EVALUATION OF THE STABILITY OF CELLULOSE NANOCRYSTALS MODIFIED WITH POLYDOPAMINE

Renewable and environmentally friendly bio-based materials derived from natural resources have gained much attention especially in recent years due to their great potential as an alternative to petrochemical-based materials. Cellulose nanocrystals (CNC), which are nanofibers of plant origin, have been extensively explored as reinforcing nanofillers in polymer matrices. However, most of the interaction at the interface between the CNC and the matrix are hydrogen bonds, which belong to weak intermolecular forces. Inspired by the adhesive protein secreted by the marine mussel, dopamine, a biomolecule containing catechol and amine functional groups, can self-polymerize, polydopamine (PDA), at alkaline pH values and spontaneously deposit on virtually any surface. Thus, this work aimed to modify the surface of CNCs with PDA and evaluate the stability of suspensions of nanostructures for possible application in polymeric matrix. Under pH 8.5 and aerobic conditions, the catechol group of dopamine was first oxidized to quinone, followed by further oxidative polymerization via deprotonation and intermolecular Michael addition reactions to form a cross-linked homopolymer PDA, which grafted onto the surface of CNCs, resulting in modified CNCs (CNC-PDA). After PDA modification, the initially whitish colored suspensions of the CNCs turned black, indicating successful PDA grafting. The hydrodynamic diameter of the CNC was initially 108.7 ± 5.1 nm and after the modification was to 142.1 ± 3.6 nm. Importantly, the hydrodynamic diameter refers to the size of a sphere that moves in the same way as the scatterer (in this case, the nanocrystals), and is dependent on the ionic strength of the scattering medium and the surface structure of the nanoparticle. Higher hydrodynamic diameter results in a low ionic concentration, where the ion layer extended around the particle, decreases the diffusion velocity, while high ionic concentration the ion layer will be compressed and the same particle will present a smaller hydrodynamic diameter. The zeta potential of the CNCs changed from -52.3 ± 0.8 mV to -43.8 ± 1.0 mV after PDA modification. The repulsive forces between the CNCs were minimized, as it obtained suspensions with lower zeta potential in absolute value, indicating a lower stability of the colloidal suspension after modification with PDA. This occurred due to the hydrogen bridges on the surface of the CNCs being weakened, thus facilitating aggregation. Although aggregate formation can negatively affect the reinforcing ability of nanofibers in the polymer matrix, the grafting of PDA on the surface of CNCs predicts an improvement in the compatibility of CNCs and the matrix, acting as a bridge to improve interfacial adhesion, thus improving the properties of the composite. In addition, the application of PDA could provide UV protection properties in composites, increasing product cycle time and has the potential to be used in photosensitive food packaging.