Craze structure and stability in oriented polystyrene

Thin polystyrene (PS) films bonded to copper grids were oriented by straining them above Tg to an extension ratio λor. These films were strained at room temperature in directions parallel to and perpendicular to the initial orientation to produce ‘parallel’ and ‘perpendicular’ crazes, respectively. The crazes were examined by transmission electron microscopy and by optical microscopy in transmitted and reflected light. The craze extension ratio λcr decreases as 1λor for parallel crazes and increases as λ12cr for perpendicular crazes, in good agreement with the values expected from changes in λmax, the maximum extension ratio of a strand in the entanglement network of the polymer glass. While the craze fibril diameter D increases with λor for parallel crazes and decreases with λor for perpendicular ones, the fibril spacing D0 is independent of λor. A modification of the model of the fibril growth mechanism, involving the strain-hardening effects, is proposed to take into account the constancy of D0 and the changes of the craze stress Sc with the orientation. Fragility test results demonstrate that fibril breakdown is the dominant step in craze fracture and that extreme perpendicular orientation strongly reduces the fibril breakdown strain.