Transport of methanol in poly(methyl methacrylate)

The transport kinetics of liquid methanol in PMMA sheet (Perspex) have been studied over the temperature range 23° to 63°C. At the lower temperatures the transport is typical Case II; the methanol penetrating the polymer behind a sharp front which moves at constant velocity. For higher temperatures the concentration of methanol at the front and the front velocity both decrease with increasing penetration. These factors, which are no doubt related, combine to give mass absorption kinetics in which the exponent of time is no longer unity (Case II) but approaches 0.5, the value typical of Fickian diffusion. Iodine has been added to the methanol to make the positions of the penetration fronts readily visible. The iodine does not affect either the rate of penetration or the equilibrium absorption. Measurements of colour density profiles on thin cross-sections using a microdensitometer give a reliable indication of the true methanol profile. The rate at which the methanol penetrates the glassy polymer is proportional to its concentration at the advancing fronts. The apparent activation energy for methanol penetration for constant concentration at the fronts is 25 kcal/mol.