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Reorientational Dynamics of a Nematogenic Liquid in Porous Sol-Gel-Glasses: Surface and Pore Size Effects

Published online by Cambridge University Press:  15 February 2011

F. W. Deeg  and
G. Schwalb
F. W. Deeg
Affiliation:
Institut für Physikalische Chemie, Universität München, Sophienstrasse 11, D-80333 München, Germany
G. Schwalb
Affiliation:
Institut für Physikalische Chemie, Universität München, Sophienstrasse 11, D-80333 München, Germany
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Abstract

Time-resolved transient grating optical Kerr effect experiments on a nanosecond timescale were used to investigate the reorientational dynamics of pentylcyanobiphenyl (5CB) confined in the nanometer lengthscale pores of silica glasses. Pore size dependent studies (25 Å ≤ d ≤ 100 Å) over a large temperature range from the bulk phase transition temperature Tni, = 35.2 ° C up to 115 ° C demonstrate a drastic influence of geometrical restriction on the cooperative reorientational motion in the nematogenic liquid. At temperatures far above the phase transition (T ≥ 70 °C) a bulk phase and a distinct surface layer with different dynamic properties are observed. With pore size reduction to 25 Å, a decrease of the surface layer thickness and an increase of the surface layer relaxation time are found. This is associated with the modification of the surface layer structure by geometrical factors, e.g., the pore curvature. Measurements at lower temperature in the range Tni ≤ T ≤ Tni = 30 ° C show a drastic decrease of the mean relaxation time τ of the order parameter fluctuations compared to the bulk. The relaxation dynamics exhibit a pronounced pore size dependence which can be explained by a crude correlation length cutoff model as well as a modified LandaudeGennes ansatz. However, these approaches cannot reproduce the nonexponentiality of the observed decays suggesting the importance of interporous interactions.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 366: Symposium N – Dynamics in Small Confining Systems , 1994 , 213
Copyright
Copyright © Materials Research Society 1995

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