What role do terminal carboxyl groups play in the degradation of polylactic acid?

Polylactic acid (PLA) is a biodegradable and bioactive thermoplastic aliphatic polyester derived from renewable resources, typically corn starch or sugarcane. The degradation of PLA occurs through a combination of hydrolysis and oxidation, and the terminal carboxyl groups play a significant role in this process. Here’s how:

Catalytic Effect: The terminal carboxyl groups in PLA can act as catalysts for the polymer’s hydrolytic degradation. The carboxyl group can protonate water molecules, making them more reactive and thereby facilitating the attack on the ester bonds within the polymer chain.

Chain Scission: The carboxyl groups can also directly participate in chain scission reactions. The free carboxyl group can react with the ester linkage in the polymer backbone, leading to the formation of shorter polymer chains.

Lower Activation Energy: The presence of carboxyl groups can lower the activation energy required for the hydrolysis reaction to occur. This makes the degradation process more facile and faster.

pH Sensitivity: The degradation rate of PLA can be influenced by the pH of the environment. Carboxyl groups can protonate or deprotonate depending on the pH, which affects their reactivity and the rate of degradation. In acidic conditions, the carboxyl groups are protonated and may be less reactive, while in basic conditions, they can be deprotonated and more readily participate in degradation reactions.

Water Absorption: Carboxyl groups can enhance the water absorption of PLA, which is a critical step in the hydrolysis process. Water molecules can penetrate the polymer matrix more easily due to the polar nature of the carboxyl groups, leading to increased degradation.

Microbial Degradation: Terminal carboxyl groups can also play a role in the microbial degradation of PLA. Certain enzymes produced by microorganisms can recognize and bind to the carboxyl groups, facilitating the degradation of the polymer by breaking down the ester bonds.

Crosslinking: In some cases, carboxyl groups can participate in crosslinking reactions, which can initially slow down the degradation process by creating a more robust polymer network. However, once the crosslinks are broken, the degradation process can accelerate.

In summary, the terminal carboxyl groups in PLA are important for initiating and accelerating the degradation process by acting as catalysts, lowering the activation energy for hydrolysis, and influencing the polymer’s interaction with water and microbial enzymes. The degradation rate and mechanism can be significantly affected by the presence and reactivity of these functional groups.

Abdelhak Maghchiche

Terminal carboxyl groups in polylactic acid (PLA) facilitate degradation by catalyzing the hydrolysis of ester linkages, leading to autocatalysis. This process accelerates breakdown, especially in acidic environments, forming lactic acid and oligomers that diffuse out of the polymer matrix.

Chaitanya Sri

Terminal carboxyl groups play a crucial role in determining the degradation kinetics and pathways of PLA, influencing its environmental impact and application.

Xi Chen

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