Morphology of the fracture surface layer of poly (methyl methacrylate) and the fracture mechanism

Abstract

AbstractOptical and electron microscopic methods were used to investigate the morphology and optical properties of the fracture surface layer of poly (methyl methacrylate). A new method developed by us made it possible to evaluate, from interference phenomena, the optical thickness of the layer which in the proximity of the secondary fracture center always exhibited a minimum and attained maximum values (1.1‐1.6 μm) at the boundaries of the conic fracture formation. After mutual correlation, the local refractive index of the surface layer was calculated and found to be 1.31 to 1.35. Although the macromolecular chains are oriented in crazes, no chain orientation in the fracture surface layer was detected. The lower refractive index of the surface layer is therefore interpreted in terms of material dilution of about 35% by volume of small holes. When the sample is subjected to thermal treatment, the profile of the optical thickness of the fracture surface layer remains unaltered, only the interference colors gradually disappear. This fact is attributed to a change in the reflectivity on the upper or lower boundary of the surface layer caused by local changes in the material density. The replica electron micrographs of the fracture surfaces showed pits in the foci of the conic markings which had various shapes and diameters about 1 μm. Using the facts established about the geometry and morphology of the fracture surface layer, a mechanism for the initiation and propagation of fracture has been suggested.