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Rheological and morphological properties as well as crystal characteristics of syndiotacticity-rich ultrahigh molecular weight poly(vinyl alcohol) (PVA) were studied with consideration of degree of saponification (DS). The solutions of syndiotacticity-rich PVA with DS higher than 90% in dimethyl sulfoxide exhibited shear-thinning behavior without lower Newtonian flow region, implying the presence of heterogeneity. From the logarithmic plot of G‘ vs G‘ ‘, the modified Casson plot, and the variation of relaxation time with DS, the syndiotacticity-rich PVA chains with high DS were elucidated to have a nature of molecular alignment under shearing deformation through the formation of microgels. These characteristic rheological features have a strong relationship with the development of the microfibrillar structure in the course of saponification of poly(vinyl pivalate). The well-oriented microfibrillar structure was observed in the syndiotacticity-rich PVA with high DS, which resulted from the hydrogen-bonding generation during saponification.

The physical gelation behavior of high molecular weight (HMW) syndiotactic poly(vinyl alcohol) (S-PVA) was investigated in consideration of stereoregularity of the polymer. To precisely identify the effect of stereoregularity, syndiotactic diad (S-diad) contents of PVAs with the similar molecular weights were controlled to 61.5, 58.2, and 55.7%, respectively. The gel point (GP) of HMW S-PVA solution was determined by observing the frequency-independent loss tangent and the crossover between the relaxation exponents from the oscillatory shear experiment. The gelation process of HMW S-PVA solution with S-diad content over 58.2% was divided into two types with respect to the concentration of solution. The first type is the gelation that took place directly from sol to gel with a clear GP which was well coincident with the macroscopic gelation temperature (Tgel). This is for the higher concentration regime. The second one is the gelation that occurred gradually accompanying the evolution of gel-like properties in sol state without a definite GP, which is for the lower concentration regime. The gel-like heterogeneity formed during the second type gelation might be related with the liquid−liquid phase separation. In contrast, HMW S-PVA solution with S-diad content of 55.7% followed only the first type gelation. The relaxation exponent of HMW S-PVA solution at GP decreased with concentration and had a lower value than the percolation value, 0.7. The higher fractal dimension indicates the structural compactness in the cross-linking system of HMW S-PVA. As compared with the other gelation system, lower gel stiffness was obtained, suggesting that the gelation of HMW S-PVA solution occurred without a serious crystallization.

The crystal and microstructures of the novel microfibrillar poly(vinyl alcohol) fibers (PVA fibrils) having different syndiotactic diad (S-diad) contents of 56−65% have been investigated by small- and wide-angle X-ray methods. The PVA fibrils were directly prepared from the saponification and in-situ fibrillation without spinning procedure. Small-angle X-ray scattering data show that these PVA fibrils had various microvoid structures due to different stereoregularities, which was similar to those found in various natural cellulose fibers. Also, the higher the syndiotacticity was, the more regular and elongated the microvoids in PVA fibril were. This might be explained by a better alignment of PVA chains with higher syndiotacticity. To effectively orient the molecular chains of high molecular weight syndiotactic PVA fibril, a high-temperature zone drawing technique was applied. Degree of crystal orientation up to 0.991 was achieved by stretching the PVA fibril with S-diad content of 63.1% and with original degree of crystal orientation of 0.887 at around 250 °C close to its melting temperature. The maximum draw ratio of the PVA fibril increased with a decrease in the S-diad content, indicating that the crystal was more stable in higher syndiotactic PVA. When the same draw ratio was applied to the fibrils, higher crystal orientation was achieved for the fibrils having higher syndiotacticity. The crystallinity, the apparent lateral and longitudinal crystal sizes, and the crystal to amorphous length in long period of drawn PVA fibrils were larger in higher syndiotacticity. It turns out that PVA fibril drawn at 230 °C had a well-oriented lamellar structure. Moreover, dimensions of these lamellae were enlarged with increasing syndiotacticity.

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 saponified 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.