Synthesis, structure and crystal morphology of nylon 2/16

The alternating copolyamide poly(glycineco-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 so-called Form II typical of nylons 2/n and was able to reproduce satisfactorily 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. Variations in the crystallization temperature produced changes in the lamellar crystal morphology consistent with the occurrence 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 found for nylon 2/16.