Crystalline Destruction Caused by Water Tree Growth in Low-density Polyethylene

The present work focuses on the effect of water tree development on crystalline structure of low-density polyethylene (LDPE). LDPE samples with 5 mm in thickness are prepared by the hot-pressing method, and then aged by the water-needle method at a voltage of 6 kV/5 kHz. After aging for 250, 500 and 750 hours, the cultivated water trees are observed in the anatomical sections of the samples. In order to study the crystalline structure, differential scanning calorimetry (DSC) and Raman spectroscopy (Raman) are adopted to analyze the LDPE samples with/without water trees. DSC results reveal that the crystallinity decreases with the growth of water tree, and the content of non-crystalline region gradually increases. It is found from Raman results that the appearance of water trees causes the decrease of crystallinity and the increase of interfacial content. As water trees grow, their values keep almost unchanged, but the distributed area with lower crystallinity and higher interfacial content is expanding. For the amorphous content, both the value and spatially distributed character almost remain constant. At last, the results obtained by the two methods are compared, and the effect of water tree growth on the crystalline destruction is discussed.