Water-State Reorganization as the Molecular Origin of the Thermo-Mechanical Anomaly in Hydrated Polymer Networks

Abstract

Hydrated polymer networks are widely used across diverse environmental conditions, including varying humidity and temperature ranges, for numerous applications. The three distinct types of water present within hydrated polymer networks, namely tightly bound water, loosely bound water, and free water, play a critical role in determining their mechanical properties under different environmental conditions. This work provides mechanistic insights into the evolution of thermo-mechanical properties associated with the redistribution of these water states within a hydrated polymer network. Under both multiaxial loading, applied through nanoindentation, and uniaxial loading, applied through molecular simulations, the hydrated chitosan polymer network exhibits a nonlinear increase in elastic modulus with temperature, followed by a subsequent decrease. Molecular simulations reveal that the thermal energy absorbed with increasing temperature overcomes both water–water interactions and water–polymer binding energies, leading to the redistribution of different water states within the polymer network. The formation and breaking of hydrogen bonds under increasing temperature and internal mechanical stress significantly influence the structural characteristics of the polymer chains across different length scales, which in turn affect the thermo-mechanical behavior of the hydrated polymer network. Molecular dynamics simulations are employed in this work to analyze how temperature influences the redistribution of thermodynamically distinct water states, along with the associated conformational changes in the polymer network.