IR spectroscopy of crystalline polymers from ab initio calculations: Nylon 6,6

The IR spectrum of Nylon 6,6 in the crystalline α phase is computed by means of dispersion-corrected density functional theory calculations carried out with periodic boundary conditions on the crystal. The results are carefully compared to experimental spectra through a detailed analysis of different frequency regions and focusing on the spectroscopic markers of crystallinity or regularity (i.e. of the regular conformation of the polymer chain). The previous assignments, based exclusively on experimental or semi-empirical investigations, are critically revised, demonstrating that state of the art computational methods in solid state chemistry can provide tools to obtain an unambiguous description of the vibrational properties of the crystalline phases of macromolecular materials. In particular, the ambiguities related to the assignment of some debated bands of crystallinity/regularity are solved. The structural and vibrational properties are interpreted on the basis of the peculiar intra and intermolecular interactions occurring in polyamides.