DIELECTRIC-SPECTROSCOPY OF LIQUID-CRYSTALS IN SMECTIC, NEMATIC, AND ISOTROPIC PHASES CONFINED IN RANDOM POROUS-MEDIA

The dielectric behavior of alkylcyanobiphenyls (5CB and 8CB) confined in porous matrices with randomly oriented, interconnected pores with t wo different mean pore sizes (1000 and 100 Angstrom) has been investig ated by means of broadband dielectric spectroscopy in the frequency ra nge from 1 mHz to 1.5 GHz. The confinement has a strong influence on t he dielectric properties of liquid crystals (LCs), which resulted in t he appearance of a low frequency relaxation process (f less than or eq ual to 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 r elaxation 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 2 0 degrees below the bulk crystallization temperature; (b) in the tempe rature range corresponding to the anisotropic phase in pores, the temp erature dependence of the relaxation times (tau) of the process due to the rotation of molecules around the short axis is non-Arrhenius; (c) the retardation factor g = tau/tau(is) is similar or equal to 1.5, wh ereas the typical value of g in bulk nematic LCs is similar or equal t o 4. At the nematic-isotropic phase transition in pores smooth and sma ll changes in tau suggest that the ''isotropic'' phase of LCs in pores is not bulklike isotropic phase with complete disorder in molecular o rientations, and some degree of orientational order still persists.