Structural changes of polylactic-acid (PLA) microspheres under hydrolytic degradation

Low molecular-weight polylactic acid (PLA) was obtained by direct polyconde nsation of a mixture of 95% L and 5% D-lactic acid isomers, without catalys t, at 195 degrees C. This polymer was used for the synthesis of microsphere s by emulsion-solvent evaporation method. Gel Permeation Chromatography (GP C), X-ray Scattering (XRD), Differential Scanning Calorimetry (DSC), and Sc anning Electron Microscopy (SEM) techniques were applied to follow morpholo gical and structural changes of particles along in vitro degradation at 37 degrees C. The original microspheres were amorphous but could crystallize p artially upon heating. Samples stored in a humid environment exhibited an i ncrease in the crystallization capability upon heating. Initial smooth-surf ace microspheres were transformed to porous particles at the time of degrad ation at pH = 7 (37 degrees C). The shape of mass loss vs, time curve suppo rts the presence of a heterogeneous bulk degradation process. After hydroly tic degradation the residual particles showed a molecular weight decrease a nd a crystallinity increase. After 90 days the crystallinity attained a val ue of 53%. The X-ray diffraction spectrum indicated the formation of a crys talline oligomeric structure. Crystallization of low molecular weight speci es will not enable the desired PLA absorption in drug delivery systems. (C) 1999 John Wiley & Sons, Inc.