Nylon 6 fibers: Changes in structure between moderate and high draw ratios

Abstract The in-phase modulus and coefficient of diffusion of nylon 6 fibers are analyzed in terms of wide-and small-angle X-ray diffraction data. Both properties are examined in directions parallel and perpendicular to the fiber axis. The anisotropy in diffusion and mechanical coupling between the crystalline and amorphous phase reaches its maximum at relatively low draw ratios of about 2.5 to 3.0X. With increasing draw ratio the anisotropy in these properties decreases monotonically and reaches its minimum value with the fibers of the highest draw ratio (5.35X). The diffusion analysis yields a heretofore undetermined structural parameter, the separation of the microfibrils. The results indicate that increases in draw ratio lead to an increase in the distance between the microfibrils, a decrease in the diameter to length of the crystallite, and a decrease in the diffusion constant of the permeable phase. The longitudinal structure of the microfibril is not affected significantly during this phase of drawing. These observations cannot be explained by the microfibrillar fiber model derived from studies of polyethylene and polypropylene fibers. A new structural model is proposed in which the strength, diffusion, and modulus are controlled by the densely packed matrix. The model is corroborated by transmission electron micrographs from thin fiber cross sections.