Degradation modelling random chain scissions

The model chosen to describe the degradation of polyethylene was random chain scission. Lenz (3,) in his section on degradation reactions of polymers cites work which supports the contention that polyethylene does thermally degrade in a random chain scission manner as opposed to depolymerization. For this model a statistical treatment has been developed by Montroll and Simha (). The extent of reaction may be related to the number average molecular weight by ... 

Figure 8.6 Time evolution of oligomer concentrations during poly(lactic acid) degradation in aqueous solution at 100°C. Symbols experimental data. Lines random chain scission model model dashed) and preferential chain end scission model continuous).

In conclusion, it appears that the hydrolytic degradation of PLA in liquid media fits a model based on random chain-scission. The rate increases when the polymer is softened or dissolved. The rate is relatively insensitive to pH of the medium. 

Several theoretical models have been published which predict the change in MWD during degradation especially for a random mechanism. For detail, the reader is referred to specialized books (see, for example, Conley [2]). Kotliar, using Monte Carlo techniques, calculated the changes in distribution for pol)miers with Schulz-Zimm distributions of varying initial breadth. He considered random scission [3-6] and scission plus cross-linking. He concluded that an increase in amount of chain scission for a broad MWD, M jMn > 2, results in a decrease in the MWD, for the case of random... 

It was suggested that the proposed model may be used to predict rates of mass loss for a vaporising polymer and also that the model may be extended to investigate other bondbreaking processes such as simultaneous random and end-chain scission, or the inclusion of recombination during degradation. 

---