Structure of poly(vinyl alcohol) microfibrils produced by saponification of copoly(vinyl pivalate/vinyl acetate)

High molecular weight specimens of syndiotacticity-rich poly(vinyl alcohol) (PVA) have been prepared by saponification of copolymers of vinyl pivalate (VPi) and vinyl acetate (VAc). VPi and VAc were copolymerized using the low-temperature initiator 2,2′-azobis(2,4-dimethylvaleronitrile) and saponi-fied in KOH/methanol/water with vigorous stirring, i.e., under shear. The PVAs obtained had number-average degrees of polymerization in the range (5.6-16.5) × 10³ and syndiotactic diad (S-diad) contents of 52.8-61.5%. The highest molecular weight and S-diad content were obtained for specimens derived from homopoly(VPi) prepared at low initiator concentration, and these parameters declined progressively with increasing VAc content in the parent copolymers. The structures of these PVAs show a consistent sharp transition between 55 and 56% S-diad content. Specimens with lower S-diad contents have shapeless morphologies, but at 56% S-diad content the polymers were fibrous, with a higher degree of crystallinity and orientation of the crystallites. The order improves steadily with the S-diad content above the transition and correlates with the observed changes in the thermal and mechanical properties. The results suggest that the small increase in S-diad content from 55 to 56% facilitates a major change in the crystallization process, such that the crystallinity improves, and leads at the same time to the development of a fibrous morphology when the saponification is performed under shear.