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Applied Surface Science 263 (2012) 532–535

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

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ynthesis and properties of PANI/SiO2 organic–inorganic hybrid films

ingjie Yu ∗, Jianming Xu, Jie Liu, Baoxia Li, Yongjun Liu, Yuanyuan Hanchool of Chemical Engineering, Huaqiao University, Xiamen, 361021, China

r t i c l e i n f o

rticle history:eceived 16 July 2012eceived in revised form0 September 2012

a b s t r a c t

PANI/SiO2 organic–inorganic hybrid films were prepared directly from polyaniline (PANI) emulsion solu-tion and tetraethyl orthosilicate (TEOS) by sol–gel process. The evolution of phase, morphology andcorrosion resistance ability of PANI/SiO2 hybrid materials were characterized by TG, FTIR, SEM andpotentiodynamic polarization analysis. It was demonstrated that PANI/SiO2 organic–inorganic hybrid

ccepted 20 September 2012vailable online 26 September 2012

eywords:ANI/SiO2 hybrid filmsmulsion polymerization

films have a homogeneous and smooth surface without detectable cracks. Potentiodynamic polarizationanalysis revealed that the hybrid films provided an exceptional barrier and corrosion protection in com-parison with untreated aluminum alloy substrates. However, the increase of TEOS content in PANI/SiO2

organic–inorganic hybrid materials tends to exhibit brittleness and micro-cracks which will lead to thedeterioration of corrosion resistance ability.

orrosion resistance ability

. Introduction

Polyaniline (PANI) has received extensive attention in recentears due to its excellent electrical conductivity, chemical or elec-rochemical redox reversibility and good environmental stability1–5]. It also has potential applications in many fields, such as elec-rodes, electromagnetic shielding materials, anticorrosion coatings,ensor devices, catalysts, and so on [6–8]. However there are someajor drawbacks for chemically obtained PANI, such as low sol-

bility in most of the organic solvents, low fusibility, and poorrocessability, which make it difficult to prepare cast films and limit

ts application [9]. Thus, compositing with conventional polymerso form hybrid materials have received great attention since theseew materials gather the advantages of the usual polymers andANI to obtain the flexible and unbroken transparent conductinglms, consequently increasing the technological potential of theseaterials [1,10–18].Sol–gel process is a suitable technology for preparing the

rganic–inorganic hybrid materials because of low cost, highfficiency, good uniformity and easy treatment of large andomplex shape substrates. It has been reported that PANI/SiO2rganic–inorganic hybrid films were prepared by incorporatingANI particles with TEOS and GPTMS precursor [19–21]. How-ver, water and ethanol are necessary reagent and solvent for

ol–gel process, but common PANI synthesized by the emulsionolymerization does not dissolve in them. Therefore, the self-ggregation of polyaniline particles during filtering and washing

∗ Corresponding author. Tel.: +86 592 6162300; fax: +86 592 6162300.E-mail address: qingjieyu@hqu.edu.cn (Q. Yu).

169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2012.09.100

© 2012 Elsevier B.V. All rights reserved.

process, and the dispersion of polyaniline particles in compositesmaterials have strongly affected on the morphology and propertyof organic–inorganic hybrid materials.

In the present work, we have developed a new method toobtain PANI/SiO2 organic–inorganic hybrid materials in which thepolyaniline emulsion solution prepared by emulsion polymeriza-tion without filtering and washing is performed in solution ofETOH and TEOS as precursor. The material obtained by using thismethodology is quite homogeneous and presents good mechanicalstability. The effects of the molar ratio of TEOS/ANI on the struc-ture, corrosion protection properties of the hybrid films were alsodiscussed.

2. Experimental

2.1. Materials

Aniline monomer (ANI, Aldrich, AR), sodium dodecyl sulfate(SDS, Aldrich, 99%), ammonium peroxodisulfate (APS, Aldrich, AR),were used as received. The other reagents such as tetraethylorthosilicate (TEOS, 97%), ethanol (ETOH, AR), and hydrochloricacid at 37% concentration were purchased from Tianjin KermelChemical Reagent Co. (China) and used as received without furtherpurification. Deionized water was used throughout the experi-ments.

2.2. Synthesis

Typical synthetic processes of PANI were as follows: in the typ-ical emulsion polymerization, 0.01 mol SDS and 0.01 mol APS werefirst dissolved in 20 ml of 0.1 M HC1 aqueous solution in a 250 ml

ce Science 263 (2012) 532–535 533

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plane bending mode, and the peak at 828 cm due to the C Hout-of-plane bending mode for a 1–4 substituted aromatic ring.

FTIR spectra of PANI/SiO2 hybrid films show more absorptionbands originated from siloxane monomer. The wide band centered

300 400 500 600 700 800 900

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PANI/SiO2

Q. Yu et al. / Applied Surfa

eaction vessel. Then, 0.01 mol ANI was added to another 20 ml of.1 M HC1 aqueous solution. The emulsion solution was initiatedy titrating 20 ml ANI aqueous solution to the above mixture. Theolymerization was carried out with magnetic stirring at 25 ◦C for4 h. At the end of this period the reaction mixture had turned darkreen, no precipitation of polyaniline being observed.

TEOS was dissolved in the required amount of ethanol. After vig-rous stirring for 1 h at 60 ◦C, the above PANI emulsion solution wasdded to complete hydrolysis. The molar ratios of TEOS/ANI/ETOHere 1:1:2, 5:1:10, and 10:1:20, respectively. The mixed solutionas constant stirring for 5 h at 60 ◦C. Subsequently the resultantixture was aged in a closed container for 1 h at 25 ◦C, and poured

nto polyethylene containers.1060 Aluminum alloy panels (Dalian Aluminum Manufacture

o., China) were cut to a size of 20 mm × 20 mm × 1 mm and useds the substrates for coating. The samples were cleaned with 800eshes silicon carbide paper, washed with a detergent and deion-

zed water, rinsed with acetone in an ultrasonic bath, and then driedy exposure to air.

Film deposition was carried out at room temperature by theertical dipping method using a draw speed in the range of0–12 cm/min. The coated film was then dried in air for 30 mint 25 ◦C and then heated to 140 ◦C at a rate of 0.5 ◦C/min in an ovennder N2 atmosphere.

.3. Characterization of coating films

Thermal stability of the hybrid gels pretreated at 85 ◦C for 3 has analyzed using an AUTO HI-TGA2950 system with a heating

ate of 10 ◦C/min in N2 atmosphere. The chemical transformationsf organic–inorganic hybrid were identified by FTIR (EQUINOX55,Br tablet). FTIR spectra were recorded in the 4000–400 cm−1

ange working with a resolution of 2 cm−1. UV–vis absorption spec-ra were recorded on the UV/vis spectrometer UV-2800H. Surface

icrograph of the coated samples was inspected using a scanninglectron microscopy (SEM, Hitachi S-500). The corrosion protectionf the coated films was evaluated by potentiodynamic polariza-ion in 0.5 mol/L sodium chloride aqueous solutions exposed to air.he potentiodynamic polarization was performed in a conventionallectrochemical cell, using a saturated calomel electrode (SCE) aseference and a platinum wire as counter-electrode. The exposedrea of the working electrode was 1.0 cm2. All potentials were mea-ured at 25 ◦C. The potential was varied from −1.0 V to positiveotentials at a scanning rate of 5 mV/s. Prior to the above measure-ents, the samples were kept in sodium chloride aqueous solution

or 10 min to attain a steady state.

. Results and discussion

The thermogravimetric curves (TG) of PANI and PANI/SiO2ybrid gels are shown in Fig. 1. As shown in TG curve of polyani-

ine, the weight loss before 140 ◦C is due to the loss of water. Theeight loss observed between 140 ◦C and 250 ◦C, which is due to

he de-doping of SDS. The weight loss stage in range of 250–400 ◦Cs attributed to the decomposition of excess SDS. The forth weightoss up to 400 ◦C is ascribed to the decomposition of polyanilinetself [22].

When the conducting polyaniline is reacted with inorganic pre-ursor, the polyaniline chain is dispersed into the stable Si O Sietwork structure through the hydrogen bonding interaction,hich improves the thermal stability of the hybrid materials [20].

omparing with PANI, the whole weight loss of PANI/SiO2 hybridels is less than that of the conducting polyaniline. The major dif-erence of TG curves between PANI and PANI/SiO2 hybrid gels ishat the weight loss PANI/SiO2 hybrid gels before 140 ◦C is higher

Fig. 1. The thermogravimetric curves of PANI (a) and PANI/SiO2 hybrid gels withTEOS/ANI molar ratio at 1:1 (b).

than that of the conducting polyaniline. This is because at this stage,beside the loss of water, the weight loss of PANI/SiO2 hybrid gelsalso contains the volatilization of solvent, the loss of absorbed waterand water produced by the condensation reaction between Si OHduring the heat treatment [23].

UV–vis absorption spectra of PANI and PANI/SiO2 hybrid gelsare shown in Fig. 2. Peaks around 340–360, and 420–435 nm areassigned to �–�* transition of the benzenoid segments and the pro-tonation of polyaniline chains, respectively. The absorption band at780–820 nm is corresponding to n–�* transition for the cis-isomerof the azo linkage. When TEOS is reacted with PANI, the hydroxylgroup of inorganic precursors will also form hydrogen bond withPANI. As a result, the absorption peak of n–�* transition shows ared shift in the UV–vis spectrum of PANI/SiO2 hybrid gels [20].

The FTIR spectra of PANI and PANI/SiO2 hybrid gels are shownin Fig. 3. The peak at 3400 cm−1 is due to N H stretching of aro-matic amine. The double peak at 2947 and 2878 cm−1 is attributedto the aliphatic C H stretching mode, which is due to the long alkyltail of SDS. The peaks at 1580 and 1490 cm−1 are corresponding toC C ring stretching of quinoid and benzenoid structure, respec-tively. The peak observed at 1300 cm−1 with weak ones at 1380and 1240 cm−1 is characteristic peak of intrinsic PANI indicatingC N stretching [18]. The peak at 1136 cm−1 due to the C H in-

−1

Wavelengh (nm)

Fig. 2. UV–vis spectra of PANI and PANI/SiO2 hybrid gels with TEOS/ANI molar ratioat 1:1.

534 Q. Yu et al. / Applied Surface Science 263 (2012) 532–535

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accumulation of shear stress and reduce the micro-cracks caused

ig. 3. FTIR spectra of PANI (a) and PANI/SiO2 hybrid gels with TEOS/ANI molar ratiot 1:1 (b).

t 3440 cm−1 and the band at 1640 cm−1 are respectively assignedo the stretching and bending absorption of OH group. A very pro-ounced band appearing at 1080 cm−1, together with the bandt 795 and 460 cm−1, are assigned to the vibration absorption ofi O Si group originated from TEOS, indicating that the sample isainly composed of silica network. The band at 960 cm−1 associ-

ted with the presence of non-bonded oxygen Si O− can be alsobserved.

SEM micrographs of PANI and PANI/SiO2 hybrid films with var-ous TEOS/ANI molar ratios are shown in Fig. 4. It is apparent fromig. 4a that the size range of PANI particles is less than 50 nm, and

hese PANI particles self-aggregate or spontaneous coalescence toorm floccule-like structure. It can be found from Fig. 4b–d thatEOS/ANI molar ratio has a strong effect on the morphology of

Fig. 4. SEM images of PANI (a) and PANI/SiO2 hybrid films with di

Fig. 5. Potentiodynamic polarization curves of Al alloy (a) and PANI/SiO2 hybridfilms with various TEOS/ANI molar ratios: (b) 10: 1; (c) 5:1; and (d) 1:1.

hybrid films. With high TEOS/ANI mole ratio (Fig. 4b), PANI/SiO2hybrid films tend to exhibit brittleness and micro-cracks. Thepresent of micro-cracks formed during heat treated process isas a result of the volume shrinkage of silicon network caused bythe dehydration of gel and the evaporation of solvent. With thedecrease of TEOS/ANI molar ratio, micro-cracks on the surfaceof PANI/SiO2 hybrid films gradually decline. Especially whileTEOS/ANI molar ratio is 1:1, PANI/SiO2 hybrid films have a homo-geneous and smooth surface without detectable cracks (Fig. 4d).This micro-structural characteristic could be explained by theincrease the flexible organic composition which could relax the

by the volume shrinkage of silicon network.Fig. 5 shows the potentiodynamic polarization curves of Al alloy

without coating and with PANI/SiO2 coatings. It is obvious that all

fferent TEOS/ANI molar ratios: (b) 10:1; (c) 5:1; and (d) 1:1.

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oated samples exhibit dramatically low corrosion current densityhan the uncoated one. The results reveal that the anticorrosionehavior of the PANI/SiO2 hybrid films is far better than that of thentreated aluminum alloy sample. The presence of the films on theubstrates can form a stable barrier layer at the interface which isery effective in reducing the critical current density, and result-ng in good barrier protection properties. Evidently, the decreasef the TEOS content will result in better corrosion resistance abil-ty as it was indicated by the decline of corrosion current density.he improvement of corrosion resistance ability will be mainlyttributed to the decrease of micro-cracks and the production ofenser films that segregate the permeation of corrosive mediumnd are correspondingly less susceptible to localized pitting.

. Conclusion

In this paper, we present a new way to synthesize PANI/SiO2rganic–inorganic hybrid materials in which the polyanilinemulsion solution prepared by emulsion polymerization with-ut filtering and washing is performed in solution of ETOH andEOS as precursor. The PANI/SiO2 organic–inorganic hybrid mate-ials obtained by using this method are quite homogeneous andresent exceptional corrosion protection properties. It was demon-trated that for PANI/SiO2 organic–inorganic hybrid materials, lowEOS/ANI molar ratio is prone to obtain a smooth surface andxceptional corrosion protection properties, while high TEOS/ANIolar ratio tends to exhibit brittleness and micro-cracks and leads

o the deterioration of corrosion resistance ability.

cknowledgements

This work was financially supported by Natural Science Foun-ation of Fujian Province of China (Grant No. 2011J01051) and the

[[[[

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Fundamental Research Funds for the Central Universities (Grant No.JB-ZR1224).

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