European Polymer Journal 48 (2012) 1374–1384

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Contents lists available at SciVerse ScienceDirect

European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Macromolecular Nanotechnology

The role of rheology of polymer solutions in predicting nanofiberformation by electrospinning

R. Rošic a, J. Pelipenko a, P. Kocbek a, S. Baumgartner a, M. Bešter-Rogac b, J. Kristl a,⇑a Faculty of Pharmacy, University of Ljubljana, Ljubljana, Sloveniab Faculty of Chemistry and Chemical Technology, University of Ljubljana, Ljubljana, Slovenia

a r t i c l e i n f o

Article history:Received 21 December 2011Received in revised form 16 April 2012Accepted 3 May 2012Available online 16 May 2012

Keywords:NanofibersElectrospinningInterfacial rheologyComplex fluidsChitosanAlginate

0014-3057/$ - see front matter � 2012 Elsevier Ltdhttp://dx.doi.org/10.1016/j.eurpolymj.2012.05.001

⇑ Corresponding author. Address: Faculty of PhaLjubljana, Askerceva cesta 7, 1000 Ljubljana, Sloven521.

E-mail address: julijana.kristl@ffa.uni-lj.si (J. Kris

a b s t r a c t

Electrospun polymer nanofibers are gaining increasing importance in tissue engineering,wound dressing and drug delivery. Here, we present a thorough rheological study of poly-mer solutions in the bulk and at the interface to find correlations between those propertiesand the electrospinnability of the solutions and the morphology of the resultant nanofibers.Our results indicate that blended solutions of chitosan or alginate with poly(ethyleneoxide) (PEO) are appropriate for electrospinning when they form conductive, unstructuredfluids displaying plasticity, rather than elasticity, in the bulk and at the interface. The inter-facial rheological parameters are three orders of magnitude lower than those in the bulk.We demonstrate for the first time that interfacial, rather than bulk, rheological parametersshow improved correlation and can be used to predict the success of the electrospinningprocess. Using the interfacial parameters of samples with homologous compositions, dif-ferent groups of solutions can be identified that form smooth nanofibers. However, rheo-logical parameters of the bulk and at the interface provide complimentary information.The bulk parameters are determined by polymer concentration and directly affect jet ini-tiation, while the interfacial behaviour determines the continuation of the jet and fibre for-mation. We propose that interfacial parameters are indispensible tools for the design ofelectrospinning experiments.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Polymer nanofibers represent an emerging class ofbiomimetic nanostructures that have shown tremendouspromise as tissue scaffolds, modern wound dressings andadvantageous drug delivery systems [1–3]. The main reasonfor the increased interest is the unique ability of nanofibersto mimic and organise biological microenvironments, sup-plement damaged or diseased tissue and stimulate tissueregeneration [4–6]. To date, three processing techniques,self-assembly, phase separation and electrospinning, havebeen developed, with the last method showing the greatest

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tl).

promise [1,7]. To prepare nanofibers by electrospinning,high voltage is applied to a polymer solution, whereupona charged jet is ejected from the needle and then undergoesextensive stretching and thinning during a rapid solventevaporation stage. While the jet travels towards thegrounded collector, polymer fibres are formed [8]. The elec-trospinning process is governed by a variety of forcesincluding the Coulomb force between the charges on thejet surface, the electrostatic force due to the external electricfield, the viscoelastic force of the solution, the surface ten-sion, the gravitational force, and the frictional force due toair drag [9].

More than one hundred polymers have been investi-gated for the design of electrospun nanofibers, with poly-mers of natural origin being generally favoured [4,5].Among those nanofibers containing chitosan and alginate(Fig. 1) have demonstrated promising properties as tissue

Fig. 1. Chemical structure of chitosan (A) and alginate (B). Chitosan is alinear polymer composed of randomly distributed b-(1-4)-D-glucosamineand N-acetyl-D-glucosamine, whereas alginate is formed by blocks of a-(1-4)-D-mannuronic acid and a-(1-3)-L-guluronic acid.

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engineering constructs due to their biocompatibility,biodegradability, safety and hydrophilicity [5,10–12].Additionally, chitosan suppresses an inflammation re-sponse during healing and expresses antimicrobial activity[13].

However, pure chitosan and alginate, as well as mostnatural polymers, are difficult or impossible to electrospininto fibrous structures due to their limited water solubility,ionic character, three-dimensional network due to hydro-gen bonds, coupled with high solution viscosity at low con-centrations [5,8]. These limitations can be overcome byblending them with non-ionogenic linear flexible poly-mers, such as poly(ethylene oxide) (PEO) and poly(vinylalcohol) (PVA) which significantly improve the naturalpolymer’s spinnability [14,15]. Moreover, such blends haveadded advantages over the electrospun nanofibers of purenatural polymers because their mechanical properties, bio-compatibility and antibacterial effects are drastically en-hanced [16]. In our study, chitosan and alginate wereblended with PEO, which was chosen due to its solubilityin water, thereby avoiding the use of organic solvents dur-ing production.

Extensive research has been conducted to determinethe spinnability of particular polymer solutions, but themechanisms of fibre formation are still not completelyunderstood. In general, a solution is spinnable if the poly-mer jet remains unbroken until dry nanofibers are formed.Numerous studies have shown that the optimal electros-pinning configuration and operational conditions differdrastically from one polymer to another, with processparameters (applied electric field, needle-to-collector dis-tance, flow rate, type of collector) and solution properties(viscosity, surface tension, conductivity) being the mainfactors influencing the transition of a polymer solution intoultrafine fibres [10,17,18]. Barnes et al. [1] demonstrated alinear relationship between electrospun polymer solutionconcentrations (in a range from 2% to 30%) and fibre

diameter for poly(glycolic acid), poly(lactic acid), theirblends or copolymers, poly(caprolactone) and gelatine. Be-cause polymer type and concentration are the mainparameters defining the rheological behaviour of the poly-mer solutions, it would be reasonable to expect that thoseare the determining factors in fibre formation. Thus, rheol-ogy is the spotlight of the present study.

The majority of published papers on electrospun nanof-ibers routinely examine the viscosity of the polymer solu-tions; with most of the collected data suggesting thathigher viscosity favours smoother nanofiber formation[16,17,19,20]. Other rheological parameters, such as theelastic (G0) and plastic (G0 0) modulus, are rarely considered.A few studies have examined the role of elasticity of thepolymer solutions [21,22]. The authors hypothesised thatelastic forces resist the bending of the jet and hinder ajet from breaking up. Su et al. [22] also concluded that fi-brous structures actually originated from the elasticity ofthe liquid jet out of the needle during the electrospinningand that greater elasticity resulted in a beadless nanofiberstructure. Even more rarely reported in the literature is therole of interfacial rheology in the electrospinning process,although the importance of this feature becomes obviouswhen considering the increase in surface area-to-volumeratio of the electrospun jet during the thinning process,where jet diameter decreases and the area increases byseveral orders of magnitude. One previously publishedstudy [21] has shown that spinnable solutions have aninterfacial G0 0 that is 10 times greater than that of solutionsfrom which fibres could not be prepared.

In this context, a deeper understanding of the rheolog-ical parameters of polymer solutions used for nanofiberformation is required. It cannot be neglected that viscosityin bulk is important in toto, but because the majority ofprocesses in the electrospinning mechanism actually takesplace at the interfaces, it would not be surprising thatinterfacial rheology may determine the docking site ofthe complex jet formation. Therefore, understanding howand when interfacial rheology properties dominate theprocess should be considered as an integral part of poly-mer solution management for electrospinning.

The aim of the present research is to present a thoroughrheological study of polymer solutions in the bulk and atthe interface in an effort to find correlations between thoseproperties and the electrospinnability of the solutions andthe morphology of the resultant nanofibers. Our hypothe-sis is that the viscoelasticity of the interfacial layer of thesolution is a better predictive parameter for jet formationrelative to bulk parameters. This analysis will permit fur-ther interpretation of the results, which are essential forjets that do not break up and that form uniform nanofibers.The conclusions of this study will be generally applicablefor investigations of nanofiber design and production.

2. Materials and methods

2.1. Materials

The chitosan used was low-viscous chitosan (viscosityof a 1% solution in acetic acid at 20 �C 6 200 mPas) from

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Fluka (Buchs, Switzerland). Sodium alginate (viscosity of a2% solution in water at 25 �C 6 250 mPas) and poly(ethyl-ene oxide) (PEO) (Mw = 400,000 g/mol) were supplied bySigma–Aldrich Chemie GmbH, Germany. Acetic acid wasprovided by Merck (Darmstadt, Germany).

2.2. Preparation of the polymer solutions

Solutions of pure polymers were prepared separatelyand then mixed in order to obtain a single blended solu-tion. A 3% (w/w) chitosan solution in 2% (w/w) acetic acidand a 3% (w/w) PEO solution in distilled water were usedto prepare blends with chitosan:PEO mass ratios of100:0, 90:10, 80:20, 70:30, 60:40, 50:50, 40:60, 30:70,20:80, 10:90, 93:7, 97:3 and 0:100, with a final polymerconcentration of 3% (w/w). A 4% (w/w) alginate solutionand a 4% (w/w) PEO solution in distilled water were mixedto gain the same mass ratios described above, with a finalpolymer concentration of 4% (w/w). The blends were stir-red at 20 rpm with a magnetic stirrer for 24 h to obtainhomogenous solutions at equilibrium. Previous studiesguided our choice of total mass concentration for the poly-mers in the final blended solution. Those studies showedthat the optimal total mass concentration for chitosan-blended solutions is 3% (w/w) and for alginate-blendedsolutions is 4% (w/w) [14,23,24].

2.3. Electrospinning of the polymer solutions

The electrospinning setup is schematically presented inFig. 2. The polymer solution was placed in a 20 ml plasticsyringe fitted with a metallic needle with an inner diameterof 0.8 mm. A syringe pump (Model R-99E, RazelTM) wasused to feed at a constant rate. High voltage at the needlewas achieved by connection to a voltage generator (modelHVG-P60-R-EU, Linari Engineering s.r.l., Italy) capable ofgenerating voltages in the range of 0–60 kV. For each solu-tion, a variety of parameters were tested for the electrospin-ning process, including applied voltage (10–30 kV), solutionflow rate (0.1–3 ml/h) and needle-to-collector distance (10–20 cm). The optimal applied voltage for both polymers was

Fig. 2. A sheme illustrating the basic principles of the electrospinningprocess.

found to be 25 kV with a needle-to-collector distance of17 cm. The optimal flow rate was determined to be1.8 ml/h for chitosan:PEO and 0.4 ml/h for alginate:PEOblends.

2.4. Characterisation of the polymer solutions

The rheological properties in the bulk and at the inter-face as well as the conductivity and surface tension weredetermined for all polymer solutions.

Rotational and oscillatory rheological tests were per-formed using a Physica MCR 301 rheometer (Anton Paar,Graz, Austria) with a cone-plate measuring system CP50-2(cone radius 24.981 mm, cone angle 2.001�, sample volume1.15 ml) at a constant temperature 25.0 ± 0.1 �C. Rotationaltests were used to determine the viscosity, which for a cone-plate measuring system is calculated as g ¼ sc= _c, where _c isthe shear rate and sC the shear stress. Oscillatory tests wereperformed to define the elastic and loss modulus, which arecalculated as G0 = (sa/ca)⁄cosd and G0 0 = (sa/ca)⁄sind, wheresa is the shear stress, ca is the deformation and d is the phaseshift angle [25,26].

The shear rate during the rotational tests ranged from 2to 100 s�1. The oscillatory shear measurements were per-formed at an amplitude within the linear region (ampli-tude 10%) with a frequency from 0.2–100 s�1.

Interfacial rheological tests were carried out using thesame rheometer with interfacial cell, with a biconical mea-suring system (cone radius 34.171 mm, cup radius40.00 mm, cone angle 4.973�, approximate sample volume104 ml), where the shear rate was set between 2 and100 s�1 and the frequency in the oscillatory measurementwas in the range of 0.2–100 s�1 (amplitude 1%). The systemwas positioned at the interface, and the sample was delib-erately kept in direct contact with the ambientatmosphere.

The presented values of viscosity in bulk and at theinterface are taken at the lowest measured shear rate (forbulk viscosity at 2 s�1, while the first measured point forinterfacial viscosity was at 9 s�1), where the load of thesystem only minimally influences viscosity. The oscillatorytests were compared at low (0.2 s�1), medium (3.98 s�1)and high frequency (100 s�1). However, the results of G0

and G0 0 presented here are taken only at the lowest fre-quency because these values show the most dramatic dif-ferences between the blended solutions.

Conductivity of solutions was measured at room tem-perature using an Iskra Conductivity meter MA 5964 (Iskra,Ljubljana, Slovenia) with an electrode conductivity con-stant of 0.7265 cm�1.

Surface tension was measured at 25 ± 0.5 �C by the platemethod with a Processor Tensiometer K-12, Version 5.05(Kruss Gmbh, Hamburg). For each solution, average valuesof parameters from at least three measurements arereported.

2.5. Characterisation of nanofibers

The diameter and morphology of the electrospun nanof-ibers were examined using a 235 Supra 35VP-24-13 high-resolution scanning electron microscope, SEM (Carl Zeiss,

R. Rošic et al. / European Polymer Journal 48 (2012) 1374–1384 1377

Germany) operated at an accelerating voltage of 1 kV witha secondary detector; no conductive coating layer was ap-plied before imaging. The obtained images were used todetermine the fibre diameter using ImageJ 1.44p software(NIH, USA) by measuring 50 fibres chosen randomly.

Fourier transform infrared (FTIR) spectroscopic analysiswas used to qualitatively characterise the interactions be-tween chitosan, or alginate, and PEO. FTIR spectra werecollected with a Nexus FT-IR Nicolet spectrophotometerat a resolution of 4 cm�1, with signal averaging over 250scans in each interferogram over the range of 500 to4000 cm�1.

Fig. 3. Surface tension and conductivity of chitosan:PEO (A) and algi-nate:PEO (B) blends as a function of solution composition. The totalamount of polymer in chitosan:PEO is 3%, and in alginate:PEO, theamount is 4%.

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3. Results and discussion

Nanofibers of chitosan or alginate combine the benefitsof the polymer properties with improved physical struc-ture, offering an advantageous approach for wound healingand tissue engineering. However, both natural polymershave proven to be challenging to electrospin. Therefore,the present paper is aimed at developing a rational expla-nation for the effect of rheological properties of polymersolutions on the electrospinning process. The results aredivided into three parts: first, evaluation of chitosan:PEOand alginate:PEO solution properties, with an emphasison solution rheology; second, the morphology of nanofi-bers formed by the electrospinning process; and third, pre-diction of nanofiber formation from the rheologicalproperties of polymer solutions.

3.1. Conductivity and surface tension of polymer solutions

The conductivity and surface tension of solutions, as afunction of the amount of chitosan and alginate in theblends, are shown in Fig. 3. The results clearly show thatthe surface tension of blends remained nearly unchangedregardless of the solution composition, whereas the con-ductivity correlated with the proportion of chitosan or algi-nate in the blends. Conductivity correlated with thepolymer concentration because both natural polymers dis-play polyelectrolyte properties in aqueous media, i.e.,chitosan is positively charged in acetic acid, and alginateis negatively charged in distilled water. The results of sur-face tension experiments may be explained by the fact thatthere was always the same total mass of polymer in theblends, which spreads uniformly across the surface.

3.2. Rheology of polymer solutions in bulk

The main emphasis of the present research was to studyrheological properties as a function of solution composi-tion. Rotational and oscillatory rheological measurementswere performed on all chitosan:PEO and alginate:PEOblended solutions as well as on solutions of the pure com-ponents. Rotational tests were used to determine the vis-cosity of the solutions as a function of shear rate, whileoscillatory tests were performed to determine dynamicmoduli, i.e., storage and loss moduli. The storage, or elastic,modulus (G0) is a measure of the elasticity of a material andrepresents the material’s ability to store energy. By

contrast, the loss, or viscous, modulus (G0 0) relates to theability of the material to dissipate energy, which is usedto change the material’s structure and is lost as heat.

Fig. 4 shows the results obtained from rotational tests,where changes in viscosity as a function of shear rate forblended solutions with different compositions are pre-sented. The shape of the viscosity curves clearly showsdecreasing viscosity values with increasing shear rate,indicating that all solutions behave as non-Newtonian(pseudoplastic), shear-thinning fluids. The data also showthat the viscosity curves of the chitosan:PEO blends alllay between the curves for the pure components over theentire compositional range, indicating that miscibility ofthe blends occurs and that compatible, one-phase blendsare formed.

The analysis defined the differences in rheologicalbehaviour between the blended solutions, since the shapeof the curves are dependent on the solution composition.Solutions with a higher percentage of chitosan or alginateshow a greater decrease in effective viscosity with increas-ing shear rate relative to solutions rich in PEO. For exam-ple, the viscosity of a 90:10 solution containing 10% PEOvaried from 2.1 to 1.0 Pas with shear rate, while a 10:90solution containing 10% chitosan varied from 0.6 to0.4 Pas. These results lead to the conclusion that thepseudoplasticity of the solutions increases with an increas-ing proportion of polyelectrolyte in the blends. The pro-nounced shear-thinning effect can be related to the

Fig. 4. Viscosity curves showing the shear-thinning behaviour for chito-san:PEO (A) and alginate:PEO (B) blended solutions obtained using arotational cone-plate measuring system.

Fig. 5. Viscosity, storage (G0) and loss (G0 0) moduli as a function ofchitosan:PEO (A) and alginate:PEO (B) blended solution compositions asdetermined using a cone-plate measuring system with a viscositymeasured at a shear rate of 2 s�1 and dynamic moduli measured at afrequency 0.2 s�1.

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structure of the polymer chains in the solutions. Withoutan external load, each single macromolecule can be foundin the shape of a three-dimensional coil because this islowest energy state. During the shear process, however,the molecules are more or less oriented parallel to thedirection of shear, resulting in elongation, which lowerstheir flow resistance and results in a decrease in the bulkviscosity.

The results presented in Fig. 5 show the viscoelasticproperties of the chitosan:PEO (Fig. 5A) and alginate:PEO

(Fig. 5B) blended solutions as a function of their composi-tion. It is observed that viscosity and G0 0 increase monoton-ically with an increase in the proportion of chitosan in theblends. Generally this trend is true for alginate:PEO blendsas well, although the slope is not as extreme. Additionally,G0 0, as well as its slope, is always much greater than G0. Thedifferences between the G0 and G0 0 values for the blends in-crease as the chitosan or alginate content increases and aregenerally higher for the chitosan relative to the alginatesamples. Notably, the viscosity, G0 and G0 0 of pure chitosanis higher than those of pure PEO (bulk viscosity of chitosan2.1 Pas, PEO 0.4 Pas), explaining the observation of lowervalues in the blends with higher PEO content. However,the experimental values are not simple proportional sumsof the individual viscosities. This finding may be attributedto the fact that the polysaccharides are able to form inter-actions between individual polymer chains.

In order to assign intermolecular interactions betweenionogenic and nonionogenic polymers, FTIR spectroscopywas used. The FTIR spectra were suitable for studying thespectral changes caused by polymer–polymer interactionsby measuring changes in the relative intensities of vibra-tional bands of the dried blended material versus the pure

R. Rošic et al. / European Polymer Journal 48 (2012) 1374–1384 1379

components. Blended samples exhibited most of the char-acteristics of the pure components (chitosan and PEO),with an additional broad band at approximately3350 cm�1 (Fig. 6). On the other hand, we are aware thatour samples contain certain amount of water, which cancontribute to formation of the same peak at 3350 cm�1.Similar changes were observed on the FTIR spectra of algi-nate, PEO and their blends (results not shown).

The presence of PEO in blends changes the inter- and in-tra-molecular interactions of the chitosan and alginatechains by interacting with the backbone of the naturalpolymer and disrupting the self-association of the chains.Murray [27] reported that such inter-polymer interactionsresult in the formation of a compact structure such that thehydrodynamic volume is smaller than the volume sum ofindividual macromolecules, and this modulation is physi-cally manifested as a decrease in the solution viscosity.

Fig. 6. FTIR spectra of pure chitosan, PEO and the chitosan:PEO blend.

The effect of reduced viscosity in solutions with a high pro-portion of alginate can be attributed to the fact that in thepresent study, alginate forms a less ordered structure dueto its stronger repulsive forces between charged groups,as proven by the observation of higher conductivity shownin Fig. 3.

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3.3. Interfacial rheology of polymer solutions

A detailed investigation of the rheological parameters ofthe bulk may not be sufficient when dealing with complexprocesses, such as electrospinning, Interfacial parametersdiffer from the bulk due to reorganisation of the polymermolecules at the interface itself. Until now, interfacial rhe-ological properties were mostly studied in surfactant solu-tions, foams and emulsions [27–29]; their use forcharacterising polymer blends for electrospinning is stud-ied here for the first time.

The results depicted in Fig. 7 show changes of interfacialviscosity as a function of the applied shear rate for chito-san:PEO and alginate:PEO blended solutions, indicatingtheir interfacial pseudoplasticity. A wide gap can be clearlyobserved between the interfacial viscosity curves ofblends, while viscosity curves in the bulk followed sequen-tially subsequently (Fig. 4). Notably, the effect is muchmore obvious with chitosan than alginate (in Fig. 7).

The results of the interfacial measurements presentedin Fig. 8 show the same parameter trends as was observedin the bulk. However, the values of the interfacial rheolog-ical parameters are three orders of magnitude lower thanthose observed in the bulk. The interfacial viscosity andloss modulus have significantly higher values as comparedto the storage modulus. Nonetheless, an impressive differ-ence is established in the slopes of the plots for interfacialviscosity and G0 0 as a function of the solution composition,a trend not observed for G0 (Fig. 8A). The values are muchmore significant in the case of chitosan as compared toalginate (Fig. 8A and B).

In the case of chitosan, the slope in the first region in-cludes solutions having a chitosan proportion in the blendsof 10% or less, the second region includes chitosan contentof up to 60%, and solutions with higher chitosan contentare placed into the third region.

According to the results from the alginate:PEO blends,the solutions having concentrations of alginate up to 30%are in the first region and those up to 60% are in the secondregion. With 70% alginate, the parameters start to decrease,although division into three regions remains consistent be-cause the parameter values remain above those of theother two regions.

3.4. Electrospinning of polymer solutions

Because the success of the electrospinning process isstrongly correlated to the properties of the solutions fromwhich the nanofibers are prepared, variation between themorphology of the obtained nanofibers was expected.Nanofibers from blended solutions and those obtainedfrom pure chitosan or alginate and PEO were preparedusing the same process parameters in order to directly

Fig. 7. Interfacial viscosity as a function of shear rate for chitosan:PEO (A)and alginate:PEO (B) blended solutions. Measurements were performedusing a biconical measuring system presented schematically above.

Fig. 8. Interfacial viscosity, storage (G0) and loss (G0 0) moduli as a functionof solution composition of chitosan:PEO (A) and alginate:PEO (B) blends.Solutions are categorised in three groups according to spinnability andnanofiber morphology. Conditions: biconical measuring system, viscosityat a shear rate of 9 s�1 and dynamic moduli at a frequency of 0.2 s�1.

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correlate the solution characteristics to the spinnabilityand morphology of the products.

Electrospinning pure chitosan and alginate solutions(without any addition of PEO) resulted in the formationof polymer droplets on the collector without any nanofiberformation as is seen in Fig. 9. A proposed explanation is theinability of jet formation during electrospinning due to thehigh bulk viscosity and pseudoplastic structure of the puresolutions. Generally, there is a critical viscosity value (closeto 1.5 Pas) required to allow for electrospinning becausebelow these values, improved results are observed as

shown in Fig. 8 and 9. In addition to the viscosity, chitosanand alginate are polyelectrolytes that are charged in solu-tion. Strong repulsive forces between ionogenic groupswithin the polyelectrolyte backbone impede the formationof continuous fibres.

The spinnability of chitosan and alginate was achievedby the addition of PEO in different ratios as suggested byothers [14,15]. The morphologies of a series of the electro-spun samples were analysed by SEM, with representativemicrographs shown in Fig. 8. It can be clearly seen thatthe composition of the spinning solutions had a significanteffect on the morphology of the nanofibers. Improved

Fig. 9. Representative SEM images of nanofibers obtained from blended solutions of chitosan:PEO (above) and alginate:PEO (below) at mass ratio (A/a)10:90, (B/b) 20:80, (C/c) 30:70, (D/d) 40:60, (E/e) 50:50, (F/f) 60:40, (G/g) 70:30, (H/h) 80:20, (I/i) 90:10 at 25 kV, needle-to-collector distance 17 cm andflow rate for chitosan 1.8 ml/h and alginate 0.4 ml/h. Total polymer concentration is 3% (w/w) for chitosan blends and 4% (w/w) for alginate blends.

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morphological changes are observed with decreasingamounts of chitosan or alginate in the polymer

blends, resulting in a decrease in the values of their rheo-logical parameters and conductivity as well. The highest

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investigated chitosan:PEO or alginate:PEO ratio resulted inno nanofiber formation (Fig. 9I and i). At an 80:20 ratio,beads with rare nanofibers were formed (Fig. 9H and h),and at the ratio of 70:30, a structure of short fibres, whichextend between the beads, was observed for both poly-mers (Fig. 9G and g). Electrospinning of other solutions re-sulted in the formation of three-dimensional nanofibernetworks. In the case of chitosan, smooth nanofibers withno beads were formed when the content of chitosan inthe blends was 10% or less (Fig. 9A), whereas solutionswith 30% alginate content still produced nanofibers with-out beads (Fig. 9a–c). Nanofibers with a bead-on-stringmorphology were produced when the content of chitosanin the blend was 40% or less (Fig. 9B–F) and at 40–60% inthe case of alginate (Fig. 9d–f). In both cases, a higheramount of polyelectrolyte triggers the formation of a high-er number of beads. Therefore, increasing the amount ofPEO in blended solutions and relatively decreasing theamount of polyelectrolyte causes a progressive transfor-mation of the electrospun sample’s morphology fromnanoparticles to bead-on-string structures to nanofibers.

The mean diameter of the obtained nanofibers was inthe range of 15–130 nm for chitosan and from 30–120 nm for alginate. In general, the diameter of the electro-spun nanofibers depended on the polymer concentration,3% or 4%, with variability caused by the proportion of bothpolymers in the blends. The nanofibers grew thinner as thecontent of chitosan or alginate increased. The reason forobtaining a reduced diameter in spite of having highersolution viscosity is due to the charge density in theejected jet, which imposed higher elongation forces lead-ing to thinner fibre formation, in agreement with previ-ously published data [14,17].

3.5. Prediction of nanofiber formation

The morphology of prepared electrospun products isdifficult to predict before completing all of the experimen-tal procedures and analyses, including SEM, which requiresa lot of time. Therefore, determining the key parametersnecessary to predict electrospinning success from solutionproperties is essential. Regarding the electrospinningmechanism, solution viscoelasticity profoundly influencesjet formation and jet stability, which is crucial for success.The solution has to maintain appropriate viscoelasticity inorder to survive stretching, acceleration and whipping.

The analysis of nanofiber morphology in relation to rhe-ological properties in the bulk suggests that polymer solu-tions from which nanofibers are produced should beconductive, unstructured shear-thinning fluids and expressboth plastic and elastic behaviour. The plasticity of theblends has to be greater than elasticity, and only a properrelation between these two features enable jet formationand stabilisation. Elasticity, however, has to be as low aspossible, but still present, to enable jet initiation. The for-mation of the electrospun jet from spinnable (Supplemen-tary 1) and unspinnable (Supplementary 2) solution wasrecorded and can be seen in supplement. The underlyingexplanation is that the increased elastic force increasesthe tendency of the jet to contract, which prevents jetinitiation and elongation. Our findings downplay the

importance of the elastic response of the polymer chainsfor jet stabilisation and support an absence of any signifi-cant bulk elasticity in shear. The Regev research group[21] has obtained similar results. Our results are not inaccordance with the traditional explanation for jet stabili-sation, which places an emphasis on the role of the elasticresponse over the plastic response [21,22]. Nevertheless,during the drying of the jet when the overall polymer con-centration increases, the proportion of elastic and plasticmoduli changes (G0 0/G0 decreases with higher overall poly-mer concentration). An important additional cause forunsuccessful nanofiber formation from solutions contain-ing a higher proportion of charged natural polymer is thegrowing repulsive forces, which viscous forces are not ableto overcome. It was determined that the minimum neces-sary proportion of spinnable carrier in the blends is 40%,which is equal to a concentration of 1.2% (w/w) in the caseof chitosan and 1.6% (w/w) in the case of alginate.

However, a much more significant discovery was madebased on interfacial rheology. The results clearly show thedependence of the morphology on the solution composi-tions, subdivided into regions as defined previously(Fig. 8). From solutions processed in the first region,smooth, uniform and bead-free nanofibers are formed,whereas the solutions in the second region form beadednanofibers, and only droplets with few nanofibers formin the solutions in the third region. In the case of alginate,the breaks between different regions are less distinct, butthe analysis revealed that the quality of the obtained elec-trospun nanofibers is better and variations between fibresprepared from different blends are smaller. To conclude,the production of nanofibers from solutions with smallervariation in interfacial parameters is easier because theproper solution parameters can now be quickly achieved.We discovered that in the case of chitosan, the formationof three-dimensional nanofiber networks was enabledfrom the solutions past the gap in interfacial viscosity(from approximately 9 to 18 mPas).

Inimitable correlation between the interfacial viscosityand the plastic modulus is not surprising when consideringthe basic physical mechanism of electrospinning. Duringthe process, the electrospun jet thins dramatically from800 lm (diameter of the needle) to a few nm (Fig. 10),causing much more dominant role for surface properties.In the region of the Taylor cone of ejected jet bulk rheolog-ical properties play a more important role due to the largerdiameter and volume, whereas thinning of the jet causesreduced importance of the bulk rheological parametersand increased the significance of interfacial rheologicalparameters. Notably, the thinner the jet is, the greaterthe surface area, and therefore, the greater the importanceof the interfacial characteristics. Moreover, due to solventevaporation, which occurs at the interface, a concentrationgradient of polymer molecules is formed, further increas-ing the interfacial effect.

Additionally, much greater sensitivity of interfacial rhe-ology enables the determination of even small differencesbetween solutions, which is also the reason why bulk rhe-ological parameters in chitosan:PEO and alginate:PEOshow continuous increase, while in the interfacial resultsthe definition of distinctive slopes is possible. Moreover,

Fig. 10. A set of pictures presenting jet development during electrospinning.

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interfacial shear rheology is so far the best rheologicalmethod which allows the determination of real dilatationalmaterial properties of the interface at extension rates thatoccur during actual electrospinning process [21,28,30]. Allaforementioned facts support the conclusion that the cor-relation between the rheological characteristics of theinterface and nanofiber morphology is far more distinctive,obvious and evident than are the correlations with rheo-logical properties in the bulk or any other solutioncharacteristic.

4. Conclusion

The present study explored correlations between therheological parameters of solutions and electrospinnabili-ty. The rheological results indicate that blended solutionsof polyelectrolyte and nonionogenic polymer for electros-pinning are effective when they are conductive, unstruc-tured fluids. The blends should show greater viscous(plastic) properties over elastic behaviour in the bulk aswell as at the interface. For the first time, we proved thatinterfacial rheological parameters correlate much betterwith the outcomes of, and predict the success of, the elec-trospinning process. Based on interfacial parameters, dif-ferent groups of solutions can be defined, predictingwhere smooth nanofibers form. Because the predictionwas proven for two distinct natural polymers, the conclu-sions may be extended to other polymer blends containingpolyelectrolytes. Still, rheological parameters in bulk andat the interface have to be taken as complementary. Bulkproperties are largely determined by polymer concentra-tion and thus allow for the prediction of jet and fibre for-mation, while the interfacial properties enable theprediction of jet continuation. We are sure that interfacialparameters will be indispensable tools in the design ofpolyelectrolyte solutions for electrospinning in the future.

Acknowledgment

The authors are grateful to the Slovenian ResearchAgency for financial support of this research work: P1 -0189, J1 - 4236, 1000 - 09 - 310085 and 1000 - 11 –310213.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.eurpolymj.2012.05.001.

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