Comparison of Structures and Behavior on Heating of Solution-Grown, Chain-Folded Lamellar Crystals of 31 Even−Even Nylons

Abstract

A wide-ranging and varied group of 31 even-even nylons, in the form of adjacent, re-entry chain-folded lamellar crystals, are compared and contrasted with respect to their room-temperature structures and behavior on heating. In this comparison, various relationships and trends emerge that provide a clearer understanding of the salient features that control the competitive interplay between the nylon chemistry, the crystal structure of the lamellar core, and the nature of the folds. For eveneven nylons with differing alkane segment lengths, only one type of hydrogen-bonded sheet is found; for those with equal alkane segment lengths (2N 2(N + 1) nylons), two types of hydrogen-bonded sheet are found and there is direct coupling with the fold chemistry. In some cases the fold chemistry and/or stereochemistry dictate the sheet structure (e.g., nylon 4 6), while in other cases the reverse is true (e.g., nylon 4 4). There is no chain directionality in these even-even nylons, and it transpires that the two distinct alkane segments, diamine and diacid, can independently influence the final structure and behavior on heating. Relatively high intrachain amide density molecules (e.g., nylon 2 4) need to incorporate amide units within the adjacent, re-entry folds, and the fold geometry can bear a resemblance to the folding found in apβ-sheet proteins. For molecules with short, dimethylene, diamine alkanes (2 Y nylons) the proximity of the intrachain amide units perturbs the all-trans conformation; however, for molecules with similarly short, dimethylene, diacid alkanes (X 4 nylons) the all-trans conformation occurs. Nylon isomer pairs with inverted amides (nylons X Y and Y-2 X+2) form sheets with the same hydrogen-bonded lattice parameters; however, these pairs usually exhibit different sheet stacking and behave differently on heating. Comparisons are made between the behavior on heating, including the Brill transformation, of the 31 even-even nylons.

Polymorphism of poly(nonamethyleneoxamide) crystal

“…The wide-angle X-ray diffraction (WAXD) patterns of these nylon crystals give two strong Bragg reflections at d-spacings ca. 4.4 and 3.7 Å, which are similar to those of the -form of even-even nylons [1,2].…”

“…The crystal structures for aliphatic nylons have been widely investigated [1][2][3][4][5]. Generally, the  form appears in the crystals of even-even nylons [1,6].…”

Hydrogen bonding, mobility, and structural transitions in aliphatic polyamides

“…92 This behavior has been observed in many other nylons. [93][94][95][96][97] Figure 9 shows some recent examples of such results. The first peak at 2h$208 corresponds to the intrasheet distances between the hydrogen-bonded chains, and the second peak at 2h$248 corresponds to intersheet distances.…”

Amorphous and crystalline phase interaction during the Brill transition in nylon 66

“…At the transition point the two diffraction maxima are replaced by one with a spacing of 0.42 nm [6][7][8]. Over the years since its discovery, the Brill transition has been studied for various types of even-even and odd-even polyamides [9][10][11][12][13] and for different crystallization conditions [10]. In a recent study on the Brill transition in transcrystalline nylon 66 we pointed out that the explanation for the effect of the crystalline structure on this first order transition must be based on the existence of reciprocal interactions between the crystalline and amorphous phases [14], which corresponded to the finding that structural changes within the crystalline domains in the lamellae (as in the Brill transition) are accompanied by changes in the packing of the amorphous chain segments outside the lamellae [15].…”