Transparent, flexible, and strong 2,3-dialdehyde cellulose films with high oxygen barrier properties

2,3-Dialdehyde cellulose (DAC) of a high degree of oxidation (92% relative to AGU units) prepared by oxidation of microcrystalline cellulose with sodium periodate (48 degrees C, 19 h) is soluble in hot water. Solution casting, slow air drying, hot pressing, and reinforcement by cellulose nanocrystals afforded films (similar to 100 mu m thickness) that feature intriguing properties: they have very smooth surfaces (SEM), are highly flexible, and have good light transmittance for both the visible and near-infrared range (89-91%), high tensile strength (81-122 MPa), and modulus of elasticity (3.4-4.0 GPa) depending on hydration state and respective water content. The extraordinarily low oxygen permeation ofPeer reviewe

Conformal Ultrathin Coating by scCO₂‑Mediated PMMA Deposition: A Facile Approach To Add Moisture Resistance to Lightweight Ordered Nanocellulose Aerogels

A facile approach for adding moisture resistance to transparent nematic aerogels composed of individualized cellulose nanofibers (<i>i</i>-CNF) at full preservation of the anisotropic aerogel structure is presented. Sequential nitroxide-mediated oxidation and mechanical cellulose fiber delamination were applied to obtain <i>i</i>-CNF dispersions in water. Nematic ordering caused by repulsive forces between <i>i</i>-CNF surface carboxylate groups was set by acid-induced hydrogen bonding and gelation, respectively. Solvent exchange to acetone, impregnation with the PMMA, scCO₂-mediated antisolvent precipitation of the secondary polymer, and scCO₂ extraction of interstitial acetone afforded highly hydrophobic nanocomposite aerogels. Birefringence studies utilizing polarized light and the Michel–Levy Chart to evaluate interference patterns revealed that nematic <i>i</i>-CNF ordering is virtually not affected by the respective surface modification and scCO₂ drying steps. Morphological studies provide evidence that the large internal surface (>500 m² g^–1) of the hybrid aerogels consists of a homogeneous ultrathin PMMA (mono)­layer that virtually does not affect the high porosity (≥99%) and transparency (>77% transmission, 600 nm, 2.13 mm thickness) inherent to <i>i</i>-CNF aerogels. The obtained materials exhibited excellent resistance toward moisture as apparent from the high water contact angle (119.4° ± 7.5). As PMMA imparts the aerogel stiffness and hydrophobicity, aggregation of nanofibrils in moist environment or under vacuum conditions can be avoided even at ultralow densities as low as 9.6 mg cm^–3