Organization of aliphatic chains grafted on nanofibrillated cellulose and influence on final properties

Chemical surface modification of nanofibrillated cellulose (NFC) was performed using a long aliphatic isocyanate chain. Different molar ratios of the coupling agents were tested, i.e., 1, 10, 30 equivalents with respect to hydroxyl groups of the NFC surface. FE-SEM analyses revealed that there were no changes in their morphology thus keeping nanofibril-like structure with about 30 nm of diameter. All these samples were characterized by different techniques (e.g., FTIR) to check the efficiency of the grafting. Hydrophobic NFC were achieved whatever the grafting agent ratio. The Degree of Substitution was determined by Elemental Analyses and the Degree of Substitution of the Surface was calculated thanks to X-ray Photoelectron Spectroscopy data. Combining these two techniques, the Internal Degree of Substitution was proposed for the first time. It indicates if the modification occurs also within NFC internal layers. Surface (contact angle), rheological (water suspension viscosity) and thermal properties (ThermoGravimetric Analysis) of grafted NFC do not follow the expected linear evolution of properties with the increase of molar ratio. X-Ray Diffraction analyses showed that the grafted aliphatic chains display crystalline waxy domains at some ratios. A model for aliphatic chain organization at the surface is proposed and clearly explained for the first time why a compromise in molar ratio is necessary to achieve best properties.