Degradation Behavior of Poly (Lactic Acid) during Accelerated Photo-Oxidation: Insights into Structural Evolution and Mechanical Properties

• Published in: Journal of polymers and the environment 2024-08, Vol.32 (8), p.3810-3821
• Main Authors: Liu, Xueping, Hua, Xiangdong, Wu, Hao
• Format: Article
• Language: English
• Subjects: Amorphous materials; Annealing; Biodegradation; Chain branching; Chain scission; Chemistry; Chemistry and Materials Science; Crystal structure; Crystallinity; Crystallization; Crystals; Degradation; Embrittlement; Environmental Chemistry; Environmental Engineering/Biotechnology; Evolution; Industrial Chemistry/Chemical Engineering; Materials Science; Mechanical properties; Molecular chains; Molecular structure; Original Paper; Oxidation; Oxidation process; Photodegradation; Photooxidation; Polylactic acid; Polymer Sciences; Polymers; Quenching
• ISSN: 1566-2543; 1572-8919
• DOI: 10.1007/s10924-024-03211-x

Poly (lactic acid) (PLA) has received considerable attention as a sustainable and biodegradable alternative to petroleum-based polymers in recent years. In general, the properties of PLA depend on its molecular chain structure, e.g., linear, branched, and aggregated structure, e.g., orthorhombic α-form and stereocomplex crystals. However, the evolution of hierarchical structure triggered by photo-oxidation degradation remain elusive for PLA. Herein, the accelerated photo-oxidation degradation behaviors of PLA samples with different thermal histories, including quenching (PLA-q), slow cooling (PLA-c), and annealing (PLA-a), were investigated by several characterization techniques. Compared to PLA-q and PLA-c, PLA-a exhibits relatively lower rates of molecular chain scission and oxygen-containing groups generation during the accelerated photo-oxidation process, suggesting that the increase in crystallinity contributes to suppressing the degradation of PLA. Changes in the molecular chain structure leads to the evolution of aggregation structure. The crystallinity of PLA samples, whether slowly cooled or annealed, increases with UV exposure time, which is attributed to the newly-formed crystals induced by chemi-crystallization. Unexpectedly, although PLA-a exhibits a slower photo-oxidation degradation rate than PLA-q and PLA-c, it undergoes embrittlement at an earlier stage. Morphological observations of photo-oxidized samples indicate that the degradation reaction of PLA-a occurs preferentially in the amorphous region, transforming the molecular chains into volatile products and eventually resulting in the embrittlement of PLA materials. This research sheds light on photo-oxidation degradation behaviors of PLA, and will serve as a valuable reference for investigating the degradation of other bio-based polymeric materials.