The synthesis, phase transition parameters, the temperature dependence of spontaneous polarization, tilt and pitch of (S)-(+)-4-(1-methylheptyloxycarbonyl)phenyl 4′-(cyanoacetoxy hexoxy)biphenyl-4-carboxylates and (S)-(+)-4-(1-methylheptyloxycarbonyl)biphenylyl 4′-(cyanoacetoxy)benzoate are described. These compounds exhibit the synclinic smectic C phase and are useful as the components of antiferroelectric mixtures with high tilt angle. The (S)-(+)-4-(1-methylheptyloxycarbonyl)phenyl 4′-(cyanoacetoxyhexoxy)biphenyl-4-carboxylate shows the ability to induce an antiferroelectric phase in mixtures based on the other ferroelectric components.

Ferroelectric Compounds with Strong Polar Cyano Group in Terminal Position

Asymmetric antiferroelectric liquid crystal displays (AAFLCD) are attractive since they show a very well defined off state and fast switching time. Moreover, they can be driven by a simple biasless DC compensated waveform. The electrooptical response of an AAFLCD allows for new addressing modes, including quasi-static intermediate greyscales maintained without applying a field and passively addressed multiplexed high-frequency displays and spatial light modulators. A new kind of asymmetric cells have been obtained by using fluorinated block copolymer (FBC) alignment, which enhances surface segregation and provides a low energy surface. In this work we combine FBC alignment with antiferroelectric liquid crystal mixtures containing strongly electronegative fluorinated components. Threshold voltages for the antiferroelectric-ferroelectric phase transition as low as 3 volts are observed. We report the time evolution of the shift of the electro-optical response.

Low threshold voltage asymmetric antiferroelectric liquid crystal cells

A new type of domain in twisted-wedge negative nematics is reported. The wedge shape allows the separation of ordinary and extraordinary beams that can be studied separately. In these conditions, at least five different regions can be detected depending on applied voltages and frequencies. Both rays are shown to, yield different diffraction patterns. The relative light intensity of several spots of these patterns is also studied.

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Electrooptical behavior of twisted-wedge nematic structures.

The ferroelectric-antiferroelectric transition in greyscale generation of antiferroelectric liquid crystal displays (AFLC) is a heterogeneous process. The process has been described as the growth of finger-like domains [1]. We have previously studied the ferroelectric-antiferroelectric phase transition, relaxation that follows the data pulse in surface stabilized asymmetric antiferroelectric liquid crystal displays using biasless video frequency waveforms [2]. This relaxation involves an intensity decay of the light transmitted by a pixel and depends on several parameters such as surface stabilization, rotational viscosity of the AFLC, magnitude of the data pulse, and bias voltage.

Effect of bias in ferroelectric-antiferroelectric relaxation

Temperature dependence of ultrasonic velocity (V) and specific volume (v) are reported for cholesteryl erucate. Physical parameters like adiabatic compressibility (βad), molar sound velocity (R), molar compressibility (B), and intermolecular freelength (Lf) are evaluated. The order parameter is evaluated using the Maier-Saupe theory and the values are compared with those obtained from optical anisotropies. Pretransitional effects in the vicinity of mesophase-isotropic phase transition are discussed in terms of thermal expansion coefficient (αT) and temperature coefficient of adiabatic compressibility (βT).



Die Temperaturabhangigkeit der Ultraschallgeschwindigkeit (V) und des spezifischen Volumens (v) werden fur Colesterylerucat mitgeteilt. Physikalische Parameter wie adiabatische Kompressibilitat (βad), molare Schallgeschwindigkeit (R), molare Kompressibilitat (B) und intermolekulare freie Lange (Lf) werden berechnet. Mit der Maier-Saupe-Theorie werden Ordnungsparameter berechnet und die Werte mit denen aus Messungen der optischen Anisotropien verglichen. Vorubergangseffekte in der Nahe des Mesophase-isotrope Phase-ubergangs werden mit dem thermischen Ausdehnungskoeffizienten (αT) und dem Temperaturkoeffizienten der adiabatischen Kompressibilitat (βT) diskutiert.

Propagation of ultrasonic waves in a smectic liquid crystal. Cholesteryl erucate

As part of our line of research into high resolution, video rate antiferroelectric displays we have studied the ferroelectric – antiferroelectric relaxation that follows the data pulse, in surface stabilised antiferroelectric liquid crystal (AFLC) displays, driven by biasless or low bias video frequency waveforms. This generally incomplete relaxation reduces the intensity of the light transmitted by a pixel in any partially ferroelectric (grey-level) state during a video frame, and hence limits the frame-averaged brightness of the AFLC display. The relaxation is a function of the surface stabilisation (i.e., the anchoring strength, pre-tilt and thickness of the device and the pitch of the AFLC), the rotational viscosity of the AFLC, the degree of relaxation of the AFLC between successive frames, the magnitude of the data pulse (grey level) and bias voltage. We have optimised the driving scheme for a number of symmetric and asymmetric (biasless) displays with different antiferroelectric materials and a var...

Relaxation of Surface Stabilised Antiferroelectric Liquid Crystals at Video Frequency

A new type of domain for nematic liquid crystals with a twisted-wedge structure is presented. This new type of domain appears from the low frequency range to 10 kHz. This behavior was observed for square and pulsed excitations. The liquid crystal was N-(p-methoxybenzylidene)-p'-butylaniline) (MBBA) used at room temperature. These domains offer a higher degree of complexity than conventional Williams domains. The corresponding stability chart is presented.

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Electrohydrodynamic Behavior in Twisted-Wedge Nematic Structures

In this paper we report the experimental results obtained when an He-Ne laser beam crosses an MBBA homeotropic sandwich structure and is modulated by the influence of another laser beam, in our case an Ar+ laser, crossing through the same region. We extend some results previously reported by us12 concerning the influence of the ratio of the diameters of the laser beams on the modulation characteristics. A theoretical model, based on the one reported in Ref6, shows good agreement with the experimental results. If the Ar+ laser is intensity chopped, the resulting He-Ne diffracted image is also intensity modulated. The highest frequency observed has been 500 p. p. s.

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Optically Induced Modulation of a Laser Beam in Nematic Liquid Crystals Structures

A new type of photonic logic, based on the use of nematic liquid crystals is proposed. The system takes advantage of the refractive-index changes induced by laser beams. Examples of AND, OR and NOR functions are presented.

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F. J. Lopez

Papers

Relaxation of Surface Stabilised Antiferroelectric Liquid Crystals at Video Frequency

Electrohydrodynamic Behavior in Twisted-Wedge Nematic Structures

Optically Induced Modulation of a Laser Beam in Nematic Liquid Crystals Structures

Photonic logic based on molecular reorientation of nematic liquid crystals