| dc.description.abstract | Dielectric absorption of a number of nonpolar,
weakly polar and fairly polar molecules in the
pure solid state are studied. Measurements of these
solids using either a General Radio 1621 Precision
Capacitance Measurement System or a General Radio 1615-A
Capacitance bridge with appropriate temperature-controllable
cells are described. The glass transition (Tg)
measurements using the Glass Transition Measurement
Apparatus are also described. The experimental data as
a function of frequency at different temperatures are
subjected to analysis by a series of computer programmes
written in the APL language. The activation energy barriers for the dielectric relaxation processes were
obtained by the application of ;the Eyring rate equation.
The dielectric absorption of some spherically shaped
nonpolar molecules are observed, the energy
barrier values of which agree well with those found from
other measurements. It is suggested that this absorption
might be due to the interaction of the radiofrequency
radiation with an induced moment which results from
multiple interaction.
Of the spherically-shaped polar molecules
examined, both molecular and co-operative relaxations
are detected in all the alkylhalides. Dielectric data
for molecular relaxations are described by the Cole-Cole
plots, whereas those for co-operative motion, in most
of the cases, cannot be accurately represented by the
Davidson-Cole skewed-arc function.
Low temperature molecular relaxations are
detected in some nonpolar and weakly polar aromatic
hydrocarbons. The results of these molecules are used
to suggest a possible correlation between enthalpy and
entropy of activation and ionization potential and enthalpy
of activation. The dielectric absorption of apparently
nonpolar aromatic hydrocarbons are explained on the basis
of quadrupole or octupole induced moment which may be
of significant magnitude when the internuclear distances
are small as in the solid state.
Co-operative relaxations of some of the nonpolar
and weakly polar hydrocarbons are found above the
glass transition temperature (Tg). The variation in the
energy barrier values for these relaxations are explained
in terms of molecular interaction which is measured either by ionization potential or by the stretching frequency
of a suitable solute molecule such as pyrrole (e.g. N-H)
with which the aromatic hydrocarbon can form hydrogen bond.
The molecular relaxation processes of a number
of rigid arylhalides in the pure solid state are found
almost in the same temperature and frequency regions as
those found in different viscous media. A similar enthalpy
of activation values are also observed. For some rigid
heterocyclic molecules, except N-methyIpyrrole, no molecular
relaxation is observed and this could be accounted for
by the crystal structure and the lack of free volume for
these molecules to relax. | |
