## 3 Results and Discussion
### Complementary Tests
The DSC results for the different material/oil combinations are shown in Figure S7, and Tables 5-7 were obtained from those curves. The enthalpy of fusion (\(\Delta\)H) for a fully crystalline PEEK would be 164.85 J/g [18], so the fresh sample, with 31.60 J/g, was not crystalline. When the PEEK was aged in the different ATFs, a rise in the enthalpy associated with an increase in crystallinity was detected (Table 5), especially for those samples aged in oils B and C. However, this change in the crystalline structure was not large enough to modify the mechanical characteristics \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & \(\Delta\)**H [J/G]** & \(\mathbf{T_{PEAK}}\) [\({}^{\circ}\)C]** & \(\mathbf{T_{ONSET}}\) [\({}^{\circ}\)C] \\ \hline Fresh & 39.18 & 336 & 324 \\ Oil A & 41.28 & 336 & 324 \\ Oil B & 40.14 & 338 & 324 \\ Oil C & 41.30 & 336 & 324 \\ Oil D & 42.95 & 335 & 325 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: DSC results for the PTFE samples (fresh and aged in the ATFs). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & \(\Delta\)**H [J/G]** & \(\mathbf{T_{PEAK}}\) [\({}^{\circ}\)C]** & \(\mathbf{T_{ONSET}}\) [\({}^{\circ}\)C] \\ \hline Fresh & 11.03 & 258 & 250 \\ Oil A & 19.30 & 258 & 248 \\ Oil B & 17.43 & 257 & 246 \\ Oil C & 29.71 & 260 & 249 \\ Oil D & 21.26 & 258 & 249 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: DSC results for the PA66 samples (fresh and aged in the ATFs). Figure 8: PCA analysis of the FT-IR spectra of the PA66 (oil A (blue), oil B (green), oil C (fuchsia), oil D (red), 672 h (empty markers) and 168 h (solid markers)). of the PEEK. The T\({}_{\text{Onset}}\) and T\({}_{\text{Peak}}\), represent the start of the fusion process and the fusion temperatures, respectively. PEEK is the material that shows the greatest difference between them, and the highest values of all the materials, although very similar to those of PTFE. The difference between T\({}_{\text{Onset}}\) and T\({}_{\text{Peak}}\) might be associated with crystallization taking place during the heating process, with the possibility of the peak being double, depending on the heating speed [18]. In the case of PTFE, the \(\Delta\)H at fusion for a crystalline sample would be 57.3 J/g [19; 20]. The results therefore show that, as in the case of PEEK, the fresh sample is not crystalline (Table 6). The slight increase in enthalpy after ageing does not suggest a notable change in the crystalline structure, confirming the results of the mechanical tests, with very small changes being seen. Finally, Table 7 shows the DSC results for PA66. This material showed the greatest changes in mechanical properties (+7 ShD points, +15% tensile strength, and -80% elongation at break) of all the materials in this study. These variations would suggest a certain degree of crystallization, which has been mentioned by [PERSON] et al. [13] and [PERSON] et al. [14] after an ageing process of PA66 in ATFs. PA66 is a semi-crystalline polymer [21; 22; 23], and the variation in the degree of crystallinity can be observed in the evolution of the melting enthalpy [24]. Table 7 shows that the melting enthalpy nearly doubles after ageing in the different ATFs, meaning that the increase in the degree of crystallinity is proportionately greater than in the other studied polymers. Although the amorphous phase is still more prevalent, according to the XRD results, the change in the structure of PA66 is enough to cause variations in its mechanical properties. Finally, according to the changes in the different mechanical properties tested for the materials after being aged in the four ATFs, PEEK and PTFE may be considered to be compatible in all cases, as the changes in volume, hardness and tensile strength are negligible. Although they presented higher variations in their elongation at break, we consider these variations to be acceptable. That is not the case for PA66, which showed a very large decrease (\(\sim\)75%) in elongation at break.