Rheological and TGA study of acylated chitosan gel-like dispersions in castor oil: Influence of acyl substituent and acylation protocol

This work deals with the development and characterization of new formulations based on different types of acylated chitosan, obtained using different acylation protocols and acyl chlorides, which are able to generate stable gel-like dispersions in castor oil. These dispersions can be proposed as promising candidates to replace current non-biodegradable lubricating grease formulations. The biopolymers and gel-like dispersions were characterized through Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). In addition to this, small-amplitude oscillatory shear (SAOS) measurements were performed on gel-like dispersions. The resulting oleogels demonstrated suitable thermal resistance although the inclusion of acyl segments into the polymer structure reduces thermal stabilities. The values of the linear viscoelastic functions and their evolution with frequency for gel-like dispersions of chitosans N-acylated with decanoyl chloride or N- and O-acylated with palmitoyl chloride were very similar to those exhibited for non-biodegradable commercial greases. A significant decrease in the storage and loss moduli was detected as the carbon chain length in the acyl group decreases in both N-acylated and N- and O-acylated chitosans. Moreover, both moduli decrease when the acylation reaction time increases, as a result of a major chitosan acylation degree, which provides weaker gels.