Comparison of structures and behavior on heating of solution-grown, chain-folded lamellar crystals of 31 even-even nylons

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 even-even 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 ape-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.