Degradation Behaviour of PLA

It was suggested by Aoyaggi et al. [5] in accordance with previous works [6] that the thermal degradation behaviour of PLA was very complex because various reactions occurred concurrently. Py-GC/MS analysis indicated acetalaldehyde, lactide monomer and oligomers of lactide as products of PLA pyrolysis. 

The hydrolytic degradation behaviour of aliphatic polyesters depends on many factors such as matrix morphology, stereocomplexation, PLA configuration, chemical composition, size and porosity, dmg loading, etc. (Li, 1999,2006 Li and Vert, 1999). The influence of the factors on the degradation is discussed in the following sections. 

The hydrolytic degradation of PLAs and copolymers has been intensively studied, and, as a result, there are more than 250 papers related to this issue [188,189]. The degradation mechanism, behaviour, and rate depend on material- and media-related factors, which are summarized in Table 8.3. In addition, the methods used to monitor the hydrolytic degradation of biodegradable polymers are summarized in Table 8.4. Detailed information regarding the hydrolytic degradation of PLA-based materials can be found in the related review articles [188-190],... 

Another problem apart from the melt degradation is that PLA has a low crystallization rate which results in an amorphous material after processing. This affects of course the number of possible commercial applications for PLA. It has, however, also been shown that the thermal history is important. PLA has completely different crystallization behaviour when cooled from the melt compared to when heated from the solid (66). This aspect is relevant for selecting suitable post-processing steps to increase the crystallinity inside the... 

In other studies, Lee et al and Paul et al. have found an increased PLA biodegradation rate in compost due to the presence of nanoparticles. This behaviour was attributed to the high relative hydrophiheity of the clays, allowing an easier permeability of water into the bulk material, thus possibly accelerating the hydrolytic degradation process of the polymer matrix. 

In this study, lactide was polymerized by Sn-oct in the presence of polyfunctional alcohols such as glycerol or pentaerythritol. The resulting PLA had multi-armed chains to yield a star-shaped architecture. The microstructure, thermal properties, and degradation behaviour were studied to compare the effect of the different architecture, linear PLA with starshaped one. In addition, it was very beneficial to study the effect of various chain end groups on the thermal and hydrolytic stability of this multi-armed PLA. Therefore, OH terminal groups of PLA were converted into Cl, NH2 or COOH groups. 

Except for the nanocomposite prepared with qC16SAP, the degree of degradation was not different for other samples. This observation indicates that MMT or alkylammonium cations, as well as other properties, has no effect on the biodegradabiUty of PBS. The accelerated degradation of PBS matrix in the presence of qClbSAP may be due to the presence of alkylphosphonium surfactant. This kind of behaviour was also observed in the case of PLA-based nanocomposites as described in the previous seetion. 

Thus, an increase in polymer crystallinity should generally decrease polymer degradation rates, as extensively reported for PLA matrices under alkaline hydrolytic degradation conditions.Other researchers reported accelerated hydrolysis in neutral media with increasing polymer crystallinity for PLLA and PLLA/PDLA blends. A possible explanation to this behaviour has been related to the consideration that, upon crystallization of PLLA, hydrophilic terminal groups (-OH and -COOH) can be condensed in the amorphous area between the crystalline regions (see... 

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