Effect of orientation on the physical properties of potato amylose and high-amylose corn starch films.

The effect of orientation on the properties of amylose and starch films was studied in order to determine if film strength, flexibility, and water resistance could be improved. Potato amylose and high (70%) amylose corn starch were peracetylated, cast into films, stretched in hot glycerol 1-6 times the original length, and deacetylated. Molecular orientation of potato amylose films was much higher than for high-amylose corn starch films as determined by optical birefringence. For potato amylose films, orientation resulted in large increases in tensile strength and elongation but little change in modulus. For high-amylose corn starch films, tensile strength and modulus did not change with draw ratio but elongation to break increased from about 8% to 27% as draw ratio increased from 1 to 5. Scanning electron micrographs revealed many small crazes in the drawn starch films, suggesting that the improved film toughness was due to energy dissipation during deformation of the crazes. Annealing of drawn films at 100% humidity resulted in partial crystallization and improved wet strength.