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

Abstract

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.