Time-resolved isothermal crystallization of absorbable PGA-co-PLA copolymer by synchrotron small-angle X-ray scattering and wide-angle X-ray diffraction

The isothermal crystallization behavior of absorbable dyed and undyed PGA-c o-PLA copolymers was investigated by time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) methods wi th synchrotron radiation. The morphological parameters extracted from time- resolved SAXS profiles show that long period and lamellar thickness decreas e slightly after primary crystallization. The unit cell parameters a and b and the apparent lateral crystal sizes L-110 and L-020 were extracted from the corresponding WAXD profiles. A significant decrease in the unit cell pa rameters and a substantial increase in the apparent crystal sizes are seen during the initial crystallization stage. Both scattering invariant (Q from SAXS) and crystallinity (X-c from WAXD) results indicate that the crystall ization rate is the fastest at 130 degreesC. These copolymers show a bell-s hape crystallization rate curve with temperature, where the dyed copolymer has a faster crystallization rate than the undyed one even though the inclu sion of the low molecular weight organic dye is very small (ca. 0.2% by wei ght). We conclude that the dye molecule, which enhances the visibility duri ng surgery, acts as a nucleating agent that increases the overall crystalli zation rate. The crystallization rate at 90 degreesC is significantly slowe r than that at 130 degreesC, however, the long period and lamellar thicknes s formed at 90 degreesC are much lower than those formed at higher temperat ures. The dyed and undyed PGA-co-PLA copolymers have almost the same morpho logical parameters at the same temperature. This indicates that morphologic al parameters of the lamellar structures in the polymers depend primarily o n the crystallization temperature rather than on the crystallization rate. It is evident that the thermodynamic factor driven by temperature principal ly determines the lamellar morphology. The final unit cell parameters a and b and the final apparent crystal sizes all increase with temperature, indi cating that crystal perfection prevails at high temperatures. (C) 2001 Else vier Science Ltd. All rights reserved.