Age-Induced Changes in the Microstructure and Selected Properties of Extruded Starch Films Plasticized with Glycerol and Stearic Acid

Aging of normal corn starch films processed by extrusion and stored at 20-22 degrees C and 50-56% RH was investigated over a period of 120 d. The changes in the structural, thermal, tensile, and water vapor permeability (WVP) properties of the films during aging were studied. The thermoplastic starch (TPS) just after extrusion processing (approximate to 4 h) was nearly amorphous in character. However, aging above the T-g produced both B- and V-type crystalline structures in the TPS within 3 d of drying. The relative crystalline content of the TPS increased from 2.6 to 17.7%, depending on the glycerol content. Post-processing, the tensile strengths of the films increased by 39.3-134.1% while the tensile strains decreased by 48-81.1%. The rates of change in tensile strength and strain were faster during the first 30 d but stabilized thereafter. The biphasic transitions in the DSC thermograms suggested phase separation in the TPS. The T-g of glycerol-rich domains decreased while those of the starch-rich regions increased during storage. The band-narrowing and changes in peak styles and intensities in the 1400-800 cm(-1) region of the Fourier transform infrared spectra suggested increases in the crystalline content of the TPS during aging. The WVP values tended to decrease with time as the crystallinity of starch increased and the free volume in the film network decreased.