Bistability, ferroelectrics

Figure 8.35 Illustration of bistable ferroelectric EO switching observed for blue focal conic domains of MHOBOW. SmCsPF structure can be assigned for these domains. In this phase director structure in all layers is oriented identically. This structure and corresponding extinction bmsh orientation for cylindrical focal conic are illustrated at bottom of figure for two bistable ferroelectric states. Note that for material with 30° tilt angle, such as MHOBOW, two ferroelectric states look very similar in still photos. In fact, two states result from large rotation of extinction brushes through 60°, as can be easily seen when observing switching in motion.

ICE. Maly, M.D. Wand, R.P. Lemieux, Bistable ferroelectric liquid crystal photoswitch triggered by a dithienylethene dopant. J. Am. Chem. Soc. 124, 7898-7899 (2002)... 

Now let us turn to another type of bistable FLC structure with a very short helical pitch [103,154]. This type of FLC material is called Short Pitch Bistable Ferroelectric (SBF) [103]. The specific feature of SBF materials is a sufficiently large value of the spontaneous polarization (Table 7.3). 

L.A. Beresnev, R. Buchecker, N.I. Chernova, V.G. Chigrinov, J. Funf-schilling, M.V. Loseva, Yu.P. Panarin, E.P. Pozhidaev, and M. Schadt, Bistable Ferroelectric Liquid Crystal Display Cell, USA Patent No. 5,327,273. Date of patent July 5, 1994. 

Short pitch bistable ferroelectric (SBF) liquid crystal [59] for simple matrix driving... 

For both polymers the switching is very fast, less than a millisecond close to the Sc -Sa transition. Also, copolymer 40 exhibits a clear bistable ferroelectric switching even at room temperature (albeit slow, I > 600 ms). For both polymers a pronounced electrolink effect in the S phase was observed. The shortest electrook switching time for the copolymer was less than 30 is at T-T -Sa lOK. Close to the Sc -Sa transition, a tilt angle of nearly 18 is induced for an applied field of 100 V across a 10-)im cell. 

M. Isogai, K. Kondo, T. Kitanum, A. Mukoh. Y. Nagae, and R Kawakami. Bistable ferroelectric liquid crystal display device using conventional ducknesa cell, Proc. 6th InL Disp Rex Conf. "Japan Display S6," Ttkyo, 1986, pp. 472>474. 

Chiral Smectic. In much the same way as a chiral compound forms the chiral nematic phase instead of the nematic phase, a compound with a chiral center forms a chiral smectic C phase rather than a smectic C phase. In a chiral smectic CHquid crystal, the angle the director is tilted away from the normal to the layers is constant, but the direction of the tilt rotates around the layer normal in going from one layer to the next. This is shown in Figure 10. The distance over which the director rotates completely around the layer normal is called the pitch, and can be as small as 250 nm and as large as desired. If the molecule contains a permanent dipole moment transverse to the long molecular axis, then the chiral smectic phase is ferroelectric. Therefore a device utilizing this phase can be intrinsically bistable, paving the way for important appHcations. 

To produce novel LC phase behavior and properties, a variety of polymer/LC composites have been developed. These include systems which employ liquid crystal polymers (5), phase separation of LC droplets in polymer dispersed liquid crystals (PDLCs) (4), incorporating both nematic (5,6) and ferroelectric liquid crystals (6-10). Polymer/LC gels have also been studied which are formed by the polymerization of small amounts of monomer solutes in a liquid crystalline solvent (11). The polymer/LC gel systems are of particular interest, rendering bistable chiral nematic devices (12) and polymer stabilized ferroelectric liquid crystals (PSFLCs) (1,13), which combine fast electro-optic response (14) with the increased mechanical stabilization imparted by the polymer (75). 

In solid state all the 10 pyroelectric crystal groups allow in principle for bistable switching behaviour. This is the proper ferroelectricity. Under certain conditions ferroelectricity (improper) can be realized in liquid crystals. This was shown by Meyer and coworkers115 in 1975. Since that time intense activities have been initiated, applying this property for flat-panel devices, switches, light modulators etc. In principle, three effects can be observed and used ... 

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... 

Szafrahski, M., Katrusiak, A. and McIntyre, G.J. (2002). Ferroelectric order of parallel bistable hydrogen bonds. Phys. Rev. Lett. 89, 215507-1-4. 

Szafrahski, M. and Katrusiak, A. (2000). Thermodynamic behaviour of bistable NH -N hydrogen bonds in monosalts of l,4-diazabicyclo[2.2.2]octane. Chem. Phys. Lett. 318,427-432. Katrusiak, A. and Szafrahski, M. (1999). Ferroelectricity in NH-N hydrogen-bonded crystals. Phys. Rev. Lett. 82, 576-579. 

A dithienylperfluorocyclopentene derivative was reported by Lemieux et al. for the photochemical switching of polarization in SmC and this system exhibited the advantage of thermally stabOity at irradiated state over the azobenzene-doped systems [136, 137]. FLC mixtures MDW950-PhP doped with 1.0 and 3.0 mol % of 30 was irradiated with UV light (Fig. 5.23), the photoisomerization of 30 caused a destabihzation in SmC and the Ps versus temperature plots shifted to lower temperatures. The phase transition temperature from ferroelectric SmC to non-ferroelectric SmA was decreased by 2 °C upon irradiation of the 3 mol % FLC mixture. The photoswitching was bistable and fatigue resistance (Fig. 5.24). 

ICE. Maly, P. Zhang, M.D. Wand, E. Buncel, R.P. Lemieux, Reversible photocyclization of achiral dithienylperfluorocyclopentene dopants in a ferroelectric liquid crystal bistable SSELC photoswitching. J. Mater. Chem. 14, 2806—2812 (2004)... 

I. Dozov, A. Boissier and T. Laboureau, Nemoptic s bistable nematic liquid-crystal technology - Cholesterics or ferroelectrics are not necessary to make a bistable LCD, and using conventional nematics offers some substantial advantages, Information Display 18(1), 10 13, (2002). 

The spontaneous molecular polarization of ferroelectric liquid crystals, arising from their structure when constrained in small cell gaps, results in unique features that can be exploited in display devices. A low electric field of only a few volts can switch the ferroelectric liquid crystal between two equally stable states with opposing polarization directions. This is commonly referred to as bistability. In contrast, nematic displays generally require the electric field to maintain the ON state. The power required to run ferroelectric liquid crystal displays is consequently much less than that required for a nematic display. Since active switching is used in both directions, ferroelectric liquid crystals can switch hundreds of times faster than a... 

---