Role of degree of saponification in the shear-induced molecular orientation of syndiotacticity-rich ultrahigh molecular weight poly(vinyl alcohol)

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.