Tailoring the Surface Modification of Cellulose Nanofibrils for Nanocomposite Applications

Located in the plant cell wall as the structural component responsible for the mechanical properties, cellulose is a unique raw material, highly attractive when targeting a variety of different applications. The use of cellulose-based materials for the production of a broad range of products such as paper, cardboard, construction materials and textiles dates back thousands of years. However, the isolation of nanoscopic forms of cellulose during the last decades has paved the way to a new spectrum of possible applications in the field of composite materials of industrial interest. Within the family of nanomaterials derived from cellulose, cellulose nanofibrils (CNFs) are distinguished by a very high aspect ratio. Their typical dimensions are: width in the range of 3–100 nanometers, length as high as 1 micrometer. Made of alternating crystalline and amorphous domains, CNFs are isolated from fibers by mechanical treatments generally preceded by chemical pre-treatments of the surface. These are often aimed at introducing a surface charge, which ensures a stable dispersion in aqueous media thanks to electrostatic repulsion.<br/><br/>Within the hierarchical structure of plant tissues, elementary fibrils of cellulose are organized in bundles embedded in an amorphous matrix (mainly consisting of lignin and hemicelluloses), giving rise to an extraordinary composite material. Thanks to this complex structure, natural fibres display impressive strength combined with high performance in terms of flexibility. Nanoscopic cellulosic materials show even superior properties due to a higher structural homogeneity, higher extent of crystallinity and extremely large specific surface area. These features make them promising candidates as low density reinforcing additives for composite materials. CNFs isolated from lignocellulosic biomass have a longitudinal Young’s modulus close to 100 GPa, as determined by both theoretical and experimental methods. Yet in order to exploit their mechanical properties in composite materials, further investigation of surface modification pathways leading to a better fibril/matrix interaction is needed, resulting in the optimization of the fibrils dispersion in the matrix. Specific challenges must be addressed when incorporating nanocelluloses in hydrophobic polymer matrices: cellulosic nanomaterials are hydrophilic and have a strong tendency for self-aggregation driven by surface hydrogen-bond interactions. Surface modification is therefore essential to improve the material’s compatibility with common polymer matrices.<br/><br/>In the current study, we aim to explore the potential of surface-modified CNFs as reinforcing additives. Nanocomposite materials containing a low amount of CNFs are obtained by melt processing as well as by curing coating formulations. To ensure compatibilization, different routes for chemical modification of the CNF surface are used, tuning the surface properties depending on the specific application. Avoiding covalent modification and favouring adsorption of compatibilizers in aqueous media, tailor-made functional block copolymers synthesized by controlled radical polymerization, as well as small hydrophobic molecules, are adsorbed onto water dispersed CNFs. Finally, the composite materials obtained are characterized with a focus on the mechanical properties, highly dependent on the effectiveness of the nanofibrils dispersion in the matrix.