Structures of monodisperse nylon 6 oligoamides. Onset of chain-folding

Crystals were grown of the 5-amide, 9-amide, and 17-amide nylon 6 monodisperse oligoamides and investigated using electron microscopy (real space and diffraction) and X-ray diffraction. Analyses of the data allowed us to determine the crystalline structures and relate them to the morphology. Under our crystallization conditions, the 5-amide chains are unfolded and crystallize in the usual nylon 6 cc-structure, i.e., an apolar arrangement of chains, directed parallel to the layer normal, within hydrogen-bonded sheets which stack via van der Waals interactions. In chain-folded nylon polymers such as nylon 6, the length of the straight stems is approximately 5 to 7 nm; this is equivalent to about six to eight amide units. Therefore it is not too surprising that the 5-amide molecule, of length 4.6 nm, remains unfolded. If the 9-amide molecules (length 8 nm) fold, we would expect them to create hairpin-like structures with four, or close to four, amide units in adjacent straight stems. Our results show that, depending on the crystallization conditions, both unfolded and once-folded conformations can occur for this 9-amide oligomer. In the unfolded conformation, the straight-stem chains crystallize in the nylon 6 gamma-phase structure as opposed to the usual ex-phase structure. The once-folded structure for the 9-amide chain represents the onset of folding in these nylon 6 monodisperse oligoamides and the lamellar stacking periodicity (LSP) is 4.60 nm. This once-folded conformation is similar to the usual nylon 6 a-phase structure. The 17-amide chains crystallize in a twice-folded conformation, also with the nylon 6 a-phase structure, reinforcing the notion that once the chains are long enough to start folding, the pattern of behavior approaches that of the nylon 6 polymer. In this case, the LSP value is 5.36 nm.