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-co-PLA copolymers was investigated by time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) methods with synchrotron radiation. The morphological parameters extracted from time-resolved SAXS profiles show that long period and lamellar thickness decrease slightly after primary crystallization. The unit cell parameters a and b and the apparent lateral crystal sizes L₁₁₀ and L₀₂₀ were extracted from the corresponding WAXD profiles. A significant decrease in the unit cell parameters 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 crystallization rate is the fastest at 130°C. These copolymers show a bell-shape crystallization rate curve with temperature, where the dyed copolymer has a faster crystallization rate than the undyed one even though the inclusion of the low molecular weight organic dye is very small (ca. 0.2% by weight). We conclude that the dye molecule, which enhances the visibility during surgery, acts as a nucleating agent that increases the overall crystallization rate. The crystallization rate at 90°C is significantly slower than that at 130°C, however, the long period and lamellar thickness formed at 90°C are much lower than those formed at higher temperatures. The dyed and undyed PGA-co-PLA copolymers have almost the same morphological parameters at the same temperature. This indicates that morphological parameters of the lamellar structures in the polymers depend primarily on the crystallization temperature rather than on the crystallization rate. It is evident that the thermodynamic factor driven by temperature principally determines the lamellar morphology. The final unit cell parameters a and b and the final apparent crystal sizes all increase with temperature, indicating that crystal perfection prevails at high temperatures.