Unusual retardation and enhancement in polymer dissolution: role of disengagement dynamics

Abstract

This work addresses the issue of resolution of some unusual observations in the swelling-dissolving problem in polymers. There are observations showing several fold increase in the rate of dissolution when the residual solvent content is increased only marginally. Additionally, by using an in situ NMR technique to study dissolution, we have found the existence of an intermediate plateau in the dissolution curve indicating an unusual retardation in the rate of dissolution under certain conditions. Interestingly, this plateau disappears on either increasing the molecular weight of the polymer or on doing dissolution under shearing rather than under stagnant conditions. An improved model is proposed to explain these results, which cannot be explained by the existing models. The new feature is the incorporation of a kinetic model to relate the disengagement rate to the swelling rate, through the changing mobility of the disengaging macromolecule at the gel-liquid interface. The present study thus enables a clearer elucidation of the role of disengagement dynamics in the dissolution process.