Electrospinning Polymers 2012

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Electrospun supramolecular polymer fibres

D. Hermida-Merino a, M. Belal b, B.W. Greenland a, P. Woodward a, A.T. Slark c, F.J. Davis a,,G.R. Mitchell b, I.W. Hamley a, W. Hayes a

a Department of Chemistry, University of Reading, Whiteknights, Reading, RG6 6AD, UKb Department of Physics, J.J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading, RG6 6AF, UKc Henkel Adhesive Technologies, 957 Buckingham Avenue, Slough, Berkshire SL1 4NL, UK

a r t i c l e i n f o

Article history:

Received 23 December 2011

Received in revised form 18 April 2012

Accepted 19 April 2012

Available online 30 April 2012

Keywords:

Electrospinning

Molecular weight

Supramolecular

Hydrogen bonding

Binding constant

Fibre

a b s t r a c t

The electrospinning of urethane based low molecular weight polymers differing only in the

nature of the hydrogen bonding end-groups has been investigated. For the end-groups with

the lowest binding constants at maximum solubility only droplets, are produced at the

electrode; in contrast, increasing the binding constant of the end-group results in electro-

spun fibres being produced. The properties of the fibres produced are subject to changes in

solvent, concentration and temperature. Typical diameters for these fibres were found to

be some 10 s oflm, rather than the sub-micron dimensions often produced in electrospin-

ning systems. Such diameters are related to the high initial concentrations required; this

also may influence the rate of solvent removal and preferential surface solidification which

feature in these examples. A simple theoretical model is used to relate the association con-

stant to the molecular weight required for fibre formation; significantly lower levels of

association are required for higher molecular weight macromonomers compared to smal-

ler molecular systems.2012 Elsevier Ltd. All rights reserved.

1. Introduction

Electrospinning occurs when a solution of polymer is

exposed to a region of high electric field. Typically the solu-

tion is passed slowly through a needle such that droplets at

the tip become charged; under these conditions, a jet is

expelled from the droplet and then follows a chaotic,

whip-like trajectory towards a grounded collection plate

[1]. The technique permits the rapid formation of fibres

with diameters typically ranging from approximately

10lm to 10 s of nanometers. At low polymer concentra-

tions, the high forces experienced by the jet prior to

becoming grounded on the collection plate, result in the

formation of undesirable discrete droplets of material,

rather than fibres. At higher concentrations (above the

critical entanglement limit (Ce) for the polymer), the

polymer chains can become entangled and consequently

are stretched and orientated whilst the solvent rapidly

evaporates, delivering high aspect ratio fibres [2]. The

resulting mesh of overlapping fibres frequently has useful

properties such as high surface area and porosity, which

has led to their investigation for a range of applications[3]

including filtration membranes[4]and tissue scaffolds[5].

As a consequence of the properties of nanoscale electro-

spun polymers, detailed studies have been conducted into

the factors which control the fibre production in order to

enable rapid optimisation of the conditions required to

generate materials with targeted properties. A range of fac-

tors have been shown to influence the nature of the fibres

produced, including voltage, solution conductivity, surface

tension and in particular, viscosity[6]. In this regard, Long

and co-workers [2,7,8] have shown that for a range of poly-

mers the diameter of the fibres produced can be related to

the ratio of the concentration of the solution used to the

critical concentration required for entanglements via a

power law. However, it has been shown that high molecu-

lar weight PMMA based materials (Mw= 183,000 g mol1)

0014-3057/$ - see front matter 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.eurpolymj.2012.04.015

Corresponding author. Fax: +44 1183786331.

E-mail address: [email protected](F.J. Davis).

European Polymer Journal 48 (2012) 12491255

Contents lists available at SciVerse ScienceDirect

European Polymer Journal

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c at e / e u r o p o l j
http://dx.doi.org/10.1016/j.eurpolymj.2012.04.015mailto:[email protected]://dx.doi.org/10.1016/j.eurpolymj.2012.04.015http://www.sciencedirect.com/science/journal/00143057http://www.elsevier.com/locate/europoljhttp://www.elsevier.com/locate/europoljhttp://www.sciencedirect.com/science/journal/00143057http://dx.doi.org/10.1016/j.eurpolymj.2012.04.015mailto:[email protected]://dx.doi.org/10.1016/j.eurpolymj.2012.04.015


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that contain multiple complementary hydrogen bonding

motifs produce fibres of greater diameter than predicted

by this simple power law [8]. This is as a consequence of

the supramolecular interactions between polymer chains

resulting in an increase in the effective molecular weight

of the polymer in solution. In a similar manner, Long has

been able to electrospin lecithin molecules by virtue of

their ability to form worm-like micelles[9]and in another

investigation the same group showed that the formation

of electrospun fibres by melt electrospinning could be

favored by the introduction of complementary adenine or

thymine hydrogen bonding units [10]. Very recently Yan

et al. [11] have published an account of supramolecular

polymer nanofibres produced by electrospinning of a hete-

roditopic monomer (based on a crown ether ammonium

salt interaction). We have followed an alternative approach

based on a series of urethane based macromonomers with

a range of hydrogen bonding end-groups.

This work focuses on a series of low molecular weight

polyurethanes (Mn< 17,000 g mol1) that have a weak (Ka

between 1.4 and 45 M1) hydrogen-bonding motif located

at each chain end [12]. Morphological and rheological

studies have highlighted the role of hydrogen bonding

and the binding constant of the end-groups in generating

materials with properties normally observed in high

molecular weight polymers[13,14]. In this account we re-

port the successful electrospinning of low molecular

weight self-assembling polyurethanes into fibres and

investigations of the relationship between the hydrogen

bonding strength of the end-group and the ability to pro-

duce fibres successfully.

2. Experimental

Electrospinning was performed using a glass syringe

mounted in a syringe pump fitted with a 22 gauge needle

of length 2.5 cm and with an internal diameter of

0.71 mm together with a flat aluminium electrode placed

normal to the needle at distances varying from 10

70 cm. A Glassman high voltage power supply was used

which allowed defined voltages over the range 7.520 kV.

Micrographs were recorded using either a FEI Quanta FEG

600 Environmental Scanning Electron Microscope or a

Cambridge 360 Stereoscan electron microscope in the Cen-

tre for Advanced Microscopy at the University of Reading.

The structures of the supramolecular polyurethanes (1

4) used in this study are shown inTable 1, detailed syn-

thetic procedures and characterisation have been described

previously [13,14]. The association constants (Ka) of the

end-groups for thesefour polymers range from 1.415 M1.

3. Results and discussion

Initial investigations into the possibility of electrospin-

ning this series of supramolecular polymers (14) into fi-

bres were conducted using THF as the solvent as a

consequence of the high solubility of each of the polymers

in this medium. For each sample electrospinning was per-

formed on solutions of polymer at the maximum possible

concentration in all cases between 40 and 47 wt.%. The

results of these experiments are shown in Table 2. As evi-

dent from Table 2, electrospinning supramolecular poly-

urethanes 1 and 2, i.e., those which contained end-groups

with the lowest binding constant, resulted in the formation

of discrete droplets, rather than the desired fibrillar struc-

tures. Beads are formed as a consequence of higher surface

tension of the cylindrical jet when compared to the volume

occupied by droplets where the polymer concentration (in

this case 4247%) lies below the chain entanglement con-

centration (Ce) [2]. In contrast, Samples 3 and 4 showed

evidence of fibre formations, albeit in the case of Sample

3, beaded fibres, which is taken as evidence that in this

case the concentration is at the lower end of that required

to produce fibres[2]; subsequent investigations were con-

ducted on polyurethane 4.

It was noted that a change in solvent from tetrahydro-

furan to dichloromethane resulted in a change in fibre

diameter (from 5243 lm). Although there are many fac-

tors that influence this fibre diameter, including conductiv-

ity and surface tension, it is viscosity which seems to play a

major role, and on this basis it seems likely that the change

in diameter arises as a result of viscosity differences conse-

quent on the slight variations in binding constants of the

polymer end-groups and the increased volatility of CH2Cl2.

On the basis of this apparent improvement in fibre forma-

tion, further investigations used this solvent.

A further study of the electrospinning of supramolecular

polymer 4 was conducted using CH2Cl2 as the solvent to

investigate the effect that polymer concentration has on

the structure of the resulting fibres. Table 3 summarises

the dimensions and products of the electrospinning process

at select concentrations from 1742 wt.%, illustrated with

selected SEM micrographs of the products. As observed

for conventional (non-associative) polymers, the morphol-

ogy of the electrospun products of supramolecular polymer

4 passes through three distinct regimes. At low concentra-

tions (>23 wt.%), discrete droplets predominate. As the

polymer concentration increases, beaded fibres predomi-

nate, and finally, above 28 wt.% polymer in CH2Cl2, high

aspect ratio fibres are formed. At 25 wt.% polymer concen-

tration, an unexpected compacted sphere product was

repeatedly observed. These results clearly represent beads

that have collapsed; their appearance suggests the forma-

tion of a skin on the surface which collapses at the elec-

trode. At the slightly lower concentration (23%) droplets

clearly flow sufficiently to allow the smeared structures

shown in the figure. Thus electrospinning this system with

differing concentrations shows many similarities to similar

experiments for non-associative polymers[6].

The results described above can be explained on the ba-

sis of the equilibrium constants for the association of the

end-groups. In the cases of polyurethanes 1 and 2 there

is no evidence for fibre formation, in contrast polyure-

thanes 3 and 4 produce fibres at similar concentrations.

The four polyurethanes are essentially the same molecular

weight in non-associated form1 since these materials are

1 Although for macromonomer 4 the presence of hydroxyl end-groups

allows for the possibility of further polymerization, the GPC data suggests

that this is minimal; the small increase in the observed value is not

necessarily significant considering the change in end-group structure, andthe synthetic procedure was designed to avoid this problem.

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Table 1

Structures and end-group binding constants of the supramolecular polymers used in this study (n 3, Mnbetween 13,800 and 17,000 g mol1).

Supramolecular polymer Mn

1

15,

2

15,

3

13,

4

17,


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made from the same precursor polymer; on this basis we

might also expect these polyurethanes to possess the same

Ce. Thus the differences arise as a result of end-groups with

greater binding capability (for 3 and 4 Ka values are 6.9

15 M1, respectively). Polyurethane 3 displayed a beaded

string morphology suggesting that the polymer solution

used was just in excess of the chain entanglement concen-

tration Ce, whereas 4 (which has the highest binding con-

stant) forms well-defined, high aspect ratio fibres. Thus,

the progressive increase in binding constants from polyure-

thanes 14 results in an increase in the apparent molecular

weights of macromolecules in solution, pushing the system

from below the Ceto above Cewithout a change in the initial

polymer concentration or molecular weight. To some extent

this result is surprising as the association constants of the

end-groups present on these polyurethanes are lower than

the values typically thought to have an influence on the de-

gree of polymerisation [15]. However, we have used a

simple model for the way in which the association equilib-

rium constantKainfluences the molecular weight to provide

a consistent explanation of the results and to predict the

likely outcome of similar experiments.

In our analysis we have used the isodesmic model as

described by Zhao and Moore[16]. Here the number aver-

age degree of polymerisation,n, can be related to the

product of the equilibrium constant and the monomer con-

centration [A] as shown in eq. (1). This product is further

defined in terms of the initial concentration, cin, and equi-

librium constant Ka as shown in eq. (2). On this basis it is

possible to predict the increase in molecular weight as a

function of the productKacin andFig. 1shows how the de-

gree of polymerisation varies for a range of values of this

product. As can be seen at low values ofKacin(Fig. 1inset)

the degree of polymerisation is approximately linear. In

fact at such low values of Kacin eqs. (1)and (2) simplify

to eq.(3); in contrast at very high values the relationship

approaches a simple square root function as shown eq.(4).

< dp>n 1=1 KaA 1

Where:

KaA 1 1=2Kacin 1=Kacin 1=2Kacin2

1=2 2

On this basis ifKacin (e.g. n 1 Kacin 3

In contrast ifKacin is large then eq. (1)reduces to:

< dp>n Kacin1=2 4

On this basis we can see that, although high molecular

weight systems require less change to the molecular

weight, there is a corresponding reduction in the molar

concentrations for equivalent weights of material per unit

volume of solvent considered in contrast, for lower molec-

ular weight systems, higher concentrations are likely to be

obtained. However this is accompanied by a reduction in

the effectiveness of this higher concentration as the system

moves from the model described by eq. (3) to that by

eq.(4). Of course higher degrees of polymerisation are re-

quired to reach molecular masses sufficient for electros-

pinning.

The combined effect of these factors can be seen if we

take as a threshold for electrospinning, the situation repre-

sented by Sample 3; i.e. a 40% w/v concentration and a va-

lue for the molecular weight of about 15,000. Of course we

note that this value reflects the specific conditions (such as

voltage and tip-to-collector distance) applied here, but on

this basis and using eqs.(1)and (2), we obtain an estimate

of the molecular weight of 17,400. As concentrations

Table 2

SEM images of the electrospun products of supramolecular polymers 14 at stated concentrations. In all cases:

concentration is in weight%, temperature 24 2 C, relative humidity 43 4%, planar electrode, working distance:

30 cm, working voltage: 12 kV.

Supramolecular polymer1

(concentration: 45%)

Supramolecular polymer2


Supramolecular polymer 3


Supramolecular polymer 4




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Table 3

Products of electrospinning supramolecular polymer 4 at increasing concentration (in dichloromethane), with selected SEM

micrographs. For all samples a planar electrode was used with a working distance of 70 cm, working voltage: 12 kV.

Supramolecular polymer

4 concentration (w/v)

Morphology Electrospinning conditions

temp (C) (Relative humidity)

17% 18 C (46%)

23% 24 C (53%)

25% 24 C (42%)

28% 16.9 lm diameter fibres 23 C (50%)

30% 18lm diameter fibres 23 C (47%)

34% 24lm diameter fibres 25 C (43%)

42% 23 C (42%)

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100 120

n

Kacin

1.45

1.35

1.25

1.15

1.05

1.4

1.3

1.2

1.1

10 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Fig. 1. Eq.(1)plotted as a function of the product Kacin. The inset shows the variations at low values of the product in the regime described by eq. 3.

D. Hermida-Merino et al. / European Polymer Journal 48 (2012) 12491255 1253


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greater than 40% are likely to be difficult to handle on the

basis of our experiences here, we can take this as a reason-

able limit to the initial concentration. If we take 17,400 as

the critical chain length for the entanglements required for

effective electrospinning, we can estimate the value for the

equilibrium constant Ka required to achieve electrospin-

ning for materials with different molecular weights. The

resulting values are shown inFig. 2(together with values

taken for a 20% solution). Of course the model does not

take account of other factors such as chain flexibility and

solutesolvent interactions, but it clearly shows that for

higher molecular weight systems the level of association

required is much lower than for smaller molecule systems.

On this basis, the threshold concentration for electrospin-

ning of polymer 4 would be 20% w/v, which is in accord

with the observations as shown in Table 3. We observe

substantial levels of solvent evaporation as the spinning

process proceeds and thus the concentration varies during

the electrospinning jet flight. On the basis of this analysis,

it appears that it is the initial monomer concentration and

the association constant which determines the viability of

the electrospinning process.

The studies above clearly indicate hydrogen bonding

facilitates fibre formation and the samples obtained

showed relatively large diameters, substantially larger,

for example, than those obtained from the associative sys-

tem recently described by Yan et al. [11]. We ascribe this to

the viscosity of the high concentrations used to obtain the

data shown in Table 2, in this respect these samples resem-

ble rather materials electrospun from the melt, where high

viscosities lead to rather broad fibres [17]. We note the

diameters decreased with reduced concentration as shown

by the data inTable 3. We also note the samples described

in by Yan et al. were prepared at a lower weight percentage

(ca. 19 wt.%); here the combination of high concentration

and higher molecular weight mitigate against the forma-

tion of submicron fibres.

The appearance of the fibres shown in Table 3suggests

some flow as they hit the collector; we attribute this to the

presence of residual solvent. However, this also reflects the

high viscosity, making, for example, bending instabilities

less effective at solvent removal. It was found that the

diameters of the fibres could be decreased both by increas-

ing the temperature and through the use of the more vol-

atile solvent dichloromethane; though of course some of

the solvent may be trapped by the formation of a surface

film. That such films are formed is apparent from the for-

mation of the hollow cylindrical structures; these appear

to be spheres formed at the electrode surface which subse-

quently collapse. By extension it may be the case that such

a skin forms on the fibres themselves. The formation of

surface layers has been discussed by Reneker et al. [18];

the presence of this solidification may have significant

influence on the final morphology of the polymer, since it

may allow the buildup of tension at the surface, whilst

the central core of any fibre remains free-flowing[19].

4. Conclusions

The results demonstrate the influence of hydrogen

bonding interactions on the electrospinning process. This

study revealed a correlation between the nature of the

end-groups and the formation of fibres. Increasing the

binding constant for the end-groups, leads to small but sig-

nificant increases in the effective degree of polymerisation

in the concentrated solutions. The studies suggest that

there are a number of factors which influence the electros-

pinning, but probably the most crucial are concentration of

the monomers and the value of the association equilibrium

constant Ka. However, with all other factors being equal

electrospinning occurs much more readily in higher molec-

ular weight systems.

Acknowledgements

The authors would like to thank Henkel UK Limited

(post-graduate studentships for PW and DHM) and EPSRC

(EP/D07434711, EP/G026203/1 post-doctoral fellowships

Fig. 2. Minimum equilibrium constant required to produce electrospun fibres predicted on the basis of eqs.(1) and (2)and the data for Sample 3; solid line

40% w/v, broken line 20% w/v.



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for BWG) and the University of Reading for financial sup-

port of this research.

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Electrospinning Polymers 2012