SmC phase are shown in Figure 12.7. P3 shows an SmA phase in the heating process and SmA and SmX phases in the cooling process. XRD analysis suggests that the SmX phase of P3 might be a higher order smectic phase. 

The phase transitions starting from the smectic phase were also observed in several doped systems [57-60]. The smectic LC host 8CB, doped with chiral molecules and azobenzene compounds, showed the phase transition from smectic to cholesteric due to the photo-induced isomerization of azobenzene molecules, and the prolonged irradiation drove the phase transition further to the isotropic phase. Matsui et al. also reported a chiral smectic C (SmC )-cholesteric (N ) phase sequence in azo-dye-doped ferroelectric LCs [57]. 

In summary, chiral smectic-C phases lack a center of symmetry. Hence they can be used as materials for second-order nonlinear optics [120-124], and possess piezoelectric and pyroelectric properties. Pyroelectric measurements have been performed on LC polymers [125] as well as on LCEs [126-128]. Irradiation of an FLCE sample with light usually leads to a temperature increase resulting in a pyroelectric signal [129]. More interesting are systems in which dye molecules like azobenzenes lead to a shift of the phase transition temperature upon isomerization [19]. 

G glass transition, Sa smectic A phase, Sc chiral smectic C phase, I isotropic phase... 

Mono- and dicyclopalladation of mesogenic pyridazines was achieved and the subsequent reaction with /3-diketones gave metallomesogens 125-128 (Scheme 19). Monometallated derivatives have a flat central core, dimetallated derivatives have a sterically induced twist in the molecule rendering them chiral. Smectic A phases are exhibited, with transition temperatures in the region of 100/300 °G. 

We should note that (8.15) is only an approximation. Because of the helical structure in the chiral smectic-C phase, the divergence of /g is in principle incomplete since a real divergence would be obtained only as a response to a helicoidal electric field [68], [69]. Including the helical structure into the Landau model leads to a modification of (8.15) and a truncation of the divergence with a finite value of /g at 7), similar to the case of a first-order transition. However, whereas the truneation at a first-order smectic-yl-smectic-C transition can be observed experimentally, measurements of Xe around a tricritical point, where the transition changes from first-order to second-order, have shown that the influence of the helix on the divergence of Xg is probably beyond experimental resolution [70]. 

Figure 13.5. Temperature-dependent pyroelectric investigations on a chiral smectic C elastomer. Two different cross-linking states were measured, (a) 0% cross-linking (polymeric state), (b) 5% cross-linking (transition temperatures (a) gi — 26°C c 18 °C Sc 85 °C i (b) gi — 28 °C c 23 °C S)) 77 °C i) (reproduced with permission from [28]).

Figure 13.6. Piezoelectric signal of a chiral smectic C elastomer as a function of the applied dynamic deformation at 35°C (A), 39°C (O), and 52°C ( ) (transition temperatures gi 1 °C S(( 81 °C i) (reproduced with permission from [15]).

Figure 13.10. Mechanical and electromechanical response of a chiral smectic C elastomer as a function of the temperature for four dilferent frequencies, (O) 0.158 Hz, ( ) 1.12DHz, (A) 11.2 Hz, (V) 100 Hz (transition temperatures g Sx 308 K Sc 333 K Sa 346 K i). (a) E mechanical storage modulus, E" mechanical loss modulus (b) g electromechanical storage coefficient, g" electromechanical loss coefficient (c) g electromechanical storage modulus, g" electromechanic storage modulus.

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