Case II sorption in glassy polymers, penetration kinetics

grantor: University of TorontoThe penetration kinetics of methanol in polymethyl methacrylate (PMMA) beads has been examined in detail, which reveals a size dependence in the induction time during the initial phase of the Case II sorption. In addition, the induction time and front penetration rate are inversely related. Similar size dependence and inverse relationship between induction time and front penetration rate have also been observed in several other systems including methanol in crosslinked PMMA beads, higher alcohols in uncrosslinked PMMA beads, and n-hexane in polystyrene (PS) beads. This suggests that the induction time is not a material constant as previously assumed by existing theories. A new phenomenological. analysis of front penetration kinetics further substantiates that the induction time and front penetration rate are inversely related for any given system. Our results also show that the volume swelling ratio in the swollen region of the bead remains constant during the propagation of the penetration front. This supports the notion of a uniform concentration distribution in the swollen region during Case II sorption. This volume swelling ratio also shows a slight increase with temperature. As confirmed by SEM and AFM observations, the dependence of induction time on size is not caused by changes in surface morphology since there is no detectable difference in surface morphology for PMMA beads of different diameters. The inverse relationship between induction time and penetration rate as derived from our phenomenological analysis is in agreement with that predicted from a molecular theory. The phenomenological analysis further confirms that the induction time and its relationship with the initial front penetration rate depend on both the geometry and dimension of the polymer specimen in the penetration direction. The length of induction time for slab and cylindrical geometry approaches that of the semi-infinite geometry more than that of the spherical geometry. Consequently, this size dependence may become insignificant when the dimension of the cylindrical discs and slabs is comparable to the diameter of the beads. Based on our results, a quantitative criterion has been formulated for the occurrence of Case II sorption. A new phase diagram has also been constructed to predict the occurrence of Case II behavior. It provides a qualitative definition of the upper temperature limit for Fickian diffusion as well as a lower temperature limit for anomalous diffusion in glassy polymers.Ph.D