1. Introduction
Electrospinning has been widely used to prepare polymer fi bers with diameters ranging from several nanometers to a few micrometers. [ 1 , 2 ] Various types of polymer fi bers
have been fabricated by electrospinning for applications such as fi ltration, [ 3 ] catalysis, [4 ] wound dressing, [ 5 ] tissue
engineering, [ 6 ] and drug delivery. [ 7 ] For different
applica-tions, the surface properties and morphologies of the elec-trospun polymer fi bers play important roles. Most studies focus on controlling the surface properties and morpholo-gies of electrospun polymer fi bers by adjusting the experi-mental parameters during the electrospinning process. The experimental parameters include the polymer type, the polymer concentration, the fl ow rate, the working distance, and the applied voltage. [ 8 ] These parameters
are crucial in obtaining electrospun fi bers with unique
properties such as superhydrophobicity. [ 9 ] For examples,
Jiang et al. [ 10 ] prepared porous microsphere/nanofi ber
composite fi lms of polystyrene (PS) by electrospinning to mimic the topography of lotus leaves and to achieve a high water contact angle (WCA). The morphologies of the com-posite fi lms were controlled by adjusting the concentra-tion of the polymer soluconcentra-tion. Superhydrophobicity (contact angle = 160.48 ° ) was achieved because of the increasing surface roughness. Rutledge and co-workers [ 11 ] also
pre-pared block copolymer poly(styrene- b -dimethylsiloxane) fi bers in the range 150 − 400 nm by electrospinning. A con-tact angle of 163 ° and contact angle hysteresis of 15 ° were achieved due to the surface roughness of the electrospun fi bers and the surface enrichment in siloxane. [ 11 ]
Despite many studies on controlling surface proper-ties and morphologies of electrospun polymer fi bers by adjusting electrospinning conditions, there have been only few studies on the effect of post-treatments on the surface properties and morphologies of electro-spun poly mer fi bers. Post-treatments such as thermal annealing have been used for polymer bulks or polymer thin fi lms. After annealing, polymer chains are mobile and can relax toward an equilibrium state. [ 12 ] Fong and
Electrospun polymer fi bers are gaining importance because of their unique properties and
applications in areas such as drug delivery, catalysis, or tissue engineering. Most studies to
control the morphology and properties of electrospun polymer fi bers focus on changing
the electrospinning conditions. The effects of post-treatment processes on the morphology
and properties of electrospun polymer fi bers, however, are little studied. Here, the effect of
thermal annealing on the surface properties of electrospun polymer fi bers is investigated.
Poly(methyl methacrylate) and polystyrene fi bers are fi st
prepared by electrospinning, followed by thermal annealing
processes. Upon thermal annealing, the surface roughness of
the electrospun polymer fi bers decreases. The driving force of
the smoothing process is the minimization of the interfacial
energy between polymer fi bers and air. The water contact
angles of the annealed polymer fi bers also decrease with the
annealing time.
Effect of Thermal Annealing on the Surface
Properties of Electrospun Polymer Fibers
Jiun-Tai Chen , * Wan-Ling Chen , Ping-Wen Fan , I-Chun Yao
Prof. J. T. Chen, W. L. Chen, P. W. Fan, I. C. Yao
Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30050
E-mail: jtchen@mail.nctu.edu.tw Tel: 886-3-5731631
rate of 0.1 mL h − 1 . The nozzle was connected to a high voltage power supply (SIMCO), and the range of the applied voltage was 10 − 30 kV. The working distance between the grounded collector and the capillary nozzle was 10 − 20 cm.
2.3. Thermal Annealing Processes of Polymer Fibers and Films
Electrospun PMMA and PS fi bers were collected on glass sub-strates (1 × 1 cm 2 ) and thermally annealed in an oven for dif-ferent temperatures and times. Polymer fi lms were prepared by blade coating or spin-coating techniques on glass substrates, followed by thermal annealing for different temperatures and times.
2.4. Structure Analysis and Characterization
The glass transition temperatures ( T g ) of PMMA and PS were measured by differential scanning calorimetry (DSC) (Seiko Instruments, EXSTAR 6000). The T g of PMMA and PS used in this work are 110 and 109 ° C, respectively. The surface mor-phologies of the polymer fi bers before and after annealing were investigated by a scanning electron microscope (SEM) (JEOL, JSM-7401F) with an accelerating voltage of 10 kV. The samples were coated with 4 nm platinum before the SEM measurement.
3. Results and Discussion
Figure 1 shows the schematic illustration of the prepa-ration of polymer fi bers by electrospinning and the annealing treatment. The sizes and morphologies of the as-spun polymer fi bers are controlled by adjusting co-workers [ 13 ] studied the crystalline morphology and
polymorphic phase transitions of electrospun nylon-6 nanofi bers by thermal annealing. Upon annealing above 150 ° C, the metastable γ -crystals gradually melted and recrystallized into thermodynamically stable α -form crys-tals. Tan and Lim [ 14 ] also studied the morphology change
of electrospun poly( L-lactic acid) (PLLA) nanofi bers by
annealing. A purely fi brillar structure was changed to a mixture of fi brillar and nanogranular structures with enhanced interfi brillar bonding. The annealing process also causes the increase of the Young’s modulus of the nanofi ber because of the crystallinity.
In spite of these studies, the effect of thermal annealing on the surface morphologies and properties of electrospun polymer fi bers is still not clear. For many applications in which annealing process is involved, it is always problematic to assume that the surface prop-erties of the annealed fi bers are equivalent to those of the unannealed fi bers. Here, we investigate this effect by annealing electrospun fi bers of two commonly used polymers, poly(methyl methacrylate) (PMMA) and PS. The surface morphologies of the as-spun polymer fi bers are controlled by the electrospinning conditions. Upon thermal annealing, we fi nd that the surface roughness of the electrospun fi bers decreases, and the WCAs of the electrospun polymer fi bers also decrease. After thermal annealing, the values of WCAs of polymer fi bers are close to those of bulk polymer fi lms. Rayleigh-instability-type transformation of the electrospun fi bers, however, is not observed in this system. [ 15 ]
2. Experimental Section
2.1. Materials
Poly(methyl methacrylate) ( M w : 75 kg mol − 1 ) and PS ( M w: 192 kg mol − 1) were obtained from Sigma-Aldrich. N,N
-dimethylforma-mide (DMF) was purchased from TEDIA.
2.2. Polymer Fibers by Electrospinning
The electrospinning processes were carried out using a vertical confi guration at room temperatures. Polymer solutions with dif-ferent concentrations were fi rst prepared (PMMA: 25, 30, 35, and 40 wt%; PS: 20, 25, 30, and 35 wt%). In a typical electrospin-ning experiment to prepare PMMA fi bers, for example, a 25 wt% of PMMA solution in DMF was added to a syringe, which was connected to a capillary nozzle (inner diam-eter: 0.41 mm). A syringe pump (KD Scien-tifi c) was used to feed the polymer solution into the capillary nozzle at a constant fl ow
Figure 1 . Schematic illustration of the electrospinning process to prepare polymer fi bers
and the thermal annealing process. After thermal annealing, the surface roughness of electrospun polymer fi bers decreases. The water contact angles of the fi bers also decrease upon thermal annealing.
electrostatic force. [ 18 ] With increasing
the applied voltage, the electrostatic force can overcome the surface tension force of the droplet, resulting in the for-mation of a charged jet of polymer solu-tion. The solution jet is further thinned by the bending instability. [ 19 ] Finally,
polymer fi bers are obtained and col-lected after the fast evaporation of the solvent.
The morphologies and sizes of the polymer fi bers can be simply controlled by the electrospinning conditions, such as the polymer concentration, the fl ow rate, the working distance, and the applied voltage. One of the most common ways to control the morpholo-gies and sizes of electrospun fi bers is by adjusting the polymer concentra-tion. [ 20 ] In general, the diameter of the
electrospun fi bers increases with the polymer concentration because of the increased solution viscosity. Tan and co-workers [ 21 ] studied that the fi ber
diameter increases with the solution concentration according to a power law relationship. Figure 2 shows the SEM images of PMMA ( M w: 75 kg mol − 1)
fi bers electrospun at different concen-trations while keeping other experi-mental parameters constant. The concentration of PMMA in DMF is changed from 25 to 40 wt% with the feed rate of 1 mL h − 1 . The applied voltage is 10 kV and the working distance is 15 cm. At 25 wt%, beads-on-string structures are observed, as shown in Figure 2 a. The formation of the beaded fi bers is caused by both the entanglement of the polymer chains and the contraction of the radius of the solution jet driven by the surface tension. [ 22 ] With higher solution concentration,
the sizes of the beads become bigger, and the distances between beads are longer. Fibers without beads can fi nally be formed once a critical poly mer concentration is reached. The diameters of the electrospun PMMA fi bers are increased from ≈ 1 − 1.5 μ m for 30 wt% to ≈ 4–4.5 μ m for 40 wt%, as shown in Figure 2 b,c, and d. Although fi bers with larger diameters can be prepared by using even higher polymer concentrations, the polymer solu-tion might be too viscous to be ejected from the capillary nozzle once the concentration is too high.
The dependency of fi ber diameters on the polymer concentration can also be seen for PS fi bers. Figure S1 (Supporting Information) shows the SEM images of PS ( M w : 192 kg mol − 1 ) fi bers electrospun at different
concen-trations while keeping other experimental parameters the electrospinning conditions including the polymer
con-centration, the fl ow rate, the working distance, and the applied voltage. [ 8 ] After the polymer fi bers are collected,
they are thermally annealed in an oven for different temperatures and times. The surface properties and mor-phologies of the samples annealed at different annealing conditions are examined by a scanning electron micro-scope and contact angle measurement.
Electrospinning is commonly used to prepare polymer fi bers. With suitable solvents, almost all polymers can be prepared as fi bers by this simple process. The aspect ratios of the electrospun polymer fi bers are much larger than other one-dimensional polymer materials prepared by methods such as using anodic aluminum oxide tem-plates. [ 16 , 17 ] In a typical electrospinning process, a polymer
solution is prepared and fed into a capillary nozzle. The polymer solution is ejected from the nozzle at constant rates controlled by a syringe pump. The polymer solution is subjected to both the surface tension force and the elec-trostatic force established between the capillary nozzle and the ground collector. A conical shape of the solution droplet, referred to as the Taylor cone, with a half-angle of 49.3 ° is formed once the droplet is destabilized by the
Figure 2 . SEM images of PMMA ( M w : 75 kg mol − 1 ) fi bers electrospun at different concen-trations: a) 25, b) 30, c) 35, and d) 40 wt%. The applied voltage is 10 kV and the working distance is 15 cm. The feed rate is 1 mL h − 1 .
process is observed, as shown in the Supporting Infor-mation. But the time for the surface of the fi ber to become smooth is shorter. In addition, the fi bers melt together with other fi bers, as shown in Figure S2f (Sup-porting Information) with a lower magnifi cation. When the annealing temperature is increased to 170 ° C, the smoothing process is even faster.
The optimal condition for annealing the electrospun PMMA fi bers without forming melted fi lms is at a lower temperature (130 ° C) for 0 to 90 min. To avoid the forma-tion of porous fi lms by thermal annealing at higher tem-perature, one possible way is to anneal the electrospun polymer fi bers in a uniform environment. For example, the fi bers can be dispersed and thermally annealed in a nonsolvent such as ethylene glycol. During the annealing process, the stirring process can avoid the aggregation of the polymer fi bers. [ 24 ] For as-spun PS fi bers, the smoothing
process is similar when the fi bers are annealed at dif-ferent temperatures. After annealed at higher tempera-tures (170 ° C) annealing, fi bers melt together and form a porous fi lm structure.
In this work, the roughness change of the electro-spun polymer fi bers is demonstrated qualitatively from the SEM images. A quantitative analysis on the rough-ness change from the SEM data is diffi cult because of the curved surface of the polymer fi bers. The quantitative surface analysis may be performed by using special soft-wares such as Mex 3D (Alicona Imaging GmbH). Using these softwares, SEM images taken at different tilt angles can be mathematically processed, and three-dimensional SEM images can be reconstructed.
constant. The concentration of PS in DMF is changed from 20 to 35 wt% with the feed rate of 1 mL h − 1 . The applied voltage is 10 kV, and the working distance is 15 cm. The diameters of the electrospun PS fi bers are increased from ≈ 2 − 3 μ m for 20 wt% to ≈ 5 − 5.5 μ m for 35 wt%, as shown in Figure S1a–d (Supporting Information).
Other than polymer concentration, the sizes of the elec-trospun fi bers can also be controlled by other electrospin-ning parameters. For example, the fi ber diameters can be controlled by changing the feed rate. The diameter of the electrospun fi bers increases with the feed rate. In addition, fi bers with larger diameters can be prepared by using a lower applied voltage because of the lower elec-tric fi eld. [ 23 ]
After the preparation of electrospun polymer fi bers, the fi bers are collected and annealed in an oven for dif-ferent temperatures and times. For PMMA, we choose the fi bers made from 35 wt% of PMMA ( M w : 75 kg mol − 1 )
in DMF with an applied voltage of 10 kV and a feed rate of 1 mL h − 1 . Under this condition, PMMA fi bers without beaded structures are prepared. The diameters of PMMA fi bers prepared by this condition are ≈ 5–5.5 μ m. Figure 3 shows the SEM images of electrospun PMMA fi bers annealed at 130 ° C for different times. Before annealing, the surface of the as-spun fi ber is rough (see Figure 3 a). After annealing, the surface rough-ness decreases gradually. By annealing at 130 ° C for 30 min, the surface of the fi ber becomes smooth. But the fi brillar shape of the fi ber is still maintained, as shown in Figure 3 f with a lower magnifi cation. At higher annealing temperatures (150 ° C), similar smoothing
Figure 3 . SEM images of electrospun PMMA fi bers annealed at 130 ° C for different times: a) 0, b) 5, c) 10, d) 30, e,f) 90 min at different
argument. [ 27 ] The change of contact angles can be
pre-dicted by the Wenzel relation:
cos θ∗ = r cos θ, (2)
where r is the roughness factor, defi ned by the ratio between the actual surface area and the projected surface area. If the roughness factor is larger than 1, a hydrophobic surface becomes more hydrophobic, and a hydrophilic sur-face becomes more hydrophilic. [ 25 ] Cassie and Baxter later
used another equation to predict the contact angle of a binary composite which contains two components:
cos θ = f1cosθ1+ f2cosθ2 (3) where θ 1 and θ 2 are the contact angles of the two
com-ponents, and f 1 and f 2 are the area fractions of the two
components. [ 28 ] Therefore, the roughness change of the
electrospun polymer fi bers is expected to cause the change of WCAs.
Figure 4 a–c shows the plot of WCA of electrospun PMMA fi bers annealed at 130, 150, and 170 ° C. It has been studied that the WCAs of as-spun fi bers depend on the sizes of the electrospun fi bers. For example, Rutledge and co-workers [ 20 ] studied that the hydrophobicity increases
monotonically up to a contact angle of ≈ 155 ° with a reduction of fi ber diameters by using initiated chemical vapor deposition (iCVD)-coated poly(caprolactone) (PCL) fi bers. [ 20 ] They found that the contact angles for both
bead-free fi bers and beaded fi bers increase as the average fi ber diameter decreases. Therefore, we use polymer fi bers prepared under the same electrospinning conditions for the annealing studies to obtain polymer fi bers with sim-ilar diameters and initial WCAs. Still, the size distribution Here, we focus on the surface morphology and
prop-erties of polymer fi bers away from the substrate, and the substrate effect is not considered. For polymer fi bers in contact with the substrate, wetting or dewetting on the substrate might occur after thermal annealing. Pre-viously, we studied that electrospun PS fi bers can wet the glass substrate after thermal annealing. [ 15 ] We also
found that a Rayleigh-instability-driven morphology transformation can be observed when the electrospun PS fi bers are annealed on a PMMA-coated substrate. The polymer fi bers transform into hemispherical polymer particles, caused by the lower surface tension of PS than that of PMMA and the interfacial tension between PS and PMMA. [ 15 ] Here, however, many layers of electrospun
fi bers on substrates are prepared, and the substrate effect is neglected.
In addition to study the surface morphologies of elec-trospun fi bers by SEM, the annealed elecelec-trospun polymer fi bers are examined by WCA measurement. The rough-ness of a solid surface can signifi cantly affect the contact angle and the contact angle hysteresis. [ 25 ] For a smooth
surface, the contact angle is determined by the Young’s equation:
γSA= γSL+ γ cosθ (1)
where θ is the contact angle, and γ SA , γ SL , and γ are the
surface tensions of solid/air, solid/liquid, and liquid/air, respectively. [ 26 ] The surface is referred to as hydrophilic
when the contact angles lie between 0 ° and 90 ° and as hydrophobic when the contact angles lie between 90 ° and 180 ° . The effect of roughness on the contact angles is fi rst appreciated by Wenzel, who used a geometrical
Figure 4 . a–c) The plot of water contact angle of electrospun PMMA fi bers annealed at 130 (a), 150 (b), and 170 ° C (c). d–f) The plot of water
of annealing electrospun polymer fi bers in contact with nanoporous anodic aluminum oxide templates. [ 33 ] After
the annealing process, the polymer chains wet the nano-pores of the templates and form hierarchical polymer structures on the polymer fi bers. The high surface energy of the pore walls of the aluminum oxide templates causes the wetting of polymer melts by capillary force.
Previously, we also studied the Rayleigh-instability-type transformation of electrospun PMMA fi bers by thermally annealing the fi bers in ethylene glycol, a non-solvent for the polymer. [ 24 ] The surface of polymer fi bers
undulates with a fi nite wavelength to reduce the interfa-cial area between fi ber and ethylene glycol. The undula-tion grows with time, and polymer fi bers transform into polymer spheres with sizes determined by the original diameters of the polymer fi bers. The driving force for the transformation is the minimization of the interfa-cial energy between polymers and ethylene glycol. [ 34 , 35 ]
This type of instability, however, is not observed when the as-spun polymer fi bers are annealed in air, as we demonstrate here. The absence of the undulation of the fi ber surface might be caused by the following reasons: First, the viscosity ratio between the polymer and air is much higher than that between the polymer and eth-ylene glycol. [ 36 ] Therefore, the driving force is not enough
to induce the structure transformation due to kinetic consideration. Second, the polymer fi bers are in contact with other fi bers during the annealing process. We expect that the transformation of electrospun polymer fi bers to polymer spheres can occur if the polymer fi bers are embedded in another polymer matrix.
The initial roughness of electrospun polymer fi bers is mainly controlled by the electrospinning conditions, such as the type of polymer, the solvent, the humidity, the fl ow rate, or the working distance. In this work, we study the roughness change of electrospun fi bers with high initial roughnesses. For many electorspun fi bers with low initial roughnesses, this work also provides useful information for the decrease of roughness by annealing treatments, which is often ignored.
4. Conclusion
We study the effect of thermal annealing on the surface properties and morphologies of electrospun PMMA and PS fi bers. After thermal annealing, the surface roughness of the electrospun polymer fi bers decreases. The WCAs of PMMA fi bers decrease from ≈ 135 ° to ≈ 75 ° , which is close to the WCA of a bulk PMMA fi lm. Similarly, the WCAs of PS fi bers also decrease from ≈ 115 ° to ≈ 95 ° , which is close to the WCA of a bulk PS fi lm. This work not only provides a simple approach to control the surface properties of elec-trospun polymer fi bers but also contributes to a better of the as-spun fi bers can result in the deviations in the
data of WCA measurement. In addition, the arrangement of the fi bers on the substrate also causes deviations in the data. Before thermal annealing, the contact angle is ≈ 135 ° . When the annealing temperature is 130 ° C, the contact angle only decreases slightly and is maintained at around 120 ° with annealing time, even though the surface of the fi ber surface becomes smooth. For the samples annealed at 150 ° C, the contact angle decreases gradually with time and reaches ≈ 75 ° for the annealing time from 30 to 90 min. When the annealing temperature is increased to 170 ° C, the WCA decreases at a faster rate than those annealed at 130 or 150 ° C. After annealing at 170 ° C for ≈ 20 min, the WCA decreases quickly from ≈ 135 ° to ≈ 85 ° and maintains at ≈ 75 ° at longer annealing time, which is close to the WCA of a bulk PMMA fi lm.
WCA measurements are also performed for electro-spun PS fi bers. The contact angles of the as-electro-spun PS fi bers are ≈ 115 ° . After annealing, similar trend in WCAs is observed for electrospun PS fi bers. The WCA of elec-trospun PS decreases upon thermal annealing, as shown in Figure 4 e–f. By annealing at 170 ° C for 30 min, for example, the WCA decreases from ≈ 115 ° to ≈ 95 ° . Similar to the case of PMMA fi bers, the decreasing rate in WCAs of PS fi bers is higher at higher annealing temperatures. After annealing for longer time, the WCA of PS fi bers is close to that of a bulk PS fi lm ( ≈ 95 ° ).
For amorphous polymers, the thermal annealing pro-cess is usually performed above the T g of the polymers.
The T g of PMMA and PS used in this work is measured to
be 110 and 109 ° C, respectively. There have been studies about the surface and interface effect on the elevation or reduction of T g in polymers. [ 29 ] Here, however, we only
consider the T g values of bulk polymers. But the T g values
of polymer chains near the surface of the electrospun polymer fi bers might be different from the T g values of
polymer chains near the core of the polymer fi bers. The roughness change of the electrospun polymer fi bers can result in other property changes of the polymer fi bers. For example, Kaneko and co-workers [ 30 , 31 ] studied
that the absorption of polycyclic aromatic hydrocarbons on single wall carbon nanotubes is strongly affected by the nanoscale curvature effect. They found that tetracene adsorption was more than six times greater than that of phenanthrene. Therefore, the nanoscale curvatures on the rough polymer fi bers varied by annealing can result in different absorption abilities of organic molecules. In addition, the information about the roughness change of the polymer fi bers can be useful for separation mem-branes based on molar mass fractionation. [ 32 ]
After annealing, the surface changes from rough to smooth. The driving force for the reduction in rough-ness is to reduce the surface area between polymer and air. This result can be compared with our previous study
understanding of the relationship between the contact angle and the surface roughness of polymer fi bers.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements : This work was supported by the National Science Council.
Received: March 25, 2013; Revised: April 23, 2013; Published online: May 31, 2013; DOI: 10.1002/marc.201300290
Keywords: annealing; contact angle; electrospinning; nano-fi bers; polymers
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