The objectives of this dissertation work were to study the film-forming behavior of corn starch and to characterize the microstructure and functional properties of the films. The effects of glycerol, water, stearic acid and extrusion temperature on the film-forming attributes of corn starch and on selected physical and functional properties of starch films were analyzed.
The tensile and water vapor properties (WVP) of starch films were significantly influenced by plasticizer content. The multiple glass transitions recorded in the thermoplastic starch were indicative of phase separation of starch into starch-rich and glycerol-rich domains. Similarly, the melting endotherms and presence of starch granules in films suggested that the starch granules were not melted completely during extrusion. Amylose-lipid complexes were formed during extrusion processing, depending on the moisture and stearic acid contents. Urea plasticized the starch more effectively than stearic acid and sucrose, and formed stable hydrogen bonds with starch.
High-performance size exclusion chromatography data confirmed that both amylopectin and amylose were fragmented during extrusion, resulting in reductions in the amylopectin content and a consequential increase in the amylose content. However, the extent of fragmentation in the amylopectin and amylose chains was not on a logarithmic scale. Regression models predicting the relative percentages of amylose and amylopectin and their molecular weights in the thermoplastic starch were developed. Fourier-transform infrared spectra revealed interactions between the functional groups of starch and glycerol.
The native 'A' type crystalline structure of starch was destroyed during extrusion. Aging studies demonstrated that the extruded amorphous starch developed 'B' and 'V' type crystalline polymorphs after just three days of storage. The tensile strengths of starch films increased progressively while both the tensile strains and WVP decreased continuously with respect to aging-time. Blending of low-density polyethylene (LDPE) into starch network improved the tensile strengths and strains of the films and decreased the WVP. Compounding of the ingredients before film-forming improved the dispersibility of starch and LDPE and the stress transfer between the two polymers.
