High oxygen barrier chitosan films neutralized by alkaline nanoparticles

Abstract

The most frequent neutralisation procedure, applied on chitosan (CS) films includes treatment with NaOH base. Such treatment endows CS films with stability in water, yet, same can significantly decrease the film performance. In the present paper, we investigate Mg(OH)2 nanoparticles as a neutralisation agent for CS solutions followed by casting into films. This is combined and compared with classical casting and film drying from non-neutralized solutions followed by NaOH treatment after film formation. The influence on the properties of resulting films is investigated in detail and large differences are found for structure and barrier properties. The stable, opaque-to-transparent CS films (depending on Mg(OH)2 content and post-treatment) were obtained by facile casting method of neat CS or CS–Mg(OH)2 dispersions, in the complete absence of cross-linkers and plasticizers. FTIR data demonstrate the Mg(OH)2 and NaOH deprotonation effect, and strongly suggest intensive H-bonding interaction between CS and Mg(OH)2. X-ray photoelectron spectroscopy showed differences in the hydroxide content and protonation of CS nitrogen. The reduction of surface roughness and increase of homogeneity, the tensile strength and elongation, as well as thermal stability and excellent oxygen barrier properties were measured for CS enclosing the Mg(OH)2 nanoparticles. Further treatment with 1 M NaOH causes re-packing of CS polymer chains, improving the crystallinity and water vapour barrier properties, degrading the mechanical properties by increasing the films brittleness and increasing the char formation due to reduced thermal stability. Graphic abstract: [Figure not available: see fulltext.]