Morphology and Mechanical Properties of Biodegradable Composite Films of Poly (vinyl alcohol) and Starch

publisher[Synopsis] In an attempt to produce biodegradation materials, poly (vinyl alcohol) (PVA)-starch (ST) blends were prepared by gelation/crystallization from semidilute solutions in dimethyl sulfoxide (Me_2SO) and water mixtures and elongated up to 8 times. The content of mixed solvent represented as Me_2SO/H_2O (volume percent) was set to be 60/40 assuring the greatest drawability of PVA homopolymer films. The PVA-ST compositions chosen were 1/1, 1/3, and 1/5. The elongation up to 8 times could be done for the 1/1 blend but any elongation was impossible for other blends whose ST content was beyond 50％. The starch particles could be observed under SEM but were not detected under polarizing microscopes for the blend films. This indicates that the starch particles were collapsed and well-mixed with PVA. The PVA crystallites for the PVA-ST (1/1) composite prepared by quenching the solution at —30℃ show a higher orientation degree than that prepared at 20℃ at the same draw ratio of 8-fold. On the other hand, the orientation degree of PVA in blend film is much lower than PVA homopolymer at the same draw ratio indicating that starch hampered the preferential orientation of PVA molecular chains along stretching direction obviously. The storage modulus at 20℃ was about 0.5 GPa and modulus for specimen prepared at —30℃ is higher than that prepared at 20℃. X-ray diffraction patterns showed the diffraction spots from PVA crystallites but no diffraction from ST crystallites. This indicates that ST crystallites disappeared under heating process to prepare the solutions. When the blends were immersed in water at 20 or 83℃, the solubility becomes considerable for an undrawn blend with 1/5 composition and a drawn 1/1 blend with λ＝8. To avoid this phenomenon, cross-linking of PVA chains was carried out by formalization under formaldehyde vapor. Significant improvement could be established by the cross-linking of PVA chains. For the 1/1 blend, the amount of ST dissolved in water at 23℃ was less than 3％ for the undrawn state and 25％ for the drawn film. The decrease in the ST content was enough for use as biodegradation materials. Namely, the water content relating to the biodegradation in soil is obviously different from such a serious experimental condition that a piece of blend film was immersed in a water bath. At temperatures above 0℃, the storage modulus of the formalization blends became slightly higher than those of the nonformalization blends. The Young's modulus of the drawn films with a draw ratio of 8 times was 2 GPa at 20℃