In vitro hydrolysis of blends from enantiomeric poly(lactide)s Part 1. Well-stereo-complexed blend and non-blended films

Well-stereo-complexed 1:1 blend and non-blended films were prepared from PLLA and PDLA both having a medium molecular weight (M-w = 1.5 x 10(5)) by solvent evaporation method and their hydrolysis in phosphate-buffered solution (pH = 7.4) at 37 degrees C was investigated up to 30 months using gel permeation chromatography (GPC), tensile tests, differential scanning calorimetry (DSC), scanning electron microscopy (SEM), optical polarizing microscopy, X-ray diffractometry, and gravimetry. The rate of reduction in molecular weight, tensile strength, Young's modulus, melting temperature, and mass remaining of the films in the course of hydrolysis was lower for the well-stereo-complexed 1:1 blend film than for the non-blended films. The induction period until the start of decrease in tensile strength, Young's modulus, and mass remaining was longer for the well-stereo-complexed 1:1 blend film than for the non-blended films. These findings strongly suggest that the well-stereo-complexed 1:1 blend film was more hydrolysis-resistant than the non-blended films. The retarded hydrolysis of the well-stereo-complexed 1:1 blend film compared with that of the non-blended films was ascribed mainly to the peculiar strong interaction between L- and D-lactide unit sequences in the amorphous region and/or the three-dimensional (3D) micro-network structure in the 1:I blend film formed by stereo-complexation in the course of solvent evaporation. The change in the molecular weight distribution and surface morphology of the I:1 blend film after hydrolysis revealed that its hydrolysis in phosphate-buffered solution proceeded homogeneously along the film cross-section mainly via the bulk erosion mechanism. (C) 2000 Elsevier Science Ltd. All rights reserved.