Structural and morphological investigations of nylon 8 chain-folded lamellar crystals

Abstract

The structure and morphology of nylon 8 chain-folded lamellar crystals have been studied using transmission electron microscopy including selected area electron diffraction. Solution grown lamellar crystals are lathe-like in appearance. Two types of diffraction pattern have been indentified, indicating two crystal forms. The normal room temperature form is monoclinic with a = 0.964nm, b = 0.803nm, c, the chain axis = 2.24 nm and γ = 115°. This is similar to the α form reported for fibres. Crystals quenched into non-solvent from the crystallization temperature show a different structure for which we propose a pseudo-hexagonal unit cell with a - b = 0.965 and γ = 120°. When monoclinic crystals are heated in the hot stage of the transmission electron microscope the diffraction pattern changes, gradually transforming towards the pseudo-hexagonal phase. For crystals prepared under some conditions the transformation is completed, but in others melting takes place before the full pseudo-hexagonal phase is reached. It is argued that the low temperature phase of nylon 8 single crystals is monoclinic and the pseudo-hexagonal phase is the stable form at high temperatures. Crystals transform from monoclinic towards pseudo-hexagonal on heating. The crystals grow in the high temperature, pseudo-hexagonal phase and this can be obtained at room temperature by quenching from the crystallization temperature into non-solvent below the glass transition temperature.