Confinement and surface effects on the molecular dynamics of a nematic mixture investigated by dielectric relaxation spectroscopy

Broadband dielectric spectroscopy (10(-2)-10(9) Hz) was employed to investigate the molecular dynamics of the liquid crystalline mixture E7 confined in both untreated and lecithin-treated 20 nm Anopore membranes. Because E7 does not crystallize, it was possible to cover a temperature range of more than 200 K, providing an exhaustive dielectric characterization of a liquid crystal confined to Anopore membranes for the first time. In the nematic state, the tumbling (alpha-) and the delta-relaxation are observed, also under confinement conditions. The analysis of their relative intensities give that the orientation of the E7 molecules is preferentially axial in untreated but opposite radial in lecithin-treated pores. The radial alignment of the liquid crystals in the modified membrane is understood as a tail-to-tail conformation of E7 molecules imposed by the adsorbed lecithin molecules. The relaxation rate of the alpha-process is enhanced for E7 confined in native Anopore compared with the bulk and E7 in treated pores. This is interpreted as resulting from a less dense molecular packing of E7 in the middle of the pore compared to the bulk. In both untreated and treated membranes, the relaxation rate of the delta-process is higher than in the bulk, and the values of the respective Vogel-Fulcher-Tammann temperatures depend on the actual surface treatment. Additionally, a surface process, due to molecular fluctuations of molecules within an adsorbed layer at the pore wall, was detected.