Importance of Primary and Secondary Hydrogen Bonding Interactions of Polyols on the Plasticization of Chitosan

Molecular-level interactions between glucosamine and glycerol have shed light on the specific binding motif required for the plasticization of chitosan with glycerol via the gel theory mechanism. Here, we describe a spectroscopic study of the intermolecular interactions between the monomeric repeat unit of chitosan, glucosamine, and simplified 1,2- and 1,3-diol units of glycerol (i.e, 1,3-propanediol and ethylene glycol). The material properties of chitosan films containing these diols at varying concentrations were characterized using ATR-IR, DMA, TGA, and SEM. The combined results indicate that these diols plasticize chitosan via the lubricity theory mechanism, which differs from glycerol that plasticizes via the gel theory mechanism. At low concentrations, this difference in mechanism has a minimal impact on the material properties. However, at high concentrations of the diols, the necessity of a secondary hydrogen bonding interaction for the retention of chitosan plasticization is observed with a significant increase in the Young's modulus of the materials. The impact of hydrophobicity within the diols was also investigated in chitosan films using 1,2-propanediol and 2-methyl-1,3-propanediol. The combined analyses provide strong evidence that both primary and secondary interactions are responsible for determining the mechanism of chitosan plasticization.