Changes of mechanical properties and phase separated structure of thermotropic polyurethane fiber under the periodic elongation and recovery

The periodic structure, which is composed of hard and soft domains, oriented along the fiber axis in the melt spun thermotropic polyurethane fiber. The phenomena of structure and mechanical properties change can be divided into three extension ratio ranges, less than 1.2, from 1.2 to 1.5, and more than 1.7. There are little changes in the structure and the properties observed for extension ratio less than 1.2. The periodic structure became apparent at the extension ratio 1.2 - 1.5. The short relaxation time component, the maximum stress, and the energy loss were steeply increased at that extension range. The periodic structure vanished by over 1.7 times extension, but it reconstructed by the relaxation process. The residual strain and the loss energy decreased obviously by the previous extension-recovery For example, the ratio of 2(nd) cycle residual strain to the 1(st) cycle one was about 30% for extension ratio 1.5 and 1.7, whereas it was 13-19% for extension ratio more than 2.