High-resolution x-ray study of the nematic - smectic-A and smectic-A - smectic-C transitions in 8barS5-aerosil gels

TL;DR

This study uses high-resolution x-ray diffraction to investigate how aerosil nanoparticle gels influence phase transitions in a specific liquid crystal, revealing that certain order parameters remain unaffected by disorder.

Contribution

It extends understanding of quenched disorder effects from well-studied liquid crystals to a different material and transition, highlighting the robustness of the smectic-C tilt order.

Findings

Smectic-A fluctuations are characterized by thermal and random-field contributions.

Smectic-C tilt order parameter is unaffected by disorder.

Large smectic correlation length may explain the robustness of tilt order.

Abstract

The effects of dispersed aerosil nanoparticles on two of the phase transitions of the thermotropic liquid crystal material 4-n-pentylphenylthiol-4'-n-octyloxybenzoate 8barS5 have been studied using high-resolution x-ray diffraction techniques. The aerosils hydrogen bond together to form a gel which imposes a weak quenched disorder on the liquid crystal. The smectic-A fluctuations are well characterized by a two-component line shape representing thermal and random-field contributions. An elaboration on this line shape is required to describe the fluctuations in the smectic-C phase; specifically the effect of the tilt on the wave-vector dependence of the thermal fluctuations must be explicitly taken into account. Both the magnitude and the temperature dependence of the smectic-C tilt order parameter are observed to be unaffected by the disorder. This may be a consequence of the large bare smectic correlation length in the direction of modulation for this transition. These results show that the understanding developed for the nematic to smectic-A transition for octylcyanobiphenyl (8CB) and octyloxycyanobiphenyl (8OCB) liquid crystals with quenched disorder can be extended to quite different materials and transitions.