Thermotropic Orientational Order of Discotic Liquid Crystals in Nanochannels: An Optical Polarimetry Study and a Landau-de Gennes Analysis

TL;DR

This study investigates the orientational order of discotic liquid crystals confined in nanochannels, revealing continuous phase transitions and defect structures, supported by optical polarimetry and Landau-de Gennes modeling.

Contribution

It provides new insights into the temperature-dependent orientational order and defect structures of confined discotic liquid crystals, combining experimental optical measurements with theoretical modeling.

Findings

Continuous evolution of order parameter contrasts with bulk discontinuous transition.

Identification of defect structures compatible with optical and diffraction data.

Prediction of phase boundary movement within channels during temperature changes.

Abstract

Optical polarimetry measurements of the orientational order of a discotic liquid crystal based on a pyrene derivative and confined in parallel-aligned nanochannels of monolithic mesoporous alumina, silica, and silicon as a function of temperature, channel radius (3 - 22 nm) and surface chemistry reveal a competition of radial and axial columnar order. The evolution of the orientational order parameter of the confined systems is continuous, in contrast to the discontinuous transition in the bulk. For channel radii larger than 10 nm we suggest several, alternative defect structures, which are compatible both with the optical experiments on the collective molecular orientation presented here and with a translational, radial columnar order reported in previous diffraction studies. For smaller channel radii our observations can semi-quantitatively be described by a Landau-de Gennes model with a nematic shell of radially ordered columns (affected by elastic splay deformations) that coexists with an orientationally disordered, isotropic core. For these structures, the cylindrical phase boundaries are predicted to move from the channel walls to the channel centres upon cooling, and vice-versa upon heating, in accord with the pronounced cooling/heating hystereses observed and the scaling behavior of the transition temperatures with channel diameter. The absence of experimental hints of a paranematic state is consistent with a biquadratic coupling of the splay deformations to the order parameter.