Effects of radiation and chain ends on fatigue behaviour of polystyrene

Fatigue lifetimes, under a given alternating stress amplitude, have been determined for a series of linear and branched polystyrenes. The branched polymers were obtained by a crosslinking reaction using γ-irradiation from a Co⁶⁰ source. By control of irradiation time, a series of branched samples of progressively increasing weight average molecular weight ($\text{M}$_w), with little change in number average molecular weight ($\text{M}$_n, were obtained. From comparison of fatigue data for these irradiated and branched samples with fatigue data obtained on a series of linear polystyrenes of increasing molecular weight, it may by concluded that appreciable increases in fatigue endurance can be achieved by increase in $\text{M}$_n and reduction in chain end density. For the irradiated samples, whether irradiated in air or in vacuum, fatigue lifetimes were comparable to or less than lifetimes to fracture for the unirradiated polymer, even though significant increases in $\text{M}$_w had occurred. It is suggested that the improved fatigue performance with increase of $\text{M}$_n is a consequence of increased craze stability resulting from the greater degree of chain entanglement and the smaller proportion of chain end defects.