Orientational Fluctuations Near the Smectic A to Smectic C Phase Transition in Two “de Vries”‐Type Liquid Crystals

Abstract

On the basis of thorough analysis of 2D X-ray diffraction patterns from smectic monodomains, we examine the influence of orientational fluctuations on the weakly first-order smectic A (SmA) to smectic C (SmC) transitions in two nonchiral organosiloxane "de Vries"-type liquid crystals. We find that these materials exhibit very large molecular tilt fluctuations with magnitudes of up to 35°--thus larger than the average tilt itself. This is essential to understand the underlying molecular mechanism behind the practical absence of smectic layer contraction in these materials: in the SmA phase, the nematic order parameter is very low (molecular fluctuations correspondingly high), and the expected layer shrinkage at the SmA to SmC transition is almost fully compensated by the increase in orientational order, as the fluctuations diminish with decreasing temperature. In addition to the general tilt fluctuations, we observe intrinsic soft-mode fluctuations. They have a λ-shaped temperature dependence that peaks at the SmA-SmC transition with a maximum amplitude of about 2°.

First Examples of de Vries‐like Smectic A to Smectic C Phase Transitions in Ionic Liquid Crystals

“…in 2009 and later experimentally confirmed by Nonnenmacher et al. in 2013 . The origin of de Vries behavior is therefore the unusual combination of strong smectic 1D translational and exceptionally low orientational order of the mesogens in the de Vries SmA phase.…”

“…Furthermore, in bilayered structures a decrease in the extent of interdigitation might also contribute to the compensation of the layer shrinkage. This de Vries mechanism was proven experimentally for several non‐ionic de Vries‐type materials . Owing to their low orientational and high translational order, ILCs should thus be perfect candidates for finding new de Vries‐type materials.…”

Tuning the mesogenic properties of 5-alkoxy-2-(4-alkoxyphenyl)pyrimidine liquid crystals: the effect of a phenoxy end-group in two sterically equivalent series

“…organosiloxane liquid crystals such as QL1-7 exhibit unusually large molecular tilt fluctuations, and that the layer contraction due to molecular tilts at the SmASmC transition is almost fully compensated by an increase in orientational order as the tilt fluctuations decrease with decreasing temperature. [40] 2. The degree of 'de Vries-like' behaviour in a smectic liquid crystal may be quantified on a scale of 0 (perfect 'de Vries') to 1 (conventional SmA-SmC transition) at a given temperature T below T AC by the reduction factor R according to the equation:…”

“…These values are the same within experimental error (±0.02) and are smaller than S 2 values typical of a conventional SmA phase (0.7-0.8), [4] but still significantly larger than those measured for 'de Vries-like' materials such as QL1-7 (S 2 = 0.38-0.43). [40] The effective molecular lengths are calculated using Equation (1) and the corresponding d values measured by SAXS to give average L eff values for QL11-10/6 and QL12-10/6 of 39.6 and 39.8 Å, respectively, which are the same within experimental error (±0.4 Å). The difference between L eff and the calculated molecular length L of QL11-10/6 (36.3 Å) is ca.…”

“…[35][36][37][38][39] In our ongoing study of liquid crystals with 'de Vries-like' properties, we developed a new class of organosiloxane and carbosilane mesogens such as QL1-7 and QL2-7 that form smectic phases with high lamellar order and low orientational order and undergo a SmA-SmC transition with a maximum layer contraction of ≤1%. [21,22,40] 1 Although siloxane and carbosilane end-groups are very effective at promoting high lamellar order, they often reduce the quality of alignment of smectic phases in electro-optical cells with rubbed polyimide layers. In order to diversify our library of 'de Vries-like' liquid crystal materials and address the alignment problems associated with siloxane and carbosilane end-groups, we sought other end-groups that might promote the formation of smectic phases and enhance lamellar ordering.…”