Rheological properties of water solutions of syndiotactic poly(vinyl alcohol) of different molecular weights

To precisely identify the effect of the molecular weight of syndiotacticity-rich poly(vinyl alcohol) (s-PVA) on the rheological properties of s-PVA/water solutions, we prepared four s-PVAs with a syndiotactic dyad content of 57%, a degree of saponification (DS) of 99.9%, and number-average degrees of polymerization (P_n′s) of 300, 1300, 2700, and 4000. Through a series of experiments, we found that the molecular weight of poly(vinyl alcohol) had a significant influence on the rheological properties of s-PVA/water solutions. Over a frequency range of 10^-1 to 10² rad/s, the s-PVA/water solution with the highest P_n value showed the largest values of the complex viscosity, storage modulus, and loss modulus at similar syndiotacticity and DS values of s-PVA, and this suggested that the higher P_n was, the stronger the internal ordered structure was in the molecules. All the s-PVA/water solutions showed shear-thinning behavior, which implied heterogeneity. In a modified Casson plot, Bingham flow behaviors, which gave rise to non-zero yield stress, were evident. This suggested that some pseudostructure existed in the s-PVA/water solutions. The yield stress increased with P_n, and this implied that the pseudostructure was developed as P_n increased.