Real-time tracking of physical changes and optical anisotropy during drying of aqueous chitosan solution: Modeling of drying

Abstract

The drying behavior of blade casting aqueous chitosan solution was investigated by means of a custom designed real time characterization system that can track the thickness, weight, temperature and in-plane and out-of-plane birefringence. Beyond a critical solvent concentration, initially isotropic cast solution rapidly develops optical anisotropy as detected by out of plane birefringence measurement while it remains isotropic in plane. Increasing drying temperature and air flow speed can both accelerate the drying rate but change the birefringence level in opposite directions. Lower birefringence was obtained at higher drying temperature while, higher birefringence was observed when the air flow speed is increased. This is due to the cumulative effects of the residual stress development in the film and polymer chains relaxation. Using a thermodynamically consistent model developed in the framework of classical irreversible thermodynamics, we quantitatively describe the drying kinetics and evolution of concentration and temperature profiles during the course of drying of the solvent. It was shown that the model can precisely describe the experimental data without using any fitting parameters.