Thermal treatment inducing the building-up of tailor-made chitosan films containing gallic acid

Abstract

The building-up of chitosan (CS) / gallic acid (GA) films was performed by casting method, thermally pre-processing the reaction mixture at several temperatures and process times. This study was focused on the thermal treatment impact over the antioxidant activity, mechanical deformation at break, water vapor diffusion and surface CS film features. It is found that the thermal energy promotes the diffusion of species through CS chains as well as the GA interaction with the polysaccharide, producing films with different structural organizations. Since the crosslinking degree depends on the energy received by the system, heterogeneous materials with a low chain crosslinking and a high free GA content are generated at 40 °C, while homogeneous films with covalently crosslinking are mostly generated from 50 °C. The free GA availability within segregated CS film regions favors the antioxidant activity, but also the polyphenol interaction with the polysaccharide decreases the crystallinity and elastic modulus, increasing the tensile strength and elongation at break. Since a threshold 516 kJ energy received regulates the transition from heterogeneous to homogeneous films can be thermally tuned. This discovery represents a controlling synthetic factor critical to the design of materials with unique characteristics such as those required for food packaging.