Characteristic rheological features of PVA solutions in water-containing solvents with different hydration states

Two organic solvent systems containing equimolar ratio of water but different association strenght, 8/2 by wt mixture of dimethyl sulfoxide (DMSO) and water and 86.7/13.3 by wt mixture of N-methyl morpholine N-oxide and water (NMMO monohydrate), were chosen and the rheological properties of polyvinyl alcohol (PVA) solutions in the solvents were investigated and compared. The associated state of water in the solvent systems had a significant effect on the rheological properties of the solutions. In dilute concentration regime, NMMO monohydrate gave higher intrinsic viscosity than DMSO/water although two solvents gave similar values of the Mark–Houwink exponent. More noticeable difference was observed on the viscosity curve in high concentration regime. DMSO/water exhibited near a Bingham behavior and gave much lower power-law index than NMMO monohydrate. On the logarithmic plot of storage modulus (G′) against loss modulus (G″), both solutions exhibited slight decrease in slope with increasing concentration. Of two solvent systems DMSO/water gave lower values of slope than NMMO monohydrate. In addition, NMMO monohydrate gave tan δ greater than one at low frequency whereas DMSO/water gave tan δ much less than one over the entire frequency range examined. Further, the frequency to cause gelation was decreased with increasing concentration in the case of NMMO monohydrate. This indicates that PVA/DMSO/water system is more heterogeneous because weakly bonded water molecules produce hydrogel structure.