Thermal degradation of poly(lactic acid) oligomer: Reaction mechanism and multistep kinetic behavior

Abstract

This study was aimed to reveal the kinetic and mechanistic features of the thermal degradation of linear poly(lactic acid) oligomer (PLAO) by analyzing the evolved gases and overall rate behavior during mass-loss processes. The thermal degradation reaction involved two partially overlapping mass-loss steps. The first mass-loss step started right after the reactant melted and was accompanied with the release of lactides as the main gaseous products while maintaining the microscopic appearance of molten PLAO. The mass-loss rate behavior was controlled by a diffusion process with an apparent activation energy (Ea) of approximately 90 kJ mol–1, with the diffusional transfer of lactides from the reaction sites to the top surface of the molten PLAO being considered as the rate-limiting step. The second mass-loss step became noticeable at reacting system temperatures above the boiling point of lactides. During this dramatically accelerated mass-loss step, carbon oxides, aldehydes, lactides, and cyclic PLAO with large molecular weights were simultaneously evolved with frequent formation and disappearance of gaseous bubbles in the molten reacting system. In this step, the overall rate behavior was largely influenced by the bubbling phenomena for which the Ea value was determined to be approximately 155 kJ mol–1. The changes in the chemical mechanism and the physico-geometrical reaction behavior midway through the thermal degradation reaction were both illustrated by the change in the molten reacting system in view of degradation-polymerization equilibrium via the boiling points of lactides.