Morphology of poly(vinyl alcohol) gels assuring the greatest significant drawability

Atactic poly(vinyl alcohol) (at-PVA) and syndiotactic poly(vinyl alcohol) (st. PVA) prepared by gelation/crystallization using dimethyl sulphoxide/water mixtures were drawn in a hot oven at 160 degrees C under nitrogen. The degrees of polymerization of at- and st-PVA were 2000 and 1980, respectively. The drawability of at- and st-PVA films was affected by the composition of the solvent mixture as well as by quenching temperature. The drawability of at- and st-PVA films prepared by using the solvent mixture containing 60% of dimethyl sulphoxide and 40% of water became more pronounced as the temperature of gelation/crystallization decreased and the draw ratio reached maximum value at - 80 degrees C. Namely, the greatest significant drawability was the same condition for at- and st-PVA films in spite of the different stereo-regularity. Even in this common best condition for significant drawability, however, the morphological properties of swollen gels and of the resultant dry gel films are different each other, dependent upon the tacticity. For at-PVA, small-angle light scattering under I-IV polarization condition could not be observed in the swollen gels and in the dry films when the solutions were quenched at temperatures < -10 degrees C. In contrast, for st-PVA, the X-type scattering pattern from swollen gels became clearer as the temperature decreased but the pattern became indistinct under drying process at ambient condition. On the other hand, the fibrillar textures within the at- and st-PVA dry films became finer and the orresponding crystallinity became lower as the temperature of gelation/crystallization decreased. Thus it turned out that the morphological properties of the swollen gels and of the dried films play an important role to assure the greatest significant drawability.