Effect of stereoregularity on the properties of syndiotactic-rich ultrahigh molecular weight poly(vinyl alcohol)/dimethyl sulfoxide/ water gel

Syndiotactic-rich ultrahigh molecular weight poly(vinyl alcohol) (UHMW S-PVA) gels having syndiotactic diad (S-diad) contents of 61.5, 58.2, and 55.7% and similar molecular weights were prepared in dimethyl sulfoxide (DMSO)/water solution. The gelation temperature of UHMW S-PVA/DMSO/water solution decreases with an increase in syndiotacticity. A great difference in the gel melting temperature (T_gm) was not found between the gels having an S-diad content over 58.2%. However, a significant decrease of T_gm was revealed in the gel having an S-diad content of 55.7%. The enthalpy of junction fusion and the number of segments participating in the junction of the UHMW S-PVA gel increases with syndiotacticity, and they are remarkably higher than the values of atactic PVAs reported in previous papers. The higher the syndiotacticity of the UHMW S-PVA gel, the higher is the dynamic storage modulus. The elasticity of the PVA gel with an S-diad content over 58.2% was recovered under cooling after heating, whereas almost no recovery was found in the gel having an S-diad content of 55.7%. The PVA gel having an S-diad content over 58.2% is opaque and exhibits a phase separated appearance in optical morphology, while the PVA gel with an S-diad content of 55.7% is almost transparent and phase separation does not occur. A highly microporous and close structure was observed for the surface of the UHMW S-PVA gels.