Effect of molecular weight distribution on fatigue crack propagation in poly(methyl methacrylate)

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Abstract
It is well established that both molecular weight (M) and its distribution (MD) affect many polymer properties such as mechanical behavior. Thus studies have shown that fatigue life is enhanced by increases inM. Research here has shown that with notched specimens fatigue crack propagation (FCP) rates are dramatically decreased by increasingM, even when theMis high enough that the static fraeture energy has essentially reached its asymptotic limit. In this study, specimens of poly(methyl methacrylate) containing either high‐ or low‐Mtails were prepared and characterized. The earlier finding that FCP rates are inversely related to averageMwas confirmed, but specific effects ofMdistribution were observed. At constantMn, a low‐Mtail had little effect on FCP resistance, while a high‐Mtail improved FCP resistance of polymers whose averageMwas too low for effective entanglements. Thus with high‐Mtails, it was possible to test specimens whose averageM's were too low to permit machining. It is proposed that the effects noted are due to relative stabilization or destabilization of crazes ahead of the crack.