## 2 Experimental Section
### Characterizations  
The morphologies were investigated using a scanning electron microscope (SEM, SU8200, Hitachi, Tokyo, Japan). The presence of different elements was confirmed using energy dispersive X-ray spectroscopy (EDX). Tensile testing was used to obtain information about a material's mechanical properties, such as elongation-at-break (EaB), Young's modulus, and strength at break, by stretching a sample until it broke while keeping track of load and extension [9]. Tensile tests were performed at 25 \({}^{\circ}\)C with a universal testing machine (UTM, SES-100, Shimazu, Kyoto, Japan) equipped with a 100 N load cell, using a crosshead speed of 25 mm/min. At least four different specimens were tested for each bath. The degree of swelling was measured to confirm the change in the degree of crosslinking of XLPE according to deterioration. Dried XLPE flakes of approximately 12 mm in diameter were first weighted and recorded as \(W_{u}\), and the flakes and Xylene were placed in a vial. The flakes were then cooked at 80 \({}^{\circ}\)C for 8 h. The weight of the wet XPLE flakes were recorded as \(W_{s}\). The degree of swelling (\(Q\)) was derived by using the equation below [20].  
\[Q(\%)=\frac{W_{s}-W_{u}}{W_{u}} \tag{1}\]  
where \(W_{s}\) is the weight of the swollen XLPE, and \(W_{u}\) is the weight of the unswollen XLPE. The structure of the prepared samples was determined through X-ray diffraction (XRD, SmartLab, Rigaku, Japan) with nickel-filtered CuK\(\alpha\) radiation (40.0 kV, 30.0 mA) at 2-theta angles ranging from 10\({}^{\circ}\) to 70\({}^{\circ}\). Fourier-transform infrared spectroscopy (FTIR, Vertex 80v, Bruker, Billerica, MA, USA) was used to analyze the structural composition of XLPE. The  
Figure 1: Insertion of dumbbell test pieces into an aging oven.  
sample thickness was measured to be around 2 mm using attenuated total reflection (ATR) mode. To eliminate the effect of moisture, the pressure in the measurement chamber was maintained below 2 hPa. Each sample was scanned 34 times in the wavenumber range of 600-4500 cm\({}^{-1}\) with a resolution of 4 cm\({}^{-1}\). Differential scanning calorimetry (DSC, Q2000, TA Instruments, New Castle, DE, USA) was used to observe the phase transition characteristics due to thermal deterioration. Approximately 1.0-3.0 mg samples were analyzed using two repeated heating and cooling cycles at a constant rate of 10 \({}^{\circ}\)C/min in a nitrogen atmosphere over a temperature range of \(-\)50-200 \({}^{\circ}\)C. The DSC thermogram obtained from the first heating was strongly influenced by the material's thermal history; therefore, the DSC thermogram obtained from the second heating/cooling was selected and reflected in the analysis results. The optical properties and yellowness index (ASTM D1925) of XLPE were characterized using a UV-vis spectrophotometer (UV-vis, Cary 500, Agilent, Santa Clara, CA, USA). The absorption spectra in the 200-800 nm wavelength range were recorded by mounting the sample in an integrating sphere. Using the measured CIE tristimulus \(X\), \(Y\), and \(Z\) values, the yellowness index can be calculated through the following equation [21]:  
\[YI=\frac{100\times(1.28\times X-1.06\times Z)}{Y} \tag{2}\]