Kinetics of the hydrolytic degradation of poly(lactic acid)

Abstract

The hydrolysis of water soluble PLA oligomers of different chain lengths and chirality was investigated at acidic pH and temperatures in the range from 40 to 120 °C. The time evolution of the concentrations of all oligomers was measured by HPLC and the corresponding degradation rates were evaluated for each specific chain length. In agreement with the preferential chain end scission mechanism suggested in the literature, the ester groups were classified as α (chain end esters) and β (backbone esters). A kinetic model was developed from the resulting kinetic scheme and it was found to well reproduce the concentration values of all different oligomers during degradation as a function of time. The corresponding rate constants kdα and kdβ were estimated over the whole temperature range, with activation energies of 73 and 58 kJ/mol and pre-exponential factors of 8.21·107 and 1.77·105 l/mol/h, respectively. It is seen that the faster hydrolysis of the ester groups close to the carboxylic and hydroxyl chain end groups (α) with respect to those inside the polymer chain (β) is mainly due to the largely different pre-exponential factors. This steric effect can be explained considering that the water approach is favoured by the hydrophilic nature of the chain end groups compared to the hydrophobic character of the polymer backbone. No dependence of kdα and kdβ on chiral composition was found, suggesting that the differences reported earlier in the literature are due to the effect of crystallinity on diffusion phenomena rather than to different reactivity of the two stereoisomers.