TY - JOUR A1 - Frunza, S. A1 - Schönhals, Andreas A1 - Frunza, L. A1 - Beica, T. A1 - Zgura, I. A1 - Ganea, P. A1 - Stoenescu, D. T1 - Dynamics of cyanophenyl alkylbenzoate molecules in the bulk and in a surface layer adsorbed onto aerosil. Variation of the lengths of the alkyl chain JF - Chemical physics N2 - The molecular mobility of 4-butyl- and 4-pentyl-4'-cyanophenyl benzoate (CP4B, CP5B) and their composites prepared from aerosil A380 was investigated by broadband dielectric spectroscopy in a large temperature range. Thermogravimetric and infrared investigations were additionally performed. High silica density (larger than 7 g aerosil/1 g of liquid crystal) was selected to observe a thin layer adsorbed on the surface of the silica particles. The data were compared with those of the member of the series with six carbon atoms in the alkyl tail. Bulk CP4B and CP5B show the dielectric behaviour expected for liquid crystals. For the composites one relaxation process is observed at frequencies much lower than those for the corresponding bulk, which was assigned to the dynamics of the molecules in a surface layer. The temperature dependence of the relaxation rates (and of the dielectric strength) shows a crossover behaviour with two distinguished regimes. At higher temperatures the data obey the Vogel-Fulcher-Tammann law, whereas an Arrhenius law is observed at lower temperature, in a close similarity to the behaviour of a constrained dynamic glass transition. The estimated Vogel and crossover temperature is independent on the tail length, while the activation energy for the low temperature branch increases weakly with increasing the alkyl tail. KW - Confined liquid crystals KW - Molecules at surfaces KW - Dielectric properties KW - Relaxation processes KW - Glassy dynamics KW - Crossover phenomenon PY - 2010 DO - https://doi.org/10.1016/j.chemphys.2010.04.031 SN - 0301-0104 SN - 1873-4421 VL - 372 IS - 1-3 SP - 51 EP - 60 PB - Elsevier CY - Amsterdam AN - OPUS4-21588 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -