Equations of state for polyethylene and its shock-driven decomposition products

Abstract

We construct new equations of state (EOS) for high density and ultrahigh molecular weight polyethylene and their chemical decomposition products under shock loading. The former were built using the SESAME framework, based in part on new specific heat and thermal expansion data reported here. The products EOS was based on thermochemical modeling under the assumption of full thermodynamic and chemical equilibrium. The products are represented as the ideal mixture of bulk carbon in the form of diamond, H2, H, and CH4. In the process of building a new EOS for the products, we recalibrated our exponential-6 pair potential for methane in order to better agree with data that have appeared since its original parameterization. The polyethylene EOS were calibrated to thermal, thermomechanical, and shock data, and their performance was evaluated in hydrodynamic modeling of deep release experiments reported previously.

Diamond and methane formation from the chemical decomposition of polyethylene at high pressures and temperatures

“…This allowed XRD to obtain the bulk moduli of the products and to detect crystalline methane, which is fluid under both decomposition and ambient conditions, in the product mixture. While our results provided no means to estimate the relative product fractions, the methane and diamond constituents of the product mixture are in good agreement with thermodynamic and chemical equilibrium predictions 10 . In contrast, polyethylene dynamically compressed to P > 100 GPa and temperatures between 2000 and 4000 K for times on the order of a few nanoseconds did not decompose 12 .…”

“…Although there is limited data on polyethylene decomposition under static high pressure conditions, investigations of polyethylene under dynamic shock loading have been performed using both gas gun driven impactors and laser drives. Under the assumption of full thermodynamic and chemical equilibrium, EOS modeling of HDPE’s shock driven decomposition products predicts the formation of equal mole fractions of diamond and methane at 25 GPa and the gradual replacement of methane with hydrogen for increasing pressures 10 . Experiments using gas gun driven impactors, which maintain high P–T conditions for hundreds of nanoseconds, observed a shock decomposition threshold of 25 GPa and products recovered from shocks reaching 28–40 GPa consisted of methane, hydrogen gas, and carbon soot 11 .…”

Shock-Driven Decomposition of Polymers and Polymeric Foams

“…Similar results were reported for PTFE, but the PE case is particularly notable due to it representing an extreme: because its chain structure is so clean, its volume collapse upon reaction is negligible and there is only a subtle change in slope in U−u. One would expect any accompanying temperature rise to be small and, indeed, preliminary calculations indicate that it actually cools upon decomposition [96]. The fact that only full decomposition products are recovered above the cusp would at least suggest a fortiori that this be the case for other polymers in which the volume collapse is larger.…”