A critical component of any mathematical model that attempts to describe the ignition or combustion of a polymer is an expression, or set of expressions, which determines the rate at which combustible products evolve from the solid. Traditional approaches to modelling thermal degradation of solid polymers in the context of combustion have involved the use of sets of rate equations of Arrhenius form. Unfortunately there is little scientific justification for this - other than a posteriori when model predictions are compared with experimental thermogravimetric analyses. In this paper a different approach is taken. Starting from the premise that an individual polymer molecule undergoes scission at a random location, a governing set of differential equations may be derived which determines how the MW distribution evolves in time. Other scission mechanisms, such as end-chain scission, can also be explored using the same technique. Also one can explore unusual scission mechanisms such as θ-scission, where a molecule breaks into two smaller molecules with MW in the ratio θ/(1-θ), or end-bond weighted random scission. The model is an extension of a simpler model published in an earlier paper. Exact solutions are possible for random scission and end-chain scission; these are discussed and compared with experimental thermogravimetric curves.
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