Solvent-Free Interaction of Gallic Acid with Chitosan via Reactive Extrusion

In this paper, we describe mechanochemical approach as an environmentally friendly method to obtain chitosan and gallic acid derivatives. Covalent cross-linking with a polysaccharide is one way to stabilize antioxidants and improve their bioavailability. Since gallic acid is a solid compound with limited solubility and a high melting point (220-240 °C), it was of interest to conduct its interaction with chitosan using solid-state synthesis technique under shear deformations. The experimental conditions were selected using a pilot twin-screw extruder designed for processing solid dispersions. The insertion of gallate groups onto the polymeric backbones was confirmed by 1H NMR, FTIR and UV–vis analyses. Dynamometry was used for study of rheological properties. Depending on the synthesis conditions, the amount of bound gallic acid was more than 600 mg per 1 g of chitosan. It was found that gallate groups are predominantly linked to chitosan via salt bonds. Carrying out the process at a moderate temperature (100-120 °C) led to the formation of amide bonds between chitosan and gallic acid. In contrast to the physical mixing of components, the resulting products swelled well and partially dissolved in water, and tended to form aggregates with an average size of 206±36 µm in aqueous media.