No abstract

We recently reported the preparation of a series of osmium(II) polypyridyl compounds.1 The compounds were remarkable because of the suggestion that the properties of their metal-to-ligand charge-transfer [MLCT; Os11 -* ir*(bpy) or (phen)] excited states including luminescence lifetimes, emission maxima, and redox potentials are systematically variable by making chemical changes.

We use the recently developed reactive force field ReaxFF with molecular dynamics to study thermal induced chemistry in RDX ͓cyclic-͓CH 2 N͑NO 2 )] 3 ] at various temperatures and densities. We find that the time evolution of the potential energy can be described reasonably well with a single exponential function from which we obtain an overall characteristic time of decomposition that increases with decreasing density and shows an Arrhenius temperature dependence. These characteristic timescales are in reasonable quantitative agreement with experimental measurements in a similar energetic material, HMX ͓cyclic-͓CH 2 N͑NO 2 )] 4 ]. Our simulations show that the equilibrium population of CO and CO 2 ͑as well as their time evolution͒ depend strongly of density: at low density almost all carbon atoms form CO molecules; as the density increases larger aggregates of carbon appear leading to a C deficient gas phase and the appearance of CO 2 molecules. The equilibrium populations of N 2 and H 2 O are more insensitive with respect to density and form in the early stages of the decomposition process with similar timescales.

To investigate the failure of the poly(dimethylsiloxane) polymer (PDMS) at high temperatures and pressures and in the presence of various additives, we have expanded the ReaxFF reactive force field to describe carbon-silicon systems. From molecular dynamics (MD) simulations using ReaxFF we find initial thermal decomposition products of PDMS to be CH3 radical and the associated polymer radical, indicating that decomposition and subsequent cross-linking of the polymer is initiated by Si-C bond cleavage, in agreement with experimental observations. Secondary reactions involving these CH3 radicals lead primarily to formation of methane. We studied temperature and pressure dependence of PDMS decomposition by following the rate of production of methane in the ReaxFF MD simulations. We tracked the temperature dependency of the methane production to extract Arrhenius parameters for the failure modes of PDMS. Furthermore, we found that at increased pressures the rate of PDMS decomposition drops considerably, leading to the formation of fewer CH 3 radicals and methane molecules. Finally, we studied the influence of various additives on PDMS stability. We found that the addition of water or a SiO2 slab has no direct effect on the short-term stability of PDMS, but addition of reactive species such as ozone leads to significantly lower PDMS decomposition temperature. The addition of nitrogen monoxide does not significantly alter the degradation temperature but does retard the initial production of methane and C 2 hydrocarbons until the nitrogen monoxide is depleted. These results, and their good agreement with available experimental data, demonstrate that ReaxFF provides a useful computational tool for studying the chemical stability of polymers.