Correlation of Tensile Properties of Tough Amorphous Polymers with Internal Friction

Abstract

The ultimate stress-strain behavior of five tough amorphous polymers was studied at temperatures from 4.2 to 300°K using an Instron tensile tester which was adapted for cryogenic measurements. The polymers were found to fail by one of three modes depending upon test temperature and sample pretreatment condition. The transition from a general ductile behavior to brittle fracture was accompanied by a maximum in toughness which could be correlated with the γ transition in these polymers. At still lower temperatures there was a change in the brittle failure which correlated with the magnitude of the internal friction intensity of δ = 0.007 – 0.020. This transition in brittle fracture mode was characterized by a maximum in the brittle fracture stress. It is proposed that the brittle fracture at very low temperature occurs at abnormally low stresses due to stress concentrations factors which can not be relieved since molecular mobility becomes greatly restricted under these cryogenic conditions.