Using nanocellulose to strengthen and toughen collagen-based film: Effect of carboxymethyl content of nanocellulose and relative humidity.

Collagen-based films possess numerous merits due to their biodegradability, edibility, and widespread existence. Although extensive research focuses on the mechanical strengthening of collagen films, the mechanisms underlying conformational changes of collagen during the film-forming stage and the impact of interface alterations on film's mechanical properties remain ill-defined. This work investigated the dynamic drying process of different-sized collagen fibers. It revealed that smaller size of collagen fiber exhibited a shorter gelatin stage and more rapid conformational transition. Subsequently, the effects of substitution degree (SD) of carboxymethylated nanocellulose (CNF) and relative humidity (RH) were analyzed on the mechanical behaviors of collagen-based film. When RH was in the range of 50 %-90 %, increasing RH and SD of CNF gradually weakens the interfacial strength between CNF and matrix, thereby increasing the toughness and decreasing strength of collagen-based film. The highest strength (110.76 ± 6.60 MPa) was achieved in COL/CNF-C2 film. Combined with water content and microstructure results, the transformation from brittle to ductile fracture could be observed in collagen-based films, ascribing the toughening of water molecules and hydration of CN with water. These results can provide a guidance for the actual production of collagen-based film and offer strategies for the adjustable mechanical properties of biopolymer films.