Mesomorphic Form (β) of Nylon 6

Abstract

The structure of poly (ε-caprolactam) (nylon 6) in the β mesomorphic form is here examined. The comparison of the diffraction intensity, calculated on modeled structures, with the experimental profiles, collected by us through an automatic diffractometer, is presented. This analysis has put into evidence the following limiting structural features. (a) The β form of nylon 6 is made of small mesomorphic aggregates of chains (where the matter scatters coherently) with axes arranged in a hexagonal lattice (a = b = 0.48 nm; γ = 120°). (b) The chains have disordered conformations (and do not have a definite chirality, as it is the case for the 21 helices in the α and γ forms) with the −CH2− chains close to nearly all-trans (antiperiplanar) and the two dihedral angles adjacent to the amide bond, ±120°) to antiperiplanar (180°). This notwithstanding, the chains are straight and extended. As a result, the mean chain periodicity is close to 0.835 nm. (c) The H-bonds are formed along lines in the [100], [010], and [11̄0] directions; they force neighboring chains within the small mesomorphic aggregates to adjust their conformation in such a way that nearly 100% of hydrogen bonds are always formed, in agreement with the IR data. In the case of the α and γ more ordered forms of nylon 6, such lines are all in a unique direction, leading to hydrogen-bonded sheets (parallel to the chain axis) of enantiomorphous anticlined, alternately up and down chains (in the α form) or of isomorphous isoclined chains, (in the γ form). (d) As a consequence, since the amide groups lie all at nearly the same height along z, in the β form of nylon 6 the lines of hydrogen bonds lie in layers perpendicular to the chain axis and have the same direction within each layer. However, consecutive layers along z may have the lines of H-bonds which are not parallel (e.g., occasionally rotated by +120 or −120° instead of the “normal” 180°). (e) In the β form of nylon 6, disorder arises also from the random substitution of up and down chains in the lattice positions.

Bio-polyamides based on renewable raw materials

“…This observation may indicate the coexistence of different crystalline forms, a-and c-triclinic and b-pseudohexagonal structure. The b-mesomorphic structure was identified, e.g., for PA 6 [36]; however, other studies [34,37] show that only one crystalline phase exists at room temperature in PA 10.12. These discrepancies may come from the different thermal history of the material, resulting in different crystallization conditions.…”

Hydrogen bonding, mobility, and structural transitions in aliphatic polyamides

“…17 It is most likely that phases other than a and c correspond to structures in which the hydrogen bonds form a 2D network, the extreme situation being the pseudohexagonal phase that persists at room temperature. 18,19 a 0 is the high-temperature phase of a, 20,21 and b is the disordered a phase. 15,19,22,23 A k phase is observed in PA-6 oligomers.…”

“…18,19 a 0 is the high-temperature phase of a, 20,21 and b is the disordered a phase. 15,19,22,23 A k phase is observed in PA-6 oligomers. 24 The a-and c-crystalline forms can be readily distinguished by their X-ray diffraction (XRD) patterns [ Fig.…”

Morphological Study by TEM on Thin Films of Nylon-6, Nylon-6,6, and Their Blends As-cast from Formic Acid Solution

“…At the present stage, however, the possibility of N66 β-form (triclinic (P1); a = 0.49 nm, b = 0.80 nm, c (chain axis) = 1.72 nm, α = 90 • , β = 77 • , γ = 67 • ) 19 might not be disregarded for the blend films when they are made by casting from solution in formic acid. 21 Although the β mesomorphic form of N6 has recently been reported for melt-spun fibers, 20 this β-form of N6 was disregarded in this study by judging from our method of specimen preparation (namely, a solution-casting method). More study is, of course, needed for further detailed structural analysis of the blend films.…”