Initiation and termination of crazes in high impact polystyrene (HIPS) were studied by transmission electron microscope. Instead of the first initiation of crazes in the vicinity of the equator or rubber particles, ribbon|shaped crazes beyond the region were observed on the TEM photos. The starting point and the end point of each ribbon were noticed not to be randomly located on the surface of the cellar particles in HIPS. The spots connecting to the ligaments between the PS rigid inclusions in the cellar particle are found to be the locations where crazes were initiated and terminated preferentially. In addition, the crazes generated at those spots were much thicker and longer. The thoughness of high impact polystyrene was enhanced by the multiple crazes formed in the matrices, through the reduction of the craze stress at room temperature with increasing the volume fraction of polybutadiene at these spots in the dispersed phase. These results conducted that not only the cell structure of the domains in HIPS was not homogeneous but also the polybutadiene ligaments between PS rigid inclusions in the domains were inhomogeneous. Therefore, the responsibility of those polybutadiene ligaments with different thickness to the stresses was different and it forced the crazing initiation and growing preferentially. The crazes were created in some regions in the matrices; On the contrary, rare crazes were formed in the other regions in spite of these regions surrounded around the same dispersed particles. The relation between the spots on the surface and the inner structure of the cellar particle was discussed in this paper.
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