The Influence of Sheath Solvent’s Flow Rate on the Quality of Electrospun Ethyl Cellulose Nanofibers

doi:10.4236/mnsms.2013.34B001

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Modeling and Numerical Simulation of Material Science, 2013, 3, 1-5

doi:10.4236/mnsms.2013.34B001 Published Online October 2013 (http://www.scirp.org/journal/mnsms)

The Influence of Sheath Solvent’s Flow Rate on the

Quality of Electrospun Ethyl Cellulose Nanofibers

Deng-Guang Yu*, Xiong-Xiu Li, Jia-Wen Ge, Peng-Peng Ye, Xia Wang*

School of Materials Science & Engineering, University of Shanghai for Science and Technology, Shanghai, China

Email: *[email protected], *[email protected]

Received May, 2013

ABSTRACT

The present research investigates the influence of sheath solvent’s flow rate on the quality of electrospun ethyl cellulose

(EC) nanofibers using a modified coaxial process. With 24 w/v % EC in ethanol as electrospinnable core fluid and

ethanol as sheath fluid, EC nanofibers generated under different sheath flow rates were generated from the modified

processes. FESEM observations demonstrate that the modified process is effective in preventing the clogging of spin-

neret for a smooth electrospinning. The key for the modified coaxial process is the reasonable selection of a sheath flow

rate matching the drawing process of core EC fluid during the electrpospinning. The EC nanofibers’ diameters (D, nm)

could be manipulated through the sheath-to-core flow rate ratio (f) as D = 819-1651f (R= 0.9754) within a suitable range

of 0 to 0.25. The present paper provides useful data for the implementation of the modified coaxial process controllably

to obtain polymer nanofibers with high quality.

Keywords: Coaxial Electrospinning; Nanofibers; Ethyl Cellulose; Sheath Fluids; Flow rate

1. Introduction

Ethylcellulose (EC) is a derivative of cellulose in which a

defined percentage of the hydroxyl groups of the repeating

glucose units are substituted with ethyl ether groups. It

fulfills all the requirements of major pharmacopoeias

(USP, EP, and JP) and food regulations. EC is an inert,

hydrophobic polymer and is essentially tasteless, odorless,

colorless, non-caloric, and physiologically inert. It has

long been used as solvent-based tablet and pellet coating,

tablet binder, to prepare microcapsules and microspheres,

and both as film and matrix-forming material for

sustained- release dosage forms [1]. Most recently, EC

was selected as the drug carrier and polymer matrix to

generate composite fibers and microparticles to achieve

sustained-release profiles [1,2].

All the above-mentioned EC-based functional nano-

materials were generated using a single fluid electrohy-

drodynamic atomization process (including electrospin-

ning, electrospraying and e-jetting printing), which is a

popular procedure for producing nanofibers or micropar-

ticles due to ease of implementation and cost-effective-

ness, and the unique properties and versatile applications

of the resultant products [1-3]. Although electrospinning

is simple and straightforward, the mechanism of fiber

formation involves complex electro-fluid-mechanical

issues and there are many factors that can affect the fiber

diameters and morphology. Controlled production of

electrospun nanofibers with uniform diameter and struc-

ture remains a challenge [4,5]. To ensure a successful

electrospinning process, the chain-entanglement density

in the working solution must be high enough to prevent

the capillary breakup and also to subdue Rayleigh insta-

bility. However, high concentration polymer solutions

often result in clogging of the spinneret and failure of the

electrospinning process. Thus polymers often have nar-

row windows of electrospinnable solution concentrations,

and the objective of obtaining finer structures (such as by

lowering polymer concentration, by adding salt or sur-

factant, and by manipulating the electrospinning pa-

rameters) is often limited and compromised by sacrific-

ing the fiber uniformity [6,7].

Clogging is a critical but common problem experi-

enced during the traditional single fluid electrospinning,

especially when a high-volatility solvent is used to pre-

pare a polymer solution. With zein as model, Kanjan-

apongkul [8,9] reported that clogging would occur even

when the zein concentration in its ethanol aqueous solu-

tions (85 wt %) decreased from an electrospinning level

to electrospraying level (< 18 wt %), 16 wt % and 10 wt

% zein solutions were also tested. It was found that clog-

ging was still observed in both cases. They have put for-

ward a method to prevent clogging simply by providing

an additional solvent through another syringe pump onto

the surface of the droplet at the needle tip. But this

*Corresponding author.

D.-G. YU ET AL.

method is difficult for precise control of the process, be-

cause the additional solvent was put on the needle tip

simply by gravity.

Coaxial electrospinning, in which a concentric spin-

neret can accommodate two different liquids, is regarded

as one of the most significant breakthroughs in this area

[10,11]. It has been applied broadly in controlling secon-

dary structures of nanofibers, encapsulating drugs or

biological agents into the polymer nanofibers, preparing

nanofibers from materials that lack filament-forming

properties, enclosing functional liquids within the fiber

matrix, manipulating the size of self-assembled nanopar-

ticles, and preparing ultrafine fibers from concentrated

polymer solutions. It is a common sense that the sheath

fluids must have enough viscosity to overcome the inter-

facial tension between the two solutions through “viscous

dragging” and “contact friction” for a successful coaxial

electrospinning process [11]. However, our group have

broken this concept and developed a modified coaxial

electrospinning process, in which only organic solvent

was used as sheath fluid. This opens a new way for ma-

nipulating the additional solvent to accompany the elec-

trospinnable fluids in a core-sheath and controllable way,

and thus should provide new possibility in controlling the

nanofiber-forming process and also the nanofibers’ quality.

2. Experimental

2.1. Materials

EC (6 mPa·s to 9 mPa·s) was obtained from Aladdin

Chemistry Co., Ltd (Shanghai, China). Methylene blue

and anhydrous ethanol were purchased from Sinopharm

Chemical Reagent Co., Ltd. (Shanghai, China). All other

chemicals used were analytical grade.

2.2. Coaxial Electrospinning

The core solutions were prepared by dissolving 24 g EC,

3 g KET and 2 mg methylene blue in 100 mL ethanol.

The sheath solvent was pure ethanol. Two syringe pumps

(KDS100 and KDS200, Cole-Parmer, IL, USA) and a

high-voltage power supply (ZGF 60kV/2 mA, Shanghai

Sute Corp., Shanghai, China) were used for coaxial elec-

trospinning. All electrospinning processes were carried

out under ambient conditions (21℃ ± 2℃ with a relative

humidity 64% ± 6%). A homemade concentric spinneret

was used to conduct the coaxial electrospinning proc-

esses. A stainless steel capillary with an inner diameter

of 0.3 mm was used to conduct the single fluid electro-

spinning.

The electrospinning process was recorded using a dig-

ital video recorder (PowerShot A490, Canon, Tokyo,

Japan). For optimization, the applied voltage was fixed at

14 kV, and the fibers were collected on an aluminum foil

at a distance of 20 cm. All other parameters are listed in

Table 1.

2.3. Characterization

The surface morphologies of electrospun fibers were

assessed using a JSM-5600LV scanning electron micro-

scope (SEM, Japan Electron Optics Laboratory Co. Ltd.).

Prior to the examination, the samples were gold sputter-

coated under argon atmosphere to render them electri-

cally conductive. The pictures were then taken at an ex-

citation voltage of 3 kV. The average fiber diameter was

determined by measuring diameters of fibers at over 100

different locations from the same SEM images using

Image J software (National Institutes of Health, USA).

3. Results and Discussion

A schematic diagram of the modified coaxial electro-

spinning process with solvent as sheath fluid is shown in

Figure 1(a). A homemade concentric spinneret was used

to carry out the modified process (Figure 1(b)). The criti-

cal voltage applied to a fluid to initiate Taylor cone forma-

tion and the straight thinning jet (Vc) have a close rela-

tionship with the diameter of sheath part of the concen-

tric spinneret [12].





Where Vc is the critical voltage for a jet emanating

from the meniscus tip, d is the electrode separation, ε is

the permittivity, γ is the surface tension, and R is the

principal curvature of the liquid meniscus. A small

Table 1. Parameters of EC nanofibers and their preparation.

Flow rate (ml/h)

No.

Sheath Core

Rsa Morphologyb Diameter

(µm)

F1Single fluid spinning Line 0.42±0.18

F20.1 0.9 0.11 Line 0.34±0.15

F30.2 0.8 0.25 Line 0.23±0.14

F40.3 0.7 0.43 Mixed --

aRsc Sheath-to-core flow rate ratio; bIn this column, “Line” morphology

refers to that nanofibers have few beads or spindles on them; “Mixed” mor-

phology refers to that there are beads/spindles on the nanofibes.

Figure 1. The modified coaxial electrospinning: (a) Sche-

matic diagram of the modified process; (b) The homemade

concentric spinneret.

D.-G. YU ET AL. 3

diameter of the spinneret’s orifice means a small R, and

thus a small Vc to imitate the coaxial electrospinning

process. The homemade spinneret here has an out and

inner diameter of 1.2 and 0.3 mm respectively (Figure

1(b)), facilitating the coaxial electrospinning process. In

addition, the inner capillary is a little project out the

sur-face of the out capillary, which should help to make

the sheath ethanol well surround the core EC solutions.

When a single fluid electrospinning of the EC solu-

tions was conducted using a stainless steel capillary, the

spin-neret was clogged from time to time, as shown in

Figure 2(a). Manual remove of semi-solid substance

hung on the nozzle of the spinneret was needed to keep

the electro-spinning process go on. The fast evaporation

of ethanol on the surface of fluid jet led to the formation

of a highly viscous semi-solid substance, which clung to

the nozzle of spinneret and thus retarded the electrospin-

ning process.

When the modified coaxial electrospinning was car-

ried out to prepare the EC nanofibers, two syringe pumps

were used to drive the sheath and core fluids independ-

ently. An alligator clip was used to connect the inner

stainless steel capillary with the high voltage supply

(Figure 2(b)). With ethanol as sheath fluid and under a

sheath-to-core flow rate ratio of 0.25, the arrangement

produced a typical fluid jet trajectory, in which a Taylor

cone followed by a straight fluid jet and a bending and

whipping instability region (Figure 2(c)). The compound

Taylor cone is clear to be composed of two parts with the

sheath solvent well surrounding the core polymer solu-

tions, indicating by the methylene blue in Figure 2 (d).

The coaxial electrospinning process could be car-ried

out without any clogging phenomena and go on smoothly

for finishing the electrospinning of the whole EC solu-

tion in the syringe. Solvent evaporation and vis-cosity of

Figure 2. Observations of the modified coaxial electrospin-

ning and a single fluid process: (a) a typical clogging of the

single fluid electrospinning; (b) a digital picture shows the

connection of the concentric spinneret with the syringe

pump and the power supply; (c) a typical coaxial electro-

spinning process with ethanol as sheath fluid and under a

sheath-to-core flow rate ratio of 0.25 (taken under a magni-

fication of 12×); (d) the compound Taylor cone.

the solution have been noted to have strong impacts on

clogging. High volatility of a solvent accelerates solvent

evaporation, thus increases the likelihood of clogging.

This is because applied electric field cannot overcome

the viscous drag force. If the viscosity is too high, it is

possible that, the higher the solution viscosity is the ap-

plied electrical force would not be adequate to overcome

the viscous drag force at the droplet-air inter-face, lead-

ing to clogging at the spinneret. When sheath ethanol

was exploited to surround the core EC solution during

the electrospinning, the sheath ethanol can effectively

prevent the fast evaporation of the ethanol from the sur-

face of the core EC solutions, retarding the formation of

“skin” surface to smooth the electrospinning of EC solu-

tions.

Nanofibers obtained from the single fluid electro-

spinning and the modified coaxial electrospinning are

showed in Figure 3, and the nanofibers’ diameter distri-

butions are given in Figure 4. All the nanofibers have

linear morphology, except F4 which has a typical beads-

on-a-string morphology resulted from the excessive

sheath solutions (Figure 3 (d)).

Nanofibers F1 obtained from the single fluid elec-

trospinning are showed in Figure 3(a), and the nanofi-

bers had an average diameter of 860 ± 230 nm (Figure

4(a)). Besides a larger diameter, nanofibers F1 also have

a wide distribution of diameters. As the flow rate of

sheath ethanol was increased from 0.1 to 0.2 mL/h, the

resultant nanofibers F2 andF3 were progressively nar-

rower (Figures 3(b) to (c) and Figures 4(b) to (c)), with

Figure 3. FESEM images of EC nanofibers: (a) F1; (b) F2;

Figure 4. Nanofiber diameters’ distributions: (a) F1; (b) F2;

and the sheath-to-core flow rate ratio.

D.-G. YU ET AL.

a diameter of 620 ± 140 nm, 410 ± 110 nm, respectively.

Moreover, all had diameters smaller than F1 fibers (Fig-

ure 4(a)) prepared from the single fluid EC solution us-

ing a traditional single fluid electrospinning process.

Both the nanofibers had good structural uniformity and a

relatively small diameter distribution, suggesting a finer

quality of the EC nanofibers. A linear relationship be-

tween the sheath-to-core flow rate ratio (f) and the resul-

tant nano-fibers’ average diameters (D, nm) was found

(Figure 4(d)) within a suitable range. The regressed equ-

ation is D = 819-1651f, with a correlation coefficient of

0.9754. The results suggest that the diameters of uniform

nanofibers can be tailored through manipulating the

sheath flow rates over a suitable range through the modi-

fied coaxial electrospinning.

Throughout the modified coaxial electrospinning, the

sheath solvent would exert the following influences on

the process: 1) facilitating the formation of Taylor cone

due to lower solvent surface tensions; 2) surrounding the

straight thinning jet of the core electrospinnable EC solu-

tions that retards the fast evaporation of the core solvent,

while the sheath solvent itself outwardly evaporates to

the open air; 3) following the core fluid to enter the in-

stability region. The primary reason for the sheath sol-

vent to thin the nanofibers should be the retarding effect

on the evaporation of solvents from the surface of the

core spinning polymer solutions prematurely (Figure 5),

and in turn to retain the core jet in a fluid state thus al-

lowing it to be subjected to electrical drawing for a long-

er period in the instability region. This should be the

reason that the modified coaxial electrospinning process

could generate thinner EC nanofibers than the single flu-

id electrospinning. The single fluid electrospinning proc-

ess is very sensitive to the environmental changes, in

particular for spinning liquid systems prepared from vo-

latile organic solvents such as ethanol. The present modi-

fied process provided a stable and robust core- sheath

interface for the core EC solutions to be drawn in the

electrical field, keeping from the disturbances of envi-

ronmental changes. Thus the modified coaxial process

could produce EC nanofibers with more uniform diame-

ter distributions.

Figure 5. The proposed mechanisms of sheath solvent on

the formation of EC nanofibers using the modified coaxial

electrospinning.

However, when an excess sheath flow was used, such

as the case of nanofibers F4, the surrounding solvent

remained with the core fluid for a relatively long time

period and would break up into separate segments along

the core EC jets due to lack of viscoelasticity (Figure 5).

It is postulated that the divided sheath solvent might mix

with the core fluid locally to form sections of the fluid jet

with different local polymer concentrations, which in turn

would result in nanofibers with beads-on-a-string struc-

ture, as showed in Figure 3(d).

4. Conclusions

A modified coaxial electrospinning process, in which

only pure ethanol was used as sheath fluid, has been

successfully developed to produce EC nanofibers with

high quality. It was evident from FESEM observa-tions

that the modified coaxial electrospinning process is an

effective method for preparing high quality nanofibers in

terms of nanofibers’ diameter and distribution, struc-tural

uniformity. The key for the modified coaxial proc-ess is

that the reasonable selection of the sheath flow rate,

which must well match the drawing process of core EC

fluid during the electrpospinning. The EC nanofibers’

diameters (D, nm) could be manipulated through the

sheath-to-core flow rate ratio (f) as D = 819-1651f (R=

0.9754) within a suitable range of 0 to 0.25. The mecha-

nisms of the sheath solvent influence on the formation of

EC nanofibers are discussed. Sheath solvents can act as a

useful tool, permitting core electrospun fluid jets sub-

jected a longer period of electrical drawing while having

little influence on the entanglement of the core spinning

solutions. They can also render the spinning process

more stable by transferring the air-spinning-solution in-

terfaces to solvent-spinning-solution interfaces and thus

avoiding any negative influence of environment on the

core fluids. The present report provides a simple method

for implementation of the modified coaxial process to

smooth the electrospinning and obtain polymer nanofi-

bers with high quality.

5. Acknowledgements

This work was supported by the Natural Science Founda-

tion of Shanghai (No.13ZR1428900), the Key project of

Shanghai Municipal Education Commission (No.13ZZ-

113), the Innovation project of University of Shanghai

for Science and Technology (No. 13XGM01), the inno-

vation project of college student fund committee (Nos.

XJ2013274 and SH201210252153).

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