(Andrady 2008; Ramakrishna 2005). Typically, the surface tension decreases with increasing polymer concentration. To further reduce the surface tension, cosolvents such as ethanol or surfactants can be added to the polymer solution. Small amounts of surfactants (concentrations ~0.01 to 1 mM) dramatically decrease surface tension and facilitate electrospinning (Andrady 2008; Ramakrishna 2005). For example, 5–15 w/v% polystyrene in DMF/tetrahydrofuran only formed bead‐free nanofibers with the addition of surfactant (dodecyl trimethyl ammonium bromide). Triton X‐100 is a common nonionic surfactant used in aqueous systems such as polyvinyl alcohol and PEO and their blends. While nonionic surfactants change in conductivity relative to the change in surface tension is minimal, ionic surfactants such as sodium dodecyl sulfate affects surface tension as well as conductivity. The relative contribution of the conductivity and surface tension cannot be determined. Practically, ionic surfactants are effective in improving fiber quality. Zwitterionic surfactants may be especially effective at increasing the surface charge density. Nanofiber size can be affected by surfactant concentration, i.e. size decreases monotonically with surfactant concentration (Lin et al. 2004). Although, most often, cosolvents/surfactants to reduce surface tension are used to reduce beading and create uniform fibers.
1.4.3.2 Conductivity
Solution conductivity is required for transfer of electric charge from the electrode to polymer solution (Andrady 2008; Ramakrishna 2005). Solvents commonly used in electrospinning have conductivity lower than water, but they play an important role in solution conductivity. For example, Uyar and coworker observed that the grade and supplier of the solvent affected solution conductivity. Solutions with a higher conductivity produced bead‐free fibers at lower polymer concentrations (Uyar and Besenbacher 2008). Polymer solutions generally have higher conductivity due to conducting ionic species (typically impurities or additives) from the polymer. Increasing polymer concentration can reduce its conductivity. With polyelectrolytes, i.e. polymers that have ionic functionalities, the solution conductivity is highly concentration‐dependent (Andrady 2008). The conductivity of the spinning solution can be increased using additives (Andrady 2008; Ramakrishna 2005). Typically, additives are inorganic salts such as sodium chloride. Organic compounds such as pyridinium formate or trialkylbenzyl ammonium chloride have also been used. Typically, additive concentrations below 2 wt% are used because at high salt concentrations, maintaining surface charge on the droplet becomes increasingly difficult. For example, aqueous solutions of poly(ethylene oxide) and CaCl2 with conductivities above 5 mS/cm could not be electrospun (Andrady 2008).
The additives increase surface charge density maintained on the jet which promotes fiber extension during whipping that can reduce beading in systems (Andrady 2008). In some cases, increased fiber extension can reduce resulting nanofiber size. The size of the conducting species is an important consideration. Ions with smaller ionic radius are thought to be more mobile and create elongational force during electrospinning. For example, solutions containing NaCl had smaller diameter than solutions with dissolved KH2PO4 or NaH2PO4 (Ramakrishna 2005). However, increasing fiber diameter with increasing solution conductivity has also been observed (Mit‐uppatham et al. 2004; Seo et al. 2009). The increases in size were salt concentration‐dependent. For example, increasing the NaCl concentration from 0.1% to 2.0% (w/v) increased the fiber diameter from ~75 to 200 nm (Arayanarakul et al. 2006). The increase in the fiber diameters has been attributed to the increased electrostatic force acting on a jet segment that either delays the occurrence of the bending instability or increases the mass throughput (Arayanarakul et al. 2006). However, it is difficult to isolate the effect of conductivity because the additive also often changes the surface tension, viscosity, and dielectric constant (Andrady 2008). Therefore, observed changes cannot be uniquely attributed to changes in conductivity.
1.5 Electrospinnable Systems
A vast range of synthetic and biopolymers have been electrospun. Electrospinning low molecular weight polymers, polyelectrolytes, and some biomacromolecules have been limited. A common approach to form fibers from such materials is to blend the nonelectrospinnable polymer with an easily electrospinnable polymer (Schiffman and Schauer 2008; Wang et al. 2015; Wendorff et al. 2012; Tang et al. 2012; Saquing et al. 2013). The carrier polymer is typically high molecular weight that forms a well‐entangled network at low concentrations (Schiffman and Schauer 2008; Saquing et al. 2013). For example, biopolymers such as chitosan or alginate are often blended with PEO (Schiffman and Schauer 2008; Saquing et al. 2013). The electrospinnable polymer increases the elasticity of the blend to enable formation of nanofibers (Yu et al. 2006).
1.5.1 Nonpolymer Electrospinning
Traditionally, electrospinning has been limited to high molecular weight polymers. Generally, polymer networks are thought to provide molecular, entanglement, and elasticity to achieve uniform fibers (Shenoy et al. 2005; Yu et al. 2006). Polymer‐free electrospinning explores fabricating nanoscale fibers from small molecules and offers potential for new applications (Table 1.2).
Phospholipids and Gemini surfactants, amphiphilic molecules with molecular weights of ~700 and ~300 g/mol, respectively, for example, have been electrospun into uniform fibers (McKee et al. 2006; Cashion et al. 2010). Both amphiphilic molecules self‐assemble into spheres or worm‐like micelles in solution. The worm‐like micelles behave analogous to polymer chains and can entangle due to hydrogen bonding and other intermolecular interactions (McKee et al. 2006). In the case of Gemini surfactants, the transition from globular to branched micelle network resulted in an increase in viscoelasticity of the solution and electrospinning of continuous fibers (Cashion et al. 2010). The fibers are of interest for controlled release applications (Cashion et al. 2010; Hemp et al. 2014).
Table 1.2 Nonpolymer electrospinning systems.
| Material | Solvent | Concentration | Fiber diameter (μm) |
|---|---|---|---|
| Phospholipids (McKee et al. 2006) | CHCl3/DMF | 43 wt% | 2.8 |
| Gemini surfactants (Cashion et al. 2010) | Water/methanol | 28–30 wt% | 0.9–7 |
| 42–44 wt% | 4–5 | ||
| Phosphonium Gemini surfactants (Hemp et al. 2014) | Chloroform | 52 wt% | 0.7–1.3 |
| HPβCD (Celebioglu and Uyar 2012; Manasco et al. 2012) | Water, DMF, DMAc | 120–160% (w/v) | 0.7–1.4 |
| Water | 70 wt% | 1–1.2 | |
| HPγCD (Celebioglu and Uyar 2012; Manasco et al. 2012) | Water, DMF, DMAc | 125–160% (w/v) | 1.2–6.4 |
| MβCD (Celebioglu and Uyar 2012; Manasco et al. 2012) | Water, DMF, DMAc | 140–160% (w/v) | 0.1–1.2 |
| Water | 70 wt% | 0.4–0.5 | |
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