Effect of sample size on Case II diffusion of methanol in poly(methyl methacrylate) beads

The effect of sample size on the characteristic Case II solvent front penetration behavior has been investigated in a methanol-PMMA bead system at 25 °C in the particle diameter range of 0.125-1.280 mm. The observed induction time generally increases, whereas the front penetration rate exhibits a weak decreasing trend, with increasing sample diameter. Such dependencies of induction time and front penetration rate on sample size, although not predicted by existing theories of Case II diffusion, can be attributed to the size-dependent nature of several physical processes originated from the Fickian precursor penetrating ahead of the moving solvent front in an inhomogeneous swelling system of finite dimension.