Morphology and Structure of Nylon 68 Single Crystals

Abstract

The structure and morphology of nylon 68 single crystals were studied by transmission electron microscopy. There are two crystal phases. A monoclinic phase with a = 0.960 f 0.005 nm, b = 0.826 f 0.005 nm, and y = 115 f lo, when viewed along the chain axis, c, is the usual form for crystals cooled slowly from the crystallization temperature. This lattice is the same, within experimental error, as that of monoclinic crystals of nylon 46, a polymer which has the same chain structure, but with amide groups more closely spaced. Nylon 68 crystals quenched into nonsolvent from the crystallization temperature are found in a pseudohexagonal phase which has parameters a = b = 0.97 nm and y = 120". When monoclinic crystals are heated, they transform, gradually, into the pseudohexagonal phase; the transformation is complete at 203 "C, and melting takes place at 234 "C. The crystals grow from solution in the pseudohexagonal phase which is stable at high temperatures; they usually revert to the monoclinic phase on cooling, but the pseudohexagonal phase can be obtained at room temperature by quenching to below the glass transition temperature. Monoclinic nylon 46 single crystals were examined on heating. They transformed to the pseudohexagonal structure at 245 "C, before melting took place at 295 "C.

Structure and morphology of nylon 2 4 lamellar crystals: Comparison with polypeptides and relationship with other nylons

“…The chemical synthesis and detailed chemical characterization has already been reported by Wang, Shao, and Yu. 18 Unlike many other aliphatic polyamides, [1][2][3][4][5][6]19,20 nylon 2 4 will not thermally crystallize from polyols, such as 1,4-butanediol, and alternative crystallization methods need to be used. The polymer was crystallized from a 0.1% w/v solution in phenol at Computer-generated space-filling models of: (a) the nylon 2 4 apolar chain; (b) the nylon 3 polar chain.…”

“…Lamellar crystals were also prepared from a 0.1% w/v solution of nylon 2 4 in formic acid in two different ways: (1) holding the solution at 90°C and then adding, at the same temperature, four parts of n-butanol to precipitate the polymer; (2) holding the solution at room temperature and adding two parts of n-butanol to precipitate the polymer.…”

“…At room temperature, the hydrogenbonded sheets stack together via van der Waals interactions. Previous studies of lamellar crystals of nylons 4 6 1 and 6 8, 2 crystallized from 1,4-butanediol, have shown that at room temperature these 2N 2(Nϩ1) nylons adopt a-sheets that stack with an alternating shear, while, under similar conditions, nylons 8 10 and 10 12 adopt p-sheets, and usually these p-sheets shear progressively. 3 We have argued 3 that p-sheet/alkane folds are more stable than a-sheet/amide folds when crystallized from solution, but that nylons 4 6 and 6 8 form a-sheets because each individual alkane segment (diamine alkane or diacid alkane) is insufficiently long to complete an unstrained fold.…”

“…Across the range, from nylons with long alkane segments, 12 12 and 6 18 for example, to nylons 4 4 and 2 6, we find similarities and also subtle and important differences in both crystal structure and behavior on heating. [1][2][3][4][5][6] The length of the alkane segments affects the melting behavior 7 and can influence the type and nature of the adjacent reentry folds; below a certain length, the alkane segments alone are not able to achieve stereochemically unstrained folds, and amide units need to be incorporated within the folds. 3 We now consider nylon 2 4, which has only two methylene units in both the diamine and diacid alkane segments, as shown in Figure 1(a).…”

“…These nylons are interesting because they are able to form two types of hydrogenbonded sheet structures. [1][2][3] For sufficiently long alkane segments, these nylons can form chainfolded hydrogen-bonded sheets with progressively sheared amide units (called p-sheets 3 ). In these sheets the chains tilt at an angle of 13°to the chain-fold normal, and there is a single alkane segment in the chain fold, 3 as shown in Figure 2(a).…”