AbstractThe structure of melt‐spun nylon 6 filaments was studied using on‐line x‐ray diffraction and birefringence measurements. Measurements were also made on as‐spun and treated filaments. On‐line wide‐angle x‐ray scattering measurements indicated that crystallization did not occur on the nylon 6 spinline at spinning rates up to 1000 m/min when spinning was done into either ambient air of 60 relative humidity or into wet saturated air. The filaments did crystalline gradually on the bobbin to a paracrystalline pseudohexagonal (γ) form. The rate of crystallization was dependent on the molecular orientation developed in the spun filaments. Crystalline orientation factors based on hexagonal symmetry were computed as a function of take‐up velocity for fibers which were conditioned 24 hr in air at 65 relative humidity. Annealing in air or treatment in water or 20 formic acid solution causes a transformation from the pseudohexagonal form to the α monoclinic form. The tangent modulus of elasticity and tensile strength of spun and conditioned filaments increase with increasing take‐up velocity and spinline stress, while elongation to break decreases w
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