Thermal deformations of oriented noncrystalline poly (ethylene terephthalate) fibers in the presence of mesophase structure

Thermally induced dimensional changes, thermal shrinkage and elongation, in oriented noncrystalline PET fibers were investigated. The fibers exhibited two very distinct thermal responses depending on the fiber orientation. The local structure of the oriented noncrystalline PET chains as studied by the X-ray diffraction and FTIR spectroscopy revealed the mesophase structure with the well extended chain conformation in some fibers of high orientation. It was suggested that the oriented noncrystalline structure of PET consists of partially oriented noncrystalline phase and chain-extended noncrystalline phase. Our results demonstrated that the evolution of mesophase structure, i.e. chain-extended noncrystalline phase in the spin line not only led the drastic increase of packing density but also had a strong effect on thermal deformations upon post heat treatment. The amount of thermal shrinkage or the elongation reduced drastically in the fibers containing the mesophase. The high population of trans conformer and the strong inter-chain interactions of the extended chains provided the dimensional stability of the fibers during the thermal treatment.