Hydrogen bonding impact on chitosan plasticization.

The use of chitosan natural polymers to replace synthetic polymers is of interest as part of the green materials movement. However, a major challenge of using chitosan material is its rigid and brittle nature associated primarily with its intra- and inter-molecular hydrogen bonds creating a hydrogen bond network. Plasticizers are able to make chitosan flexible, hypothetically by destroying its hydrogen bond networks. Herein, we showed the importance and complicated nature of the chitosan's hydrogen bond network with respect to its flexibility, through a comparative study of glycerol and ionic liquids plasticizers. The results demonstrated that glycerol's hydrogen bonding was able to disrupt the chitosan's hydrogen bond network resulting in a flexible film, but ionic liquids, even with their very strong hydrogen bonding, were not able to plasticize chitosan. This result suggested that the plasticization phenomenon was more complicated than hydrogen bond disruption. A molecular level study by quantum chemistry calculation showed that the efficiency of glycerol as chitosan plasticizer was due to its single hydrogen bonding site, which breaks down the chitosan hydrogen bonding networks, and leave hydrophobic C-H ending groups to limit the formation of inter molecular hydrogen bonds.