Polymorphic forms of nylon 3

Abstract

The crystal structure of nylon 3 was studied, and four crystalline modifications were observed. Modification I, as determined from the x‐ray diffraction pattern of drawn fibers, is similar to the α crystal structure of nylon 6. The unit cell is monoclinic; a = 9.33 Å, b = 4.78 Å, (fiber identity period), c = 8.73 Å, and β = 60°. The theoretical density for nylon 3 with four monomeric units in the unit cell is 1.39 g/cm3, and the observed density is 1.33 g/cm3. The space group is P21. The nylon 3 chains are in the extended planar zigzag conformation. Although other odd‐numbered nylon form triclinic or pseudohexagonal crystals when oriented, drawn nylon 3 crystals are monoclinic. In addition to modification I, modifications II, III, and IV were studied. Lattice spacings of modifications II and III are equal to those of modification I. However x‐ray diffraction intensities are different. Infrared spectra of those forms indicate an extended planar zigzag conformation of the chains. Modification IV is thought to correspond to the so‐called smectic hexagonal form. No γ crystals were found, and it appears that polyamide chains with short sequences of methylene groups cannot form crystals of this type.

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

“…The ␣-and ␤-angles were both set to 90°because the chains are known to be parallel to the lamellar surface normal. The c-value (chain axis) was taken to be twice the value reported for nylon 3, 16,17 and is consistent with an all-trans chain conformation. Form II nylon 2 4 crystals.…”

“…Because the diffraction signals in both crystalline forms are almost identical to those reported for the two forms of nylon 3 chain-folded lamellar crystals, 17 we have followed the same assignment of Miller indices (see Table I). The diffraction signals are all of the type hk0, suggesting that the polymer chains are normal to the chain-folded surface.…”

“…(An identical notation is used for nylon 3. 17 ) Electron diffraction of whole aggregates gave strong diffraction rings at 0.41 and 0.39 nm, and a weaker ring at 0.48 nm. We believe this weaker ring at 0.48 nm (and another at 0.38 nm, masked by the 0.39 nm signal, see Table I) indicates that a minor proportion of another form (form II) is also present.…”

“…Such a diffraction fingerprint is characteristic of the hk0 reciprocal lattice net of many nylon structures (c is chain axis) with hydrogen-bonded sheets stacking via van der Waals interactions; for example, the even-even nylons [1][2][3][4][5][6] and nylon 3. 17 We will refer to this as crystalline form I. (An identical notation is used for nylon 3.…”

“…16,17 Although nylon 3 [see Fig. 1(b)] has the same linear density of amide units as nylon 2 4, it is a polar chain, and as a consequence, the detailed arrangement of amide units within the sheets and the intrasheet hydrogen bonding are subtly different.…”