Polymer Properties

There are several polymer properties that will effect the electrospinning process and the outcome nanofibrous structure. One of the most important parameter is the molecular weight of the polymer. Higher molecular weight is generally preferred as there will be greater chain entanglement which facilitate the formation of fibers during spinning. In contrast, lower molecular weight polymer solution may break up into droplets or form beads. Where higher molecular weight polymer is not available, increasing the concentration may facilitate the formation of fibers. If it is not possible to get fiber out of the preferred material, a simple option is to blend with another higher moleculer weight polymer.

Apart from the molecular weight, there are specific polymer characteristics that give rise to subtle differences in the integrity of the nanofibrous mesh. Polysulfone for example will give a less compact nanofibrous coating compared to most other polymers. Electrospinning of polyacrylonitrile with carbon nanotubes has been shown to give rise to fluffy and "high rise" structure. While the reason for this phenomena has not been investigate in-depth, it has been suggested that the interaction between the residual charges on the nanofibers and intrinsic stiffness of the polymer may play a part in this. A preliminary study has shown that by neutralizing the charges on the deposited fibers during electrospinning, the resultant structure becomes more compact [Yousefzadeh et al 2012].

Polyelectrolytes in contrast to neutral polymer have been shown to influence its electrospinnability by negative or positive high voltage. 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.