Properties and morphology of poly(L-lactide). III. Effects of initial crystallinity on long-termin vitro hydrolysis of high molecular weight poly(L-lactide) film in phosphate-buffered solution

Abstract

The effects of crystallinity (xc) on the hydrolysis of high molecular weight poly(L-lactide) (PLLA) films in a phosphate-buffered solution at 37°C was investigated by gel permeation chromatography, tensile testing, differential scanning calorimetry, scanning electron microscopy, and polarizing optical microscopy. The change in molecular weight distribution and surface morphology of the PLLA films after hydrolysis revealed that the hydrolysis of PLLA film in a phosphate-buffered solution proceeded homogeneously along the film cross section, mainly via the bulk-erosion mechanism. The induction period until the start of the decrease in mass remaining and the tensile strength became longer with a decrease in the initial xc of the PLLA films. The rate of molecular weight reduction was higher as the initial xc of the PLLA films increased when hydrolysis was carried out up to 24 months. Melting and glass transition temperatures of the PLLA films increased in the first 12 months of hydrolysis, while they decreased in another 24 months, irrespective of the initial xc. The xc value of the PLLA films increased monotonously by hydrolysis. The lamella disorientation in PLLA spherulites after hydrolysis implied that the hydrolysis of PLLA chains occurred predominantly in the amorphous region between the crystalline regions in the spherulites. The area of a specific molecular weight in GPC spectra at 36 months increased with increase in the initial xc of the PLLA film, suggesting that the specific peak should be due to the component of one fold in the crystalline region. The reason for enhanced hydrolysis of PLLA films having higher initial crystallinities was discussed in terms of tie chains and terminal groups of PLLA. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1452–1464, 2000