Hydrolytic degradation of poly( dl-lactic acid) in the presence of caffeine base

Abstract

The mechanism of hydrolytic degradation of poly( dl-lactic acid) (PLA 50) matrix was examined in the presence of a tertiary amine, namely caffeine, in order to elucidate the influence of this basic compound on the hydrolytic cleavage of polyester chains. Caffeine was incorporated into PLA 50 in various contents (0 to 20%) by blending in acetone followed by solvent evaporation. The resulting blends were processed to 1.5 mm thick plates and 0.3 mm thin films by compression moulding. Degradation was carried out under standardized conditions, i.e. in isoosmolar pH 7.4 phosphate buffer at 37°C. The effects of caffeine on degradation characteristics were rather complex and largely depended on the blend composition. For low contents (≤ 2%), it is suggested that caffeine was molecularly dispersed in matrices and accelerated considerably the degradation with respect to caffeine-free devices. However, the increase of degradation rate was not proportional to caffeine content due to the combined effects of base/carboxyl end group interaction, crystallization and matrix-controlled or channeling-controlled diffusion of caffeine. In the early stages of degradation, the overall catalytic effect was larger for devices with low caffeine contents than for highly loaded ones where caffeine was in crystallized state and thus less available for basic catalysis. At the later stages, however, the neutralization of carboxyl end groups became predominant and governed the degradation in the case of highly loaded devices. From a general viewpoint, plates degraded slightly faster than films. Moreover, crystalline residues composed of oligomeric stereocomplex were obtained for both plates and films.