The preparation of polyaniline waterborne latex nanoparticles and their films with anti-corrosivity and semi-conductivity

Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 111–120

The preparation of polyaniline waterborne latex nanoparticles and their films with anti-corrosivity and semi-conductivity Xin-Gui Lia,b,c,∗ , Mei-Rong Huanga , Jian-Feng Zenga , Mei-Fang Zhub a

b

Institute of Materials Chemistry, The Key Laboratory of Concrete Materials Research, College of Materials Science Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China The State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 200051, China c The Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, China Received 16 March 2004; accepted 28 August 2004

Abstract An emulsion polymerization of aniline was performed in micellar solution of sodium dodecylbenzene sulfonate (SDBS), dodecylbenzene sulfonic acid (DBSA), or sodium dodecylsulfate (SDS) as both emulsifier and dopant to obtain stable nano-polyaniline (PAN) waterborne latexes. The direct film formability and anti-corrosivity of PAN latexes were studied. The uniform PAN–SDS/polyvinyl alcohol (PVA) composite films were prepared by casting a direct mixture of the PAN latex and PVA aqueous solution. The emulsifier species and concentration, mixing way of emulsifier with aniline, and polymerization solution acidity, have a remarkable influence on the diameter and stability of PAN latex particles. The molar ratio of emulsifier over aniline was optimized for the synthesis of the PAN latex with small particle size and good properties including film formability, anti-corrosivity, and electrical conductivity. The smallest diameters of PAN–SDBS, PAN–DBSA, and PAN–SDS spherical particles determined by laser particle analyzer and electron microscopes are 2500, 40 and 5 nm, respectively. A good direct film formability of the PAN latex nanoparticles was found. The film formed by solution-casting method is thin, smooth and metal-lustrous. The electrochemical impedance spectroscopy was used to evaluate the impedance of the PAN latex films sandwiched between acrylic resin and iron sheet. It is found that the PAN has excellent anti-corrosivity, implying a possibility of direct application as metal anti-corrosive coating. A semi-conductive PAN/PVA nanocomposite film containing a very small amount of PAN nanoparticles exhibits low percolation threshold of PAN concentration of 0.1 wt.%. © 2004 Elsevier B.V. All rights reserved. Keywords: Emulsion polymerization; Polyaniline nanoparticle; Metal anti-corrosion; Polyaniline/polyvinyl alcohol composite film; Semi-conductivity

1. Introduction Polyaniline (PAN) has been considered as one of the most important conducting polymers for various electrochemical, electrorheological and electronic applications to rechargeable batteries, sensors, controlling systems and organic displays [1–4] because of its facile synthetic process, good environmental stability, easy conductivity control and cheap production in large quantities. However, the conductive form of PAN is difficult to be processed because it is ∗

Corresponding author. Tel.: +86 21 65980524; fax: +86 21 65980530. E-mail address: [email protected] (X.-G. Li).

0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.08.077

both insoluble in common organic solvents except for Nmethylpyrrolidone (NMP) and unstable at melt-processing temperature, which greatly limits its future wide applications. To solve the intractable processing problem, several modification researches such as introducing side groups [5–10], doping with functional dopants [11,12], blend and composite [13,14], and preparing dispersed particles [15–21], have been done. PAN dispersion is one of the attractive alternatives to overcome the poor processability, which could also be directly utilized as a key component of the blend and composite materials exhibiting high performance. Specially, the nano-PAN waterborne latex not only remains the good property of PAN

112

X.-G. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 111–120

but also greatly improves its processing capability as well as reduce environmental impact. Furthermore, it possesses some special properties of the nanoparticles. As to applications, this kind of waterborne latex is suitable to be used as PAN coating, which is much better than the coatings based on organic solvent or strong acid because the water in the PAN coating does not pollute the environment. Therefore, the nano-PAN waterborne latex has become a hot research point in materials science. The synthesis of nanoscopic PAN in colloids form in the presence of a water-soluble polymers such as poly(vinyl alcohol) (PVA), poly(N-vinylpyrrolidone), poly(vinyl methyl ether), poly(ethylene oxide) (PEO) or cellulose ethers has been reported [15–17,22]. The water-soluble polymers acted not only as steric stabilizers but also as co-monomers. However, sometimes the dispersion polymerization still resulted in macroscopic precipitation and, in certain cases, in a low yield of small colloidal particles. Thus, it is not easy to get the pure polymers with these methods and the nanoscopic microspheres obtained are often copolymers or composites because the separation between the PAN and the water-soluble polymers is very difficult. One facile method of synthesizing nanoscopic PAN as a waterborne coating is microemulsion polymerization containing a large amount of nanoscopic micelles. The micelles can change the local environment by aligning and absorbing the monomer and adjusting polymerization reaction, and may yield polymer microspheres with improved properties. Han et al. have prepared PAN nanoparticles in DBSA and SDS micellar solutions, respectively [18,19]. However, a direct film formability and anti-corrosion property of this kind of PAN latex have not been investigated. A study on a facile fabrication of a nanocomposite film of the nano-PAN latex and PVA exhibiting low percolation threshold is not found either. The overall goal of this article is to synthesize nano-PAN waterborne latexes and investigate their structure, properties and possible applications. Two series of waterborne latexes, nano-PAN–DBSA and nano-PAN–SDS, were prepared by emulsion and microemulsion polymerisation, respectively, in which DBSA and SDS were used both as surfactants and dopants. The size and shape of PAN micelles were observed in detail. These two kinds of nano-PAN latexes were directly cast into homogeneous films and nanocomposite films. The electrochemical impendence, anti-corrosion ability, and electrical conductivity of the films were characterized for the first time. The potential application of the PAN latexes is mentioned.

2. Experimental 2.1. Chemicals Aniline (AN), ammonium persulfate (APS), sodium dodecylbenzene sulfonate (SDBS), dodecylbenzene sulfonic acid

(DBSA), sodium dodecylsulfate (SDS), acrylic resin, and HCl were of analytical reagent and commercially obtained. 2.2. Preparation of nano-PAN waterborne latexes 2.2.1. Emulsion polymerization in dodecylbenzene sulfonic acid (DBSA) micellar solution Aqueous micellar dispersions were prepared by introducing DBSA into distilled water (80 mL) with a slow stirring. Then AN monomer was added drop-wise to the solution and a white turbid emulsion of DBSA and AN was formed. After the micellar dispersion was kept stirring for longer than 120 min, an oxidative polymerization was performed at 5 ◦ C for 12 h by a drop-wise addition of APS aqueous solution (20 mL) into the micellar solution at a fixed total APS/AN molar ratio of 0.5. 2.2.2. Microemulsion polymerization in sodium dodecylsulfate (SDS) micellar solution The 0.1 M HCl solution was used as an aqueous phase in SDS micellar solution. First, 100 mL 0.1 M HCl was used to dissolve SDS for the preparation of aqueous micellar dispersions. Then AN monomer was added drop-wise to the dispersions. After stirring this AN transparent microemulsion for some time, 10 mL 0.1 M HCl solution containing APS as oxidant was added drop-wise at an adding rate of a drop (around 60 ␮L) per 3 s into the SDS micellar solution. After the induction period of about 40–60 min, the homogeneous transparent reaction mixtures turned into blue and its coloration was pronounced as polymerization proceeded. The molar ratio of APS to AN was kept as 0.5. Since the Krafft point of SDS is around 16 ◦ C, the polymerization was performed at 20 ◦ C. 2.3. Direct film fabrication from nano-PAN waterborne latex The free-standing films were made by casting nano-PAN waterborne latex on a clean, dry glass with the area of 2 cm × 2 cm and drying under an infrared lamp at 60 ◦ C to remove the solvent. After drying for at least 24 h, these films were immersed in water for 30 min to peel the films from the glass. 2.4. Preparation of PAN/ACR coating on iron plate A tinplate containing 99.9% iron with respective area and thickness of 2 cm × 10 cm and 0.8 mm was used as testing samples. One side of the samples was polished by 100 grit emery paper. Prior to coating, all samples were degreased with acetone to remove impurities. The nano-PAN waterborne latex was cast drop-wise on the pretreated iron samples. After drying in oven at 60 ◦ C for 24 h, these PAN/iron samples were coated with acrylic resin as a top layer. These composite coatings were then dried in the same condition

X.-G. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 111–120

for another 24 h. The other side and edges of the iron samples were sealed with olefin before being characterized by electrochemical impedance spectroscopy (EIS). 2.5. Preparation of PAN/PVA composite films The 5-wt.% PVA solution was obtained by adding 5 g PVA into 95 g distilled water with vigorous stirring at 85 ◦ C. A mixture of nano-PAN waterborne latex and PVA solution was prepared by mixing these two solutions directly at room temperature. The resultant volume and solid content of the whole mixture were controlled to 2 mL and 3%, respectively. The PAN/PVA composite films were prepared by casting the mixture solutions onto 5 cm × 5 cm glass substrates and drying at 40 ◦ C for 48 h prior to measurements. 2.6. Measurements The size and morphology of PAN particles in their latexes were observed by HITACHI model H800 transmission electron microscope (TEM) and field emission scanning electron microscopy (FE-SEM, Jeol Model JSM-6340F). Samples for TEM observation were prepared by dropping highly diluted latex onto the carbon-coated copper grid and dried in a desiccator at room temperature. SEM samples were sputter-coated with gold for 60 s. The size of the PAN latex particles in water was analyzed using LS230 laser particle size analyzer from Beckman Coulter Inc. Samples for FT-IR characterization were made with two different methods. Method 1: original PAN waterborne latex was directly dried in an oven at 60 ◦ C for 48 h to get solid PAN particles. Method 2: original PAN waterborne latex was first precipitated by excess amount of methanol, then washed with ethanol, and finally dried at 60 ◦ C for 48 h. The FT-IR spectra (Nicolet Magna-IRTM 550 spectroscopy) of these two kinds of powders were recorded by the KBr pellet technique. EIS experiments were carried out in a three-electrode cell using 1 M NaCl as background electrolyte [23,24]. A saturated calomel electrode (SCE) was used as reference electrode. The ACR/PAN/iron samples were used as working electrode and a platinum electrode with a foil of area 0.8 cm2 as counter electrode. The EIS testing system consists of EG&G Amplifier & Filter model 5208. The potentiostatic model M273 and the Impedance Spectrum Analyzer with EIS M398 software were employed to measure and analyze their electrochemical impedance spectra at ambient temperature. In this work, the potential amplitude of ac was kept at 5 mV and its frequency ranged from 100 kHz to 10 Hz. In our study, the dried samples were immersed in the 5% NaCl solution for 10 min before the testing, so that the open-circuit potential of the coated substrate could be stable during EIS measurements. The electrical conductivity of PAN/PVA composite films was measured using a digital multimeter. The bulk resistance was measured when the film was clamped tightly between the two electrodes with the area of 0.785 cm2 . The film thickness was measured with a continuous thickness detector. The DC conductivity (␴) could be

113

calculated according to the following Eq. (1): σ=

L RS

(1)

where R represents bulk resistance; L the film thickness; and S the electrode area. Therefore, the specific sheet resistance (Rs ) could be calculated according to Eq. (2): Rs =

1 σL

(2)

3. Results and discussion 3.1. Emulsion polymerization of AN in DBSA and SDS–HCl micellar solutions The emulsification of AN and surfactants and emulsion polymerization are followed by the pH values of the reaction solutions, which might provide an insight into polymerization process. It is seen from Fig. 1 that the emulsification of AN–DBSA aqueous solution is accompanied by a remarkable increase in pH value, suggesting that a strong interaction between alkaline AN and acidic DBSA occurs, such as the protonation of AN. As the APS solution was dropped, the solution pH increased further and then gradually reached up to 5.4. The solution color also changed from yellow, lightly green to green at about 8 h. These phenomena strongly imply an occurrence of the AN oxidative polymerization and the formation of PAN emeraldine. The variation of the solution pH with reaction time shows that the oxidative polymerization was substantially finished at 8 h. As the polymerization proceeded, the color of the AN–SDS solution also changed from yellow, brown, blue, and finally to green at 5 h, which indicates the formation of PAN emeraldine salt (ES). Note that the reaction mixture was transparent from the beginning to the end, which is quite different from turbid emulsion polymerization of AN in DBSA solution. The pH of this solution in Fig. 1 slightly increased as the reaction proceeded. Apparently, the enhancement of the pH value is much lower than that in DBSA micellar solution, because of the presence of HCl as polymerization medium. It

Fig. 1. The changes of solution pH value with reaction time. (a) In DBSA micellar solution at 5 ◦ C; and (b) in SDS micellar solution at 20 ◦ C.

114

X.-G. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 111–120

Fig. 2. FT-IR spectra for PAN–DBSA and –SDS particles of the sample numbers D3 and S8.

appears that the solution pH is basically constant after oxidative polymerization of 10 h, indicating a completion of the oxidative polymerization at 10 h. Apparently, the polymerization rate of AN is slower in SDS–HCl aqueous solution than in DBSA aqueous solution. 3.2. Molecular structure and size of PAN latex nanoparticles

Fig. 3. SEM microphotographs of PAN–DBSA latex particles of (a) D2 sample and (b) D5 sample.

FT-IR spectra for two kinds of powder samples of PAN–DBS A and PAN–SDS are shown in Fig. 2. The purified samples corresponding to curves b and d exhibit substantially same IR spectra as general PAN [25] except for a peak at 1125 cm−1 assigned to S O stretching mode of sulfonic acid that indicates a doped PAN by DBSA. However, unpurified original samples illustrate stronger peaks from 2800 to 3000 cm−1 due to aliphatic C H stretching mode on long alkyl tail of DBSA and SDS. Two unpurified samples also illustrate different shapes and wave numbers of the peaks attributed to C C stretching of benzenoid and quinoid

rings, suggesting their different structures each other, possibly due to the different interactions between the PAN chains and DBSA or SDS. Table 1 summarized the size of PAN latex particles formed at five DBSA/AN molar ratios. Fig. 3 shows the SEM micrographs of PAN latex particles of samples D2 and D5. It can be seen from Fig. 3(a) that the shape of PAN latex particles is basically globular and their diameter ranges from 80 to 100 nm. These particles are different from larger needle-like crystals of polyanilinium-DBSA complex at higher molar ratio of

Table 1 The size and direct film formability of PAN particles obtained by emulsion polymerization of AN in DBSA micellar system at a fixed APS/AN molar ratio of 0.5 Sample numbers

DBS A/AN molar ratio Particle size determined by SEM (nm) Film-forming ability Film appearance Film color Film metal luster

D1

D2

D3

D4

D5

0.5 800–1000 Fair Porous, rough Dark green No

0.8 80–100 Nice Sparse, slightly smooth Green Slight

1.0 60–70 Good Dense, smooth Green Evident

1.2 50–60 Good Dense, smooth Green Evident

1.5 40–50 Good Dense, smooth Green Evident

X.-G. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 111–120

115

Table 2 The size of PAN particles obtained by microemulsion polymerization of AN/SDS micellar system at APS/AN molar ratio of 0.5 in 0.1 M HCl Sample numbers

AN concentration (M) SDS/AN molar ratio Particle size determined by TEM (nm)

S1

S2

S3

S4

S5

S6

S7

S8

S9

0.12 1.7 >2000

0.03 1.7 2000

0.09 2.2 >2000

0.03 2.2 1000

0.06 3.3 >2000

0.03 3.3 300–400

0.04 5.0 800–1000

0.03 5.0 5–20

0.03 6.7 10–20 [17]

DBSA/AN (1/1), which even could be observed by optical microscopy [26]. But a few particles are not definitely regular in shape and the their size is up to one micrometer. The formation of large particles may be ascribed to the aggregation of small particles during the precipitation of the latexes for SEM observation. The particles of sample D5 shown in Fig. 3(b) exhibit rice-like shape. The size of some isolated particles seems to be below 50 nm, which is smaller than that of sample D2, probably owing to the comparatively higher DBSA content in sample D5 than D2. At the same time, the reaction speed is accelerated in lower pH media caused by relatively higher DBSA content. As a result, the deviation of latex particle shape from sphere could not be excluded. TEM microphotographs of PAN latex particles from SDS micellar system are shown in Fig. 4. PAN–SDS particles are clearly observed to be spherical and exhibit much smaller size and much narrower size distribution than PAN–DBSA particles in Fig. 3. In particular, the diameter of PAN–SDS particles formed at lower SDS/AN ratio of 5.0 in this study is 5–20 nm which is even smaller than that of the PAN–SDS particles prepared at higher SDS/AN ratio of 6.7 [19].

the synthesis of nano-PAN waterborne latexes should be 0.8 and 3.9, respectively. That is to say, the DBSA is much more efficient for the preparation of the nano-PAN waterborne latexes than the SDS–HCl. However, the PAN latex particles

3.3. Influence of polymerization conditions on the formation of nano-PAN latexes As can be seen from Tables 1 and 2 and Fig. 5, the molar ratio of emulsifier to AN has a great affection on the PAN particle size. The size of both PAN–DBSA and PAN–SDS particles decreased with increasing DBSA/AN or SDS/AN molar ratio. According to the nucleation mechanism of micelles in emulsion, nucleation plays an important role in the formation of particles, and the number of the resultant latex particles was controlled by the number of micelles. In principle, at a fixed AN concentration, the larger the number of micelles, the smaller the size of latex micelles is. So, as the emulsifier content increases, the number of micelles increases, leading to a reduced particle size. On the contrary, it can also be found from Fig. 5 that the PAN latex particle size increased sharply to one or a few micrometers at the DBSA/AN molar ratio below 0.8 and SDS/AN below 5. The reason could be attributed to lower emulsifier content, resulting in the instability of smaller latex particles. To stabilize the smaller latex particles, it is necessary for the particles to absorb more emulsifier molecules from the solution nearby. This suggests that at fixed oxidant content, the critical DBSA/AN and SDS/AN molar ratios for

Fig. 4. TEM microphotographs of PAN–SDS latex particles of S8 sample.

116

X.-G. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 111–120

revealing a great discrepancy in formation and stabilization of PAN latex particles caused by DBSA, SDS, and SDBS. Therefore, it can be concluded that a reduced order of the size of the PAN latex particles stabilized by three emulsifiers usually ranks as follows: SDBS–HCl > DBSA > SDS–HCl

Fig. 5. The influence of emulsifier/AN molar ratio on dried PAN particle sizes in the DBSA or SDS micellar system.

Fig. 6. The effect of HCl concentration on the size of PAN–SDBS latex particles in water at a fixed SDBS/AN and APS/AN molar ratios of 1 and 0.5, respectively, at 1–2 ◦ C by laser particle size analysis.

formed at the SDS/AN molar ratio of 5 exhibit the smallest size. As shown in Fig. 6, the size of PAN latex particles dramatically gets larger with the increase of HCl concentration. This suggests that the ionization of SDBS molecules in HCl solution would be depressed with elevating HCl concentration, which weakens the electric repulsion between surfactant head groups. This weak electric repulsion might prevent the aggregation of smaller particles with difficulty and then the formation of larger particles. In addition, at higher HCl concentration, the formation rate of AN radical cations at a fixed AN concentration becomes slower, resulting in less polymerizing sites or reactive nuclei, and then larger PAN particles. Figs. 5 and 6 clearly illustrate a great difference in the sizes of PAN–DBSA, PAN–SDS, and PAN–SDBS particles,

The way of adding the monomer into the micellar solution significantly affected the stability of PAN waterborne latexes. It is found that the addition of the AN monomer into the micellar solution in one portion generally resulted in the formation of big PAN particles that can be seen by naked eyes. This result may be explained by the followed two causes. One is that some latex particles aggregated together because of very exquisite polymerization. The other is that there is not enough time for some AN monomer to enter into the DBSA or SDS micelles. Therefore, the AN monomer still stayed in the continuous aqueous phases. Apparently, it is much easier for PAN particles to get together in the absence of protection from emulsifier molecules. Contrary to the way of adding monomer one time, the way to add monomer drop-wise could obtain homogeneous and transparent solution-like nano-PAN waterborne latex. Furthermore, this kind of latex could keep high homogeneity for at least several months. That is to say, the nano-PAN latexes formed in this way exhibit much higher stability than those in literature [27]. The way to add AN drop by drop could always form smaller and more stable nano-PAN latexes because the AN monomer has enough time to diffuse into micelles homogenously, finally leading to a smooth polymerization in the micelles. As listed in Table 3, stirring time of AN emulsion before adding oxidant strongly influences the stability of PAN–DBSA latexes obtained later. It is found that the latexes get more homogeneous and transparent with prolongating stirring time. This is related to the degree of uniformity extent of AN diffusion into micelles. If the stirring time is too short, the AN would not have adequate time to uniformly and sufficiently enter into the micelles, resulting in the polymerization of AN outside the micelles, and finally affecting the stability of PAN latexes. Approximate stirring time of AN emulsion before adding oxidant should range from 120 to 150 min for the preparation of homogeneous and semitransparent PAN nano-latexes. Table 3 also shows that the stability of the PAN–DBSA and PAN–SDS latexes exhibits similar dependency of stirring time.

Table 3 The effect of stirring time of aniline emulsion before polymerization on the stability of PAN–DBSA micellar system Stirring time of aniline emulsion before adding oxidant (min)

Appearance of PAN–DBSA latex after polymerization for 12 h

Appearance of PAN–DBSA latex on the 30th day after polymerization

30 60 120 150

Some aggregates Some floc Basically homogeneous and semi-transparent Homogeneous and semi-transparent

Demulsion or precipitation Demulsion or precipitation Homogeneous and semi-transparent Homogeneous and semi-transparent

X.-G. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 111–120

117

Table 4 Direct film formability of nano-PAN waterborne latexes obtained using SDS–HCl as emulsifier Sample numbers

Film formability Film appearance Film color Film metal luster

S2

S4

S6

S8

S9

Bad Porous, rough Dark green No

Bad Porous, rough Dark green No

Good Sparse, slightly smooth Dark green Fair

Excellent Dense, smooth Dark green Evident

Excellent Dense, smooth Dark green Evident

3.4. Direct film formability of nano-PAN waterborne latexes Tables 1 and 4 present the apparent characters of the films formed by directly casting nano-PAN–DBSA and –SDS waterborne latexes. It appears that the film-forming ability of the latexes becomes better with increasing emulsifier content or decreasing PAN particle size. The films of PAN latexes with smaller size exhibit evident metal luster as well as dense and homogeneous structure that has been observed by optical microscope. A possible process of film formation of latex nanoparticles could be described as follows: the evaporation of water will first result in an appropinquity between the PAN latex particles. With a further water evaporation, the latex particles may shrink gradually and then get closer and closer, leading to a uniform mergence of the adjacent particles, finally forming homogeneous film. Note that the emergence of the larger latex particles must accompany with the formation of some defects, such as microvoids and pinholes between the particles, leading to higher surface roughness. In summary, the smaller the particle size, the stronger the film formability is, because PAN nanoparticles possess tremendous specific surface area as well as the surfaces of the doped PAN nanoparticles are highly charged. These specific features of PAN nanoparticles provide an original driving force for the self-assembly of nanomaterials including anticorrosion coating and semi-conducting composite films.

tail’ demonstrates that the corrosion process has been transformed from electrochemical control to diffusion control, indicating that the coatings have been at the last immersion stage. Zre– and Zim–ω−1/2 curves present a characteristic

Fig. 7. The dependency of the modulus of complex number of the impedance of ACR/iron and ACR/PAN/iron on the testing frequency.

3.5. Inhibition of iron corrosion by PAN/ACR coating The modulus (|Z|) of complex number of the impedance of ACR/iron and ACR/PAN/iron shows a dependency of the testing frequency in Fig. 7. It is seen that ACR/PAN/iron exhibits much higher |Z| than ACR/iron. Among them, ACR/PAN(D5)/iron containing the smallest latex particles (D5) exhibits the highest |Z| value in the higher frequency range. Figs. 8 and 9 show the electrochemical impedance spectra of ACR/iron and ACR/PAN/iron. Fig. 8 is a Nyquist diagram that illustrates the relationship between real part and imaginary units of impedance (Zre and Zim, respectively). Fig. 9 displays a relationship between Zre or Zim and ω−1/2 , where ω represents angle frequency. From the Nyquist diagram, it can be seen that all impedance curves have a semicircle in the high-frequency region and a ‘diffusing tail’ in the low-frequency region. This impedance arc is attributable to the transport resistance through the coating. The ‘diffusing

Fig. 8. Nyquist diagram for ACR and PAN/ACR coatings on iron after immersed in 1 M NaCl for 120 min. (a) ACR and (b) ACR/PAN(D2) and ACR/PAN(D5).

118

X.-G. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 111–120

Fig. 9. Zre– or Zim–ω−1/2 diagram for (a) ACR and (b) ACR/PAN(S8) coatings.

Fig. 10. Equivalent circuits for iron panel coated with ACR or ACR/PAN coatings.

beeline in the low-frequency region, also verifying the diffusion control. Therefore, it can be deduced that the equivalent circuit for iron panel coated with PAN/ACR coatings should be illustrated in Fig. 10. The electrochemical parameters of corrosion educed by the electrochemical impedance spectra in Figs. 8 and 9 with EIS M398 software were summarized in Table 5. Chargetransfer resistance (Rt ), coating resistance (Rc ), and doublelayer capacitance (Cdl ) can be used to jointly represent the electrochemistry of corrosion at the coating/metal interface after coating penetration by the corrosion species [28]. It is found that ACR/S4 and ACR/S6 coatings exhibit basically similar corrosion parameters as single pure ACR coating, in-

dicative that the insertion of S4 and S6 intermediate layers could not improve anti-corrosion performance. However, the insertion of S8, particularly, D2 and D5 coatings, will largely increase Rt , Rc and impedance coefficient or decrease Cdl , suggesting that S8, D2, and D5 coatings indeed offer much higher anti-corrosive performance than pure ACR layer if a small difference in thickness is neglected. Therefore, it is reasonable to infer that there has been a dense passive ironoxide layer formulated on the metal surface. According to metal corrosion theory, there is an interface capacitance between metal and electrolyte solution. Its value has something to do with the some factors such as metal surface state or solution components. In a given system, the change of interface capacitance could reflect the transition of metal surface state. There is a ‘space charge layer’ in the passive iron-oxide layer, which works as a new capacitance. When the passive ironoxide layer came into being, it corresponded to inserting a new capacitance between metal and electrolyte. At this time, the interface capacitance is composed of the new capacitance in series with the double-layer capacitance. Since the space charge layer exhibits much lower capacitance than the double layer, the interface capacitance decreases sharply. The formulation of passive layer further approves that nano-PAN waterborne latex film possesses excellent protection properties and anti-corrosive performance. And a passivation condition at the coating–metal interface created by the PAN coating is also confirmed. Note that both the Rt and Cdl values in this work are higher than literature value [29] at the analogical conditions. Note that the S8 coating gives stronger anti-corrosivity than S4 and S6 coatings because of its smallest latex particle size and higher film formability. D2 and D5 coatings show better anti-corrosive performance than S8 coating because the D2 and D5 coatings contain much less emulsifier, leading to relatively higher film-forming ability and then more homogeneous and denser film structure. 3.6. Semi-conductivity of PAN/PVA composite films The semi-conductivity and percolation threshold of PANcontaining composite films depend significantly on the size and morphology of PAN particles. The direct availability of PAN latex nanoparticles provides the possibility to facilely prepare semi-conductive composites with low percolation threshold. The effect of the PAN–SDS or –DBSA loading

Table 5 The best-fitting values of the equivalent circuit element analyzed from Nyquist diagram and Zre– or Zim–ω−1/2 diagram of the ACR/PAN/tin plate samples ACR/PAN (layer thickness, (␮m)

Coating resistance Rc (k /cm2 )

Charge transfer resistance Rt (k /cm2 )

Double-layer capacitance Cdl (␮F/cm2 )

Warburg impedance coefficient (k /cm2 s1/2 )

ACR(217) ACR/S4(220/10) ACR/S6(234/10) ACR/S8(243/10) ACR/D2(220/10) ACR/D5(215/10)

1.817 0.250 1.245 375 515.2 607.0

1.482 1.374 3.799 517.2 596.8 628.1

9.831 10.56 25.45 0.102 4.30 × 10−6 5.43 × 10−6

1.515 2.738 2.225 834 3149 2865

X.-G. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 111–120

119

est PAN–SDS content (0.1 wt.%). This foreshows that the PAN/PVA composite films have good antistatic performance.

4. Conclusions

Fig. 11. Effect of PAN–SDS and –DBSA loading on the conductivity and sheet resistance of PAN/PVA composite films with thickness of 10–22 ␮m.

on the room-temperature conductivity of PAN–SDS(S8) or –DBSA(D5)/PVA composite films is shown in Fig. 11. As the PAN–SDS content in the films increased from 0 to 0.1 wt.%, the conductivity sharply increases from 1.65 × l0−15 to 1.10 × l0−7 S/cm for PAN–SDS(S8)/PVA film and to 4.93 × l0−12 S/cm for PAN–DBSA(D5)/PVA film. With further increasing PAN content to 2.0 wt.%, the conductivity of the both films gradually increases to 4.4 × l0−5 for PAN–SDS(S8)/PVA film and 1.2 × l0−8 S/cm for PAN–DBSA(D5)/PVA film. Therefore, the percolation threshold (Pc ) of the PAN concentration for film conductivity was found to be 0.1 wt.%. This Pc value is much lower than that in composite films containing PAN particles prepared by dispersion polymerisation [30–32]. Note that Banerjee and Mandal [15] have depressed the Pc to an extremely low value of 0.043 wt.%. Such a low Pc value should be attributed to the novel properties of carefully prepared PAN nanoparticles and is not commonly obtainable through a direct addition of PAN latex nanoparticles in the present work. It should be noticed that the PAN–SDS (2.0 wt.%)/PVA film exhibits higher conductivity of 4.4 × 10−5 S/cm than that (5.6 × l0−6 S/cm) of PAN (7.8 wt.% or higher)/PVA film reported in refs. [32,33]. The establishment and enhancement of conductivity of the composite films with introducing PAN nanoparticles are attributed to the formation of conductive paths through the films. The direct addition of PAN latex nanoparticles into PVA solution is not only simple but also predominant for the fabrication of PAN nanoparticle composite film exhibiting higher electrical conductivity. Specific sheet resistance (Rs ) is defined as the resistance per unit area, which can be used to evaluate the antistatic performance of materials to some extent. The (Rs ) value has been calculated based on the electrical conductivity of PAN–SDS and –DBSA composite films and shown in Fig. 11. It is seen that the (Rs ) value of the films sharply decreases with inducting PAN. Particularly, the PAN–SDS/PVA composite film exhibits much lower specific sheet resistance (7.58 × 109 ) than the pure PVA film (5.51 × 1017 ) even at the low-

PAN waterborne nano-latexes have been successfully prepared by using DBSA and SDS as surfactant and dopant, respectively. The mean diameter of PAN–DBSA and PAN–SDS spherical particles is 40–50 and 5–20 nm, respectively. The optimal molar ratios of DBSA and SDS to AN for the synthesis of the PAN latex with nanoscopic particle size and good properties are 1.5 and 5.0, respectively. The stirring time of AN emulsion before adding oxidant and the way to add monomer or oxidant have a great influence on the properties of resultant latex particles. Relatively long stirring time of resulting AN emulsion and a drop-wise addition of AN into surfactant solution are preferred. The size of the PAN latex particles decreases significantly with increasing emulsifier concentration or decreasing the system acidity. The article gives a good method of synthesizing very small nanoparticles with the diameter down to 5 nm that is usually very difficult to be achieved. The PAN waterborne latex nanoparticles exhibit good direct film formability. The film obtained is smooth and also metal-lustrous. The PAN nano-latex has better anticorrosivity, which could be directly applied as anti-corrosion coatings of metals. The PAN–SDS/PVA composite films containing a very small amount of PAN nanoparticles show semiconductivity with a low percolation threshold of 0.1 wt.%. As compared with pure PVA film, the PAN–SDS/PVA composite films show much lower sheet resistance, implying an ability to dissipate electric charge, and realizing a facile semiconducting functionalization of traditional PVA.

Acknowledgements The project was supported by (1) the National Natural Science Foundation of China (20174028); (2) the Foundation of Nano Science and Technology Project of Shanghai China (0259nm022); (3) the Foundation of Shanghai Leading Academic Discipline, Donghua University, China; and (4) the Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, China. The authors would like to thank Prof. Dr. Dong-Yuan Zhao and He-Yong He (Fudan University) for their valuable assistance.

References [1] A.J. Heeger, Synth. Met. 125 (2002) 23–42. [2] M.S. Cho, Y.H. Cho, H.J. Choi, M.S. Jhon, Langmuir 19 (2003) 5875–5881. [3] X.-G. Li, M.-R. Huang, W. Duan, Y.-L. Yang, Chem. Rev. 102 (2002) 2925–3030. [4] M.-R. Huang, X.-G. Li, J.F. Zeng, W. Zhang, Mod. Chem. Ind. 22 (12) (2002) 10–22 (Chinese).

120

X.-G. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 111–120

[5] A. Falcou, A. Longeau, D. Marsacq, P. Hourquebie, A. Duchene, Synth. Met. 101 (1999) 647–648. [6] M.-R. Huang, X.-G. Li, Y.-L. Yang, X.-S. Wang, D. Yan, J. Appl. Polym. Sci. 81 (2001) 1838–1847. [7] M.-R. Huang, X.-G. Li, Y.-L. Yang, Polym. Degrad. Stabil. 71 (2001) 31–38. [8] X.-G. Li, M.-R. Huang, W. Feng, M.-F. Zhu, Y.-M. Chen, Polymer 45 (2004) 101–115. [9] X.-G. Li, H.-J. Zhou, M.-R. Huang, M.-F. Zhu, Y.-M. Chen, J. Polym. Sci. Part A, Polym. Chem. 42 (2004) 3380–3394. [10] X.-G. Li, M.-R. Huang, Y.-M. Hua, M.-F. Zhu, Q. Chen, Polymer 45 (2004) 4693–4704. [11] S.M. Ahmed, S.A. Ahmed, Eur. Polym. J. 38 (2002) 25–31. [12] J. Laska, J. Widlarz, Synth. Met. 135–136 (2003) 261–262. [13] S.-G. Oh, S.-S. Im, Curr. Appl. Phys. 2 (2002) 273–277. [14] I. Sapurina, J. Stejskal, Z. Tuzar, Colloids Surf. A 180 (2001) 193–198. [15] P. Banerjee, B.M. Mandal, Macromolecules 28 (1995) 3940–3943. [16] H. Eisazadeh, K.J. Gilmore, A.J. Hodgson, Colloids Surf. A 103 (1995) 281–288. [17] B.D. Chin, O.O. Park, J. Colloid Interface Sci. 234 (2001) 344–350. [18] M.G. Han, S.K. Cho, S.G. Oh, S.S. Im, Synth. Met. 126 (2002) 53–60.

[19] B.J. Kim, S.G. Oh, M.G. Han, S.S. Im, Synth. Met. 122 (2001) 297–304. [20] J. Stejskal, M. Spirkova, A. Riede, M. Helmstedt, P. Mokreva, J. Prokes, Polymer 40 (1999) 2487–2492. [21] D. Ichinohe, T. Arai, H. Kise, Synth. Met. 84 (1997) 75–76. [22] M.-R. Huang, X.-G. Li, J. Wang, Petrochem. Technol. 33 (2004) 231–238 (Chinese). [23] W.-C. Chen, T.-C. Wen, A. Gopalan, Synth. Met. 28 (2002) 179–189. [24] K.R. Prasad, N. Munichandraiah, Synth. Met. 126 (2002) 61–66. [25] X.-G. Li, M.-R. Huang, F. Li, W.-J. Cai, Z. Jin, Y.-L. Yang, J. Polym. Sci. Part A, Polym. Chem. 38 (2000) 4407–4418. [26] Y. Haba, E. Segal, M. Narkis, G.I. Titelman, A. Siegmann, Synth. Met. 106 (1999) 59–66. [27] N. Kuramoto, E.M. Gebies, Synth. Met. 68 (1995) 191–194. [28] P. Li, T.C. Tan, J.Y. Lee, Synth. Met. 88 (1997) 237–242. [29] B. Wessling, J. Posdorfer, Electrochim. Acta 44 (1999) 2139–2147. [30] P. Ghosh, A. Chakrabarti, S.K. Siddhanta, Eur. Polym. J. 35 (1999) 803–813. [31] P. Ghosh, S.K. Siddhanta, A. Chakrabarti, Eur. Polym. J. 35 (1999) 699–710. [32] M.S. Cho, S.Y. Park, J.Y. Hwang, H.J. Choi, Mater. Sci. Eng. C 24 (2004) 15–18. [33] R. Gangopadhyay, A. De, Synth. Met. 132 (2002) 21–28.