A study of diffusion in polymers using C-14 labelled molecules

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.