Development of high-strength and high-modulus polyethylene-starch composite films and biodegradation of the composite films

Composite films of polyethylene with ultrahigh molecular weight (6 x 10(6)) and corn starch were prepared by gelation-crystallization from dilute solutions. The polyethylene-starch compositions chosen were 3/1, 1/1, 1/3, and 1/5. The elongation was carried out in a hot oven at 135 degrees C. The maximum drawability was affected by the composition. The maximum draw ratios of the 1/5 composite reached 80, and the corresponding Young's modulus was 10 GPa. The values of the films with the 3/1 and 1/1 composites were about 75 and 105 GPa, respectively, which are higher than the moduli of aluminum (70 GPa) and cast iron (90 GPa). In spite of the elongation up to such high draw ratios, most starch particles remained within the composite films. This interesting phenomenon is discussed in terms of the morphology of the composite films as studied by wide-angle x-ray diffraction, small-angle x-ray scattering, and scanning electron microscopy. Furthermore, the composite films with different compositions were put on a decarbonized agarose bench containing mineral nutrients, which had been prepared as isolate filamentous fungi medium in a Petri dish. Then two kinds of filamentous fungi, Myrothecium verrucaria IFO 6531 and Chaetomium globosum IFO 6347, were put on each film for 25 days. Observation by scanning electron microscopy revealed that the number of starch particles in the fibrous tissue decreased, and the sizes of residual particles were much smaller than the original sizes, indicating the biodegradation of starch by the filamentous fungi. This tendency is significant for M. verrucaria. Disruption of the polyethylene fibrous textures could not be confirmed by scanning electron microscopy, in spite of the decrease in Young's modulus and tensile strength in comparison with those of the original specimens.