20—TEMPERATURE-DEPENDENCE OF BIREFRINGENCE OF NYLON 6.6 AND THE STRUCTURE OF SPHERULITES

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Abstract
The reversible temperature-dependence of birefringence was measured for fibre-oriented drawn filaments, doubly oriented rolled filaments and their cross-sections, and positive and negative spherulites. The pseudo-hexagonal symmetry of the unit cell, known from X-ray results to be attained above 160°C (Brill transition), is achieved by anisotropic thermal expansion and not by break-up of the planes of associated amide groups to give a three-dimensional random array of hydrogen bonds as suggested by earlier workers. The changes in refractive index with temperature are consistent with increasing torsional oscillations of hydrocarbon segments. The birefringence data for spherulites are consistent with negative spherulites growing with 3 to 2 degrees of orientational freedom of the unit cell (‘c’ axis tangential) while positive spherulites grow with only one degree of freedom of unit cell orientation (‘a’ axis radial, ‘c’ axis roughly tangential). The variation in magnitude and sign of birefringence with temperature of growth of the spherulites is consistent with this proposed morphology.

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