Chain-folding and structures in nylon 6 oligoamide lamellar crystals

A series of carefully selected monodisperse nylon 6 oligomers, terminated with short alkane segments, have been crystallised from solution. This contribution contains an overview of the folding and structures in nylon oligomer crystals, recorded at room temperature, using transmission electron microscopy and X-ray diffraction (both wide- and small-angle); together with important conclusions drawn from studies of these molecules. The work is set in the context of much earlier studies by Professors Zahn and Keller and their co-workers. The oligomers contained 2, 3, 5, 9, 10 and 17 amide units, with a length range of 2 nm to 15 nm. The shorter oligomers (with 2, 3 and 5 amide units) crystallised with non-folded chains; the longer 9- and 10-amide oligomers were once-folded, following rapid crystallisation; whilst the 17-amide oligomer was found to fold twice. Thus chain folding sets in on rapid crystallisation, with the 9-amide molecule (length ∼8 nm). Studies of the once folded 9- and 10-amide oligomers show that both amide unit and alkane segment folds occur, in this instant the symmetry of the hairpin and the requirement for saturated hydrogen bonds between straight stems are the controlling factors; the nature of the fold is subservient. This shows that nylon 6 chains can stereochemically accommodate either alkane or amide folds. All the folded molecules were observed in the nylon 6 α-structure, where chains are antiparallel within the hydrogen-bonded sheets, which have alternating chain (c)-axis shear alternately. In the shorter oligoamides (2-, 3- and 5-amide), where the molecules do not fold, the crystallisation is less restricted (e.g. the chains can form a parallel array) and two new crystal structures have been identified. In the 2- and 3-amide crystals, hydrogen bonds can occur in two directions between parallel chains and orthogonal to the molecular axis (c-axis), which is inclined at a substantial angle to the lamellar surface (the κ-structure). The 5-amide oligomer also does not fold. In this case, the hydrogen bonds are made between antiparallel chains, as in the nylon 6 α-structure; however, these sheets progressively c-axis shear to give a new structure (the λ-structure). Chain-folded molecules can crystallise in the λ-structure although not in the κ-structure because it requires parallel chains.