LAMELLAR STRUCTURE AND MORPHOLOGY OF NYLON-46 CRYSTALS - A NEW CHAIN FOLDING MECHANISM FOR NYLONS

The structure and morphology of solution grown nylon 46 crystals have been determined and compared with those of other polyamides. They have similar lath-shaped crystallization products, possessing the characteristics of chain-folded lamellar crystals with a thicknesses of approximately 6 nm, and often aggregate to form sheaves. Electron diffraction patterns show that the chains lie perpendicular to the lamellar surfaces and that crystal twinning occurs. Surface decoration techniques reveal for the first time in a nylon that the fold plane corresponds to the plane of the hydrogen bonds, themselves lying along the long axis of the crystal in agreement with the electron diffraction results. Sedimented mats of the crystals were examined using both wide- and low-angle X-ray diffraction in directions perpendicular to the lamellar normals. The lamellar thickness remains constant up to 125-degrees-C and then increases with crystallization temperature T(cr). Analyses of the X-ray diffraction patterns confirms without doubt that the chain stems in nylon 46 chain-folded crystals are parallel to the lamellar normal in contrast to previous results from other polyamides, e.g. nylon 66, 6.10, and 6.12 where the chain stems are inclined at substantial angles to the lamellar normal. The consequences of these findings demand an entirely new mechanism for folding in nylons, where an amide group is incorporated in the fold, rather than alkane segments as in previous chain-folded nylons. The fold exhibits similarity with the beta-bend in proteins. The lamellar thickness is sufficient to accommodate four structural repeats of the chain. The crystallographic subcell is monoclinic, with parameters a = 0.96, b = 0.826, and c (chain axis) = 1.47 nm and gamma = 115-degrees, and contains four chain segments (two sheet segments), which bears similarity with the unit cell of nylon 6.