Degradation of Lactic Acid-based Polymers

Basically, lactic acid-based polymers can be degraded chemically or enzymatically. Although some exotic enzymes can degrade lactic acid stereocopolymers in vitro [54] and occasionally some microorganisms in the environment [55, 56], these aliphatic polyester chains are primarily degraded by the hydrolytic action of absorbed water molecules, carboxylic chain ends and any acid or base additive or impurity acting as an occasional catalyst [26]. In the case of chain end catalysis, one talks of autocatalysis, a well-known phenomenon in organic chemistry that has particularly complex consequences in the case of polymers [15]. 

The effects of chemical composition, MW, polydispersity, chiral repeating unit distribution and crystallinity are well documented in the literature [62]. However, one of the consequences of the general mechanism of hydrolytic degradation described above is the existence of dramatic differences of behaviour depending on many other factors (Table 9.2). 

Gross composition of L- and D-lactate units Distribution of chiral units Molar mass 

Distribution of molar mass Intrachain defects Mechanism of chain growth Extent of purification 

Presence of low molar mass additives and/or monomer and solvent residues, and/or acidic, basic or buffering low molar mass molecules Glass transition temperature 

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The hydrolysis of the polymers leads to molecular fragmentation. The hydrolytic degradation of lactic acid based polymers is an undesired phenomenon under certain circumstances, such as during processing or material storage. It is however, beneficial in other applications, for example in medical devices or compostable packages (12). 

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