polyacrylonitrile composites

European Polymer Journal 41 (2005) 2127–2133

EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj

Electrical and structural analysis of conductive polyaniline/polyacrylonitrile composites Wei Pan a

a,b

, Sheng Lin Yang

a,* ,

Guang Li a, Jian Ming Jiang

a

State Key Laboratory of Modification Chemical Fibers and Polymer Materials, Donghua University, Shanghai 200051, PR China b Zhongyuan Institute of Technology, Zhengzhou 450007, PR China Received 23 October 2004; received in revised form 26 March 2005; accepted 4 April 2005 Available online 13 June 2005

Abstract Conducting composites of polyacrylonitrile (PAN) copolymer containing 10% mass ratio methylacrylate and dodecylbenzene sulfonic acid doped polyaniline (PANI–DBSA) were prepared by solution blending. Electrical properties of the blends were characterized by means of electrical conductivity measurements and the phase structures were investigated via scanning electron microscopy (SEM), X-ray diffraction (XRD), FT-IR spectroscopy, differential scanning calorimetry (DSC) and dynamical mechanical analysis (DMA). It was found that the electrical conductivity of the composites increased with the increase of PANI–DBSA content and the percolation threshold lay around 3.2 wt%. DSC and DMA measurements showed that there was only one Tg for each blend and the values of Tg varied with the PANI–DBSA content, implying that the PANI–DBSA/PAN blend was at least partially compatible. The formation of the hydrogen bonding between the carbonyl groups in PAN copolymer and the imine groups in PANI–DBSA was identified by the FT-IR spectra. XRD demonstrated that the intrinsic layered arrangement of PANI–DBSA was disaggregated in the blends. Nanosize network structure of PANI–DBSA dispersing in PAN matrix and the so-called phase reverse occurring in the skin layer of the film samples at low PANI–DBSA loading were observed by SEM.
2005 Published by Elsevier Ltd. Keywords: Polyaniline; Polyacrylonitrile; Composites; Conductivity; Compatibility

1. Introduction In recent years, intrinsic electrically conductive polymers (ICP) have been studied enthusiastically because of their potential application in light-emitting diodes, batteries, electromagnetic shielding, antistatic coating, gas sensors and activators. Polyaniline (PANI) is one of the most intensively investigated ICPs due to its environmental stability, low cost of raw material and ease of

*

Corresponding author. E-mail address: [email protected] (S.L. Yang).

0014-3057/$ - see front matter
2005 Published by Elsevier Ltd. doi:10.1016/j.eurpolymj.2005.04.003

synthesis [1]. However, as other ICPs, PANI is insoluble, infusible and almost non-processable, which retard its potential applications. In order to improve the processability of PANI, a large number of methods have been studied, of which the most widely adopted strategy is to dope PANI with organic acids with long alkyl chain such as camphor sulfuric acid (CSA) or dodecylbenzene sulfonic acid (DBSA). The bulk non-polar tail renders the polyanilines in conducting form to be soluble in some ordinary organic solvent such as m-cresol, chloroform and xylene [2,3]. Therefore, such doped PANI can be solution processed together with common insulating polymers in

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proper solvent. As a matter of fact, solution blending of PANI doped by camphor sulfonic acid (PANI-CSA) with polar polymer matrixes such as PMMA [4–6] or PA [7], and PANI doped by DBSA (PANI–DBSA) with PS [8], EPDM [9], PVC [10], PI [11] and SEBS [12] have been reported. The relationship between conductivity and volume fraction of PANI can be described with the classical law of percolation theory, i.e., r(f) = c(f
fp)t, where c is a constant, t is critical exponent of the equation f is the volume fraction of the filler particles and fp is the volume fraction at percolation threshold. The classical percolation theory predicts a percolation threshold of fp = 0.16 for conducting particles dispersed in an insulating matrix in three dimensions. This prediction is found to be true with some conducting polymer blends. However, there have also been some reports on making conducting polymer blends with fp  0.16 [6,13]. The varied value of fp for different blends may be attributed to different compatibilities between conducting filler and insulating matrix. Polyacrylonitrile (PAN) is one of the most important fiber-forming polymers and has been widely used because of its high strength, high abrasion resistance, and good insect resistance. However, the PAN based conducting blends has not been explored, and it is expected to be a competitive system for making conductive fiber. In our study, the conductive complex PANI–DBSA was prepared by a method known as emulsion polymerization, in which the DBSA acted as a surfactant and a protonating agent for the resulting electrically conducting polyaniline. The composite films were obtained by casting the mixed solution with PANI–DBSA dissolved in chloroform and PAN in DMSO. The structural characteristics and morphologies of the PANI–DBSA/PAN composites were also investigated.

2. Experimental 2.1. Synthesis of PANI–DBSA The emulsion polymerization of aniline was performed via a procedure described in literature [14]. In a typical polymerization, a solution of 4.65 ml (0.05 mol) of aniline and 24.5 g (0.075 mol) of DBSA and chloroform was prepared in a 250 ml Erlenmeyer flask under stirring. The medium was kept at 0 C and an aqueous solution containing 4.68 g (0.02 mol) ammonium peroxide sulfate (APS) in 20 ml of water was slowly added. The total polymerization time was 24 h. The polymerization was terminated by pouring the emulsion into acetone. The precipitate as a dark green powder was filtered, washed with acetone and dried under vacuum at room temperature.

2.2. Blend preparation Firstly, PANI–DBSA was dissolved in chloroform and PAN (supplied by Shanghai petroleum chemical Company, Mw = 5.5 · 104) was dissolved in DMSO. Then these two solutions were mixed together to obtain the desired composition, keeping the total concentration of the PANI–DBSA and the PAN as 18 wt% mass ratio. The mixture solutions were cast and left to dry in vacuum at 60 C for 48 h. 2.3. Characterization and testing 2.3.1. FT-IR spectra Fourier transform infrared absorption spectra for the composite powder of PANI–DBSA and PAN copolymer were taken with a Nicolet 20sx-B FT-IR spectrometer. The scanning ranged from 4000 cm
1 to 400 cm
1 with 16 times of scanning. 2.3.2. Differential scanning calorimetric DSC thermograms of various samples were obtained from heating the samples from 40 C to 180 C under N2 purging at 20 C/min by a Perkin–Elmer 1 differential scanning calorimeter. 2.3.3. Dynamic mechanical analysis The dynamic storage modulus and tan d of sample were obtained from a TA 2980 DMA at 3 C/min heating rate. The sample was 12 · 2 mm and the damping node frequency were extended and 1 Hz, respectively. 2.3.4. Scanning electronic microscopy Morphological studies were performed via SEM observation of the cross section of film samples with a JSM-5600LV scanning electron microscope. 2.3.5. X-ray diffraction Wide angle X-ray diffraction was carried out using a BRUKER-AXC08 X-ray diffractometer and filtered Cu Ka radiation. The diffraction patterns of the composite powder of the PANI–DBSA and PAN compolymer were obtained by scanning the samples in an interval of 2h = 1–40. 2.3.6. Electrical conductivity Electrical conductivity of the blend films was measured by the usual four-probe method.

3. Results and discussion 3.1. DSC The Tg of NMP-solution-cast film of PANIEB was strongly depended on the content of the residual solvent.

W. Pan et al. / European Polymer Journal 41 (2005) 2127–2133 10 3

c

endo

d e f

0

20 40 60 80 100 120 140 160 180 Temperature ( oC)

Fig. 1. DSC thermograms (a) PAN itself, the PANI–DBSA/ PAN blends of PANI–DBSA content, (b) 2.5%, (c) 5%, (d) 10%, (e) 15% and (f) PANI–DBSA itself.

For films containing about 16–0% residual solvent, the Tg was determined to be between 105 C and 220 C [15]. The rigid PANI backbone as well as their tightly arranging may be responsible to the very high Tg. However, when doped with a long alkyl chain acid, such as DBSA, CSA, the resultant doped PANI exhibits the Tg that could be detected at relatively low temperature. Long alkyl chains bonded to the backbone of PANI act as plastisers and make PANI segment to move at low temperature. So it should be easy to understand that the Tg of PANI–DBSA is dependent on the DBSA concentration. Ikkala et al. [16] found that the Tg values of PANI–DBSA could vary from 65 C to 130 C as the long alkyl chain acid content decrease. Tsocheva et al. reported that when the molar ratio of PANI over DBSA is about 1:1, the Tg of PANI–DBSA is 65 C. In our study, the Tg of PANI–DBSA is around 82 C, whereas the Tg of PAN copolymer is 100 C (Fig. 1). The DSC thermograms of PANI–DBSA/PAN blends are also illustrated in Fig. 1. It is found that there is only one Tg for each blends, and Tg values gradually get lower with increase of PANI–DBSA content in the blends. It is well known that the Tg of a polymer blend is one of the most important criteria for compatibility of components. Two Tgs were observed for some composites with introduction of PANI–DBSA, such as PANI– DBSA/PU [17], where percolation threshold of PANI– DBSA was high as 20–30% mass ratio. The shape of dispersed PANI–DBSA in above mentioned system is spherical-like. On the contrary, only one Tg appears in our PANI–DBSA/PAN composites, which implies better compatibility between PANI–DBSA and PAN. With a better dispersion of PANI–DBSA in PAN matrix, a lower percolation threshold could be expected. 3.2. DMA analysis The change of storage modulus (E 0 ) and tangent delta (tan d) of PAN and PANI–DBSA/PAN film (10%

a

0.6

a

0.5

b

b

0.4 10 2

0.3 0.2

Tan δ

b

Storage Modulus (MPa)

a

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0.1 10

1

20

40

60

80

100

120

140

0.0

o

Temperature ( C)

Fig. 2. Dynamic storage modulus and tan d as a function of temperature for (a) PAN and (b) PANI–DBSA/PAN (10 wt% PANI–DBSA content).

mass ratio PANI–DBSA) are given in Fig. 2. The glass transition temperatures for PAN and PANI–DBSA lie about 112 C and 107 C respectively. The varied values of Tgs obtained from DSC and DMA may be due to different measuring technique. Anyway, it is agreeable that there is only one Tg for the PANI–DBSA/PAN system form either DSC or DMA. Therefore, it could be concluded that the two components, PANI–DBSA and PAN, are compatible or at least partially compatible. 3.3. FT-IR analysis Theoretically, PANI has the potential to form hydrogen bonds with carbonyl polymers. Because of H-donating in imine groups, PANI has been found to be compatible or partially compatible with some polymer counterparts [18,19]. FT-IR spectroscopy is a very useful and convenient technique to detect the formation of hydrogen bonding. As mentioned above, PAN employed in this study is a copolymer, containing methylacrylate segments. That is to say, there exit carbonyl groups in the system. With FT-IR spectroscopy, the presence of carbonyl groups could be detected, as shown in Fig. 3, the band (1743 cm
1) is assigned to free carbonyl group absorption. Furthermore, upon mixing PAN with PANI– DBSA, the band is observed to slightly shift to low wave numbers. The more the PANI–DBSA content, the lower the wave numbers. For 10% PANI–DBSA loading composite, the carbonyl band is observed to be 1732 cm
1, 11 cm
1 lower than the band of free carbonyl. Benson et al. [20] studied the change of carbonyl groups in PU by FT-IR and found that the formation of hydrogen bonding between carbonyl groups and H-donating groups make carbonyl group band shift to a lower frequency. We believed that hydrogen bonding between imine of PANI and carbonyl of PAN should be assigned to the shifts of carbonyl group band.

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[

Transmittance

a b c

CH2

CH ...... CH2

CH ...... CH2

CH

CN

C

CN

1743

O CH3

]

O

d

[

NH

H

H N +. SO3

NH

N +

.

]

SO3

1732

1680

1700

1720

1740

1760

1780

Fig. 3. FT-IR spectra of the carbonyl (C@O) group in (a) PAN; PANI–DBSA/PAN with PANI content, (b) 2.5 wt%, (c) 5 wt% and (d) 10 wt%.

CH2 (CH2)10 CH3

CH3 (CH2)10 CH2

Wavenumbers (cm-1)

Fig. 5. Proposed scheme of the hydrogen bonding between PANI–DBSA and PAN.

3.4. Wide-angle X-ray diffraction study

Transmittance

a b c d

The WAXD patterns of PANI–DBSA, PAN and PANI–DBSA/PAN blends with different weight fraction of PANI–DBSA are given in Fig. 6. PAN shows only one scattering peak at 2h = 17. PANI–DBSA shows a sharp scattering peak at 2h = 2.5 together with a broad amorphous scattering around 2h = 19 and a scattering peak at 2h = 26. It should be noted that the peak at the low angle is due to the layered structure of PANI– DBSA. That is, the rigid PANI backbones are packed into the platelike layers whereas alkyl side chains form spaces between the layers [21]. From Fig. 6 it can be seen that the crystalline characteristic peak of PAN at 2h = 17 are maintained in all blends, which means that low content PANI–DBSA does not affect the packing structure of PAN. The peak at 2h = 2.5 which attributed to long-range ordered structure of PANI becomes rather weak and almost

Arbitrary Intensity

Fig. 4 shows the corresponding imine stretching bands of PANI–DBSA and PANI–DBSA/PAN blends. The peaks around 3450 cm
1 come from free imine groups (–NH–) of PANI–DBSA, whereas shoulder peaks at lower wave number (3285 cm
1) could be observed for PANI–DBSA/PAN blends and the intensity of these peaks increased with the PANI–DBSA content. Considering the shift of carbonyl groups in PAN sample, these shoulder peaks should be attributed to the hydrogen-bonding between imine and carbonyl group. The hydrogen bonding formed in PANI–DBSA/PAN composite could be schematically explained with Fig. 5. Thermodynamically speaking, hydrogen bonding among different components of polymer blends will make pronounced contribution to compatibilities, because it makes entropy change (DH) negative. From this point of view it can be now well understand that why there is only one Tg for PANI–DBSA/PAN blends, because the hydrogen bonding builds up the interactions between the PANI–DBSA filler and the PAN matrix.

3450

a b c d e

3285

0 3000

3200

3400

3600

3800

4000

5

10

15

20

25

30

35

2θ/o

Wavenumbers (cm-1)

Fig. 4. FT-IR spectra of the amine (N–H) group in (a) PANI– DBSA itself; PANI–DBSA/PAN blends with PANI content, (b) 2.5 wt%, (c) 5 wt% and (d) 10 wt%.

Fig. 6. WAXD patterns of (a) PANI–DBSA, (b) 15 wt% PANI–DBSA content, (c) 10 wt% PANI–DBSA content, (d) 2.5 wt% PANI–DBSA content and (e) PAN and PANI–DBSA– PAN.

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Fig. 7. Cross section SEM micrographs of PANI–DBSA/PAN blend films containing 2.5 wt%, 5 wt% and 7.5 wt% of PANI–DBSA (from top to bottom). In each row, left column is the micrograph of core layer and right column is that of skin layer.

disappear in the PANI–DBSA/PAN blends, which implies that the PANI–DBSA dispersed well in the PAN matrix and the aggregating domains of PANI– DBSA are very small. This is agreeable with the DSC and DMA measurement. 3.5. Morphology The cross section SEM micrographs of the PANI– DBSA/PAN blend films containing 2.5, 5 and 7.5 wt% PANI–DBSA respectively are listed in Fig. 7. The dark regions are related to the PAN phase while the bright regions to conductive PANI–DBSA phase. Comparing the PANI–DBSA distribution in core and skin layer, it is observed that the content of PANI–DBSA located in the skin layer is more then that in the core layer. Because

the solvent of PANI–DBSA, i.e., chloroform, evaporates more rapidly than DMSO, more PANI–DBSA will move to the skin layer during the diffusion process of chloroform. Even at low loading of PANI–DBSA, in the skin layer, the so-called phase reverse occurred, i.e., the rare PANI–DBSA phase dispersed as network structure embodying the enriched PAN phase. However, in the core layer PANI–DBSA dispersed as isolated particles with size ranged in 20–50 nm. With the increased loading of PANI–DBSA, PANI–DBSA phase in the core layer is also apt to aggregate and into a network structure. Similarly, in PANI-CSA/PMMA blends [5], a high volume fraction of such a network was reported to be the origin of low percolation threshold. The low percolation threshold of conductivity of the PANI– DBSA-PAN films maybe attribute to the network

W. Pan et al. / European Polymer Journal 41 (2005) 2127–2133

Conductivity (S/cm)

2132 100

References

10-2

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10-4 10-6 10-8 10-10 10-12 0

20 40 60 80 100 PANI-DBSA content (wt%)

Fig. 8. Electrical conductivity of PANI–DBSA/PAN composites as a function of the PANI–DBSA content.

morphology in the skin layer, which offer a connected pathway for the electric current. From the above SEM micrographs, it is clearly that PANI–DBSA dispersion size in PAN matrix is on nanolevel. This excellent dispersion is assigned to the interaction (hydrogen bonding) between two components, which is the cause of low percolation threshold. 3.6. Electrical conductivity The effect of PANI–DBSA content on the electrical conductivity of the PANI–DBSA/PAN blend films is plotted in Fig. 8. When the mass fraction of PANI– DBSA is just 2.5%, the conductivity of the composite increases from 10
13 S/cm to 10
7 S/cm. If the PANI– DBSA is added up to 5% mass ratio, the conductivity can even reach to 10
3 S/cm. The conductivity increases sharply when the mass ratio of PANI–DBSA is less then 4%, after which it will gradually reach to 10
1 S/cm when more PANI–DBSA is added. It is shown that the percolation threshold of the blend system should be less than 5% mass ratio. The data presented in Fig. 8 could be fit into the scaling law of percolation theory mentioned above. By fitting the experimental data to a plot of log r versus log (f
fp), it is possible to estimate the percolation threshold of the system. The method yields a value of 3.2 wt% for fp and 3.8 for t. As expected, these composites really have a low percolation threshold.

4. Conclusions For PANI–DBSA/PAN composite films obtained from solution casting, there may be interaction (hydrogen-bonding) between two components, which makes good compatibility and an nanolevel dispersion of PANI–DBSA in PAN. The network structure of PANI–DBSA in PAN matrix should be responsible to low percolation threshold and high conductivity.

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