A Reactive Molecular Dynamics Model of Thermal Decomposition in Polymers: II. Polyisobutylene
S I. Stoliarov, R Lyon, Marc R. Nyden
This is the second manuscript in a series of publications continuing the development and application of a method for investigation of the thermal decomposition of polymeric materials. This method, which is called reactive molecular dynamics, belongs to a relatively new class of classical molecular dynamics techniques, where the force field methodology has been extended to modeling chemical reactions. In this paper, reactive molecular dynamics is used to investigate the thermal decomposition of polyisobutylene. In all of the experimental studies, homolytic scission followed by depolymerization were found to be important reactions in the thermal decomposition of polyisobutylene. The stable product of these reactions, the monomer, is the major product of the thermal degradation of this polymer under most experimental conditions. There are, however, notable disagreements between investigators regarding the importance of hydrogen transfer and termination reactions. These disagreements arise primarily from ambiguities in the mechanistic interpretation of experimental data, which consist of distributions of stable products accumulated during pyrolyses of the polymer. These data provide no direct information on highly reactive intermediates and elementary chemical reactions that operate in the polymer melt. In principle, the details of the thermal decomposition chemistry are captured in the reactive molecular dynamics simulations. Thus, they offer a unique outlook on this complex process. The results of these simulations indicate that, at least at high temperatures (1300-1750 K), reactions involving the transfer of hydrogen atoms do not play a significant role in the decomposition. Furthermore, the results indicate that the Arrhenius pre-exponential factor and activation energy of the key initiation reaction, backbone scission, depends on the size of the molecular model. This observation is probably the most important outcome of this study because it implies that the kinetics of some elementary reactions that take place in a polymer melt are influenced by the macromolecular nature of the environment.