1,721,107 research outputs found
A revisitation of the polymorphism of poly(butylene-2,6-naphthalate) from periodic first-principles calculations
Full text linkPeriodic density functional theory calculations are here used to investigate polymorphism in poly(-butylene-2,6-naphthalate) (PBN) and to understand the intra- and inter-molecular effects which are responsible for its behavior. Despite its similarity to poly(butylene terephthalate), the larger p-electron conjugation promoted by the presence of naphtyl rings generates peculiar intramolecular effects and stronger interchain packing interactions which cause some differences between the two polymers. This is particularly evident by comparing the structural and spectroscopic data predicted for the a and b crystals with respect to the respective one dimensional infinite chain models. Two different interpretations have been proposed in the previous literature to describe the structural transitions from the a to the b
polymorph of PBN upon mechanical deformation or thermal treatments: from one hand, the setting on of a transplanar conformation on the methylene chains has been proposed while, on the other hand, a larger coplanarity of the ester groups and the naphtyl rings has been suggested. Our calculations reveal that both these effects are present in b-PBN and should be both taken into account to give an interpretation of the trends observed by IR spectroscopy and structural characterization
Modeling of molecular charge distribution on the basis of experimental infrared intensities andfirst-principles calculations: the case of CH bonds
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Unpolarized and Polarized Raman Spectroscopy of Nylon-6 Polymorphs: A Quantum Chemical Approach
Full text linkExploiting the very recent potentialities of state-of-the-art quantum chemical simulations of crystalline solids, unpolarized Raman spectra of α and γ polymorphs of Nylon-6 obtained through periodic density functional theory calculations are presented for the first time. The computed spectra are compared with the experimental spectra reported in the literature and allow a detailed interpretation to be proposed of the patterns observed, identifying unambiguous Raman marker bands of the different phases. The calculations of single crystal directional intensities gave the further possibility to predict the polarization properties of the Raman spectra of these polymorphs: considering in particular the α phase, polarized Raman spectra have been computed and showed a very good agreement with measurements previously reported for uniaxially oriented samples
Polymorphism of Poly(butylene terephthalate) Investigated by Means of Periodic Density Functional Theory Calculations
Full text linkThe conformation and solid state structure of the two α and β polymorphs of poly(butylene terephthalate) are here studied by means of state-of-the-art first principles calculations, carried out both for the crystals and the infinite one dimensional chain models. Focusing in details on the debated β form, induced by mechanical deformation, we verified the setting on of an all-trans conformation, as also supported by the simulation of the infrared spectra of the different polymorphs compared to the available experimental spectra. The transition from the α to the β form is also simulated by applying increasing strains to the infinite polymer chain: a peculiar evolution of the intramolecular structure is indeed predicted, showing a transition from the GTG′ conformation found for the α form to the TTT conformation of the strained β form
Crystal Structure and Vibrational Spectra of Poly(trimethylene terephthalate) from Periodic Density Functional Theory Calculations
Full text linkThe crystal structure and the IR spectrum of crystalline poly(trimethylene terephthalate), PTT, have been
investigated by means of periodic density functional theory
calculations including Grimme’s correction for dispersion interactions. Both structural and spectroscopic results have been critically compared to the experimental data taken from the literature, showing very good agreement between theory and the experiments. The previous spectral assignments, based only on experimental investigations, have been revised, and further insights have been obtained. Furthermore, spectroscopic markers of crystallinity or
regularity (i.e., of the regular conformation of the polymer chain) have been proposed. In addition to the analysis of the IR spectra, the effect of computational parameters on the crystal structure determination (basis sets and parameters for Grimme’s correction) have been analyzed. This work demonstrates that state of-the-art computational methods can provide an unambiguous description of the structural and vibrational properties of crystalline polymers on the basis of the peculiar intra- and intermolecular interactions occurring in different macromolecular materials
Polymorphism of even nylons revisited through periodic quantum chemical calculations
Full text linkDensity functional theory calculations with periodic boundary conditions have been carried out for the 3D crystal cells of alpha and gamma polymorphs of even polyamides (ranging from nylon-4 up to nylon-12) in order to investigate the relative stability of the two forms. The infinite 1D polymer chains possessing alpha and gamma conformations have been also simulated to shed light on the intramolecular effects taking place in these systems and to discriminate the intermolecular effects in terms of hydrogen bonding and packing of CH2 units. The results of this analysis allowed to give a detailed interpretation of the polymorphism properties observed in even nylons, predicting a larger stability for the cc form in nylon-4 and nylon-6 and a stabilization of the gamma phase from nylon-8 up to nylons having a larger number of CH2 unit in the chain
Structural diagnosis of perfluorinated polymers for advanced applications: the joint use of vibrational spectroscopy and first-principles calculations
No full textExperimental IR spectra of perfluoropolyethers show many features which cannot be interpreted by means of simple correlations commonly used for band assignments. First-principles calculations on suitable molecular model should be used to this aim and reveal the existence of peculiar intra and intermolecular interactions which are responsible for the modulation of both the structural and spectroscopic properties of these polymers
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