Raman Evaluation of the Crystallinity Degree of Poly(L-Lactide)-Based Materials

We carried out a Raman study of a series of poly(L-lactide) (PLLA) samples annealed for different periods of time and therefore having different crystallinity degree. We compared the results with our recent study of the series of poly(L-lactide-co-ε-caprolactone) (PLCL) copolymers with the ε-caprolactone (CL) content ranging from 5 to 30 mol %. X-ray diffraction (XRD) analysis showed that the crystallinity degree of the analyzed PLLA-based materials is in the range of 0–86%. We suggest using the ratio of the peak intensities of the PLLA Raman bands at 411 and 874 cm^–1 to evaluate the crystallinity degree of PLLA homopolymers as well as PLLA blocks in the PLCL copolymers. This ratio does not depend on the CL content in the copolymers, it strongly depends on the crystallinity degree of PLLA (PLLA blocks in the PLCL copolymers) and it is a linear function of the crystallinity degree, measured by XRD analysis. We carried out quantum chemical calculations of the optimized geometries and Raman spectra of PLLA oligomers in the conformation of helix 10₃ with the number of monomeric units from 5 to 12. The results of the calculations revealed that the ratio of the intensities of the bands at 411 and 874 cm^–1 weakly depends on the oligomer length for the number of the PLLA monomeric units more than 7.