Halloysite nanotubes sandwiched between chitosan layers: novel bionanocomposites with multilayer structures

This work is a contribution to the design of multilayer biocomposites based on halloysite nanotubes (HNTs) and chitosan. Both the polymer and nanotubular inorganic additive have been selected among easily available green materials. An innovative preparation procedure based on the sequential casting of chitosan and HNTs has been proposed in order to obtain multilayer composite biofilms. A physicochemical investigation (contact angle measurements, differential scanning calorimetry, thermogravimetry) has been conducted to characterize the bionanocomposites. As evidenced by scanning electron microscopy, the nanocomposites possess an intermediate halloysite layer between the chitosan ones. The multilayer morphology of the prepared chitosan/HNT nanocomposites has been confirmed by water contact angle measurements, which revealed that the hybrid films present a hydrophobic surface. The peculiar sandwich-like morphology of the chitosan/HNT hybrid materials has been correlated to their thermal behavior under inert and oxidative atmospheres. The kinetic aspects of chitosan degradation have been studied by a non-isothermal thermogravimetric approach (Friedman's method), while the suitability of HNTs as flame retardant fillers of multilayer nanocomposites has been estimated by the thermodynamic parameters of oxidative degradation. According to the thermogravimetric data, the formation of a well-compacted middle layer of HNTs has induced a reliable decrease in the activation energy of the degradation of chitosan. Differential scanning calorimetry experiments showed that the nanocomposites possess an enhanced ignition temperature compared with pure chitosan. This paper opens new sustainable prospects for the preparation of novel nanocomposites with layered structures that can be strategic for packaging, tissue engineering, and pharmaceutical applications.