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 mol ecular motion during dissolution of poly(vinyl alcohol) (PVA) in water. One -dimensional water concentration profiles were measured as a function of di stance 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 t oward the glassy core of the polymer. Self-diffusion coefficient values wit hin the dissolving polymer increase with increasing dissolution time, while those near the polymer-solvent interface remain fairly constant. The effec t of PVA molecular weight on the dissolution mechanism was investigated, wi th (M) over bar(n) varying between 35 740 and 133 000. Comparisons were mad e with a mathematical model for polymer dissolution. The water concentratio n profiles predicted by the model are qualitatively consistent with the exp erimentally obtained profiles. In addition, the scaling laws proposed in th e model for the polymer diffusion coefficients are verified. The calculated polymer diffusion coefficients (based on the experimental data) yield D (p olymer self-diffusion coefficient) similar to M-1.9 (theory predicts an exp onent of -2) and D similar to M-0.6 (theory predicts an exponent of -0.5) n ear the glassy-rubbery and the rubbery-solvent interfaces, respectively, pr oviding supporting evidence for the hypothesis that phenomena such as repta tion are important near the glassy-rubbery interface while Zimm-type diffus ion occurs near the polymer-solvent interface. The results also point to th e existence of a change in the mode of diffusion as solvent penetrates into the polymer.