Electrospinning of Polyacrylonitrile Solutions at Elevated Temperatures

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 (ηo) 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 ηo, 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. High-temperature 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, df, was also altered from df = 14.8ηo0.52 (unit: df in nm and ηo in cP) at room temperature to df = 3.0ηo0.74 at 88.7 °C, suggesting that high-temperature electrospinning was an effective method to produce ultrathin fibers.