Thermotropic interface and core relaxation dynamics of liquid crystals in silica glass nanochannels: A dielectric spectroscopy study

TL;DR

This study investigates the dielectric relaxation dynamics of liquid crystals confined in silica nanochannels, revealing distinct surface and core relaxation processes and their temperature-dependent molecular ordering behaviors.

Contribution

It provides new insights into the separate surface and core relaxation mechanisms and their influence on molecular ordering in confined liquid crystals.

Findings

Two distinct relaxation processes identified: slow surface and fast core relaxations.

Core region exhibits bulk-like nematic ordering behavior.

Surface ordering evolves continuously with a kink near the transition temperature.

Abstract

We report dielectric relaxation spectroscopy experiments on two rod-like liquid crystals of the cyanobiphenyl family (5CB and 6CB) confined in tubular nanochannels with 7 nm radius and 340 micrometer length in a monolithic, mesoporous silica membrane. The measurements were performed on composites for two distinct regimes of fractional filling: monolayer coverage at the pore walls and complete filling of the pores. For the layer coverage a slow surface relaxation dominates the dielectric properties. For the entirely filled channels the dielectric spectra are governed by two thermally-activated relaxation processes with considerably different relaxation rates: A slow relaxation in the interface layer next to the channel walls and a fast relaxation in the core region of the channel filling. The strengths and characteristic frequencies of both relaxation processes have been extracted and analysed as a function of temperature. Whereas the temperature dependence of the static capacitance reflects the effective (average) molecular ordering over the pore volume and is well described within a Landau-de Gennes theory, the extracted relaxation strengths of the slow and fast relaxation processes provide an access to distinct local molecular ordering mechanisms. The order parameter in the core region exhibits a bulk-like behaviour with a strong increase in the nematic ordering just below the paranematic-to-nematic transition temperature T_PN and subsequent saturation during cooling. By contrast, the surface ordering evolves continuously with a kink near T_PN. A comparison of the thermotropic behaviour of the monolayer with the complete filling reveals that the molecular order in the core region of the pore filling affects the order of the peripheral molecular layers at the wall.