Degradation modelling random scission

Staggs, J. E. J. Modelling random scission of linear polymers. Polymer Degradation and Stability 2002 76 37. 

A better model of thermal degradation by depropagation may be obtained if we distinguish between polymer molecules and radical fragments in the population. Here, we consider radical species being formed from an initial random scission reaction. Once formed, the radical species then undergo... 

Barbuzzi et al. [28] analyzed a copolymer with 3-hydroxybutyrate (3HB) and 3-mercaptopropionate (3MP) or 3-mercaptobutyrate (SMB) units by ESI-MS. The samples were partially methanolyzed.The methanolysis process can be modeled as a random scission, which is known to broaden the MMD and, in fact, the polydispersity index is not small. For obvious reasons, it would be interesting to analyze the native copolymer prior to partial degradation by methanolysis. 

Kinetic studies have been made on the thermal decomposition of a poly(oxypropylene)triol-toluene di-isocyanate copolymer foam. Following a diffusion rate-controlled step, the cellular structure collapses to a viscous liquid and degradation then occurs on a random scission basis. Products of degradation of A-monosubstituted and A A-disubstituted polyurethanes have been analysed by direct pyrolysis in the ion source of a mass spectrometer. The mono-substituted polymers depolymerize quantitatively to di-isocyanates and diols, whereas the disubstituted materials decompose selectively to secondary amines, olefins, and carbon dioxide. The behaviour of the monosubstituted polymers has been confirmed in an i.r. study of the degradation of model compounds. A study of the thermal degradation in vacuum of polyurethanes prepared from butanediol, methylene bis(4-phenylisocyanate), and hexanedioic acid-ethylene glycol-propylene glycol polyesters has been reported and reaction mechanisms proposed. ... 

A mathematical model to describe the molecular weight and pofydispersity index in poly (L-lactide) (PLLA) thermal degradation has been developed [10], Based on the random scission mechanism, effects of temperature and time on the molecular weight and polydispersity index are included in this model. 

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... 

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).

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. 

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