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Electrospun Micro- and Nanostructured Polymer Particles Jing Liu, Asif Rasheed, Hongming Dong, Wallace W. Carr, Mark D. Dadmun, Satish Kumar*
Electrospinning at relatively low polymer concentrations produces polymer particles rather than fibers. To study the relationship between solvent characteristics and particle morphologies, PMMA was electrospun from seven different solvents and PSVPh random copolymers were electrospun from solutions in MEK. High-speed photography was used to visualize the particle-formation process. Based on these studies, a qualitative relationship between the solvent properties and the electrospun particle morphologies is discussed. By tailoring the solution properties and electrospinning conditions, particles with different morphologies (porous polygonal particles, solid polygonal particles, hollow spheres, cups etc) can be produced.
Introduction Electrospinning is used for making nanometer- to micrometer-diameter polymeric fibers[1–7] for a variety of applications.[8–11] Electrospinning at relatively low polymer concentrations results in particles rather than fibers. This particle-formation process can be termed as electrospray.[12,13] For example, poly(methyl methacrylate)
J. Liu, A. Rasheed, H. Dong, W. W. Carr, S. Kumar School of Polymer, Textile & Fiber Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0295, USA E-mail:
[email protected] M. D. Dadmun Department of Chemistry, University of Tennessee, Knoxville, USA
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(PMMA)/ethyl acetate solution[14] forms particles of decreasing size with increasing voltage. Poly(vinylidene fluoride) (PVDF)[15] particles have been processed by electrospraying its N,N-dimethylformamide (DMF) solution. It was concluded that the droplet size is controlled by the conductivity and flow rate of the solution. Silica nanocups have been produced by electrospinning poly (vinylpyrrolidone) (PVP) sol/gel solutions (50:50% mixture of ethanol and water)[12] followed by calcination. In our previous study, we reported PMMA particles with a cuplike[16] morphology. In this paper an attempt has been made to better understand various electrospun particle morphologies. PMMA was electrospun from seven different solvents and a series of poly[styrene-co-(4-vinylphenol)] (PSVPh) copolymers with varying vinyl phenol component were electrospun from 2-butanone (methyl ethyl ketone, MEK). Based on these studies, a qualitative relationship is discussed between the solvent properties and the electrospun particle morphologies.
DOI: 10.1002/macp.200800396
Electrospun Micro- and Nanostructured Polymer Particles
Experimental Part PMMA (Mw 95 000–150 000 g mol1) was obtained from Cyro Industries. PSVPh random copolymers were synthesized by freeradical polymerization.[17] The composition, molecular weight and polydispersity index of the PSVPh copolymers are listed in Table 1. All of the the solvents were purchased from Sigma-Aldrich Co. Electrospinning was carried out in the horizontal mode in a chemical hood with a flow rate of 2 mL h1 via an 18-gauge stainless-steel needle (inner diameter ¼ 0.84 mm, length ¼ 51 mm) at 22 kV; the distance between the needle tip and the grounded aluminum-foil target was 10 cm. Scanning electron microscopy was carried out on gold-coated samples in a LEO 1530 thermally assisted field-emission gun scanning-electron microscope at 10 kV. High-speed photographs were taken using a CCD camera (model: Photron FASTCAM-X 1280 PSI) with a speed of 1 000 frames per second (FPS). In order to obtain a sharp image on the micrometer-scale field of view, a pulsed Cu-vapor laser (Oxford Lasers), which emits green/yellow (510/578 nm) flash pulses with pulse duration of about 25 ns, was used.
Table 1. Vinyl phenol content measured by NMR spectroscopy, and Mn , and polydispersity index of PSVPh copolymers, measured by gel permeation chromatography (GPC).[17]
Copolymer sample
Vinylphenol content
Mn
Polydispersity index
mol-%
g molS1
PSVPh0
0.0
94 000
1.6
PSVPh10
13.2
118 000
1.8
PSVPh20
19.0
108 000
1.9
PSVPh30
32.5
105 000
2.4
PSVPh40
42.0
116 900
2.2
Table 2. Solvents and PMMA solution concentrations used.
Solvent
Solution concentration wt.-%
Results and Discussion nitromethane
PMMA Electrospinning
2, 4, 6, 8, 10, 12, 14, 16, 18, 20
acrylonitrile
The solvents and PMMA solution concentrations used for making the electrospun samples are given in Table 2. The properties of the solvents used in electrospinning the PMMA are listed in Table 3. Electrospinning of PMMA using nitromethane over a concentration range of 6 to 12 wt.-%, resulted in the formation of cups (Figure 1). The relationship between the outer diameter of the cup and solution concentration is plotted in Figure 2. In order to investigate the solvents effects on the PMMA electrospunparticle morphology, 8 wt.-% PMMA solutions were electrospun from different solvents. At the same concentration and electrospinning conditions, particles with various morphologies were produced by employing different solvents (Figure 3 and Table 4). The particle dimension generally increased with increasing polymer
8
acetone
8
DMF
8
formic acid
8
methylene chloride (MC)
1, 8
THF
1, 8
concentration. At 8 wt.-% concentration in methylene chloride (MC), particles with dimensions of 25 mm were obtained, while at 1 wt.-% concentration, the particle dimension was 8 mm. The dielectric constant of MC is quite small and its evaporation rate is the highest among the solvents used in the study. For solvents with intermediate dielectric constants (acetone, DMF, acrylonitrile,
Table 3. Properties of various solvents at room temperature.[28]
Solvent
methylene chloride (MC) THF acetone
Dielectric constant
Viscosity
Surface tension
10S3 Pa s
(mN mS1)
0.44
28.1
9.1
Evaporation rate (butyl acetate ¼ 1)
R2ij (PMMA/solvent) MPa
14.5
20
7.6
0.46
26.4
6.3
37
20.7
0.30
25.2
5.6
39
DMF
38.3
3.80
37.1
0.2
30
acrylonitrile
38.0
0.34
26.6
4.5
66
nitromethane
39.4
0.63
36.8
1.4
106
formic acid
58.0
1.80
58.2
2.1
159
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Table 4. The morphology and size of the electrospun PMMA particles from different solvents at 8 wt.-% concentration.
Solvent
Particle morphology
Particle size
methylene chloride
porous polygonal
25
THF
porous polygonal
40
acetone
solid polygonal
3
DMF
solid polygonal
2
acrylonitrile
cup
5
nitromethane
cup
formic Acid
solid polygonal
mm
4 0.5–1
with fibrous tails
Figure 1. Cups electrospun from PMMA/nitromethane solutions at different concentrations: (a) 6, (b) 8, (c) 10, and (d) 12 wt.-%. The scale bar is 2 mm.
or nitromethane), an 8 wt.-% polymer concentration results in particles with dimensions of 2 to 6 mm. The relationship between the PMMA particle size and the dielectric constant of the solvents is given in Figure 4. The larger the dielectric constant, the smaller the particle size, which is consistent with the effect of the dielectric constant on fiber diameter.[18] Formic acid, the solvent with highest dielectric constant, yielded particles (dimension 500 nm to 1 mm) with fibrous tails at 8 wt.-% concentration. The solvent evaporation rate is another important factor: fast evaporation introduces local phase separation, and the solvent-rich regions transform into pores. PMMA electrospun using MC produces porous polygonal particles (Figure 5). Solvents with a low evaporation rate, such as DMF, result in solid particles (Figure 3). Figure 2. The diameter of electrospun cups as a function of the The quality of the solvent (denoted by the R2ij value PMMA solution concentration in nitromethane. listed in Table 3) also affects the particle morphology. The R2ij value is defined as R2ij ¼ 4ðdd1 dd2 Þ2 þ ðdp1 dp2 Þ2 þðdh1 dh2 Þ2 (where dd1, dp1, and dh1 are the three-dimensional solubility parameter values for the solvent; and dd2, dp2, and dh2 are those for the polymer).[19] The smaller the R2ij value, the better the solvent for the polymer. The R2ij of methylene chloride, acetone, and DMF for PMMA are 20, 39, and 30 MPa (Table 3), respectively. These solvents are considered to be good solvents for PMMA, while acrylonitrile and nitromethane have R2ij values of 66 and 106 MPa, respectively, and thus are poor solvents for PMMA. As can be seen from the large R2ij value of 159 MPa, Figure 3. Particles electrospun from PMMA solution (8 wt.-%) in different solvents: (a) methylene chloride, (b) THF, (c) acetone, (d) DMF, (e) nitromethane, (f) acrylonitrile, (g) formic acid is the poorest PMMA solvent used in this study. At a given concentration, formic acid. The scale bars of (a) and (b) are 10 mm, the others are 2 mm.
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DOI: 10.1002/macp.200800396
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Figure 4. PMMA particle size (electrospun from different solvents at 8 wt.-% concentration using the same electrospinning conditions) as a function of solvent dielectric constant. Figure 6. PMMA cups electrospun from nitromethane solution at 8 wt.-% polymer concentration.
Figure 5. Electrospun porous polygonal PMMA particles at 1 wt.-% concentration from (a) methylene chloride and (b) tetrahydrofuran.
this difference in viscosity. The Brunauer-Emmett-Teller (BET) specific surface area of the microscopic cups processed from nitromethane at 8 wt.-% concentration, measured by nitrogen gas adsorption is 13.7 m2 g1. The adsorbed N2 quantity as a function of relative pressure of isothermal N2 adsorption (77 K) is shown in Figure 7a. The pore size and pore-size distribution determined, based on the nitrogen-gas adsorption study, are shown in Figure 7b. The cup-formation process was visualized by high-speed photography (1 000 frames per second). The distance between the needle tip and the collection target was 10 cm. Figure 8 shows the electrospinning process of PMMA/nitromethane solution at 8 wt.-% concentration. Figure 8a is the charged-polymer-solution jet initiating from the Taylor cone. At about 0.5 cm from the Taylor cone, the jet begins whipping (Figure 8b), and finally breaks up into droplets (Figure 8c). The phase separation into a solvent-rich region and a polymer-rich region appears to start at this stage, which leads to the formation of a hole and finally, complete evaporation of solvent results in cup
poor solvents result in a lower viscosity, thereby allowing for jet break up into smaller particles. Particles with similar morphology can be produced from solvents with similar properties. PMMA electrospun from tetrahydrofuran (THF) or MC results in porous polygonal particles (Figure 5). THF and MC have similar values for dielectric constant, viscosity, surface tension, and both have high evaporation rates (Table 3). In addition, THF and MC have smaller R2ij value with PMMA as compared to the other solvents used in this study. Acrylonitrile and nitromethane have similar properties (Table 3). Electrospinning PMMA from these two solvents produces cup-shaped particles. At a given concentration and electrospinning conditions, the diameter of these microscopic cups is highly uniform (Figure 6). At 8 wt.-% concentration, the cup diameter from acrylonitrile is about 20% larger and less porous than that produced from nitromethane. The dielectric constants of acrylonitrile and nitromethane are very comparable; however, based on their R2ij values; acrylonitrile is a better solvent to PMMA than nitromethane. Therefore at the same concentration, acrylonitrile solution has higher viscosity Figure 7. Nitrogen gas adsorption analysis of cups electrospun from PMMA/nitrothan the nitromethane solution. The methane solution at 8 wt.-% concentration: (a) adsorbed N2 quantity as a function larger cup diameter obtained from acryof the relative pressure of the isothermal N2 adsorption (77 K) and (b) pore size distribution of PMMA cups. lonitrile, at least in part, is attributed to Macromol. Chem. Phys. 2008, 209, 2390–2398 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 8. High-speed photographs of the PMMA/nitromethane (8 wt.-%) electrospinning process: (a) charged-polymer-solution jet initiating from the Taylor cone, (b) jet whipping, (c) jet breaking up into separate droplets. Figure 10. The angle u between the two red lines is measured as the Taylor cone angle.
Taylor cone was stable with an angle of around 70 8 (this angle is consistent with Yarin’s report[21]), and electrospinning resulted in cup morphology. At high concentrations (14 and 20 wt.-%), the Taylor cone was stable with an angle of around 110 8, and resulted in fibers. Figure 9. High-speed photographs of the PMMA/acetone (8 wt.%) electrospinning process: (a) charged-polymer-solution jet initiating from the Taylor cone, (b) jet breaking up into separate droplets.
Elctrospinning of PSVPh Copolymers PSVPh copolymers were electrospun in 2-butanone (MEK) solutions to further explore cup-formation conditions. The solvent-evaporation rate of a polymer solution depends not only on the solvent’s own characteristics, but also on the solution concentration and the interaction between
formation. For PMMA/nitromethane electrospinning, the jet break up starts in the whipping region, while for PMMA/acetone, the jet breaks up within the straight region. The electrospinning process of PMMA/acetone solution at 8 wt.-% concentration is shown in Figure 9. Figure 9a shows the charged-polymersolution jet initiating from the Taylor cone, which then breaks up into droplets (Figure 9b). Each droplet turns into a polygonal particle after solvent evaporation. In both cases, break-up occurs within 1 cm of the Taylor cone tip. The Taylor cone angle was measured as shown in Figure 10. For PMMA/ nitromethane electrospinning at various concentrations, the Taylor cone is shown in Figure 11. At the low polymer concentrations (0, 0.5, and 2.0 wt.-%), branching occurs at the Taylor cone. Branching has also been reported in carbon nanotube/poly(acrylonitrile)/ DMF solution electrospinning.[20] At these concentrations, the electrospun Figure 11. Taylor cones of electrospinning PMMA/nitromethane solutions at different morphology was a cup with a small tail polymer concentrations. The Taylor angle for the 6.0, 8.0, and 10.0 wt.-% polymer (photograph not shown). At moderate solutions is about 70 8, while for the 14.0 and 20.0 wt.-% solutions it is about 110 8. Jet branching occurred in nitromethane with the 0, 0.5 and 2.0 wt.-% polymer solutions. concentrations (6, 8, and 10 wt.-%), the
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Table 5. Three-dimensional solubility parameters of the PSVPh copolymers and R2ij between the copolymers and MEK.
energy; V: molar volume), and are listed in Table 5. The calculated values of the three-dimensional solubility parameter for polystyrene (PSVPH0) based on the group PSVPh copolymer dd dp dh R2ij contribution method are dd ¼ 18.05 (MPa)0.5, dp ¼ 1.12 (MPa)0.5, and dh ¼ 0.00 (MPa)0.5, while the corresponding MPa0.5 MPa0.5 MPa0.5 MPa experimental values for polystyrene are 21.3 (MPa)0.5, 5.8 (MPa)0.5, and 4.3 (MPa)0.5. Although there is significant PSVPh0 18.05 1.12 0.00 104.8 difference between calculated and experimental solubiPSVPh10 17.82 1.58 1.76 79.5 lity-parameter values,[23,24] the trend with increasing vinyl PSVPh20 17.72 1.78 2.54 70.5 phenol component leading to the changes in the solubility PSVPh30 17.48 2.24 4.34 55.0 parameters can be meaningfully used to assess the relative PSVPh40 17.32 2.57 5.61 48.5 solubility of various copolymers in MEK. With increasing MEK 16.00 9.00 5.10 0.0 vinyl phenol, the value of R2ij between the PSVPh copolymer and MEK decreases. This is consistent with dissolution rate and suggests that MEK becomes a better solvent for the copolymer containing a larger vinyl phenol component. the polymer and the solvent. With increasing concentraWe think that increasing amount of hydrogen bonds tion, the solvent is surrounded by more and more polymer between the –OH group in the copolymer and the –CO molecules, which slows down the solvent evaporation. group in MEK plays a role here. In cases where hydrogen bonding or other specific interactions exist between the polymer and the solvent, Figure 12(a–e) shows that the particle morphology changes from porous hollow spheres to deformed the solvent evaporation is further hindered. cups with increasing concentration of PSVPh0 in The three-dimensional solubility parameters of the PSVPh MEK. Figure 12(f–j) shows that the particle morphocopolymers were calculated by the group-contribution P Fdi [22] logy changes from hollow porous spheres to cups methods the following equations: dd ¼ V , qffiffiffiffiffiffiffiffiffiffi ffi P using qffiffiffiffiffiffiffiffiffiffi P ffi 2 Fpi with increasing concentration of PSVPh20 in MEK. Ehi dp ¼ (F , d ¼ : dispersion component force; h d Figure 12(k–o) shows that morphology changes from V V Fp: polar component force; Eh: hydrogen-bond component hollow spheres to beaded fibers to bead-free fibers with increasing PSVPh40 concentration in MEK. These morphology changes result from two factors: (1) for one specific copolymer, the MEK-evaporation rate decreases with increasing polymer concentration; (2) based on the R2ij calculation, MEK is a relatively poor solvent for PSVPh0, PSVPh10 and PSVPh20, while it is a relatively good solvent for PSVPh30 and PSVPh40. Therefore, at the same polymer concentrations, the solvent evaporation rate decreases from PSVPh0 to PSVPh40. PSVPh10 and PSVPh20 have similar morphology changes with increasing polymer concentration. PSVPh30 and PSVPh40 have comparable morphology changes with increasing concentration. Fibers were produced for PSVPh30/MEK or PSVPh40/MEK solutions at concentrations above 6 wt.-%. Due to hydrogen-bond saturation between the polymer and the solvent at a certain critical vinyl phenol Figure 12. Electrospinning of PSVPh0/MEK solution with increasing concentration of content,[25] there is no obvious electroPSVPh0: (a) 1.0, (b) 2.0, (c) 4.0, (d) 6.0, and (e) 8.0 wt.-%. Electrospinning of PSVPh20/ MEK solution, with increasing concentration of PSVPh20: (f) 1.0, (g) 2.0, (h) 4.0, (i) 6.0, spun morphology difference between and (j) 8.0 wt.-%. Electrospinning of PSVPh40/MEK solution, with increasing concen- PSVPh30 and PSVPh40 at the same tration of PSVPh40: (k) 1.0, (l) 2.0, (m) 4.0, (n) 6.0, and (o) 8.0 wt.-%. The scale bar is 2 mm. concentration. Macromol. Chem. Phys. 2008, 209, 2390–2398 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 13. Polymer-particle formation conditions. The solvents are divided into poor and good solvents. The evaporation rates of the solvents are divided in three categories: fast, medium, and slow. The particle sizes in brackets are for the PMMA particle electrospun from the 8 wt.-% concentration solutions, while for others the size is for particles electrospun from 2 wt.-% concentration solutions.
Electrospun-Particle Formation Conditions Based on electrospun PMMA and PSVPh particles, qualitative relationships between the solvent properties and the electrospun particle morphologies are shown in Figure 13. After the charged-polymer-solution jet comes out of the Taylor cone, it breaks up into droplets due to low viscosity and high surface tension. The various parameters that determine the particle size and its morphology include the polymer molecular weight, and the solvent’s dielectric constant, evaporation rate, and quality. Here we discuss the effects of these factors on particle morphology. (1) Molecular weight: low-molecular-weight polymer broadens the concentration window suitable for producing electrospun particles, which has been demonstrated by electrospinning PMMA with different molecular weights/nitromethane under the identical electrospinning conditions. Low-molecular-weight (LMW) PMMA (Mw 95 000–150 000 g mol1)/
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nitromethane produces ladles and cups at the concentration ranges from 1–12 wt.-%, and from 14 wt.-% it produces cups connected by fine fibers, while high-molecular-weight (HMW) PMMA (Mw ¼ 350 000 g mol1)/nitromethane produces cups connected by fine fibers even at very low concentrations (1–3 wt.-%).[26] (2) Dielectric constant: a solvent with a high dielectric constant results in smaller particles (Figure 4). (3) Quickly evaporating solvents result in porous particles (Figure 5). The evaporation rate of the solvent is also affected by the concentration and the interaction between the solvent and the polymer. (4) Solvent quality: the solvent quality for the polymer can be roughly predicted by the R2ij value. By choosing a good solvent with a fast evaporation rate for the polymer, polygonal porous particles can be produced (Figure 13a). For a medium-evaporation-rate solvent, solid particles with a polygonal morphology are
DOI: 10.1002/macp.200800396
Electrospun Micro- and Nanostructured Polymer Particles
Figure 14. PMMA particles electrospun from methylene chloride at 1 wt.-% concentration: the solution was maintained at room temperature while the temperature of the target was maintained at about 5 8C.
formed (Figure 13b). For a solvent with a slow evaporation rate, solid near-spherical particles (Figure 13c) were obtained. When a poor solvent is used for the polymer, porous hollow spheres are produced for solvents with a fast evaporation rate (Figure 13d). For a solvent with a medium evaporation rate, hollow spheres with open mouths (Figure 13e) are processed. For a poor solvent with a slow evaporation rate, cups (Figure 13f) are obtained. Cups can be produced by adjusting the electrospinning conditions. Hollow porous spheres (Figure 12a) from PSVPh0/MEK change into deformed cups (Figure 12e) by increasing the solution concentration. The change in morphology is attributed to hindered solvent evaporation caused by the increase in concentration. PSVPh20/MEK results in hollow spheres with an opening (Figure 12e) at 4 wt.-%, while cups (Figure 12f) are formed at 8 wt.-% solution concentration. The solvent evaporation is slowed down by the presence of more hydrogen bonds and the increased concentration, making the solution meet the cup-formation requirements. The PMMA/MC solution has an upper critical solution temperature behavior.[27] At low temperature, MC might become a poor solvent for PMMA and also the decreased temperature retards the evaporation rate. PMMA cups are obtained by electrospinning PMMA/MC solution at low temperature (5 8C) (Figure 14). By choosing the proper solvent and by tailoring the electrospinning conditions, the polymer particle morphology can be tailored.
Conclusion Electrospinning produces particles of various morphologies at relatively low polymer concentrations. Microscopic PMMA polymer cups were produced from electrospinning in nitromethane or acrylonitrile solutions. High-speed Macromol. Chem. Phys. 2008, 209, 2390–2398 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
photographs can visualize the particle formation process. In order to further explore cup-formation conditions, PSVPh random copolymers were electrospun from solutions in MEK. Cups were obtained from PSVPh10 and PSVPh20, while PSVPh30 and PSVPh40 resulted in beaded fibers at the same concentration and the same electrospinning conditions. Based on the PMMA and PSVPh electrospun particles, the qualitative relationship between the solvent properties and the morphology of the electrospun particles is shown. The electrospinning parameters determine the particle morphology. By tailoring the solution properties and the electrospinning conditions, the particle morphology can be controlled based on the desired application.
Acknowledgements: This work was supported by the Air Force Office of Scientific Research. Chongfu Zhou’s assistance for the surface area measurements and Tong Wang’s experimental assistance in electrospinning are gratefully acknowledged.
Received: July 30, 2008; Revised: September 12, 2008; Accepted: September 15, 2008; DOI: 10.1002/macp.200800396 Keywords: cups; electrospinning; high-speed photography; morphology; poly(methyl methacrylate) (PMMA); poly[styrene-co-(4vinyl phenol)] (PSVPh) copolymer
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DOI: 10.1002/macp.200800396
