Inhomogeneous Relaxation Dynamics and Phase Behaviour of a Liquid Crystal Confined in a Nanoporous Solid

TL;DR

This study investigates how liquid crystal molecules confined in nanopores exhibit inhomogeneous relaxation dynamics and phase transitions, revealing a continuous paranematic-to-nematic transition influenced by confinement and tensile pressure.

Contribution

It provides a detailed analysis of the inhomogeneous relaxation and phase behavior of liquid crystals in nanopores, supported by dielectric spectroscopy and a Landau-de-Gennes model, highlighting the effects of confinement and pressure.

Findings

Identification of slow interface and fast core relaxations.

Observation of a continuous paranematic-to-nematic transition.

Quantitative explanation of transition shifts due to tensile pressure.

Abstract

We report filling-fraction dependent dielectric spectroscopy measurements on the relaxation dynamics of the rod-like nematogen 7CB condensed in 13 nm silica nanochannels. In the film-condensed regime, a slow interface relaxation dominates the dielectric spectra, whereas from the capillary-condensed state up to complete filling an additional, fast relaxation in the core of the channels is found. The temperature-dependence of the static capacitance, representative of the averaged, collective molecular orientational ordering, indicates a continuous, paranematic-to-nematic (P-N) transition, in contrast to the discontinuous bulk behaviour. It is well described by a Landau-de-Gennes free energy model for a phase transition in cylindrical confinement. The large tensile pressure of 10 MPa in the capillary-condensed state, resulting from the Young-Laplace pressure at highly curved liquid menisci, quantitatively accounts for a downward-shift of the P-N transition and an increased molecular mobility in comparison to the unstretched liquid state of the complete filling. The strengths of the slow and fast relaxations provide local information on the orientational order: The thermotropic behaviour in the core region is bulk-like, i.e. it is characterized by an abrupt onset of the nematic order at the P-N transition. By contrast, the interface ordering exhibits a continuous evolution at the P-N transition. Thus, the phase behaviour of the entirely filled liquid crystal-silica nanocomposite can be quantitatively described by a linear superposition of these distinct nematic order contributions.