Fiber Diameter control by parameters optimization

Electrospinning has gained widespread attention due to its ability to spin nanofibers from a wide variety of materials. Electrospun nanofiber as fine as 9 nm in diameter has been recorded [Tan et al 2005]. However electrospun fiber diameter is typically in the hundreds of nanometer or more. In many cases, the diameters of electrospun fibers are in the micrometer scale. Optimization of the electrospinning parameters are necessary to bring down the fiber diameter to the nanometers scale.

Viscosity

It is widely known that reducing the solution concentration or the molecular weight will reduce the fiber diameter due to lower viscosity [Eda 2006, Tan et al 2005]. The viscosity of the solution generates an opposing force to the electrostatic repulsion responsible for stretching and thinning the solution jet. Unlike most other parameters, it has been consistently shown that higher viscosity results in greater fiber diameter regardless of material used. However, the limitation to this method is the transition from smooth to beaded fibers when the viscosity of the solution is too low. In this case, alternative parameters and methods need to be used to further reduce the fiber diameter.

Solution Properties

Solution conductivity is essential for electrospinning to take place. A more conductive solution will allow greater stretching of the electrospinning jet due to the presence of more charge carriers. This would favor a reduction in fiber diameter [Zhong et al 2002]. There are several ways of increasing the solution conductivity. The most commonly used method is to use more conductive solvent or to spike the solution with more conductive solvent or salt.

Using solvent mixtures or spiking the solution with salt may also be used to reduce the solution surface tension. A lower surface tension encourages greater fiber stretching as less forces is required to overcome the surface tension [Fong et al 1999] .

Voltage

Thinning of the spinning jet is partly dependent on the stretching force applied to it. In electrospinning, the stretching force is due to the charges on the spinning jet within the electric field. Therefore, increasing the applied voltage has been shown to reduce the fiber diameter [Lee et al 2004, Buchko et al 1999]. Higher voltage may also encourage spinning jet splitting which also leads to smaller fiber diameter [Wang et al 2006]. However, beyond an optimum voltage, the fiber diameter may start to increase instead due to reduction in flight time [Zhao et al 2004].

General effect of increasing voltage on electrospinning process and fiber diameter .

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Distance between tip and collector

Varying the distance between electrospinning tip and collector typically affects the electric field strength between them and the electrospinning flight duration. Increasing the distance between tip and collector may reduce the fiber diameter as there is a greater stretching distance [Mazoochi et al 2012]. However, beyond a certain distance, the fiber diameter may increase instead due to the significantly weakened field strength [Ding et al 2010, Bosworth 2012].

Environment

The effect of humidity on fiber diameter is dependent on the interaction between the solution and the surrounding water vapor. Higher humidity was found to result in larger diameter polystyrene fibers spun from the same concentration [Kim et al 2004; Fashandi and Karimi 2012]. The same observation was made by Icoglu et al (2013) using polyetherimide. A possible reason for this is rapid precipitation of the polymer when water condenses on the surface of the electrospinning jet especially at high relative humidity prevents further elongation of the polymer thus resulting in thicker fibers [Icoglu et al 2013; Fashandi and Karimi 2012]. Ironically, lower relative humidity may also led to faster solvent vaporization and the resultant increase in solidification rate may also lead to larger fiber diameter. Golin (2014) demonstrated this effect on a core-shell nanofibers composed of a poly(caprolactone) shell and a poly(ethylene glycol) core. For some polymers, a higher relative humidity has been shown to result in reduced fiber diameter instead. Htike et al (2012) demonstrated the effect of humidity on water absorption ability of the solution. Using polyvinyl alcohol, there was a slight reduction in the fiber diameter when the relative humidity was increased from under 20% to 70%. When water soluble egg-shell membrane was added to the polymer solution, water absorption ability of the solution increases. Electrospinning of the solution mixture with increasing humidity showed a gradual evolution of clogging at the needle tip at low relative humidity (20% relative humidity) to stable electrospinning at 40% relative humidity. Mean fiber diameter was shown to reduce from 300 nm at 20% relative humidity to 256 nm at 50% relative humidity. The reduction in fiber diameter was attributed to lower viscosity and concentration when water was absorbed into the solution from the environment during electrospinning. Similar reduction in fiber diameter with higher humidity has been reported for cellulose acetate [Hardick et al 2010]. Morsy (2022) showed that at a higher collector temperature, the diameter of the collected electrospun gelatin fiber is reduced. A heated collector will increase the ambient temperature around the electrospinning jet which increases evaporation of the solvent and reduces the viscosity of the electrospinning solution. When the collector surface temperature was below 50 °C, the average diameter of the gelatin fibers was about 680 nm. At 50 °C, the fiber diameters were reduced to 420 nm. Further increase in temperature to 75 °C and 100 °C gave a stable fiber diameter of around 380 nm. The gelatin for electrospinning was dissolved in acetic acid which has a boiling point of 118 °C. At temperatures between 50 to 100 °C, there may be a significant reduction in the viscosity of the gelatin solution without a significant increase in the evaporation of the solvent near the tip of the nozzle. Therefore, significant thinning of the fibers were observed without disruption to the electrospinning process. However, for water sensitive polymer, a balance must be reached for the relative humidity.

Table 1: Summary of effect of humidity on fiber diameter

Humidity	Fiber Diameter	Explanation	Reference
Higher	Increase	Precipitation effect especially for water-insoluble polymer	Icoglu et al 2013
	Decrease	Water absorption leading to reduced concentration especially for water soluble polymer	Htike et al 2012
Lower	Increase	Rapid solvent vaporization	Golin 2014
	Decrease	Reduction in precipitation effect	Icoglu et al 2013

Diameters of electrospun fiber have been shown to be influenced by the ambient temperature. Polymer solution viscosity is likely to reduce when it is spun in a higher temperature environment. This will generally allows greater stretching of the solution resulting in smaller fiber diameter. Icolgu et al (2013) showed that with higher spinning temperature ranging from 10 °C to 35 °C, the diameter of the polyetherimide (PEI) fibers may be reduced by more than 50%. Similar reduction in fiber diameter with higher temperature has been reported for cellulose acetate [Hardick et al 2010].

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Nozzle Diameter

Increasing nozzle diameter has the effect of increasing fiber diameter [Heikkila et al 2008]. This may be due to larger amount of mass available for the spinning process. Using regression analysis, Thompson et al [2007] showed that a larger initial spinning radius will lead to a larger final jet radius, in other words, larger fiber diameter. Certainly, a larger nozzle diameter will contribute to a larger Taylor cone and corresponding initial jet radius at initiation. Experimental result has demonstrated that significantly larger nozzle diameter does led to a significant increase in fiber diameter as demonstrated by Kizildag et al (2012) using silk fibroin.

Solution Feedrate

Secondary jet splitting off from main jet when there is excess solution. [Abdel-Hady et al. ISRN Nanotechnology, vol. 2011, Article ID 851317, 14 pages, 2011. doi:10.5402/2011/851317. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Several studies have reported an increase in fiber diameter [Zargham et al 2012, Milleret et al 2011, Wang et al 2009] with increasing feed-rate. This is due to increased volume and initial radius of the electrospinning jet leading to reduced bending instability and subsequently increases in fiber diameter. Using regression analysis, Thompson et al [2007] showed that a larger initial spinning radius will lead to a larger final jet radius, in other words, larger fiber diameter. In some cases, increasing the feed-rate does not lead to an increase in average fiber diameter. In a study by Schoenmaker et al [2012], the fiber diameter first increase then decrease when the feed-rate was increased. The reduction in average fiber diameter with high feedrate has been attributed to the emergence of secondary jets from the main jet as solidified solution at the tip of the nozzle forces jet eruption from unsolidified surfaces. The secondary jets will result in smaller diameter fibers compared to the initial primary jet.

Solution Temperature

Raising the temperature of the solution during electrospinning has been shown to reduce fiber diameter [Wang et al 2009]. With a higher solution temperature, the viscosity of the solution will be lowered and this reduces the resistance to stretching under the same stretching force. However, higher temperature would also increase the solidification rate of the spinning jet and this may accelerate the "stiffening" of the jet and therefore its resistance to stretching.

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Salt additive

The influence on salt additive on fiber diameter is similar to that of applied voltage. Generally, the addition of salt reduces the fiber diameter [Choi et al 2004] unless there are interactions with the solution which results in higher solution viscosity. However, with higher charges on the electrospinning jet, the attraction towards the collector would increase and this may lead to shorter flight duration and less stretching [Arayanarakul et al 2006, Ding et al 2010].

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Polarity

The effect of applied voltage polarity on electrospun fiber diameter is still poorly understood with conflicting results. While Angammana (2011) obtained smaller fiber diameter with negative high voltage, other researchers collected fibers with larger diameter compared with fibers spun from 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. 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).

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