Controlling Polylactide Degradation through Stereocomplexation and Lactic Acid Based Additives

Abstract: The successful use of degradable materials in new applications depends on our ability to control the degradation process. A number of aspects need to be evaluated including degradation rate, the onset of degradation and the nature, formation and release of degradation products. In this study, the possibility of tuning the properties and degradation of polylactide (PLA) through stereocomplexation between the L- and D-enantiomers of PLA was investigated. The influence of the oligo(D-lactic acid) architecture on the stereocomplexation and subsequent degradation rate was evaluated. The dependence of stereocomplex formation on the structure of the side-group and the effect of oligo(L-lactide) additives on hydrolytic degradation and thermal aging were also investigated. One advantage of these additives is that no foreign migrants are introduced into the degradation product pattern. This reduces the risk of a harmful impact on the environment. The degradation process was monitored through analyses of the remaining material and the released degradation products. Depending on the modification, both increased and decreased degradation rates could be obtained. Resistance against degradation was increased by stereocomplexation, which is explained by the strong interactions between the complementary L- and D- chain structures. On the other hand, even though the mass loss was low, larger amounts of short hydroxy acids were released from the stereocomplex materials, resulting in a faster decrease in the pH. This can be explained by an increase in the amount of intermolecular stereocomplex crystallites resulting in a large number of tie-chains connecting the crystallites. These chains are more exposed to hydrolysis, and hence short degradation products are released. The architecture of the added oligo(D-lactic acid) had a considerable impact on material properties such as crystallinity and degradation. Acidic end-groups increased the degradation rate, while alcoholic end-groups had the opposite effect. The addition of hydrophilic linear oligo(L-lactide) to poly(L-lactide) (PLLA) resulted in a rapid migration of additives from the material during hydrolytic aging and a faster loss of mass and molar mass than from a material containing cyclic analogues. During thermal aging, however, the opposite effect was observed as the linear oligo(L-lactide) additives interacted more strongly with PLLA, which resulted in smaller mass loss. Stereocomplexation was also revealed to take place between oligomers of the two enantiomers of the lactic acid-like monomer α-hydroxyisovaleric acid.