Structure and morphology changes in absorbable poly(glycolide) and poly(glycolide-co-lactide) during in vitro degradation

The changes of crystal structure and morphology in poly(glycolide), PGA, ho mopolymer and poly(glycolide-co-L-lactide), PGA-co-PLA, (90/10) random copo lymer during in vitro degradation were investigated by gel permeation chrom atography (GPC), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). GPC results showed that the molecular weight and polydi spersity decreased significantly during the first 2 weeks of degradation. I n contrast, the mass degree of crystallinity, phi(mc), determined from WAXD showed an Avrami-like behavior in both polymers, increasing rapidly within the first 2 weeks and gradually reaching a plateau value. The effect of de gradation on the crystal unit cell dimensions was noticeable and anisotropi c, which may reflect the process of crystal perfection in vitro. Correspond ing SAXS results also showed that the long period (L), lamellar thickness ( l(c)), and amorphous layer thickness (l(a)) from the crystal lamellar stack s all decreased appreciably in both PGA and PGA-co-PLA. samples during the first 3 weeks of degradation. By comparing molecular weight and lamellar th ickness results, we conclude that the fully degraded chain fragments have a n average length of about 3 times the crystal thickness. On the basis of th ese results, we propose that degradation proceeds through the combined proc esses of chain scission and cleavage-induced crystallization in the amorpho us regions via two pathways. (1) The degradation occurs in the amorphous ga ps between the crystal lamellar stacks, where the amorphous chains are brok en leading to greater mobility to form new crystal lamellar stacks with thi nner thickness. This process significantly reduces the averaged values of L , L-c, and l(a). (2) The degradation process also occurs in the amorphous l ayer domain between the adjacent lamellae within the lamellar stacks, where chain scission causes the rapid decrease in polydispersity.