Crystal structure of the nylon-6 α form as established by wide-angle X-ray and neutron diffraction data analyses
摘要
About 70 years ago, the crystal structure of the nylon 6 α form was proposed by Bunn et al. (Bunn’s model) on the basis of wide-angle X-ray diffraction (WAXD) data analysis. The nearly extended planar-zigzag chains with alternating upward and downward orientations are regularly connected by intermolecular hydrogen bonds to form a sheet plane. These sheets are stacked regularly under the space group symmetry of P21. The quantitative analyses of WAXD and WAND data, which were measured for the first time in the present study, revealed the following. (i) Bunn’s model can be constructed using the space group P21/n of higher symmetry (this revision is important in the structure‒property discussion), (ii) but their regular model cannot reproduce both WAXD and WAND data without any issues. Moreover, (iii) the introduction of a statistically disordered up/down chain packing concept can reproduce these observed diffraction data consistently. As one local structure, the aggregation of domains has been proposed, where the domain itself consists of regular chain packing, but these domains are randomly aggregated together with the ±a/2 shift and a small c-axial shift to form a crystalline lamella. In this way, the crystal structure of the nylon-6 α form has been conclusively established.