Random chain scission model

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).

For the initiation step, the free radicals formed may consist of one free radical chain plus one monomeric free radical, one free radical chain plus one low molecular weight free radical different from the monomer, or may consist of two free radical chains. The random chain scission could take place truly randomly or at the weaker link. Some possibilities are exemplified below with poly-isoprene taken as the model ... 

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 ... 

Computer modelling for polyesters — end chain scissions and random chain scissions... 

Although the success of the proposed model, there is no spectral evidence for the presence of cis double bonds in the yellow solution. Moreover, all attempts to produce the cis-structure by photo-induced or catalytic cis trans isomerisation led to a random chain scission rather than the intended structure. Probably increased amount of the ciis-double bonds decrease the stability of the polymer chain due to the repulsive interaction between the side groups and cause the observed chain scission. 

The random chain scission occurs by photocleavage of the weak N—O bond (53kcalmol ). The photolysis mechanism has been studied using a low molecular weight model compound, i.e. benzophenone oxime-diphenyl urethane (4.76). 

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. 

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