Application of broad-band dielectric spectroscopy for investigations of liquid crystal - porous media microcomposites

摘要

We applied ultra broad-band dielectric spectroscopy in the frequency range from 10~(-3) Hz to 10~9 Hz to investigate the effect of size, shape and volume fraction of the pores in the porous matrices on the dielectric properties of liquid crystals (LC) dispersed in these matrices. Measurements in such a broad frequency range make it possible to obtain detailed information on the important aspects of the electrical behavior of heterogeneous materials such as: conductivity, surface polarization, and influence of confinement on dynamics of molecular motion of polar molecules forming LC. We investigated alkylcyanobiphenyls in the isotropic, nematic and smectic phases dispersed in porous glasses (average pore sizes - 100 A and 1000 A) which have randomly oriented, interconnected pores, and anopore membranes (pore diameters - 200 A and 2000 A) with parallel cylindrical pores. Dispersion of LC resulted in qualitative changes of their dielectric properties. Analysis of broad-band dielectric spectra shows that in organic (LC) - inorganic (porous matrix) heterogeneous composites conductivity plays an important role at f <1 Hz. We observe the appearance of new dielectric modes: a very slow process with characteristic frequency approx= (1 - 10) Hz and a second process in frequency range about (10~3 - 10~6) Hz. The slow process arises due to the relaxation of interfacial polarization at pore wall -LC interface. The origin of this could be due to absorption of ions at the interface. Another possibility is the preferential orientation of the permanent dipoles at pore surface. The second new mode is due to the hindered rotation of the molecules near the interface. Additionally we observed two bulk like modes due to the rotation of the molecules around their short and long axii which are modified.