Assessment of surface accessibility of nanocellulosic structures using surface sensitive methods

Due to the high surface area and high surface energy of nanoscaled cellulosic materials, high reactivity is the rule rather than an exception as it is with all nanomaterials. Here the aim is to illuminate how the surface properties and surface accessibility of nanocellulosic materials are changed due to the surface passivation and/or flocculation subsequent to solvent exchanges, drying in air or after chemical modifications. Tools to obtain deeper understanding on the surface behaviour of cellulosic nanofibrils (CNF) with respect to formation of functional structures using a set of surface sensitive methods will be presented. Methods include X-Ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM), Contact Angle analysis and Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) equipped with the humidity module. With XPS indirect information on CNF surface reactivity can be achieved by following the surface composition, especially the relative abundance of non-cellulosic carbon-carbon bonds. When combined with contact angle measurements, direct information on wetting behaviour can be revealed which can be linked to available surface OH groups, i.e. surface reactivity of CNF. AFM topography and phase contrast images reveal structural information on modified fibrils simultaneously with an indication of evenness and quality of the surface modification. The overall goal of this work is to overcome the challenges related to nanocellulose modifications, i.e. how the strong self association tendency via hydrogen bonding within nanofibrils can be controlled and how chemicals can effectively reach the fibril surface to ensure the sufficient degree of modification without deteriorating the nanoscale fine structure and with the optimal consumption of chemicals.