ACTIVATION BARRIERS FOR OXYGEN DIFFUSION IN POLYSTYRENE AND POLYCARBONATE GLASSES - EFFECTS OF CODISSOLVED ARGON, HELIUM, AND NITROGEN

A recently developed spectroscopic technique was used to determine oxy gen diffusion coefficients as a function of temperature for polystyren e and polycarbonate films. Data were recorded at total pressures <300 Torr over the temperature range 5-45 degrees C under conditions in whi ch argon, helium, and nitrogen, respectively, were copenetrants. In al l cases, the presence of the additional gas caused an increase in the oxygen diffusion coefficient. Arrhenius plots of the data yield (a) a diffusion activation barrier, E(act), and (b) a diffusion coefficient, D-0, that represents the condition of ''barrier-free'' gas transport for the temperature domain over which the Arrhenius plot is linear. Fo r all cases examined in both polystyrene and polycarbonate, D-0 increa sed with an increase in the partial pressure of added gas. In polystyr ene, the presence of an additional gas did not E(act). In polycarbonat e, E(act) obtained in the presence of helium and argon likewise did no t differ from that obtained in the absence of the copenetrant. When ni trogen was the added gas, however, a larger value of E(act) was obtain ed. This latter observation is interpreted to reflect the plasticizati on of polycarbonate by nitrogen. E(act) and D-0 data are discussed wit hin the context of a model that distinguishes between dynamic and stat ic elements of free volume in the polymer matrix.