Crystallinity and Morphology of Starch Polymers in Films
Starch is a renewable material that can be converted into biodegradable plastic products, or films with excellent oxygen barrier properties. Films of potato starch, amylose, amylopectin and blends thereof were made by solution casting. Their crystallinity, morphology, water content, thermal transitions and surface composition were studied and related to film composition and film formation conditions.
An increase in air humidity during film formation increased the crystallinity of starch films, whereas the crystallinity of amylose films was unaffected and amylopectin films were amorphous, as determined by wide-angle X-ray diffraction. Blending of amylose and amylopectin resulted in films with a considerably higher crystallinity than could be predicted. We propose that the reason for this is co-crystallization between amylose and amylopectin. Melting of co-crystals is suggested to give rise to an endotherm observed with differential scanning calorimetry.
The starch films and films with low amylose content were phase separated, with amylose domains in an amylopectin matrix. In films with high amylose content, the phase separation was prevented by the formation of a continuous network. Transmission electron microscopy of amylose rich films revealed an amylose network structure consisting of stiff strands and pores. Water is present in the pores, in the amorphous phase and in the crystalline structure. The water content of the films was therefore dependent both on the degree of crystallinity and on the morphology. Redistribution of water from crystalline regions and from network pores into the amorphous phase is suggested to be the reason for the lower glass transition temperature (Tg) observed after heating as compared to the initially observed Tg.
The film surfaces were covered with protrusions of nanometer size as observed with atomic force microscopy. These are proposed to be proteins, native in starch, that have migrated to the surface during film formation. The possibility of surface modification by gas plasma treatment was explored.
This research work provides broad knowledge of starch polymer behavior and inter-actions with water in films that can be used in the development of new and environmentally friendly plastic materials as well as in other areas with starch applications.