Temperature dependence of methanol transport in spherical PMMA beads

Abstract

The effect of temperature on the kinetics of methanol transport in PMMA has been investigated for the first time in spherical sample geometry over the temperature range of 25–50°C. The observed penetration of methanol in PMMA beads of 1.045 ± 0.03 mm diameter exhibits a geometry-dependent Case II transport behaviour up to 50°C. This is characterized by an induction period followed by an extended region of linear front penetration before an acceleration of front movement towards the centre. This front acceleration, being absent in sheet samples, was shown to be characteristic of the solvent penetration behaviour in spheres. The induction period has been observed to decrease, whereas the front velocity to increase, with increasing temperature, consistent with their known dependencies on the solvent diffusivity and the viscosity of the polymer glass. Both the induction time and the velocity of the methanol front are thermally activated, with apparent activation energies of 27.8 kcal mol −1 and 26.4 kcal mol −1, respectively. No further increase in the PMMA bead diameter has been observed after the penetrating methanol fronts have met at the centre, suggesting a negligible concentration gradient behind the Case II swelling front. In addition, the continuous increase of PMMA bead diameter during the induction period for methanol front penetration provides new supporting evidence for the existence of a Fickian precursor penetrating ahead of the sharp Case II front in the PMMA-methanol system.