Modulating layer-by-layer assembled sodium alginate-chitosan film properties through incorporation of cellulose nanocrystals with different surface charge densities

In this work, 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose nanocrystals (TOCNs) were loaded into sodium alginate/chitosan multilayer film as nanofillers to investigate the modulation of the surface charge density of TOCNs on the film properties. First, the surface charge density of TOCNs was controlled by adjusting the carboxyl content and morphological size by varying the oxidant dosage. After oxidation, TOCN with higher surface charge density was observed to display a higher crystallinity, a more open internal structure, a better dispersibility and a slightly weaker thermal stability. In addition, a 15-layer film composed of sodium alginate and chitosan, called (SA/CH)15, was constructed by layer-by-layer assembly. Both in situ deposition monitoring and free-standing multilayer film formation indicated that TOCNs relied on strong electrostatic interactions and hydrogen bonding to achieve a compact and uniform interlayer and a thinner thickness of (SA/CH)15, which was more evident at a high surface charge density. The addition of TOCNs also enhanced the mechanical properties, thermal stability, hydrophobicity, and barrier properties of (SA/CH)15. In particular, the resulting sodium alginate/chitosan multilayer film exhibited an improved packaging performance when nanocomposite was performed using TOCN with a surface charge density of 3.22 ± 0.11 e nm−2.