THE ROLE OF DOUBLE-HELIX FORMATION IN WATER DIFFUSION AND AGING OF INJECTION-MOLDED STARCH

• Published in: Journal of macromolecular science. Physics 2001-11, Vol.40 (5), p.733-747
• Main Authors: Bayer, R. K., Lindemann, S., Dunkel, M., Cagiao, M. E., Ania, F.
• Format: Article
• Language: English
• Subjects: Aging; Applied sciences; Exact sciences and technology; Mechanical properties; Natural polymers; Physicochemistry of polymers; Starch; Starch and polysaccharides; Structural model; Water diffusion
• ISSN: 0022-2348; 1525-609X
• DOI: 10.1081/MB-100107558

Native potato starch has been processed without the use of any additive. Moldings of initial high strength and ductility have been achieved. However, mechanical properties change with time. Thermogravimetric experiments at different temperatures show that aging takes place in three different steps. The first is characterized by a strong formation of double helices (as revealed by wide-angle X-ray scattering [WAXS]) in parallel with strong water loss. Hardness, strength, and elongation at break increase in this time range: 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. In a third step, the starch moldings become quite brittle due to the lack of molecular mobility. At room conditions, it takes approximately 1 year (beginning of range 3) to lose the good mechanical properties. It is proposed that the mechanical behavior of starch moldings is controlled by a molecular network that consists of singlehelix molecules bound by double helices within an amorphous structure. * Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th birthday.