Surface Depletion of the Fluorine Content of Electrospun Fibers of

Feb 9, 2009 - State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and ... However, for the electrospun fibrous...
0 downloads 0 Views 356KB Size
Download PDF
3178

Langmuir 2009, 25, 3178-3183

Surface Depletion of the Fluorine Content of Electrospun Fibers of Fluorinated Polyurethane Wanling Wu, Guangcui Yuan, Aihua He,* and Charles C. Han* State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Materials, 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 to aggregate in the outer surface because of the low surface energy, which endows the fluorinated materials with special surface properties such as self-cleaning, superhydrophobicity, and so forth. However, for the electrospun fibrous membranes of polyurethane elastomers containing perfluoropolyether segments (FPU), abnormal fluorine aggregations in the core of the electrospun fibers were observed. The XPS analysis indicated a rather low fluorine content at the surface of the electrospun FPU fibers. Further study with dynamic light scattering and fluorescence showed that FPU chains can form aggregates in the concentrated solution. Therefore, it can be deduced that the rapid evaporation of solvent and fast formation of fibers during the electrospinning process could result in the freeze-in of the aggregated chain conformation and the depletion of fluorine units on the surface of the electrospun FPU fibers.

Introduction Material surfaces with certain special properties have drawn a lot of interest for their practical applications in many fields. Fluorinated polymers, which have outstanding surface properties such as good hydrophobicity/oleophobicity, chemical resistance, and low friction coefficient, are undoubtedly among the most important candidates. Because of the low surface energy, fluorinated segments and end groups tend to segregate to the polymer-air interface1-6 resulting in differences in chemical composition between the surface and the bulk. Besides conventional fluoropolymers such as poly(tetrafluoroethylene) (PTFE) and poly(vinylidene fluoride) (PVDF), many researchers are interested in copolymers composed of fluorinated segments and some common polymers for their processibility and properties.3,5-7 Among these copolymers, fluorinated polyurethane has received great interest for its wide application in plastics, elastomers, fibers, foams, coatings and adhesives in construction, transport, and biomedical fields. Fluorinated polyurethane can be synthesized by the reactions of diisocyanates with diols, polyesters, and polyethers, when at least one of the reactants is fluorinated. The fluorinated groups can be hard segments,8 soft segments,9-15 chain extenders,16-18 or end groups.19,20 * Corresponding authors. E-mail: [email protected]. Tel: +86-1082618089. Fax: +86-10-62521519. (1) Elman, J. F.; Johs, B. D.; Long, T. E.; Koberstein, J. T. Macromolecules 1994, 27, 5341. (2) Lei, Y. G.; Cheung, Z. L.; Ng, K. M.; Weng, L. T.; Chan, C. M. Polymer 2003, 44, 3883. (3) Ming, W.; Tian, M.; van de Grampel, R. D.; Melis, F.; Jia, X.; Loos, J.; van der Linde, R. Macromolecules 2002, 35, 6920. (4) Affrossman, S.; Bertrand, P.; Hartshorne, Kiff, T.; Leonard, D.; Pethrick, R. A.; Richards, R. W. Macromolecules 1996, 29, 5432. (5) Casazza, E.; Mariani, A.; Ricco, L.; Russo, S. Polymer 2002, 43, 1207. (6) Bottino, F. A.; Pasquale, G. Di.; Pollicino, A.; Pilati, F.; Toselli, M.; Tonelli, C. Macromolecules 1998, 31, 7814. (7) Cheung, Z. L.; Ng, K. M.; Weng, L. T.; Chan, C. M.; Li, L. Polymer 2006, 47, 3164. (8) Holander, J.; Trischler, F. D.; Gosnell, R. B. J. Polym. Sci. 1967, 5(A-1), 2757. (9) Keller, T. M. J. Polym. Sci., Chem. Ed. 1985, 23, 2557. (10) Ho, T.; Wynne, K. J. Macromolecules 1992, 25, 3521. (11) Honeychuck, R. V.; Ho, T.; Wynne, K. J.; Nissan, R. A. Polym. Mater. Sci. Eng. 1992, 66, 521. (12) Honeychuck, R. V.; Ho, T.; Wynne, K. J. Chem. Mater. 1993, 5, 1299.

Electrospinning, as a simple and effective way to fabricate nanofibers, has been developed rapidly in recent years. A large number of polymers, including synthesized polymers21-27,29-34 and natural polymers,28 have been electrospun into nanofibrous materials. Currently, the electrospun nanofibrous mats of polyurethane have attracted great interest for their excellent mechanical properties and good biocompatibility,29-34 particularly in the research on PU nanofibrous mats as materials with enhanced mechanical properties,30,31 antimicrobial nanofilters,32,33 wound dressing materials, sensors,34 and so forth. (13) Tonelli, C.; Trombetta, T.; Scicchitano, M.; Castiglioni, G. J. Appl. Polym. Sci. 1995, 57, 1031. (14) Tonelli, C.; Trombetta, T.; Scicchitano, M.; Simeone, G.; Ajroldi, G. J. Appl. Polym. Sci. 1996, 59, 311. (15) Yu, X. H.; Okkema, A. Z.; Cooper, S. L. J. Appl. Polym. Sci. 1990, 41, 1777. (16) Takahara, A.; Jo, N. J.; Takamori, K.; Kajiyama, T. Progress in Biomedical Polymers, Gebelein C. G. and Dunn R. L., Eds.; Plenum Press: NewYork, 1990; p 317. (17) Chen, K. Y.; Kuo, J. F. Macromol. Chem. Phys. 2000, 201, 2676. (18) Hearn, M. J.; Briggs, D.; Yoon, S. C.; Patner, B. D. Surf. Interface Anal. 1987, 10, 384. (19) Khayet, M.; Suk, D. E.; Narbaitz, R. M.; Santerre, J. P.; Matsuura, T. J. Appl. Polym. Sci. 2003, 89, 2902. (20) Xie, Q. D.; Xu, J.; Feng, L.; Jiang, L.; Tang, W.; Luo, X.; Han, C. C. AdV. Mater. 2004, 16, 302. (21) Fong, H.; Chun, I.; Reneker, D. H. Polymer 1999, 40, 4585. (22) Koombhongse, S.; Liu, W.; Reneker, D. H. J. Polym. Sci.: Polym. Phys. Ed. 2001, 39, 2598. (23) Choi, S. W.; Jo, S. M.; Lee, W. S.; Kim, Y. R. AdV. Mater. 2003, 15, 2027. (24) Mckee, M. G.; Wilkes, G. L.; Colby, R. H.; Long, T. E. Macromolecules 2004, 37, 1760. (25) Boland, E. D.; Wnek, G. E.; Simpson, D. G.; Pawlowski, K. J.; Bowlin, G. L. J. Macromol. Sci. Pure Appl. Chem. 2001, A38, 1231. (26) Kalra, V.; Kakad, P. A.; Mendez, S.; Ivannikov, T.; Kamperman, M.; Joo, Y. L. Macromolecules 2006, 39, 5453. (27) Zong, X. H.; Bien, H.; Chung, C. Y.; Yin, L. H.; Fang, D. F.; Hsiao, B. S.; Chu, B.; Entcheva, E. Biomaterials 2005, 26, 5330. (28) Matthews, J. A.; Wnek, G. E.; Simpson, D. G.; Bowlin, G. L. Biomacromolecules 2002, 3, 232. (29) Demir, M. M.; Yilgor, I.; Yilgor, E.; Erman, B. Polymer 2002, 43, 3303. (30) Pedicini, A.; Farris, R. J. Polymer 2003, 44, 6857. (31) Mckee, M. G.; Park, T.; Unal, S.; Yilgor, I.; Long, T. E. Polymer 2005, 46, 2011. (32) Lee, K. H.; Kim, D. J.; Min, B. G.; Lee, S. H. Biomed. MicrodeVices 2007, 9, 435. (33) Jeong, E. H.; Yang, J.; Youk, J. H. Mater. Lett. 2007, 61, 3991. (34) Wang, X. Y.; Drew, C.; Lee, S. H.; Senecal, K. J.; Kumar, J.; Sarnuelson, L. A. Nano Lett. 2002, 2, 1273.

10.1021/la803580g CCC: $40.75  2009 American Chemical Society Published on Web 02/09/2009

Electrospun Fibers of Fluorinated Polyurethane

Langmuir, Vol. 25, No. 5, 2009 3179 Scheme 1. Chemical Structure of FPU

In a previous study, we successfully electrospun polyurethane elastomers containing perfluoropolyether segments (FPU) as a superhydrophobic material.35 The chemical structure of FPU is showed in Scheme 1. As many researches have shown, surface chemical composition may be one of the main factors that affect the surface wetting behavior.36-38 In addition, only a few researchers have focused on the surface chemistry of the electrospun fluorinated materials.39 Therefore, the surface chemistry of the electrospun FPU membranes were studied by XPS in our present research. Instead of the fluorine enrichment, an abnormal low fluorine content was found in the surface of the electrospun FPU fibers. To better understand this phenomenon, the solution properties of FPU were studied using dynamic light scattering and fluorescence, and then in combination with the actual process during electrospinning, a conceivable explanation was presented in this paper.

Experimental Section Materials. Polytetrahydrofuran (Mn ) 2000, Sigma-Aldrich, Inc., USA) in DMAc (25.0 wt %) was added to 4,4′-methylenebis(phenylisocyanate) (MDI, Acros Organics, USA) solutions in DMAc (10.0 wt %) in a three-necked, round-bottom flask under argon atmosphere. The polymerization was carried out at 70-80 °C for 2 h. Stoichiometric perfluoropolyether alcohol (CF3CF2CF2O(CFCF3CF2O)2CFCF3CH2OH, Shanghai Institute of Organic Chemistry) in DMAc (10.0 wt %) was added drop by drop to the prepolymer solution and the reaction was kept for 1 h. Then, a small amount of 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 resulting polymers were precipitated in excess water and dried in a vacuum oven at 60 °C. They were redissolved in DMAc and precipitated in an excess methanol-water mixture to remove low molecular weight materials. The purified polymers were washed with methanol, then deionized water, and dried in a vacuum at 60 °C. The resulting polymer is a light yellow elastomer, and the detailed characterization of the FPU can be found elsewhere.35 Electrospinning. FPU solutions with different concentrations were prepared for electrospinning with a mixture of N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) as solvent.The dc highvoltage generator (The Beijing machinery and electricity institute, China) was employed to produce voltages ranging 0-50 kV. The electrospinning solutions were placed into a 5 mL syringe with a capillary tip having an inner diameter of 0.3 mm. A syringe pump was used to feed polymer solutions into the needle tip. A sheet of aluminum foil, connected to the ground, was placed under the syringe as 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 this study. The normal casting sample was made by dropping the polymer solution onto the silicon substrate and drying the film at 50 °C. The other two kinds of casting films were made under electric field. A (35) Wu, W. L.; Zhu, Q. Z.; Qing, F. L.; Han, C. C. Langmuir (la-200803089y), 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. (38) Iyengar, D. R.; Perutz, S. M.; Dai, C. A.; Ober, C. K.; Kramer, E. J. Macromolecules 1996, 29, 1229. (39) Deitzel, J. M.; Kosik, W.; Mcknight, S. H.; BeckTan, N. C.; DeSimone, J. M.; Crette, S. Polymer 2002, 43, 1025.

silicon substrate was placed in a dc high-voltage field with a distance of 7 cm between the two poles, the voltage applied was the same as in the electrospinning process, and the ambient temperature was 50 °C; then, the FPU solution was dropped onto the silicon substrate and dried in ambient temperature. Negative and positive voltages were used, respectively, and the cast films were marked “casting film(-)” and “casting film(+)”, correspondingly. Characterization. Gel permeation chromatography was performed to estimate the molecular weights of FPU using a 1515 system (Waters) equipped with 2414 refractive index and Styragel gel columns calibrated with narrow-molecular-weight polystyrene standards. The surface morphologies of these electrospun membranes were observed using a scanning electron microscope (JSM6300F, JEOL, Japan). X-ray photoelectron spectroscopy data were obtained with an electron spectrometer (ESCALab220i-XL, VG Scientific) using 300 W Al KR radiation. The binding energies were referenced to the C1s line at 285.0 eV from adventitious carbon. The casting samples and the electrospun samples were characterized directly without any treatment except as indicated otherwise. Dynamic light scattering measurements were carried out by a commercial LLS spectrometer (ALV/DLS/SLS-5022F) equipped with a multi-τ digital time correlator (ALV5000) and a 22 mW UNIPHASE He-Ne laser (λ0 ) 632.8 nm). The LLS cell is held in a thermostat index matching vat filled with purified and dust-free toluene, with the temperature controlled at 25 ( 0.02 °C. In dynamic LLS, the intensity-intensity time correlation function G(2)(t, q) in the self-beating mode was measured, where t is time and q is scattering vector (q ) (4πn/λ0) sin(θ/2)). G(2)(t, q) can be related to the normalized first-order electric field time correlation function |g(1)(t, q)| via the Siegert relation as

G(2)(t,q) ) A[1 + β|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 at 340 nm, and emission spectra were recorded from 360 to 600 nm; the scan rate was 600 nm/min. Bandwidth slits for excitation and emission were both 5 nm. Excitation spectra of the samples were performed at room temperature. Pyrene in DMF with a concentration of 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 molecular weight of 220 000 (g/mol) and ploydispersity index of 1.94 characterized by GPC. Polyurethane is a polymer composed of hard segments (diisocyanate) and soft segments (polyether or polyester). THF is a good solvent for the flexible soft segments, and DMF is a good solvent for the hard segments. A DMF/THF mixture was used as the solvent for FPU in this study. The concentration of the FPU solution was fixed at 25 mg/mL, and the volume ratio of DMF to THF was varied. Figure 1a-c shows the SEM images of electrospun FPU nanofibers. It can be seen from Figure 1 that uniform FPU fibers could be fabricated successfully. The average diameter of the FPU fibers was decreased from 580 nm, 330 to 220 nm when the ratio (v/v) of DMF/THF was increased from 30/70 to 100/0. It seems that the fiber diameter decreases with an increasing amount of DMF in the solvent. This might be due to the higher conductivity of DMF

3180 Langmuir, Vol. 25, No. 5, 2009

Wu et al. Table 1. XPS Results of Electrospun FPU Membranes from Different Solutions sample

concentration (mg/mL)

DMF/THF (v/v)

F/C

1 2 3 4a 5 6

25 25 25 25 20 30

3:7 7:3 10:0 10:0 3:7 3:7

0.059 0.036 0.018 0.053 0.039 0.068

a

Note: 4a is annealed membrane of sample 3.

Table 2. XPS Results of Casting Films Prepared under Different Conditions F/C sample Figure 1. SEM images of electrospun FPU membranes at concentration of 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) sample of (c) after annealing.

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

than that of THF, which will lead to a decrease in fiber diameter.40 Figure 1d is the sample of (c) that underwent an annealing treatment at 100 °C for 2 h. It can be seen that the morphology of the film is unchanged after annealing. The morphologies of samples (c) and (d) are different from those of the other samples, which were attributed to the slow volatilizing rate of DMF and the easy adhesion of wet jets to each other. When the volume ratio of DMF to THF was fixed at 30/70, the concentration of the FPU solution was varied. Figure 2a-c shows the SEM images of FPU nanofibers with different concentrations. It can be seen that the fiber diameter increased with increasing concentration. The average diameters of those fibers were 460, 590, and 840 nm, respectively, which indicated that FPU nanofibers with different fiber diameter could be fabricated successfully by electrospinning. Surface Chemistry of Electrospun FPU Fibers. The electrospun films are expected to have potential applications for their special surface properties. The previous studies on electrospinning of fluorinated materials mainly focus on the morphology of the films; few focus on the surface compositions. (40) Zheng, J. F.; He, A. H.; Li, J. X.; Xu, J.; Han, C. C. Polymer 2006, 47, 7095.

bulk normal casting casting casting

casting film film (-) film (+) film (water)

takeoff angle 30°

takeoff angle 90°

--0.668 0.715 0.709 ---

0.117 0.419 0.434 0.459 0.041

Generally speaking, fluorine units prefer to enrich in the surface, which will lead to lower surface energy. In this study, the surface chemistry of the electrospun FPU films was investigated using XPS. Table 1 is the XPS results of the electrospun samples, with a takeoff angle of 90°. In the case of planar film, the XPS data of 90° takeoff angle usually represent the composition of the outermost 10 nm of the surface. However, in our case, the surface of the membranes was made up of column-like fibers instead of a plane, so only a small part of the surface was perpendicular to the incident X-ray, which meant that the data we obtained was from the surface composition of the outermost 10 nm or less. The mole ratio of fluorine to carbon was used to represent the fluorine content of the outermost surface of the electrospun FPU fibers in this study. It can be seen from Table 1 that the measured fluorine content in the outermost surface of FPU fibers was rather low (lower than 0.07). In addition, it can be seen that the fluorine content tends to decrease as the amount of DMF increases in the mixed solvent. The fluorine content of sample (c) is a little higher after annealing. When the volume ratio of mixed solvent was fixed, fluorine content of the surface increased with increasing concentration. For comparison, normal casting film was prepared at 50 °C from a FPU solution with concentration of 30 mg/mL in mixed solvent of DMF/THF (30/70 v/v). From the XPS data in Table 2, we can see that the fluorine content in the normal casting film is much higher than that of electrospun membranes, and the data of 30° takeoff angle is higher than that of 90°, indicating a remarkable enrichment of the fluorine content in the surface of the casting film. This indicated that fluorine was actually depleted at the surface of the electrospun fibers, which was a really abnormal phenomenon. Then, the question comes up: how did this happen? As we know, a high voltage is used in the electrospinning process. To study the effect of the voltage on the fluorine content in the fiber surface, another two casting films were prepared under electric field as described in the Experimental Section. Both negative and positive voltage were used to investigate the effect of the electric field on the fluorine enrichment in the surface. From the XPS data in Table 2, we can see that the fluorine content of the casting films under electric field shows little difference from that of the normal casting film, which means high voltage was not the key factor that affects the fluorine enrichment in the surface.

Electrospun Fibers of Fluorinated Polyurethane

Langmuir, Vol. 25, No. 5, 2009 3181

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.

Besides the high voltage, the electrospinning process is also a rapid volatilization process of the solvent. When the FPU solution is pumped out of the syringe, the solvent evaporates quickly and the polymer fibers are collected in a very short time. So, we designed an experimental model to investigate how the drying of solvents affects the fluorine content on the surface. The procedure is the following: We dripped a drop of FPU solution with concentration of 75 mg/mL in mixed solvent of DMF/THF (30/70 v/v) into water, and a film was formed almost instantaneously because FPU is not a water-soluble polymer, while both solvents can mix with water at any ratio. So, it is a rapid diffusion process for solvents to leach out. Finally, the film was freeze-dried, and the sample is marked “casting film (water)”. From the XPS data in Table 2, we can see that the fluorine content on the surface of the casting film (water) is close to that of the electrospun films. Meanwhile, we cut a piece of polymer from the bulk and obtained the fluorine content in the bulk FPU as 0.117, which was shown in Table 2. This value is between the values of the electrospun films and the casting films. The fluorine content on the surface of the electrospun films is much lower than the chemical composition of the bulk, which indicates that the abnormal low fluorine content might be related to the original chain conformation of FPU in the mixed solvent. If the molecular conformation of polymer chains is such that the hydrophobic fluorine groups are aggregated like the core of the micelle, then this conformation could be retained because of the rapid volatilization of solvent during the elecrospinning process. The FPU solution was then studied by dynamic laser light scattering. Figure 3 shows the typical intensity-intensity correlation function of FPU solutions in solvent of DMF/THF (30/70 v/v) (a) and in pure DMF (b). When the concentration was low, only the intensity of the correlation function increases as the concentration increases. As the concentration becomes higher, not only the intensity but also the decay time of the

correlation function changes. It becomes larger and larger, which indicates that aggregates begin to form in the FPU solution at certain concentration, and their size becomes larger as the concentration grows higher. It is more obvious in the relative intensity of different concentrations. As we can see in Figure 3c,d, there is a transition of the slope of the relative intensity between 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. Solutions used for electrospinning are more concentrated; in other words, the polymer chain should be overlapped. In this study, the FPU solution with a lowest concentration of 25 mg/mL was used for electrospinning. Therefore, it can be deduced that aggregation occurred in the electrospinning solution. We call it “aggregate” because the measured second virial coefficient (A2) values of the FPU solutions in two kinds of solvents were below zero, indicating that mixed solvent and DMF are poor solvents for FPU. We know that pyrene, as a fluorescence probe, can reflect the polarity of the solvent environment around it via the “3/1 ratio”, which is the relative intensity of peak III to peak Ι with reference to the 0-0 band of pyrene. It is widely used in the micellar solution to determine the critical micelle concentration (cmc).41 In our study, we used it to measure the change of solution property base on the same mechanism. Figure 4 shows the pyrene fluorescence spectra of FPU solutios with different concentrations. It is obvious that the 3/1 ratio becomes larger when the concentration increases. This can be reflected straightforwardly in Figure 5, where the 3/1 ratio has a sharp change between 3 and 4 mg/mL, which indicates that there is a sharp change of the environment polarity of pyrene because of the formation of FPU aggregates. This result is in accordance with the previous dynamic light scattering data. (41) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.

3182 Langmuir, Vol. 25, No. 5, 2009

Wu et al.

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) 36.9 mg/mL.

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

According to the experimental data above, the mechanism of the abnormal low fluorine content phenomenon can be explained as follows: The concentration of our electrospinning solutions is in the range 25-40 mg/mL, which is higher than the aggregation transition concentration. FPU polymers existed as aggregates in the solvent with the perfluoropolyether segments wrapped inside the hydrogen segments, because the solvent is not a good solvent for the fluorinated segments. Then, the solution is electrospun and the solvents evaporate very rapidly from the solution, THF first and then DMF. This process may be too fast for the perfluoropolyether segments to be unwrapped and move to the outer surface. Then, the structure of the aggregates in the solution is “frozen” in the fiber and finally forms the electrospun membranes. That is probably why we cannot observe the fluorine enrichment at the surface of the electrospun FPU membranes. This explanation can be further confirmed by the following results shown in Figure 6. A growing trend of the fluorine content with an increase in fiber diameter can be seen from Figure 6. This can be understood with the proposed mechanism above: for the thinner fibers, the surface area to volume ratio of the jet is larger and

Figure 6. Fluorine content with increasing fiber diameter.

the solvent will evaporate faster, which means only a very short time is available for the perfluoropolyether segments to become relaxed and come to the surface, resulting in a lower fluorine content at the surface; while for the thicker fibers, the surface area of the jet decreases, and the evaporation rate of the solvent becomes reduced accordingly, which means longer time is available for fluorine segments to become relaxed and results in a higher fluorine content at the surface. Although the solvent compositions in Tables 1 and 2 are not exactly the same, we think the influence of the change of solvent composition is smaller than the change of fiber diameter in our case.

Conclusion Fluorinated polyurethane nanofibers were successfully electrospun with mixed DMF/THF as solvent. The surface compositions of the nanofibers were studied using XPS, and an abnormal low fluorine content was found in the surface of the FPU fibers. Through a model experiment, the freezing of the original chain

Electrospun Fibers of Fluorinated Polyurethane

conformation due to the rapid evaporation of solvent during the electrospinning process was proven to be the major cause of the low fluorine content at the surface of the fibers. Then, the solution property of the FPU polymer was studied by dynamic light scattering and fluorescence, and a transition concentration was observed that corresponds to the aggregation formation of the FPU chains in solution with increased FPU concentration. Therefore, it can be deduced that the aggregate formation of FPU in concentrated solutions and the fast evaporation of solvent during the electrospinning process that resulted in the freeze-in of the aggregated chain conformation might be the mechanism of formation of the abnormal low fluorine content at the surface of the electrospun FPU fibers. The increasing fluorine content

Langmuir, Vol. 25, No. 5, 2009 3183

of the surface with thicker fiber diameter agrees well with the above mechanism. This study has provided a fundamental understanding of the mechanism of the variation of the hydrophobic group composition at a membrane surface, which could lead to some potential applications in functionalizing a surface and in biomedical applications. Acknowledgment. This work was financially supported by the National Nature Science Foundation of China (No. 50503023-50521302), and the CMS Creative Project of CAS in the Yong Talent Field (CMS-Y200709). LA803580G