Linkage between tailored crystalline morphology and charge transport in LDPE: Insights from fractions

Abstract

High-voltage direct-current (HVDC) power cables predominantly rely on cross-linked polyethylene as the key insulating material, and the performance is determined by the properties of low-density polyethylene (LDPE). The absence of a scientific basis for regulating direct-current (DC) conductivity via molecular chain architecture and crystallization has impeded the advancement of HVDC cable insulation. Herein, preparative temperature rising elution fractionation was employed to fractionate LDPE resins, obtaining fractions with more homogeneous chain structures. The results demonstrate that the DC conductivity of LDPE exhibits a skewed V-shaped dependence on molecular weight. For the samples investigated in this study, LDPE exhibits a minimum DC conductivity of approximately 7.5 × 10−15 S/m at 70 °C, corresponding to a molecular weight of about 130 kg/mol and a spherulitic morphology characterized by small spherulites with an average size of approximately 2 μm. The crystalline morphology, including lamellar dimensions and stacking manner, is tailored by molecular structure and modulates the charge transport in LDPE. Charge traps in pure LDPE are found to originate predominantly from the amorphous regions. The thinner amorphous thickness, together with small, non-banded spherulitic morphology, contributes to a lower trap density and deeper trap energy levels, thus ensuring optimal insulation performance. This study establishes a critical linkage between the chain structure, crystalline morphology, and charge transport behavior in LDPE, providing fundamental insights for material selection and the development of high-performance HVDC cable insulation.