Thermodynamics of Interfacial Water Confinement Reveal Altered Mechanical Properties in Polysaccharides Films

In this work, we introduce a water-driven thermodynamic mechanism, herein referred to as hydro-softening, rendering polysaccharide films soft and stretchable during thin-film processing. Distinct from conventional water-mediated plasticization or swelling, we show that substrate effects during processing can reorganize hydration clusters on the polymer chain, generating disjoining pressure and penalizing configurational entropy, with mechanical softening thereby arising as a consequence. Chemical, thermal, and mechanical characterization corroborates the presence of confined water molecules as the primary driver of hydro-softening. Notably, we observed distinct cold crystallization of confined water under hydro-softened conditions. We also found greater mass loss at temperatures above 100 °C─the boiling point of unbound water─coupled with pronounced decreases in both elastic modulus and hardness under hydro-softened conditions. Furthermore, a physically informed simulation is introduced to structurally model the emergent confined water, qualitatively revealing how steric confinement alters the hydration landscape, leading to anisotropic hydration clustering. Although water-induced softening has long been observed in polymers, the absence of mechanistic insight has constrained the development of functional materials. The thermodynamic framework established here addresses this limitation, enabling a more deliberate and extensible application of these systems.