Rheological Behavior of Aqueous Poly(vinyl alcohol) Solution during a Freeze–Thaw Gelation Process
Nidhi Joshi, Khushboo Suman, Yogesh M. Joshi
Abstract
In this work, we study the rheological behavior of the aqueous solution of poly(vinyl alcohol) (PVOH) having different molecular weights and concentrations, which are subjected to the freeze–thaw process. We observe that, during the freezing step, PVOH solution undergoes a sol–gel transition upon decreasing the temperature as defined by the Winter criterion, wherein at the critical point dynamic moduli show identical power-law dependence on frequency. During the thawing step, with an increase in temperature, the material undergoes the gel–sol transition but at a higher temperature than during the freezing step. Interestingly, the fractal dimension and the gel strength of the critical state while thawing are observed to be higher than those during freezing despite the temperature of the former being larger. We propose that, while gradually decreasing the temperature, the polymer segments indulge in hydrogen bonding, which forms nuclei for microcrystalline domain formation. Consequently, a point comes when the hydrogen-bonded polymer segments, where the microcrystalline domains act as junction knots, form a three-dimensional percolated network leading to the critical gel transition. Upon increasing the temperature of the consolidated PVOH gel, the segments participating in the crystalline junctions dissolve to bring about the gel–sol transition. The rheological experiments suggest that the number of PVOH segments participating in the crystalline junction points increases exponentially during the freezing step, while it decreases exponentially during the thawing step in the vicinity of the critical point.