The formation of negatively birefringent spherulites in polyhexamethylene adipamide (nylon 66)

Abstract

Crystallization of nylon 66 at and above the optical melting point of the polymer has been studied. When heated at constant temperature in the ∼250–265°C. range, the polymer, originally crystallized in the form of positively birefringent spherulites, melts and subsequently crystallizes in the form of negatively birefringent spherulites. The number of spherulites formed and their birefringence decrease as the temperature at which the polymer is heated in the ∼250–265°C. range is increased. In the limit at 265–266°C., non‐birefringent spherulites are formed in the polymer. No crystallization has been detected in polymer heated at constant temperature above 266°C. Negative spherulites are found to grow from predetermined nuclei which persist in the melt above the optical melting point of the polymer. The decrease in the number of spherulites, and hence of effective nucleation centers, with increasing temperature is taken to indicate that nucleation is from polymer aggregates or crystallites and not from foreign particles. Additional evidence is provided to support this view. The optical melting point of the negative spherulites is 268–270°C. Spherulites formed at 260°C. have a degree of crystallite perfection comparable to that of undrawn annealed nylon yarn. Etching experiments indicate that the crystallinity of the spherulites varies inversely with the temperature at which they are formed.