Nanoindentation of high performance semicrystalline polymers: A case study on PEEK

Semicrystalline high performance polymers (HPPs) exhibit desirable thermomechanical properties which make them suitable materials in several industries. However, when their mechanical characteristics are investigated at nanoscale, the inhomogeneous nature of semicrystalline HPPs results in measurement of localized properties which might not represent the overall material response. In this paper, the nanomechanics of poly(ether-ether-ketone) (PEEK) as a tough HPP is scrutinized using the nanoindentation technique. It is demonstrated how surface properties of this polymer can affect the nanoindentation measurements and reveal significant anomalies compared to the nanoindentation of fully crystalline or amorphous solids. In addition, polishing and annealing procedures are introduced as simple techniques that can be used for eliminating the inhomogeneity of the mechanical response of PEEK. Lastly, the capability of depth sensing indentation for determining the distribution of crystalline and amorphous sub-regions within semicrystalline solids is scrutinized and critically discussed.