Electrospun poly(butylene terephthalate) fibers: Entanglement density effect on fiber diameter and fiber nucleating ability towards isotactic polypropylene

Abstract

Electrospun poly(butylene terephthalate) (PBT) fibers were prepared from trifluoroacetic acid (TFA)-based solvent. Rheological studies revealed the concentration (ϕ, volume fraction) dependence of zero-shear viscosity (ηo) to be ηo ∼ ϕ3.87 in the entangled solution regime, and the entanglement concentration (ϕe) was 7.84 vol.%. Effects of ηo on the straight jet-end diameter (dj) and fiber diameter (df) were investigated; two scaling laws existed for PBT/TFA solutions, i.e., dj ∼ ηo0.06 and df ∼ ηo0.73. Moreover, df can be scaled with entanglement density and solution plateau modulus (GN) through df ∼ (ϕ/ϕe)2.6 ∼ GN. Results of differential scanning calorimetry and wide-angle X-ray diffraction revealed that as-spun PBT fibers were not amorphous but contained imperfect small α crystallites. During stepwise annealing, crystal perfection gradually occurred, and well-ordered, α-form crystal was developed at temperatures above 160 °C. When electrospun PBT fibers were embedded in isotactic polypropylene (iPP) matrix, fiber surface-induced crystallization readily occurred to develop a transcrsytalline layer of iPP monoclinic crystals at the interface. Similar effects were found for electrospun fibers of poly(ethylene terephthalate) (PET) and poly(trimethylene terephthalate) (PTT). The conventional method of counting iPP nuclei occurring on the fiber surface to determine fiber nucleation rate was infeasible because of the fine fiber diameter. A new approach to characterize the nucleating ability of polyester fibers with submicron diameter was developed. By measuring the intensity of depolarized light during cooling from the melt, the initial temperature (Ti) for iPP transcrystallization was determined and compared. Based on the derived Ti, the nucleating ability of electrospun fibers toward iPP followed the order PBT > PET ∼ PTT. This finding was consistent with isothermal crystallization results. Moreover, the maximum temperature for PBT fiber to induce crystallization of iPP was ca. 157 °C, which was 20 °C higher than those of PET and PTT fibers. The remarkable nucleating ability of PBT fibers was likely associated with epitaxial crystallization and the large substrate crystal dimension in the matching direction, as revealed by atomic force microscopy.