  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.
