Marta Bermúdez

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.

Synthesis, structure and crystal morphology of nylon 16

The alternating copolyamide poly(glycine-co-16-aminohexadecanoic acid) (nylon 2/16) was synthesized by polycondensation of the dimer glycyl 16-aminohexadecanoic acid pentachlorophenyl ester hydrobromide in a N,N-dimethylformamide solution. Both X-ray diffraction and electron microscopy studies showed that this new 2/n copolyamide crystallized in a hexagonal lattice of a = 0.479 nm with a residue height of about 2.4 nm. The crystal structure was modeled for a 6-fold helix with a period of 14.4 nm that is linked to its six neighbors by a three-dimensional network of hydrogen bonds. The structure had the overall features of the socalled Form II typical of nylons 2/n and was able to reproduce satisfactorilly the experimental diffraction data. 4.5 nm-thick lamellar single crystals accommodating only two nylon 2/16 chemical residues in height were obtained by crystallization from solution. Varations in the crystallization temperature produced changes in the lamellar crystal morphology consistent with the occurence of preferential hydrogen bonding directions. Compared with all other members of the 2/n family studied already by us, no trace of crystal form with chains in fully extended conformation (Form I) was for nylon 2/16.

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.