The structure of poly (epsilon-caprolactam) (nylon 6) in the beta 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 beta 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; gamma = 120 degrees). (b) The chains have disordered conformations (and do not have a definite chirality, as it is the case for the 2(1) helices in the alpha and gamma forms) with the -CH2- chains close to nearly all-trans (antiperiplanar) and the two dihedral angles adjacent to the amide bond, +/-120 degrees) to antiperiplanar (180 degrees). 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 [1(1) over bar0$] 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 alpha and gamma 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 alpha form) or of isomorphous isoclined chains, (in the gamma form), (d) As a consequence, since the amide groups lie all at nearly the same height along z, in the beta form of nylon 6 the lines of hydrogen bonds lie in layers perpendicular to the chain asis 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 degrees instead of the ''normal'' 180 degrees). (e) In the beta form of nylon 6, disorder arises also from the random substitution of up and down chains in the lattice positions.
