Diffusion of deuterated toluene in polystyrene

The diffusion of deuterated toluene (d-toluene) from the vapour into glassy polystyrene has been studied using forward recoil spectrometry (FRES) over a range of temperatures (20–35°C) and d-toluene activities (0.10 to 0.35). At the highest d-toluene activity a Case II diffusion front is observed to form and propagate at a constant velocity into the polymer. The velocity is thermally activated with an activation enthalpy, ΔH v, of 1.49 eV. The diffusion coefficient, D, of d-toluene into the glass can be extracted from the measured concentration profile ahead of the front and was also found to be thermally activated with an activation enthalpy, ΔH D, of 1.12 eV. Because the Thomas and Windle model of Case II diffusion predicts that ΔH v = (ΔH D + ΔH η) 2 , where ΔH η is the activation enthalpy for viscous creep of glassy polystyrene, previously determined to be 1.76 eV, we can compare the predictions of this model with experiment. The good agreement observed gives further evidence that the basic premise of the Thomas and Windle model of Case II diffusion is correct. Measurements of D at low d-toluene activities as well as the exchange diffusion coefficient were made and yield values that are in reasonable agreement with those extracted from the concentration profile ahead of the Case II front. These results show that D is approximately independent of d-toluene volume fraction below the critical volume fraction, ø c, for Case II front formation.