Infrared Spectrum of Poly(l-lactide): Application to Crystallinity Studies

Abstract

The spectrum of PLLA is analyzed in order to investigate its crystallinity and crystalline morphologies. The carbonyl and ester bands of PLLA have been analyzed, and individual components have been successfully assigned. Nucleation always proceeds through curved lamellar crystals, this crystalline morphology being exclusive for low crystallization temperatures. At higher crystallization temperatures, a transition from curved crystals to flat lozenge-shaped lamellae is observed. Curved crystals with edge-on orientation and flat crystals with flat-on orientation affect the intensity of spectral bands. The total crystallinity has been obtained from a skeletal band at 955 cm-1. In addition, intensity changes observed in the CO stretching region during crystallization provide a simple procedure to obtain the relative population of the two crystalline morphologies. As crystallization temperature increases, the relative population of curved edge-on crystals is observed to decrease, but their population remains important even at the higher crystallization temperatures. The CO stretching region shows a complex profile that can be fully explained assuming intramolecular through bond coupling and factor group splitting. The latter is also affected by crystalline perfection; hence, the observed crystalline components strongly depend on the crystallization temperature. In the CO stretching region, perfectly flat crystals give two narrow components at 1767 and 1758 cm-1. Curved crystals obtained at low crystallization temperatures give a broader band located at 1760 cm-1 attributed to factor group splitting averaged over the different curvatures shown by this crystalline morphology. This contribution is expected to depend on crystallization temperature according to theoretical considerations (larger nuclei sizes). DSC melting shows a shoulder at lower temperature attributed to the presence of the less stable edge-on crystalline morphology. Finally, the ester C−O stretching region also shows factor group splitting in both the perpendicular (split ∼ 10 cm-1) and parallel (split ∼ 18 cm-1) components.