Evidence for Crystalline Structure in Dynamically-Compressed Polyethylene up to 200 GPa

Abstract

We investigated the high-pressure behavior of polyethylene (CH2) by probing dynamically-compressed samples with X-ray diffraction. At pressures up to 200 GPa, comparable to those present inside icy giant planets (Uranus, Neptune), shock-compressed polyethylene retains a polymer crystal structure, from which we infer the presence of significant covalent bonding. The A2/m structure which we observe has previously been seen at significantly lower pressures, and the equation of state measured agrees with our findings. This result appears to contrast with recent data from shock-compressed polystyrene (CH) at higher temperatures, which demonstrated demixing and recrystallization into a diamond lattice, implying the breaking of the original chemical bonds. As such chemical processes have significant implications for the structure and energy transfer within ice giants, our results highlight the need for a deeper understanding of the chemistry of high pressure hydrocarbons, and the importance of better constraining planetary temperature profiles.

High Pressure Hydrocarbons Revisited: From van der Waals Compounds to Diamond

“…A similar experiment on polyethylene ([C 2 H 4 ] n ) should in principle give very different answers from polystyrene: the stability of CH 2 should prevent diamond formation up to 280 GPa at low temperature. Indeed that seems the case, as polystyrene has been reported as a stable solid up to 190 GPa [40]. The interpretation of dynamic compression experiments and static lattice calculations is further compounded by possible time scale effects: the formation of stable reaction products (diamond or other hydrocarbons) might not be possible on the experimental time scale (about 10 ns), as it involves major rearrangements of carbon-carbon and carbon-hydrogen bonds.…”

“…However, these decompositions are not favoured by much: 30 meV/CH 2 at 100 GPa, and only 13 meV/CH 2 at 260 GPa. These small enthalpy differences might explain why compressed CH 2 in experiments cannot overcome kinetic barriers towards diamond formation, and remains in a polyethylene phase up to almost 200 GPa [40]. However, diamond and pure hydrogen have a much larger enthalpy-pressure gradient, and the enthalpic drive towards decomposition of CH 2 increases strongly above 280 GPa; this should make diamond formation in polyethylene samples much more likely above 300 GPa.…”

“…Recent shock experiments on polystyrene ([C 8 H 8 ] n ) [37,38] reported its decomposition into diamond, however only above 140 GPa and 4000 K in an apparent disagreement with diamond anvil cell experiments [23,24]. On the other hand, compressed polyethylene did not form diamond up to 200 GPa and 5000 K, instead retaining a crystalline polyethylene structure up to 190 GPa and 3500 K [40]. Kraus et al [37] compare their polystyrene measurements to the diamond formation conditions found in static methane compression as well as an extrapolation of the diamond formation line (C 4 H 10 + 3 H 2 → 4 C + 8 H 2 ) obtained in first-principles calculations [18].…”

“…Dynamic compression experiments on hydrocarbons are used to probe their properties at combined high-pressure and -temperature conditions [33][34][35][36][37][38][39][40], both to investigate their potential decomposition, but also to better understand a critical ablator material used in inertial confinement fusion experiments. The thermal equation of state of hydrocarbons has therefore been a subject of several ab initio molecular dynamics studies [41][42][43][44][45][46].…”

Dynamics of bond breaking and formation in polyethylene near shock front

“…Recently, phase separation of carbon and hydrogen in hydrocarbon materials has attracted much attention [13,40,41]. It is considered as a necessary premise for the formation of diamonds.…”