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The charge effect of cationic surfactants on the elimination of fibre beads in the
electrospinning of polystyrene
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INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY
Nanotechnology 15 (2004) 1375–1381 PII: S0957-4484(04)80029-1
The charge effect of cationic surfactants
on the elimination of fibre beads in the
electrospinning of polystyrene
Tong Lin1 , Hongxia Wang, Huimin Wang and Xungai Wang
School of Engineering and Technology, Deakin University, Geelong, Vic 3217, Australia
E-mail: tongl@deakin.edu.au
Received 29 April 2004
Published 13 August 2004
Online at stacks.iop.org/Nano/15/1375
doi:10.1088/0957-4484/15/9/044
Abstract
Polystyrene nanofibres were electrospun with the inclusion of cationic
surfactants, dodecyltrimethylammonium bromide (DTAB) or
tetrabutylammonium chloride (TBAC), in the polymer solution. A small
amount of cationic surfactant effectively stopped the formation of beaded
fibres during the electrospinning. The cationic surfactants were also found
to improve the solution conductivity, but had no effect on the viscosity. Only
DTAB had an effect on the surface tension of the polymer solution, the
surface tension decreasing slightly with an increase in the concentration of
DTAB.
The formation of beaded fibres was attributed to an insufficient stretch of
the filaments during the whipping of the jet, due to a low charge density.
Adding the cationic surfactants improved the net charge density that
enhanced the whipping instability. The jet was stretched under stronger
charge repulsion and at a higher speed, resulting in an exhaustion of the
bead structure. In addition, a polymer/surfactant interaction was found in
the polystyrene–DTAB solution system, while this interaction was not found
in the polystyrene–TBAC system. The polymer/surfactant interaction led to
the formation of thinner fibres than those formed in the absence of the
interaction.
The effects of a non-ionic surfactant, Triton X-405, on the electrospun
fibres were also studied. The addition of Triton X-405 did not eliminate the
fibre beads, but reduced the bead numbers and changed the morphology.
Triton X-405 slightly improved the solution conductivity, and had a minor
effect on the surface tension, but no effect on the viscosity.
1. Introduction A solution droplet at the tip of the nozzle gets deformed
due to the electrostatic attraction of the opposite electrode,
The generation of ultrafine fibres using electrospinning has forming a so-called ‘Taylor Cone’. Increasing the strength
been known since the 1930s [1] and has received a great deal of the electric field tends to increase the electrical attraction.
of attention in the last few decades [2, 3]. The electrospinning When the electric field is large enough, the droplet overcomes
involves the introduction of electrostatic charges to a stream of the restriction of the surface tension to create a solution jet.
polymer fluid in the presence of a strong electric field. When This jet is further stretched into a single filament or split into
a high voltage is applied to the polymer solution, charges multiple fibres/filaments while travelling through the electric
accumulate on the surface and the bulk of the polymer solution. field.
Since the electrospun nanofibres have diameters of
1 Author to whom any correspondence should be addressed.
nanometres and are usually collected as a non-woven fabric,
0957-4484/04/091375+07$30.00 © 2004 IOP Publishing Ltd Printed in the UK 1375
T Lin et al
(PVA), improved both the onset voltage and the reproducibility
of electrospinning.
This paper reports on electrospinning polystyrene with
the addition of a small amount of surfactants to the solution.
The beaded fibres disappeared when a cationic surfactant was
present in the polymer solution. Reasons as to why the
cationic surfactants prevent the formation of beaded fibres in
the electrospinning are discussed.
Figure 1. Apparatus for electrospinning.
(This figure is in colour only in the electronic version)
2. Experimental details
2.1. Materials and measurements
porous structures with interconnecting pores are inherent
in the electrospun fibre fabric. Such fabrics have an Polystyrene with an average molecular weight of
extremely high surface area and therefore have many potential 100 000 g mol−1 was obtained from BDH Chemicals Ltd and
applications in filters [4, 5], protective membranes [6–8], and used without further purification. N ,N -dimethyl formamide
in bioengineering as scaffolds [9–13] and composite material (DMF), tetrahydrofuran (THF), dodecyltrimethylammonium
bromide (DTAB), tetrabutylammonium chloride (TBAC) and
areas [14].
Triton X-405 were used as received from Aldrich. The
Polystyrene represents a high electrical resistance polymer
polystyrene solution used an equal-volume percentage mix-
with a low dielectric loss. It is also stiff and brittle in nature.
ture of DMF and THF as the solvent, and the concentration
Electrospinning of polystyrene nanofibres has been reported
of polystyrene was in the range of 5–15% (w/v). The surfac-
by some researchers [15–18]. However, defects such as
tants were dissolved separately into the polymer solutions, the
beads or necklace-like fibres were often concomitant with the
concentration ranging from 0.03 to 30 mmol l−1 for cationic
electrospun fibres; these adversely affected the reproducibility
surfactants, and from 0.001% to 1% (w/v) for Triton X-405.
of the electrospinning system and the evenness of the resultant
The viscosities and conductivities of the polymer solution
fabrics. The existence of defective fibres remains an issue
were determined by a digital rotational viscometer (D443
of concern and has also been found in other electrospinning
Rheology International) and a conductivity meter (LF330
polymer systems [19–23].
Merck), respectively. This viscometer has 5% experimental
The formation of beaded fibres has been attributed to
error and good sensitivity. The surface tension was measured
the operating conditions and the material properties, but there by the Du Nouy Ring method, using a platinum ring (Cole
has not been any unanimous agreement on the precise causes Parmer) and a highly precise electronic balance (AEA 160
so far. Fong et al [19], for example, reported that the DA ADAM). The morphology of the electrospun fibres
beaded fibres came from a capillary breakup of the jet in was observed under a scanning electron microscope (SEM,
the electrospinning, and that surface tension and viscoelastic LEO1530 microscope). The average diameter of the fibres was
properties were the key parameters for forming the beads. Lee calculated based on the SEM pictures by measuring at least 50
and his co-workers [15] suggested that the beads occurred single fibres, with the aid of computer software (ImagePro plus
only in a low viscosity polymer solution, and emphasized 4.5).
that the viscosity was the main reason for the formation
of beads. Jun et al [23] reported that the addition of an
2.2. Electrospinning
organic salt, pyridinium formiate, to poly-L-lactide solution
caused a significant reduction of the bead formation in the The electrospinning apparatus included a high voltage power
electrospinning. supply (ES30P, from Gamma High Voltage Research), a
Surfactants have been used in a variety of ways due to their syringe pump (Aldrich) and some accessories, as shown in
ability to lower the surface/interfacial tension of the medium figure 1. A high voltage was applied to the polymer solution
in which they are dissolved. An ionic surfactant, having by connecting the electrode with the metal syringe needle. A
an ionic hydrophilic head, also improves the conductivity of ground aluminium sheet was placed about 15 cm away from
the dissolved solution. When a solution contains polymer the tip of the needle. The electrospinning was conducted
and surfactant, if the polymer is capable of associating with under the ambient temperature, and at 1.5 kV cm−1 of applied
the surfactants, it will have a so-called ‘polymer/surfactant electric field and a polymer flow rate of 3 ml h−1 , except where
interaction’. The strength of the polymer/surfactant interaction specifically indicated.
depends on the polymer, the surfactant, the solvent and
their relative concentrations. A strong polymer/surfactant 3. Results and discussions
interaction could alter the rheological properties of the polymer
solution. Upon electrospinning a polystyrene solution without the
Electrospinning and the resultant nanofibres could be presence of any surfactant, beads-on-string structures emerged
affected by adding a surfactant to the polymer solution, since in the spun fibres. As shown in figure 2, the beaded fibres
the surfactant could have an influence on the electrostatic covered all the electrospun area. Increasing the flow rate
or rheological properties of the polymer solution. Yao resulted in more beads. Efforts to eliminate the beaded
et al [24] have reported that a small amount of non-ionic fibres by adjusting the operating conditions and changing the
surfactant, Triton X-100, in aqueous poly(vinyl alcohol) polymer concentrations were unsuccessful.
1376
The charge effect of cationic surfactants on the elimination of fibre beads in the electrospinning of polystyrene
Figure 2. Polystyrene nanofibres electrospun from a 10% polystyrene solution (w/v, DMF/THF 1:1 v/v).
(a) (b)
(c) (d)
Figure 3. Polystyrene nanofibres electrospun with the cationic surfactants in the polystyrene solution. In (a) and (b) TBAC 0.1 mmol l−1 ,
(c) TBAC 10 mmol l−1 , (d) DTAB 10 mmol l−1 .
When a small amount of TBAC or DTAB was added action of surface tension was decelerated abruptly, preventing
in the polymer solution, the same electrospinning process rapid breakup.
produced non-beaded fibres. The SEM images shown in A theoretical description of an electrospinning mechanism
figure 3 revealed that the addition of the surfactants led to bead- showed that under the action of the electric field the jet involved
free and homogeneous fibres. No isolated beads and beads-on- a whipping instability process during its propagation in air [26–
string structures were found. The surfactant was so effective 28]. Charges repulsing in the jet resulted in bending the jet
that a concentration as low as 10−6 mol l−1 was enough to and stretching it thinner. The whipping instability mainly
prevent the formation of the beaded fibres. depended on two reversed actions, surface tension and charge
repulsion. The surface tension tended to stabilize the jet and
Yarin et al [25] have simulated the formation of a beads-
minimize its surface, while the charge repulsion tended to
on-string structure during a free polymer fluid jet propagating
destabilize the jet. When charge repulsion dominated, the
in air. They suggested that an elongational flow arising perturbation to the jet centreline grew, and the jet became bent.
in the thin section of the jet transformed the thin section As the jet thinned away from the nozzle, the charge density
into thread where the polymer jet was hyper-stabilized due grew until charge repulsion overcame the surface tension,
to the orientation and appreciable stretching of the polymer after which the jet whipping began. The whipping instability
macromolecules. This would strengthen the liquid into a resulted in stretching the jet thinner, until it exactly balanced
filament and make it thinner. Its outflow into droplets under the the surface tension.
1377
T Lin et al
49.0
1000
Polystyrene solution with DTAB
Polystyrene solution with TBAC 48.5
100 48.0
γ ( dyn/cm )
κ ( µS/cm )
47.5
10 47.0
46.5
Polystyrene solution with DTAB
Polystyrene solution with TBAC
1 46.0
1E-3 0.01 0.1 1 10 0.01 0.1 1 10
C ( mM ) C ( mM )
(a) (b)
Figure 4. Conductivity and surface tension change with the concentration of cationic surfactants. The polymer solutions contain 10%
polystyrene (w/v in DMF/THF).
1600 1400
1300
1400 DTAB
1200 TBAC
1100
1200
1000
Diameter ( nm )
Diameter ( nm )
1000 900
800
800 700
600
600
500
400 400
300
200 200
100
0 5 10 15 20 25 30 35 4 6 8 10 12 14 16
Csurfactant ( mM ) Cpolystyrene ( %, w/v )
(a) (b)
Figure 5. Average fibre diameter change with the solution concentrations. (a) The solutions contain 10% polystyrene and different
concentrations of TBAC; (b) the solutions contain different concentrations of polystyrene, while keeping the ratio of polystyrene/surfactant
(w/mol) constant (100 g/0.715 mmol).
During the whipping, the jet was stretched under the the solvent properties. Increasing the whipping instability led
action of axisymmetric and non-axisymmetric instabilities due to stronger stretching forces that resulted in the formation of
to the perturbations of surface charges. The axisymmetric uniform threads.
instability came from the perturbations of the surface charges As shown in figure 4, the conductivity of the
along the jet axis. Different sections were affected by different polymer solution was largely improved with increase in
strengths of force that resulted in an uneven jet. However, the the concentration of DTAB. The conductivity value was
non-axisymmetric instability came from the perturbations of increased by 330 µS cm−1 , when the concentration of DTAB
surface charges around the circumference of the jet. It led to was changed from 0.003 to 30 mM. A similar result was
a localized torque and accounts for the whipping motion of found for TBAC. When the concentration was larger than
the jet. Under the action of the non-axisymmetric instability, 10 mmol l−1 , TBAC led to a higher conductivity value than
the jet tended to be uniform. The formation of beads-on- DTAB. Increasing the solution conductivity suggested that the
string structures could be related to a strong axisymmetric net charge density of the jet was increased [29]. The whipping
instability, especially when the net charge density was low. The instability was thus enhanced and the jet was stretched under
jet stretched under weak non-axisymmetric instabilities and the stronger force, resulting in the exhaustion of any bead-like
strong axisymmetric instabilities could lead to the formation fluid in the whipping process.
of beads. Further increasing the concentration of surfactant led to a
Solvent evaporation is another important factor in the larger charge repulsion that could result in stretching the thread
formation of beads. The stretched liquid threads solidified thinner. As shown in figure 5(a), the average diameter of the
rapidly due to the evaporation of solvent in the whipping. The electrospun fibres decreased slightly with the increase in the
solid fibres lost their mobility and were not deformed under the concentration of TBAC. This result confirmed that the effect
action of the surface tension. The beads-on-string structures of cationic surfactants on the whipping process came from the
could also be formed if the jet was not stretched to a uniform ionic nature.
thread before solidification. When the concentration of surfactant was larger than
Therefore, the formation of beads is related to the strength 10 mmol l−1 , the conductivity of the polymer solution reached
of the whipping instability, the stretching velocity, the viscosity a very high value. Long and branched fibres were observed for
and the surface tension of the polymer solution, as well as the TBAC involved fibres (figure 3(c)), indicating a split of the
1378
The charge effect of cationic surfactants on the elimination of fibre beads in the electrospinning of polystyrene
(a) (b)
(c)
Figure 6. Polystyrene nanofibres electrospun with the presence of Triton X-405. In (a)–(c), the concentrations of Triton X-405 are 0.01%,
0.1% and 1% (w/v) respectively.
single fibre in the electrospinning. However, such branched Similar to other polymer systems, the concentration of
fibres did not appear when the solution contained the same polymer depends on the average fibre diameters. It was
concentration of DTAB (figure 3(d)). observed that the average fibre diameters increased with the
The polymer/surfactant interaction in the aqueous system concentration of polystyrene for both of the two surfactants,
has been studied intensively [30, 31]. This interaction occurs as shown in figure 5(b). However, it was interesting to note that
when a non-ionic polymer associates with ionic surfactants by the same concentration of DTAB led to thinner fibres than with
wrapping the individual polymer chain around the surfactant TBAC, which suggested that thinner fibres were electrospun
molecules. It makes the polymer/surfactant behave like a in the system involving a polymer/surfactant interaction.
polyanion of the absorbed species. The absorbed surfactant The effects of the surfactants on the solution viscosity
increases charges and maintains the polymer chain in a more and the surface tension were also studied. It was found that
expanded structure than in the presence of an equivalent the addition of cationic surfactants did not affect the solution
amount of salt, such as NaCl [32]. viscosity. For a 10% polystyrene solution, the viscosity value
was about 14.24 centipoises (cp). This value only fluctuated
For a system with a polymer/surfactant interaction,
within the range of experimental error when the concentration
it is generally observed that the surfactant self-associates
of TBAC was changed within 0.003–30 mM. A similar result
cooperatively, i.e., in the form of aggregations, at a so-called
was found for DTAB. The effect of the surfactant on the surface
critical aggregation concentration (cac). This cac value is
tension is shown in figure 4(b). DTAB led to a slight decrease in
usually lower than the critical micelle concentration (cmc) the surface tension, by 2 dyn cm−1 , when its concentration was
of the surfactant by a factor of say between 1 and 10. The increased from 0.003 to 30 mM. However, in the same range
strength of the interaction between a polymer and a surfactant of concentration, TBAC showed no influence on the surface
can be characterized by the ratio of cac/cmc [33, 34]. The tension.
cac/cmc for the studied polymer–surfactant–solvent systems In order to confirm the charge effect of cationic surfactants
was determined according to the reported method [34]. on the elimination of beaded fibres, a similar electrospinning
The cac/cmc value for the polystyrene–DTAB system is process was also conducted by replacing the cationic surfactant
about 0.8 (cac 1.68 mmol l−1 and cmc 2.10 mmol l−1 ), with a non-ionic surfactant, Triton X-405. Figure 6 shows the
indicating that a polymer/surfactant interaction occurs between SEM images of fibres electrospun with the presence of different
polystyrene and DTAB. However, the cac/cmc value for the concentrations of Triton X-405. Obviously, the addition of a
polystyrene–TBAC system is about 0.97 (cac 1.75 mmol l−1 non-ionic surfactant did not stop the formation of beaded fibres,
and cmc 1.81 mmol l−1 ), which indicates that the cac and cmc but it largely reduced it. The beads showed a long elongated
are too close to have the polymer/surfactant interaction in the ellipsoid structure, the morphology being different to those
system. that were electrospun with the absence of any surfactants (in
1379
T Lin et al
5 49.5
49.0
4
48.5
γ ( dyn/cm )
48.0
κ ( µS/cm )
47.5
2
47.0
46.5
1
46.0
0 45.5
1E-3 0.01 0.1 1 0.01 0.1 1 10
CTriton X-405 ( %, w/v ) CTriton X-405 ( %, w/v )
(a) (b)
Figure 7. Conductivity and surface tension change with the concentration of Triton X-405. The polymer solutions contain 10% polystyrene
(w/v, in DMF/THF).
figure 2(b)). Increasing the concentration of the Triton tended structures on the filaments. Increasing the concentration
to elongate the beads. of cationic surfactant leads to thinner fibres. However, the
The addition of the Triton led to a minor decrease in the presence of a non-ionic surfactant cannot stop the formation
surface tension (figure 7(b)) but had no effect on the solution of bead fibres, but reduces the bead numbers and changes the
viscosity. Although Triton X-405 is a non-ionic surfactant, fibre morphology.
it made the solution conductivity increase slightly, from 0.7 The polymer/surfactant interaction occurs between the
to 2.7 µS cm−1 when the concentration changed from zero to polystyrene and DTAB. Such an interaction tends to produce
1% (w/v) (figure 7(a)). The improvement in the conductivity thinner fibres than a system without a polymer/surfactant
could be attributed to the weak conductivity of the Triton, interaction. It is expected that a strong polymer/surfactant
due to the existence of polarity groups, such as the hydroxyl interaction between a non-ionic polymer and an ionic
group and ethylene oxide units, in the molecule. However, surfactant can be used to tune the diameter of the electrospun
the improvement is very limited, because even for pure Triton fibres.
X-405, the conductivity is as low as 19.8 µS cm−1 . The small
improvement in the conductivity accounted for the reduction
Acknowledgments
of beads in the electrospinning.
The authors appreciate assistance given by Mr Graeme
CH3 CH3
Keating, Mr Chris Hurren and Mr Geoff Giles in constructing
H3C C C C (OCH2CH2)40OH the electrospinning apparatus.
H2
CH3 CH3
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