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Water-Stable Nonwovens Composed ofElectrospun Fibers from Aqueous Dispersionsby Photo-Cross-Linking

Elisabeth Giebel, Andreas Greiner*

Copolymers of MMA, BA, and MABP are prepared by radical emulsion polymerization in water.As a result aqueous dispersions of these copolymers are obtained with particle sizes around60–120 nm. After addition of some PVA the dispersions are electrospun. The resulting fibersdisplay different morphologies depending on the copolymercomposition. Inter- and intra-particle cross-linking is achievedby photo-cross-linking induced by the MABP moieties yieldingfibers with good thermomechanical properties depending onthe content of MABP. With this approach, thermomechanicallystable electrospun fibers with smooth surface structure can beobtained by electrospinning from water.

1. Introduction

Electrospinning is a versatile method for the preparation of

highly functional nonwovens. The extremely fast develop-

ment of this field is mostly driven by a large variety of

potential applications and simplicity of electrospinning

itself, although the process is of complex nature.[1–6]

Nevertheless, practical application of electrospinning is

hindered for several reasons, depending on the require-

ments of the particular application. One of the major

concerns in technical application of electrospinning is the

need of organic solvents, which are mostly harmful, for

the preparation of water-stable electrospun nonwovens.

Furthermore, the usage of water as solvent in the

electrospinning process opens the way to applications

where toxic or harmful solvents cannot be used, e.g.,

medical applications[7–9] or plant protection.[10] In contrast,

electrospinning of aqueous solutions of water-soluble

polymer result in water-soluble nonwovens unless they

are stabilized by cross-linking or other chemical modifica-

E. Giebel, Prof. A. GreinerDepartment of Chemistry and Scientific Center of MaterialsScience, Philipps-Universitat Marburg, Hans-Meerwein Str.,D-35032 Marburg, GermanyE-mail: [email protected]

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlin

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tions of the polymer.[11–14] However, the cross-linking of

water-soluble polymers requires considerable energy and

can result in swelling of the electrospun fibers or brittleness

depending on the degree of cross-linking. An elegant

alternative to water-based electrospinning is electrospin-

ning of aqueous polymer dispersions.[15–20] Electrospinning

of aqueous polymer dispersions offers several advantages

including harmless solvent conditions and higher overall

polymer concentrations due to the inherently lower

viscosities as compared to analogous polymer solutions.

It has been shown that the merging of dispersion particles

upon fiber formation is crucial for electrospun fibers with

decent mechanical properties, which depends on the

adjustment of the polymer glass transition temperature

close to room temperature.[17] However, at temperatures

above room temperature the polymer will flow, leading to a

loss of fiber structure. Thus, the thermomechanical stability

of such fibers is insufficient for many applications. Recently,

dispersion born electrospun fibers with improved thermo-

mechanical properties were obtained by cross-linking of

dispersion particles or by cross-linking between dispersion

particles after electrospinning.[20] The intraparticle

cross-linking was performed during the preparation of

the polymer dispersions by emulsion polymerization

with cross-linking agents. The interparticle cross-linking

occurred during the electrospinning process itself between

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E. Giebel, A. Greiner

functional groups bound to the particles and a cross-link

agent added to the spinning solution.

We wondered whether aqueous polymer dispersions

for electrospinning of electrospun fibers with good

thermomechanical stability could be tailored in that

way that intraparticle and interparticle cross-linking

takes place without the need of addition of extra cross-

linking agents. Building on the previous concepts we

have investigated dispersions with particles which

can be undergo both, interparticle and intraparticle

cross-linking by photo cross-linking as shown in

Figure 1C in comparison to previously published concepts

(Figure 1A, B).

2. Experimental Section

2.1. Materials

Methylmethacrylate (MMA) and butylacrylate (BA, Aldrich) were

purified by distillation over calcium-hydride at reduced pressure.

Sodium dodecylsulfate (SDS), potassium peroxodisulfate, 4-hydro-

xybenzophenone, and methacryloyl chloride (Aldrich) were used

as received. Deionized water was degassed by refluxing under

argon for 6 h. Poly(vinyl alcohol) (PVA, Mw¼ 195 000, hydrolysis

grade 98.0–98.8%, Mowiol 56–98) was used as received (Kuraray

Europe).

Figure 1. Different concepts for the preparation of electrospun fiberspublished previously[20] and concept C is reported here in detail.

Macromol. Mater. Eng. 2012, DOI

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2.2. Methods

Characterization of latex fibers was done with a JSM-7500F (JEOL)

scanning electron microscope operating at accelerating voltages of

4 kV.

The solid content of dispersions was determined by TGA using a

Mettler ToledoTGA/STD A 851 at a heating rate of 10 K �min�1

under air. Evaluation was done with 2 STARe software.

Glass transition temperatures were measured by a Mettler

Toledo DSC 821c at heating/cooling rates of 10 K �min�1 under a

nitrogen atmosphere. Evaluation was done with 2 STARe software.

IR-Spectra were taken by a Digilab (Excalibur series) instrument

with ATR crystal ZnSe and WinIRPro software version 3.3.

Stress/strain experiments were performed by a Zwick/Roell BT1-

Fr0.5TN-D14. The test speed was 50 mm �min�1. Evaluation was

done using testXpert II V3.0 software.1H NMR spectroscopy was carried out using a Bruker ARX300-

spectrometer in CDCl3 solution.

2.3. Preparation of 4-Methacryloyloxybenzophenone

100 g (505 mmol) of 4-hydroxy-benzophenone was dissolved in

1 L of dichloromethane, the mixture was cooled in an ice bath

and a solution of 51 mL (525 mmol) of methacryloyl chloride

and 77.5 mL (560 mmol) of triethylamine in 200 mL of dichloro-

methane was added dropwise.[21] The reaction was stirred

at room temperature for 4 h. The solvent was evaporated and

from aqueous dispersions by cross-linking. Concept A and B were

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Table 2. Properties of the electrospinning formulations.

Amount

MABP

[mol%]

Conductivity

[mS cm�1]

Surface

tension

[mN m�1]

Viscosity at

a shear rate

of 3000 s�1

0 2.76 52 2.64

1 2.78 56 2.31

2 2.96 57 3.15

Water-Stable Nonwovens Composed of Electrospun Fibers from Aqueous Dispersions . . .

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the residue was dissolved in diethyl ether. The non-soluble

triethylammonium salt was removed by filtration. The organic

phase was washed with water three times and afterwards dried

over Na2SO4. The solvent was evaporated and the crude product

was chromatographed over (7/3 dichloromethane/hexane). The

product was recrystallized from a mixture of dichloromethane

and hexane (2:8).1H NMR (CDCl3, d): 2.08 (3H, s), 5.81 (1H, s), 6.39 (1H, s), 7.18–7.27

(2H, m), 7.46–7.51 (2H, m) 7.57–7.62 (1H, m), 7.77–7.81(2H, m), 7.85–

7.88 (2H, m).
5 3.06 52 2.26
10 2.94 54 2.32

2.4. Preparation of Dispersions by Emulsion

Polymerization

A mixture of MMA, BA, and MABP were mixed in ratios as given in

Table 1. 50 mL of water and 114.5 mg of SDS were added to this

mixture and stirred at 1500 rpm in a 250 mL glass reactor under

argon atmosphere at 75 8C. After 15 min of stirring, 68 mg

potassium peroxodisulfate dispersed in 1 mL of water was

added and the stirring speed was reduced to 250 rpm. The mixture

was stirred for additional 60 min and was then allowed to cool

to 20 8C. The resulting dispersion was used without any further

purification.

For DSC and 1H NMR measurement a small amount of dispersion

was precipitated in saturated CaCl2 solution and washed three

times with water and one time with methanol. Afterward it was

dried in vacuum at 20 of 8C.

The experimental ratios of MABP and MMA/BA were analyzed

by 1H NMR spectroscopy by comparing the integrals of the aromatic

protons of the MABP (d¼7.2–7.9), the �OCH2 protons of BA at

d¼ 4.0 and the �OCH3 protons of the MMA at d¼3.6.1H NMR (CDCl3, d): 0.85–2.4 (m), 3.6 (s), 4.0 (s), 7.18–7.27 (m), 7.46–

7.51 (m), 7.57–7.62 (m), 7.77–7.81 (m), 7.85–7.88 (m).

2.5. Electrospinning of Dispersions

A solution of 25 wt% PVA in deionized water was mixed with the

dispersion in a ratio matrix polymer/dispersion (solid content)

1:4. This formulation was electrospun on alumina foil with a

voltage of 40 kV at a distance of 20 cm and a feed rate of

0.05 mL �min�1 through a needle with a diameter of 0.9 mm. The

properties of the electrospinning formulation are shown in

Table 2.

Table 1. Overview of the amount of monomers used in the reaction

Amount of

MABP [%]

BA

[mL] [mmol] [m

0 8.6 61 9

1 8.5 60 9

2 8.4 59 9

5 8.2 57 9

10 7.7 54 8

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Macromol. Mater. Eng. 2012, DO



2.6. UV Irradiation

UV irradiation was performed with a medium pressure mercury

lamp TQ 150 (power input 150 Watt) by Heraeus and a quartz

cooling tube. The distance between radiation source and

sample was 20 cm. Irradiation times were varied between 5 and

30 min. For UV irradiation during electrospinning the UV source

was placed in a distance of 60 cm from the electrospinning

setup.

2.7. Sample Preparation for Mechanical Tests

The samples were prepared by collecting the fibers on a rotating

wheel turning at 1000 rpm with a voltage of 20 kV at a distance of

10 cm and a feed rate of 0.05 mL �min�1 through a needle with a

diameter of 0.9 mm. The resulting ribbon with a width of 1 cm was

cut in samples of 2 cm length. A part of the samples was irradiated

with UV light for 30 min on each side. For each MABP content ten

samples were taken before and after UV irradiation.

3. Results and Discussion

MABP was selected as a well-established polymerizable

photo-cross-linker.[21–27] MABP was synthesized in high

yields in a one-step process from 4-hydroxybenzophenone

and methacryloyl chloride according to literature.[21]

Dispersions with different amounts of MABP were obtained

by emulsion copolymerization with MMA and BA according

to Scheme 1.

.

MMA MABP

L] [mmol] [g] [mmol]

.7 91 –

.6 90 0.381 1.5

.5 89 0.762 3

.2 86 1.905 7.5

.7 82 3.81 15

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Scheme 1. Preparation of dispersion particles by emulsion copolymerization of MABP,MMA, and BA.

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In all polymers the molar ratio of MMA to BA was always

kept at 3:2. SDS was used as surfactant and potassium

persulfate as radical initiator. The experimental amount of

MABP was determined via 1H NMR spectroscopy. Disper-

sions with solid contents around 25 wt% were obtained. The

glass transition temperature of the dispersion particles was

adjusted just below 40 8C. Particle sizes were between 60

and 120 nm and the z potential of the dispersions was

between �55 and þ72 mV (Table 3).

The viscosity of the dispersion were adjusted for

electrospinning by addition of PVA in a weight ratio of

1:4 to the dispersion polymer. Electrospinning was

performed in a standard one-needle electrospinning set-

up with parameters as given in the experimental part.

Electrospun fibers were characterized using scanning

electron microscopy (SEM). No structural details of disper-

sion particles were observed for fiber samples, which were

composed of 0–5% of MABP (Figure 2A–D). Obviously,

the particles merged and formed smooth fibers despite

showing a glass transition temperature of 40 8C. The

formation of smooth fibers during green electrospinning

of polymers with melting points more than 30 8C above

processing temperature was reported before.[18] A possible

Table 3. Synthesis and properties of dispersions.

Amount MABP

[mol%]

Glass transition

temperature latex

polymer [-C]

Solid

content

[wt%]Calc. Exp.

0 0 39 23

1 1 35 23

2 2 36 24

5 5 41 26

10 a) 54 27

a)The polymer was not completely soluble.

Macromol. Mater. Eng. 2012, DOI: 10.1002/mame.20110

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explanation is that water and surfactants

plasticize the polymer. In contrast, fibers

with uneven structure were obtained

with dispersions of higher glass transi-

tion temperature containing 10 wt%

MABP (Figure 2E).

After electrospinning the fibers were

cross-linked by UV irradiation. The cross-

linking process was monitored by

infrared spectroscopy. Excerpts of the IR

spectra after different times of UV-

irradiation time focusing on the signal

of the keto group of MABP at 1665 cm�1

showed a steady decrease of the keto

group indicating the cross-linking

reaction (Figure 3). No signal of the keto

group of MABP could be observed after 15 min UV

irradiation.

The influence of the cross-linking on the mechanical

properties was investigated by stress/strain experiments.

For the investigations fibers were collected on a rotating

wheel to obtain an aligned fiber mats. Before and after

UV irradiation for 30 min of each side of the sample the

E-modulus was measured. To prevent a structure loss of

the non-cross-linked fibers due to removal of the matrix

polymer the fibers were still containing the PVA. The

E-modulus was calculated using the effective area (Aeff) of

the cross-section. This area was calculated from the density

(r) after measuring the weight (m) and length (l) of the

samples by using

Aeff ¼m

lp(1)

�3
A hypothetical density of 1 g � cm was used for the
calculation. The results are shown in Table 4.

The fibers without MABP showed no change of E modulus

due to the UV irradiation. The E modulus of samples with 1–

5% MABP showed a significant increase after UV irradiation.

Particle

size

[nm]

Zeta

potential

[mV]

65 �55

76 �64

74 �57

78 �55

112 �72

0401

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se DOI for citation !!

Figure 2. SEM images of the untreated electrospun fiber mats (A) 0, (B) 1, (C) 2, (D) 5, and (E) 10% MABP.


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The fibers consisting of the particles containing 10% MABP

showed only a slight increase of the E modulus which could

be explained by the incomplete merger of the dispersion

particles upon fiber formation resulting in a reduced degree

of interparticle cross-linking.

The thermomechanical stability of the fibers was probed

by immersion of the photo-cross-linked fibers in water at

60 8C for 1 h. Water was removed three times in order to

assure complete removal of the non-cross-linked PVA. SEM

images of the samples with different amount of the cross-

linker MABP showed that hot water treatment proved

Figure 3. Excerpt of the IR spectra of electrospun fibers containing5% MABP before and after UV irradiation at different timeintervals.





shape persistence for fibers with MABP but a significant loss

of fiber shape without MABP and thereby without cross-

linking (Figure 4). Fibers with 10% MABP became smoother

in shape but still showed an uneven surface in comparison

to fibers with less MABP.

After the test with hot water the same fibers were

annealed at 100 8C in an oven for 24 h. SEM images of the

annealed fibers showed a good persistence of the fiber

shape when MABP was present in the original fibers and

confirmed the results by hot water treatment (Figure 5).

Upon water treatment the fibers containing no MABP

showed a increase in fiber diameter, due to the flowing of

the polymer. The fibers containing MABP showed a

decrease in diameter, which can be attributed to the loss

of the PVA. Heat treatment resulted in a further increase of

the fiber diameter of the sample containing no MABP. The

Table 4. Change of the mechanical properties of electrospun fibermats due to photo cross-linking.

Amount

MABP [%]

E Modulus [MPa]

Before UV

treatment

After UV

treatment

0 140� 31 148� 20

1 175� 42 243� 51

2 176� 41 230� 26

5 162� 44 250� 50

10 166� 42 194� 32

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Figure 4. SEM images of the electrospun fiber mats after photo-cross-linking and removal of the matrix polymer (A) 0, (B) 1, (C) 2, (D) 5, and(E) 10% MABP.

Figure 5. SEM-images of the photo-cross-linked electrospun fiber mats after heat treatment (A) 0, (B) 1, (C) 2, (D) 5, (E) 10% MABP.

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fibers containing MABP showed no significant change of

the fiber diameter (Table 5).

A nonwoven with an area weight of 11 mg � cm�2 was

prepared by using the dispersion containing 5% MABP. One

side of this sample was irradiated for 15 min. Figure 6 shows

excepts of the IR spectrum of the untreated fiber mat (a),




compared to the IR spectrum of the irradiated side (b), and

the non-irradiated back side (c). The non-irradiated side

showed no change in the intensity of the signal of the keto

group of MABP compared to the untreated sample,

indicating that the UV light did not penetrate the sample.

Figure 6d shows the IR spectra of the backside of a

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Table 5. Change of the fiber diameter due to water and heattreatment.

Amount

MABP [%]

Diameter [nm]

Before

treatment

After water

treatment

After heat

treatment

0 410� 120 654� 233 790� 265

1 646� 153 558� 169 597� 175

2 629� 157 536� 136 590� 209

5 571� 131 547� 109 493� 115

10 558� 277 447� 143 378� 94

Figure 6. Excerpt of the IR-spectra of a nonwoven with an areaweight of 11 g � cm�2 containing 5% MABP (a) without UV treat-ment, (b) irradiated side of a nonwoven, (c) non-irradiated back-side of a nonwoven, (d) backside of a nonwoven irradiated duringelectrospinning.


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nonwoven spun in the presence of a UV source. The reduced

signal of the keto group proves, that combining electro-

spinning and photo-cross-linking in one step is possible and

will result in a homogeneous cross-linking reaction

throughout the fiber. However, a stronger UV source is

needed to obtain complete cross-linking.

4. Conclusion

Following the concept for cross-linkable dispersion parti-

cles for electrospinning, copolymers of MMA and BA

were successfully functionalized with UV cross-linker

by emulsion polymerization in water. The resulting

dispersions could be mixed with PVA and process to

fibers by electrospinning. UV irradiation of the fibers





resulted in intra- and intermolecular cross-linking as

proven by the persistence of the fiber shape at higher

temperatures in comparison to un-cross-linked fibers. UV

cross-linking of the dispersion-based fibers resulted in a

significant increase of mechanical properties and thermal

stability. Time required for UV cross-linking was in a

relatively short range and could be significantly shortened

for technical applications, e.g., by stronger UV sources. In

conclusion, for the preparation of smooth and stable

fibers by dispersion electrospinning low glass transition

temperatures and cross-linking is required, which can be

achieved with chemically bound cross-linking sites on the

dispersion particles.

Acknowledgements: The authors are indebted to the Bundesmi-nisterium fur Bildung und Forschung for financial support.

Received: November 15, 2011; Revised: January 20, 2012;Published online: DOI: 10.1002/mame.201100401

Keywords: dispersions; electrospinning; synthesis

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