Detailed mechanistic modeling of polymer degradation: Application to polystyrene

The degradation of polystyrene was modeled at the mechanistic level using population balance equations formulated via the method of moments. Five degradation models of varying complexity were developed. For all models, the conversion among the species was described using typical free radical reaction types, including hydrogen abstraction, mid-chain β-scission, end-chain β-scission, 1,5-hydrogen transfer, radical addition, bond fission, radical recombination, and disproportionation. The five models differed in their resolution of the structural characteristics of the "dead" and "live" polymeric species and whether they explicitly tracked low molecular weight species. The most complex model included over 4500 reactions and tracked 93 polymeric and low molecular weight species, both dead and live species. To facilitate model construction, programs were developed using the programming language Perl to assemble population balance equations from specific reaction mechanism input. The intrinsic kinetic parameters (a frequency factor and activation energy for each reaction) were obtained from previous modeling work in which the decomposition of a polystyrene mimic was described at the mechanistic level (Woo, Ph.D. dissertion, Northwestern University, 1999; Woo and Broadbelt, 1998, Catal. Today 40, 121) to link reactivity directly to structure. As the complexity of the models increased to include branching reactions and branched species, the model predictions improved. The separation between M_n and M_w observed experimentally was reproduced well for the three most complex models. The general modeling framework established may be easily extended to other single-component and multicomponent polymeric systems.