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

Abstract

This paper reports the analysis of the C=O 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 Coupled Oscilator (SCO) model, which only accounts for mechanical coupling, has been found to predict adequately the relative intensity of the symmetric and asymmetric bands of dicarbonyl compounds. The dispersion curves predicted for PLLA by the SCO model also match those given by more general treatments, such as Miyazawa's first-order perturbation theory. Hence, the SCO model is adopted here as an adequate yet simple tool for the interpretation of band splitting caused by intramolecular coupling of polylactide. The four components observed in the C=O stretching band of semicrystalline PLLA are attributed to the four possible conformers: gt, gg, tt, and tg. The narrow bands observed for the interlamellar material are attributed to highly ordered chains, indicating the absence of a truly amorphous phase in the crystalline polymer. The interphase seems to extend over the whole interlamellar region, showing the features of a semiordered metastable phase. In amorphous PLLA, bands corresponding to gt, gg, and tt conformers also can be resolved by second derivative techniques, and curve-fitting results provide information about the conformational population at different temperatures.

Effect of Film Formation Method and Annealing on Crystallinity of Poly(L-Lactic Acid) Films

“…The carbonyl (C=O) stretching region, CH 3 and CH bending region, and the skeletal stretching region (C-O-C) show prominent peaks. Along the molecular chain of PLLA, there are three skeletal bonds: C-O (ester), O-C α , and C α -C, in which the ester bond can be assumed trans because of increased stability due to C=O double bond conjugation [37]. The O-C α bond has two minimal energies if its bond rotation angle φ is -160° and -73°, while C α -C bonds has two minimal energies if its bond rotation angle ψ is 160° and -48°.…”

“…The gt conformation has the lowest energy and constitutes the crystal structure. Because of the four conformations, the carbonyl region can be split into four peaks due to intramolecular and intermolecular interactions, and the four peaks are located at 1776 cm -1 , 1767 cm -1 , 1759 cm -1 , and 1749 cm -1 , corresponding to gg, tg, gt, and tt conformations, respectively [37]. The spectra of the C=O stretching region of solvent cast and spin coated films, before and after annealing, are given in Figures 9(a), 9(b), and 10.…”

Effect of Film Formation Method and Annealing on Morphology and Crystal Structure of Poly(L-Lactic Acid) Films

“…Along the molecular chain, there are three skeletal bonds: C-O (ester), O-C α , and C α -C, in which the ester bond can be assumed trans [43]. The O-C α bond has two minimal energies when its bond rotation angle φ is -160° and -73°, while the C α -C bond has two minimal energies when its bond rotation angle ψ is 160° and -48°.…”

“…Because of the four conformers, the C=O stretching region can be split into four peaks due to intramolecular and intermolecular interactions, and the four peaks are located at 1776 cm , and 1749 cm -1 , corresponding to the gg, tg, gt, and tt conformers, respectively [43]. The spectra of this region of solvent cast and spin coated films are given in Figs.…”

PARAFAC Analysis for Temperature-Dependent NMR Spectra of Poly(Lactic Acid) Nanocomposite

“…PLA essentially undergoes highly convoluted transition process, when temperature and its constitution are altered. These transitions include the melting of ordered molecular segments, as well as the grass-to-rubber transition and other relaxation of process of the amorphous component (Zhang et al, 2005;Meaurio et al, 2006). The CP-MAS technique is ideal for the observation of 13 C spectra of solid samples.…”