Solution-Electrospun Poly(ethylene terephthalate) Fibers: Processing and Characterization

Electrospun poly­(ethylene terephthalate) (PET) fibers were prepared from a trifluoroacetic acid (TFA)-based solvent. Rheological studies revealed the concentration (ϕ) dependence of the specific viscosity (η_sp) to be η_sp ∼ ϕ^3.7 for PET/TFA solutions in the entangled regime. The determined entanglement concentration (ϕ_e) was higher using a lower-molecular-weight PET. To obtain bead-free fibers, the minimum concentration for the electrospinning was 0.8–1.0ϕ_e owing to the high volatility of TFA solvent, which significantly enhanced the chain network strength during jet whipping. The double-logarithmic plots of the jet (<i>d</i>_j) and fiber (<i>d</i>_f) diameters versus the zero-shear viscosity (η₀) revealed that two scaling laws existed for the present solutions, i.e., <i>d</i>_j ∼ η₀^0.06 and <i>d</i>_f ∼ η₀^0.77. The microstructural evolution of the electrospun PET fibers from stepwise annealing to crystal melting was investigated by simultaneous small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) measurements using synchrotron radiation sources. The conformer transformation from gauche to trans was monitored by in-situ Fourier transform infrared spectral measurement. In the absence of any WAXD reflection, the as-spun PET fibers possessed a SAXS scattering peak, indicating the presence of a mesomorphic phase with an interdomain distance of 6.8 nm. At annealing temperatures (<i>T</i>_a) higher than 100 °C, the mesomorphic phase gradually transformed into imperfect triclinic crystals and reached its saturation at 130 °C. Further increased <i>T</i>_a perfected the triclinic structure without altering fiber crystallinity until the initial crystal melting at 218 °C, at which a significantly increased long period was detected. When the electrospun PET fibers were embedded in an isotactic polypropylene (iPP) matrix, surface-induced crystallization occurred to develop a transcrsytalline layer of iPP monoclinic crystals at the interface