Influence of chitosan protonation degree in nanofibrillated cellulose/ chitosan composite films and their morphological, mechanical, and surface properties

Composite films of nanofibrillated cellulose (NFC) and chitosan (CS) were prepared by spray deposition method, and the influence of polymers ratio and protonation degree (alpha) of chitosan was evaluated. Films were characterized using morphological, mechanical, and surface techniques. Higher NFC content increased Young's modulus of film composites and reduced air permeability, while higher CS content increased water contact angle. Variations in the degree of protonation of chitosan from non-protonated (alpha = 0) to fully protonated (alpha = 1) in the NFC/CS composite film with a fixed composition allowed to modulate surface, mechanical, and structural properties, such as water contact angle (31.3-109.2 degrees), Young's modulus (1.7-5.3 GPa), elongation at break (3.1-1.2 %), oxygen transmission rate (9.0-5.5 cm3/m2day) and air permeability (2074-426 s). Highly protonated chitosan composite films showed similar contact angles to pure chitosan films, while low protonated chitosan composite films presented contact angles similar to pure NFC films, suggesting a possible coating effect of NFC by CS through electrostatic interactions, evidenced by microscopy and spectroscopy analysis. By mixing both polymers and adjusting composition and protonation degree it was possible to enhance their properties, making pH adjustment a useful tool for NFC/CS composite films formation.