Interaction of crazes with pre-existing shear bands in glassy polymers

Abstract

Shear bands have been grown in bulk specimens of P₃O(poly 2,6 diphenyl 1,4 phenylene oxide) and in thin films of two blends of polystyrene with poly(xylenyl ether). The subsequent interaction of crazes with these shear bands has been characterized by transmission electron microscopy. For the case of shear bands grown under the plane stress conditions of thin films, it is found that the bands act as preferential sites for craze nucleation. A fairly regularly-spaced array of short crazes grows within the shear bands and these crazes may thicken sufficiently to coalesce. When the crazes reach the end of the shear band they emerge and propagate into the unoriented polymer matrix. Within the shear band the craze growth direction does not lie normal to the tensile axis, but is rotated due to the molecular orientation of the shear band. The direction of craze growth is also affected under the plane strain conditions of bulk specimens. In this case the craze is diverted along the shear band before re-emerging into the matrix. Measurements of the craze fibril extension ratio, λ, within the shear band show an increase over typical values obtained outside the shear band in the same polymer. This high value of λ leads to an increased likelihood of craze break-down and crack nucleation within the shear band.