Bg. Sumpter et Dw. Noid, COMPUTATIONAL EXPERIMENTS ON THE MIGRATION OF INTERNAL ENERGY IN MACROMOLECULAR SYSTEMS, Chemical physics, 186(2-3), 1994, pp. 323-353
The mechanistic details of internal energy flow in models of polyethyl
ene containing up to 300 000 atoms (with explicit treatment of hydroge
ns) is discussed. The intra- and intermolecular dynamics of the macrom
olecular systems is studied as a function of CH-stretch excitation, te
mperature, pressure, crystal structure, and phase (solid, melt, or gas
) by employing the quasiclassical trajectory method coupled with compu
tational neural networks. The rate of energy flow from local and norma
l CH stretching modes is found to be very rapid and irreversible, occu
rring on a time scale of less than 1.0 picosecond at low temperatures
and increases with rising temperature. The flow of energy follows a pa
thway that traces out multiple stages, with an initial rapid flow due
to the decay of the excitation followed by a slower flow related to re
distribution throughout the system. The mechanism for the facile energ
y flow is shown to involve strong nonlinear couplings dominated by a C
H-stretch/HCH bend Fermi (1:2) resonance. This strong dynamical intera
ction facilitates the overall process of energy flow away from CH stre
tching sites in all of the various macromolecular systems that were ex
amined. A second type of energy relaxation process is observed in the
long-time dynamics which demonstrates two primary components: the time
required to redistribute the initial energy intramolecularly (within
a chain) and the time associated for complete redistribution among all
of the available vibrational modes (intermolecularly, chain to chain)
. Intramolecular redistribution occurs on a 2 picosecond time scale wh
ile the intermolecular process requires up to 270 ps (two orders of ma
gnitude longer). However, both processes are coupled, even on a picose
cond time scale, thereby leading to intermolecularly assisted intramol
ecular energy transfer. Overall, the results demonstrate that there ar
e invariant pathways for energy redistribution in polyethylene (marked
by strong nonlinear couplings such as Fermi resonances). However, due
to the differences in the intermolecular interactions for the various
environments (different phases), the processes occur over a range of
time scales. A thermal conductivity of 0.253 J/(K m s) and a rate for
heat diffusion of 1.6 km/s were determined for a highly crystalline mo
del of polyethylene based on the simulations. The thermal conductivity
tends to decrease substantially as the model is allowed to have more
amorphous content and approaches a value near 0. 15 J/ (K m s).