| dc.description.abstract | Dielectric relaxation studies of a number of diaryl molecules
dispersed in atactic polystyrene and in some cases as solid discs have
been carried out. Preparation of the polystyrene matrices and the
solid disks, and the dielectric measurements using a General Radio 1615“A
capacitance bridge and a Hewlett-Packard Q-meter with appropriate
temperature-controllable cells have been described. The experimental
data as a function of frequency at different temperatures were 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.
A number of diary! molecules with different central bridging
atom or group, and different ring substituents have been studied. Attempt
have been made to Identify and study a unique type of rapid, intramolecular
rotation process termed "double-internal rotation", in
which the two aryl rings undergo simultaneous, coupled rotation.
Studies have been made to correlate molecular properties such
as symmetry, and electronic and sterlc effects to the occurrence of
double-internal rotation. Symmetric diaryl molecules such as sulfones,
sulfoxides, and dibenzothiophenes were found to undergo predominantly
over-all molecular rotation in polystyrene matrices. Asymmetric diaryl ethers and ketones displayed a major contribution to dielectric
relaxation from ovei*-al 1 molecular rotation. Symmetric diaryl ethers,
ketones, and sulfides (e.g., diphenyl ether, benzophenone, and
diphenyl sulfide) were found to undergo primarily double-internal
rotation, both in polystyrene and as compressed solids.
Some large molecules such as 1,2-diphenylcyclopropenone
underwent co-operative motion with the polymer chains near the glasstransition
temperature of polystyrene. A linear correlation was observed
between the enthalpy of activation and the entropy of activation for
molecules undergoing the same type of relaxation process. There was
also found to be a linear correlation between the volume swept out
during molecular reorientation and the energy of activation for the
process in rigid symmetric diaryl molecules.
Energy barriers for the molecular rotation of non-planar symmetric diaryl molecules (e.g., diphenyl sulfone and diphenyl sulfoxide) were found to be >= 60 kJ mol [superscript -1] depending on the size of the substituents.
Planar molecules the size of-dlbenzothlophene were found to have energy
barriers for molecular rotation of about 40 kJ mol [superscript -1]. The energy barrier for symmetric diaryl molecules which undergo double-internal rotation were observed in most cases to be in the range 5 15 kJ mol [superscript -1] at temperatures below 100 K. | |
