Establishing whether the structural feature controlling the mechanical properties of starch films is molecular or crystalline

The effects of molecular and crystalline structures on the tensile mechanical properties of thermoplas-tic starch (TPS) films from waxy, normal, and high-amylose maize were investigated. Starch structuralvariations were obtained through extrusion and hydrothermal treatment (HTT). The molecular and crys-talline structures were characterized using size-exclusion chromatography and X-ray diffractometry,respectively. TPS from high-amylose maize showed higher elongation at break and tensile strength thanthose from normal maize and waxy maize starches when processed with 40% plasticizer. Within thesame amylose content, the mechanical properties were not affected by amylopectin molecular size orthe crystallinity of TPS prior to HTT. This lack of correlation between the molecular size, crystallinity andmechanical properties may be due to the dominant effect of the plasticizer on the mechanical properties.Further crystallization of normal maize TPS by HTT increased the tensile strength and Youngs modulus,while decreasing the elongation at break. The results suggest that the crystallinity from the remainingungelatinized starch granules has less significant effect on the mechanical properties than that resultingfrom starch recrystallization, possibly due to a stronger network from leached-out amylose surroundingthe remaining starch granules.