T. Bruckert et al., DIELECTRIC STUDIES OF OCTYLOXY-CYANOBIPHENYL (8OCB) UNDER HIGH-PRESSURE, Berichte der Bunsengesellschaft fur Physikalische Chemie, 100(7), 1996, pp. 1133-1137
Citations number
35
Journal title
Berichte der Bunsengesellschaft fur Physikalische Chemie
The static and complex permittivity of octyloxy-cyanobiphenyl (8OCB) h
as been measured for the smectic, nematic and isotropic phase as a fun
ction of temperature and pressure. The main part of the work is concer
ned with the study of epsilon(parallel to), the component of the permi
ttivity parallel to the director n, and its frequency dependence up to
13 MHz, yielding the dielectric relaxation time tau(parallel to) conn
ected with the rotation of the molecules around their short axes. The
temperature and pressure dependence of tau(parallel to) allows to calc
ulate the activation enthalpy, Delta double dagger H-parallel to and a
ctivation volume, Delta double dagger V-parallel to, respectively. The
activation parameters for the nematic phase (Delta double dagger H-pa
rallel to = 75-55 kJ/mol, decreasing with increasing pressure, Delta d
ouble dagger V-parallel to = 65-45 cm(3)/mol, decreasing with increasi
ng temperature) are similar to previous results for octyl-cyanobipheny
l (8 CB), however, for the smectic A(d) phase (Delta double dagger H-p
arallel to approximate to 40 kJ/mol, Delta double dagger V-parallel to
= 40-50 cm(3)/mol) the p, T-dependencies are opposite to 8 CB. A comp
arison of the ii relaxation times for the isotropic and nematic phase
enables us to estimate retardation factors and the nematic potential q
in the frame of theories of Meier-Saupe and Coffey et al. With the ai
d of literature data for the order parameter S the linear relationship
q similar to S was checked.