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Kinetics and Mechanisms of Elementary Reactive Processes in Polymer Pyrolysis.


pdf icon Kinetics and Mechanisms of Elementary Reactive Processes in Polymer Pyrolysis. (202 K)
Knyazev, V. D.

NIST GCR 09-923; 14 p. March 2009.

Sponsor:

National Institute of Standards and Technology, Gaithersburg, MD

Keywords:

polymers; pyrolysis; kinetics; polyethylenes; dissociation rates; prototypes; molecular dynamics; decomposition; melting

Abstract:

Polymer backbone scission reactions were studied computationally using polyethylene macromolecules as prototypes. Classical RRKM modeling, molecular dynamics, transition state theory, and molecular mechanics were used as research tools. The results demonstrate significant effects of polymer chain length and conformation on backbone dissociation rates. The results of molecular dynamics modeling performed for single molecules in vacuum show that per-bond rates of C-C bond dissociation increase with the increasing alkane chain length. Moreover, per-bond rates of C-C bond dissociation further increase (by orders of magnitude) in the cases where the motion of polymer chain ends is restricted. Effects of macromolecular surroundings under the conditions of polymer melt on the rates of carbon-carbon bond scission reactions were observed: rate constants obtained under the condensed phase conditions are approximately an order of magnitude lower than those of single molecules in the gas phase. The condensed phase rate constants display a strong dependence on the polymer melt density, with lower density resulting in larger rate constant values. Research was started on the kinetics of the reactions of H atom transfer: chain radical isomerization and inter-chain H atom transfer. An experimental study of the initial stages of polyethylene pyrolysis was started using 1H NMR and GC/MS as analytical tools applied to the condensed phase and the gas phase products, respectively.