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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.

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An investigation was carried out on the crystallization behavior of pdioxanone polymers using differential scanning calorimetry (DSC). Kinetic analyses were performed on data collected primarily during isothermal crystallization. Isothermal data were treated within the framework of the classical Avrami equation. Using this approach, both the Avrami exponent, n, and the crystallization half-time, t 1/ 2 , were evaluated and their implications are discussed for each system studied. It is shown that a small change in the polymer's composition greatly affects the crystallization kinetics, as well as the crystallizability of the materials. Additionally, nonisothermal crystallization under controlled heating and cooling rates was explored. In the case of cooling from the melt, the Ozawa theory and the recently proposed Calculus method were employed to describe the nonisothermal crystallization kinetics. In view of our results, the validity of these two estimation techniques for determining important kinetic and morphological parameters is also discussed.

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A significant reduction in the glass transition temperature (Tg) is found for poly(L-(-) lactide) (PLLA), crystallized under partially constrained conditions, where the polymer is prevented from shrinking. It is proposed that the constrained crystallization process results in increasing the net free volume in the amorphous phase. This is a result of the fixed sample volume in which there is an increasing crystal volume fraction at a higher density than the amorphous fraction, depleting polymer mass from the amorphous fraction, which is not able to contract to maintain its nominal density. The T g depression increases with increasing degree of crystallization or crystallization temperature, Tc. On the other hand, unconstrained PLLA samples (free to shrink during crystallization) exhibit the conventional trend of increasing Tg with crystallinity or Tc. Although the difference in Tg between samples prepared by the two methods could be large, as much as 30 °C, the melting point, heat of fusion, and overall degree of crystallinity in PLLA are not influenced. Finally, shear-induced crystallization does not affect any of the physical properties studied here but only modifies the crystallization rates, increasing them by enhancing nucleation.