Poly(lactic Acid)-Regulated Hydration Balances Stiffness and Energy Dissipation in Cellulose-Alginate Porous Networks

Humidity-induced plasticization limits the mechanical reliability of porous polysaccharide networks, yet how water organization influences their compressive response under humid conditions remains insufficiently resolved. Here, oxidized cellulose nanofibers derived from waste paper were assembled with sodium alginate into a calcium-cross-linked porous scaffold and subsequently impregnated with poly(lactic acid) (PLA). PLA incorporation produced an apparent water contact angle of 107.3° and reduced moisture uptake during 24 h exposure to humid air. Following conditioning at approximately 30% relative humidity (RH) for 24 h, compressive stiffness and volumetric energy absorption increased with PLA impregnation concentration. SCFP-12.5 exhibited the highest values, whereas SCFP-10 achieved a volumetric energy absorption of 465.32 ± 35.00 kJ m–3 at 50% strain. Relative to the dried state, conditioning at approximately 30% RH increased the specific energy absorption of all three formulations, whereas further increases to approximately 58% and 97% RH reduced energy absorption. However, the difference between SCFP-10 and SCFP-12.5 narrowed at 97% RH. Given their similar 24 h moisture uptake, SCFP-10 was selected because it combined lower density and thermal conductivity with good mechanical performance. Qualitative molecular dynamics simulations indicated preferential hydration of alginate-rich regions, changes in water domain organization after PLA incorporation, and an overall tendency toward reduced long-range water mobility. Taken together, the experimental observations and simulation trends are consistent with a proposed PLA-mediated hydration regulation mechanism in which structural reinforcement and changes in water organization jointly influence the mechanical response under humid conditions. These findings provide a framework for regulating the relationship among structure, hydration, and mechanics in renewable porous cushioning materials.