Molecular
Dynamics Simulation of C–C Bond Scission
in Polyethylene and Linear Alkanes: Effects of the Condensed Phase
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Abstract
The
reaction of C–C bond scission in polyethylene chains
of various lengths was studied using molecular dynamics under the
conditions of vacuum and condensed phase (polymer melt). A method
of assigning meaningful rate constant values to condensed-phase bond
scission reactions based on a kinetic mechanism accounting for dissociation,
reverse recombination, and diffusional separation of fragments was
developed. The developed method accounts for such condensed-phase
phenomena as cage effects and diffusion of the decay products away
from the reaction site. The results of C–C scission simulations
indicate that per-bond rate constants decrease by an order of magnitude
as the density of the system increases from vacuum to the normal density
of a polyethylene melt. Additional calculations were performed to
study the dependence of the rate constant on the length of the polymer
chain under the conditions of the condensed phase. The calculations
demonstrate that the rate constant is independent of the degree of
polymerization if polyethylene samples of different lengths are kept
at the same pressure. However, if instead molecular systems of different
polyethylene chain lengths decompose under the conditions of the same
density, shorter chains result in higher pressures and lower rate
constants. The observed effect is attributed to a higher degree of
molecular crowding (lower fraction of free intermolecular space available
for molecular motion) in the case of shorter molecules