Supercritical Fluid Assisted Impregnation Directs Solid-State Organization, Additive Localization, and Release...

Supercritical CO2-assisted impregnation (SCI) offers a sustainable route for embedding functional small molecules into polymer substrates, but its capacity to direct additive localization, interfacial organization, and solid-state structure, key determinants of optical, barrier, and controlled-release performance in multifunctional polymer composites and coatings, remains insufficiently understood. Herein, we elucidate how SCI processing governs the sorption, interfacial interactions, spatial localization, and release kinetics of vanillin, a model solid-phase bioactive compound, within semicrystalline cellulose. SCI enabled vanillin loadings ranging from 0.15 to 0.60 g g-1, with addition of polar cosolvents enhancing uptake by nearly 10-fold relative to neat CO2. By adjusting cosolvent environment and batch time, SCI directs the formation of distinct additive domains: surface-confined crystalline deposits at short times, bulk-dispersed amorphous regions at intermediate times, and recrystallized nanostructured domains once local additive concentrations exceed the dispersive capacity of cellulose. FTIR spectroscopic, X-ray photoelectron spectroscopic, X-ray diffraction, calorimetric, and electron microscopic characterization reveal that these SCI-driven interfacial interactions and phase states govern cellulose composite hydration, moisture-sorption hysteresis, and UV-shielding performance, where UVB transmittance ranges from 4-6% (surface crystalline) to 13-15% (matrix-embedded crystalline). Release experiments conducted in FDA/EU food simulants show that amorphous vanillin exhibits rapid, diffusion-controlled dissolution, reaching equilibrium (∼33 μg mL-1) within 180 min, whereas crystalline vanillin dispersed throughout the matrix exhibits delayed, anomalous release, showing a 30 min lag and ∼90 μg mL-1 equilibrium concentration after ∼10,000 min. Collectively, these results establish SCI as a versatile materials engineering platform that couples supercritical-fluid processing with control over interfacial chemistry and solid-state organization, enabling the design of sustainable cellulose-based composites with programmable optical, hygroscopic, and transport properties.