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 capillarycondensed 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-deGennes 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.

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

“…Therefore, both relaxation processes can be discriminated and the corresponding dipolar relaxation strengths provide then access to local molecular ordering. Employing this distinct dynamical behaviour, we could recently show for 7CB, a prominent member of the rod-like cyanobiphenyl nematogens that the ordering in the core region is reminiscent of the bulk behaviour 47 . It is characterised by an abrupt increase in the nematic ordering in the transition region.…”

“…Dielectric relaxation spectroscopy has proven as particularly suitable to achieve such local information on confined LCs [38][39][40][41][42][43][44][45][46][47] . A number of studies on LCs in porous media evi- Sketch of the capacitor geometry for the dielectric relaxation experiments: Gold electrodes were evaporated onto the mesoporous silica membrane, so that the sample forms a simple parallel circuit both in the regime of monomolecular layer coverage (a) and for the completely filled channels (b).…”

Self-assembly of liquid crystals in nanoporous solids for adaptive photonic metamaterials

“…Previous experimental studies of DLCs confined in cylindrical nanopores have revealed a remarkably complex, sometimes very surprising self-assembly behaviour [24,26,[38][39][40][41][42][43]. Experimental studies on HAT6 in AAO indicate that for planar anchoring, i.e., the molecules lie flat on the hydrophilic pore walls, no long-range intercolumnar hexagonal order is detectable for pore diameters d below 20 nm [44].…”

High-resolution dielectric study reveals pore-size-dependent orientational order of a discotic liquid crystal confined in tubular nanopores

“…Diffraction experiments and Molecular Dynamics simulations on partial fillings [45,68] may allow for complementary structural information on the propagation of the phase boundaries in channel space. This will shed light on the remarkably different pathways found here for freezing and melting of discotics.…”