Micromechanical fracture analysis of amylose

Amylose films were prepared with two distinctly different microstructures, and these specimens were subsequently tensile-tested at different levels of moisture content. Measurement of the strain energy release rate, G_l, was approached from three separate methods. Two of these methods, the J integral and stress intensity analyses, are based on bulk tensile properties. The third method involves direct calculation of the work expended in producing the microplastic zone at the crack tip. Birefringence measurements were used in mapping the magnitude and shape of the plastic zones. Although the semicrystalline material would not lend itself to the bulk tensile analyses, G_l values were obtained via the optical analysis. For disordered amylose, G_l values derived from this optical analysis were found to correlate with those obtained from bulk measurements, and the fracture toughness was found to be comparable to that of other engineering polymers. Surprisingly, moisture content within the range investigated herein has no great effect on the strain energy release rate. This insensitivity is attributed to active interaction between the absorbed water and amylose macromolecules.