Structure and Morphology Changes in Absorbable Poly(glycolide) and Poly(glycolide-co-lactide) during in Vitro Degradation

Abstract

The changes of crystal structure and morphology in poly(glycolide), PGA, homopolymer and poly(glycolide-co-l-lactide), PGA-co-PLA, (90/10) random copolymer during in vitro degradation were investigated by gel permeation chromatography (GPC), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). GPC results showed that the molecular weight and polydispersity decreased significantly during the first 2 weeks of degradation. In contrast, the mass degree of crystallinity, φ 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 degradation on the crystal unit cell dimensions was noticeable and anisotropic, which may reflect the process of crystal perfection in vitro. Corresponding SAXS results also showed that the long period (L), lamellar thickness (l c), and amorphous layer thickness (l a) from the crystal lamellar stacks all decreased appreciably in both PGA and PGA-co-PLA samples during the first 3 weeks of degradation. By comparing molecular weight and lamellar thickness results, we conclude that the fully degraded chain fragments have an average length of about 3 times the crystal thickness. On the basis of these results, we propose that degradation proceeds through the combined processes of chain scission and cleavage-induced crystallization in the amorphous regions via two pathways. (1) The degradation occurs in the amorphous gaps between the crystal lamellar stacks, where the amorphous chains are broken leading to greater mobility to form new crystal lamellar stacks with thinner thickness. This process significantly reduces the averaged values of L, l c , and l a. (2) The degradation process also occurs in the amorphous layer domain between the adjacent lamellae within the lamellar stacks, where chain scission causes the rapid decrease in polydispersity.

A model for simultaneous crystallisation and biodegradation of biodegradable polymers

“…It has been widely observed that the degree of crystallinity in commonly used biodegradable polymers increases significantly during both short term and long term 6 degradation [7][8][9]. The degradation-induced crystallisation can occur in either initially amorphous or semi-crystalline polymers.…”

“…Therefore even if the total volume of the crystalline phase remains constant, the observed degree of crystallinity still increases due to the loss of the amorphous phase. More importantly, cleavage of the long and amorphous polymer chains provides higher mobility for the polymer chains, facilitating the crystallisation of the amorphous polymer [7]. The detailed degradation pathways have been suggested [7] and long term experimental data of simultaneous degradation and crystallisation are available in the literature [8,9].…”

“…More importantly, cleavage of the long and amorphous polymer chains provides higher mobility for the polymer chains, facilitating the crystallisation of the amorphous polymer [7]. The detailed degradation pathways have been suggested [7] and long term experimental data of simultaneous degradation and crystallisation are available in the literature [8,9]. However a mathematical model for the degradation-induced crystallisation does not exist as far as the authors are aware.…”

“…(12) always predicts to a significant incubation period for crystallisation which is very short during biodegradation [7][8][9]. The second problem is that Eq.…”

“…PGA, PLA and their copolymers are certainly not materials easy to work with. Among many other things, two complications have to be considered: (a) the biodegradation of polymers containing PLA is heterogeneous due to the autocatalytic nature of the hydrolysis reaction [6], and (b) many of these polymers, PGA and poly(L-lactide) (PLLA) for example, crystallise during biodegradation [7][8][9]. A complicated interplay between the hydrolysis reaction, diffusion of the reaction products and crystallisation makes the mechanical and functional properties of the biodegradable devices difficult to predict.…”