electrospun micro- and nanostructured polymer particles

Full Paper

2390

Electrospun Micro- and NanostructuredPolymer Particles

Jing Liu, Asif Rasheed, Hongming Dong, Wallace W. Carr, Mark D. Dadmun,Satish Kumar*

Electrospinning at relatively low polymer concentrations produces polymer particles ratherthan fibers. To study the relationship between solvent characteristics and particlemorphologies, PMMA was electrospun from seven different solvents and PSVPh randomcopolymers were electrospun from solutions inMEK. High-speed photographywas used to visual-ize the particle-formation process. Based on thesestudies, a qualitative relationship between thesolvent properties and the electrospun particlemorphologies is discussed. By tailoring thesolution properties and electrospinning con-ditions, particles with different morphologies(porous polygonal particles, solid polygonalparticles, hollow spheres, cups etc) can be pro-duced.

Introduction

Electrospinning is used for making nanometer- to micro-

meter-diameter polymeric fibers[1–7] for a variety of

applications.[8–11] Electrospinning at relatively low poly-

mer concentrations results in particles rather than fibers.

This particle-formation process can be termed as electro-

spray.[12,13] For example, poly(methyl methacrylate)

J. Liu, A. Rasheed, H. Dong, W. W. Carr, S. KumarSchool of Polymer, Textile & Fiber Engineering, Georgia Instituteof Technology,Atlanta, GA 30332-0295, USAE-mail: [email protected]. D. DadmunDepartment of Chemistry, University of Tennessee, Knoxville,USA

Macromol. Chem. Phys. 2008, 209, 2390–2398

� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

(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) solu-

tion. 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 cup-

like[16] morphology. In this paper an attempt has been

made to better understand various electrospun particle

morphologies. PMMA was electrospun from seven differ-

ent solvents and a series of poly[styrene-co-(4-vinylphe-

nol)] (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

Table 1. Vinyl phenol content measured by NMR spectroscopy,andMn, and polydispersity index of PSVPh copolymers, measuredby gel permeation chromatography (GPC).[17]

Copolymer

sample

Vinylphenol

contentMn 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.

Experimental Part

PMMA (Mw �95 000–150000 g �mol�1) was obtained from Cyro

Industries. PSVPh random copolymers were synthesized by free-

radical polymerization.[17] The composition,molecularweight 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 �h�1 via an 18-gauge

stainless-steel needle (inner diameter¼0.84mm, length¼ 51mm)

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.

Solvent Solution concentration

wt.-%

nitromethane 2, 4, 6, 8, 10, 12, 14, 16, 18, 20

acrylonitrile 8

acetone 8

DMF 8

formic acid 8

methylene chloride (MC) 1, 8

THF 1, 8

Results and Discussion

PMMA Electrospinning

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 electrospun-

particle morphology, 8 wt.-% PMMA solutions were

electrospun from different solvents. At the same concen-

tration 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

Table 3. Properties of various solvents at room temperature.[28]

Solvent Dielectric

constant

Viscosity Surfac

10S3 Pa � s (mN

methylene chloride (MC) 9.1 0.44 2

THF 7.6 0.46 2

acetone 20.7 0.30 2

DMF 38.3 3.80 3

acrylonitrile 38.0 0.34 2

nitromethane 39.4 0.63 3

formic acid 58.0 1.80 5



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 inter-

mediate dielectric constants (acetone, DMF, acrylonitrile,

e tension Evaporation

rate (butyl acetate¼ 1)R2ij (PMMA/solvent)

�mS1) MPa

8.1 14.5 20

6.4 6.3 37

5.2 5.6 39

7.1 0.2 30

6.6 4.5 66

6.8 1.4 106

8.2 2.1 159

www.mcp-journal.de 2391

J. Liu, A. Rasheed, H. Dong, W. W. Carr, M. D. Dadmun, S. Kumar

Figure 1. Cups electrospun from PMMA/nitromethane solutionsat different concentrations: (a) 6, (b) 8, (c) 10, and (d) 12 wt.-%. Thescale bar is 2 mm.

Figure 2. The diameter of electrospun cups as a function of thePMMA solution concentration in nitromethane.

Figure 3. Particles electrospun from PMMA solution (8 wt.-%) in diffemethylene chloride, (b) THF, (c) acetone, (d) DMF, (e) nitromethane, (formic acid. The scale bars of (a) and (b) are 10 mm, the others are 2

Table 4. The morphology and size of the electrospun PMMAparticles from different solvents at 8 wt.-% concentration.

Solvent Particle

morphology

Particle size

mm

methylene chloride porous polygonal �25

THF porous polygonal �40

acetone solid polygonal �3

DMF solid polygonal �2

acrylonitrile cup �5

nitromethane cup �4

formic Acid solid polygonal

with fibrous tails

�0.5–1

2392Macromol. Chem. Phys. 2008, 209, 2390–2398


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 (dimen-

sion 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).

The quality of the solvent (denoted by the R2ij value

listed in Table 3) also affects the particle morpho-

logy. The R2ij value is defined as R2

ij ¼ 4ðdd1 � dd2Þ2 þ ðdp1 �

rent solvents: (a)f) acrylonitrile, (g)mm.

dp2Þ2 þðdh1 � dh2Þ2 (where dd1, dp1, and dh1are 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,

formic acid is the poorest PMMA solvent

used in this study.At a given concentration,

DOI: 10.1002/macp.200800396


Figure 4. PMMA particle size (electrospun from different solventsat 8 wt.-% concentration using the same electrospinning con-ditions) as a function of solvent dielectric constant.

Figure 5. Electrospun porous polygonal PMMA particles at 1 wt.-%concentration from (a) methylene chloride and (b) tetrahydro-furan.

Figure 6. PMMA cups electrospun from nitromethane solution at8 wt.-% polymer concentration.

poor solvents result in a lower viscosity, thereby allowing

for jet break up into smaller particles. Particleswith 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

Figure 7. Nitrogen gas adsorption analysis of cups electrospun from PMMA/nitro-methane solution at 8 wt.-% concentration: (a) adsorbed N2 quantity as a functionof the relative pressure of the isothermal N2 adsorption (77 K) and (b) pore sizedistribution of PMMA cups.

produces cup-shaped particles. At a given

concentration and electrospinning con-

ditions, the diameter of these micro-

scopic cups is highly uniform (Figure 6).

At 8 wt.-% concentration, the cup dia-

meter from acrylonitrile is about 20%

larger and less porous than that produced

from nitromethane. The dielectric con-

stants 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

than the nitromethane solution. The

larger cup diameter obtained from acry-

lonitrile, at least in part, is attributed to



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 � g�1. 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 processwas 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



Figure 8. High-speed photographs of the PMMA/nitromethane(8 wt.-%) electrospinning process: (a) charged-polymer-solutionjet initiating from the Taylor cone, (b) jet whipping, (c) jetbreaking up into separate droplets.

Figure 9. High-speed photographs of the PMMA/acetone (8 wt.-%) electrospinning process: (a) charged-polymer-solution jet initi-ating from the Taylor cone, (b) jet breaking up into separatedroplets.

Figure 10. The angle u between the two red lines is measured asthe Taylor cone angle.

2394

formation. For PMMA/nitromethane electrospinning, the

jet break up starts in the whipping region, while for

Figure 11. Taylor cones of electrospinning PMMA/nitromethane solutions at differentpolymer concentrations. The Taylor angle for the 6.0, 8.0, and 10.0 wt.-% polymersolutions is about 70 8, while for the 14.0 and 20.0 wt.-% solutions it is about 110 8. Jetbranching occurred in nitromethane with the 0, 0.5 and 2.0 wt.-% polymer solutions.

PMMA/acetone, the jet breaks up within

the straight region. The electrospinning

process of PMMA/acetone solution at

8wt.-% concentration is shown in Figure 9.

Figure 9a shows the charged-polymer-

solution jet initiating from the Taylor

cone, which then breaks up into droplets

(Figure 9b). Each droplet turns into a

polygonal particle after solvent evapora-

tion. 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 con-

centrations (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

morphology was a cup with a small tail

(photograph not shown). At moderate

concentrations (6, 8, and 10 wt.-%), the



Taylor cone was stable with an angle of around 70 8 (thisangle is consistent with Yarin’s report[21]), and electro-

spinning resulted in cup morphology. At high concentra-

tions (14 and 20 wt.-%), the Taylor cone was stable with an

angle of around 110 8, and resulted in fibers.

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

DOI: 10.1002/macp.200800396


Table 5. Three-dimensional solubility parameters of the PSVPhcopolymers and R2ij between the copolymers and MEK.

PSVPh copolymer dd dp dh R2ij

MPa0.5 MPa0.5 MPa0.5 MPa

PSVPh0 18.05 1.12 0.00 104.8

PSVPh10 17.82 1.58 1.76 79.5

PSVPh20 17.72 1.78 2.54 70.5

PSVPh30 17.48 2.24 4.34 55.0

PSVPh40 17.32 2.57 5.61 48.5

MEK 16.00 9.00 5.10 0.0

the polymer and the solvent. With increasing concentra-

tion, the solvent is surrounded by more and more polymer

molecules, which slows down the solvent evaporation.

In cases where hydrogen bonding or other specific

interactions exist between the polymer and the solvent,

the solvent evaporation is further hindered.

The three-dimensional solubility parameters of the PSVPh

copolymers were calculated by the group-contribution

methods using the following equations:[22] dd ¼P

FdiV ,

dp ¼

ffiffiffiffiffiffiffiffiffiffiffiPF2pi

q

V , dh ¼ffiffiffiffiffiffiffiffiffiffiffiP

EhiV

q(Fd: dispersion component force;

Fp: polar component force; Eh: hydrogen-bond component

Figure 12. Electrospinning of PSVPh0/MEK solution with increasingPSVPh0: (a) 1.0, (b) 2.0, (c) 4.0, (d) 6.0, and (e) 8.0 wt.-%. ElectrospinMEK solution, with increasing concentration of PSVPh20: (f) 1.0, (g) 2and (j) 8.0 wt.-%. Electrospinning of PSVPh40/MEK solution, with intration of PSVPh40: (k) 1.0, (l) 2.0, (m) 4.0, (n) 6.0, and (o) 8.0wt.-%. The



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

contribution method are dd¼ 18.05 (MPa)0.5, dp¼ 1.12

(MPa)0.5, and dh¼ 0.00 (MPa)0.5, while the corresponding

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

difference between calculated and experimental solubi-

lity-parameter values,[23,24] the trendwith increasing vinyl

phenol component leading to the changes in the solubility

parameters can bemeaningfully used to assess the relative

solubility of various copolymers in MEK. With increasing

vinyl phenol, the value of R2ij between the PSVPh copolymer

andMEK decreases. This is consistent with dissolution rate

and suggests that MEK becomes a better solvent for the

copolymer containing a larger vinyl phenol component.

We think that increasing amount of hydrogen bonds

between the –OH group in the copolymer and the –CO

group in MEK plays a role here.

Figure 12(a–e) shows that the particle morphology

changes from porous hollow spheres to deformed

cups with increasing concentration of PSVPh0 in

MEK. Figure 12(f–j) shows that the particle morpho-

logy changes from hollow porous spheres to cups

with increasing concentration of PSVPh20 in MEK.

Figure 12(k–o) shows that morphology changes from

hollow spheres to beaded fibers to bead-free fibers with

concentration ofning of PSVPh20/.0, (h) 4.0, (i) 6.0,creasing concen-scale bar is 2mm.

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 con-

centration; (2) based on the R2ij calcula-

tion, 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 satura-

tion between the polymer and the

solvent at a certain critical vinyl phenol

content,[25] there is no obvious electro-

spun morphology difference between

PSVPh30 and PSVPh40 at the same

concentration.



Figure 13. Polymer-particle formation conditions. The solvents are divided into poor and good solvents. The evaporation rates of the solventsare divided in three categories: fast, medium, and slow. The particle sizes in brackets are for the PMMAparticle electrospun from the 8wt.-%concentration solutions, while for others the size is for particles electrospun from 2 wt.-% concentration solutions.

2396

Electrospun-Particle Formation Conditions

Based on electrospun PMMA and PSVPh particles, quali-

tative 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) M

Mac

� 20

olecular 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 �mol�1)/

romol. Chem. Phys. 2008, 209, 2390–2398

08 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

nitromethane produces ladles and cups at the con-

centration ranges from �1–12 wt.-%, and from

14 wt.-% it produces cups connected by fine fibers,

while high-molecular-weight (HMW) PMMA (Mw ¼350000 g �mol�1)/nitromethane produces cups con-

nected by fine fibers even at very low concentrations

(�1–3 wt.-%).[26]

(2) D
ielectric constant: a solvent with a high dielectric
constant results in smaller particles (Figure 4).

(3) Q
uickly 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) S
olvent 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


Figure 14. PMMA particles electrospun from methylene chlorideat 1 wt.-% concentration: the solution was maintained at roomtemperature while the temperature of the target wasmaintainedat about 5 8C.

Mac

� 20

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 openmouths (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 electrospin-

ning 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 morpholo-

gies at relatively low polymer concentrations. Microscopic

PMMA polymer cups were produced from electrospinning

in nitromethane or acrylonitrile solutions. High-speed

romol. Chem. Phys. 2008, 209, 2390–2398

08 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 solu-

tions in MEK. Cups were obtained from PSVPh10 and

PSVPh20, while PSVPh30 and PSVPh40 resulted in beaded

fibers at the same concentration and the same electro-

spinning 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 para-

meters determine the particlemorphology. By tailoring the

solution properties and the electrospinning conditions, the

particlemorphology can be controlled based on the desired

application.

Acknowledgements: This work was supported by the Air ForceOffice of Scientific Research. Chongfu Zhou’s assistance for thesurface area measurements and Tong Wang’s experimentalassistance 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; mor-phology; poly(methyl methacrylate) (PMMA); poly[styrene-co-(4-vinyl phenol)] (PSVPh) copolymer

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DOI: 10.1002/macp.200800396

electrospun micro- and nanostructured polymer particles

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