Discovery in nylon-8 chain-folded lamellar crystals of new structure??-structure?with progressive intersheet shear along the chain axis

Abstract

Structural studies on solution-grown, chain-folded nylon-8 lamellar crystals, using transmission electron microscopy (imaging and diffraction) together with X-ray diffraction from oriented mats, have led to the discovery of a new crystal structure. This has been termed the λ-structure because of its similarity to the recently reported λ-structure found for the nonfolded, monodisperse 5-amide nylon-6 oligomer. In the stable nylon-8 monoclinic α-structure, the flat hydrogen-bonded sheets (ac plane; chain axis c), composed of antiparallel chains, stack via van der Waals interactions with alternating c-axis shear. Thus, in chain-folded lamellae, the chain direction is orthogonal to the lamellar surface. However, in the nylon-8 λ-structure the same hydrogen-bonded sheets stack with severe progressive c-axis intersheet shear (44°), generating an antiparallel two-chain triclinic unit cell with the parameters: a (hydrogen-bonding direction) = 0.980 nm; b = 0.544 nm; c = 2.24 nm; α = 49°; β = 90°; γ = 71.4°. The structure bears a resemblance to the classic, heavily intersheet-sheared (41°) nylon-6 6 α-structure reported by Bunn et al. in 1947. As far as we are aware, this is the first time the λ-structure has been identified in polymeric even nylons and in chain-folded lamellar crystals. Changes to the principal diffraction signal spacings and the unit cell parameters with increasing temperature have been recorded. On heating, the chain-folded hydrogen-bonded sheets remain intact and move apart. Two regimes of intersheet movement have been identified: (1) the low-temperature regime (T 120 °C), where the sheets move along both the b and c directions. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3302–3308, 2000