Solution Temperature

It is well understood that at higher solution temperature, the viscosity of the solution decreases. With lower viscosity, it is expected that the diameter of the fiber will be reduced. Nangrejo et al (2012) showed that with increasing temperature of polyurethane solution (Dimethyl formamide as solvent), the viscosity and surface tension of the solution were reduced while the electrical conductivity increases. The resultant electrospun fibers correspondingly showed a reduction in fiber diameter and its standard deviation. The reduction in fiber diameter with higher solution temperature has been demonstrated earlier by Wang et al [2009]. However, there is a limit to which the diameter can be reduced with increasing spinning temperature [Wang et al 2009] as higher temperature will also lead to increased solvent vaporization which in turn increases the viscosity. Rodoplu et al (2012) showed that polyvinyl alcohol fibers changed from beaded to flat fiber when the temperature increases from 40 °C to more than 60 °C. It is hypothesized that the flat fiber structure is due to insufficient drying of the fiber during the travel from the spinneret to the collector [Rodoplu et al 2012]. Transition of fiber morphology from round cross-section to flat cross section has also been observed when electrospinning in an elevated temperature environment [Amiraliyan et al 2009]. In a comprehensive study of the effect of solution temperature on electrospinning, Yang et al (2017) investigated its effect of electrospinning polyacrylonitrile (PAN) and polyvinyl pyrrolidone (PVP). With PAN dissolved in dimethylacetamide (DMAc), the optimum temperature was found to be 69 ° C which gave the smallest fiber diameter of 250 nm. For PVP in ethanol, the optimal temperature was 39 ° C and a corresponding fiber diameter of 415 nm. In terms of fiber appearance, electrospinning PAN at lower temperature produces wrinkled fibers while at higher temperatures, the fiber surfaces are smooth. Wrinkled fiber surface at lower electrospinning temperature can be attributed to the presence of residual solvents and this forms skin layer which later collapsed generating a wrinkled surface. When evaporation rate was faster, quick solidification of the fibers prevents the formation of skin layer and this produces a smooth fiber surface. With PVP in ethanol, low boiling point of ethanol means that the evaporation rate will be fast even at relatively low temperature and this produces smooth PVP fibers across the temperature range. Apart from fiber physical characteristic, electrospinning of solution at higher temperature may also alter its molecular structure.

Reduction in viscosity with higher solution temperature is due to greater moleculer chain mobility. This may affect the molecular arrangement and crystallinity of the resultant material and its corresponding properties. Nangrejo et al (2012) showed that when higher temperature polyurethane solution was electrospun, there was a clear shift in the melting endotherm values to a higher melting temperature. At 25 °C processing temperature, T_m of the polyurethane fibers was 333 °C while at 100 °C processing temperature, T_m of the polyurethane fibers was 347 °C. This has been attributed to increased crystallinity of the electrospun polyurethane fibers at higher processing temperature.

Heating the electrospinning solution at the source is challenging as the high voltage supplied at the spinneret may interfere with the electrical heating element meant for warming up the solution reservoir. To overcome this problem, Worarutariyachai et al (2020) used water to warm up the heat jacket surrounding the syringe. Insulated pipes were used to carry the heated water from the source. This setup ensures that electrical charges at the syringe are isolated from the electrical heating elements. With this, they were able to maintain a temperature of 80 °C for better electrospinnability of their lignin solution.

Electrospinning with a water-heated jacket [Worarutariyachai et al 2020].

While most investigation of temperature in electrospinning is about increasing the temperature of the solution, reducing the temperature of the spinning solution may be helpful for more volatile solvents. Abbas et al (2022) investigated the electrospinning of poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) using acetone, which is a highly volatile solvent. Using acetone as the solvent, electrospinning at a low temperature of 5 °C is not possible as the solution quickly solidifies at the nozzle tip due to rapid evaporation of acetone. It was only at temperatures below -5 °C that smooth PVDF-HFP fibers were formed and it required the temperature to drop to -19 °C before the electrospinning process was stable.