Issue 16, 2015

Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

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.

Graphical abstract: Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

Supplementary files

Article information

Article type
Paper
Submitted
14 Jan 2015
Accepted
02 Mar 2015
First published
02 Mar 2015

Soft Matter, 2015,11, 3176-3187

Author version available

Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

S. Całus, A. V. Kityk, M. Eich and P. Huber, Soft Matter, 2015, 11, 3176 DOI: 10.1039/C5SM00108K

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