A team of researchers at Tianjin University has reported a strikingly effective way to upgrade one of the most important plastics in the modern power grid. By chemically grafting a small functional molecule called 4-allyloxybenzil, or AOB, onto impact polypropylene copolymer, a material widely viewed as a leading candidate to replace cross-linked polyethylene in high-voltage direct current cable insulation, the group achieved improvements that would normally require entirely new polymer chemistries. Space charge accumulation inside the material dropped by 85 percent, electrical conductivity fell by roughly an order of magnitude, and direct current breakdown strength rose by 30 percent. The work, published in Advanced Composites and Hybrid Materials, is notable not only for the magnitude of these gains but for the rigor of the analysis behind them.
The challenge the researchers set out to address is fundamental to how direct current cables fail. In an HVDC transmission system, insulation must withstand hundreds of kilovolts across just a few centimeters of polymer. Under such intense fields, charge carriers are injected from the electrodes into the material, where they become trapped, migrate, and accumulate as space charge. This trapped charge distorts the local electric field, creating hot spots that can be far more intense than the applied field itself. Over years of service, these distortions accelerate aging, trigger partial discharges, and ultimately drive the insulation toward breakdown. Any material that hopes to serve as the next generation of cable insulation must therefore suppress charge injection, slow carrier mobility, and keep the internal field as uniform as possible.
Polypropylene has long been attractive for this role. It offers excellent electrical properties, high melting temperature, low cost, and, crucially, it is thermoplastic and fully recyclable, unlike cross-linked polyethylene, which is permanently cured and difficult to reprocess. Impact polypropylene copolymer, which blends a polypropylene matrix with an ethylene-propylene rubber phase for toughness, is a particularly promising variant. Yet until now, its optimization has been hampered by a lack of rigorous, quantitative correlations between structure and dielectric performance. Modifications were often evaluated only at the macroscopic level, leaving researchers to guess at the molecular and microscopic mechanisms responsible. The Tianjin team, led by You Wu, Zhonglei Li, and Boxue Du, set out to close that gap.
Their approach centered on a deliberately designed monomer. AOB packs three chemically distinct features into a single molecule: benzene rings, carbonyl groups, and ether bonds. Each of these contributes polarizable electrons and permanent dipoles, and together they create energy states within the polymer that can capture injected charge carriers. Using an aqueous suspension grafting method, the researchers covalently attached AOB onto the polypropylene chains, ensuring that the modifier would not simply migrate or bleed out of the insulation over decades of thermal and electrical stress, a known weakness of physically blended additives.
To understand what the grafted molecules were doing at the smallest scale, the team turned to density functional theory calculations and Kelvin probe force microscopy. The DFT simulations showed that the specific molecular architecture of AOB introduces deep traps for both electrons and holes, corresponding to the highest occupied and lowest unoccupied molecular orbitals of the modified system. These deep energy wells capture charge carriers that would otherwise roam freely through the polymer. The experimental signature came from KPFM measurements of surface potential decay: when the modified material was charged and then allowed to discharge, the surface potential lingered far longer than in the unmodified polymer. The decay time stretched from 3.9 seconds in the pristine material to 9.2 seconds after grafting, a clear indication that carriers were being held in place rather than moving and recombining rapidly.
Deep traps might sound like a liability, since trapped charge is precisely what forms space charge accumulation. The subtlety lies in where and how the charge is held. By capturing carriers close to where they are injected and immobilizing them in a dense distribution of shallow-to-deep trap sites, the grafted molecules prevent large, coherent packets of charge from building up deep inside the insulation. The net effect, confirmed at the macroscopic scale, was the dramatic 85 percent reduction in space charge accumulation. Slower carrier mobility also translated directly into lower measured conductivity, since conduction in these polymers is largely a matter of charge transport through amorphous regions and along interfaces.
The second half of the story unfolds at the mesoscale, in the polymer’s crystalline morphology. When AOB was grafted onto the material, it acted as a nucleating agent, prompting the formation of more numerous, smaller spherulites and increasing the overall crystalline density. This refinement matters because crystalline and amorphous regions of a semicrystalline polymer have different dielectric constants. Wherever these phases meet, the mismatch in permittivity causes local distortion of the electric field, concentrating stress at phase boundaries. Using finite element simulations built on a Voronoi algorithm to model the realistic geometry of the spherulitic structure, the researchers quantified this effect: the morphological changes induced by AOB reduced the dielectric mismatch between the crystalline and amorphous phases, cutting interfacial electric field distortion from 43 percent to 21 percent.
What distinguishes this study from much of the prior literature is how these scales were connected. Rather than treating molecular, microscopic, and macroscopic observations as separate narratives, the team first established direct quantitative correlations within each specific scale and only then integrated them into a comprehensive model. The deep traps characterized at the molecular scale explain the suppressed injection and mobility seen in surface potential measurements; the refined spherulitic morphology measured at the mesoscale explains the homogenized internal field; and the combination of these two mechanisms accounts for the macroscopic outcomes. The synergy is the point. Trap capture alone would slow charge but leave field distortions intact, while morphology refinement alone would homogenize the field but do nothing about the sheer quantity of injected charge. Together, they attack the failure cascade from both ends.
The macroscopic results follow logically from that dual mechanism. With fewer carriers injected and those carriers immobilized, space charge accumulation collapsed by 85 percent. With carrier transport suppressed throughout the bulk, conductivity dropped by an order of magnitude, which in turn reduces Joule heating and thermal aging of the insulation. And with the internal field distribution smoothed by the refined crystalline structure, the material withstood a direct current breakdown strength 30 percent higher than the unmodified polymer before failure. Each of these metrics is directly relevant to cable design, where higher breakdown strength allows thinner insulation walls, and lower conductivity and space charge translate into longer service life and higher permissible operating temperatures and voltages.
Beyond the specific numbers, the study offers a template for rational materials design in electrical insulation. Instead of screening additives empirically, the framework demonstrated here, from molecular orbital engineering through trap spectroscopy and morphological modeling to macroscopic dielectric testing, provides a quantitative theoretical basis for predicting how a given chemical modification will perform. As grids worldwide expand high-voltage direct current transmission to move renewable electricity over long distances, the demand for recyclable, high-performance insulation will only grow. A modified polypropylene that can be grafted, characterized, and optimized with this level of mechanistic clarity brings that future measurably closer, and suggests that the next generation of power cables may be built on plastics engineered one molecular feature at a time.
Subject of Research: Multiscale structural modulation of grafted polypropylene for high-voltage direct current cable insulation
Article Title: Enhanced dielectric performance of polypropylene for hvdc cable insulation via 4‑allyloxybenzil grafting for multiscale structural modulation
Article References: Wu, Y., Li, Z., Wang, H., Cao, G., Zheng, Z., Du, H., & Du, B. (2026). Enhanced dielectric performance of polypropylene for hvdc cable insulation via 4‑allyloxybenzil grafting for multiscale structural modulation. Advanced Composites and Hybrid Materials. https://doi.org/10.1007/s42114-026-02115-9
Image Credits: AI Generated
DOI: 10.1007/s42114-026-02115-9
Keywords: polypropylene, HVDC cable insulation, dielectric properties, 4-allyloxybenzil, chemical grafting, space charge, deep traps, spherulites, breakdown strength, density functional theory, Kelvin probe force microscopy, multiscale modeling
News Source: Denise Maddox. (October 7, 2026). Grafted Molecule Tames Electric Fields in Next-Generation Power Cable Plastic. Scienmag.
