16105

2007, 111, 16105-16108

Published on Web 10/10/2007

Polymer-Micelle Complex as an Aid to Electrospinning Nanofibers from Aqueous Solutions

R. Nagarajan,*,† C. Drew,‡ and C. M. Mello†

Macromolecular Science Team and Nanomaterials Team, Natick Soldier Research, DeVelopment & Engineering
Center, Kansas Street, Natick, Massachusetts 01760
ReceiVed: September 14, 2007

We propose and demonstrate the concept that a polymer-micelle complex can be used as an aid to preparing
nanofibers from aqueous solutions by the electrospinning process. This is based on the recognition that a
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polymer-micelle complex can simultaneously contribute to an increase in solution viscosity, decrease in

surface tension, and increase in electrical conductivity, all of which favor the formation of nanofibers by
electrospinning. By incorporating the polymer-micelle complex as a secondary ingredient, electrospinning
of sparingly soluble or low molecular weight polymers is made possible, as is illustrated here using a PEO-
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SDS complex to electrospin a gel forming, genetically engineered silk-elastin biopolymer. The wide choice
of polymers and surfactants available to form the polymer-micelle complex with a range of viscosity, surface
tension and electrical conductivity properties, potentially allows for a variety of solutions and dispersions to
be electrospun from the aqueous medium.

Introduction surface tension favors the formation of fibers without beads.

Also, a direct dependence of the fiber diameter on the surface
Polymeric nanofibers are a new class of nanomaterials in the
size range of 10-1000 nm that have been prepared from nearly tension has been established by fluid mechanical analysis.17 The
50 or more synthetic as well as biological polymers.1 Numerous electrical forces are responsible for the initiation of the jet and
applications in membrane systems,2 catalysis,3 immobilized the stretching during whipping instability. Therefore, high
enzymes,4 chemical and biological defense,5,6 fiber-reinforced solution conductivity and large solvent dielectric constant are
composite materials,7 tissue engineering,8 wound healing,9 favorable to produce thinner fibers.
sensors and photonics,10 etc, are being developed taking Many solvents including formic acid, dimethyl formamide,
advantage of the large surface-to-volume ratio of these nanofi- dimethyl acetamide, chloroform, tetrahydrofuran, methylene
bers and the ability to modify and functionalize the nanofiber chloride, ethanol, isopropanol, hydrochloric acid, camphorsul-
surfaces. Polymeric nanofibers are efficiently produced by the fonic acid, trifluoroacetic acid, carbon disulfide, hexafluoroiso-
process of electrospinning, a process invented about 70 years propanol, water, and mixtures of these solvents have been
ago by Formhals.11 In electrospinning, a fluid jet is generated employed for electrospinning.7 In practice, the solvent is selected
by the application of an electric field and the jet, subjected to mainly on the basis that the polymer can dissolve in large
electrostatic, viscous, and surface tension forces, experiences enough concentration to make the solution adequately viscous.
various instabilities. Fluid mechanical analysis12-17 suggests that For all organic solvents listed here, the surface tensions are in
in most cases, the jet experiences a whipping instability giving the range 20-40 mN/m, dielectric constants lie between 5 and
rise to the bending and stretching of the jet and resulting in the 40, and the electrical conductivities are quite small, typically
generation of nanofibers. below 4 mS/m (in most cases, below 1 mS/m).
One can expect the morphology and the properties of the For the large scale production of nanofibers, electrospinning
nanofiber to be influenced by (i) the properties of the polymer from aqueous solutions is desirable, from an environmental point
solution such as the viscosity, dielectric constant, surface tension, of view. Common water-soluble polymers such as polyethylene
density, and solvent vapor pressure, (ii) the operational variables oxide (PEO), polyvinyl pyrrolidone (PVP), and polyvinyl
such as the solution flow rate, applied electric field, and the alcohol (PVOH) have been successfully electrospun into nanofi-
electric current, and (iii) the equipment variables such as the bers from aqueous solutions. However, polymers that have low
nozzle size and the distance between the nozzle end and the aqueous solubility, low molecular weight polymers, and poly-
fiber collector. Uniform nanofibers are not produced from all mers with rigid or globular structures do not generate necessary
polymer solutions. The stability of the jet depends on the viscous viscosity in aqueous solutions and are not easily electrospun
and viscoelastic properties of the polymer solution, and unstable into uniform fibers. Some polymers may undergo gelation at
jets (corresponding to low viscosity solutions) result in breakup low concentrations, making available only solutions of low
into droplets and the formation of beaded structures.17 Reduced viscosity for electrospinning. To overcome the limitations listed
above and to facilitate electrospinning from aqueous solutions,
* Corresponding author. E-mail: Ramanathan.Nagarajan@us.army.mil.
† Macromolecular Science Team. we propose an approach that makes use of a polymer-micelle
‡ Nanomaterials Team. complex as a secondary ingredient in the aqueous solution.
10.1021/jp077421o This article not subject to U.S. Copyright. Published 2007 by the American Chemical Society
16106 J. Phys. Chem. C, Vol. 111, No. 44, 2007 Letters

Figure 3. Schematic representation of electrospinning process with

the main components being a high voltage power supply, a syringe
pump system for controlled flow, and a grounded collector.
Figure 1. Schematic representation of a polymer-micelle complex
formed from the nonionic polymer, polyethylene oxide and the anionic
surfactant, sodium dodecyl sulfate, in aqueous solutions.
Experimentally determined19-21 relative viscosities of the
polyethylene oxide-sodium dodecyl sulfate solution (defined as
the ratio between the viscosity of the polymer + surfactant
solution and the viscosity of the polymer solution, both at the
same polymer concentration) have been plotted in Figure 2 as
a function of the surfactant concentration for different molecular
weights and concentrations of polyethylene oxide. In addition
to impacting the solution viscosity, the type and amount
of surfactant also determines the surface tension22 and elec-
trical conductivity23 of the aqueous solution. Therefore, the
polymer-micelle complex can be designed to manipulate the
solution viscosity, surface tension, and conductivity so as to
make the aqueous solution properties favorable for electrospin-
ning.
To illustrate the use of a polymer-micelle complex as an
aid to electrospinning, we demonstrate the electrospinning of
the gel forming, silk-elastin protein polymers (SELP). These
polymers combine the superior mechanical properties contrib-
uted by silk units and the controlled solubility and elastic
properties contributed by elastin units. A wide range of
biomedical applications of SELP including drug delivery, gene
Figure 2. Relative viscosity of a PEO + SDS aqueous solution
delivery, and as material for vascular grafting is currently being
containing polymer-micelle complex (defined with respect to a PEO explored.24 The protein polymers are water soluble, and they
solution with no SDS). Circles:19 0.1 wt % PEO (Mw ) 5 × 106), form irreversible gels due to physical interactions25,26 even at
Diamonds:20 0.5% w/v PEO (Mw ) 885 000), Squares:21 1.15% w/v ambient temperatures. The onset of gel formation is sufficient
PEO (Mw ) 250 000) plus 0.02 M NaCl. All measurements are at to cause problems with electrospinning. The gelation is avoided
25 °C. or at least the rate of gelation is retarded only when the
A schematic representation of the nonionic polymer-ionic biopolymer concentration in the aqueous phase is significantly
micelle complex18,19 is presented in Figure 1. The segments of decreased. But this causes the solution viscosity to also decrease
the polymer molecule wrap around the micelles with the polymer below a level suitable for electrospinning.
segments partially penetrating the polar head group region of
Experimental Section
the micelles and reducing the hydrophobic micelle core-water
contact. The size of the polymer-bound micelle depends on the Materials. The biopolymer sample received from Genencor
nature of polymer-micelle interactions,19 whereas the number is a genetically engineered, repeat block copolymer, of protein
of micelles bound per polymer depends on the polymer sequences representative of silk and elastin. It has the structure
molecular weight. The ionic micelles make the nonionic polymer H2N-(S3E4EKE3S3)1-13-COOH, where S stands for the silk-
behave like a polyelectrolyte because of inter-micelle repulsions like peptide block GAGAGS, E stands for the elastin-like
and contribute to significant expansion of the polymer coil upon peptide block GVGVP, and EK stands for the modified elastin-
complex formation. As the surfactant concentration continues like peptide block, GKGVP (one-letter amino acid abbreviations
to increase, the ionic strength of the solution also increases, are used, G for glycine, A for alanine, S for serine, V for valine,
which reduces the inter-micelle repulsions and causes a reduc- P for praline, K for lysine). The molecular weight of the polymer
tion in the magnitude of polymer expansion. As a result, the is about 70 000 Da. All other chemicals including polyethylene
solution viscosity first increases, exhibits a maximum, and then oxide, sodium dodecyl sulfate, formic acid are commercial
decreases with increasing concentration of the surfactant. products and used as received.
Letters J. Phys. Chem. C, Vol. 111, No. 44, 2007 16107

Figure 4. SEM images of electrospun nanofibers of silk-elastin protein polymer, SELP67K prepared at an applied voltage of 20 kV and a polymer
solution flow rate of 120 µL/h. (a) 15 wt % SELP67K in formic acid, (b) 8 wt % SELP67K in formic acid, (c) 6.7 wt % SELP67K, 2.1 wt % PEO
(Mw ) 900 000) and 1 wt % SDS in aqueous solution, (d) fibers from (c) were dried overnight in air and then exposed to air saturated with
methanol at 25 °C for 1 h, (e) fibers from (c) were dried overnight in air and then thermally treated by incubation at 140 °C for 30 min.

Electrospinning Setup. A schematic of the electrospinning like in the image are spots where fiber-fiber contact had
setup is shown in Figure 3. The polymer solution is placed in occurred between not fully dry fibers. The same system was
a syringe pump with a needle attached to the end. As the pump electrospun, but at a lower polymer concentration of about 8
displaces the fluid, a droplet of the solution becomes suspended wt %, corresponding to which the solution viscosity is lower.
from the needle tip where it is held by surface tension forces. The resulting electrospun nanofibers are imaged in Figure 4b.
An electrode from a high voltage power supply is placed in One can see significant presence of beads forming in the system.
contact with the needle tip, applying an electrical potential, Aqueous solutions of SELP polymers undergo gelation and
which induces free charges in the polymer solution. These the fraction of polymer incorporated into the gel is a function
charged ions move in response to the applied electric field. This of the polymer molecular weight, initial concentration of the
introduces a tensile force in the polymer solution. When the polymer, temperature, and time.26 Given adequate time, say 24
tensile force overcomes the surface tension force associated with h, most of the polymer becomes part of the gel when the
the pendent drop of the liquid at the capillary tip, a jet of liquid polymer molecular weight is as high as that of the product used
is ejected from the tip. As the jet travels the short distance of in this study. We determined that an aqueous solution of SELP,
about 10∼20 cm between the nozzle tip and the collector, the up to about 7 wt %, did not lead to any gelation at room
contour length of the jet dramatically increases by orders of temperature for the duration of our experiments (2-3 h),
magnitude and the jet thins to nanometer scale. The solvent whereas for higher concentrations, gelation was more rapid.
evaporates as the jet travels from the tip to the collector. The However, for this concentration range of up to 7 wt % SELP,
evaporation of the solvent leaves dry nanofibers at the collector
the solution viscosity is quite small, and therefore, the aqueous
surface. Aluminum foils or glass slides were used as collector
solution cannot be processed into fibers by electrospinning. The
surfaces.
consequences of low solution viscosity could not be compen-
SEM Imaging. The electrospun fibers were sputtered with a sated by changes in other operational variables accessible for
15 nm layer of gold-palladium and imaged at 20 kV accelerating manipulation, such as the applied voltage (in the range 10-30
voltage using a Zeiss EVO 60 scanning electron microscope. kV) or the solution flow rate (in the range 50 to 250 µL/hr).
Keeping SELP as the main polymer component, we added
Results and Discussion
2.1 wt % polyethylene oxide (Mw ) 900 000) and 1 wt %
Electrospinning of SELP. SELP biopolymer is soluble in sodium dodecyl sulfate (approximately 35 mM) to an aqueous
formic acid without the formation of any gels. The polymer solution of SELP at 6.7 wt % concentration. The polymer-
was electrospun from a 15 wt % solution in formic acid at an micelle complex formation is spontaneous and the solution
applied voltage of 20 kV and a polymer solution flow rate of becomes significantly viscous. As mentioned before, the polymer
120 µL/h. The distance between the nozzle end and the collector PEO contributes to increasing the solution viscosity. The
surface was approximately 15 cm and a grounded aluminum addition of the surfactant SDS further increases the solution
foil was used as the collector. Nanofibers of 200-300 nm viscosity by the formation of extended PEO-SDS micelle
diameter are produced as shown by the SEM image in Figure complexes, decreases the surface tension of the solution,
4a. No bead formation is observed. What appear to be bead- increases the solution conductivity, and also increases the
16108 J. Phys. Chem. C, Vol. 111, No. 44, 2007 Letters

clouding/gelation temperatures, thus retarding any possible spin nanofibers from aqueous solutions. The main contribution
gelation. Each one of these changes is conducive to the to the physical chemistry literature rests on the simple observa-
formation of fibers. Nanofibers of 200-300 nm diameter are tion that the addition of a polymer-micelle complex simulta-
formed as shown by the SEM image in Figure 4c. The fibers neously changes the solution viscosity, electrical conductivity,
are more nonuniform in thickness compared to the fibers and surface tension, all in the direction favorable for electro-
produced in formic acid (Figure 4a) and appear not to be fully spinning of nanofibers. An additional advantage that could result
dry. from the polymer-micelle complex is the reduction or delay
Making Electrospun Nanofibers Water-Resistant. To make in the gelation rate for gel-forming polymers. Further, since the
the dry nanofibers stable when contacted with water, the polymer-micelle complex can act also as a colloidal dispersant,
secondary structure of silk-like blocks can be modified to induce it may be possible to electrospin other polymer molecules,
formation of a larger fraction of hydrogen-bonded beta sheets nanoparticles or enzymes in their dispersed state using polymer-
or beta strands. Such increased crystallinity in the silk-like micelle complex as an aid. The present approach is designed to
blocks will make the nanofiber more stable when contacted with keep the polymer-micelle complex as a secondary ingredient
water vapor in practical applications and also impart improved in the total solids that will form nanofibers. Although in the
mechanical properties. One approach to induce such structural example presented here, the polymer-micelle complex accounts
changes in the silk-like blocks is by treatment with methanol.27 for a mass fraction of 0.32 based on total solids, this fraction
From our experiments using liquid methanol, we found that the can be significantly reduced to less than 0.10 by choosing a
nanofiber morphology was compromised and liquid methanol PEO of larger molecular weight.
transformed a part of the nanofibers into a thin polymer film
with the remaining fibers incorporated into the film. This could Acknowledgment. R.N. gratefully acknowledges a NRC
be because the liquid methanol treatment was done on freshly Senior Research Associateship. The bioengineered SELP sample
spun nanofibers which were not completely dry. To avoid this was kindly supplied by Genencor.
result, the fibers were treated by contact with methanol vapor
overnight. The resulting nanofiber morphology is shown in References and Notes
Figure 4d. The fibers are swollen compared to the untreated (1) Li, D.; Xia, Y. AdV. Mater. 2004, 16, 1151.
fibers (Figure 4c) and the fiber diameter is relatively larger. (2) Gibson, P. W.; Schreuder-Gibson, H. L.; Pentheny, C. J. Coated
An alternate approach to change the morphology of silk Fabrics 1998, 28, 63.
(3) Demir, M. M.; Gulgun, M. A.; Menceloglu, Y. Z.; Erman, B.;
domains is though thermal annealing.28 The fibers were annealed Abramchuk, S. S.; Makhaeva, E. E.; Khokhlov, A. R.; Matveeva, V. G.;
at 140 °C for 30 min. The thermal treatment also results in some Sulman, M. G. Macromolecules 2004, 37, 573.
fiber swelling (Figure 4e) but not as much as in the case with (4) Jia, H.; Zhu, G.; Vugrinovich, B.; Kataphinan, W.; Reneker, D.
methanol vapor treatment. The fiber diameter is somewhat larger H.; Wang, P. Biotechnol Prog. 2002, 18, 1027.
(5) Schreuder-Gibson, H. L.; Gibson, P. H.; Senecal, K.; Sennett, M.;
than that of the untreated fibers. In these exploratory experi- Walker, J.; Yeomans, W.; Ziegler, D.; Tsai, P. P. J. AdV. Mater. 2002, 34,
ments, no attempt has been made to optimize the polymer- 44.
micelle complex. (6) Schreuder-Gibson, H. L.; Truong, Q.; Walker, J. E.; Owens, J. R.;
Wander, J. D.; Jones, W. E., Jr. MRS Bull. 2003, August, 574.
Choice of Polymer-Micelle Complex Systems. It is clear (7) Huang, Z. M.; Zhang, Y. Z.; Kotaki, M.; Ramakrishna, S.
from Figure 2 that the polymer molecular weight, polymer Composites Sci. Technol. 2003, 63, 2223.
concentration, and surfactant concentration are variables that (8) Li, W. J.; Laurencin, C. T.; Caterson, E. J.; Tuan, R. S.; Ko, F. K.
can be manipulated to obtain any desired viscosity value. In J. Biomed. Mater. Res. 2002, 60, 613.
(9) Katti, D. S.; Robinson, K. W.; Ko, F. K.; Laurencin, C. T. J.
general, when the polymer molecular weight is lower, higher Biomed. Mater. Res. 2004, 70B, 286-296.
polymer and surfactant concentrations are needed to attain the (10) Wang, X. Y.; Drew, C.; Lee, S. H.; Senecal, K. J.; Kumar, J.;
same viscosity level. A number of nonionic polymers including Samuelson, L. A. Nano Lett. 2002, 2, 1273.
(11) Formhals, A. U.S. Patent No. 1,975,504, 1934.
PEO, PVP, hydroxyl propyl cellulose (HPC), polypropylene (12) Doshi, J.; Reneker, D. H. J. Electrostatics 1995, 35, 151.
oxide (PPO), and ethyl hydroxylethyl cellulose (EHEC) have (13) Hohman, M. H.; Shin, M.; Rutledge, G.; Brenner, M. P. Phys. Fluids
been shown to form complexes with ionic micelles. The choice 2001, 13, 2201.
of the surfactant mainly controls the surface tension of the (14) Hohman, M. H.; Shin, M.; Rutledge, G.; Brenner, M. P. Phys. Fluids
2001, 13, 2221.
aqueous solution since the surface tension reduction with the (15) Feng, J. J. Phys. Fluids 2002, 14, 3912.
surfactant is much larger than the reduction achieved with the (16) Shin, M.; Hohman, M. H.; Rutledge, G.; Brenner, M. P. Appl. Phys.
polymer alone. Compared to the surface tension of ∼72 mN/m Lett. 2001, 78, 1149.
(17) Fridrikh, S. V.; Yu, J. H.; Brenner, M. P.; Rutledge, G. C. Phys.
for water, surface tensions of 30-40 mN/m are attainable with ReV. Lett. 2003, 90, 144502.
hydrocarbon type surfactants, whereas for fluorinated surfac- (18) Nagarajan, R. Colloids Surf. 1985, 13, 1.
tants, surface tensions in the range 17-24 mN/m can be (19) Nagarajan, R.; Kalpakci, B. Polym. Prepr. 1982, 23, 41.
achieved.22 The conductivity of the aqueous solution depends (20) Chari, K.; Antalek, B.; Lin, M. Y.; Sinha, S. K. J. Chem. Phys.
1994, 100, 5294.
on the concentration of the surfactant and has been measured23 (21) Chari, K.; Kowalczyk, J.; Lal, J. J. Phys. Chem. B 2004, 108, 2857.
to be 33 mS/m at 5 mM SDS, 60 mS/m at 10 mM SDS, and 75 (22) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.;
mS/m at 16 mM SDS. Similar values are achieved for various Wiley: New York, 1989.
(23) Perez-Rodriguez, M.; Varela, L. M.; Garcia, M.; Mosquera, V.;
ionic surfactants, both of the hydrocarbon type and the Sarmiento, F. J. Chem. Eng. Data 1999, 44, 944.
fluorocarbon type. Thus, the solution containing polymer- (24) Haider, M.; Megeed, Z.; Ghandehari, H. J. Controlled Release 2004,
micelle complex can be designed to have viscosity, surface 95, 1.
tension, and conductivity properties covering a wide range, (25) Cappello, J.; Crissman, J. W.; Crissman, M.; Ferrari, F. A.; Textor,
G.; Wallis, O.; Whitledge, J. R.; Zhou, X.; Burman, D.; Aukerman, L.;
suitable for electrospinning, by the choice of the polymer- Stedronsky, E. R. J. Controlled Release 1998, 53, 105.
micelle complex system. (26) Dinerman, A. A.; Cappello, J.; Ghandehari, H.; Hoag, S. W.
Biomaterials 2002, 23, 4203.
Conclusions (27) Magoshi, J.; Magoshi, Y.; Nakamura, S. J. Polym. Sci. Polym. Phys.
Ed. 1981, 19, 185.
We propose and demonstrate the concept that the self- (28) Drummy, L. F.; Phillips, D. M.; Stone, M. O.; Farmer, B. L.; Naik,
assembled polymer-micelle complex can be an aid to electro- R. R. Biomacromolecules 2005, 6, 3328.