Hydrolytic behavior of enantiomeric poly(lactide) mixed monolayer films at the air/water interface: stereocomplexation effects.

The Langmuir film balance technique was used to determine the hydrolytic kinetics of monolayers of the stereocomplex formed from mixtures of enantiomeric polylactides, poly(L-lactide) (L-PLA) and poly(D-lactide) (D-PLA), spread at the air-water interface. The present study investigated parameters such as degradation medium, mixture composition, and time on the relative degradation rate. The pi-A isotherms of monolayers of the mixtures provide clear evidence for the presence of a stereocomplex; the isotherms of monolayers of individual polyenantiomer show a transition at about 8.5 mN/m, whereas the transition of monolayers containing a stereocomplex formed from the equimolar mixture shifted to higher surface pressure, about 11 mN/ m. The rate of hydrolysis was recorded by a change in occupied area when the monolayer is maintained at a constant surface pressure. The hydrolysis of the mixture monolayers under basic conditions was slower than that of individual polyenantiomer monolayers, depending on the composition or the degree of complexation. In the presence of proteinase K, the enzymatic hydrolysis rate of mixture monolayers with >50 mol % l-PLA was much slower than that of the single-component L-PLA monolayer. The monolayers formed from mixtures with < or =50 mol % L-PLA did not show any change of occupied areas. This result is explained by the inactivity of D-PLA and stereocomplexed chains to the enzyme. From both results, it can be concluded that the retardation of the hydrolysis of mixture monolayers is mainly due to a strong interaction between D- and L-lactide unit sequences, which prevents the penetration of water or enzyme into the bulk.