Using a jacket-type heat exchanger to control the solution temperature, the electrospinning of polyacrylonitrile/dimethylformamide (PAN/DMF) solutions with various concentrations was carried out at temperatures ranging from ambient to 88.7 °C. The purpose of this is to investigate the temperature effect on the cone/jet/fiber morphologies that developed. By varying the solution temperature, the chain entanglement status existing in the solution (which is the prerequisite condition for preparing uniform fibers) remained intact. However, the solution properties were significantly altered, thereby giving rise to a feasible route to manipulate the as-spun fiber diameter. By increasing the solution temperature, it was found that the viscosity (η o ) and surface tension (γ) of the PAN/DMF solutions were decreased, but the solution conductivity (κ) was increased; all these trends favored the development of thinner electrospun PAN fibers at high electrospinning temperatures. For instance, with the 6 wt % solutions, PAN fibers with a diameter of 65-85 nm were readily prepared by electrospinning at 88.7 °C, whereas larger fibers with a diameter of 190-240 nm were frequently obtained at room temperature. The temperature dependence of η o , γ, and κ followed the Arrhenius equation, and the corresponding activation energies were composition dependent and found to be ca. 15-28, ∼10 and ∼3.7 kJ/mol, respectively. Hightemperature electrospinning eventually produced PAN fibers with less crystallinity but higher chain orientation as revealed by the wide-angle X-ray diffraction and birefringence measurements. Moreover, the scaling law for the viscosity dependence of fiber diameter, d f , was also altered from d f ) 14.8η o 0.52 (unit: d f in nm and η o in cP) at room temperature to d f ) 3.0η o 0.74 at 88.7 °C, suggesting that high-temperature electrospinning was an effective method to produce ultrathin fibers.

Electrospun syndiotactic polystyrene (sPS) fibers with a diameter of 150−400 nm were successfully prepared via high-temperature electrospinning of solutions with different concentrations in an o-dichlorobenzene solvent. From the polarized FTIR spectra, the sPS chains in the as-spun fibers were amorphous and exhibited a preferential chain orientation along the fiber axis. Upon thermal annealing at a temperature lower than 270 °C, sPS crystals with the α-form modification were developed, as revealed by the electron diffraction patterns of the individual fiber segments. The orientation factor of the (002) plane was determined to be 0.94−0.97, relatively independent of the annealing temperature. Solvent-induced crystallization readily took place in these submicrometer-sized fibers, giving rise to the δ-form sPS crystals, as revealed by the FTIR spectra and WAXD patterns. The absorbed solvents could subsequently be expelled by boiling in ethanol, leaving the cavities available for the next solvent treatment. In contrast with the bulk films, sPS fiber mats exhibit a promising application in chemical separation and water purification due to their large surface area as well as high solvent permeation resulting from its small-fiber diameter character.