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Electrospun supramolecular polymer fibres
European Polymer Journal 48 (2012) 1249–1255
Contents lists available at SciVerse ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
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, UK Department of Physics, J.J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading, RG6 6AF, UK c Henkel Adhesive Technologies, 957 Buckingham Avenue, Slough, Berkshire SL1 4NL, UK b
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 electrospun 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 of lm, rather than the sub-micron dimensions often produced in electrospinning 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 constant to the molecular weight required for fibre formation; significantly lower levels of association are required for higher molecular weight macromonomers compared to smaller molecular systems. Ó 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 solution 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 10 lm to 10 s of nanometers. At low polymer concentrations, 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 ⇑ Corresponding author. Fax: +44 1183786331. E-mail address: [email protected] (F.J. Davis). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.04.015
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 electrospun 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 factors 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 polymers 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 molecular weight PMMA based materials (Mw = 183,000 g mol1)
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D. Hermida-Merino et al. / European Polymer Journal 48 (2012) 1249–1255
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 heteroditopic 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 report 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 produce 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 7.5–20 kV. Micrographs were recorded using either a FEI Quanta FEG 600 Environmental Scanning Electron Microscope or a Cambridge 360 Stereoscan electron microscope in the Centre for Advanced Microscopy at the University of Reading. The structures of the supramolecular polyurethanes (1– 4) used in this study are shown in Table 1, detailed synthetic procedures and characterisation have been described previously [13,14]. The association constants (Ka) of the end-groups for these four polymers range from 1.4–15 M1. 3. Results and discussion Initial investigations into the possibility of electrospinning this series of supramolecular polymers (1–4) into fibres 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 performed 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 polyurethanes 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 structures. 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 42–47%) lies below the chain entanglement concentration (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 conducted on polyurethane 4. It was noted that a change in solvent from tetrahydrofuran to dichloromethane resulted in a change in fibre diameter (from 52–43 lm). Although there are many factors that influence this fibre diameter, including conductivity 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 consequent 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 formation, 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 17–42 wt.%, illustrated with selected SEM micrographs of the products. As observed for conventional (non-associative) polymers, the morphology of the electrospun products of supramolecular polymer 4 passes through three distinct regimes. At low concentrations (>23 wt.%), discrete droplets predominate. As the polymer concentration increases, beaded fibres predominate, and finally, above 28 wt.% polymer in CH2Cl2, high aspect ratio fibres are formed. At 25 wt.% polymer concentration, an unexpected ‘compacted sphere’ product was repeatedly observed. These results clearly represent beads that have collapsed; their appearance suggests the formation of a skin on the surface which collapses at the electrode. 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 basis 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 polyurethanes 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, and the synthetic procedure was designed to avoid this problem.
Table 1 Structures and end-group binding constants of the supramolecular polymers used in this study (n 3, Mn between 13,800 and 17,000 g mol1). Supramolecular polymer
Mn
Ka of end-group (M1) 1.4
15,100
1.5
13,800
6.9
1
2
3 17,000
D. Hermida-Merino et al. / European Polymer Journal 48 (2012) 1249–1255
15,200
15
4 1251
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D. Hermida-Merino et al. / European Polymer Journal 48 (2012) 1249–1255 Table 2 SEM images of the electrospun products of supramolecular polymers 1–4 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 polymer 1
Supramolecular polymer 2
(concentration: 45%)
(concentration: 47%)
Supramolecular polymer 3
Supramolecular polymer 4
(concentration: 40%)
(concentration: 42%)
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 concentration Ce, whereas 4 (which has the highest binding constant) forms well-defined, high aspect ratio fibres. Thus, the progressive increase in binding constants from polyurethanes 1–4 results in an increase in the apparent molecular weights of macromolecules in solution, pushing the system from below the Ce to above Ce without 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 degree of polymerisation [15]. However, we have used a simple model for the way in which the association equilibrium constant Ka influences 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 average degree of polymerisation,n, can be related to the product of the equilibrium constant and the monomer concentration [A] as shown in eq. (1). This product is further defined in terms of the initial concentration, cin, and equilibrium 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 product Kacin and Fig. 1 shows how the degree of polymerisation varies for a range of values of this product. As can be seen at low values of Kacin (Fig. 1 inset) 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 ¼ 1=ð1 Ka ½AÞ
ð1Þ
Where:
Ka ½A ¼ 1 þ 1=2Ka cin ½1=ðKa cin Þ þ 1=ð2Ka cin Þ2 1=2
ð2Þ
On this basis if Kacin (e.g. <1) is small then eq. (1) reduces to:
< dp>n ¼ 1 þ Ka cin
ð3Þ
In contrast if Kacin is large then eq. (1) reduces to:
< dp>n ¼ ðKa cin Þ1=2
ð4Þ
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 molecular 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 required to reach molecular masses sufficient for electrospinning. The combined effect of these factors can be seen if we take as a threshold for electrospinning, the situation represented by Sample 3; i.e. a 40% w/v concentration and a value 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
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D. Hermida-Merino et al. / European Polymer Journal 48 (2012) 1249–1255 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%)
16.9 lm diameter fibres 18 lm diameter fibres 24 lm diameter fibres
28% 30% 34%
23 °C (50%) 23 °C (47%) 25 °C (43%) 23 °C (42%)
42%
10 9 8
1.45 1.4
n
7
1.35 1.3 1.25
6
1.2 1.15
5
1.1 1.05
4
1 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
3 2 1 0
20
40
60
80
100
120
Kacin 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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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.
greater than 40% are likely to be difficult to handle on the basis of our experiences here, we can take this as a reasonable 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 electrospinning for materials with different molecular weights. The resulting values are shown in Fig. 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 solute–solvent 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 electrospinning 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 system 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 resemble 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 in Table 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 formation of submicron fibres. The appearance of the fibres shown in Table 3 suggests 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 increasing the temperature and through the use of the more volatile 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 formation of the hollow cylindrical structures; these appear to be spheres formed at the electrode surface which subsequently 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 significant increases in the effective degree of polymerisation in the concentrated solutions. The studies suggest that there are a number of factors which influence the electrospinning, 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 molecular 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
D. Hermida-Merino et al. / European Polymer Journal 48 (2012) 1249–1255
for BWG) and the University of Reading for financial support of this research. References [1] Reneker DH, Chun I. Nanometer diameter fibres of polymers, produced by electrospinning. Nanotechnology 1996;7:216–23. [2] McKee MG, Wilkes GL, Colby RH, Long TE. Correlations of solution rheology with electrospun fiber formation of linear and branched polyesters. Macromolecules 2004;37(5):1760–7. [3] Greiner A, Wendorff JH. Electrospinning: a fascinating method for the preparation of ultrathin fibres. Angew Chem Int Ed 2007;46(30): 5670–703. [4] Barhate RS, Ramakrishna S. Fabrication of nanofibers with antimicrobial functionality used as filters: protection against bacterial contaminants. J Membrane Sci 2007;296(1–2):1–8. [5] Chong EJ, Phan TT, Lim IJ, Zhang YZ, Bay BH, Ramakrishna S, et al. Evaluation of electrospun PCL/gelatin nanofibrous scaffold for wound healing and layered dermal reconstruction. Acta Biomaterialia 2007;3(3):321–30. [6] Wang C, Hsu C-H, Lin J-H. Scaling laws in electrospinning of polystyrene solutions. Macromolecules 2006;39(22):7662–72. [7] Gupta P, Elkins CL, Long TE, Wilkes GL. Electrospinning of linear homopolymers of poly(methyl methacrylate):exploring relationships between fiber formation, viscosity, molecular weight and concentration in a good solvent. Polymer 2005;46(13):4799–810. [8] McKee MG, Elkins CL, Long TE. Electrospinning of linear and highly branched segmented poly(urethane urea)s. Polymer 2004;45(26): 8705–15. [9] McKee MG, Layman JM, Cashion MP, Long TE. Phospholipid nonwoven electrospun membranes. Science 2006;311(5759):353–5.
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[10] Huntly MT, Karikari AS, Mckee MG, Mather BD, Layman JM, Fornof AR, et al. Taking advantage of tailored electrostatics and complementary hydrogen bonding in the design of nanostructures for biomedical applications. Macromol Symp 2008;270:1–7. [11] Yan X, Zhou M, Chen J, Chi X, Dong S, Zhang M, et al. Supramolecular polymer nanofibers via electrospinning of a heteroditopic monomer. Chem Commun 2011;47(25):7086–8. [12] Hermida-Merino D, Slark AT, Colquhoun HM, Hayes W, Hamley IW. Thermo-responsive microphase separated supramolecular polyurethanes. Polym Chem 2010;1(8):1263–71. [13] Woodward P, Hermida-Merino D, Greenland BW, Hamley IW, Light Z, Slark AT, et al. Hydrogen bonded supramolecular elastomers: correlating hydrogen bonding strength with morphology and rheology. Macromolecules 2010;43(5):2512–7. [14] Woodward P, Clarke A, Greenland BW, Hermida-Merino D, Yates L, Slark AT, et al. Facile bisurethane supramolecular polymers containing flexible alicyclic receptor units. Soft Matter 2009;5(10): 2000–10. [15] De Greef TFA, Smulders MMJ, Wolffs M, Schenning APHJ, Sijbesma RP, Meijer EW. Supramolecular polymerisation. Chem Rev 2009;109(11):5687–754. [16] Zhao D, Moore JS. Nucleation-elongation: a mechanism for cooperative supramolecular polymerisation. Org Biomol Chem 2003;1(20):3471–91. [17] Dalton PD, Grafahrend D, Klinkhammer K, Klee D, Möller M. Electrospinning of polymer melts: phenomenological observations. Polymer 2007;48(23):6823–33. [18] Koombhongse, Liu W and D. H, Reneker J. Flat polymer ribbons and other shapes by electrospinning. Polym Sci, Part B: Polym Phys 2001;39(21): 2598–2606. [19] Mohan SD, Mitchell GR, Davis FJ. Chain extension in electrospun polystyrene fibres: a SANS study. Soft Matter 2011;7(9):4397–404.
Contents lists available at SciVerse ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
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, UK Department of Physics, J.J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading, RG6 6AF, UK c Henkel Adhesive Technologies, 957 Buckingham Avenue, Slough, Berkshire SL1 4NL, UK b
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 electrospun 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 of lm, rather than the sub-micron dimensions often produced in electrospinning 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 constant to the molecular weight required for fibre formation; significantly lower levels of association are required for higher molecular weight macromonomers compared to smaller molecular systems. Ó 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 solution 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 10 lm to 10 s of nanometers. At low polymer concentrations, 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 ⇑ Corresponding author. Fax: +44 1183786331. E-mail address: [email protected] (F.J. Davis). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.04.015
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 electrospun 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 factors 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 polymers 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 molecular weight PMMA based materials (Mw = 183,000 g mol1)
1250
D. Hermida-Merino et al. / European Polymer Journal 48 (2012) 1249–1255
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 heteroditopic 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 report 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 produce 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 7.5–20 kV. Micrographs were recorded using either a FEI Quanta FEG 600 Environmental Scanning Electron Microscope or a Cambridge 360 Stereoscan electron microscope in the Centre for Advanced Microscopy at the University of Reading. The structures of the supramolecular polyurethanes (1– 4) used in this study are shown in Table 1, detailed synthetic procedures and characterisation have been described previously [13,14]. The association constants (Ka) of the end-groups for these four polymers range from 1.4–15 M1. 3. Results and discussion Initial investigations into the possibility of electrospinning this series of supramolecular polymers (1–4) into fibres 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 performed 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 polyurethanes 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 structures. 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 42–47%) lies below the chain entanglement concentration (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 conducted on polyurethane 4. It was noted that a change in solvent from tetrahydrofuran to dichloromethane resulted in a change in fibre diameter (from 52–43 lm). Although there are many factors that influence this fibre diameter, including conductivity 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 consequent 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 formation, 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 17–42 wt.%, illustrated with selected SEM micrographs of the products. As observed for conventional (non-associative) polymers, the morphology of the electrospun products of supramolecular polymer 4 passes through three distinct regimes. At low concentrations (>23 wt.%), discrete droplets predominate. As the polymer concentration increases, beaded fibres predominate, and finally, above 28 wt.% polymer in CH2Cl2, high aspect ratio fibres are formed. At 25 wt.% polymer concentration, an unexpected ‘compacted sphere’ product was repeatedly observed. These results clearly represent beads that have collapsed; their appearance suggests the formation of a skin on the surface which collapses at the electrode. 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 basis 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 polyurethanes 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, and the synthetic procedure was designed to avoid this problem.
Table 1 Structures and end-group binding constants of the supramolecular polymers used in this study (n 3, Mn between 13,800 and 17,000 g mol1). Supramolecular polymer
Mn
Ka of end-group (M1) 1.4
15,100
1.5
13,800
6.9
1
2
3 17,000
D. Hermida-Merino et al. / European Polymer Journal 48 (2012) 1249–1255
15,200
15
4 1251
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D. Hermida-Merino et al. / European Polymer Journal 48 (2012) 1249–1255 Table 2 SEM images of the electrospun products of supramolecular polymers 1–4 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 polymer 1
Supramolecular polymer 2
(concentration: 45%)
(concentration: 47%)
Supramolecular polymer 3
Supramolecular polymer 4
(concentration: 40%)
(concentration: 42%)
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 concentration Ce, whereas 4 (which has the highest binding constant) forms well-defined, high aspect ratio fibres. Thus, the progressive increase in binding constants from polyurethanes 1–4 results in an increase in the apparent molecular weights of macromolecules in solution, pushing the system from below the Ce to above Ce without 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 degree of polymerisation [15]. However, we have used a simple model for the way in which the association equilibrium constant Ka influences 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 average degree of polymerisation,
to eq. (3); in contrast at very high values the relationship approaches a simple square root function as shown eq. (4).
< dp>n ¼ 1=ð1 Ka ½AÞ
ð1Þ
Where:
Ka ½A ¼ 1 þ 1=2Ka cin ½1=ðKa cin Þ þ 1=ð2Ka cin Þ2 1=2
ð2Þ
On this basis if Kacin (e.g. <1) is small then eq. (1) reduces to:
< dp>n ¼ 1 þ Ka cin
ð3Þ
In contrast if Kacin is large then eq. (1) reduces to:
< dp>n ¼ ðKa cin Þ1=2
ð4Þ
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 molecular 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 required to reach molecular masses sufficient for electrospinning. The combined effect of these factors can be seen if we take as a threshold for electrospinning, the situation represented by Sample 3; i.e. a 40% w/v concentration and a value 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
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D. Hermida-Merino et al. / European Polymer Journal 48 (2012) 1249–1255 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%)
16.9 lm diameter fibres 18 lm diameter fibres 24 lm diameter fibres
28% 30% 34%
23 °C (50%) 23 °C (47%) 25 °C (43%) 23 °C (42%)
42%
10 9 8
1.45 1.4
7
1.35 1.3 1.25
6
1.2 1.15
5
1.1 1.05
4
1 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
3 2 1 0
20
40
60
80
100
120
Kacin 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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D. Hermida-Merino et al. / European Polymer Journal 48 (2012) 1249–1255
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.
greater than 40% are likely to be difficult to handle on the basis of our experiences here, we can take this as a reasonable 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 electrospinning for materials with different molecular weights. The resulting values are shown in Fig. 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 solute–solvent 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 electrospinning 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 system 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 resemble 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 in Table 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 formation of submicron fibres. The appearance of the fibres shown in Table 3 suggests 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 increasing the temperature and through the use of the more volatile 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 formation of the hollow cylindrical structures; these appear to be spheres formed at the electrode surface which subsequently 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 significant increases in the effective degree of polymerisation in the concentrated solutions. The studies suggest that there are a number of factors which influence the electrospinning, 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 molecular 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
D. Hermida-Merino et al. / European Polymer Journal 48 (2012) 1249–1255
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