Surface Depletion of the Fluorine Content of Electrospun Fibers ofFluorinated Polyurethane

Wanling Wu, Guangcui Yuan, Aihua He,* and Charles C. Han*

State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science andMaterials, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of

Science, Beijing 100190, China

ReceiVed October 28, 2008. ReVised Manuscript ReceiVed December 18, 2008

For materials containing fluorine, it has been generally accepted that fluorinated segments or end groups tend toaggregate in the outer surface because of the low surface energy, which endows the fluorinated materials with specialsurface properties such as self-cleaning, superhydrophobicity, and so forth. However, for the electrospun fibrousmembranes of polyurethane elastomers containing perfluoropolyether segments (FPU), abnormal fluorine aggregationsin the core of the electrospun fibers were observed. The XPS analysis indicated a rather low fluorine content at thesurface of the electrospun FPU fibers. Further study with dynamic light scattering and fluorescence showed that FPUchains can form aggregates in the concentrated solution. Therefore, it can be deduced that the rapid evaporation ofsolvent and fast formation of fibers during the electrospinning process could result in the freeze-in of the aggregatedchain conformation and the depletion of fluorine units on the surface of the electrospun FPU fibers.

Introduction

Material surfaces with certain special properties have drawna lot of interest for their practical applications in many fields.Fluorinated polymers, which have outstanding surface propertiessuch as good hydrophobicity/oleophobicity, chemical resistance,and low friction coefficient, are undoubtedly among the mostimportant candidates. Because of the low surface energy,fluorinated segments and end groups tend to segregate to thepolymer-air interface1-6 resulting in differences in chemicalcomposition between the surface and the bulk. Besides con-ventional fluoropolymers such as poly(tetrafluoroethylene)(PTFE) and poly(vinylidene fluoride) (PVDF), many researchersare interested in copolymers composed of fluorinated segmentsand some common polymers for their processibility andproperties.3,5-7 Among these copolymers, fluorinated polyure-thane has received great interest for its wide application in plastics,elastomers, fibers, foams, coatings and adhesives in construction,transport, and biomedical fields. Fluorinated polyurethane canbe synthesized by the reactions of diisocyanates with diols,polyesters, and polyethers, when at least one of the reactants isfluorinated. The fluorinated groups can be hard segments,8 softsegments,9-15 chain extenders,16-18 or end groups.19,20

Electrospinning, as a simple and effective way to fabricatenanofibers, has been developed rapidly in recent years. A largenumber of polymers, including synthesized polymers21-27,29-34

and natural polymers,28 have been electrospun into nanofibrousmaterials. Currently, the electrospun nanofibrous mats ofpolyurethane have attracted great interest for their excellentmechanical properties and good biocompatibility,29-34 particu-larly in the research on PU nanofibrous mats as materials withenhanced mechanical properties,30,31 antimicrobial nanofil-ters,32,33 wound dressing materials, sensors,34 and so forth.

* Corresponding authors. E-mail: c.c.han@iccas.ac.cn. Tel: +86-10-82618089. Fax: +86-10-62521519.

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3178 Langmuir 2009, 25, 3178-3183

10.1021/la803580g CCC: $40.75 2009 American Chemical SocietyPublished on Web 02/09/2009

In a previous study, we successfully electrospun polyurethaneelastomers containing perfluoropolyether segments (FPU) as asuperhydrophobic material.35 The chemical structure of FPU isshowed in Scheme 1. As many researches have shown, surfacechemical composition may be one of the main factors that affectthe surface wetting behavior.36-38 In addition, only a fewresearchers have focused on the surface chemistry of theelectrospun fluorinated materials.39 Therefore, the surfacechemistry of the electrospun FPU membranes were studied byXPS in our present research. Instead of the fluorine enrichment,an abnormal low fluorine content was found in the surface of theelectrospun FPU fibers. To better understand this phenomenon,the solution properties of FPU were studied using dynamic lightscattering and fluorescence, and then in combination with theactual process during electrospinning, a conceivable explanationwas presented in this paper.

Experimental SectionMaterials. Polytetrahydrofuran (Mn)2000, Sigma-Aldrich, Inc.,

USA) in DMAc (25.0 wt %) was added to 4,4′-methylene-bis(phenylisocyanate) (MDI, Acros Organics, USA) solutions inDMAc (10.0 wt %) in a three-necked, round-bottom flask underargon atmosphere. The polymerization was carried out at 70-80 °Cfor 2 h. Stoichiometric perfluoropolyether alcohol (CF3CF2CF2O-(CFCF3CF2O)2CFCF3CH2OH, Shanghai Institute of Organic Chem-istry) in DMAc (10.0 wt %) was added drop by drop to the prepolymersolution and the reaction was kept for 1 h. Then, a small amountof 1,4-butanediol dissolved in DMAc as a chain extender was added.The reaction was continued for 1-2 h at 70-80 °C. The resultingpolymers were precipitated in excess water and dried in a vacuumoven at 60 °C. They were redissolved in DMAc and precipitated inan excess methanol-water mixture to remove low molecular weightmaterials. The purified polymers were washed with methanol, thendeionized water, and dried in a vacuum at 60 °C. The resultingpolymer is a light yellow elastomer, and the detailed characterizationof the FPU can be found elsewhere.35

Electrospinning. FPU solutions with different concentrations wereprepared for electrospinning with a mixture of N,N-dimethylfor-mamide (DMF) and tetrahydrofuran (THF) as solvent.The dc high-voltage generator (The Beijing machinery and electricity institute,China) was employed to produce voltages ranging 0-50 kV. Theelectrospinning solutions were placed into a 5 mL syringe with acapillary tip having an inner diameter of 0.3 mm. A syringe pumpwas used to feed polymer solutions into the needle tip. A sheet ofaluminum foil, connected to the ground, was placed under the syringeas a collector. The dc high voltage applied was fixed at 13.5 kV,the environmental temperature at 50 °C, the feeding rate at 50 µL/min, and the distance between the tip and the collector at 13 cm.

Casting Film. Three kinds of casting films were prepared in thisstudy. The normal casting sample was made by dropping the polymersolution onto the silicon substrate and drying the film at 50 °C. Theother two kinds of casting films were made under electric field. A

silicon substrate was placed in a dc high-voltage field with a distanceof 7 cm between the two poles, the voltage applied was the sameas in the electrospinning process, and the ambient temperature was50 °C; then, the FPU solution was dropped onto the silicon substrateand dried in ambient temperature. Negative and positive voltageswere used, respectively, and the cast films were marked “castingfilm(-)” and “casting film(+)”, correspondingly.

Characterization. Gel permeation chromatography was per-formed to estimate the molecular weights of FPU using a 1515system (Waters) equipped with 2414 refractive index and Styragelgel columns calibrated with narrow-molecular-weight polystyrenestandards. The surface morphologies of these electrospun membraneswere observed using a scanning electron microscope (JSM6300F,JEOL, Japan).

X-ray photoelectron spectroscopy data were obtained with anelectron spectrometer (ESCALab220i-XL, VG Scientific) using 300W Al KR radiation. The binding energies were referenced to the C1sline at 285.0 eV from adventitious carbon. The casting samples andthe electrospun samples were characterized directly without anytreatment except as indicated otherwise.

Dynamic light scattering measurements were carried out by acommercial LLS spectrometer (ALV/DLS/SLS-5022F) equippedwith a multi-τ digital time correlator (ALV5000) and a 22 mWUNIPHASE He-Ne laser (λ0 ) 632.8 nm). The LLS cell is heldin a thermostat index matching vat filled with purified and dust-freetoluene, with the temperature controlled at 25( 0.02 °C. In dynamicLLS, the intensity-intensity time correlation function G(2)(t, q) inthe self-beating mode was measured, where t is time and q is scatteringvector (q ) (4πn/λ0) sin(θ/2)). G(2)(t, q) can be related to thenormalized first-order electric field time correlation function |g(1)(t,q)| via the Siegert relation as

G(2)(t,q))A[1+ �|g(1)(t,q)|2]

where A (≡⟨I(0)⟩2) is the measured baseline.Steady-state fluorescence measurements were performed using a

Varian FLR025 fluorimeter. The excitation wavelength was set at340 nm, and emission spectra were recorded from 360 to 600 nm;the scan rate was 600 nm/min. Bandwidth slits for excitation andemission were both 5 nm. Excitation spectra of the samples wereperformed at room temperature. Pyrene in DMF with a concentrationof 3 × 10-5 g/mL was used as solvent for all the solution samples.

Results and Discussion

Electrospinning of FPU. The synthesized FPU has a molecularweight of 220 000 (g/mol) and ploydispersity index of 1.94characterized by GPC. Polyurethane is a polymer composed ofhard segments (diisocyanate) and soft segments (polyether orpolyester). THF is a good solvent for the flexible soft segments,and DMF is a good solvent for the hard segments. A DMF/THFmixture was used as the solvent for FPU in this study. Theconcentration of the FPU solution was fixed at 25 mg/mL, andthe volume ratio of DMF to THF was varied. Figure 1a-c showsthe SEM images of electrospun FPU nanofibers. It can be seenfrom Figure 1 that uniform FPU fibers could be fabricatedsuccessfully. The average diameter of the FPU fibers wasdecreased from 580 nm, 330 to 220 nm when the ratio (v/v) ofDMF/THF was increased from 30/70 to 100/0. It seems that thefiber diameter decreases with an increasing amount of DMF inthe solvent. This might be due to the higher conductivity of DMF

(35) Wu, W. L.; Zhu, Q. Z.; Qing, F. L.; Han, C. C. Langmuir (la-2008-03089y), in press.

(36) Kassis, C. M.; Steehler, J. K.; Betts, D. E.; Guan, Z.; Romack, T. J.;DeSimone, J. M.; Linton, R. W. Macromolecules 1996, 29, 3247.

(37) Schmidt, D. L.; Dekoven, B. M.; Coburn, C. E.; Potter, G. E.; Mayers,G. F.; Fischer, D. A. Langmuir 1996, 12, 518.

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(39) Deitzel, J. M.; Kosik, W.; Mcknight, S. H.; BeckTan, N. C.; DeSimone,J. M.; Crette, S. Polymer 2002, 43, 1025.

Scheme 1. Chemical Structure of FPU

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than that of THF, which will lead to a decrease in fiber diameter.40

Figure 1d is the sample of (c) that underwent an annealingtreatment at 100 °C for 2 h. It can be seen that the morphologyof the film is unchanged after annealing. The morphologies ofsamples (c) and (d) are different from those of the other samples,which were attributed to the slow volatilizing rate of DMF andthe easy adhesion of wet jets to each other. When the volumeratio of DMF to THF was fixed at 30/70, the concentration ofthe FPU solution was varied. Figure 2a-c shows the SEM imagesof FPU nanofibers with different concentrations. It can be seenthat the fiber diameter increased with increasing concentration.The average diameters of those fibers were 460, 590, and 840nm, respectively, which indicated that FPU nanofibers withdifferent fiber diameter could be fabricated successfully byelectrospinning.

Surface Chemistry of Electrospun FPU Fibers. The elec-trospun films are expected to have potential applications fortheir special surface properties. The previous studies on elec-trospinning of fluorinated materials mainly focus on themorphology of the films; few focus on the surface compositions.

Generally speaking, fluorine units prefer to enrich in the surface,which will lead to lower surface energy. In this study, the surfacechemistry of the electrospun FPU films was investigated usingXPS. Table 1 is the XPS results of the electrospun samples, witha takeoff angle of 90°. In the case of planar film, the XPS dataof 90° takeoff angle usually represent the composition of theoutermost 10 nm of the surface. However, in our case, the surfaceof the membranes was made up of column-like fibers instead ofa plane, so only a small part of the surface was perpendicularto the incident X-ray, which meant that the data we obtained wasfrom the surface composition of the outermost 10 nm or less.The mole ratio of fluorine to carbon was used to represent thefluorine content of the outermost surface of the electrospun FPUfibers in this study. It can be seen from Table 1 that the measuredfluorine content in the outermost surface of FPU fibers was ratherlow (lower than 0.07). In addition, it can be seen that the fluorinecontent tends to decrease as the amount of DMF increases in themixed solvent. The fluorine content of sample (c) is a little higherafter annealing. When the volume ratio of mixed solvent wasfixed, fluorine content of the surface increased with increasingconcentration.

For comparison, normal casting film was prepared at 50 °Cfrom a FPU solution with concentration of 30 mg/mL in mixedsolvent of DMF/THF (30/70 v/v). From the XPS data in Table2, we can see that the fluorine content in the normal casting filmis much higher than that of electrospun membranes, and the dataof 30° takeoff angle is higher than that of 90°, indicating aremarkable enrichment of the fluorine content in the surface ofthe casting film. This indicated that fluorine was actually depletedat the surface of the electrospun fibers, which was a reallyabnormal phenomenon. Then, the question comes up: how didthis happen?

As we know, a high voltage is used in the electrospinningprocess. To study the effect of the voltage on the fluorine contentin the fiber surface, another two casting films were preparedunder electric field as described in the Experimental Section.Both negative and positive voltage were used to investigate theeffect of the electric field on the fluorine enrichment in the surface.From the XPS data in Table 2, we can see that the fluorinecontent of the casting films under electric field shows littledifference from that of the normal casting film, which meanshigh voltage was not the key factor that affects the fluorineenrichment in the surface.

(40) Zheng, J. F.; He, A. H.; Li, J. X.; Xu, J.; Han, C. C. Polymer 2006, 47,7095.

Figure 1. SEM images of electrospun FPU membranes at concentrationof 25 mg/mL in mixed solvent of (a) DMF/THF ) 30/70 (v/v), (b)DMF/THF ) 70/30 (v/v), (c) DMF/THF ) 10/0 (v/v), and (d) sampleof (c) after annealing.

Figure 2. SEM images of electrospun FPU membranes in mixed solventof DMF/THF (30/70 v/v) with concentrations (a) 20 mg/mL, (b) 25mg/mL, and (c) 30 mg/mL.

Table 1. XPS Results of Electrospun FPU Membranes fromDifferent Solutions

sampleconcentration

(mg/mL)DMF/THF

(v/v) F/C

1 25 3:7 0.0592 25 7:3 0.0363 25 10:0 0.0184a 25 10:0 0.0535 20 3:7 0.0396 30 3:7 0.068

a Note: 4a is annealed membrane of sample 3.

Table 2. XPS Results of Casting Films Prepared under DifferentConditions

F/C

sample takeoff angle 30° takeoff angle 90°

bulk --- 0.117normal casting film 0.668 0.419casting film (-) 0.715 0.434casting film (+) 0.709 0.459casting film (water) --- 0.041

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Besides the high voltage, the electrospinning process is alsoa rapid volatilization process of the solvent. When the FPUsolution is pumped out of the syringe, the solvent evaporatesquickly and the polymer fibers are collected in a very short time.So, we designed an experimental model to investigate how thedrying of solvents affects the fluorine content on the surface. Theprocedure is the following: We dripped a drop of FPU solutionwith concentration of 75 mg/mL in mixed solvent of DMF/THF(30/70 v/v) into water, and a film was formed almost instan-taneously because FPU is not a water-soluble polymer, whileboth solvents can mix with water at any ratio. So, it is a rapiddiffusion process for solvents to leach out. Finally, the film wasfreeze-dried, and the sample is marked “casting film (water)”.From the XPS data in Table 2, we can see that the fluorinecontent on the surface of the casting film (water) is close to thatof the electrospun films. Meanwhile, we cut a piece of polymerfrom the bulk and obtained the fluorine content in the bulk FPUas 0.117, which was shown in Table 2. This value is betweenthe values of the electrospun films and the casting films. Thefluorine content on the surface of the electrospun films is muchlower than the chemical composition of the bulk, which indicatesthat the abnormal low fluorine content might be related to theoriginal chain conformation of FPU in the mixed solvent. If themolecular conformation of polymer chains is such thatthe hydrophobic fluorine groups are aggregated like the core ofthe micelle, then this conformation could be retained because -of the rapid volatilization of solvent during the elecrospinningprocess.

The FPU solution was then studied by dynamic laser lightscattering. Figure 3 shows the typical intensity-intensitycorrelation function of FPU solutions in solvent of DMF/THF(30/70 v/v) (a) and in pure DMF (b). When the concentrationwas low, only the intensity of the correlation function increasesas the concentration increases. As the concentration becomeshigher, not only the intensity but also the decay time of the

correlation function changes. It becomes larger and larger, whichindicates that aggregates begin to form in the FPU solution atcertain concentration, and their size becomes larger as theconcentration grows higher. It is more obvious in the relativeintensity of different concentrations. As we can see in Figure3c,d, there is a transition of the slope of the relative intensitybetween 4 mg/mL and 10 mg/mL in the DMF/THF mixed solvent,while it is between 3 mg/mL and 4 mg/mL in pure DMF. Solutionsused for electrospinning are more concentrated; in other words,the polymer chain should be overlapped. In this study, the FPUsolution with a lowest concentration of 25 mg/mL was used forelectrospinning. Therefore, it can be deduced that aggregationoccurred in the electrospinning solution. We call it “aggregate”because the measured second virial coefficient (A2) values ofthe FPU solutions in two kinds of solvents were below zero,indicating that mixed solvent and DMF are poor solvents forFPU.

We know that pyrene, as a fluorescence probe, can reflect thepolarity of the solvent environment around it via the “3/1 ratio”,which is the relative intensity of peak III to peak Ι with referenceto the 0-0 band of pyrene. It is widely used in the micellarsolution to determine the critical micelle concentration (cmc).41

In our study, we used it to measure the change of solution propertybase on the same mechanism. Figure 4 shows the pyrenefluorescence spectra of FPU solutios with different concentrations.It is obvious that the 3/1 ratio becomes larger when theconcentration increases. This can be reflected straightforwardlyin Figure 5, where the 3/1 ratio has a sharp change between 3and 4 mg/mL, which indicates that there is a sharp change of theenvironment polarity of pyrene because of the formation of FPUaggregates. This result is in accordance with the previous dynamiclight scattering data.

(41) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.

Figure 3. Dynamic light scattering data of FPU solutions: intensity-intensity correlation function of different concentrations in solvent of (a)DMF/THF ) 30/70 (v/v), (b) DMF; relative intensity of different concentrations in solvent of (c) DMF/THF ) 30/70 (v/v), (d) DMF.

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According to the experimental data above, the mechanism ofthe abnormal low fluorine content phenomenon can be explainedas follows: The concentration of our electrospinning solutionsis in the range 25-40 mg/mL, which is higher than the aggregationtransition concentration. FPU polymers existed as aggregates inthe solvent with the perfluoropolyether segments wrapped insidethe hydrogen segments, because the solvent is not a good solventfor the fluorinated segments. Then, the solution is electrospunand the solvents evaporate very rapidly from the solution, THFfirst and then DMF. This process may be too fast for theperfluoropolyether segments to be unwrapped and move to theouter surface. Then, the structure of the aggregates in the solutionis “frozen” in the fiber and finally forms the electrospunmembranes. That is probably why we cannot observe the fluorineenrichment at the surface of the electrospun FPU membranes.This explanation can be further confirmed by the following resultsshown in Figure 6. A growing trend of the fluorine content withan increase in fiber diameter can be seen from Figure 6. This canbe understood with the proposed mechanism above: for the thinnerfibers, the surface area to volume ratio of the jet is larger and

the solvent will evaporate faster, which means only a very shorttime is available for the perfluoropolyether segments to becomerelaxed and come to the surface, resulting in a lower fluorinecontent at the surface; while for the thicker fibers, the surfacearea of the jet decreases, and the evaporation rate of the solventbecomes reduced accordingly, which means longer time isavailable for fluorine segments to become relaxed and results ina higher fluorine content at the surface. Although the solventcompositions in Tables 1 and 2 are not exactly the same, wethink the influence of the change of solvent composition is smallerthan the change of fiber diameter in our case.

Conclusion

Fluorinated polyurethane nanofibers were successfully elec-trospun with mixed DMF/THF as solvent. The surface composi-tions of the nanofibers were studied using XPS, and an abnormallow fluorine content was found in the surface of the FPU fibers.Through a model experiment, the freezing of the original chain

Figure 4. Pyrene fluorescence spectra of FPU solutions in DMF with different concentrations: (a) 2 mg/mL, (b) 4 mg/mL, (c) 12 mg/mL, (d) 36.9mg/mL.

Figure 5. Concentration dependence of the 3/1 ratio in DMF.

Figure 6. Fluorine content with increasing fiber diameter.

3182 Langmuir, Vol. 25, No. 5, 2009 Wu et al.

conformation due to the rapid evaporation of solvent during theelectrospinning process was proven to be the major cause of thelow fluorine content at the surface of the fibers. Then, the solutionproperty of the FPU polymer was studied by dynamic lightscattering and fluorescence, and a transition concentration wasobserved that corresponds to the aggregation formation of theFPU chains in solution with increased FPU concentration.Therefore, it can be deduced that the aggregate formation ofFPU in concentrated solutions and the fast evaporation of solventduring the electrospinning process that resulted in the freeze-inof the aggregated chain conformation might be the mechanismof formation of the abnormal low fluorine content at the sur-face of the electrospun FPU fibers. The increasing fluorine content

of the surface with thicker fiber diameter agrees well with theabove mechanism. This study has provided a fundamentalunderstanding of the mechanism of the variation of thehydrophobic group composition at a membrane surface, whichcould lead to some potential applications in functionalizing asurface and in biomedical applications.

Acknowledgment. This work was financially supported bythe National Nature Science Foundation of China (No.50503023-50521302), and the CMS Creative Project of CASin the Yong Talent Field (CMS-Y200709).

LA803580G

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