Phase behavior and physical gelation of high molecular weight syndiotactic poly(vinyl alcohol) solution

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 (T-gel). 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.