COMPUTATIONAL EXPERIMENTS ON THE MIGRATION OF INTERNAL ENERGY IN MACROMOLECULAR SYSTEMS

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).