Title: Confinement effects on strongly polar alkylcyanobiphenyl liquid crystals probed by dielectric spectroscopy
Authors: Leys, Jan ×
Glorieux, Christ
Thoen, Jan #
Issue Date: 2008
Publisher: IOP Pub.
Series Title: Journal of physics: condensed matter vol:20 pages:1-7
Article number: 244111
Abstract: Dielectric spectroscopy has often been used to study confinement effects in alkylcyanobiphenyl
liquid crystals. In this paper, we highlight some of the effects that have been discovered
previously and add new data and interpretation. Aerosil nanoparticles form a hydrogen bonded
random porous network. In dispersions of alkylcyanobiphenyls with aerosils, an additional slow
process arises, that we ascribe to the relaxation of liquid crystal molecules in close interaction
with these nanoparticles. Their relaxation is retarded by a hydrogen bond interaction between
the cyano group of the liquid crystals and an aerosil surface hydroxyl group. A similar surface
process is also observed in Vycor porous glass, a random rigid structure with small pores. A
comparison of the temperature dependence of the relaxation times of the surface processes in
decylcyanobiphenyl and isopentylcyanobiphenyl is made, both for Vycor and aerosil
confinement. In decylcyanobiphenyl, the temperature dependence for the bulk and surface
processes is Arrhenius (in a limited temperature range above the melting point), except in
Vycor, where it is a Vogel–Fulcher–Tamman dependence (over a much broader temperature
range). In bulk and confined isopentylcyanobiphenyl, the molecular processes have a
Vogel–Fulcher–Tamman dependence, whereas the surface processes have an Arrhenius one.
Another effect is the acceleration of the rotation around the short molecular axis in confinement,
and particularly in aerosil dispersions. This is a consequence of the disorder introduced in the
liquid crystalline phase. The disorder drives the relaxation time towards a more isotropic value,
resulting in an acceleration for the short axis rotation.
ISSN: 0953-8984
Publication status: published
KU Leuven publication type: IT
Appears in Collections:Soft Matter and Biophysics
× corresponding author
# (joint) last author

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