Effect of degree of saponification on the rheological properties of syndiotactic poly(vinyl alcohol)/water solution

To identify the effect of degree of saponification (DS) of syndiotactic poly(vinyl alcohol)s (s-PVA)s having similar tacticity and molecular weight on the rheological properties of s-PVA/water solution, four kinds of (s-PVA)s with assigned (DS)s, from 93.1 to 97.5%, were prepared by copolymerization of vinyl pivalate (VPi) and vinyl acetate (VAc), followed by saponifying the corresponding copoly(VPi/VAc). The DS played a significant role in rheological behavior. Over the frequency range of 10^-1 to 10² rad/s s-PVA with higher DS shows more shear thinning at similar molecular weight and tacticity of polymer, suggesting that PVA molecules are more readily oriented as DS increases. This may provide indirect evidence of the spontaneous in situ orientation of s-PVA molecules at the late stage of saponification. Yield stress is higher for s-PVA with higher DS at similar molecular weight and tacticity of s-PVA. This indicates that more domains with internal order are produced at higher saponification. These facts result from increase in stiffness of s-PVA molecules with proceeding the saponification reaction.