Mechanistic Modeling of the Thermal Degradation of Hydroxyl-Containing Polymers. Part 1: Mechanism and Model Development

In part one of this two-part series, the method of moments (MoM) modeling framework for describing the complex chemistry of polymer decomposition is expanded to capture the evolving mid-chain topology of functionalized polymers, expanding the suite of depolymerization chemistries that can be described with MoM. This has been achieved by tracking the evolution of deterministically arranged dyad (DAD) concentrations as reactions consume and produce functional groups stoichiometrically. Additionally, reaction site probabilities are calculated using the ratios of concentrations of different DAD types, allowing reaction rates to be a function of the average structure in addition to temperature and species concentration. Two models were constructed, one for the pyrolysis of a homopolymer, poly(vinyl alcohol) (PVA), and another for the pyrolysis of a related copolymer, ethylene vinyl alcohol (EVOH), to demonstrate the robustness of this approach. This work demonstrates both the wide applicability of this methodology as well as the importance of including the influence of evolving polymer structure on the competing degradation chemistries that govern the depolymerization of these systems. This first installment focuses on uncovering the specific mechanistic pathways by which PVA and EVOH decompose under pyrolytic conditions, the generalized method for initializing and tracking the evolution of DADs, and the calculation of the expected concentrations of the reaction sites.