CTA-mediated free radical cross-linking behavior toward highly cross-linked polyethylene with enhanced dielectric performance

Developing highly cross-linked low-density polyethylene to improve dielectric performance is the key to ensuring the reliability and stability of high-voltage cables. However, achieving a high cross-linking degree at lower levels of cross-linking agents toward excellent dielectric performance of cross-linked polyethylene remains a challenge. Herein, an improvement in dielectric performance is achieved by introducing the chain transfer agent (CTA), 2,4-diphenyl-4-methyl-1-pentene (MSD), at a reduced level of cross-linking agent. The 0.2 phr MSD significantly increases the cross-linking degree and enhances the breakdown strength of XLPE from 470 to 485 kV/mm at room temperature. The structural characterization and density functional theory calculation results demonstrate that the addition–fragmentation reaction of MSD effectively modulates the cross-linking behavior, with the formed reactive styrene polymer radicals leading to a star-cross-linked structure. This cross-linking behavior renders the network unique in that MSD provides chemical cross-linking at the cost of physical entanglements, as evidenced by the low-field nuclear magnetic resonance results. The maximum extent of chemical and physical cross-linking is achieved in XLPE when the MSD content is 0.2 phr, reducing the fractional free volume and further improving the breakdown strength. This strategy shows great potential for applications in high-voltage cable insulation.