Nonlinear Relaxation of PVA/Borax Solution under Step Strain

Aqueous solutions of poly(vinyl alcohol) (PVA) and Borax exhibit significant viscoelasticity because of association of PVA chains. In unentangled PVA solutions with a high Borax concentration C_B, the chains formed a percolated transient network bridged by Borax ions thereby exhibiting sticky Rouse-type LVE relaxation much slower than the relaxation of the neat PVA solution. This high-C_B solution showed nonlinear damping under large step strain γ, and its nonlinear relaxation modulus G(t,γ) was time-strain separable at long time (t). The γ dependence of its damping function h(γ) = G(t,γ)/G(t,0) was stronger than that of the unentangled neat solution but weaker than the Doi-Edwards-type γ dependence commonly observed for entangled polymers. This nonlinear damping of the high-C_B PVA/Borax solution was analyzed on the basis of the sticky Rouse model wherein the dynamic bridges between chains are treated as beads with a high friction. It appeared that the Rouse-type chain dynamics itself is not affected by the strain but the initial chain conformation just after imposition of the strain changes through the strain-induced disruption of the bridges followed by rapid and (almost) isotropic reformation. In fact, h(γ) calculated from the model with this initial conformation was in good agreement with the h(γ) data, suggesting a simple but unexplored nonlinear damping mechanism of the PVA/Borax solution.