STRUCTURE AND MORPHOLOGY OF NYLON-4 CHAIN-FOLDED LAMELLAR CRYSTALS

The structure and morphology of nylon 4 chain-folded lamellar crystals has been investigated using transmission electron microscopy (TEM), s elected area electron diffraction, and X-ray diffraction. Solution-gro wn crystals are seen in the electron microscope in the form of sheaves with lathlike extremities. Electron diffraction patterns show the 0.4 4 (200) and 0.37 nm (020) reflections which are representative of the interchain distance within a hydrogen-bonded sheet and the intersheet distance, respectively, and are characteristic of the broader family o f polyamides. The orientation of the electron diffraction patterns fro m individual lamellar crystals establishes that the polymer chains lie perpendicular to the lamellar surface and that the folding plane is p arallel to the long axis of the crystal. Sedimented mats of the crysta ls were examined using wide-angle X-ray diffraction in a direction per pendicular to the lamellar normals. The X-ray diffraction patterns con firm that the nylon 4 chains are parallel to the lamellar normal. As a result of these findings and owing to the relatively high amide conte nt of nylon 4, a unique mechanism for folding in nylons is proposed in which an amide group is incorporated in the fold, similar to that rec ently proposed for nylon 46. The fold exhibits similarity with the bet a-bend in proteins rather than with the previous folding mechanism for nylons in which the alkane segments are in the fold. The crystallogra phic unit cell is monoclinic with a = 0.979 (+/-0.002) nm, b = 0.831 ( +/0.002) nm, c (chain axis) = 1.225 (+/-0.002) nm, and gamma = 117-deg rees and contains four chain segments or two sheet segments, similar t o the unit cell of bulk nylon 6 and nylon 46. The effects of temperatu re on the lattice spacing show that the 200 and 020 reflections conver ge with increasing temperature but do not meet prior to the melting te mperature, and two distinct reflections are observed up to the melt.