Design and Electro-optic investigations of de Vries smectics for exhibiting broad temperature ranges of SmA* and SmC* phases and fast electro-optic switching
Abstract In this paper we report on the design, synthesis and characterisation of two new ferroelectric liquid crystalline compounds. These are based on using cores of (i) 5-phenyl-pyrimidine benzene and (ii) 5-phenyl-pyrimidine benzoate both terminated by achiral trisiloxane chain, and chiral (S) alkyl chain on the opposite ends of their respective mesogen. These compounds exhibit broad temperature ranges of SmA* and SmC* phases, whereas temperature range of the latter includes room temperature down to 11 oC. This is a highly desirable feature of a compound for perspective applications in devices. Differential scanning calorimetry shows a weak first order SmA* - SmC* transition in both compounds. This observation is supported by critical exponents ~0.2 of the power laws that describe the dependence of the tilt angle and of the spontaneous polarization on temperature. Both compounds show excellent de Vries characteristics as the maximum layer shrinkage in SmC* phase in both cases is ~ 1.5%. Furthermore (i) the birefringence decreases with a reduction in temperature instead of increasing, and (ii) a large field-induced birefringence is observed in SmA* phase close to the SmA*-SmC* transition temperature, (iii) much reduced maximum birefringence. The characteristics of fast switching speed are compared to a conventional ferroelectric liquid crystalline mixture. The mixtures that can possibly be developed using these compounds are likely to surpass in performance in having (i) a wider temperature range of both of SmA* and SmC* phases and (ii) of higher speed of switching. The layer thickness in SmA* and SmC* is measured using the techniques of (a) small-angle X ray diffraction, and (b) the optical interferometry on free standing smectic films. A comparison of the results from these techniques are used to determine the tilt angle as a function of temperature. Significantly, the tilt angle is found without applying electric field which is unusually exceptional. The apparent tilt angle is seen to emerge even in the SmA* phase without the application of the electric field –de Vries SmA* phase.