## 3 Results and Discussions
160 \({}^{\circ}\)C showed a rapid decrease in crystallization in both cases. The degrees of crystallinity (%) derived from the XRD of the sample that deteriorated for 60 days at each temperature were confirmed to be 34.07%, 33.48%, 14.34%, and 9.87%, respectively. Figure 6 depicts the degree of swelling as a result of XLPE degradation. It is an important characteristic that indicates the degree of polymer crosslinking. The degree of swelling in the undegraded XLPE sample was 10.38%. However, it increased with exposure to high heat. An increase in the degree of swelling indicates that a large amount of solvent had penetrated into the sample. This phenomenon occurs when the cross-linking of XLPE  
Figure 5: XRD results of XLPE samples: (**a**) X-ray diffraction patterns, (**b**) Gaussian fitting of the crystalline peaks and the amorphous halo, and (**c**) crystallinity variation depending on aging time. is broken due to deterioration and the cross-linking density is lowered, thereby increasing solvent penetration. Figure 7 shows the FTIR spectra of the thermally deteriorated XLPE material. Since XLPE is a PE-based material, a peak for C-H bonding was confirmed. The peaks for symmetric and asymmetric stretching vibrations of the -CH\({}_{2}\) group were identified at 2916 and 2848 cm\({}^{-1}\), respectively. Peaks corresponding to wagging vibration and the rocking methylene group of -CH\({}_{2}\) were identified at 1463 and 719 cm\({}^{-1}\), respectively [24]. There were no significant changes in the XLPE FTIR results after exposure to heat at 100 and 120 \({}^{\circ}\)C for 60 days. However, when XLPE was exposed to 140 and 160 \({}^{\circ}\)C, a peak corresponding to the carbonyl functional group (C=O) was identified between 1650 and 1800 cm\({}^{-1}\). The absorbance of the C=O peak gradually increased with exposure to heat. In addition, the C=O peak was observed as being divided into three. The C=O peak at 1712 cm\({}^{-1}\), the lowest wavenumber, corresponds to carboxylic acid and ketones. Secondly, the peak appearing at 1732 cm\({}^{-1}\) corresponds to that present in aldehyde or ester. Finally, the peak observed at 1770 cm\({}^{-1}\) corresponds to carboxylic anhydride [25]. At the beginning of degradation, the C=O peak absorbance at 1712 cm\({}^{-1}\) was the highest. However, as the period of exposure to heat increased, the C=O peak absorbance at 1731 cm\({}^{-1}\) increased, showing the highest value. Due to the deterioration of XLPE, bonds between C and H in the polymer chain were broken and carbon radicals were generated. Then, it reacted with oxygen to form peroxide, and each peroxide was decomposed into radicals. This alkoxy radical breaks other polymer bonds. Due to the continuous degradation reaction, various carbonyl structures were generated in the polymer chain. At this time, the carboxylic acid identified at 1712 cm\({}^{-1}\) was converted into an ester by reacting with another alkoxy radical, or ROH, formed from the decomposition of peroxide in the polymer chain generated during deterioration. Therefore, as the deterioration progressed, the absorbance detected at 1731 cm\({}^{-1}\) became higher than the carbonyl absorbance detected at 1712 cm\({}^{-1}\). Figure 6: Degree of swelling of XLPE samples depending on aging time and temperature. In the samples exposed to heat at 140 and 160 \({}^{\circ}\)C, a peak was observed between 1100 and 1250 cm\({}^{-1}\) due to the vibration of the C-O-C bond [24]. The peak absorbance also increased with increasing exposure to heat. In addition, it was confirmed that the absorption peaks at 800 to 1128 cm\({}^{-1}\) and 1560 to 1650 cm\({}^{-1}\) increased with deterioration. These peaks correspond to the vinylidene and vinylene groups of the C=C bond, respectively [24,26]. Finally, as it was exposed to heat, the peak caused by the -OH functional group was confirmed and increased from 3000 to 3500 cm\({}^{-1}\)[24,26].