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Dielectric Relaxation in a Deeply Supercooled Liquid Crystals Confined in Random Porous Media

  • G. P. Sinha (a1) and F. M. Aliev (a1)

Abstract

Liquid crystals (LCs)—pentylcyanobiphenyl (5CB) and octylcyanobiphenyl (8CB)—were confined in random porous media with narrow pores of mean size equal to 100 Å and investigated by means of broad band dielectric spectroscopy in deeply supercooled state. In liquid crystalline phases, bulk 5CB and 8CB have two dielectrically active modes. The main mode with the relaxation time τ ∼ 10”8s is due to the rotation of molecules about their short axis, and the secondary mode is due to the tumbling motion of molecules with the relaxation time ツ ∼ 10”10s. Bulk 5CB and 8CB are nonglass formers and they crystallize at cooling. The confinement strongly influences the dynamical behavior of LCs and is resulted in qualitative changes in their properties. Deep supercooling of LCs in pores up to ∼ 150 degrees below the bulk crystallization temperature was observed. The relaxation rate of the process due to the molecular rotation in deeply supercooled state is slower than at the temperatures corresponding to nematic phase by many orders of magnitude. This slowing down is accompanied by anomalous broadening of the dielectric spectra.

Other new properties observed in confined LCs are two low frequency relaxation processes absent in bulk LCs. One of these processes is due to the molecular relaxation in the surface layers at liquid crystal-solid pore wall interface. The second process is probably a collective mode due to the relaxation of the surface induced polarization. The collective process due to surface polarization and the surface molecular mode show features typical for glass formers.

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Dielectric Relaxation in a Deeply Supercooled Liquid Crystals Confined in Random Porous Media

  • G. P. Sinha (a1) and F. M. Aliev (a1)

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