Nature and structure of once-folded nylon 6 monodisperse oligoamides in lamellar crystals

Lamellar crystals of the monodisperse 10-amide nylon 6 oligomer were prepared from solution. The crystal structure and morphology were studied using electron microscopy (diffraction and imaging) from individual lamella and X-ray diffraction data from sedimented oriented mate. The lamellar stacking periodicity (LSP) was found to be 4.77 nm, approximately half the length of the all-trans conformation (8.9 nm) for the 10-amide molecule. Under the rapid crystallization conditions used, the molecules fold at the midpoint to form a symmetric hairpin conformation. The straight stems are aligned orthogonal to the lamellar surface and crystallize in the nylon 6 polymeric a-structure, i.e., hydrogen-bonded sheets stacking via van der Waals interactions. A tight fold occurs in the central alkane segment, and the five amide units in each of the straight stems form intramolecular hydrogen bonds. The once-folded conformation and lamellar morphology of the 10-amide molecules are compared with the structure reported for once-folded 9-amide nylon 6 molecules, crystallized under the same conditions; these molecules fold via the central amide unit to form a symmetric hairpin conformation. Thus, the combined results, from both the once-folded 10-amide and 9-amide crystals, suggest that the requirement for all the amide units in the straight-stem pair to form intramolecular hydrogen bonds, together with fully saturated intermolecular hydrogen bonding in the crystalline straight stems, results in symmetric hairpin conformations; the nature of the fold, alkane or amide, is coupled to the conformation of the once-folded molecules. There are two possible alkane fold types (1 and 2) for the 10-amide molecule. Crystalline lamellae with mixed fold types have thicknesses at least 4% greater than the measured mean LSP value. Type 1 folded molecules are favored for energetic reasons and can only juxtaposition side by side; thus, within the apolar lamellae the hydrogen-bonded sheets are polar. The implications of this feature are discussed in terms of chain-folding in the nylon 6 polymer. The evidence shows that the nylon 6 molecule can form tight, adjacent re-entry folds via amide units or via alkane segments on crystallization.