Modeling the evolution of the full polystyrene molecular weight distribution during polystyrene pyrolysis

The degradation of polystyrene was modeled at the mechanistic level using the method of moments to track structurally distinct polymer species. To keep the model size manageable, polymer species were lumped into groups, and within these groups, the necessary polymeric features for capturing the degradation chemistry were tracked. The pyrolysis reactions incorporated into the model included hydrogen abstraction, midchain β-scission, end-chain β-scission, 1,5-hydrogen transfer, 1,3-hydrogen transfer, radical addition, bond fission, radical recombination, and disproportionation. From the evolution of the zeroth, first, and second moments tracked for each dead species, polymer molecular weight distributions were constructed by summing the Schultz (Teymour, F.; Campbell, J. Macromolecules 1994, 27, 2460) and Wesslau (Pladis, P.; Kiparissides, C. Chem. Eng. Sci. 1998, 53 (18), 3315) distributions for the polymer groups. Model results were compared to experimental data collected in our laboratory, where polystyrene samples that differed in the shape and breadth of their initial distributions were pyrolyzed. The model was able to predict the formation of a bimodal distribution during the pyrolysis of polystyrene samples (molecular weight range of 10 000 - 500 000 g/mol) with narrow unimodal molecular weight distributions (polydispersity index < 1.1). This was accomplished by distinguishing the initial polymer from the polymer formed from midchain β-scission reactions within the model. At high conversions, all of the polystyrene samples investigated evolved to unimodal distributions, and these distributions were best captured by the Schultz distribution.