Physical evaluation of pure zein films by atomic force microscopy and thermal mechanical analysis

Biodegradable edible films can be made from corn protein, alpha-nein. Pure zein films are cast from an organic solution of alpha-zein. This report outlines the surface conditions of such pure zein films. First, the transition temperature, T-t' of the pure zein film was measured with a thermomechanical analyzer. T-t was between 167.0 and 172.7 degrees C. The thermal elongation of the films depended on the drying conditions used during film preparation. Second, the surface microstructure of pure zein films, produced under several different drying conditions, was observed by atomic force microscopy; The surface had a morphology that showed depressions either with acutely (90-120 degrees) or obtusely (121-180 degrees) angled features, depending upon the drying conditions. On the other hand, the surface microstructure after thermal elongation analysis appeared to have a pattern of projections that was repeated every 25 nm. Third, we measured the contact angle of the pure zein films. We found a correlation between surface microstructure and contact angle. Pure zein films with projections smaller than 200 nm in base diameter on the surface had a high contact angle (>70 degrees).