Influence of pH and Temperature on the Kinetics of Polylactic Acid Hydrolysis

This study investigates the hydrolytic degradation of polylactic acid (PLA) under different operating conditions to define the microkinetics of polymer decomposition through hydrolysis. The experimental study accounts for the effect of various factors such as temperature, pH, and initial molar mass of the PLA analyzed. High-performance liquid chromatography (HPLC) was adopted to track the concentrations of reagents and degradation products over time. A comprehensive kinetic model was developed, which integrated random chain scission, preferential end-chain scission, and backbiting reactions. The kinetic parameters were derived by fitting experimental data, yielding a generalized expression for the observed rate constant of each reaction as a function of chain length, temperature, and pH. Specifically, the reaction rates were broken down into three distinct contributions: alkaline-catalyzed, uncatalyzed, and acid-catalyzed mechanisms. Ultimately, this functional form is validated across several lactic-acid-based macromolecules, underscoring its applicability to polyesters and polyamides. The resulting mechanistic model exhibits robust predictivity, serving as a valuable tool for the design and optimization of composting processes.