2025 AIChE Annual Meeting

(516e) Upcycling of Aliphatic Polyesters: Mechanistic Insight into Polylactic Acid Degradation

Authors

Saleta Powell, University of South Florida
Elamthal Abuobaida, University of South Florida
Weizhong Zou, Massachusetts Institute of Technology
In the year 2024 alone, approximately 500 million tons of plastics were produced globally. With less than 10% being recycled, a majority of these plastics will be landfilled or released to our ecosystem where it naturally takes 20 to 500 years to complete their decomposition. This, unfortunately, results in significant waste accumulation and social-environmental loss. As an appealing solution to the above plastic crisis, the use of biodegradable polymers, such as poly lactic acid (PLA), has sparked great interest due to their biocompatible, non-toxic and renewable properties. However, the widespread application of PLA is limited by poor resistance to thermo-degradation and insufficient mechanical strength at elevated temperatures. Thus, substantial efforts are needed to understand the thermal induced degradation mechanism and kinetics of PLA.

Our research therefore aims to quantify how thermo-degradation affects the microstructure of PLA and how this can be tuned in favor of recyclability and mechanical performance. Given the complexities in reaction pathways, stereochemistry, and solvation, quantum theory was used to evaluate the kinetics of chain scission and transesterification and their relationships to molecular weight and architecture of molten PLA. Our preliminary results showed the thermal radical induced b-scission at the chain end is kinetically inefficient given the high energy barrier (~140 kJ/mol) to produce end-shredded PLA with small molecules (CO and acetaldehyde) at mild reaction temperature (500K). A further computational investigation on transesterification suggests an alternative degradation mechanism with the formation of oligomeric rings, whose size and thermal stability can be regulated by the type and the state of metal ions. The configuration of the rings and the energy of transforming a linear segment of PLA into a cyclic form (termed ring strain energy) are found to be strongly affected by the stereochemistry and the solvation. These findings are of particular importance in rebuilding PLA chain via “reopening” of the aforementioned cyclic structures and in copolymerization to create more thermally stable PLA chains.