EUROPEAN

European Polymer Journal 43 (2007) 2171–2176

www.elsevier.com/locate/europolj

POLYMERJOURNAL

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Macromolecular Nanotechnology

Radiolytic synthesis and characterizationof Ag-PVA nanocomposites

Aleksandra N. Krkljes, Milena T. Marinovic-Cincovic,Zorica M. Kacarevic-Popovic *, Jovan M. Nedeljkovic

Institute for Nuclear Sciences Vinca, Laboratory for Radiation Chemistry, Mike Alasa 12-14, 11001 Belgrade, Serbia and Montenegro

Received 27 February 2007; accepted 13 March 2007Available online 20 March 2007

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Abstract

The Ag-PVA nanocomposites with different contents of inorganic phase were prepared by reduction of Ag+ ions inaqueous PVA solution by gamma irradiation followed by solvent evaporation. Optical properties of the colloidal solutionsand the nanocomposite films were investigated using UV–vis spectroscopy. Structural characterization of the Ag nanopar-ticles was performed by TEM and XRD. Interaction of the Ag nanoparticles with polymer matrix and the heat resistanceof the nanocomposites were followed by IR spectroscopy and DSC analysis. IR spectra indicated that Ag nanofiller inter-act with PVA chain over OH groups. The changes of heat resistance upon the increase of the content of inorganic phase arecorrelated to the adsorption of polymer chains on the surface of Ag nanoparticles.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Ag nanoparticles; PVA; Polymer nanocomposites; Radiation chemistry; Thermooxidative degradation

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1. Introduction

The preparation of metal nanoparticles hasreceived increasing attention due to their uniqueproperties. Noble metal particles such as silver andgold are of great significance due to their size-depen-dent optical properties [1–3]. Moreover, size effectwas also observed for antibacterial activity of silvernanoparticles [4]. A large number of synthetic pro-cedures have been employed in order to synthesize

0014-3057/$ - see front matter � 2007 Elsevier Ltd. All rights reserved

doi:10.1016/j.eurpolymj.2007.03.023

* Corresponding author. Tel.: +381 11 8066428; fax: +381 112453986.

E-mail address: zkacar@vin.bg.ac.yu (Z.M. Kacarevic-Popovic).

silver nanoparticles and/or nanocomposites. Usageof strong reducing agents in water [5,6] and organicsolvents [7], laser ablation [8], sonochemical treat-ment [9] and UV irradiation [10] have been appliedfor synthesis of silver nanoparticles, while polymers[11] and membranes [12] have been used as matricesfor preparation of nanocomposites, and DNA [13]and carbon nanotubes [14] as templates. It has beenshown that morphology, particle size distribution,stability and properties of silver nanoparticles aswell as corresponding nanocomposites are stronglydependent on the method of preparation and spe-cific experimental conditions.

Colloidal silver nanoparticles protected by poly-(vinyl alcohol) (PVA) have been extensively studied

.

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[15–17]. PVA has been widely used as a matrix forpreparation of nanocomposites due to its easy pro-cessability, high optical clarity [18] and biocompat-ibility [19]. However, the synthesis of silvernanoparticles of desired shape and uniform size dis-tribution within the matrix remains challenging.The radiolytic method is suitable for generation ofmetal particles, particularly silver, in solution [20–24]. The radiolytically generated species, solvatedelectrons and secondary radicals, exhibit strongreducing potentials, and consequently metal ionsare reduced at each encounter. The control of parti-cle size is commonly achieved by the use of cappingagents such as polymers, which are present in thesolution during the formation of metal clusters.Polymer molecules interact with the growing metalparticles inhibiting the aggregation process. In addi-tion, casting of film becomes easier and the particlesize can be well controlled within the desired regime.

In this work the Ag-PVA nanocomposites wereprepared by the simple radiolytic procedure usingsteady state gamma irradiation. The aim of thisstudy was investigation of the optical and structuralproperties of nanocomposites as well as the influ-ence of interaction between polymer chains andAg nanoparticles on the heat properties of nano-composites. The anchoring of chain segments ontothe nanoparticles is accompanied by the decreaseof configurational entropy [18], changes of thesupermolecular structure of polymer and conse-quently the different heat properties of the nano-composites compared to the pure polymer matrix.

2. Materials and methods

AgNO3, 2-propanol and PVA with Mw = 72 000and 99% of minimal degree of hydrolysis were prod-uct of Merck. All chemicals were of analytical gradeand they were used as received. Water from Milli-pore Milli-Q system was used in all experiments.

The polymer (1.4 mass%) was completely dis-solved in boiling water. Solutions containing PVA(1.4 mass%), appropriate concentration of AgNO3

and 2-propanol (0.2 M) were bubbled with argonin order to remove oxygen. Gamma irradiationwas performed in 60Co radiation facility at roomtemperature up to the integral radiation doses of0.27, 0.53, 1.08, 2.86, 5.64, 9.32 and 12.04 kGy inorder to reduce 0.17, 0.34, 0.67, 1.71, 3.51 and5.79 mM of Ag+ ions, respectively. Dose rate wasapproximately 12 kGy/h. Nanocomposite films withcontent of inorganic phase in the concentration

range from 0.2 up to 10 mass% were obtained aftersolvent evaporation.

The content of the Ag in the Ag-PVA nano-composites was determined by atomic emissionspectrometry with inductively coupled plasma(ICP-AES, Perkin–Elmer model ICP/6500). Thecontents of inorganic phase in the nanocompositefilms were found to be 0.20, 0.40, 1.0, 2.5, 5.0, 8.0and 10.0 mass%.

Absorption spectra of Ag colloids in water andthe Ag-PVA nanocomposite films were recordedusing Perkin–Elmer Lambda 5 spectrophotometer.

Transmission electron microscopy (TEM) mea-surements were performed using Philips EM 400microscope operated at 120 kV. Samples for micros-copy analysis were deposited on C-coated Cu grid.

The X-ray diffraction (XRD) measurements ofthe Ag-PVA nanocomposites were performed onPhilips PW 1710 diffractometer.

The IR spectra of the pure PVA and the Ag-PVAnanocomposites were recorded using Perkin–Elmer983 G instrument.

Differential scanning calorimetric (DSC) analysisof the pure PVA and the Ag-PVA nanocompositeswere performed on Perkin–Elmer DSC-2 systemunder nitrogen or oxygen atmosphere. The samplesweighting about 5 mg were heated from 50 to250 �C at the heating rate of 10 �C/min.

3. Results and discussion

The advantage of gamma irradiation method forthe synthesis of metallic nanoparticles lies in the factthat desired highly reducing radicals can be gener-ated without formation of any byproduct. The pri-mary radicals and molecules produced in waterupon gamma irradiation are e�aq (2.7), OH� (2.7),H� (0.6), H2 (0.45) and H2O2 (0.7). The number inparentheses represents the respective G values, i.e.number of species formed per 100 eV of absorbedenergy. The OH� and H� radicals are capable toabstract hydrogen from the alcohol producing analcohol radical [20]. This way oxidizing OH� radicalsare transformed into reducing alcohol radicals. Inthis study 2-propanol was used to scavenge OH�

radicals. Under the stated experimental conditionsthe reduction of Ag+ ions takes place by electrontransfer from hydrated electrons and 2-propanolradicals.

Radiolytic reduction of Ag+ ions in PVA solu-tions yielded yellow colored colloids. Typicalabsorption spectra of primary Ag colloid, corre-

Fig. 1. Absorption spectra of primary Ag colloid (a), corre-sponding Ag-PVA nanocomposite film (b), and colloid obtainedafter dissolution of Ag-PVA nanocomposite film (c).

Fig. 2. Typical TEM image (a) and size distribution (b) of Agnanoparticles.

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sponding Ag-PVA film and colloid obtained afterdissolution of Ag-PVA film are shown in Fig. 1(curves a–c, respectively). Absorption spectrum ofprimary Ag colloid has surface plasmon absorptionband peaking at 410 nm. The peak position does notchange in the entire concentration range, while theintensity is proportional to the concentration ofAg nanoparticles. Absorption spectra of the Ag-PVA nanocomposite films are red shifted comparedto corresponding primary Ag colloids. Position ofthe surface plasmon absorption band in the Ag-PVA nanocomposites is independent on concentra-tion of Ag nanofiller and it is around 430 nm. Thered shift of the position of surface plasmon absorp-tion band can be explained by the change of thedielectric property of surrounding medium [3].Refractive index of water is nwater = 1.33, whilerefractive index of PVA is nPVA = 1.53 [25,26]. Itis important to point out that after dissolution ofAg-PVA nanocomposite films position of surfaceplasmon absorption band is identical with the posi-tion of starting Ag colloids (see Fig. 1c). This resultindicates that evaporation of solvent and formationof Ag-PVA nanocomposites did not induce anychange in particle size distribution.

Typical TEM image and size distribution of Agnanoparticles are shown in Fig. 2. The average sizeof Ag nanospheres was found to be 6.1 nm, esti-mated from the histogram. The particle size distri-bution is broad (the particle size is in the rangefrom 3 to 12 nm), and that is main reason why halfwidth of the surface plasmon absorption band is rel-atively large (around 90 nm).

The XRD spectrum of Ag nanoparticles incorpo-rated in PVA matrix is shown in Fig. 3. The XRDpeaks exactly matched 111, 200 and 220 crystalplanes of Ag with cubic crystal structure. The Sher-rer diffraction formula was used to estimate thecrystalline domain size (D):

D ¼ kk=b cos h ð1Þwhere k = 0.9 is for the Ag cubic structure,k = 1.541 A is the X-ray wavelength, b is the peakangular width and h is the diffraction angle. Thecrystalline domain size was found to be 7.5 nmand this result is in agreement with average diameterof Ag nanoparticles estimated by TEM.

To determine if chemical bonding between thePVA matrix and the Ag nanofiller takes place, IRmeasurements of the pure PVA and the Ag-PVAnanocomposites were performed (see Fig. 4).

Fig. 3. Typical XRD spectrum of the Ag-PVA nanocompositewith 10 mass% of inorganic phase.

Fig. 4. IR spectra of the pure PVA (a), and the Ag-PVAnanocomposite with 10 mass% of inorganic phase (b).

Fig. 5. DSC thermograms of the pure PVA (a) and the Ag-PVAnanocomposites with 1.0 and 8.0 mass% of inorganic phase (band c, respectively) in nitrogen atmosphere.

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Change in the IR spectrum of the Ag-PVA nano-composite was observed for the band peaking at1322 cm�1. These changes are more pronouncedfor the Ag-PVA nanocomposites with higher con-tent of inorganic phase. In alcohols, this band isthe result of the coupling of O–H in plane vibration(strong line at 1420 cm�1) with C–H wagging vibra-tions. Therefore, the decrease in the ratio betweenthe intensities of this band and the band at1420 cm�1 upon incorporation of the Ag nanofillerindicates decoupling between the correspondingvibrations due to interaction between Ag nanoparti-cles and the OH groups originating from the PVAchains. This result is in agreement with previouslyreported data [3,15,20].

Another change in the IR spectrum of the Ag-PVA nanocomposite was observed for the band

peaking at 1141 cm�1 which occurs as a result of asymmetric C–C stretching mode that correspondsto the crystalline regions in PVA [27]. Smalldecrease in the intensity of band peaking at1141 cm�1 was observed upon incorporation ofthe Ag nanofiller in the PVA matrix indicating thedecrease of the content of crystalline phase.

In order to study the influence of interactionbetween Ag nanofiller and PVA chains on the heatresistance and the stability of supermolecular struc-ture the changes of the specific heat capacity weredetermined by DSC measurements in nitrogen andoxygen atmosphere. DSC results from the purePVA and the Ag-PVA nanocomposites with differ-ent content of inorganic phase are shown in Fig. 5.Broad signals in the temperature range from 50 to90 �C are due to vaporization of water. It is impor-tant to notice that melting temperature (Tm) of thepure PVA film in nitrogen is higher compared tothe Tm values of the Ag-PVA nanocomposites.The Tm value decreased almost 20 �C for the high-est content of inorganic phase indicating decreaseof heat resistance of nanocomposites in nitrogen.The decrease of the Tm values of semicrystallinepolymers can be explained by the decrease ofwidth and thickness of the lamellae during thecrystallization process. The Tm value of semicrys-talline polymer is given by well-known equation[27,28]:

T m ¼ T 0m 1� 2

DH 0

rf

Lþ 2rs

D

� �� �ð2Þ

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where T 0m is the equilibrium melting temperature of

an infinitely thick crystal, DH0 is enthalpy of melt-ing, rf and rs are crystal surface free energies perunit area of fold surface and side surface, respec-tively, and L and D are thickness and width of thelamellae, respectively. PVA crystals are formed bythe folding of polymer chains. These folded chainsarrange themselves parallel to each other to forma crystal. The adsorption of polymer chains on thenanofiller may decrease the mobility of chain seg-ments resulting in decrease of crystal perfectionand consequently the heat resistance of nanocom-posite. In parallel, changes of melting temperaturecan be affected by the changes of the entropy of meltinduced by the presence of nanoparticles. Accordingto the equation for the thermodynamic melting tem-perature (Tm = DH0/DSm � DSc), where DSm is theentropy of melt and DSc is the entropy of crystalphase, anchoring of polymer segments on the nano-particles can cause decrease of the melt entropy dueto configurational constrains, thus increasing themelting temperature. In other words, the measuredmelting temperature depends on the relative ratiobetween the changes of lamellae dimensions end en-tropy of melt induced by the nanofiller.

Comparison of the specific heat capacities in oxy-gen between the pure PVA film and the Ag-PVAnanocomposites with various content of inorganicphase is shown in Fig. 6. Although heat conductiv-ity of the Ag-PVA nanocomposites are larger com-pared to the pure PVA due to the presence of silvernanoparticles (silver itself exhibits the highest ther-

Fig. 6. DSC thermograms of the pure PVA (a) and the Ag-PVAnanocomposites with 1.0 and 8.0 mass% of inorganic phase(b and c, respectively) in oxygen atmosphere.

mal conductivities among all the metals) some pres-ervation of the supermolecular structure(appearance of melting endotherm) is observed innanocomposites with higher content of inorganicphase. It seems that Ag nanofiller to some extentact as stabilizer of the supermolecular structureincreasing the heat resistance of nanocomposite.Thermooxidative degradation is accompanied byadsorption of oxygen. Most likely the presence ofadsorbed polymer chains on the Ag nanoparticlescan decrease the oxygen diffusivity and its actionin polymer degradation.

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4. Conclusion

In this study the Ag-PVA nanocomposites weresynthesized using highly reducing radicals generatedby steady state gamma irradiation followed by sol-vent evaporation. XRD and TEM measurementsrevealed that Ag particles are in nanometer sizedomain. Difference in absorption spectra betweenAg colloids and corresponding nanocomposite filmswas explained by the change of dielectric propertyof surrounding medium. IR measurements indicatedinteraction between Ag nanoparticles and PVAchain over OH groups, as well as decrease of thecontent of crystalline PVA phase with the increaseof the content of inorganic phase. Calorimetric datain nitrogen indicated decrease of the heat resistanceof nanocomposites compared to the pure PVA. Onthe contrary, in oxygen some preservation of thesupermolecular structure was observed in nanocom-posites with higher content of inorganic phase.

Acknowledgements

The authors are grateful to Dr. NikolaCvjeticanin, Dr. N. Bibic and Dr. Slavica Zec forhelp in performing the experiments. Financial sup-port for this study was granted by the Ministry ofScience and Environmental Protection of theRepublic of Serbia (Project 142066).

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