Surface Modification of Nanocellulose towards Composite Applications

The desire to develop high-value end-products derived from renewable resources is continuously growing as a result of environmental awareness and the depletion of fossil resources. In this context, nanocelluloses have gained great interest during recent decades owing to their renewability, abundancy and remarkable physical and mechanical properties. The aim of the present work was to investigate new strategies for surface modification and functionalization of nanocelluloses and their subsequent incorporation in polymer-host matrices. Nanocomposites of cellulose nanofibrils (CNF) and polycaprolactone (PCL) were produced by employing CNF nanopaper (NP) as a template and surface-initiated ring-opening polymerization (SI-ROP) of ε-caprolactone (ε-CL). SI-ROP of ε-CL from filter paper (FP), which has a low surface area compared with NP, was also carried out for comparison. A larger amount of PCL was grafted from NP than from FP as a result of more available hydroxyl groups. The grafted NP had stronger mechanical properties than a neat PCL film. Cellulose nanocrystal (CNC)-reinforced polyvinyl acetate (PVAc) nanocomposites were also investigated. CNC were modified via “SI-reversible additionfragmentation chain transfer and macromolecular design via the interchange of xanthate” (SI-RAFT/MADIX) polymerization of vinyl acetate (VAc). The PVAcgrafted CNC and pristine CNC were incorporated into a PVAc matrix via solvent casting. The resulting nanocomposites exhibited improved mechanical performance than the unmodified CNC due to the greater compatibility between the nanoreinforcing-agent and the matrix. It is generally agreed that covalent grafting is superior to physical adsorption for the modification of a reinforcing agent. However, this hypothesis has never been thoroughly investigated. In the present work, CNC was modified either through covalent grafting or through physical adsorption of poly(butyl methacrylate) (PBMA). The two surface modification approaches were compared by incorporating the modified CNC in a PCL matrix via extrusion. Both methods resulted in improved mechanical performance than that of pure PCL or PCL containing unmodified CNC. However, covalent grafting gave the best mechanical performance even at high relative humidity. Functionalized CNC (F-CNC) were obtained through a versatile methodology employing organic acids bearing a functional group: double bond, triple bond, atom transfer radical polymerization (ATRP) initiator and thiol were employed for the simultaneous acid hydrolysis and esterification of cellulose fibers. This provided a facile route for the preparation of F-CNC.