Real time small-angle X-ray scattering from polystyrene crazes during fatigue

Abstract

Real time small-angle X-ray scattering (SAXS) from polystyrene (PS) crazed in cyclic three-point bending is investigated using intense X-ray radiation from the Cornell High Energy Synchrotron Source, CHESS. The SAXS patterns are recorded using a two-dimensional image intensifier/TV camera/video tape recorder system, operating at 30 frames/sec. At the maximum of the load cycle the SAXS pattern has a well defined streak normal to the craze fibrils. During the unloading portion of the cycle, however, the streak decreases in intensity and is spread into a diffuse fan. The loss of intensity is due to the decrease in volume of craze matter in the beam as the craze closes while the spreading of the diffraction pattern is due to the disorientation of the craze fibrils as they buckle in response to compression by the surrounding polymer matrix. Whilst reloading of the sample causes a relatively narrow SAXS streak to reappear, at the maximum load irreversible changes occur in the pattern from one cycle to the next. These changes are due to both an increase in craze fibril volume in the beam (craze growth) and to fibril breakdown and permanent disorientation.