Computational experiments on the migration of internal energy in macromolecular systems

Abstract

The mechanistic details of internal energy flow in models of polyethylene containing up to 300 000 atoms (with explicit treatment of hydrogens) is discussed. The intra- and intermolecular dynamics of the macromolecular systems is studied as a function of CH-stretch excitation, temperature, pressure, crystal structure, and phase (solid, melt, or gas) by employing the quasiclassifical trajectory method coupled with computational neural networks. The rate of energy flow local and normal CH stretching modes is found to be very rapid and irreversible, occuring on a time scale of less than 1.0 picosecond at low temperatures and increases with rising temperature. The flow of energy follows a pathway that traces out multiple stages, with an initial rapid flow due to the decay of the excitation followed by a slower flow related to redistribution throughout the system. The mechanism for the facile energy flow is shown to involve strong nonlinear couplings dominated by a CH-stretch/HCH bend Fermi (1:2) resonance. This strong dynamical interaction facilitates the overall process of energy flow away from CH stretching sites in all of the various macromolecular systems that were examined. 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 while the intermolecular process requires up to 270 ps (two orders of magnitude longer). However, both processes are coupled, even on a picosecond time scale, thereby leading to intermolecularly assisted intramolecular energy transfer. Overall, the results demonstrate that there are invariant pathways for energy redistribution in polyethylene (marked by strong nonlinear coupling 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 model 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).