POLYAMIDES WITH A CHOICE OF STRUCTURE AND CRYSTAL-SURFACE CHEMISTRY -STUDIES OF CHAIN-FOLDED LAMELLAE OF NYLON-8-10 AND NYLON-10-12 AND COMPARISON WITH THE OTHER 2N2(N-4-6 AND NYLON-6-8(1) NYLON)

Nylons 8 10 and 10 12 have been synthesized and. crystallized as chain -folded lamellae from 1,4-butanediol and the results compared with pre vious studies on Nylons 4 6 and 6 8. In 2N 2(N + 1) Nylons, the length s of the two alkane segments are equal and two different hydrogen-bond ed sheet schemes are possible: progressive or alternating shear. At ro om temperature, Nylons 8 10 and 10 12 adopt the progressive scheme and the adjacent re-entry folds in the crystals must be in the alkane cha in segments. In contrast, Nylons 4 6 and 6 8 lamellae, crystallized fr om the same solvent, exhibit the alternating hydrogen bonding scheme a nd each adjacent re-entry fold must contain an amide group. The transi tion in the chemical nature of the lamellar surface, from the amide fo ld to the alkane fold, occurs in passing from Nylon 6 8 to 8 10. Thus, the progressive hydrogen-bonded sheet/alkane fold structure is energe tically more favorable, provided the alkane-folding geometry is suffic iently relaxed; this comes with increasing alkane segment length. For each hydrogen-bonded sheet structure there are still two principal int ersheet stacking modes in lamellar crystals: the progressively sheared alpha-phase or the alternatingly sheared beta-phase, both of which ha ve been found in the 8 10 and 10 12 Nylons. The 2N 2(N + 1) Nylons hav e the choice of four possible structures. The melting points of soluti on grown crystals of Nylons 4 6, 6 8, 8 10, and 10 12 decrease with de creasing intrachain amide density. When lamellar crystals of these Nyl ons are heated, the two characteristic interchain diffraction signals move together and meet at their Brill temperature; for Nylon 10 12 it appears to be close to the melting point.