Microphase Stabilized Ferroelectric Liquid Crystal Displays

1 Microphase Stabilized Ferroelectric Liquid Crystal Displays 

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 

However, the bistable switching was not reported in Omenat et al. s FLC-coil block copolymers [79]. We speculate that it may be due to a relatively small LC segments and small domain sizes in their system. Ober et al. have prepared a FLC-coil diblock copolymer and demonstrated its bistable switching behavior using the concept of MSFLC [109, 130] although their system is far from practical application due to the slow response time (=0.1 s) and high volt-age (Vpp=700 V/10 jim cell) needed for the bistable switching. 

It can be seen clearly from Fig. 15 that MSFLC cells offer many advantages. For example, a large gap of 10 Xm can be used [109, 130] instead of 2 pm used in SSFLC cells [131]. The polymeric nature of the LC materials offers excellent shock resistance and mechanical stability. No rubbing of the substrate is needed. We believe that practically useful electro-optical cells can be made utilizing this concept if the FLC-coil diblock copolymers are properly designed, synthesized, and processed. 

Since the ODT of LC block copolymers is in general much higher than the LC isotropic transition temperature (unless the total molecular weight is too small, i. e. a small [43, 62], all the LC mesophase transitions lie below the block microdomain boundary conditions. These microdomain boundary conditions were shown [111] to stabilize the LC mesophase. The orientation of the mesogen within the block microdomain can be recovered when cooling from the isotropic state as already shown in Fig. 14. On the other hand, the texture of the LC block copolymer is very homogeneous and almost defect-free compared to that of 

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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. 

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