## 3 Results and Discussion
### Complementary Tests  
Complementary tests (XRD, FT-IR, and DSC) were performed on the polymers. X-ray diffraction (XRD) experiments were used to search for changes in the crystalline structure of the materials. However, the XRD spectra (Figure 5) showed no noticeable changes in the  
Figure 4: Changes in elongation at break of, PEEK, PA66, and PTFE after immersion in the ATFs. crystalline/amorphous areas ratio, which means that it is not possible to correlate these results with the reported changes in the mechanical properties. Infrared spectroscopy (FTIR) was used to detect possible changes in the polymeric structure of the materials. In the FTIR spectra, the qualitative differences (mainly peak positions) in the spectra of the same material subjected to different ageing conditions are almost negligible. Nevertheless, possible quantitative differences were evaluated by comparing peak areas. Tables S1-S3 record the peak areas of PEEK, PTFE, and PA66 when fresh and under different ageing conditions. The area ratio of aged material vs. fresh material is calculated for every sample at every peak. In principle, this ratio should be similar for all the peaks within a sample. We considered deviations to be significant in those samples with at least one area acting as outlier according to a two-sided Grubb's test (\(p\) = 0.05) [16]. With this criterion, PTFE and PA66 did not show significant differences. However, PEEK in oil C (at both tested times) showed an increase in the absorbances at 2850 cm\({}^{-1}\) and 2915 cm\({}^{-1}\), attributable to -CH- stretching vibrations, suggesting a particular influence of this oil. The sample aged in oil B for 672 h showed a similar trend, whereas the sample aged in oil A for 168 h showed a decrease in absorbance at 2915 cm\({}^{-1}\). Principal component analysis is a data analysis methodology that allows the conversion of a group of correlated variables (in this case, IR absorbance at different wavenumbers) into a smaller number of uncorrelated variables which, in addition, retain most of the variability of the original data [17]. According to PCA analysis of the spectra of the different polymers, the samples of PEEK with 168 h of ageing (solid markers) group together (Figure 6), as do the samples with 672 h of ageing (empty markers), except for oil B (green markers). These results suggest  
Figure 5: XRD spectra for the polymers (fresh, after 168 h, and after 672 h) in the ATFs. that, in the case of PEEK, time causes greater effects the oil type does. This would explain, in general, the similarity of results in the PEEK mechanical tests after ageing in the ATFs. However, the PCA analysis of aged PTFE samples (Figure 7) revealed the following three groups: oil C (fuchsia markers) after 168 and 672 h, oil D (red markers) after 168 and 672 h, and oil A (blue markers) after 672 h and oil B after 168 h. According to these findings, in the case of oils C and D, the oil type is more important than the ageing time. This is well represented in the dispersion of results for the mechanical properties variation tests, depending on the ATF used in the PTFE ageing. PA66 showed similar behaviour to that of PEEK (Figure 8). Samples with shorter ageing times grouped together, as did samples with longer ageing times, except with oil B (green markers). This suggests, again, that the ageing time is more important for the PA66 samples than the oil type. These would explain the similarity of results in the PA66 mechanical tests after ageing in the different ATFs. Figure 6: PCA analysis of the FT-IR spectra of the PEEK (oil A (blue), oil B (green), oil C (fuchsia), oil D (red), 672 h (empty markers) and 168 h (solid markers)). Figure 7: PCA analysis of the FT-IR spectra of the PTFE (oil A (blue), oil B (green), oil C (fuchsia), oil D (red), 672 h (empty markers) and 168 h (solid markers)). Differential scanning calorimetry (DSC) was used to check if there were changes in the fusion processes of the materials and, thus, in their crystalline structure.