(57a) Mechanistic Models of Polymer Degradation Chemistry

Pyrolysis is a promising method for resource recovery from plastic waste that thermally converts polymers in the absence of oxygen into valuable chemical feedstocks and monomer. It is also a detrimental process of degradation when polymeric materials are exposed to high temperatures. To provide further insight into polymer pyrolysis, a greater understanding of the mechanistic and kinetic details of the underlying reaction network is needed. To handle the complexity of mechanistic modeling of polymer degradation, we have developed both continuum and kinetic Monte Carlo (kMC) models. To facilitate creation of continuum models, we have formulated an automated modeling framework that assembles population balance models based on the method of moments from minimal user input. The population balances track the moments of a large number of radical and dead species, including monomer and small molecules that are relevant to degradation chemistry. We have applied this framework to study the degradation of polystyrene, polypropylene, polyisoprene, polystyrene peroxide and binary mixtures, and we are able to capture diverse experimental measures, including yields of individual low molecular weight products, as a function of reaction conditions. In order to solve the large models that are created, values of the rate coefficients for O(10⁵) reactions may be required. The approach that we have developed to specify rate coefficients is hierarchical, based on a combination of literature values, estimation methods, and computational chemistry.