Tuning the polylactide hydrolysis rate without introducing new migrants

Abstract: The possibility to control and tune the hydrolytic degradation rate of polylactide without introducing any new degradation products was investigated by subjecting polylactide with cyclic or linear oligolactic acid additives, and a stereocomplex between the L- and D-enantiomers of polylactide to hydrolytic degradation at 37 and 60 °C for up to 39 weeks. The degradation was monitored by measuring mass loss, molar mass changes, water uptake, changes of surface structure, crystallinity and thermal properties. The degradation product pattern was followed through pH measurements and by electrospray ionization-mass spectrometry (ESI-MS).Rapid migration of additives from the material into the aging medium was observed in the case of the more hydrophilic linear oligolactic acid additives. The mass loss at 37 °C was generally 10-20 % greater for the material containing linear additives instead of cyclic additives. The hydrolysis accelerating effect of the linear additives may be counteracted by the facilitated crystal formation of the short chains. Micrographs showed formation of holes on the surface of the material containing linear additives during degradation. This may be a result of migration of phase separated linear additives. Phase separation might take place in the material as the crystallinity increases. The stereocomplex had a higher hydrolytic stability, which is explained by the strong interactions between the complementary chain structures. At 37 °C, the observed mass loss was generally 15 % lower for the stereocomplex compared to the material containing the cyclic additives. However, a larger amount of short hydroxy acids was released from the stereocomplex material, as shown by the large pH drop and the degradation product pattern analysed using ESI-MS. This can be explained by increased intermolecular stereocomplex crystallisation, which results in a larger number of tie-chains between crystals which are susceptible to hydrolysis and facilitate the formation of shorter hydrolysis products. Hydrolysis at 60 °C, i.e. above the glass transition temperature, was drastically faster and the differences between the materials were not as obvious as at the lower temperature. A large advantage using different forms of lactide or lactic acid as additives is that no new migrants are introduced into the degradation product pattern.