Dynamic SAXS studies of the crazing and fracture of polystyrene

Small-angle x-ray scattering (SAXS) studies of bulk polystyrene (PS) under uniaxial tension have been made at room temperature to investigate the stress induced changes in microstructure. The SAXS facilities in the National Center for Small Angle Scattering Research at Oak Ridge National Laboratory were used. The effects of molecular weight, molecular weight distribution, and molecular orientation on the fracture mechanism have been investigated. The formation of crazes is indicated by an abrupt increase in the scattering power. The excess scattering power induced by stress can be resolved into the surface scattering and the volume scattering components. Surface scattering, which is caused by specular reflection from the planar microcrack interfaces, shows as an intense streak on the intersection of the incident plane and the detector plane and contributes most of the excess scattering power. By converting the observed scattering intensity into absolute units, the surface scattering invariant relating to the interfacial area of microcracks and the volume fraction of microvoids are determined. From the surface scattering component, the average length of the steps caused by the microcrack jumping from one craze-matrix interface to the other is determined and is found to increase with the degree of molecular orientation. When the tensile force is not parallel nor perpendicular to the molecular orientation direction, the interfaces of the crazes are no longer perpendicular to the tensile direction. This suggests the strain-based criteria for crazing is more suitable than the stress-based criteria.