The role of double-helix formation in water diffusion and aging of injection-molded starch

Native potato starch has been processed without the use of any additive, Mo ldings of initial high strength and ductility have been achieved. However, mechanical properties change with time. Thermogravimetric experiments at di fferent temperatures show that aging takes place in three different steps. The first is characterized by a strong formation of double helices (as reve aled by wide-angle X-ray scattering [WAXS]) in parallel with strong water l oss. Hardness, strength, and elongation at break increase in this time rang e: Double helices act as reinforcing elements. When the yield elongation is surpassed, double helices are unraveled, providing a higher elongation at break. With further aging (drying), due to a shrinking process, a temporary reduction of double helices, causing a resoftening, seems to take place. I n a third step, the starch moldings become quite brittle due to the lack of molecular mobility. At room conditions, it takes approximately 1 year (beg inning of range 3) to lose the good mechanical properties. It is proposed t hat the mechanical behavior of starch moldings is controlled by a molecular network that consists of single-helix molecules bound by double helices wi thin an amorphous structure.