Improving solution electrospinnability

Inorganic salt, commonly used to dope electrospinning solution to improve fiber quality.

Electrospinnability of a material depends very much on the solution property. Surface tension, conductivity and viscosity of the solution are the fundamental properties influencing electrospinnability and quality of the fibers. Increasing polymer solution concentration is the most direct method of enabling fiber formation using electrospinning. However, this often result in oversize fibers or in some cases, may fail to yield fibers. Therefore, other methods may be used to improve solution electrospinnability.

Additives to increase solution conductivity

Having conductivity in the solution is vital for electrospinning. Solvents such as methanol and N,N-dimethylformamide are often added to form a mixed solvent system to improve electrospinnability of the solution or to improve the quality of the fiber output. An advantage of this system is that all the solvents will be vaporized to leave behind pure polymers. Jahangir et al (2017) tested a variety of solvent systems for electrospinning poly (lactic acid) (PLA). With single solvent system, they were unable to collect good quality fibers as either the conductivity of the solvent is poor (acetone, chloroform, 1,4-dioxane DX, tetrahydrofuran, dichloromethane) or the solution showed high surface tension (DMF, dimethylacetamide). However, binary solvent systems significantly improves the quality of the fiber collected when single solvent systems fails. Using a mixture of acetone and DMF (60:40), they were able to collect high quality fibers with low diameter and low diameter distribution. Acetone is a good solvent for dissolution of PLA while DMF increases the solution conductivity.

Most studies showed that addition of small amount of salt will bring about a significant decrease in fiber diameter [Choi et al 2004] which is probably due to higher conductivity and corresponding stretching of the fiber. The same reason probably explain the spinning of smooth fibers when salt is added compared with beaded fibers from neat solution of the same concentration [Choi et al 2004, Arayanarakul et al 2006]. Helgeson et al (2008) found that there is a significant decrease in the initial jet radius and a 3-fold increase in extension rate with the addition of NaCl to polyethylene oxide solution. The combination of an initial smaller radius and greater extension rate will favor smaller fiber diameter. However, there is a limit which the repulsive charges can exert versus the viscoelastic force of the solution and no further reduction of fiber diameter is possible with increasing salt content. Several authors reported an initial reduction in fiber diameter when NaCl is added but the trend reverses when more salt is added in their aqueous-based polymer solution [Arayanarakul et al 2006, Ding et al 2010]. Fiber quality may also deteriorate with more beads and greater variance in diameter beyond an optimum salt load [Ding et al 2010

Additives to reduce surface tension

Spraying particles instead of spinning fibers and beads on electrospun fibers are often the result of high surface tension in the solution or insufficient concentration. While increasing solution concentration may help, this will also lead to greater fiber diameter. Surfactant may be used to reduce solution surface tension so that small diameter, smooth fibers may be fabricated.

Cationic surfactant such as hexadecyl trimethyl ammonium bromide (HTAB) not only reduces the surface tension of the solution, it also introduces additional charge carriers to the solution. This has the dual function of reducing beads formation while increasing fiber stretching to produce finer fibers. Zheng et al (2014) was able to produce polyvinylidene fluoride electrospun fibers with average diameter less than 65 nm with the addition of HTAB.

Support polymer

While there are a wide range of materials that can be electrospun in its pure form, there are also many others that does not form fibers despite best efforts in optimizing electrospinning conditions. A notable example is in electrospinning inorganic precursors. Inorganic precursors and other polymers with short molecular chain does not easily form fibers. One method is to use an easily electrospinnable polymer as a support such that their mixture is able to be electrospun to form fibers. Following fiber formation, the support polymer may be removed by sintering or by dissolution in appropriate solvent.

In electrospinning inorganic precursors, polyvinyl pyrrolidone (PVP) is commonly used as it shares the same solvent as the inorganic salt. Electrospinning of PVP with the precursor readily forms fibers and PVP can be removed during calcination. One way of fabricating mesoporous TiO₂ is by mixing TiO₂ precursor such as titanium isopropoxide [Mondal et al 2014] or tetrabutyl titanate (TBT) [Li et al 2012] to a polymer support followed by electrospinning. Using a solution of V₂O₅ powder and poly(vinylpyrrodidone) (PVP), mesoporous vanadium pentoxide nanofibers has been fabricated after annealing at 500 °C in air for 1 h. The mesoporous fibers were found to exhibit excellent Li-ion storage capacity [Yu et al 2011].

Plasma treatment

The fundamental purpose of plasma treatment is to create free radicals on polymers. This has the effect of altering the polymer properties. Rezaei et al (2017) used plasma treatment on prepared polylactic acid (PLA) solution and this has been found to increase solution viscosity and conductivity. Such properties are generally favoured in electrospinning. Increase in viscosity and conductivity may be due to cross-linking and formation of new oxygen containing groups. With this, a lower concentration may be used for electrospinning smooth fibers. However, beyond 5 mins of plasma treatment, beads begin to form from electrospinning.

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Content [Electrospinning Parameters]