Surface-stabilized ferroelectric liquid crystal applications

The surface-stabilized ferroelectric liquid crystals in the smectic C (SmC ) phase are among the most interesting types of liquid-crystalline systems because of their potential applications in high-resolution flat panel displays and fast electro-optical devices [73-76]. Within this class of compounds, ferroelectric liquid-crystalline polymers (FLCPs) have gained theoretical and practical interest as systems which combine the properties of polymers and ferroelectric liquid crystals. This combination is achieved by attaching the ferroelectric mesogen to a main chain via a flexible spacer... 

In the sections on smectic liquid crystals, first the alignment and molecular orientation of surface stabilized ferroelectric liquid crystals (SSFLCs) are treated in detail. Next, the alignment technologies needed for the occurrence of bistability are detailed. Furthermore, liquid crystalline devices made of AFLC materials and the applications of FLC and AFLC materials to active matrix devices are discussed. 

At first in this chapter, ferroelectric liquid crystals (FLCs) and their most interesting application as surface stabilized ferroelectric liquid crystals (SSFLCs) are briefly explained. 

So far, four display modes have been proposed in ferroelectric and antiferroelectric display applications, as shown in Figure 9.34. A bistable switching in surface stabilized ferroelectric liquid crystals (SSFLCs) has been manufactured as a passive matrix liquid crystal display (PM-LCD). The counterpart of AFLC is a tristable switching, which is also a promising candidate for PM-LCD. In addition to these PM-LCDs, active matrix displays (AM-LCDs) are also proposed in FLC and AFLC materials, i.e., deformed helix FLCD (DHFLC) and V-shaped LCD (VLCD). In this section, PM-AFLCD and AM-VLCD will be described. 

It is our belief that block copolymers containing LC segments are materials with novel and unencountered properties which will offer great opportunities for developing high performance materials. Here we would like to give two examples. One example is a microphase stabilized ferroelectric liquid crystal (MSFLC) [109] for potential flat panel display applications, while the other is a material for stable, low surface energy [110] application. 

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

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... 

There are also electro-optic effects using either a different geometry of surface stabilization or a completely different mechanism In the twisted ferroelectric smectic-C cell [54] the moleeules form in the zero field state a quarter helix which is removed when a dc field of either polarity is applied the optical effect is achieved in the same way as in a twisted nematic cell. Compounds with a short chiral smectic-C pitch in a thick cell are used for the distorted helix ferroelectric (DHF) device [55] this effect uses the optical difference between the zero-field state eharacterized by a fully developed short-pitch helix, and structures with a distorted or almost unwound helix in the presence of an applied field optically addressed spatial light modulators can take advantage of the DHF effect [56]. Further applications of ferroelectric liquid crystals are switchable diffraction gratings [57]. 

Until the mid-1990s and after 20 years of intense research on nematic field-effect LCDs it was still uncertain whether LCDs and LC materials could indeed meet the short response time requirements and the optical quality required for LCD television. Therefore, parallel to nematic LCD research, strong efforts were made to find effects based on the inherently faster responding ferroelectric liquid crystals (FLCs). Unfortunately, FLCs proved to be difficult to surface-align, rendering them up to now commercially applicable only for niche products such as electronic eye shutters or time sequential LCD projection. FLC examples are the surface-stabilized ferroelectric (SSF)-LCD of Clark and Lagerwall [40] which initiated FLC-LCD development and the deformed helix ferroelectric (DHF)-LCD of Beresnev et al. [41], In 1995 a TFT-addressed black-white DHF-LCD television prototype with 20 ps response time and broad field of view was developed by the author and coworkers in collaboration with Philips [42] (Fig. 6.5a). 

---