MATHEMATICAL-MODELING AND EXPERIMENTAL CHARACTERIZATION OF POLYMER DISSOLUTION

A new polymer dissolution model was developed by incorporating the polymer chain disentanglement mechanism into the relevant transport equations. The disentanglement time was used as a dissolution characteristic time controlling the moving position of the solvent-polymer boundary. A dimensionless dissolution number was defined as the ratio of the characteristic polymer disentanglement time to the characteristic solvent diffusion time. The dissolution number was shown to be proportional to the square of the gel layer thickness. Scaling law expressions for the dependence of the gel layer thickness and the polymer dissolution rate on polymer molecular weight were also derived. Solution of the model for one-dimensional dissolution showed three distinct dissolution stages and confirmed the proposed scaling law relations for the gel layer thickness and the dissolution rate. Experimental studies of dissolution of polystyrene and poly(methyl methacrylate) in methyl ethyl ketone were used to verify the model, and two types of polymer dissolution behavior were observed. For dissolution of polystyrene in MEK, the solvent diffusion behavior was Fickian and a constant gel layer thickness was observed during the stationary dissolution stage. The effect of polymer molecular weight on the gel layer thickness was investigated for nine monodisperse samples, with MBAR(n) ranging from 28 000 to 2 830 000. The experimental results showed that the dependence of the gel layer thickness on molecular weight is more prominent in the high molecular weight region. The polystyrene data verified the new dissolution model. The dissolution of PMMA in MEK was controlled by crack propagation as no significant gel layer was observed.