When regarded in the broad context of industrial engineering applications, the mechanical properties of PEEK generally decrease with elevated temperatures up to 250°C, with a pronounced drop-off in properties above 150°C (ie, for temperatures exceeding the glass transition temperature) [ 57 , 80 , 82 - 84 ]. However, within the context of biomaterial applications, where the expected operating thermal environment is around 37°C (body temperature), the elastic behavior of PEEK is relatively insensitive to temperature. The yielding, plastic flow, and fracture behavior of PEEK display greater sensitivity to test temperature below the glass transition than elastic properties ( Figure 4A ). Implant applications that can involve heat generation, such as impact loading during installation, or frictional contact in a joint replacement, may involve more detailed consideration of thermal effects on mechanical behavior.
As shown in Figs. Figs.3 3 and and4, 4 , unless the material exceeds the elastic limit, temperature as well as strain rate should not be primary material concerns for PEEK biomaterials in clinical use. The elastic properties of PEEK are relatively uneffected by rate effects at body temperature, which is below the glass transition [ 82 - 84 ]. However, the yielding and plastic flow behavior is slightly affected by strain rate at physiological temperatures. In uniaxial compression, varying the strain rate by seven orders of magnitude (from 10 −4 s −1 , corresponding to nearly quasistatic loading, to 10 3 s −1 , corresponding to impact loading) increases the yield strength by around 30% [ 84 ]. A number of interesting thermo-mechanical phenomena, including changes in crystallinity, deformation-induced heating, macroscopic discoloration, and viscoelastic recovery-induced rupture, can all accompany high strain rate, large deformations of PEEK associated with impact [ 84 ]. The relevance of rate sensitivity should be considered when performing mechanical test evaluations of devices that may be implanted by impact loading, such as PEEK hip stems and bone anchors.
When comparing virgin PEEK materials with the same molecular weight, the elastic modulus, yield stress, and plastic flow behavior will be strongly influenced by crystallinity [ 85 ]. The crystallinity, in turn, reflects the thermal processing history of PEEK, as discussed in a previous section of this review. Injection molded parts, in which the cooling rate varies with thickness, will thus be susceptible to heterogeneous material properties, because of their spatially varying crystallinity [ 79 ]. The formation of a lower-crystallinity surface “skin” can be addressed by subsequent thermal treatments, by machining away of any amorphous skin, or by molding test specimens of sufficient thickness as to render the presence of a thin surface skin negligible [ 79 ].