and higher viscosities, the applied voltages required to electrospin are higher than in solution electrospinning (Zhang et al. 2016). Lyons et al. used between 10 and 15 kV/cm to form polypropylene fibers, which is at least 10 times stronger than the electric field strength required for comparable solution spinning (Lyons et al. 2004). A strong electric field is necessary to overcome surface tension and viscoelastic forces of the polymer melt. Increasing the electric field strength from 10 to 15 kV/cm was shown to substantially decrease diameter of polypropylene fibers (Lyons et al. 2004; Zhou et al. 2006). Flow rate of the polymer melt has been identified as one of the most influential process parameters for tuning fiber diameter. An optimal polymer flow rate is essential for producing a stable Taylor cone (Dalton et al. 2007). Higher flow rates have also been shown to produce larger diameter fibers (Zhang et al. 2016; Detta et al. 2010). The high polymer viscosity associated with melt electrospinning necessitates larger‐diameter spinnerets (Zhang et al. 2016). Zhou et al. found that decreasing the spinneret diameter also decreases PLA fiber diameter (Zhou et al. 2006). Qin et al. found that varying collector distance will also alter fiber diameter. As collector distance increases, fiber diameter decreases to a point. With further increases in distance, the diameter increases due as the electrostatic drawing force weakens (Zhang et al. 2016; Qin et al. 2015).
Low conductivity and high viscosity tend to promote jet stability and suppress whipping/bucking instabilities (Hutmacher and Dalton 2011). Therefore, melt electrospun fibers are generally much larger than solution spun fibers (Dalton et al. 2007; Hutmacher and Dalton 2011; Zhou et al. 2006; Brown et al. 2011; Schaefer et al. 2007). Fiber diameters when melt electrospinning are generally larger than when solution spinning (approximately 100 nm to 500 μm compared to ~50 nm to 10 μm, respectively) (Brown et al. 2015). The jet stability provides greater control over fiber collection (Brown et al. 2011) and fiber uniformity. Fiber uniformity is achieved with the ability to establish and maintain a stable jet, as well as a balance of polymer parameters (Brown et al. 2011). Thus, melt electrospinning is often combined with mechanical drawing to reduce fiber size. For example, using the “gap method of alignment,” Dalton et al. produced 270 ± 100 nm fibers from a blend of poly(ethylene glycol)‐block‐poly(ε‐caprolactone) (PEG47‐b‐PCL95 ) and poly(ε‐caprolactone) (PCL) (Dalton et al. 2007). Submicron diameters were achieved by collecting the fibers in a gap between two collectors. As the gap approached 1 mm, the fibers became thinner and oriented.
Polymers that have been melt electrospun have been recently reviewed (Brown et al. 2015); nonpolymer fibers as small as 100 nm have also been produced through electrospinning from glass melt (Praeger et al. 2012). Boron oxide (B2O3) which forms glass has been electrospun into uniform fibers. The surface tension of boron oxide is slightly higher than water and viscosity can be controlled with temperature allows for electrospinning under low voltages (Praeger et al. 2012). Electrospinning such materials opens new possibilities such as integration into lab‐on‐chip devices. Other small molecules have also been electrospin into fibers from the melt. Trisamides, a class of liquid crystals, can be melt electrospun from the nematic liquid crystal phase or isotropic melt. The ability to form fibers was attributed to supramolecular self‐assembly, i.e. columnar stacking due to hydrogen bonds of the three amide groups. Similarly, perylene bisimides with an extended π‐conjugated structure also formed fibers when melt electrospun due to π–π interactions (Singer et al. 2012, 2015).
1.7.2 Needleless or “Free‐Surface” Electrospinning
Alternatively, multijet electrospinning can be achieved by free surface electrospinning or “needleless electrospinning” (Niu et al. 2011; Guo et al. 2010). In needleless electrospinning, an electric field is applied to a thin layer of polymeric solution in the presence as a field concentrator (e.g. cleft made from metal, rotating cylinder) forming a liquid jet. Similar to conventional electrospinning, the jet whips as the polymer solution travels to the collector. When there is sufficient electric field strength, several jets of polymer solution eject from the liquid surface. This jetting from a planar surface typically requires high electric field strength which results in corona discharge in the air impeding the process. To overcome this challenge, perturbations are often introduced to the liquid surface. When an electric field is applied, each bump concentrates charge accumulation on the surface of the polymer solution and becomes the origin of an electrospun jet. Throughput using various needleless setups has been on the order of ~5 g/h. This approach has been commercialized by El Marco. The production rate of the Nanospider™ (El Marco) is at least ~100 g/h (Persano et al. 2013; Niu et al. 2011; Guo et al. 2010).
Comparing traditional needle electrospinning with needleless electrospinning with a model PEO system, the fiber diameters were comparable (Thoppey et al. 2012). Further, nanofiber sizes from needleless electrospinning follow similar trends with processing parameters. Fiber size is proportional to polymer concentration and inversely proportional to collection distance. Examining the applied voltage, the average fiber diameter decreases and fiber size distribution increases with increasing voltage (Han et al. 2014; Nurwaha and Wang 2015). The number of jets increase as the applied voltage increases. The flow rate per jet (i.e. the jet diameter) also increases with increasing voltage. The geometry of the charged electrode affects the electric field and thus also can significantly affect fiber diameter (Han et al. 2014; Nurwaha and Wang 2015). Solution properties also affects the needleless electrospinning process. Adding a significant amount of salt to the polymer solution can decrease the jet diameter. Decreasing the solution viscosity increases the number of jets by minimizing jet‐to‐jet interactions. Decreasing the surface tension maximizes the number of jets to provide the highest throughput (Thoppey et al. 2012). These results qualitatively match modeling approaches to predict the maximum density of jets using a leaky dielectric model, static cone‐jet. Assuming the electric field is balanced by surface tension, the wavelength (λ) with the shortest formation time is
(1.12)
where γ is surface tension, ε 0 is the permittivity of free space, E 0 is the electric field at the edge of the fluid, and
1.7.3 Alternative Fiber Production Methods
Alternative methods to nanofiber production have also been considered including solution blowing and centrifugal spinning and are compared to electrospinning methods in Table 1.3. In solution blowing, compressed air drives fiber formation. The most common setup is a concentric nozzle system with the polymer solution in the inner nozzle and a high‐pressure gas is delivered through the outer nozzles. As the polymer solution exits the nozzle, the gas shears the solutions and the solvent evaporates. The high shear forces perturb the liquid jet to a bending instability as it forms a solid nanofiber. Therefore, similar to electrospinning, the resulting fibers are affected by the bending instability, stretching, and evaporation rate of the solvent. Analogous to electrospinning, the polymer solution properties (concentration, viscosity, molecular weight, surface tension, and vapor pressure) and ambient conditions (temperature, humidity, and pressure) also affect fiber formation. The process parameters that affect the fibers are flow rate, gas flow pressure, nozzle‐to‐collector distance, and the nozzle geometry. Randomly oriented fiber with diameters between 40 nm and several μm have been achieved, comparable to electrospinning. The production rate is ~fourfold higher than electrospinning (Stojanovska et al. 2016).
Centrifugal spinning is an alternative approach with high mass production
