Conformational Behavior of Poly(l-lactide) Studied by Infrared Spectroscopy

This paper reports the analysis of the CO stretching region of poly(l-lactide). This spectral band splits into up to four components, a phenomenon that a priori can be explained in terms of carbonyl−carbonyl coupling or specific interactions (such as C−H···O hydrogen bonding or dipole−dipole). Hydrogen bonding can be discarded from the analysis of the C−H stretching spectral region. In addition, low molecular weight dicarbonyl compounds of chemical structure similar to that of PLLA, such as diacyl peroxides, show a remarkable splitting of the carbonyl band attributed to intramolecular carbonyl−carbonyl coupling. Several mechanisms can be responsible for this behavior, such as mechanical coupling, electronic effects, or through-space intramolecular TDC (transition dipole coupling) interactions. Intermolecular dipole−dipole interactions (possible in the form of interchain TDC interactions) are proven to be of minor relevance taking into account the spatial structure of the PLLA conformers. The Simply Couple...