Effect of Thermal History on Gas Transport

Films of an atactic polystyrene were giventwo different thermal histories by treating them under different conditions, and effect of the thermal history was studied by measurements of gas transport parameters and mechanical relaxation behavior. After having been treated at 130°C for 2 hrs, (A) the polymer films were simply quenched to room temperature, the quenched sample, and (B) the films were annealed at 85°C for 10 days, the annealed sample. The permeability and time-lag experiments were made using helium, argon, krypton, and methane as penetrants in the temperature region from 23 to 130°C. Permeation and diffusion behavior of helium was found to be independent of the thremal history. In transport behavior of argon and krypton major change caused by the subsequent annealing was decrease of permeability and diffusion coefficients, while for methane transport these coefficients increased by the subsequent annealing. For all systems investigated a clear break in the Arrhenius plots of diffusion coefficient was found at the glass transition temperature Tg, and activation energies for diffusion were greater above Tg than were below Tg. Furthermore, for diffusion of argon, krypton, and methane in the annealed sample, another break was exhibited in the plots at temperatures Tb which are close to the so-called 9 transition of polystyrene. Activation energies for diffusion below Tb were smaller than those above T5. A discontinuity was observed at Tg in the Arrhenius plots fo