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.

SYNOPSISYarns of different inherent viscosities, in the range 0.6-1.1 dL/g, spun in industrial plants, and drawn at room temperature to obtain mesomorphic samples, have been characterized. The evolution from the mesomorphic form toward the triclinic crystalline form has also been studied by combined differential scanning calorimetry ( DSC ) , dynamic mechanical analyses (DMA), and accurate wide angle X-ray diffraction experiments. The DMA analysis of the mesomorphic samples allows better resolution of the glass transition and crystallization phenomena, which are superimposed in the DSC scans. The degree of molecular orientation in the mesomorphic samples, and the temperature of crystallization from the mesomorphic form (SO-80°C) , are essentially independent of the polymer molar mass.

SYNOPSISThe literature methods for the determination of the mean of the crystallite orientation distribution for the c axis, that is of the orientation coefficient f,, for poly(ethy1ene terephthalate) (PET), based on the azimuthal scan of the (i05) reflection, are reviewed. These methods appear unsuitable for samples presenting the "tilted orientation"; that is, the molecular chain axis inclined by some degrees with respect to the fiber axis, as frequently occurs for P E T fibers. A new method for the determination of fe for PET, also based on the azimuthal scan of the (i05) reflection (which can be applied also to samples with "tilted orientation"), is proposed. This method implies as a first step the determination of the tilt angle, for which the complete fiber pattern is required. A possible simplifying assumption, which allows use of the sole azimuthal (705) profile and makes the method also applicable to poorly oriented samples (for which the determination of the tilt angle is not easy), is also discussed. 0 1995 John Wiley & Sons, Inc.

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