Spontaneous Orientation of Molecules during Saponification of Poly(vinyl pivalate)

Abstract

Two kinds of poly(vinyl alcohol) (PVA) with assigned syndiotactic diad content, 52 to 53% (denoted as atactic) and 63 to 65% (denoted as syndiotactic), were prepared by low temperature UV-radiation bulk polymerization of vinyl acetate and vinyl pivalate, followed by saponifying the corresponding poly(vinyl acetate) (PVAc) and poly(vinyl pivalate) (PVPi), respectively. Saponification of PVPi produced highly crystalline and oriented PVA fibers whereas PVAc did not develop any fibrillar structure. The tacticity of the polymer seemed to play a significant role in the in situ fiber formation. Syndiotactic PVA gave positive yield stress while atactic PVA did not exhibit yield behavior. Relaxation time increased with syndiotactic content or molecular weight of polymer. Even by slight shearing between glass plates, syndiotactic PVA solutions readily developed a shear-induced fibrillar structure, while atactic PVA solutions did not exhibit any domain structure. Actual in situ fibrillation at the late stage of saponification of PVPi was simulated by two syndiotactic (PVA)s of similar molecular weight but with different degrees of saponification, 95.4 and 99.0%.