Polyaniline (GP) composites: Synthesis and characterization

Polyaniline (GP) composites: Synthesis and characterization

Carbon 43 (2005) 2983–2988 www.elsevier.com/locate/carbon Graphite/Polyaniline (GP) composites: Synthesis and characterization S.E. Bourdo, T. Viswan...

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Carbon 43 (2005) 2983–2988 www.elsevier.com/locate/carbon

Graphite/Polyaniline (GP) composites: Synthesis and characterization S.E. Bourdo, T. Viswanathan

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Department of Chemistry, University of Arkansas at Little Rock, 2801 South University Ave, Little Rock, AR 72204, USA Received 21 July 2004; accepted 7 June 2005 Available online 1 August 2005

Abstract Ever since the discovery of inherently conducting polymers (ICPs), research dealing with the applications of these unique materials continues to grow. The use of ICPs, especially polyaniline (PANi) and polypyrrole (PPy), and carbon black (CB) as conductive additives in the thermoplastics industry have been limited due to undesirable properties of each at high temperatures. Carbon blackICP composites, however, have shown improved properties at higher temperatures. The applications of these composites are still limited because the conductivities are below that of carbon black alone and about the same order of magnitude as PANi. Graphite/ICP composites have also been touted as possible electrode materials in rechargeable batteries and have numerous other applications. The exploration of graphite/PANi composites in our research lab has yielded conducting composites which exhibit conductivities greater than the graphite or PANi alone. In addition to higher conductivities, these graphite/PANi composites exhibit controllable conductivities as a function of pH. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon composites; Graphite; Infrared Spectroscopy; Electrical properties

1. Introduction The use of intrinsically conducting polymers (ICPs) for a wide array of applications has prompted intense research in this field. When ICPs are combined with other conducting materials, some unique properties can be observed. The use of ICPs, especially polyaniline (PANi) and polypyrrole (PPy), and carbon black (CB) as conductive additives in the thermoplastics industry are limited due to undesirable properties of each at high temperatures. Carbon black-ICP composites, however, have shown improved properties at higher temperatures [1]. The applications of these composites are limited due

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Corresponding author. Tel.: +501 569 8825; fax: +501 569 8838. E-mail addresses: [email protected] (S.E. Bourdo), [email protected] (T. Viswanathan). 0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.06.016

to conductivities being below that of carbon black alone and about the same order of magnitude as PANi. Graphite has been used as a conductive filler in the matrix of insulating polymers to render conductive polymers [2,3]. Some recent attention has focused on using compositions of graphite and ICPs for use as both anodes and cathodes in rechargeable batteries. The inclusion of PPy into the graphite electrode material provides for more particle-to-particle contact due to the ICP providing a conducting backbone between the graphite particles. These graphite/PPy composites have shown promising results for use as anode materials in batteries [4]. Composites using a mixture of PANi, graphite and acetylene black for cathodes in dry rechargeable batteries also show improved efficiency [5]. This study has focused on the bulk electrical conductivity of graphite/PANi (GP) composites and the surface resistivity of the composites in films. The exploration of graphite/PANi (GP) composites has yielded conducting

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composites which exhibit conductivities greater than the graphite or PANi alone [6]. Due to the higher conductivities, GP composites have potential applications in a wide range of fields including EMI shielding, radar evasion, rechargeable batteries, conductive inks and anti-static textiles [5,7,8]. Research into the intercalation of PANi into the interlayer spacing of graphitic oxide has also been reported but the conductivities of these materials are low due to the modification of the graphite layers with hydroxyl, carbonyl, and ether group functionalities [8,9]. Some research into PANi/carbon nanotubes composites has provided insight into the interaction of the graphitic sheets and the ICP moieties. Two groups have reported a doping effect or charge-transfer interaction using FTIR and Raman Spectroscopy. The interaction probably occurs between the quinoid ring of the PANi chain and the carbon nanotube due to charge transfer [10,11]. A similar type of charge-transfer complex may be forming between the graphite planes and the quinoid ring of the PANi in the GP composites as well. Recent research of a graphite/PANi system has shown that a percolation of graphite particles into the PANi matrix occurs at about 1.5 wt.% graphite [12]. Similar research presented within this article does not contradict this finding, but instead may serve as complementary evidence of two percolations in the graphite/PANi composite.

2. Experimental

uum to press pellets and to determine percent solids of the wet cakes. Some samples were dedoped by stirring the dry solids in 1 M NaOH overnight, vacuum filtering through a Whatman #4 filter paper, and washing with distilled water until the pH of the filtrate was neutral. 2.2.1. Other syntheses of 80/20 GP using different dopants Syntheses according to the above method were carried out using several different dopants. The other dopants used were isopropyl phosphate(mono- and diester mixture) [TCI America], o-phosphoric acid [Fisher], sulfuric acid [Fisher], hydrochloric acid [Fisher], (±)-10-camphorsulfonic acid (HCSA) [Aldrich], 4,5-dihydroxy-2,7-naphthalenedisulfonic acid(4,5-DH2,7-NDSA) [Aldrich], and 6,7-dihydroxy-2-naphthalenesulfonic acid(6,7-DH-2-NSA) [Aldrich]. Each of these dopants were added as a 1 M aqueous solution to the reaction vessel and the aforementioned procedure was followed. These samples were dedoped by taking a sample of the dry composite and stirring in 1 M NaOH overnight. These were then vacuum filtered and washed with distilled water until the pH of the filtrate was neutral. 2.3. Dispersing of composites Composites were diluted to 25% total solids and shaken in a container with grinding media overnight. This produced a very well-dispersed product that could then be blended into a resin then cast into a film.

2.1. Materials 2.4. Instrumentation Water dispersible colloidal graphite (25–28% solids) of a proprietary nature was obtained from Acheson Colloids (Port Huron, MI) and used in composites. Aniline, sodium persulfate, and methanesulfonic acid (HMSA) were obtained from Aldrich Chemical Company. A waterborne polyurethane (Permax 200) was obtained from Noveon, Inc. (Cleveland, OH). 2.2. Synthesis of GP An aliquot of colloidal graphite was added according to the amount of solid graphite desired in the final product. Control reactions with 0% and 100% graphite were also performed. A 1 M HMSA solution was added in a ratio of 50 mL:1 mL to aniline. Aniline was added in an amount determined by the weight ratio of graphite to PANi desired. The solution was cooled to 0 °C. Sodium persulfate was added in a 1.1:1 mole ratio to aniline. The reactions were allowed to stir overnight. The solutions were then filtered using vacuum suction through a Whatman #4 filter paper and washed with distilled water. The cake was then washed twice with 50 mL 1 M HMSA. The samples were dried under vac-

2.4.1. DC conductivity measurements DC conductivity measurements were made on pressed pellets with an Alessi four-point conductivity probe connected to a Keithly voltmeter and programmable current source. 2.4.2. Surface resistivities Dispersed composites were blended in a polyurethane resin and applied onto glass substrates at a wet thickness of 15–20 mils using a draw-down bar. The films were cured at room temperature overnight. Surface resistivities were measured using a Keithly 617 Programmable electrometer and two metal electrodes to create a square surface on the film. 2.4.3. Spectroscopic data A Nicolet MAGNA-IR 550 Series 2 Spectrometer was used to analyze the FTIR spectra of the samples as pressed pellets in KBr. A Perkin-Elmer UV/Vis/ NIR Lambda 19 Spectrometer was used to analyze the ultraviolet and visible spectrum of the samples in distilled water.

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3. Results 3.1. Electrical conductivities Generally in composite materials, the electrical conductivity of the material is dependent on the less conductive moiety, but the observation here is quite different. The synthesis of these composites resulted in materials with conductivities not only higher than the ICP, but also higher than the graphite alone. Paratoluenesulfonic acid (p-TSA) and methanesulfonic acid (HMSA) were initally used as dopants due to possible improvements in the properties of the PANi moiety such as conductivity and solubility [13]. Conductivity results of 80/20 GP composites synthesized in a 1 M acid solution showed that using HMSA as the dopant resulted in a composite with desired high conductivity (r = 275 S/ cm) in comparison to p-TSA (r = 190 S/cm) [14]. For this reason, the majority of the composites have been synthesized with HMSA as the dopant. In order to elucidate the properties of the composite material, the amount of graphite was varied in the composites themselves for both doped and dedoped samples. Fig. 1

G-Pani Conductivity Study 400 350

σ (S/cm)

300 250 200 150 100 50 0 0

10

20

30

40

50

60

70

80

90

100

% graphite

Fig. 1. Conductivity study of graphite/PANi composites: (r) doped GP, (e) dedoped GP.

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shows the results of a study done comparing the conductivity of composite with respect to varying amounts of graphite. The relatively low conductivity value for the graphite (150 S/cm) could be attributed to the dispersion process as well as any surfactant(s) present, both of which may contribute to increased grain boundaries between graphite particles. The graphical representation of the bulk conductivity of composite versus percent graphite in the composite reinforces that an increase in conductivity is observed when polyaniline doped with HMSA is added to a graphite matrix. The highest conductivity measured was 350 S/cm for a 90/10 GP composite. The higher conductivity of the composite may be due to the polymer providing a matrix for the colloidal graphite to align, or vise versa, in such a way that the transfer of electrons between PANi and graphite becomes less restricted. The observation of two significant increases in conductivity suggests that two percolations may exist in the system: Graphite into the PANi matrix [12] and PANi into the graphite matrix. Each of the samples was also dedoped by stirring in 1 M NaOH overnight. There is a dramatic decrease in conductivity when the samples are dedoped. A change from 1 to 2 orders of magnitude is seen in some composites (see Fig. 1). This variation due to pH change is quite different from the previously studied carbon/ICP composites [1]. The change we see with doping and dedoping again supports the ICP having a significant role in the mechanism of conduction in these composite materials. The ability to control the electrical properties of these materials could be advantageous to many applications. Table 1 shows the results of the study performed varying the dopants used in the synthesis of 80/20 GP composites. By varying the dopants, the conductivity of the PANi is affected due to the size and nature of the anion of the protonic acids. These results were very significant to support the claim that the ICP plays a role in the high conductivity of the samples. The change in conductivities of the doped samples change as the dopant is varied. This can only be due to the conductivity

Table 1 Pressed pellet conductivities of graphite, PANi, and 80/20 GP composites with varying dopants-both doped and dedoped Dopant used

r of Graphite (S/cm)

r of PANi (S/cm)

r Doped 80/20 GP (S/cm)

r Dedoped 80/20 GP (S/cm)

HCl HMSA p-TSA HCSA 6,7-DH-2-NSA 4,5-DH-2,7-NDSA Isopropyl Phosphate H3PO4 H2SO4

154 154 154 154 154 154 154 154 154

10 15 10 1a N/Ab N/Ab N/Ac 7 N/Ac

314 275 108 80.7 192 171 54.4c N/Ac N/Ac

69.97 91.9 64.9 71.0 191 203 79.4 76.0 86.0

a b c

Literature value [15]. Incomplete/unsuccessful polymerization. Poor pellet integrity.

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3.2. Surface resistivities The synthesized composites were dispersed in water and subsequently blended in a polyurethane resin to study film properties. The surface resistivities were measured as a function of composite loading in polyurethane and the results of these studies are shown below in Figs. 2–5. The more conductive composites gave lower resistivities as would be expected. At loadings higher than 50% for the 50/50, 70/30, and 80/20 GP composites there is no significant change in resistivity. The film quality also dramatically changes at loadings above 50% as some films do not stay intact when removed from the glass substrate. Though this may be a disadvantage in some instances, the films do still exhibit good adherence to the substrate at these higher loadings. The composites with a higher amount of PANi do stay intact as free standing films at slightly higher loadings

Resistivity (ohms/sq)

5.00E+02 4.00E+02 3.00E+02 2.00E+02 1.00E+02 0.00E+00 20

30

40

50

60

70

80

loading of composite (%TS w/w)

Fig. 3. Surface resistivities of 50/50 GP in Permax 200.

70/30 GP in Polyurethane 6.00E+02 5.00E+02 4.00E+02 3.00E+02 2.00E+02 1.00E+02 0.00E+00 20

30

40

50

60

70

80

loading of composite (%TS w/w)

Fig. 4. Surface resistivities of 70/30 GP in Permax 200.

80/20 GP in Polyurethane 6.00E+02 5.00E+02 4.00E+02 3.00E+02 2.00E+02 1.00E+02 0.00E+00

20/80 GP in Polyurethane

20

6.00E+02

Resistivity (ohms/sq)

50/50 GP in Polyurethane 6.00E+02

Resistivity (ohms/sq)

of the ICP because the conductivity of graphite alone does not change upon doping as opposed to ICPs. Several conductivity values for doped PANi with the various dopants are presented in Table 1. The value for camphorsulfonic acid-doped PANi was obtained from the literature [15]. The dedoping of the samples reinforces this claim since the conductivity of the samples was lower for the dedoped composites as would be expected due to the ICP no longer being as conductive. It is to be noted that for many of the dedoped samples the conductivity was 80 S/cm; this is significant because irrespective of the type of dopant used in the synthesis, the dedoped samples all show relatively the same conductivity. The GP composites that do not exhibit this are the two samples synthesized in the presence of substituted naphthalenesulfonic acids (NSA and NDSA). Polymerization of aniline using either the NSA or NDSA as a dopant was incomplete/unsuccessful as evidenced by lack of color change during polymerization and is therefore assumed that complete polymerization did not occur in the presence of graphite.

Resistivity (ohms/sq)

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30

40 50 60 loading of composite (%TS w/w)

70

80

Fig. 5. Surface resistivities of 80/20 GP in Permax 200.

5.00E+02 4.00E+02

than the ones with less PANi, but during the curing of some films (>70% loading) cracks formed prohibiting the determination of surface resistivities.

3.00E+02 2.00E+02 1.00E+02

3.3. Spectroscopy results

0.00E+00 20

30

40

50

60

70

loading of composite (%TS w/w)

Fig. 2. Surface resistivities of 20/80 GP in Permax 200.

80

The FTIR spectrum of the GP composites exhibit many peaks that are representative of the spectrum obtained for MSA-PANi. The spectrum of the neat graph-

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Fig. 6. IR spectrum of (a) HMSA doped Polyaniline and (b) graphite control sample.

a

Relative Absorbance

ite sample does not exhibit many peaks, and a graphite control was run under the same reaction conditions as the composites in order to compare the results. The spectra were obtained for MSA-PANi and graphite as controls (Fig. 6) and three GP composites (Fig. 7). The peaks located at 1050 cm 1 and 1200 cm 1 in all spectra most likely correspond to the sulfonic acid dopant. An explanation for these peaks and the peak at 3400 cm 1 in the spectrum for the graphite control sample (Fig. 6b) may be due to residual sulfonic acid from the reaction conditions or the proprietary process used to disperse the graphite. Two peaks characteristic of PANi are present in the GP composites at 3400 cm 1 and 1150 cm 1 and are representative of the N-H stretch and an ‘‘electronic-like band’’, respectively [11,16]. The 1500 cm 1 and 1600 cm 1 peaks corresponding to the benzenoid and quinoid vibrations in PANi, respectively,

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b

c

d

300

350

400

450

500

550

600

650

700

750

800

850

Wavelength (nm)

Fig. 8. Visible spectra of (a) MSA-PANi, (b) 20/80 GP, (c) 50/50 GP, and (d) 80/20 GP.

do not exhibit any inverse relationship in the GP composites compared to the MSA-PANi, and therefore it does not appear that charge-transfer is occurring between the quinoid moiety of the PANi and the graphite. In light of these results, the previously reported charge transfer doping between graphitic structures and the quinoid unit of the PANi in the FTIR spectra [11] may not be evident in this case, but some other interactions may be occurring that allow for such high conductivities to be achieved. The absorption peaks in the visible spectrum observed in the composites correspond to those characteristic of PANi in the conducting emeraldine salt form. Fig. 8 displays the spectra obtained from MSA-PANi and three GP composites. A noticeable shift from the MSA-PANI doped benzenoid peak at 452 nm occurs as the amount of graphite is increased in the composites. The 20/80, 50/50, and 80/20 GP composites have relative maximums at 438 nm, 445 nm, and 420 nm, respectively. Though previous research has seen shifts due to interactions of the quinoid units of the PANi with graphitic sheets [10,11], this shift corresponds to the benzenoid position of PANi in the emeraldine salt form.

4. Conclusions

Fig. 7. IR spectrum of (a) 20/80 GP composite, (b) 50/50 GP composite, and (c) 80/20 GP composite.

The work completed thus far has led to very interesting results. The ability to control the conductivity of composites through several means could provide for its use in many applications. Current studies are ongoing in the synthesis of other ICPs, such as PPy and polyethylene-dioxythiophene (PEDOT), in the presence of graphite. While this research has focused primarily on water-dispersible colloidal graphite as the conducting template on which PANi is synthesized, methods of synthesis using solid graphite as a starting material are currently being explored as well. Recent work in our lab

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suggests that the nature of graphite as well as presence of surfactant do not play a significant role in the observed increase in conductivity of the composite [17]. These materials are unique in that they possess the high conductivity of graphite but still show many of the desired characteristics of an inherently conducting polymer. References [1] Avlyanov JK, Dahman S. Thermally stable intrinsically conductive polymer-carbon black composites as new additives for plastics. ACS Symp Ser 1999;735(Semiconducting Polymers): 270–9. [2] Chen G-H, Wu D-J, Weng W-G, He B, Yan W-l. Preparation of polystyrene-graphite conducting nanocomposites via intercalation polymerization. Polym Int 2001;50:980–5. [3] Xu J, Hu Y, Song L, Wang Q, Fan W. Preparation and characterization of polyacrylamide-intercalated graphite oxide. Mater Res Bull 2001;36:1833–6. [4] Veeraraghavan B, Paul J, Haran B, Popov B. Study of polypyrrole graphite composite as anode material for secondary lithium-ion batteries. J Power Sources 2002;109(2):377–87. [5] Karami H, Mousavi MF, Shamsipur M. A new design for dry polyaniline rechargeable batteries. J Power Sources 2003;117(1–2): 255–9. [6] Viswanathan T. Highly conductive carbon/inherently conductive polymer composites. US patent application 20040232390, 2004. [7] Kuhn HH, Child AD. Electrically Conducting Textiles. In: Skotheim TA, Elsenbaumer RL, Reynolds JR, editors. Handbook of Conducting Polymers. New York: Marcel Dekker; 1998. p. 993–1013. Chapter 35.

[8] Xiao P, Xiao M, Liu P, Gong K. Direct synthesis of a polyanilineintercalated graphite oxide nanocomposite. Carbon 2000;38: 623–8. [9] Liu P, Gong K. Synthesis of polyaniline-intercalated graphite oxide by an in situ oxidative polymerization reaction. Carbon 1999;37:701–11. [10] Cochet M, Maser WK, Benito AM, Callejas MA, Martinez MT, Benoit J-M, et al. Synthesis of a new polyaniline/nanotube composite: in situ polymerisation and charge transfer through site selective interaction. Chem Comm 2001;16:1450–1. [11] Zengin H, Zhou W, Jin J, Czerw R, Dennis W, Smith J, et al. Doping Effect of Carbon Nanotubes on Polyaniline. Polym Prepr (Am Chem Soc Div Polym Chem) 2003;44(1):1104–5. [12] Du XS, Xiao M, Meng YZ. Synthesis and Characterization of Polyaniline/Graphite Conducting Nanocomposites. J Polym Sci Part B Polym Phys 2004;42:1972–8. [13] Rasmussen PG, Hopkins AR. Characterization of solution and solid state properties of undoped and doped polyanilines processed from hexafluoro-2-propanol. Macromolecules 1996;29: 7838–46. [14] Bourdo SE, Viswanathan T. Highly Conductive Graphite/ICP Composites. Polym Prepr (Am Chem Soc Div Polym Chem) 2004;45(1):236–7. [15] Ruckenstein E, Yin W. Polyaniline co-doped with camphor sulfonic and hydrochloric acids by chemical oxidation in aqueous solution. J Appl Polym Sci 2000;79(1):80–5. [16] Quillard S, Louarn G, Lefrant S, Macdiarmid AG. Vibrational analysis of polyaniline: A comparative study of leucoemeraldine, emeraldine, and pernigraniline bases. Phys Rev B Condens Matter 1994;50(17):12496–508. [17] Bourdo SE, Viswanathan T. Graphite/ICP composites of high conductivity. Polym Prepr (Am Chem Soc Div Polym Chem) 2005;46(1):656–7.