Formation of micro-crystals in poly(ethylene terephthalate) fiber by a short heat treatment and their influence on the mechanical properties

The morphological change of poly(ethylene terephthalate) (PET) fibers by a short heat treatment under free-to-relax condition (i.e., without mechanical constraints imposed on the fibers during the treatment) was investigated. Heat treatment on polymeric fibers, in particular free-to-relax condition, has been known to lower the initial elastic modulus due to the relaxation of the amorphous molecules; however a short heat treatment at 190 °C for 1.2 s in the present study increased the initial modulus, whereas the yield strength was decreased significantly. During the short heat treatment, the PET molecules in the PET fibers were relaxed and became crystallized to some extent. The PET chains in the amorphous regions were also relaxed, promoting the formation of micro-crystals. These micro-crystals in the amorphous region can explain the increase in the initial modulus. The mechanism for such unusual behavior was investigated using mechanical tests, thermal stress analysis, wide and small angle X-ray diffraction, and FTIR spectrum analysis. Furthermore, a morphological model for the molecular arrangements in the PET fibers due to the short heat treatment is proposed, offering the possibility of developing PET fibers with shape retention function that can behave similarly to metal fibers.