Molecular weight distribution effects on craze micromechanics

Quantitative transmission electron microscopy has been used to investigate the micromechanics of crazes in monodisperse and polydisperse polystyrene (PS). In the monodisperse PS, a large stress is observed at the craze tip, but this stress falls rapidly with distance behind the tip. In the polydisperse PS (or in blends in which a small percentage of monodisperse low molecular weight PS has been added to monodisperse high molecular weight PS) the stress at the craze tip is lower and falls much less rapidly with distance behind the craze tip. These changes are attributed to the effects of diluting the network of entangled high molecular weight chains with molecules that are too short to entangle. This dilution increases the molecular stress on the load bearing chains of the network, thus increasing the ease of craze fibrillation.