Effect of hydrolytic degradation and dehydration on the microstructure of 50:50 poly(glycolide-co-D,L-lactide)

Abstract

The effect of hydrolytic degradation on the microstructure of unoriented, random 50: 50 poly(glycolic acid-co-D,L-lactic acid) was examined using simultaneous small and wide angle X-ray scattering (SAXS/WAXS) and differential scanning calorimetry (DSC). Samples were degraded in phosphate-buffered saline solution at 37.5°C and studied wet and after dehydration. There was no evidence of crystalline material within the sample at any stage of degradation or dehydration from either X-ray scattering or thermal analysis. Thus, chain scission does not enable crystallization of the copolymer, and the glycolic acid and lactic acid fragments formed on degradation do not crystallize, even when the samples are dehydrated. Because such fragments are clearly formed (Hakkarainen, M., Albertsson, A. C. & Karlsson, S., Polym. Deg. Stab., 52 (1996) 283), and because they are crystalline in the dry state, it must be assumed either that these species are not present in any quantity in the degrading sample and that they diffuse easily from the bulk into the surrounding medium, or that the bulk polymer prevents them from crystallizing. SAXS gave evidence of small voids within the structure. Unlike dehydrated degraded semi-crystalline samples, there is no evidence for voiding on a macroscopic scale. The number and size of the small voids in the dehydrated samples rises with degradation. The voids close as samples are heated above the glass transition temperature and the amorphous chains gain mobility. The glass transition, although clearly visible in the undegraded samples, becomes less visible by DSC on degradation. After 28 days' degradation, there is some evidence that the structure begins to close up, perhaps as a result of reduced viscosity arising from the increased fraction of low molecular weight material.