Activation barriers for oxygen diffusion in polystyrene and polycarbonate glasses: effects of codissolved argon, helium, and nitrogen

Abstract

A recently developed spectroscopic technique was used to determine oxygen diffusion coefficients as a function of temperature for polystyrene and polycarbonate films. Data were recorded at total pressures <300 Torr over the temperature range 5–45 °C under conditions in which argon, helium, and nitrogen, respectively, were copenetrants. In all 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, Eact, and (b) a diffusion coefficient, D0, that represents the condition of "barrier-free" gas transport for the temperature domain over which the Arrhenius plot is linear. For all cases examined in both polystyrene and polycarbonate, D0 increased with an increase in the partial pressure of added gas. In polystyrene, the presence of an additional gas did not change Eact. In polycarbonate, Eact obtained in the presence of helium and argon likewise did not differ from that obtained in the absence of the copenetrant. When nitrogen was the added gas, however, a larger value of Eact was obtained. This latter observation is interpreted to reflect the plasticization of polycarbonate by nitrogen. Eact and D0 data are discussed within the context of a model that distinguishes between dynamic and static elements of free volume in the polymer matrix. Keywords: oxygen diffusion, polystyrene, polycarbonate, activation barrier.

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“…The data obtained indicate an indiscernible SNS‐dependent change in E act : E act (2% SNS) = 31.3 ± 0.7 kJ mol −1 , E act (neat PS) = 30 ± 1 kJ mol −1 . The values of E act obtained in this work from SNS‐free samples likewise compare favorably with those previously obtained from PS films 12, 29…”

“…Oxygen diffusion coefficients obtained for the various SNS‐containing films are listed in Table II. Data obtained from SNS‐free samples compare favorably with data previously obtained from PS films 12, 23, 24, 29. Although the SNS‐dependent changes in D are subtle, there is nevertheless a clear trend showing that the magnitude of D systematically decreases as the amount of SNS in the PS sample is increased.…”

“…(4)] can be obtained over comparatively large temperature ranges below T g 12, 29, 30. In this way, one can obtain an apparent activation energy, E act , that can be useful in characterizing features and properties of the polymer that influence oxygen diffusion 12, 29–31 …”

“…In the context of this latter statement, it is interesting to note that, within the errors of our measurements, we are unable to discern a significant difference in the values of E act for samples containing 0 and 2% added SNS. It is generally assumed that the value of E act derived from an oxygen diffusion experiment reflects an activation barrier for polymer motions that influence translation of oxygen through the polymer matrix 12, 29. Admittedly, there is also a noticeable amount of curvature in the Arrhenius plots recorded (Fig.…”

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“…However, this temporary microenvironment is not completely isolated from atmospheric O 2 because of the potential for exchange with the surrounding media (i.e., the piston-like sensor probes do not form a seal when lowered into the well) and the plastic comprising the microplate wells (i.e., polystyrene is permeable to O 2 [13,14]). Consequently, as [O 2 ] declines in the chamber volume above the cells during longer measurement periods, a gradient is created favoring diffusion of O 2 from the media surrounding the sensor probe and the walls of the well into the measurement volume.…”