Dielectric spectroscopy of liquid crystals in smectic, nematic, and isotropic phases confined in random porous media

Abstract

The dielectric behavior of alkylcyanobiphenyls (5CB and 8CB) confined in porous matrices with randomly oriented, interconnected pores with two different mean pore sizes (1000 and 100 \AA{}) has been investigated by means of broadband dielectric spectroscopy in the frequency range from 1 mHz to 1.5 GHz. The confinement has a strong influence on the dielectric properties of liquid crystals (LCs), which resulted in the appearance of a low frequency relaxation process $(f<~$ 10 kHz) not present in bulk and a second new process due to the presence of surface layer at solid pore wall-LC interface. Bulklike relaxation processes due to the rotation of molecules around the short axis and due to tumbling of molecules are also observed. All observed relaxation processes are of non-Debye type. Other observed differences between bulk and confined behavior are as follows: (a) the relaxation processes in confined LCs are not frozen even at temperatures about 20 degrees below the bulk crystallization temperature; (b) in the temperature range corresponding to the anisotropic phase in pores, the temperature dependence of the relaxation times $(\ensuremath{\tau})$ of the process due to the rotation of molecules around the short axis is non-Arrhenius; (c) the retardation factor $g=\ensuremath{\tau}/{\ensuremath{\tau}}_{\mathrm{is}}$ is $\ensuremath{\simeq}1.5,$ whereas the typical value of $g$ in bulk nematic LCs is $\ensuremath{\simeq}4.$ At the nematic-isotropic phase transition in pores smooth and small changes in $\ensuremath{\tau}$ suggest that the "isotropic" phase of LCs in pores is not bulklike isotropic phase with complete disorder in molecular orientations, and some degree of orientational order still persists.