Philippe Martinot-Lagarde

Photoinduced orientation in poly(vinylcinnamate) and poly(7-methacryloyloxycoumarin) thin films and the consequences on liquid crystal alignment

Thin films of poly(vinylcinnamate) and poly(7-methacryloyloxycoumarin) have been exposed to linearly polarized UV light. The resulting anisotropic films have been characterized by using UV, conventional and polarization modulation FTIR spectroscopies. In particular, several internal vibrational modes have been used as structural probes to examine the orientation of groups in the irradiated polymers. These experiments provide new information about the photoinduced anisotropy in these photocrosslinkable polymers upon irradiation with linearly polarized UV light, and an orientation mechanism is proposed. This mechanism is confirmed by studying the liquid crystal alignment induced by PVCi and poly(7-methacryloyloxycoumarin). Finally, the stability of the photoalignment process is discussed.

The Binem display: a nematic bistable device switched by surface anchoring breaking

For mobile devices, permanent image displays are very attractive. We present one: the Binem® display which is close to appear on the market. A first demonstrator was shown at SID’ 97. An important research and development work has been made in the Paris-Sud University and in Nemoptic to increase the temperature domain and define industrial processes to realise mean size displays. The Binem uses two equal lower energy states of a chiral nematic liquid crystal. The molecules are oriented almost parallel to the cell plates by monostable surface treatments. One state (U) is uniform, the other (T), is twisted by 180°.To switch, a high field pulse orients the molecules on one of the plate perpendicular to it: the anchoring is broken. After the pulse, this state is unstable, the molecules on the plate will relax to the anchoring orientation. If they fall in one direction the U state builds, if they fall in the opposite direction the T state is realized. The fall time of the pulse and the pretilts on the two plates control this process. We explain the mechanism of switching by surface anchoring breaking and the obtained display performances.

Technique for Local Pretilt Measurement in Nematic Liquid Crystals

We propose a simple technique for local measurement of the nematic pretilt on a solid substrate, using standard planar orientation on the opposite surface. We obtain the pretilt from the variation of the cell birefringence under weak electric field. We apply experimentally the proposed technique to measure the pretilt of the nematic 5CB on various substrates. We discuss the advantages and the limitations of our technique.

Development of low anchoring strength liquid crystal mixtures for bistable nematic displays

Abstract The recent Bistable Nematic (BiNem®) LCD technology presents long term bistability, high level passive matrix multiplexing, gray levels capabilities and high optical quality. The BiNem® device, based on anchoring breaking, needs specific low anchoring strength materials – alignment layers and liquid crystal mixtures. We present here our approach to develop nematic mixtures with wide enough temperature range and low zenithal anchoring energy.

16.1: Recent Improvements of Bistable Nematic Displays Switched By Anchoring Breaking (BiNem®)

We present recent major improvements of 180° bistable twisted nematics displays (BiNem®), a new nematic bistable display technology characterized by the breaking of monostable anchoring. This technology allows the switching between 0° and 180°-twisted stable textures. We show that optimization of the liquid crystal material, suitable rubbed polymer anchoring layers, and the use of a simple multiplexing scheme allow, for the first time, to demonstrate a high contrast with a medium resolution black and white passive matrix. Excellent contrast ratio, wide view angle, pure black and white optical states, good response times and low electrical consumption are achieved.

Demonstration of a technological prototype of an active-matrix BiNem liquid-crystal display

— The first implementation of active-matrix addressing on a BiNem bistable nematic liquid-crystal display has been demonstrated. Compared to previous passive-matrix addressing, major improvements have been made for the bistable mode: a smoother image refreshment, shorter refreshing frame time compatible with that of high-resolution e-book displays, the use of a touch screen for interactive functions such as hand-writing, a virtual keyboard, improved functionality for scrolling menus, partial refreshment, etc. On the same display but in the monostable mode, by using electrically controlled birefringence (ECB), the potential of displaying moving pictures at a rate of 25 images/sec, at an extended room temperature, has been demonstrated. In the ECB monostable mode, static images can be displayed down to −20°C. A new transflective single-polarizer optical mode has been developed to optimize the indoor and outdoor readability of the display. By lowering the surface reflection and enhancing the panel transmission, a contrast ratio of 94:1 and a transmittance of 17% have been achieved.

Surface Polarization and Flexoelectricity in Nematic Liquid Crystals

Abstract We propose a technique for the measurement of the surface electric polarization in nematic liquid crystals from the birefringence variations of the texture under weak vertical electric field. We study experimentally 5CB aligned on SiO layer with strong tilted anchoring. We show that in this system the surface polarization is one order of magnitude stronger than the bulk flexoelectric polarization of 5CB reported before. We discuss the sign and the magnitude of the surface polarization and the reliability of the proposed method.