Cellulose Nanofibril Aerogels: Synergistic Improvement of Hydrophobicity, Strength, and Thermal Stability via Cross-Linking with Diisocyanate.

A facile gelation cross-linking approach was devised to fabricate meso- and macroporous cellulose nanofibril (CNF) aerogels with multiple improved properties. CNF hydrogels made using a freezing-thawing method with a 94 kPa modulus were solvent exchanged with acetone and then cross-linked with methylene diphenyl diisocyanate (MDI) to produce aerogels with significantly improved compressive properties that follow a power law increment against aerogel density with impressive 1.69, 2.49, and 1.43 scaling factors for Young's modulus, yield stress, and ultimate stress, respectively. The optimally cross-linked aerogels had nearly tripled specific surface area (228 m/g) and doubled pore volume (1 m/g) from numerous new 9-12 nm wide mesopores as well as significantly improved thermal stability (43% char residue at 500 C vs 9.1% for un-cross-linked aerogel). Cross-linking also made the amphiphilic CNF aerogel highly hydrophobic and capable of completely separating chloroform from water via simple filtration. These nanocellulose aerogels show great promise for efficient and continuous separation of oils and hydrophobic liquids from water.