Effect of Internal Stress on Lattice Strain in Relaxed Heat Set PET Filaments

Using the method of resolving overlapping peaks, wide angle x-ray scattering data for relaxed heat set poly(ethylene terephthalate) (PET) filaments were analyzed and the characteristic parameters of some main crystal peaks were obtained precisely. The relative lattice strains of the filaments treated at different heat setting temperatures were thus determined. With the help of a thermomechanical analyzer, the thermal shrinkage stresses were also measured. Analysis of these experimental results indicated that the crystal structure of polymer fibers differs from that of low molecular weight substances, in that the unit cell parameters of the former are much more influenced by the internal stresses left in the fiber during the heat setting of the drawn polymer fibers. The series model is suitable for explaining these phenomena.