Squeezing Polypropylene Flips Its Insulation From Rubbery to Glassy Conduction

Deep beneath the ocean surface, where 500-kilovolt submarine cables carry power across continents and 10,000-meter-class soft robots probe the hadal trenches, insulating polymers endure pressures that no laboratory bench test at atmospheric conditions can approximate. A new study of polypropylene, one of the world’s most widely used dielectric materials, now reveals exactly what happens to its internal structure and its electrical conduction behavior as hydrostatic pressure climbs from everyday conditions to 600 megapascals, roughly six thousand times atmospheric pressure. The work, published in Polymer Bulletin by a team at Harbin University of Science and Technology together with a collaborator at Tongji University, provides the kind of quantitative data that engineers designing deep-sea electrical systems have long lacked.

The research team, led by Weizhuo Li with Xuan Wang and Yewen Zhang as corresponding authors, approached the problem from a deliberately pressure-oriented perspective. Rather than treating pressure as a secondary variable, they built a custom apparatus in which pressure itself is the primary experimental knob, allowing them to track both the mechanical compression of polypropylene and its electrical conduction current as the material is squeezed continuously from a rubbery state into a glassy one. This continuous pressure domain is the crucial innovation. Earlier studies of polymer glass transition under pressure typically examined discrete pressure points, leaving engineers without a smooth, predictive picture of how properties evolve between those points.

The measurements spanned a pressure range from one atmosphere to 600 megapascals and a temperature range from 20 to 50 degrees Celsius for the volume-pressure data, while conduction current was tracked from one atmosphere up to 300 megapascals. From these isothermal compression curves, the researchers extracted the glass transition pressure of polypropylene across the temperature window, finding values between 229.63 and 290.15 megapascals for the 20 to 50 degree Celsius range. In other words, at room temperature, polypropylene does not become glassy because it is cooled below its conventional glass transition temperature; it becomes glassy because the surrounding pressure compresses its molecular framework until segmental motion freezes, a phenomenon known as an isothermal, pressure-induced glass transition.

To understand what this transition means at the molecular scale, the team turned to two established theoretical frameworks: the Tait equation of state, a classical empirical description of polymer compressibility, and the locally correlated lattice model, a more recent statistical-mechanical treatment of polymer thermodynamics. Combining the measured volume-pressure data with these models, they calculated the critical free volume fraction of polypropylene at the glass transition to be between 11.83 and 13.88 percent. Free volume is the unoccupied space between polymer chains, the voids that allow chain segments to wiggle, rotate, and rearrange. When hydrostatic pressure squeezes that void space below a critical threshold, the cooperative segmental motions that define the rubbery state can no longer occur, and the material vitrifies even though its temperature has not changed.

The most striking result concerns what this structural change does to electrical conduction. The researchers found that polypropylene exhibits piecewise conduction behavior, with a sharp change in mechanism at the pressure-induced glass transition. In the low-pressure rubbery regime, charge transport follows thermally activated hopping-like conduction, the familiar behavior in which charge carriers hop between localized sites with an energy barrier, and the rate of hopping is governed primarily by thermal energy. This is the regime in which nearly all conventional insulation testing is performed, and it is the regime that most design rules implicitly assume.

Above the glass transition pressure, however, the conduction mechanism transforms into variable-range hopping, a fundamentally different transport regime first described by Mott for disordered systems. In variable-range hopping, carriers do not simply hop to the nearest neighboring site over a fixed energy barrier; instead, they tunnel to more distant sites that happen to be energetically favorable, with the optimal hopping distance and energy trade-off depending on temperature in a characteristic non-Arrhenius way. The dense, frozen glassy structure changes the landscape of localized states available to charge carriers, favoring this longer-range tunneling process over simple nearest-neighbor thermally activated hops. For insulation engineers, the practical consequence is that the mathematical models used to predict leakage current and charge accumulation at atmospheric pressure may simply not apply at abyssal depths.

The magnitude of the change is significant as well as mechanistic. The team measured that the steady-state conductivity of polypropylene in the high-pressure glassy state is approximately 36 percent lower than the conductivity in the rubbery state at atmospheric pressure. Lower leakage current is, on its face, good news for insulation designers, since it means less dissipative loss and potentially slower charge accumulation under deep-sea operating conditions. But the 36 percent figure comes bundled with a change in the underlying transport law, which means the temperature dependence, the field dependence, and the long-term aging behavior of the material all shift in ways that must be modeled with the correct glassy-state physics rather than extrapolated from rubbery-state data.

Within the experimental pressure domain, the researchers went a step further and established a direct quantitative relationship between conductivity and the free volume fraction of the polymer. This linkage is the conceptual heart of the paper. It ties together two threads that have usually been studied separately: the thermodynamics of the glass transition, expressed through free volume theory tracing back to the classic Turnbull and Cohen model of the amorphous phase, and the dielectric physics of charge transport in insulating polymers. By expressing conductivity as a function of free volume fraction, the study offers a single material variable that engineers can track across the entire pressure range, from a submarine cable’s atmospheric-pressure factory tests to its 100-megapascal service environment at full ocean depth.

The motivation for this level of detail is not academic. China’s National Natural Science Foundation and the Natural Science Foundation of Heilongjiang Province funded the work, and the authors explicitly frame it as support for the insulation design of power systems in deep-sea underwater robots and for improving the long-term stability of polymer devices in high-pressure environments. The deep ocean presents a uniquely unforgiving combination of stresses: at a depth of 10,000 meters, hydrostatic pressure approaches 100 megapascals, temperatures hover near 2 degrees Celsius, and equipment must operate reliably for years without maintenance. Self-powered soft robots that have already operated in the Mariana Trench demonstrate that the technology exists, but the insulating materials inside their electronics and power systems are still largely characterized using terrestrial assumptions.

What the new study delivers is a map of the terrain. It tells designers where, along the pressure axis at a given temperature, polypropylene crosses from a rubbery, thermally activated conductor into a glassy, variable-range hopping conductor; it quantifies the free volume threshold that governs that crossing; and it measures the resulting conductivity change. The authors suggest that these findings also offer a new research perspective for performance optimization and material selection of polymer insulating materials under extreme conditions more broadly, implying that the pressure-oriented methodology could be extended to other dielectrics such as cross-linked polyethylene or fluoropolymers used in submarine and aerospace applications. As humanity pushes its electrical infrastructure deeper into the ocean and its instruments into ever more extreme environments, the lesson of this work is clear: the glass transition is not only a temperature phenomenon, and the polymers that insulate our machines behave in ways that only reveal themselves under pressure.

Subject of Research: Pressure-induced glass transition, free volume changes, and electrical conduction mechanism shifts in polypropylene under high hydrostatic pressure

Article Title: Pressure-induced changes in free volume and conductive property transitions of polypropylene

Article References: Li, W., Luan, T., Dong, Z., Wang, X., & Zhang, Y. (2026). Pressure-induced changes in free volume and conductive property transitions of polypropylene. Polymer Bulletin, 83(11), Article 616. https://doi.org/10.1007/s00289-026-06578-0

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06578-0

Keywords: polypropylene, high pressure, glass transition, free volume, electrical conductivity, variable-range hopping, polymer insulation, deep-sea cables, Tait equation of state, dielectric materials, submarine cables, underwater robots