Stacked-lamellar structure of electrospun poly(heptamethylene terephthalate) nanofibers

Poly(heptamethylene terephthalate) (poly(7GT)), which is an aromatic polyesters was synthesized, and nanofibers of poly(7GT) were prepared via electrospinning from its solution in 1,1,1,3,3,3-hexafluoro-2-propanol. Uniaxially oriented thin films were also prepared by applying shear strain to molten poly(7GT). Morphology of as-spun and annealed nanofibers and that of uniaxially oriented thin films were investigated by transmission electron microscopy. Selected-area electron diffraction (SAED) of bundles of the annealed nanofibers gave a highly oriented fiber pattern. In addition, dark-field images of the poly(7GT) nanofibers, which had been annealed at 85 °C for 48 h, were taken by using some of the reflections on/near the equator. The images showed a stacked-lamellar structure, in which crystalline lamellae appearing as bright striations oriented perpendicularly to the fiber axis were stacked in the direction of the fiber axis, and the corresponding average long period was estimated at about 19 nm. As for the uniaxially oriented thin films, SAED also gave an oriented fiber pattern. When the annealing of the films was performed similar to nanofibers, crystallization occurred and a stacked-lamellar structure was constructed parallel to the shearing direction. The corresponding average long period was estimated at about 27 nm. By comparing the fiber patterns between annealed nanofibers and thin films, it seems that electrospinning is more effective than uniaxial stretching in enhancing the molecular orientation in the case of poly(7GT).