ORIGINAL PAPER

Fluorescent poly(p-phenylene vinylene)/poly(ethylene oxide)nanofibers obtained by electrospinning

Yi Xin & Zonghao Huang & Zijiang Jiang &

Liguang Che & Man Sun & Cheng Wang & Sidong Liu

Received: 26 October 2009 /Accepted: 6 April 2010 /Published online: 27 April 2010# Springer Science+Business Media B.V. 2010

Abstract Poly(p-phenylene vinylene) (PPV)/poly(ethyleneoxide) (PEO) hybrid nanofibers were prepared by electro-spinning a composite solution of PPV precursor/PEO in amixture of ethanol and water, followed by thermalconversion. The precursor/PEO composite solutions weresuccessfully electrospun into nanofibers with diversehelical, helical and linear, and helical bead-on-stringmorphologies by controlling the amount of aqueous PEOsolution in a composite solution. Moreover, adding aqueousPEO solution to a precursor ethanol solution decreased thediameters of the fibers. The experimental data suggest thatthe viscosity, conductivity, and surface tension of theelectrospinning solution are the main factors that influencethe morphology of the fibers. Fourier transform infrared(FT-IR) and X-ray diffraction (XRD) investigations indi-cated that the PPV precursor reacts with PEO duringthermal conversion. Ultraviolet–visible (UV-vis) and photo-luminescence (PL) spectra of the PPV-PEO nanofibersexhibited appreciable blue shifts with the addition of PEO,which made it possible to fabricate nanofibers withfluorescence ranging from yellow-green to blue. Thesehighly fluorescent PPV/PEO nanofibers with various

morphologies are potentially interesting for many applica-tions, such as micro- and nanooptoelectronic devices andsystems.

Keywords Electrospinning . Nanofibers . Semiconductors .

Poly(p-phenylene vinylene) (PPV)

Introduction

The production of and control over one-dimensional(1D) nanostructures with various morphologies andfunctionalities play important roles in the constructionof nano-/microscale devices, such as nano-/microoptoe-lectronic devices [1]. Notably, in recent decades, electro-spinning (a drawing process based on electrostatic force)was developed as a method to fabricate nanofibers [2, 3].Many electrospun nanofibers with interesting morphologies,such as bead-on-string [5] and helical [6] structures, havepotential applications in many areas, including advancedphotonic components, structural or inductive components inmicroelectromechanical system devices, and drug deliverysystems.

Poly(p-phenylene vinylene) (PPV) is a conjugatedpolymer that possesses excellent photo- and electrolumi-nescent [7], photovoltaic (PV) [8], and nonlinear optical [9]properties. Therefore, it is desirable to fabricate pure PPVinto various morphologies with nanoscale dimensions, andthis may be advantageous for both fundamental researchand practical applications. In this work, PPV-PEO hybridnanofibers with outstanding emission properties as well asdiverse and interesting morphologies were prepared byelectrospinning a composite solution of PPV precursor/PEOin a mixture of ethanol and water, followed by thermalconversion. These fluorescent nanofibers will have

Y. Xin : Z. Huang (*) : L. Che :M. Sun : S. LiuDepartment of Chemistry, Northeast Normal University,Changchun 130024, People’s Republic of Chinae-mail: huangzh295@nenu.edu.cn

Z. JiangChangchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun 130022, People’s Republic of China

C. WangDepartment of Chemistry and Materials Science,Heilongjiang University,Harbin 150080, People’s Republic of China

J Polym Res (2011) 18:477–482DOI 10.1007/s10965-010-9439-8

potential applications for the fabrication of electronic andphotonic devices.

Experimental section

P-Xylylene dichloride (98%) and tetrahydrothiophene(98%), purchased from Aldrich, were used to preparethe PPV precursor. The PPV precursor polymer wassynthesized according to the standard polyelectrolyteroute [10] and dialyzed against ethanol for three days.PPV precursor solution was then obtained. The concen-tration of precursor was 0.4 wt%. The weight ratio of thesolvents was 95/5 (ethanol/water w/w), as determined bygas chromatograph spectrometry with flame ionizationdetection (FID).

At room temperature, PEO (Mw=1,000,000 g/mol) wasdissolved in water at a concentration of 4 wt%. Then acontrolled amount of this prepared PEO solution was addedto the 0.4 wt% precursor solution under vigorous stirringfor over 12 h. PPV precursor/PEO blend solutions withdifferent weight ratios (PPV-PEO1: 5/1; PPV-PEO2: 1/1;PPV-PEO3: 1/5) were then prepared for electrospinning.The compositions of these three different PPV precursor/PEO mixed solutions are shown in Table 1.

In a typical electrospinning experiment, the electro-spinning solutions were placed into a 5-ml syringe with acapillary tip and an inner diameter of 0.6 mm. A wireinserted into the solution was connected to a high-voltagepower supply, and the applied voltage was fixed at 12 kV.All solutions were electrospun under the same conditions.The electrospun nanofibers were collected for about 1 minon a grounded aluminum foil, which was placed 20 cmfrom the capillary tip. The produced fibers were heated at180 °C for 2 h in a vacuum oven to convert the precursor toPPV.

The viscosity, conductivity, and surface tension of eachPPV precursor/PEO blend solution were measured using adigital viscometer (SNB-1, China), a conductive meter(DDSJ-308A, China), and an automatic tension meter(JYW-200B, China), respectively, at 25 °C. All of the fibersamples were characterized by fluorescence microscopy

(Nikon, TE2000-U, Japan), ultraviolet-visible spectroscopy(UV-vis, Cary 500 UV–vis–NIR spectrophotometer), X-raydiffraction (XRD, D/max-3c), eclipse fluorescence spec-trometry (Varian Corp., USA), and Fourier transforminfrared spectroscopy (FT-IR, Magna 560, Nicolet Corp.,USA).

Results and discussion

Fluorescence microscopy images of the PPV/PEO nano-fibers electrospun from the PPV precursor/PEO blendsolutions with different weight ratios are shown in Fig. 1.Pure PPV fibers collected on grounded aluminum foil arealso shown. All of the samples were electrospun under thesame conditions. As seen from Figs. 1a–d, strong fluores-cence is seen throughout each whole fiber under UV lightexcitation. As the blending ratio (precursor/PEO) decreases,the fluorescence of the fibers gradually changes fromyellow-green to blue. In addition, interesting morphologiesare observed. The pure PPVand PPV-PEO1 (weight ratio of5/1) fibers all exhibit a helical morphology. The PPV-PEO2fibers with a blending ratio of 1/1 consist of helical andlinear structures. When the weight ratio between theprecursor and PEO decreases to 1/5, uniform fibers with ahelical bead-on-string morphology are obtained.

Figure 2 shows the diameter distributions of these PPV/PEO hybrid fibers. The diameter distribution of each typeof electrospun fiber was obtained from measurements of200 fibers. The average diameters of the PPV, PPV-PEO1,and PPV-PEO2 fibers are all about 240 nm. When the PEOcontent is increased to 83.3% (weight ratio of 1/5), the fiberdiameter of PPV-PEO3 drops significantly, resulting in anaverage value of 106 nm.

In order to understand the mechanism of formation forthese intriguing morphologies and the diameter distribu-tions of hybrid the PPV-PEO fibers, we measured theviscosity, conductivity, and surface tension of eachelectrospinning solution, as listed in Fig. 3. These PPVprecursor/PEO blend solutions have higher conductivitiesthan many other electrospinning solutions, such as 4 wt%PVP solution in ethanol (conductivity of 16.3 µS/cm).Their high solution conductivities result in relatively highstretching forces in the electrospinning jet, which arebalanced by viscoelastic restoring forces. Therefore, theformation of these interesting helical structures may resultfrom the rapid discharge of these highly conductive fiberswhen they land on the conductor and subsequentlyundergo viscoelastic contraction. The helical structurewould appear to be caused by the electrospinningsolutions having relatively high values of both viscosityand conductivity. When the PEO aqueous solution isadded to the blend solution, the values of the solution

Table 1 The compositions of the three precursor/PEO mixedsolutions tested

Solution Amount of 0.4wt%precursor solution(g)

Amount of 4wt%PEO solution(g)

Precursor:PEO ratio

PPV-PEO1 1.96 0.04 5:1

PPV-PEO2 1.82 0.18 1:1

PPV-PEO3 1.33 0.67 1:5

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properties (viscosity, surface tension, and conductivity) allincrease. When the PEO content increases to 83.3%, theconductivity and surface tension increase significantly

relative to the viscosity. The solvent used in the PPVprecursor solution is ethanol. The solvent used in the PEOsolution is water. Therefore, the solvent in the PPV

Fig. 1 Fluorescence microsco-py images of electrospun PPV/PEO nanofibers that were col-lected on grounded aluminumfoil: a pure PPV; b PPV-PEO1;c PPV-PEO2; d PPV-PEO3

Fig. 2 The size distributions ofthe different types of PPV/PEOnanofibers: a pure PPV; b PPV-PEO1; c PPV-PEO2; d PPV-PEO3

Fluorescent poly(p-phenylene vinylene)/poly(ethylene oxide) nanofibers obtained by electrospinning 479

precursor–PEO solution is a mixture of ethanol and water.The conductivity and surface tension of the PPV precursorsolution in water are much higher than they are in ethanol.Therefore, the conductivity and surface tension of theprecursor-PEO solution increase with the addition of PEOaqueous solution. It is well known that high electricalconductivities result in high degrees of elongation duringelectrospinning and the formation of thinner fibers [11].Therefore, the formation of thin PPV-PEO3 fibers may beattributed to the high conductivity of the solution. In

addition, high surface tension favors the formation offibers with beads [12]. Thus, thin, helical PPV-PEO3fibers with beads were obtained.

To determine the interactions between PPV and PEO inthe hybrid fibers, FTIR spectra of pure PPV, PEO, andPPV-PEO hybrid fibers were obtained, and these are shownin Fig. 4. The characteristic absorption peaks of PPV occurat 961 cm−1 (trans-vinylene δ C–H out-of-plane stretchingmode), 834 cm−1 (p-phenylene C–H out-of-plane bendingmode), 1513 cm−1 (p-phenylene C–C ring stretching

Fig. 3 Changes in viscosity, conductivity, and surface tension in PPV precursor/PEO solutions with different PEO contents

Fig. 4 FTIR spectra of a PPV, b PEO, and c PPV-PEO3 fibers,respectively

Fig. 5 XRD spectra of the PEO, PPV, PPV-PEO1, PPV-PEO1, PPV-PEO2 and PPV-PEO3 samples

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mode), and 3024 cm−1 (trans-vinylene C–H stretchingmode) [13]. These absorption peaks are seen in both PPVand PPV-PEO fibers. In Fig. 4b and c, the peaks at about2888 cm−1 can be assigned to the aliphatic C–H stretchingmode of the PEO units. The peak at around 1600 cm−1 inFig. 4c may be from the PPV polymer [14]. An extraabsorption band at around 3430 cm−1 assigned to the –OHstretching mode is observed in the spectrum of the PPV-PEO fibers (Fig. 4c), which suggests that the ethylene oxideof PEO may break down in the PPV-PEO fibers duringthermal conversion. The as-generated hydroxyl groupscould react with the ethylene radical of the PPV precursorto form a C–O–C ether linkage between the two polymersduring the elimination of the sulfonium groups fromprecursor [15]. Characteristic stretching vibrations of theC–O–C are observed at about 1100 cm−1 in the spectra ofthe PEO and PPV-PEO fibers (Fig. 4b and c). Theabsorption broadened when PEO was added to the PPVmatrix, confirming the formation of a crosslinked complexin the PPV-PEO fibers.

X-ray diffraction (XRD) spectra for the PEO, PPV,and PPV-PEO samples are shown in Fig. 5. Neat PEOproduces sharp characteristic crystal peaks. The spectrumof neat PPV indicates a paracrystalline structure. Since theas-spun precursor fibers are amorphous, crystallizationtake places during the heat treatment. The diffractionpeaks of PPV-PEO1 (5/1) and PPV-PEO3 (1/5) are muchweaker and broader than those of the neat PPV and PEOsamples. When the weight ratio between PPV and PEO is1/1, the characteristic peaks of PPV and PEO are notobserved. These phenomena confirm that the PPV precur-sor reacts with PEO during thermal conversion, and thatthis reaction prevents crystal formation by PPV and PEO.

The photophysical properties of these PPV-PEO hybridfibers were investigated via their UV-vis and PL spectra. Asshown in Fig. 6a, absorption bands at about 400 nm areobserved, which can be attributed to the π–π* transition ofthe PPV component. The absorption peak becomes slightly

broader and shifts to a higher energy level as the PEOcontent in the hybrid fiber increases. This means that thePPV-PEO fibers with higher PEO contents have shorterconjugation lengths. The formation of C–O–C linkages, asshown by the FTIR spectra of the PPV-PEO fibers, resultsin an interruption to the π-conjugated system of the PPVstructure. The results obtained from the UV-vis spectracorrelated with those from the FTIR absorption spectra.Figure 6b shows the PL emission spectra of the PPV-PEOhybrid nanofibers. The pure PPV nanofibers emit yellow-green fluorescence at 555 nm with vibrant side bands at520 nm and 590 nm, which can be assigned to 0–1, 0–0, 0–2transitions, respectively [16]. In the PL spectra of the PPV-PEO nanofibers, the 0–0 peak is enhanced and the 0–1 peakreduced by the addition of PEO. Also, significant blue shiftsare observed in the PL spectra, which are confirmed byfluorescence image analysis (Fig. 1). These significant blueshifts in the PL spectra are associated with the PEO contentand, in turn, with the shorter conjugated chain length in thehybrids [17].

Conclusion

In summary, fluorescent PPV-PEO nanofibers withinteresting morphologies and tunable emission propertieshave been prepared by electrospinning PPV polyelectrolyteprecursor/PEO composite solutions and subsequent thermalconversion. The effect of the aqueous PEO solution on themorphology and diameter of the PPV nanofibers wasinvestigated. FTIR spectra revealed that the PEO reacts withthe ethylene radical of the PPV precursor to form a C–O–Cether linkage between the two polymers during the elimina-tion of the sulfonium groups from the precursor. This reactionprevents the PPVand PEO from forming crystals and resultsin blue shifts in the UV-vis and PL spectra of PPV. ThesePPV-PEO nanofibers possess great potential for use in manyapplications, including the fabrication of nanodevices.

Fig. 6 a UV–vis absorption andb PL spectra of the PPV-PEOhybrid fibers

Fluorescent poly(p-phenylene vinylene)/poly(ethylene oxide) nanofibers obtained by electrospinning 481

Acknowledgements This work was supported by NationalNatural Science Foundation of China (Grant: 20872030 and20774017), and the Analysis and Testing Foundation of NortheastNormal University.

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