Salvador León

Comparison of lamellar crystal structure and morphology of nylon 46 and nylon 5

A comparative study of the crystal structure of lamellar crystals of the isomeric nylons 46 and 5 was carried out. A lattice made of hydrogen-bonded sheets with chains in fully extended conformation and hydrogen bonds in the normal scheme was proposed for the two nylons according to the great similarities in morphology and diffraction data observed for their solution grown crystals. Energy calculations and modeling simulations showed that a model consisting of a statistical arrangement of progressive and alternating intersheet shearing is the most appropriate and consistent to describe the structure of both nylons. A preliminary investigation on the possibility of cocrystallizing nylon 46 and nylon 5 was undertaken. First experimental results were found to be compatible with the occurrence of cocrystallization. Molecular mechanics calculations indicated that insertion of one nylon in the crystal lattice of the other is energetically disfavored. However, cocrystals would be acceptable if they were made of homogeneous blocks of each nylon comprised of about 10 or more hydrogen-bonded sheets.

Structure and morphology of nylon 46 lamellar crystals: Electron microscopy and energy calculations

A detailed electron microscopy study of the structure and morphology of lamellar crystals of nylon 46 obtained by crystallization from solution has been carried out. Electron diffraction of crystals supported by X-ray diffraction of their sediments revealed that they consist of a twinned crystal lattice made of hydrogen-bonded sheets separated 0.376 nm and shifted along the a-axis (H-bond direction) with a shearing angle of 65°. The interchain distance within the sheets is 0.482 nm. These parameters are similar to those previously described for nylon 46 lamellar crystals grown at lower temperatures. A combined energy calculation and modeling simulation analysis of all possible arrangements for the crystal-packing of nylon 46 chains, in fully extended conformation, was performed. Molecular mechanics calculations showed very small energy differences between α (alternating intersheet shearing) and β (progressive intersheet shearing) structures with energy minima for successive sheets sheared at approximately 1/6 c and 1/3 c. A mixed lattice composed of a statistical array of α and β structures with such sheet displacements was found to be fully compatible with experimental data and most appropriate to describe nylon 46 lamellar crystals. Annealing of the crystals at temperatures closely below the Brill transition induced enrichment in β structure and increased chain-folding order.

PCSP : a computer program to predict and analyze the packing in crystalline polymers

A computational strategy based on force-field calculations was developed to predict and analyze the packing in crystalline polymers. The strategy, which was incorporated in a computer program called Prediction of the Crystal Structure of Polymers (PCSP), takes advantage of the information provided by X-ray and/or electron diffraction. The program evaluates all the modes of packing for a given lattice dimensions, being able to predict the lowest energy one. The ability of the method to provide reliable results was proved by analyzing the crystal structure of four different polymers. Further, an insight on the applicability of the PCSP program to estimate the mechanical properties of polymers was provided by computing the elastic constants of the orthorhombic polyethylene.

Theoretical study of the conformational and electrostatic properties of C4-monosubstituted 2-azetidinones

The conformational preferences and electrostatic properties of 2-azetidinone, 4-(S)-methoxycarbonyl-2-azetidinone and 4-(R)-methyl-2-azetidinone have been studied in gas-phase, aqueous solution and CCl4 solution using quantum mechanical methods. Gas-phase calculations were performed at the ab initio HF, MP2, and MP4 levels and solvent effects were investigated using a self-consistent reaction-field procedure adapted to the AM1 Hamiltonian. An almost planar arrangement was adopted by theβ-lactam ring in the three cases, whereas the alkoxycarbonyl side group was found to display a large conformational flexibility. The effects of the different solvents on the electrostatic properties of the three compounds were investigated by following the changes in both molecular electrostatic potentials and induced dipole moments. The resulting electrostatic parameters were used as static reactivity indices to predict the response of the systems to the attack of nucleophilic reagents. Theoretical results were compared with experimental data available on the structure and properties ofβ-lactams. The validity of the method as a predicting tool was critically discussed.

On the structure of the phase A of comb-like poly(α-alkyl-β, L-aspartate)s : a molecular modelling study

Abstract The phase A of the comb-like poly(α-n-octadecyl-β, l -aspartate) has been investigated using atomistically detailed computer simulations. Four independent polymer chains of 13 residues were constructed and packed in an orthogonal simulation box with periodic continuation conditions. A set of microstructures was obtained by Configurational Bias Monte Carlo generation. Some torsional angles of the alkyl side chains were constrained in order to facilitate the crystallization of paraffinic chains. The resulting microstructures have provided a detailed description of the structural behaviour of this phase allowing to characterize some related properties. The most relevant results have been compared with those recently obtained for the phase B and with the available experimental data.

Molecular simulation of gas solubilities in crystalline poly(α-alkyl-β-L-aspartate)s

Abstract We have studied the solubility of a small probe particle, the atoms He and Ar and the molecule CH4 in the helical poly(α-alkyl-β- l -aspartate)s using molecular simulations. The hexagonal and tetragonal crystal forms of poly(α-n-butyl-β- l -aspartate) and the hexagonal form of the poly(α-methyl-β- l -aspartate) have been chosen for the study as the most representative structures for this family of polymers. An important characteristic of these structures is that the helical conformations are retained at high temperatures. The excess chemical potentials have been determined using Widoms test-particle insertion method considering an ensemble of microstructures generated by Monte Carlo calculations. The solubility of the different penetrants in the polymer systems was explained in terms of the distribution of the unoccupied space.

Sugar-based bicyclic monomers for aliphatic polyesters: a comparative appraisal of acetalized alditols and isosorbide

Abstract Three series of polyalkanoates (adipates, suberates and sebacates) were synthesized using as monomers three sugar-based bicyclic diols derived from D-glucose (Glux-diol and isosorbide) and D-mannose (Manx-diol). Polycondensations were conducted in the melt applying similar reaction conditions for all cases. The aim was to compare the three bicyclic diols regarding their suitability to render aliphatic polyesters with enhanced thermal and mechanical properties. The ensuing polyesters had molecular weights (Mw) in the 25,000–50,000 g mol−1 range with highest values being attained for Glux-diol. All the polyesters started to decompose above 300 °C and most of them did not display perceivable crystallinity. On the contrary, they had glass transition temperatures much higher than usually found in homologous polyesters made of alkanediols, and showed a stress–strain behavior consistent with their Tg values. Glux-diol was particularly effective in increasing the Tg and to render therefore polyesters with high elastic modulus and considerable mechanical strength.

The crystal structure of nylon 3,5 and nylon 5,7: a comparative study

A comparative analysis of the crystal structure of nylon 3,5 and nylon 5,7 was carried out. Lamellar crystals of both compounds were grown from diluted solutions and examined by electron microscopy and X-ray diffraction. A structure with chains in fully extended conformation was the only modification found for nylon 3,5. A similar structure was found also for nylon 5,7 although the structure largely preferred by this nylon is the contracted γ-form. A rational explanation for the dissimilar behavior displayed by the two nylons was provided by a comparative estimation of their packing energies using molecular mechanics methods. Details concerning the displacement of sheets in nylon 3,5 and the arrangement of hydrogen bonds in the γ-form of nylon 5,7 were further unraveled by energy calculations. All these features were discussed with regard to the chemical constitution of the polymers and to the structures reported for other closely related nylons.

Force-field parametrization and molecular dynamics simulations of p-menthan-3,9-diols: a family of amphiphilic compounds derived from terpenoids.

A set of amphiphilic p-menthan-3,9-diols have been investigated by molecular dynamics simulations. These are four stereoisomers than can be specifically obtained from two terpenoids widely used in biorganic chemistry. For this purpose, the p-menthan-3,9-diols have been explicitly parametrized using both semiempirical and ab initio quantum mechanical calculations. The reliability of these parameters has been validated by predicting different molecular and thermodynamic properties. Molecular dynamics simulations in aqueous solution have been performed with the new parameters. The results provide useful insights about the conformational properties of this family of compounds and the formation of intra- and intermolecular hydrogen bonds.

Thermal behavior of long-chain alkanoylcholine soaps

Long-chain alkanoylcholines prepared from fatty acids (nACh) are fully sustainable cationic surfactants that are known for their biological and medicinal properties. In the present work the thermal behavior of the homologous series of alkanoylcholine iodides with n = 12, 14, 16 and 18, has been examined within the 25–200 °C range of temperatures. Up to three thermotropic phases have been identified, and the thermal transitions implied in their interconversion have been characterized by DSC and simultaneous WAXS and SAXS analysis carried out in real-time. All three phases consist of a bilayered structure with alkanoyl chains confined in the space between the head group layers and interdigitated to a greater or lesser extent. Melting–crystallization of either the polymethylene segments or the choline iodide groups is involved in such transitions. Additionally, a crystal phase consisting also of a bilayered structure but excluding side chain interdigitation was observed upon crystallization from solution and its structure was elucidated by single-crystal X-ray diffraction direct methods. The close correlation existing between thermal properties, phase structure and n has been brought into evidence.