Craze Breakdown Morphology

Similar craze breakdown morphologies have been observed for dust-free films of polymethylmethacrylate (PMMA), poly(a-methylstyrene) (PaMS) and poly(styrene-acrylonitrile) (PSAN) Large pear-shaped voids nucleate at the craze-bulk polymer interface, never in the craze mid-rib, and thus this mode of craze breakdown seems to be a dominant one for all glassy polymers. 

TEM micrographs of craze fibril breakdown in the absence of a dust inclusion a and associated with a dust inclusion b. Note, in both cases the site of craze breakdown is at the craze-bulk boundarv 

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In homo-polymers, when crazes are surface-initiated, generally imder a substantial stress, premature fracture follows from craze breakdown initiated from entrapped dust particles of low adhesion to craze matter. To counteract this, various types of compliant particles having a variety of morphologies that are effective in craze initiation under lower stresses are incorporated into the homo-polymers through synthesis or blending. We discuss this practice in Chapter 13 on toughening of brittle polymers. 

Failure Morphologies. Ductile failure of notched polycarbonate specimens has long been recognized to occur with shear yielding from the notch tip (6). This occurs for the block polymers for all rates of test. Hull and Owen (5) recently reported from micrographic studies of impact fracture surfaces that the brittle failure of polycarbonate involves the formation and breakdown of a craze at the notch tip. The ductile-... 

A final comparison of low temperature crazes with shear bands reveals that both deformation phenomena are related. The surface morphologies are quite similar because both modes of plastic deformation depend upon the relative displacement of domains of a size of 10 to 100 nm. However, crazing is controlled by a tensile stress and the fibrous matter contains voids. Shear banding, on the other hand, is controlled by a shear stress which encourages lateral movements without voiding. The final breakdown process may then be initiated in both cases by a random rupture at the upper or lower edge of the deformation zone (Fig. 39 a, b). 

Below, we consider first the basic principles of toughening of brittle glassy polymers that clarify the role of the different particle morphologies, and of their flexibility and size, as well as volume fraction, that act together in maximizing craze plasticity at levels of flow stress that avoid craze-matter breakdown. 

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