Differences in the CH3⋯OC interactions among poly(l-lactide), poly(l-lactide)/poly(d-lactide) stereocomplex, and poly(3-hydroxybutyrate) studied by infrared spectroscopy

Time-dependent infrared (IR) spectra were measured for isothermal melt-crystallization process of poly(l-lactide) (PLLA), poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) stereocomplex, and poly(3-hydroxybutyrate) (PHB), and the spectra were analyzed for the C-H and CO stretching band regions to explore the CH3⋯OC interactions in the above polymers. Both PLA and PHB are well known biodegradable polymers and have similar chemical structures, however, they showed quite different physical properties and spectral patterns in the C-H and CO stretching band regions. For example, PLLA, the PLLA/PDLA stereocomplex and PHB, respectively, give a CO stretching band due to the crystalline components at 1749, 1760, and 1723 cm−1. The CO stretching band of PLLA does not show a shift during the melt-crystallization process while that of the stereocomplex shows a 5 cm−1 low-frequency shift. PLLA and the PLLA/PDLA stereocomplex yield a CH3 asymmetric stretching band at very similar positions (2997 and 2994 cm−1, respectively), however, the former (the PLLA CH3 band) shows a 2 cm−1 higher frequency shift during the melt crystallization, while the latter (the PLLA/PDLA CH3 band) shows a 1 cm−1 lower frequency shift. PLLA and the stereocomplex give only one CH3 asymmetric stretching band, while PHB shows four bands. One of the CH3 asymmetric stretching bands of PHB appears at an unusually high wavenumber (3007 cm−1). These marked differences in the C-H and CO stretching band regions among PLLA, the PLLA/PDLA stereocomplex, and PHB suggest that they have different CH3⋯OC interactions. Based on the detailed analysis of the C-H and CO stretching band regions, we conclude that the PLLA/PDLA stereocomplex, and PHB have quite different types of C-H⋯O hydrogen bonds in the crystalline parts, and that PLLA has a only weak CH3⋯OC interaction.