Magnetic Resonance Imaging Analysis of Molecular Mobility during Dissolution of Poly(vinyl alcohol) in Water

Magnetic resonance imaging is used to study changing microstructure and molecular motion during dissolution of poly(vinyl alcohol) (PVA) in water. One-dimensional water concentration profiles were measured as a function of distance from the polymer-solvent interface. Diffusion-weighted profiles were used to calculate the spatial variation of the self-diffusion coefficient of water. The results indicate that diffusion coefficient values decrease toward the glassy core of the polymer. Self-diffusion coefficient values within the dissolving polymer increase with increasing dissolution time, while those near the polymer-solvent interface remain fairly constant. The effect of PVA molecular weight on the dissolution mechanism was investigated, with Mh n varying between 35 740 and 133 000. Comparisons were made with a mathematical model for polymer dissolution. The water concentration profiles predicted by the model are qualitatively consistent with the experimentally obtained profiles. In addition, the scaling laws proposed in the model for the polymer diffusion coefficients are verified. The calculated polymer diffusion coefficients (based on the experimental data) yield D (polymer self-diffusion coefficient) ∼ M-1.9 (theory predicts an exponent of -2) and D ∼ M-0.6 (theory predicts an exponent of -0.5) near the glassyrubbery and the rubbery-solvent interfaces, respectively, providing supporting evidence for the hypothesis that phenomena such as reptation are important near the glassy-rubbery interface while Zimm-type diffusion occurs near the polymer-solvent interface. The results also point to the existence of a change in the mode of diffusion as solvent penetrates into the polymer.