Surface Modification of Cellulose Nanocrystals and Their Application as a Reinforcing Phase for Polylactic Acid Films

Cellulose nanocrystals (CNC) have the potential to serve as reinforcement for polylactic acid (PLA), which has become a popular polymer but some deficiencies still need to be overcome. The main goal of this work is to improve the mechanical properties of the PLA– especially toughness – by the addition of surface-modified CNC in the polymer matrix and thus obtaining a polymer with broader applications. The surface modification of the CNC and the phase behavior in organic media are integral components of this work. CNC were isolated via acid hydrolysis combined with an esterification process and further functionalized with surfactants including decylamine (DA) and cetyltrimethylammonium bromide (CTAB). Transmission Electron Microscopy was used to demonstrate the isolation of nanocrystals. Phase behavior of modified CNC was studied in organic solvents by visual examination and cross-polarized observations. PLA-CNC films were prepared and tested in an Instron universal machine. It was found that the stability of the surface-modified CNC with DA and CTAB improved in organic solvents. No settling was observed in these suspensions for at least three weeks and the CNC self-assembly studies confirmed the enhanced molecular interactions with the solvents. The toughness of PLA-CNC nanocomposites were improved by a two-fold increase compared to plain PLA, while the tensile strength and modulus were not significantly affected by the addition of crystals. It can be concluded that surface modification of CNC with surfactants improves the interactions of the crystals with organic media and enhances the mechanical properties of PLA upon addition of CNC.