Structure analysis of chain‐folded lamellar polyamide crystals from X‐ray diffraction

Abstract

Folded‐chain crystals of certain polyamides present some novel diffraction effects due to the small number of repeat units within the lamellar thickness. X‐ray diffraction evidence is available in the complete range from low to wide angles. This information is interpreted in terms of the structure factor of an individual lamella together with the lattice factor appropriate for the stacking of lamellae. When due account is taken of the lattice factor, whose effect can be detected even at large angles, three features of the lamellar structure can be deduced. First, the evidence is in favor of the straight‐chain stems traversing almost the total thickness of each lamella, implying sharp folds at the lamellar surfaces. Some consequences of this result on the interpretation of data obtained from annealed mats are mentioned. Second, the detailed determination of the stem structure demands that the majority of the folds in nylon 66 lie in the acid group. Third, there are regions of depleted electron density at the lamellar surfaces, though features of the crystal structure are still retained. This indicates the presence of some folds deeper in the crystal than the majority.

A comparative DSC, X-Ray and NMR study on the crystallinity of isomeric aliphatic polyamides

“…patterns between these two sets of isomers is in the relative intensity of the 001, 002 and 003 reflections. It can be shown that this depends on the relative position of the planes -parallel to the lamellar surfaces -that contain the hydrogen bonds [21]. There are two such planes per crystalline unit cell and the distances between them are identical for the polyamides 4.12 and 10.6 on the one hand and for the polyamides 6.10 and 8.8 on the other hand.…”

Temperature-induced changes in chain-folded lamellar crystals of aliphatic polyamides. Investigation of nylons 2 6, 2 8, 2 10, and 2 12

“…The unit cell parameters are given in Table Ib and and 002 there are two subsidiary maxima (marked 's') indicating four repeats within the chain-folded lamellar crystals. 2 Subsidiary maxima also occur in the region between the 001 peak and the origin; one of these (marked 's') is visible between the inner 001 peak and the beam stop. The 5.48 nm small-angle diffraction signal, representing the interlamellar stacking periodicity, is shown as an inset in lower left-hand corner.…”

“…In addition, two subsidiary maxima occur between the even nylons with longer diamine alkane segments. [1][2][3][4][5][6][7][8][9][10][11] The observed spacings for the 2 Y ny-001 and 002 diffraction signals. 2 Two more subsidiary maxima also occur between the 001 and the lons are collated in Table Ia.…”

“…Directionally specific arated, at regular intervals, by alkane sequences interchain hydrogen bonds are formed between neighboring amide groups and it is these hydro-the case of nylon 6 6 single crystals, hydrogen bonds can be linear only if the chains progressively shear within each hydrogen-bonded sheet, as illustrated in Figure 2, and this is the structure found experimentally. [1][2][3] This hydrogen bonding pattern will give linear hydrogen bonds for all even-even nylons.…”

Structure of Polymer Single Crystals

“…Mats of such lamellae give rise to the strong reflections and subsidiary maxima characteristic of the lamellar and unit cell structure. Their analysis (Atkins et al, 1972) indicates that folding takes place in the diacid part of the polyamide 6-6, and therefore locates very precisely the fold along the chains. The fact that folding occurs in the methylenic segment of the chemical sequence is not unexpected: the original feature however rests in the selection, among two possible methylene sequences of the shorter C 4 of the diacid, rather than the longer C 6 one of the diamine part.…”