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. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1426–1431, 2004