AbstractA systematic study of thermo‐mechanical properties of high speed spun poly(ethylene terephthalate) fibers was undertaken as part of a broad quantitative process—structure—property examination of semicrystalline polymers through appropriate composite models. Calorimetric experiments on a series of fibers spun at different speeds show that crystallinity increases with takeup speed rapidly below 4500 m/min but at a slower rate above that speed. The glass transition, cold crystallization, and melting temperatures were also found to increase with speed. A similar increase is exhibited by both the refractive index of these fibers along the fiber axis and the fiber birefringence. Comparison of birefringence measurements with those predicted from a composite two‐phase optical model led to the conclusion that form birefringence may be a significant contributor to the overall birefringence and that it may not be ignored in calculations of orientation functions with this experimental te
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