Rheological behaviors of PVA/H₂O solutions of high-polymer concentration

The dynamic viscoelastic properties of poly(vinyl alcohol) (PVA)/H ₂O solutions with concentrations of 10 to 25 wt % have been characterized by controlled-stress rheometry at 30°C. Parameters relating to the linear and nonlinear viscoelasticities include complex viscosity (η^*), storage modulus (G′), loss tangent (tan δ), relaxation time (λ), thixotropy, and creep. Change curves of η^*, G′, tanδ, and λ with frequency (ω) have been obtained for the PVA/H₂O solutions. Creep and recovery testing yielded compliance (J′) curves with loading and unloading. Shear stress versus rate profiles of the PVA solutions have been obtained through thixotropic measurements. The PVA concentration has been found to have a profound influence on the rheological properties of the aqueous solutions. Four aqueous solutions of 10, 15, 20, and 25 wt % PVA at 30°C exhibited shear-thinning and showed different transition behaviors of η^* and G′ with frequency, and different degrees of creep under constant stress to recovery with time. The 10 wt % PVA solution was viscous and displayed the lowest η^* and G′; the 25 wt % PVA solution was visco-elastic and displayed the highest η ^* and G′; the 15 and 20 wt % PVA solutions showed η^* and G′ values and creep to recovery behaviors intermediate between those of the 10 wt % and 25 wt % PVA solutions. The different rheologi-cal properties of these PVA/H₂O solutions are considered to correlate with interchain hydrogen bonds and shear-induced orientation in the solutions. Shearing is able to break the intrachain and interchain hydrogen bonds, and, at the same time, the orientation creates new interchain hydrogen bonding. The reorganization of hydrogen-bonding mode contributes to the transitions of the macroscopic viscoelasticity with frequency.