Renewed focus on the small temperature change of smectic layer spacing in ferroelectric and antiferroelectric LCs

Abstract

We aim at becoming released from the invisible hand of de Vries scenario that a small temperature change in the smectic layer spacing must be caused by ‘the far-fetched orientational distribution called the de Vries diffused cone’ or ‘the in-layer directors statically tilted and randomly distributed around the smectic layer normal’. First, we show in a prototypal compound MC513 that all the unusual properties suggesting the revival of a de Vries-type SmA-Sm transition can be explained by ordinary SmA emergence just below the isotropic phase only in a narrow temperature range. Second, we take up the unconventional molecular structure of TSiKN65, thus far regarded as a typical de Vries material; the core part makes a large angle of with respect to the average molecular long axis. We insist that a slight change in can explain almost all the unusual properties without taking account of the de Vries scenario. Last but not least is a practical point of view. During a thermal shock cyclic reliability test between −20°C and 60°C in prototyping antiferroelectric liquid crystal displays (AFLCDs), DENSO noticed the importance of the transition from Sm to the hexatic or crystalline phase; the cell quality critically depends on the layer spacing change in the lower-temperature part of Sm. We point out that the relative displacements of molecules along their long axes and hence the layer undulational fluctuations play an important role in addition to the director tilting ones.