Cooper SJ, Atkins EDT, Hill MJ
H.H.Wills Physics Laboratory University of Bristol, Tyndall Avenue, Bristol BS8 1TL, U.K.
Macromolecules. 1998 Jul 28;31(15):5032-42. doi: 10.1021/ma9805194.
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 alpha-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 alpha-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 alpha-phase structure. The 17-amide chains crystallize in a twice-folded conformation, also with the nylon 6 alpha-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.
5-酰胺基、9-酰胺基和17-酰胺基尼龙6单分散低聚酰胺的晶体生长，并使用电子显微镜（实空间和衍射）以及X射线衍射进行研究。对数据的分析使我们能够确定晶体结构并将其与形态学联系起来。在我们的结晶条件下，5-酰胺基链展开并以通常的尼龙6α-结构结晶，即链的非极性排列，平行于层法线方向，在通过范德华相互作用堆叠的氢键片层内。在诸如尼龙6的链折叠尼龙聚合物中，直链茎的长度约为5至7纳米；这相当于大约六至八个酰胺单元。因此，长度为4.6纳米的5-酰胺基分子保持展开状态并不太令人惊讶。如果9-酰胺基分子（长度8纳米）折叠，我们预计它们会形成发夹状结构，相邻直链茎中有四个或接近四个酰胺单元。我们的结果表明，根据结晶条件，这种9-酰胺基低聚物可能出现展开和一次折叠构象。在展开构象中，直链茎链以尼龙6γ-相结构结晶，与通常的α-相结构相反。9-酰胺基链的一次折叠结构代表了这些尼龙6单分散低聚酰胺中折叠的开始，层状堆叠周期（LSP）为4.60纳米。这种一次折叠构象类似于通常的尼龙6α-相结构。17-酰胺基链以两次折叠构象结晶，同样具有尼龙6α-相结构，强化了这样一种观点，即一旦链长到足以开始折叠，其行为模式就接近尼龙6聚合物。在这种情况下，LSP值为5.36纳米。
