Electrospinning Polarity

Polarity of voltage applied to spinneret

While electrospinning may be carried out using positive, negative or alternating current voltage, there are some difference in the spinning behavior and the fiber diameter. Using polyethylene oxide, Angammana [2011] showed that a stable electrospinning jet was formed using positive high voltage but no stable jet was seen for negative voltage. For fiber diameter and distribution of fiber diameter, there conflicting results were reported. While Angammana [2011] obtained smaller fiber diameter with negative high voltage, other researchers collected fibers with larger diameter compared with fibers spun froom positive high voltage [Yang et al 2006, Wu et al 2012, Mit-uppatham et al 2004]. Experiment by Yang et al [2006] showed with increasing positive voltage, the fiber diameter and distribution of diameter starts to increase for polyethylene oxide. With negative voltage, the mean fiber diameter starts to reduce and the increment in the scatter of the fiber diameter is less. The scatter in fiber diameter corresponds to the scatter in the measured jet current at higher voltage. Tong et al [2012] also reported increasing fiber diameter with increasing applied positive voltage (Gelatin, chitosan, PLGA, PBT) but there is no significant changes in the fiber diameter when negative voltage was used. Using aqueous polyvinyl acetate solution, Wu et al [2012] found that fiber diameter reduces with increasing voltage to a minimum before the trend reverses with further increase in voltage for both positive and negative high voltages. However, there is no observable trend in the coefficient of variation in the fiber diameter when the positive voltage is increased although a higher coefficient of variation is observed when higher negative voltage is applied which is contrary to the results from Yang et al [2006].

Beyond influencing the physical property of electrospinning and nanofibers, the polarity of applied charge was also found to affect the distribution of charged molecules within the solution. Using positive voltage and negative voltage for electrospinning nylon 6, a difference in the total surface free energy and the distribution of oxygen was observed. Surface free energy of negatively electrospun nylon 6 nanofibers was almost 20% lower than nanofibers spun from positive high voltage. The percentage of electronegative oxygen atom on the surface (measured at grazing angles) was also much lower with fibers spun from negative polarity [Stachewicz U et al 2012]. Using a blend of alginate and polyethylene oxide, XPS characterization showed that alginate with its negatively charged carboxylic acid group were found near the surface of the fibers when a positive high voltage was used [Bonino et al 2012].

Electrospinnability of a material is also dependent on the polarity of the applied charge, in particular, polyelectrolytes. For electrospinning chitosan solution, Tong et al (2012) found that only electrospinning with positive high voltage is able to generate fibers while negative high voltage is unable to form fibers. This has been attributed to the positively charged ions of chitosan molecules which adhered strongly to the negatively charged spinneret surface resulting in the failure of Taylor cone formation. However, a separate study by Terada et al (2012) showed that both positive and negative high voltage is able to generate chitosan nanofibers from electrospinning. Their study demonstrated that with positive high voltage, they were unable to produce defect-free fibers at voltages between +12 and +20 kV. However, defect-free, smooth chitosan nanofibers can be produced with negative voltages (-14 and -16 kV). With positive high voltage, multiple jets were observed at the tip of the nozzle but with negative high voltage, a single jet from the tip of a Taylor cone erupts from the nozzle tip. In both studies, it was hypothesized that electrospinning using positive high voltage creates a strong electrostatic repulsion between the positively charged chitosan chains and thus enabling electrospinning of chitosan fibers. Terada et al (2012) used this hypothesis to support the observation of multiple jets from the tip of the nozzle. With negative high voltage, Tong et al (2012) was unable to obtain nanofibers while Tereda et al (2012) was able to produce defect free nanofibers. Tereda et al (2012) hypothesized that negative high voltage temporarily neutralizes the cationic chitosan in its solution and encourages chain entanglements. This leads to defect-free nanofibers and supports a stable electrospinning process as witness by the formation of a Taylor cone. The difference in the results between Tong et al (2012) and Tereda et al (2012) may lies in the molecular weight of the chitosan used in their test. Tong et al used chitosan with molecular weight of 190,000 g/mol while Tereda et al (2012) used chitosan with a higher molecular weight of 1,000,000 g/mol. It is understood that molecular weight has an influence in the electrospinnability of a polymer. With negative high voltage, the solution assumes a neutral charge polymer solution [Tereda et al 2012] and the collective chain entanglements may be insufficient to form fibers with a lower molecular weight polymer. With a positive high voltage, strong repulsive forces between the molecular chains may result in rapid but localized chain entanglements which still allows the formation of fibers at low molecular weight but there will be region of inadequate chain entanglements as shown by the defects.