imi

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different amounts of AgTFA were studied, and the crystal structure of silver nanoparticles was also presented.

of the polymers, opened a new gateway in developing

precursor and temperature in this chemical process [69].In recent years, ultra-fine polymer fibers prepared by

(PI) constitutes an important class of polymers due to its

2. Experimental

Pyromellitic dianhydride (PMDA, 99%) and 4,4-Oxydiani-line (4,4-ODA, 99%) were purchased from Shanghai Research

Materials Letters 61 (2007)electrospinning of a polymer solution have been drawingnanocomposite systems with improved performances [5].Generally speaking, there are two approaches to prepare

polymer/silver nanocomposites. One is to disperse silvernanoparticles into polymer matrices, being simple but difficultto obtain well-dispersed nanoparticles due to easy aggregationof nanoparticles. The other is in situ chemical formation ofsilver nanoparticles by the reduction of silver ions which areintroduced into polymer matrices. The tailored size and sizedistribution of silver particles can be achieved by choosingsome parameters such as reduction method, concentration of

superior thermal and chemical resistance as well as mechanicalproperties. Nah et al. [14] have reported the preparation of PIultra-fine fibers through electrospinning.

In this paper, we present an effective method to prepare ultra-fine PI fibers containing silver nanoparticles via electrospinningpoly(amic acid) (PAA)/AgTFA solution followed by thermalimidization as well as silver reduction. Their morphologicalcharacterization was carried out by X-ray diffraction, SEM andTEM.1. Introduction

Silver nanoparticles are widely used due to their superioroptical [1,2], electrical, catalytic [3] and antimicrobial [4]properties. Polymer/silver nanoparticles composites, combiningthe advantages of the silver nanoparticles and the processability

great attention because of their unique properties, such as highsurface area-to-volume and ease of fabrication [10]. Someresearches on ultra-fine polymer fibers containing silvernanoparticles have been reported in various polymer matrices:cellulose acetate [4], poly (N-vinylpyrrolidone) [6], polyacry-lonitrile [9,11,12] and poly(4-vinylpyridine) [13]. Polyimide 2007 Elsevier B.V. All rights reserved.

Keywords: Polyimide; Silver; Nanoparticles; Electrospun fibersPreparation of ultra-fine polynanoparticles via

Qian Zhang, Dezhen Wu , Shengli Qi, Z

State Key Laboratory of Chemical Resource Engineering, Beiji

Received 28 July 2006Available onlin

Abstract

Ultra-fine polyimide fibers containing silver nanoparticles were presilver(I) (PAA/AgTFA) solution. Thermal curing of the silver(I)-contaiwith concomitant silver(I) reduction. The polyimidesilver fibers wereand transmission electron microscopy. The average diameters of silv Corresponding author.E-mail address: wdz@mail.buct.edu.cn (D. Wu).

0167-577X/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.01.011ide fibers containing silvern situ technique

npeng Wu, Xiaoping Yang, Riguang Jin

niversity of Chemical Technology, Beijing 100029, PR China

cepted 5 January 2007January 2007

ed by electrospinning from poly(amic acid)/(trifluoroacetylacetonoto)g fibers led to cycloimidization of the poly(amic acid) into polyimideracterized by FT-IR, X-ray diffraction, scanning electron microscopy,anoparticles and ultra-fine PI fibers electrospun from solutions with

40274030www.elsevier.com/locate/matletInstitute of Synthetic Resins and used as received. N,N-dimethylformamide (DMF, analytic pure, 99.5%) waspurchased from Tianjin Fine Chemical Co., China and used as

3. Results and discussion

Thermal curing leads to the cycloimidization of PAA into PI withconcomitant silver reduction and aggregation yielding ultra-finepolyimidesilver fibers. Fig. 1 shows the FT-IR spectra of the as-deposited electrospun PAA/AgTFA fibers, the final PI/Ag fibers andpure PI fibers. Prior to heating, the IR spectrum for PAA/AgTFA fibersexhibits characteristic bands at 29003200 cm1, 1660 cm 1 and1550 cm1, which respond to COOH and NH2, C_O in a CONHgroup and the CNH stretch bands, respectively. Following thermaltreatment, there are four absorbance peaks at 1780 cm 1, 1725 cm 1,1380 cm 1 and 725 cm 1 on the PI/Ag spectrum, responding to C_Osymmetric stretch, C_O asymmetric stretch, CN stretch and C_Obending respectively, which indicate the formation of the polyimidematrix. It is noteworthy to mention that the relative intensity of the peakat 1780 cm 1 in the PI/Ag fibers is lower than in pure PI fibers,implying the decrease of electron density of the carboxyl (C_O)oxygen atom, probably due to the interaction between Ag nanoparticles

Letters 61 (2007) 40274030received. Silver(I) acetate (AgAC, analytic pure, 99.0%) wasobtained from Shanghai Shiyi Chemicals Reagent Co. Ltd. andused as received. 1,1,1-Trifluoro-2,4-pentanedione (TFAH,98%) was purchased from Acros Organics and used as received.

The poly(amic acid) (PAA) solution of 10 wt.%, a precursorof PI, was synthesized from its monomers, PMDA and ODA inDMF. The resin was stirred for a minimum of 2 h beforeincorporating the silver complex. The AgTFA complex wasfreshly produced by dissolving AgAC in a small volume ofDMF containing TFAH with three times mole equivalent to thesilver acetate.

After incorporating different amounts of AgTFA (Ag was 17 wt.% of the amount of PI/Ag fiber) into PAA resin, thesolution was electrospun using an electrostatic spinning

Fig. 1. FT-IR spectra of PAA/AgTFA, PI/Ag fibers electrospun from 10 wt.%PAA solution with 7 wt.% Ag and pure PI electrospun fibers.

4028 Q. Zhang et al. / Materialsapparatus. The apparatus consisted of a syringe and a needle(ID=6.5 mm), an aluminum collecting plate and a high voltagesupply. A syringe pump (TOP5300) connected to the syringecontrolled the flow rate. The PAA/AgTFA solution waselectrospun at a positive voltage of 15 kV, a working distanceof 10 cm (the distance between the needle tip and the collectingplate), and a solution flow rate of 0.2 ml/h.

Thermal curing was performed in a forced air oven toaccomplish the conversion of the PAA fibers into PI ones aswell as the formation of silver nanoparticles. The cure cycle washeating over 1 h from normal temperature to 135 C andremaining at 135 C for 1 h, followed by heating to 300 C over2 h and remaining at 300 C for 3 h.

The PAA/AgTFA and PI/Ag fibers obtained were character-ized by FT-IR spectroscopy (Nexus 670) to identify themolecular structure. X-ray photoelectron spectra were obtainedusing an ESCALAB 250 spectrometer (Thermo ElectronCorporation) in the fixed analyzer transmission mode. Themorphologies of the ultra-fine PI fibers were observed on ascanning electron microscopy (SEM) (HITACHI S-800) afterplatinum coating. Transmission electron microscopy (TEM)images were obtained with a HITACHI H-800TEM instrumentusing samples deposited on carbon coated copper grids.and the C_O group [15] of the polyimide.X-ray diffraction pattern for the 7 wt.% silver doped fibers is shown

in Fig. 2. Four obvious peaks are detected at 2=38.1, 44.3, 64.4and 77.5, which are corresponding to the crystal faces of (111), (200),(220), and (311) of silver, consistent with the formation of face-centered-cubic silver crystallite in the polyimide ultra-fine fibers afterheating. The full width at half-maximum (FWHM) of the strongestcharacteristic peak (111) is used to estimate the average crystallite sizeby applying the Scherrer equation: D= / cos. Here X-raywavelength is 1.54056, is the shape factor which is often assigneda value of 0.89 if the shape is unknown, D is the average diameter ofthe crystals in angstroms, is the Bragg angle in degrees, and is thefull width at half-maximum of the strongest characteristic peak inradians. The result shows that the average silver particle size in theultra-fine fibers is approximately 20.2 nm.

SEM micrographs for the typical non-woven fabric PI/Ag fiberselectrospunwith different silver content are shown in Fig. 3. It is clear thatthe average diameters of the ultra-fine PI/Ag fibers decreased graduallywith increasing silver concentration, with fiber diameters on the order of200 nm, 190 nm, 180 nmand 60 nm for the pure, 1wt.%, 2wt.% and 7wt.% silver doped polyimide fibers, respectively. It is suggested that theaddition of a salt would increase the charge density of the polymersolution, which will cause stronger elongation forces on the ejected jetsFig. 2. X-ray diffraction peaks of PI fiber mat electrospun from 10 wt.% PAAsolution with 7 wt.% Ag.

4029Q. Zhang et al. / Materials Letters 61 (2007) 40274030under the same electrical field, resulting in the formation of substantiallystraighter and finer fibers [16]. In addition, as Ag amount increases, moresilver particles can be observed on SEM images.

Fig. 4 shows the corresponding TEM images of the ultra-fine PI/Agfibers containing different Ag. It can be clearly seen that the silvernanoparticles are embedded on the fiber surface. Nearly spherical silverparticles were observed on the TEM images for the 1 wt.% and 2 wt.%silver doped polyimide fibers, with average diameters of 10 and 12 nm,respectively. When the silver concentration increased to 7 wt.%, theaverage diameter of silver particles increased in the range of 1420 nm,in agreement with the XRD result presented above. However, the silverparticles become irregular and non-spherical, which is attributed to the

Fig. 3. SEM images of PI/Ag fibers electrospun from 10 wt.% PAA solution with diff

Fig. 4. TEM images of PI/Ag fibers electrospun from 10 wt.% PAA solution wimassive silver migration and aggregation. The migration andaggregation of silver particles are probably driven largely by theinstability of silver atoms due to their high surface free energy. Theiraggregation would produce thermodynamically stable particles withbigger sizes [17,18].

4. Conclusion

Ultra-fine polyimide (PI) fibers containing Ag nanoparticleswere prepared by electrospinning from PAA/AgTFA solutionfollowed by thermal curing as well as silver reduction. The Ag

erent amounts of AgTFA: Ag (a) 0 wt.%, (b) 1 wt.%, (c) 2 wt.% and (d) 7 wt.%.

th different amounts of AgTFA: Ag (a) 1 wt.%, (b) 2 wt.% and (c) 7 wt.%.

nanoparticles in the PI fibers possessed a face-centered-cubicstructure. The average diameters of the ultra-fine PI fibers withdifferent amounts of AgTFA, in which Ag were 0, 1, 2, and7 wt.%, were 200, 190, 180, and 60 nm, respectively. Thenumber of Ag nanoparticles in the ultra-fine PI fibers increasedas the amount of AgTFA increased and the average diameters ofthe Ag nanoparticles were 10, 12, and 1420 nm when 1, 2 and7 wt.% Ag were doped, respectively.

Acknowledgements

This research was supported by the Program for NewCentury Excellent Talents in University (NCET) and theNational Natural Science Foundation of China (NSFC, ProjectNo. 50573007).

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Preparation of ultra-fine polyimide fibers containing silver nanoparticles via in situ techniqu.....IntroductionExperimentalResults and discussionConclusionAcknowledgementsReferences