Infrared bands to distinguish amorphous, meso and crystalline phases of poly(lactide)s: Crystallization and phase transition pathways of amorphous, meso and co-crystal phases of poly(ʟ-lactide) in the heating process

Abstract

The infrared spectral region 1280–1330 cm−1 assigned to the δs(CH3) + δ(CH) + ν(C–O–C) vibrational modes is found to be sensitive to the chain conformation, intermolecular interactions and chain packing mode of poly(lactide)s. Herein, we used the aforementioned spectral region to distinguish amorphous, meso and crystalline phases of poly(lactide)s. A single absorption band at 1302 cm−1 is observed for the amorphous poly(ʟ-lactide) (PLLA) and upon the crystallization, this band splits into two peaks, where the band positions are significantly different for the mesophase, crystalline phases of PLLA and stereocomplex. The difference between the band positions of α and α′ forms of PLLA in this spectral region is due to the slight difference in the chain conformation, interchain interactions and chain packing. Although the chain conformation of the crystalline complexes (ε form) is similar to that of the α form, distinctly different infrared band positions were observed in this spectral region. Further, the infrared spectra of PLLA ε forms prepared using different solvent molecules (N,N-dimethylformamide, tetrahydrofuran, 1,3-dioxolane and cyclopentanone) are found to be sensitive to the solvent trapped within the crystal lattice (polymer-solvent interactions). The stereocomplex formed between the equimolar mixtures of PLLA and poly(ᴅ-lactide) (PDLA) showed a new pair of infrared bands at 1306 and 1318 cm−1, which are different from the other crystalline forms of PLLA due to the intermolecular interactions between PLLA and PDLA. In addition, this spectral region is successfully used to understand the structural regularization process during the crystallization of PLLA from the amorphous and mesophases. It was also shown that this spectral region is useful to clarify the structural phase transitions that occur during the heating of the crystalline complexes (ε form). The identified infrared region is complementary to X-ray diffraction in understanding the structural regularization and structural phase transitions of poly(lactide)s during in situ measurements.