| Abstract
| - Crystals were grown of the 5-amide, 9-amide, and 17-amide nylon 6 monodisperseoligoamides 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 nylon6 α-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 nylon6, the length of the straight stems is approximately 5 to 7 nm; this is equivalent to about six to eightamide units. Therefore it is not too surprising that the 5-amide molecule, of length 4.6 nm, remainsunfolded. If the 9-amide molecules (length 8 nm) fold, we would expect them to create hairpin-likestructures 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 forthis 9-amide oligomer. In the unfolded conformation, the straight-stem chains crystallize in the nylon 6γ-phase structure as opposed to the usual α-phase structure. The once-folded structure for the 9-amidechain represents the onsetoffolding in these nylon 6 monodisperse oligoamides and the lamellar stackingperiodicity (LSP) is 4.60 nm. This once-folded conformation is similar to the usual nylon 6 α-phasestructure. The 17-amide chains crystallize in a twice-folded conformation, also with the nylon 6 α-phasestructure, reinforcing the notion that once the chains are long enough to start folding, the pattern ofbehavior approaches that of the nylon 6 polymer. In this case, the LSP value is 5.36 nm.
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