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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
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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
(concentration: 47%)
Supramolecular polymer 3
(concentration: 40%)
Supramolecular polymer 4
(concentration: 42%)
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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.
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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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