Water Adsorption Thermodynamical Analysis and Mechanical Characterization of Chitosan and Polyvinyl Alcohol-Based Films

Chitosan and polyvinyl alcohol films were prepared by the casting method. Moisture adsorption isotherms were obtained at 15 °C, 20 °C, 25 °C and 30 °C. Experimental isotherms were well fitted using the GAB model (E < 10%). The thermodynamic analysis showed a minimum integral entropy (∆S_int) at the 0.3–0.6 range of water activity (a_w) in all films. Pore radius values of the films ranged from 1.005 nm to 20.766 nm, which correspond to the micropores and mesopores classification. Compensation enthalpy-entropy in films showed that the water vapor adsorption process was driven by enthalpy at low a_w values (0.3–0.5 for CS and 0.2–0.3 for PVA). Most of the mechanical properties showed constant values at the zone of minimum ∆S_int, confirming that thermodynamic properties could be used to predict the stability of films. The study of thermodynamic water adsorption and the mechanical properties allows understanding the preparation process of stable films with adequate parameters intended for food packaging materials.