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

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 TPN and subsequent saturation during cooling. By contrast, the surface ordering evolves continuously with a kink near TPN. 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.

Dynamic Kerr and Pockels electro-optics of liquid crystals in nanopores for active photonic metamaterials

“…It has been demonstrated in a series of experimental studies that molecules show a slow molecular dynamics near the pore wall and a fast relaxation in the core region of the pore filling. [62][63][64][65][66] Thus the fast quadratic Kerr electro-optic response is dominated by the molecular dynamics originating from the isotropic (centrosymmetric) core region, whereas the linear Pockels effect owes its slow dynamics to the slow interfacial dipolar relaxations.…”

“…The origin of this peculiarity deserves a more detailed discussion. According to dielectric studies 62,64,65,73 slow relaxations of confined LCs suggest that the mobility of molecules in the surface layers is strongly affected by the boundary con-ditions. Near-interfacial layers are characterized by a considerably larger effective viscosity compared to the bulk one which solely slows down the dipolar relaxation.…”

Dynamic Kerr and Pockels Electro-Optics of Liquid Crystals in Nanopores for Active Photonic Metamaterials

“…These distinct electro-optical dynamics originates in the het-erogeneous molecular dynamics of molecular matter confined in nanopores. It has been demonstrated in a series of experimental studies that molecules show a slow molecular dynamics, in particular relaxation behavior near the LC-pore wall interface and a fast relaxation in the core region of the pore filling [56][57][58][59][60] . Thus the fast quadratic Kerr electro-optic response is dominated by the molecular dynamics originating from the isotropic (centrosymmetric) core region, whereas the linear Pockels effect ows his slow dynamics to the slow interfacial dipolar relaxations, in agreement with our conclusions above regarding its origin.…”

Molecular Ordering of Nematic Liquid Crystals in Tubular Nanopores: Tailoring of Optical Anisotropy at the Nanoscale by Polymer Pore-surface Grafting

“…The molecular ordering of LC molecules in nanopores is considerably affected by interfacial interactions. Quenched disorder and geometrical confinement effects, on the other hand, have crucial influence on both their static and dynamic properties as has been demonstrated in recent optical [1,2] and dielectric studies [7,8]. Indeed a competition of these effects define a confined molecular ordering configuration.…”