EFFECT OF MOLECULAR ENTANGLEMENTS ON CRAZE MICROSTRUCTURE IN GLASSY-POLYMERS

In the reported experiments, thin films of ten glassy polymers are bonded to copper grids and strained in tension to produce crazes, which are then examined in the transmission electron microscope. The average craze fibril extension ratio lambda for each polymer is determined from microdensitometer measurements of the mass thickness contrast of the crazes. The results are analyzed by comparing them with lambda //m//a//x, the maximum extension ratio of an entanglement network in which polymer chains neither break nor reptate. The craze extension ratios fall somewhat below lambda //m//a//x at low lambda but increase to well above lambda //m//a//x for polymers with high l//e. This comparison suggests a significant contribution due to chain breakage in the higher- lambda crazes of large-l//e polymers, which may arise from the higher true stresses in the craze fibrils. The results also imply that a useful way to increase the ″brittle″ fracture stress and decrease the ductile-to-brittle transition temperature of a glassy polymer is to decrease its entanglement contour length l//e.