A novel method of measuring the self-diffusion coefficient of a
penetrant molecule in various polymers was devised. This method
makes use of a permeation experiment, where the radioactively
labelled moleculee exchange with the chemically identical but nonradioactive
molecules through the polymer membrane specimen. The
rate of permeation is measured by the excitation of the non-radioactive
molecules which act as the solvent in a liquid scintillation
mixture, with subsequent excitation and fluorescence of the
dissolved scintillator solutes. By maintaining the concentrations
of the radioactive and non-radioactive molecules in the vapour phase at
the same level, a self-diffusion coefficient D* at one precise
penetrant concentration level can be determined.
The diffusion coefficients measured were compared to those
obtained from conventional sorption-desorption methods, and the
comparison was discussed in terms of the basic definitions of the
different diffusion coefficients.
The diffusional behaviour in silicone rubber and S-B-S block
copolymer was discussed mainly with reference to the free volume theory
and the activated zone theory, with particular emphasis on chain
mobility. Dynamic mechanical studies were also obtained in these
polymers and related to the diffusion characteristics through chain
mobility and free volume concepts.
Diffusion in filled systems and the two-phase S-B-S copolymers was
discussed with the help of mathematical models derived for analogous
electrical conductivity through hetereogeneous medium, and a certain
order of polystyrene domain distribution was indicated in the latter.
Silica-silicone rubber interaction was considered and some
conclusions were made.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.