Toughness measures in solid-state composites of ultra-high molecular weight polyethylene

Solid-state composites of ultra-high molecular weight polyethylene (UHMWPE) are increasingly being investigated for various therapeutic and sensing use cases in arthroplasty. Due to its extremely high melt viscosity, composites of UHMWPE exhibit a much more distinct phase segregation than lower-viscosity polymer composites. This phase segregation may influence UHMWPE behavior in a different way than microstructural factors studied previously (e.g. crosslinking). The purpose of this study was to quantify UHMWPE nanocomposites' resistance to failure via several distinct measures, across a range of carbon black (CB) filler loadings. Results showed that tensile and impact toughness followed similar trends: both maintained or increased in magnitude over neat controls at low filler concentration, then decreased at higher filler contents. On the other hand, fatigue crack propagation resistance (CPR) demonstrated similar behavior to tensile and impact toughness at lower concentrations but diverged at higher concentrations. From 2.5 wt% CB to the highest concentration tested (10 wt% CB), fatigue properties improved-unlike tensile or impact toughness. The observed transitions in macroscopic behavior with increasing filler content coincided with microstructural phenomena observed via scanning electron microscopy (SEM) and fracture surface imaging. These phenomena included a transition from transgranular fracture to intergranular fracture, the onset of complete granule coating, and the formation of intergranular voids. This work demonstrates that tensile and impact toughness are not necessarily indicative of fatigue CPR in these materials. Broadly, the findings presented in this study motivate further investigation of structure-property relationships for phase-segregated polymer composites and demonstrate promise for their use in high-load scenarios.