role of electrical conductivity of spinning solution on enhancement...

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Copyright © 2013 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 13, 4193–4202, 2013

Role of Electrical Conductivity ofSpinning Solution on Enhancement of

Electrospinnability of Polyamide 6,6 Nanofibers

Su-Yeol Ryu and Seung-Yeop Kwak∗

Department of Materials Science and Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea

Optimal conditions for electrospinning of uniform polyamide 6,6 (PA66) nanofibers were determinedby the control of various parameters, such as polymer solution concentration, flow rate, tip-to-collector distance (TCD), applied voltage, and electrical conductivity of the polymer solution. Anorganic salt, benzyl trimethyl ammonium chloride (C10H16ClN, BTMAC), was added to the solutionsfor increase of electrical conductivity. When no salt was added to the PA66 solution, the uniformnanofibers were electrospun only at limited conditions, such as flow rate of 0.5 mL/h and elec-tric fields greater than 2.0–3.5 kV/cm. In contrast, by the addition of BTMAC, range of optimalconditions for uniform nanofibers was expanded; uniform nanofibers were obtained at flow rate of0.5–1.5 mL/h and electric fields greater than 1.3–1.6 kV/cm. This means that the electrospinnabilityof the nanofibers is improved by increasing electrical conductivity of the solutions. Furthermore, theaddition of BTMAC affected the increasing of number average diameters and standard deviationof the nanofibers. On the other hand, the process variables, such as flow rate, TCD, and appliedvoltage, exerted little influence on the increase of diameters.

Keywords: Polyamide 6,6, BTMAC, Nanofiber, Electrospinning, Electrospinnability.

1. INTRODUCTION

Nanomaterials demonstrate interesting physical and chem-ical properties compared with conventional materials.Various types of new materials, such as nanoparticle,nanorod, nanosphere and nanofiber, have been investi-gated. For the synthesis of new functional nanomateri-als, materials’ structures, compositions and orientationsin nanoscale have been actively controlled using vari-ous nanoscience and nanotechnologies.1–3 In particular,various methods, such as self-assembly,4�5 layer-by-layer(LBL) assembly,6�7 Langmuir-Blodgett (LB) technique,8�9

template synthesis,10 vapor-phase deposition11 and elec-trostatic processing technique,12�13 have been conductedfor fabrication of unique nanostructures. The creation ofnew functional nanomaterials contributes significantly tothe innovation of nanoscience and nanotechnologies.A nanofiber, which is fiber with diameter less than

1 �m,14–16 has many features including small diameter,high surface area-to-volume ratio, light weight and con-trollable pore structures compared to other conventional

∗Author to whom correspondence should be addressed.

fibers.17 So, nanofibers have been studied for variousapplications in fields of filtration, tissue engineering, sen-sor, protective materials, electronic and photonic materialsand drug delivery.18–27 These nanofibers have been fabri-cated using various ways, such as self-assembly, templatesynthesis, phase separation method, drawing method andelectrospinning.28–32 Among them, electrospinning is anelectrostatic processing method for fabricating non-wovenmats by applying high voltage to the polymer melt or solu-tion. When the electric stretching forces overcome the sur-face tension of the polymer solution, electrically chargedpolymer jets are ejected. Electrospinning has many advan-tages, such as easy process, simple apparatus, and possi-bility of using various types of polymers in the melt orsolution state. Also it is possible to prepare compositematerials using various kinds of materials. There are manyvariables affecting the electrospinning process: (1) materialvariables, such as polymer solution concentration, solventcomposition, viscosity and vapor pressure of the solution,polymer molecular weight, and electric conductivity ofthe solution, (2) process variables, such as the solutionflow rate, tip-to-collector distance (TCD) and applied volt-age, and (3) ambient variables, such as temperature and

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Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of PA66 Nanofibers Ryu and Kwak

humidity. Through the control of these parameters, manyresearchers have looked upon the optimization of thesenanofibers.33–44

In particular, some researchers have studied the optimalelectrospinning conditions for polyamide 6,6 (PA66). Tsaiet al. electrospun PA66 nanofibers with an average diame-ter of 78 nm using formic acid as solvent.37 Guerrini et al.also electrospun PA66 nanofibers with average diametersof 140–190 nm using formic acid.38 This work discussedthe influence of the PA66 molecular weight on the elec-trospinning. De Vrieze et al. studied the effect of the sol-vent system on the electrospinning.39 This work presentedthe steady state electrospinning parameters for PA66 nano-fibers. These efforts are closely related to the establishmentof electrospinning conditions for uniform nanofibers andimproving of the electrospinnability.Improvement of electrospinnability influences on the

nanofiber productivity. Among the variables affecting theelectrospinnability, in particular, electrical conductivity ofpolymer solution is an important variable. Enhancing theelectrical conductivity reduces critical voltage applied tothe polymer solution. So, the electric stretching forcescan overcome the surface tension of the polymer solutionunder low electric field conditions. In addition, it is pos-sible to produce uniform nanofibers under the faster solu-tion flow rate. The present study focuses on presenting theoptimized electrospinning conditions for producing uni-form nanofibers using PA66 solution with improved elec-trical conductivity. To enhance the conductivity, an organicsalt is added to PA66 solution. Then, the solution con-centration, applied voltage, tip-to-collector distance (TCD)and the solution flow rate are controlled to determinethe optimized electrospinning conditions. The influence ofthe conductivity on the electrospinnability enhancement isstudied. And the influence of variables on the nanofibermorphology is discussed.

2. EXPERIMENTAL DETAILS

2.1. Preparation of PA66 Solutions

Polyamide 6,6 (PA66, average molecular weight 30.5 kDa,Sigma Aldrich, USA) was dissolved in formic acid(≥95%, Sigma Aldrich, USA) to prepare polymer solu-tions of 10, 15 and 20 wt%. Benzyl trimethyl ammoniumchloride (BTMAC, ≥99.0%, Tokyo Chemical Industry,Japan), as organic salt, was added in some of polymersolutions for enhancing the conductivity (1 wt%/PA66).These solutions were stirred for 12–24 h at 50 �C untiltransparent and homogeneous solutions were obtained. Allcomponents were used directly without further purifica-tion. The viscosities of the solutions were measured usinga Brookfield RVDV-II viscometer at room temperature.The conductivity of the solutions was measured using a

conductivity meter (CP-500L, istek, Inc., Korea) at roomtemperature.

2.2. Electrospinning of PA66 Nanofibers

PA66 polymer solutions were electrospun to nanofibersusing electrospinning apparatus (NanoNC, Korea). Theequipment was composed of a rotating drum collector,a high DC-voltage supply and a syringe pump basically.The polymer solution was placed in a 10-mL syringe hav-ing a steel needle with an internal diameter of 0.33 mm.A drum-type collector covered with aluminium foil wasplace 6–12 cm horizontally from the needle tip. The flowrates were controlled at 0.5–1.5 mL/h by the syringepump during electrospinning. For preparation of elec-trospun nanofibers, the syringe needle tip was directlycharged by a positive voltage from 15 to 25 kV and thecollector was grounded by copper wire. This is the typ-ical electrospinning setup that has been studied by manyresearch workers. All PA66 nanofibers were electrospun at20–25 �C and 45–55% humidity condition in the cham-ber. The optimized electrospinning conditions for uniformPA66 nanofibers were determined based on nanofiber mor-phology with no beads or drops and stability of the Taylorcone during electrospinning.

2.3. Characterization of PA66 Nanofibers

The morphologies of PA66 nanofibers were investigatedby using a field-emission scanning electron microscope(FE-SEM, JEOL JSM-6330F) at an accelerating voltage of5 kV. The samples were placed on an aluminium holderusing double-sided adhesive carbon tape and sputter-deposited with a thin platinum layer at 15 mA for 300 s.

3. RESULTS AND DISCUSSION

3.1. Viscosity of PA66 Solutions for Electrospinning

Previous studies used sodium chloride (NaCl) as a saltfor polyamide nanofibers.40 In this study, benzyl trimethylammonium chloride (C10H16ClN, BTMAC) was used asan organic salt. It was selected by considering the solu-bility and functionality. It has excellent solubility charac-teristics for various solvents such as formic acid, aceticacid, dimethylformamide (DMF), tetrahydrofuran (THF)and deionized water. And, it is studied and used for dis-infection applications. The viscosity of the solution ofpolyamide 6,6 (PA66) in formic acid was measured andis shown in Figure 1. As more and more PA66 con-centration increases, the viscosity increases significantly.For PA66/formic acid solution, we have determined thatthe addition of BTMAC has little effect on the viscositychange. Additionally, as the retention time of the PA66solutions increased, the viscosities decrease gradually.This is due to the PA66 polymer chain scission during

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Ryu and Kwak Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of PA66 Nanofibers

Fig. 1. Effect the BTMAC addition on stability over time for fully dis-solved PA66 solutions.

the retention time. In the case of 20 wt% PA66 solu-tions with or without BTMAC, the viscosities significantlydecreased. As the concentration of PA66 increased, degreeof viscosity decrease is increased. Therefore, for efficientelectrospinning process, we conducted the electrospinningimmediately as soon as the PA66 was completely dissolvedin formic acid.

3.2. Establishment of Optimal ElectrospinningConditions

We conducted an electrospinning process based on thevarious parameters, and selected the optimized electro-spinning conditions for uniform PA66 nanofibers with nodefects. Addition of salt affects electrical conductivity of

Table I. Optimal electrospinning conditions for 10 wt% PA66 Solution without salt.

Flow rate (mL/h) / Tip-to-collector distance (TCD) (cm)

Voltage(kV) 0.5 /12 0.5 / 9 0.5 / 6 1.0 / 12 1.0 / 9 1.0 / 6 1.5 / 12 1.5 / 9 1.5 / 6

15

16

17

18

19

20

21

22

23

24

25

polymer solution. It influences the electric stretching forceexerted on the polymer jet. PA66 solution concentrationaffects the solvent evaporation rate during electrospinning.So it influences the nanofiber diameter and morphology.Applied voltage affects the strength of electric field thatstretches the polymer jet into nanofiber. Hence it influ-ences the nanofiber morphology. Tip-to-collector distance(TCD) is related to electric field. It affects the stretchingduration and traveling distance of the polymer jet. Flowrate of polymer solution is closely connected with the vol-ume of solution applied to the Taylor cone. It influencesthe nanofiber morphology. We controlled the parametersfor electrospinning: whether the addition of the salt, PA66solution concentration of 10 to 20 wt%, DC voltage of 15to 25 kV, tip-to-collector distance (TCD) of 6 to 12 cmand flow rate of 0.5 to 1.5 mL/h.The optimized electrospinning conditions for uniform

PA66 nanofibers were selected by observing nanofibersmorphology. It was verified with FE-SEM images. Alsothe conditions were selected by observing the shape ofthe Taylor cones and electrospun jets emitted from Taylorcones during electrospinning. It was confirmed visually.The electrospinning results are summarized in Tables I–VI.The optimized conditions are shown in light gray regions,and the non-optimized conditions are shown in dark grayregions (Tables I–VI).

3.3. Optimal Electrospinning Conditions forPA66 Nanofiber Without Salt

The optimized electrospinning conditions for prepara-tion of PA66 nanofibers without salt are summarized in

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Table II. Optimal electrospinning conditions for 15 wt% PA66 Solution without salt.


Voltage(kV) 0.5 /12 0.5 / 9 0.5 / 6 1.0 / 12 1.0 / 9 1.0 / 6 1.5 / 12 1.5 / 9 1.5 / 6

15

16

17

18

19

20

21

22

23

24

25

Table III. Optimal electrospinning conditions for 20 wt% PA66 Solution without salt.


Voltage(kV) 0.5 /12 0.5 / 9 0.5 / 6 1.0 / 12 1.0 / 9 1.0 / 6 1.5 / 12 1.5 / 9 1.5 / 6

15

16

17

18

19

20

21

22

23

24

25

Tables I–III. For 10 wt% PA66 solution without salt, asshown in Table I, uniform nanofibers were electrospunonly at flow rate of 0.5 mL/h. In this case, uniform nano-fibers were prepared under electric fields greater than2.0–3.0 kV/cm. The morphology of electrospun nanofiberin these parameters is shown in Figure 2. Although itwas under high electric fields, uniform nanofibers wereable to be prepared. The average diameter of the nano-fibers is 92.75± 15.91 nm. In contrast, at flow rate of1.0 mL/h, Taylor cones were formed unstable at the end oftip. Nanofibers with beads were obtained in this condition.At flow rate of 1.5 mL/h, small droplets were formed and

spattered from the end of tip. It did not generated sufficientelectrostatic repulsion for the faster flow rate.For 15 wt% PA66 solution without salt, the range

of conditions which shows the possibility of prepara-tion for the uniform nanofibers is significantly reduced(Table II). In this case, uniform nanofibers were elec-trospun under stronger electric field, compared with theresults of Table I. At TCD of 6 cm, the nanofibers wereelectrospun regardless of the flow rate. The morphology ofthe uniform nanofiber is shown in Figure 3. The averagediameter of the nanofibers is 151.86±15.73 nm. The nano-fibers diameter increases with increasing of PA66 solution




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Table IV. Optimal electrospinning conditions for 10 wt% PA66 Solution with BTMAC.


Voltage(kV) 0.5 /12 0.5 / 9 0.5 / 6 1.0 / 12 1.0 / 9 1.0 / 6 1.5 / 12 1.5 / 9 1.5 / 6

15

16

17

18

19

20

21

22

23

24

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Table V. Optimal electrospinning conditions for 15 wt% PA66 Solution with BTMAC.


Voltage(kV) 0.5 /12 0.5 / 9 0.5 / 6 1.0 / 12 1.0 / 9 1.0 / 6 1.5 / 12 1.5 / 9 1.5 / 6

15

16

17

18

19

20

21

22

23

24

25

concentration from 10 to 15 wt%. During the electrospin-ning, when a jet is extracted from the Taylor cone by theelectrostatic repulsion, the solvent begins to evaporate. Forlarge amount of the PA66 solution concentration, more sol-vent is needed to keep the PA66 dissolved. So, it quicklyreaches a critical amount of solvent to keep the PA66 dis-solved in the liquid phase. This phenomenon occurs fasterwhen the concentration is higher. It means that a fastercoagulation of PA66 nanofibers and formation of thickernanofibers occurred.

For 20 wt% PA66 solution without salt, we were notable to prepared uniform nanofibers under most condi-tions. Uniform nanofibers were prepared under limitedconditions (Table III). The morphology of the nanofiber isshown in Figure 4. The average diameter of the nanofibersis 162.21± 19.40 nm. The nanofibers diameter slightlyincreases with increasing of PA66 solution concentrationfrom 15 to 20 wt%.With increasing concentration of polymer solution,

the viscosity of the solution is increased gradually. So, the




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Table VI. Optimal electrospinning conditions for 20 wt% PA66 Solution with BTMAC.


Voltage(kV)

0.5 /12 0.5 / 9 0.5 / 6 1.0 / 12 1.0 / 9 1.0 / 6 1.5 / 12 1.5 / 9 1.5 / 6

15

16

17

18

19

20

21

22

23

24

25

Fig. 2. Morphologies of electrospun PA66 nanofibers without salt at10 wt%, 0.5 mL/h, 9 cm and 25 kV: (a) low magnification and (b) highmagnification.





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surface tension of polymer solution droplet, formed at theend of nozzle tip, is increased. However, the electrostaticrepulsion was not strong to overcome the surface ten-sion under these electrospinning parameters. When volt-age above 25 kV was applied, the electrospinning systemwas unstable. It was unable to prepare the uniform nano-fibers just by changing the strength of the electric fieldthrough the control of TCD and applied voltage. In orderto produce uniform nanofibers effectively, materials vari-ables such as solution conductivity, had to be controlledrather than process variables. So, benzyl trimethyl ammo-nium chloride (C10H16ClN, BTMAC) was selected as a saltto increase the electrical conductivity of the PA66 solution.

3.4. Optimal Electrospinning Conditions for PA66Nanofiber with BTMAC

The electrical conductivities of PA66 solutions wereincreased by BTMAC addition (1 wt%/PA66). Absoluteamount of the salt was increased with increasing concen-tration of PA66 solutions. The solutions with the salt showhigher electrical conductivity (6.5–8.1 mS/cm) than that ofthe solutions without the salt (4.1–4.3 mS/cm). So, many

changes of electrospinning conditions for uniform nano-fibers were expected because of the increase of electricalconductivity of the solutions.The optimized electrospinning conditions for prepara-

tion of PA66 nanofibers with BTMAC are summarizedin Tables IV–VI. The control parameters for electrospin-ning were the same as that of PA66 solutions withoutsalt. Effect of the salt addition on electrospinning con-ditions was confirmed. For 10 wt% PA66 solution withBTMAC, as shown in Table IV, uniform nanofibers wereelectrospun only at a flow rate of 0.5 mL/h. The mor-phology of electrospun nanofiber in these parameters isshown in Figure 5. The average diameter of the nanofibersis 128.85±21.34 nm. These uniform nanofibers were pre-pared under electric fields greater than 1.3–1.6 kV/cm.Comparing for 10 wt% PA66 solution without the salt(Table I), the nanofibers were possible to be electrospununder lower electric field. Addition of BTMAC increasesthe charge density in extracted jets from the Taylor cone.The jet is formed more easily due to the self-repulsionof the excess charges under the electric field. So, uniformnanofibers are able to be electrospun even under lowerelectric field. For flow rate of 1.0 mL/h, Taylor cones were

Fig. 5. Morphologies of electrospun PA66 nanofibers with BTMAC at10 wt%, 0.5 mL/h, 12 cm and 25 kV: (a) low magnification and (b) highmagnification.




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stable at the end of tip. But nanofibers with some beadswere electrospun. At flow rate of 1.5 mL/h, Taylor coneswere unstable. Also, small droplets were formed and spat-tered from the end of tip.For 15 wt% PA66 solution with BTMAC (Table V), at

flow rate of 0.5 mL/h, optimal electrospinning conditionsshowed a similar trend in the case of 10 wt% PA66 solu-tion with BTMAC. At flow rate of 1.0 mL/h, the range ofelectrospinning conditions for uniform nanofibers is moreexpanded than that of 15 wt% PA66 solution without salt.Also, electrospinning of uniform nanofibers were enabledat flow rate of 1.5 mL/h. The morphology of the uniformnanofiber is shown in Figure 6. The average diameter ofthe nanofibers is 213.96±46.77 nm. When the concentra-tion is increased from 10 wt% to 15 wt%, the increaseof nanofiber diameter is larger compared with nanofiberselectrospun form PA66 solutions without salt.For 20 wt% PA66 solution with BTMAC, optimal

electrospinning conditions are similar to the case of15 wt% PA66 solution with salt. Under most of the con-ditions, except for parameters of 1.5 mL/h flow rate and12 cm TCD, uniform nanofibers were obtained (Table VI).The morphology of the nanofiber is shown in Figure 7.



The average diameter of the nanofibers is 353.65 ±62.22 nm. The nanofibers diameter increases with increas-ing of PA66 solution concentration of 10 to 20 wt%. Phadkeet al. studied the effects of salt addition on average diam-eter of polyacrylonitrile (PAN) nanofibers.45 They foundthat salt addition increases the viscosity of low-molecular-weight PAN (50 kDa) solution. But, in our case, the viscos-ity is not increased with the addition of BTMAC. For PA66solution with BTMAC, the effect of the salt on nanofiberdiameter should be considered as well as the increase ofthe concentration of the PA66 solutions. We think thatentanglement of interpolymer chains of PA66 is increasedduring electrospinning with interaction between the amidegroups of PA66 and BTMAC. So, the results of increasingdiameter of the BTMAC-added PA66 nanofibers are resultas both addition of the salt and increasing concentrationof PA66 solution.Comparing the average diameter of nanofibers electro-

spun form P66 solution without the salt, we conclude thataddition of BTMAC influence on the diameter distributionand number average diameter. PA66 nanofiber distribu-tions are shown in Figure 8. Nanofibers with BTMAC havebroader diameter distributions than those of nanofibers




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Fig. 8. Diameter distributions of PA66 nanofibers without salt and with BTMAC.

without salt. The number average diameter and standarddeviation of PA66 nanofibers were calculated based onthe diameter distribution (Fig. 9). The nanofibers diame-ter increases with increasing of PA66 solution concentra-tion regardless of having or not BTMAC. However, thedegree of diameter increases is larger with addition ofBTMAC. There are little differences in standard deviationsof PA66 nanofibers without salt. For PA66 nanofibers with

Fig. 9. The number average diameter and standard deviation of PA66nanofibers.

BTMAC, the standard deviations increase with addition ofthe salt.

4. CONCLUSIONS

The wide establishment of optimal electrospinning condi-tions for uniform nanofibers is important because electro-spinning conditions are affected by materials, process andambient variables. It allows for comprehensive understand-ing of the variables for the electrospinning and optimalelectrospinning conditions.In this study, electrical conductivity of PA66 solutions

was increased by adding BTMAC as an organic salt. Theoptimal electrospinning conditions were expanded as theconductivity was improved: when no salt was added tothe PA66 solution, the uniform nanofibers were electro-spun only at flow rate of 0.5 mL/h and electric fieldsgreater than 2.0–3.5 kV/cm. On the other hand, whenthe salt was added, uniform nanofibers were obtained atflow rate of 0.5–1.5 mL/h and electric fields greater than1.3–1.6 kV/cm. The expansion of optimal electrospinningconditions by improving conductivity, it means enhance-ment of electrospinnability and contribute to productivityimprovement of uniform nanofibers.The addition of BTMAC affected the change of the aver-

age diameter of uniform nanofibers. When the salt was not




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added, the nanofibers had an average diameter of 92.75±15.91 to 162.21± 19.40 nm. In contrast, the nanofiberswith increased electrical conductivity showed an averagediameter of 128.85± 21.34 to 353.65± 62.22 nm. Also,addition of BTMAC influenced on the increase of diameterdistribution.

Acknowledgment: This research was supported by agrant from the Fundamental R&D Program for Technol-ogy of World Premier Materials funded by the Ministry ofKnowledge Economy, Republic of Korea.

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Received: 1 December 2011. Accepted: 16 January 2012.


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