Mechanics of ZnO micro-rod and ZnO nanoparticle reinforcement in ultra-high molecular weight polyethylene biocomposite

Ultra-high molecular weight polyethylene (UHMWPE) is one of the most promising materials for cartilage replacement as an acetabular cup liner. Implant failure due to infection is a serious issue and ZnO is a well-known antibacterial agent. In the current work, the effect of the morphology of ZnO on the mechanical properties of UHMWPE is studied, where ZnO is incorporated both as nanoparticles (ZnO(NP)) and micro-rods (ZnO(R)) at 5, 10, 15 and 20 wt%. Uniaxial tensile testing of compression-moulded composites elicited a decrease of 8.8% in the Young's modulus in UHMWPE-ZnO(R) (named ZnO(R)-PE), whereas an increase of 21.1% in the Young's modulus was observed for UHMWPE-ZnO(NP) (named ZnO(NP)-PE). This contrasting effect on the Young's modulus arising due to differences in ZnO morphology is discussed and analysed using the rule of mixture and the Halpin-Tsai equation. Even when accounting for inherent porosity, and with similar crystallinity to that of base UHMWPE, these models fail to explain the decrease in the Young's modulus of compression-moulded ZnO-PE composites. Estimation of Young's modulus via a modified geometric factor is followed by proposing an empirical relation to account for interfacial strength and narrow the bounds of the predicted elastic modulus, thus making the Halpin-Tsai estimations reach the actual experimental values.