Microstructure and mechanical properties of orientated thermoplastic starches

Abstract

Effect of orientation on microstructure and mechanical properties of thermoplastic starches with different amylose/amylopectin ratios was studied to understand the relationship between structure and properties in starch-based materials. Hydrogen bonds and the highly branched microstructure in amylopectin resist the orientation of the polymer chains. The unique microstructures of amylopectin form gel-balls and super-globes after gelatinization. A gel-ball contains mainly the chains from same sub-main chain in amylopectin, and remains in regular pattern and keeps a certain “memory”. The gel-balls and super-globes can be deformed under shear stress. However, the deformation does not alter the orientation ofthe polymer chains inside the gel-balls significantly. Orientation increases both modulus and yield stress but decreases the elongation, which is mainly contributed to by the orientation of amorphous phase. The oriented super-globe has large interior stress after retrogradation (crystallization) that results in micro-cracks and poor mechanical properties.