Mechanistic investigation of drying regimes during solvent removal from poly(vinyl alcohol) films

摘要

The mechanism of removal of solvents such as water and methanol from semicrystalline poly(vinyl alcohol) films was investigated using various thermal analysis techniques to obtain a mechanistic understanding of the drying process. Various drying regimes were identified, which correspond to different rate-controlling steps, and characterized by differences in drying rates and mechanisms. The kinetics of solvent removal was measured gravimetrically using thermogravimetric analysis (TGA). The drying kinetics studies yielded four distinct drying regimes hypothesized to be due to (1) removal of free solvent, (2) elimination of bound solvent, (3) solvent removal during the rubbery-glassy transition, and (4) drying when the polymer is in the glassy state. The amounts of free and bound water present in the polymer were measured using differential scanning calorimetry and compared with the TGA results, and they were found to verify the proposed hypothesis. The rubbery-glassy transition observed in TGA results was confirmed using dynamic mechanical analysis. The thickness of the films was also measured as a function of drying time using thermomechanical analysis. The drying rate plots were found to be qualitatively similar to the plots of changes in film thicknesses as functions of drying time. The results of these various techniques were analyzed to propose a comprehensive mechanism of solvent removal from poly(vinyl alcohol) films.