Abstract
The swelling behaviour of dried and annealed poly(vinyl alcohol) (PVA) films was investigated in several mixed solvents of dimethylsulfoxide (DMSO) and water with different solvent compositions. The swelling ratio versus time curves obtained in DMSO-rich solvents were shown to be of anomalous swelling type: the swelling ratio first increased in proportion to time for a while and then catastrophically expanded on approaching the equilibrium ratio. Further, crazing of the films occurred before the catastrophic expansion. These results indicated that the swelling obeyed case II diffusion of solvent through glassy PVA. It was shown that the relaxation rate of the glassy PVA molecules was the key factor that dominated the nature of swelling. The relaxation rate was sufficiently low in the DMSO-rich solvents and too high in the water-rich solvents for the anomalous swelling to be detected. The crystallinity of PVA was also a factor by which the relaxation rate and thus the swelling rate could be controlled over a wide range.
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