Behaviors of water molecules in polyvinyl alcohol gel amid stretch and temperature changes: A molecular dynamics study

Mechanical performance of polyvinyl alcohol (PVA) gel features temperature dependence, which is closely related to the behaviors of water molecules at the microscale. In this study, we use molecular dynamics simulation to study the effect of temperature on hydrogen bonding and tensile properties of PVA gel. By analyzing the states of water molecules in the polymer network, we reveal the mechanism behind the temperature dependence of tensile properties of PVA gel. Based on a method that classifies water into different states, we find that there are 3 states of water in PVA gel, including non-freezing water, freezing water, and free water. It is shown that the conversion of water molecules from bonded to free states is a critical behavior for the decline of tensile properties of PVA gel at elevated temperatures. Furthermore, we investigate the changes of hydrogen bond contents in PVA gel at different temperatures. The analyses show that increasing temperature decreases the content of hydrogen bonds between solvent water molecules and polymer chains, which destabilizes the cross-linking points and weakens the integrity of the gel, thereby degrading the mechanical properties of PVA gel. The mechanism behind the temperature dependence is revealed in this work.