Mechanistic Modeling of Polymer Degradation:  A Comprehensive Study of Polystyrene

Abstract

The degradation of polystyrene was modeled at the mechanistic level by developing differential equations describing the evolution of the moments of structurally distinct polymer species. This work extends our previous modeling work by incorporating chain-length-dependent rate parameters, tracking branched species more explicitly, using rate parameters primarily from the literature, and comparing the model results to extensive experimental data on the degradation of polymers of different molecular weights and at different temperatures. Unique polymer groups were devised that allowed the necessary polymeric features for capturing the degradation chemistry to be tracked, while maintaining a manageable model size. The conversion among the species was described using typical free radical reaction types, including 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. The model included over 2700 reactions and tracked 64 species. Programs were developed using the programming language Perl to assemble moment equations from input of the polymeric features to be tracked. The intrinsic kinetic parameters (a frequency factor and activation energy for each reaction) were obtained from data in the literature and previous modeling work in our laboratory. − The model predictions for the evolution of M n and M w and the yields of styrene, dimer, and trimer compare very well with experimental data obtained in our laboratory for the degradation of polystyrene over a large temperature range and with different initial molecular weights. Evolution of low molecular weight products from experiments reported in the literature is also captured.

Composition of Liquid Fuels Derived from the Pyrolysis of Plastics

“…The pyrolysis product distribution is consistent with the same free radical decomposition mechanism that has been described for PS. Although the presumption explaining the minimal contribution of intermolecular radical transfer by the hindrance of the bulky phenyl group [31] is not confirmed. The nitrile group is not as large as phenyl, and about the half the substituents are nitriles in ABS.…”

“…The size and the chemical composition of the substituent influence the reactions of the secondary macroradical, the main reactive competitor of the thermal decomposition process in vinyl polymers. A bulky substituent such as the phenyl group in PS may hinder the intermolecular radical transfer by shielding the easily abstractable hydrogen atoms [31] while at the same time a phenyl group is able to give a higher stability to the secondary macroradicals by resonance over the aromatic ring. The secondary macroradical plays the main role in the pyrolysis process of several vinyl polymers [19,28] which is either depolymerized to monomer or transferred to the third and fifth carbon atom from the radical end of the macromolecule evolving dimer and trimer after β-scission, respectively, reproducing the secondary macroradical in a chain reaction as drawn in Scheme 12.4.…”

Development of materials and technologies for control of polymer recycling

“…Depending on experimental conditions, various oligomeric compounds such as dimer and trimer are formed as well as monomer St. 17 Sawaguchi and Seno 18 investigated simplified volatile products, mainly consisting of monomer, dimer and trimer in a low-temperature range of 310-350 1C under vacuum. They found that the lower the degradation temperature, the larger the trimer formed up to 76.5 wt%.…”

Kinetic Modeling of Semi‐Interpenetrating Polymer Network (SIPN) Process ‐ A Comprehensive Study on the Case of Polyethylene/Polystyrene Semi‐I IPN

“…Smoluchowski equation was adapted, [62] where the termination rate was assumed to be proportional to the average of the inverse chain length. In our study, to maintain the simplicity and validity of the model, a lumped rate is used in the form of Equation (63):…”

“…Finally, although an inversely proportional relationship between polymer chain transfer rate and size of the abstracting radical for branched polystyrene is suggested, [62] we do not consider the difference of chain transfer mobility for the polystyrene radical in this model, due to the (mostly) linear structure of the polystyrene chain. Instead an average reaction rate which follows Arrhenius relationship is applied for k fs .…”