Drawing and annealing of nylon-6 fibres: studies of crystal growth, orientation of amorphous and crystalline domains and their influence on properties

Structural changes in nylon-6 fibres that occur during processing are analysed using X-ray diffraction (X.r.d.), optical birefringence, density and infra-red (i.r.) spectroscopy methods. Amorphous orientation is described completely by a single parameter, degree of orientation, on the basis of i.r. and optical birefringence measurements. But an additional parameter, the fraction of oriented or anisotropic amorphous component, was necessary to fully interpret the X.r.d. data. The amorphous orientation increases as expected upon drawing, but decreases upon annealing at all temperatures. Crystalline orientation increases rapidly at small draw ratios (<3) and reaches a plateau at higher draw ratios. Crystalline orientation decreases upon annealing in highly drawn fibres but increases in others. The crystallinity is ∼15% in the undrawn fibre, and continues to increase beyond the ∼50% value measured for the fibre with the highest (4.5x) draw ratios used here. Secondary crystallization during drawing occurs at the expense of unoriented amorphous fraction, and in the initial stages of drawing (3x) is accompanied by the transformation of the γ into the α crystalline form. Secondary crystallization during annealing occurs at the expense of the oriented amorphous fraction. The ease of γ to α conversion during annealing depends on the initial state of the γ form. The glass transition temperature (Tg) appears to be determined solely by the amorphous orientation, being higher in fibres with a higher anisotropic component. The improved mechanical properties observed at higher draw ratios are attributed to higher crystallinity and lower fraction of the unoriented amorphous component. The shrinkage observed upon annealing of fibres with smaller draw ratios (∼3) is likely to be the result of the crystallization of the oriented amorphous chain segments.