Crystallization and transient mesophase structure in cold-drawn PET fibers

Transient structure and crystallization behaviors in cold-drawn PET fibers were studied by synchrotron wide-angle X-ray diffraction, IR spectroscopy, and thermomechanical analysis. As-spun PET fibers were mechanically cold-drawn up to the breaking point, and those stretched ones beyond the strain hardening point in the stress-strain curve exhibited the transient layer structure. Evidence suggesting the tilted PET chains within the transient layer structure, and therefore the smectic C mesophase, was discussed. The tilt angle of the chains within the layer was 6° against the fiber axis. The tilt angle, however, decreased rapidly with the crystallization. Results of X-ray diffraction, IR absorbance, and thermomechanical analysis suggested two-stage crystallization in oriented PET chains. Bundles of highly oriented chains including the mesophase were responsible for the first stage crystallization (80-100°C), but those of less ordered or nonoriented chains were for the second stage crystallization beyond 140°C. The structural transformation from the transient mesophase to the triclinic crystal structure was interpreted by the chain sliding mechanism, leading to the tilted (001) planes against the fiber axis.