Dynamics of Cyanophenyl Alkylbenzoate Molecules in a Surface Layer Adsorbed onto Aerosil. I. Cyanophenyl Hexylbenzoate

Abstract

A composite prepared from aerosil A380 and the liquid crystal (LC) 4-hexyl-4′-cyanophenyl benzoate (CP6B) was investigated by broadband dielectric spectroscopy in a large temperature range. The selected high silica density (ca. 7 g aerosil/1 g of CP6B) allows the observation of a thin layer (two-monolayer structure) adsorbed on the surface of the silica particles. For the composite one relaxation process is observed at frequencies much lower than that of the processes found for bulk CP6B. It is assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its relaxation rates obeys the Vogel–Fulcher–Tammann law, which is characteristic for glass-forming liquids. The quasi 2D character of the observed glassy dynamics in the surface layer is discussed. The temperature dependence of the CP6B relaxation in the composite is compared with that of related hexylcyanobiphenyl molecules in the surface layer of aerosil composite with a similar concentration.

Keywords:

• aerosil composites
• cyanophenyl-alkylbenzoates
• dielectric spectroscopy
• infrared spectroscopy
• liquid crystals
• thermal analysis

Acknowledgments

SF, LF, TB, RM and DMM gratefully acknowledge the financial support from Romanian Ministry of Education and Research (Project CEEX 76/C1). LF and SF are also grateful for their fellowships from Federal Institute of Materials Research and Testing (BAM). The authors thank Degussa-Hüls for the aerosil samples.