Molecular Degradation Mechanism

In marked contrast to enzymatic degradation, the position of the ester group (chain end or within the chain) does not affect the hydrolytic degradability of PLA-based materials, as long as the polymers are composed of one monomer unit. An exception is the report on the acidic hydrolysis of amorphous poly(DL-lactide) (poly(DL-lactic acid) (PDLLA)) in the solid state by Shih [110,111], where the ester group adj acent to the chain terminal is more susceptible to hydrolytic degradation than that in the middle of the chain. Degradation at the chain end and in the middle of the chain is, respectively, called exo- 

TABLE 21.1 Reported Articles on Hydrolytic Degradation of PLA-Based Materials [14-265] 

Type Polymer Temp. (°C) Medium Specimen State Reference 

Acetylated L-lactic acid 37 Phosphate-buffered solution Solid [78] 

L-Lactic acid ohgomers 37 Acetonitrile - - buffer Dissolved [79] 

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In random degradation molecular mass decreases early, while in chain degradation the molecular mass of the polymer remains almost constant. Characterisation methods for molecular mass are thus very sensitive methods to follow random degradation. In contrast, as monomer is produced in chain depolymerisation, weight loss measurement techniques are the best methods to follow this kind of degradation. (Chapters 10-12, in Section IV, of this book focus on the methods used in the molecular characterisation and analysis of polymer degradation and polymer degradation mechanisms.)... 

Elastic collision, determined by mass ratio, is a very inefficient process. By default, it is the only available mechanism in rare gases. Rotations are not easily excited in nonpolar molecules, especially in the condensed phase. They can be a contributing factor in molecular gases (vide infra). In polar media, rotations are an important degradation mechanism (Frohlich and Platzman, 1953). 

Since the environmental degradation of polyethylene starts with abiotic oxidation, the determination of abiotic oxidation products is an important step towards establishing the environmental degradation mechanisms and environmental impact of the material. In a secondary process, microorganisms may utilise these abiotic degradation products and the low molecular weight... 

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