Electrospinning solvent evaporation

Fiber jet speed and material elasticity are two of the most important parameters involved in the jet-mandrel interaction and each of these properties are influenced by multiple electrospinning parameters, such as solution conductivity, viscosity, voltage, and feed rate. In addition, material properties cannot be accurately predicted without knowing the exact degree of solvent evaporation at the point when fibers are taken up by the collector. 

Electrospinning uses a high-voltage electrical field (typically 10-20 kV) to form micro- and even nanoscale fibres from a suspended droplet of polymer melt or solution [118]. When the repulsive electrostatic interactions overcome the droplet s surface tension, a Taylor cone is formed and a polymer jet is ejected from the tip of this Taylor cone [119]. The polymer jet is then accelerated towards a grounded collector screen. As the jet moves through the air, a stretching process occurs and the solvent evaporates which results in a non-woven polymer fabric or polymer mat [120]. Electrospinning has already been applied for both synthetic as well... 

While there is no disagreement on the enhancement of the p phase of PVDF during electrospinning, there is no clear evidence as yet on the mechanism of the conversion of the predominant non-polar a phase to the ferroelectric p phase. There is ambiguity in the literature on whether the rapid solvent evaporation rate or the high stretching forces experienced by the nanofibres prepared from low polymer concentration solutions are responsible for this. 

Solvent volatility is also an important factor in determining the properties of fibrous structures produced by electrospinning. In the electrospinning process, solvent evaporation occurs while the jet travels from the tip of the syringe to the collector. If all of the solvent evaporates on the way, fibers can be formed and deposited on the collector. However, if some solvent remains on the pol5mier, instead of dry fibers, wet fibers or thin films can be produced. Solvent volatility might play a role on the formation of pores in the fibers. A decrease in the solvent volatility resulted in a smoother fiber surface. Low-boiling-point solvents are desirable because evaporation of the solvent is enhanced and deposition of the fibers becomes easier. A rapid evaporation rate of the solvent can cause the fibers to form as ribbons with various cross sections. 

In the PANI.TSA/PLA blended electrospun nanofibers no phase segregation of PANI in a PLA matrix was observed, while phase segregation was observed in cast films with the same composition. Due to rapid solvent evaporation in the electrospinning process, no crystalline structures in fiber mats were formed compared to cast films. Highly homogeneous electroactive fibers can be useful in the construction of electronic devices and sensors. Similar behavior was observed in the PVDF-TrFE/PANI-PSSA electrospun nanofibers. 

Electrospinning can create surface textures needed for superhydrophobicity. Electrospinning is a widely used technique to make ultra-thin polymer fibers. In this process, a polymer solution is charged in a capillary tube and electrically biased with respect to a grounded collector surface located 10 cm from the needle. The polymer solution is then ejected from the capillary into a jet form and the solvent evaporates leaving polymer fibers. By electrospinning polymers with hydrophobic properties, one can easily produce superhydrophobic fiber mats on the collector [89-92]. Of course, the electrospinning can also be used to make porous fiber mats... 

Analogous to HME, electrospinning is also a widely used technique in the polymer industry. A schematic of electrospinning process is shown in Fig. 3.11. A polymer solution is drawn through a capillary tube that is subjected to an electric field. As the electric field increases, the feed solution forms a Taylor cone at the tip of the capillary. Once the electric field overcomes the surface tension of the solution, the polymer solution is ejected as an electrically charged jet. Due to the increase in surface area, the solvent evaporates leaving thin filaments of material (50 nm to 5 microns). These fibers are then collected on collector screens for further processing. This technique has been applied for pharmaceutical systems by several researchers... 

---