Composite films of nanostructured polyaniline with poly(vinyl alcohol)

Composite films of nanostructured polyaniline with poly(vinyl alcohol)

Synthetic Metals 128 (2002) 83–89 Composite films of nanostructured polyaniline with poly(vinyl alcohol) Zhiming Zhang, Meixiang Wan* Organic Solid L...

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Synthetic Metals 128 (2002) 83–89

Composite films of nanostructured polyaniline with poly(vinyl alcohol) Zhiming Zhang, Meixiang Wan* Organic Solid Laboratory, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China Received 29 June 2001; received in revised form 19 October 2001; accepted 26 November 2001

Abstract The uniform composite films of nanostructured polyaniline (PANI) (e.g. nanotubes or nanorods with 60–80 nm in diameter) were successfully fabricated by blending with water-soluble poly(vinyl alcohol) (PVA) as a matrix. The PANI nanostructures were synthesized by a template-free method in the presence of b-naphthalene sulfonic acid (b-NSA) as a dopant. The molecular structures of PANI–b-NSA and the related composite films were characterized by UV–Vis absorption spectrum, FTIR spectrum and X-ray diffraction. It was found that the electrical, thermal and mechanical properties of the composite films were affected by the content of nanostructured PANI–b-NSA in the PVA matrix. The composite film with 16% PANI–b-NSA showed the following physical properties: room-temperature conductivity is in the range 102 S/cm, tensile strength 603 kg/cm2, tensile modulus 4:36  105 kg/cm2 and ultimate elongation 80%. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Nanostructure; Poly(vinyl alcohol); Composite film; Conductivity

1. Introduction Nanostructures such as nanotubes or nanowires have attracted considerable attention because of their unique properties and promising potential applications in nanodevices [1–6]. Conducting polymers are an excellent choice as molecular wires because of their metal-like conductivity as high as 103–105 S/cm and large p-conjugation length [7,8]. In general, template synthesis is a common and effective method to synthesize nanostructures of conducting polymers [9–12], but a post-treatment is required after polymerization [13]. Thus, it is particularly interesting to form nanotubes or nanowires by a self-assembly process, in which the nanostructures are formed through H-bonding, metal-ligating, Van der Waal’s interaction or electrostatic interaction [14,15]. Recently, Huang and Wan [16] have prepared microtubes of polyaniline (PANI) using (NH4)2S2O8 as an oxidant in the presence of b-naphthalene sulfonic acid (b-NSA) as a dopant without any template. This method is termed as template-free method. It is proposed that the formation of these PANI microtubes can be attributed to self-assembling

* Corresponding author. Tel.: þ86-10-6256-5821; fax: þ86-10-6255-9373. E-mail addresses: [email protected], [email protected] (M.X. Wan).

of b-NSA molecules and/or their aniline salts into microstructural intermediate [17]which acts as both a supermolecular template [18] and a self-doping agent. Compared with template synthesis method, obviously, the templatefree method is simple and cheap since one can leave out the microporous membrane as template and ‘‘molecular anchor’’ to bind the polymer to the wall of microporous membrane. Moreover, we found that the resulting microtubes exhibit both electrical and magnetic loss at the microwave frequency (f ¼ 118 GHz) [19], which are quite different from granular PANI. While granular PANI only has electrical loss at the microwave frequency. Furthermore, it was found that the diameter and length of those tubes strongly depended on the polymerization conditions [18,20]. Recently, we found that the microtubes synthesized by template-free method can dissolve in some organic solvents, but their tubular morphology disappears. This indicates that the processability of the microtubes is poor, which limits their applications. Thus the improvement of the processability of the microtubes is necessary. In fact, many methods, such as covalent substitution [21,22], doping with functionalized protonic acid [23–25] and blending with soluble polymers [26–30], have been developed for improving the processability of PANI. Among these methods, it is reasonable to believe that blending with soluble polymers is suitable for improving the processability of PANI micro- or nanotubes.

0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 6 6 9 - 5

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In this paper, composite films of nanostructures (e.g. nanotubes or nanorods) of PANI blended with water-soluble poly(vinyl alcohol) (PVA) as a matrix are reported. The electrical, thermal and mechanical properties of the resulting composite films are discussed.

2. Experimental Aniline monomer was distilled under reduced pressure. Other reagents, such as b-NSA as a dopant, ammonium peroxydisulfate (APS) as an oxidant and PVA as a matrix were used as-received without further treatment. The nanostructures (e.g. nanotubes or nanorods) of PANI were synthesized by a template-free method with some modifications [16]. A typical synthesis process for PANI– b-NSA with nanostructures is as followed: 0.2 ml of aniline monomer was mixed with b-NSA dissolved in 10 ml of deionized water under supersonic stirring at room temperature to form a white emulsion of aniline/b-NSA complex. Then aqueous solution of APS (0.46 g in 5 ml deionized water) was added to the above reaction mixture and stirred for 2 h. The mixture was left overnight. The product was washed with water, methanol and ether three times, respectively, and then dried in vacuum for 24 h to obtain green powder of PANI–b-NSA. Composites of PANI–b-NSA nanostructures with PVA were prepared by simple mixing PANI–b-NSA dispersion in water with aqueous PVA solution and followed by film casting to obtain composite films. The nanostructures (e.g. nanotubes or nanorods) were confirmed using a scanning electron microscope (SEM Hitachi-530) and transmission electron microscope (TEM Hitachi-9000). The molecular structures of the resulting composite films were characterized by FTIR, UV–Vis absorption spectrum and X-ray diffraction. FTIR spectrum was carried on a DIFS-113V spectrometer (Bruker). UV–Vis absorption spectra of PANI–b-NSA film, PVA film and the resulting composite films were recorded on an UV-3100 spectrometer. X-ray scattering patterns were measured on a M18AHF (Japan, MAC Science) instrument. The conductivity of PANI–b-NSA and the composite films was measured by a four-probe method using Keithley 196 SYSTEM DMM digital multimeter and ADVANTEST R6142 Programmable DC Voltage/Current Generator as the current source. The thermal properties were measured by thermogravimetric analysis (TGA, Perkin Elmer TGA), while the mechanical properties were measured using an Instron-1122 Materials Testing Device.

3. Results and discussion 3.1. PANI–b-NSA nanostructures Fig. 1 shows typical SEM and TEM images of PANI–bNSA synthesized by the template-free method. SEM images

Fig. 1. SEM and TEM images of PANI–b-NSA synthesized by templatefree method: (a) SEM; (b, c) TEM.

show that PANI–b-NSA is fibrous and TEM images indicate that most of those fibers are hollow, but a little part is solid. Moreover, it was found that the diameter of the tubes increases with decreasing the aniline/b-NSA molar ratio as shown in Fig. 2. It is noted that nanotubes or nanorods with an average of 70–80 nm were observed at a higher aniline/b-NSA molar ratio (e.g. 1:0.5). However, the tubular morphology of PANI–b-NSA was not affected by aniline/bNSA molar ratios. This indicates that the diameter and/or the inner diameter of the tubes could be controlled by changing the polymerization conditions, especially the concentration of b-NSA. It is well known that b-NSA has a sulfonic acid group attached to the naphthalene ring. Thus, b-NSA has both a doping and a surfactant function. As described in Section 2, an emulsion was formed when aniline monomer was mixed with b-NSA under supersonic stirring due to the hydrophobic behavior of aniline monomer and hydrophilic behavior of b-NSA. Thus, the formed micelles of aniline/b-NSA play a role of template-like in forming nanostructures of PANI–b-NSA. As a result, it is expected that the size and morphology of these micelles would affect the diameter and morphology of the PANI–b-NSA nanostructures. Thus the concentration of b-NSA which has an important role in the formation of aniline/b-NSA salt strongly affects the size and morphology of the resulting PANI–b-NSA. In addition, it was found that room-temperature conductivity of PANI–b-NSA increases with increasing aniline/ b-NSA molar ratios as shown in Fig. 3, which is consistent

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Fig. 2. Effect of aniline/b-NSA molar ratio on the morphology of PANI–b-NSA nanostructures: (a) 1:0.5; (b) 1:1; (c) 1:2; (d) 1:3.

Fig. 3. Effect of b-NSA/aniline ratio on the room-temperature conductivity of PANI–b-NSA nanostructures.

with PANI doped with HCl [31]. Enhancement of the conductivity with increasing the concentration of b-NSA is due to the increase in doping level, which is confirmed by elemental analysis as given in Table 1. It was found that lower doping level (i.e. low concentration of b-NSA) is favorable for the formation of nanotubes or nanorods of PANI–b-NSA. In particular, PANI–b-NSA nanostructures show unusual high crystallinity as shown in Fig. 4, which is

consistent with our previous results [16]. A sharp peak at 2y ¼ 8:8 , which is assigned as the scattering along the orientation parallel to the PANI chain [32], was observed, while it is absent in PANI doped with HCl [33]. Moreover, some sharp peaks located at 2y ¼ 2330 , which are attributed to the periodicity perpendicular to the chain direction [32] were also observed. It is noted that the intensity of these sharp peaks is reduced when aniline/b-NSA molar ratio

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Table 1 Elemental analysis data of PANI–b-NSA Aniline/b-NSA (molar ratio)

C (%)

H (%)

N (%)

S (%)

S/N

Conductivity (S/cm)

1:0.5 1:1 1:2 1:3

61.53 64.00 63.80 64.04

4.42 4.58 4.58 4.68

8.46 6.86 6.72 6.32

6.42 7.30 7.92 8.34

0.332 0.444 0.516 0.577

7:31  102 6:13  101 4.81 7.65

Fig. 4. Influence of aniline/b-NSA molar ratios on X-ray scattering patterns of PANI–b-NSA nanostructures: (a) 1:0.5; (b) 1:1; (c) 1:2; (d) 1:3.

changed from 1:3 to 1:0.5, especially those peaks located at 2y ¼ 2330 . It is apparent that the crystallinity of PANI–bNSA(1/3) is higher comparing with PANI–b-NSA(1/0.5), which is consistent with the conductivity of PANI–b-NSA. 3.2. Characteristics of the composite films Fig. 5 shows typical SEM images of the composite films. It clearly showed that PANI–b-NSA was dispersed equally among PVA matrix when the content of PANI–b-NSA was

lower than 12.6% (Fig. 5a), and obvious aggregates or agglomerates of PANI–b-NSA were seen when 49% of PANI–b-NSA was used (Fig. 5b). As shown in Fig. 6, PVA is almost transparent in the wavelength region of 400–1400 nm, while PANI–b-NSA has two bands at 420 and 850 nm with a long tail. The band at 850 nm is due to the formation of polarons [34]. UV–Vis absorption spectrum of PANI–b-NSA/PVA composite film was similar to that of PANI–b-NSA and the intensity of these bands increases with increasing the content of PANI–b-NSA.

Fig. 5. SEM images of PANI–b-NSA/PVA composite films: (a) 12.6% of PANI–b-NSA; (b) 49% of PANI–b-NSA.

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Fig. 6. UV–Vis absorption spectra of PVA film, PANI–b-NSA film and PANI–b-NSA/PVA films: (a) PVA film; (b) PANI–b-NSA/PVA film (16%); (c) PANI–b-NSA/PVA film (49%); (d) PANI–b-NSA film.

Fig. 7 shows FTIR of PVA, PANI–b-NSA and PANI–bNSA/PVA composite film. For pure PVA, a broad band at 3400–3100 cm1 due to O–H stretching vibration and another band at 2930 cm1 assigned as C–H stretching vibration were observed. It was found that FTIR spectrum of PANI–b-NSA nanostructures, for instance, 1567 and 1493 cm1 assigned as stretching vibration of quinoid ring and benzenoid ring, respectively, were identical to that of PANI synthesized by a common method [31]. The characteristic bands of both PANI–b-NSA and PVAwere observed in the composite films. However, the bands at 1305 and 1246 cm1 corresponding to

C–H stretching vibration with aromatic conjugation become weaker in PANI–b-NSA/PVA composite film. The band at 830 cm1 due to the vibration of symmetrically substituted benzene was also observed in PANI–b-NSA/PVA composite film and the band at 3400–3100 cm1 in the composite film is almost the same as that of PVA. 3.3. Physical and thermal properties of the composite films The effect of the PANI–b-NSA loading on the roomtemperature conductivity of PANI–b-NSA/PVA composite

Fig. 7. FTIR spectra of PVA, PANI–b-NSA and PANI–b-NSA/PVA film: (a) PANI–b-NSA; (b) PVA; (c) PANI–b-NSA/PVA film (49%).

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Fig. 8. Effect of PANI–b-NSA loading on room-temperature conductivity of PANI–b-NSA/PVA composite films.

films is shown in Fig. 8. The conductivity of the composite film increases with increasing PANI–b-NSA loading, not showing a typical percolation threshold behavior [35]. The composite film become electrically conductive (5  103 S/cm) at a PANI–b-NSA content of 24%. The enhancement of conductivity of the composite films with increasing PANI–b-NSA loading is due to the formation of conductive paths through the blend. TGA was used to measure the thermal properties of the composite films. The pure PVA exhibits a process of a mass loss probably due to loss of water or solvent, and another loss at 280 8C. PANI–b-NSA undergoes three steps: loss of water or solvent, de-doping and decomposition of PANI chain, which is consistent with the previous results [36,37]. The onset temperature of the decomposition of PANI–b-NSA (321 8C) is slightly higher than that of PANI doped by common acids (300 8C) [38,39], which may be due to high crystallinity of PANI–b-NSA nanostructures. However, the composite films present a thermogram showing a mass loss at 200 8C and another loss at 445 8C, leaving a residue of 22.5%. So the PVA/PANI–b-NSA composite has a lower thermal stability. This is presumably due to the formation of H-bonding between PANI–b-NSA and PVA, which decreases H-bonding interactions among PVA molecules. The mechanical properties of PANI–b-NSA/PVA films were measured and the results are shown in Table 2. In case of PVA, the ultimate tensile strength and tensile modulus are Table 2 Mechanical properties of PANI–b-NSA/PVA films

Tensile strength (kg/cm2) Modulus (105 kg/cm2) Ultimate elongation (%)

802 and 6:26  105 kg/cm2, respectively. However, the inclusion of PANI–b-NSA led to a significant decrease in both tensile strength and tensile modulus. For the ultimate tensile strength, e.g., it decreases from 802 to 482 kg/cm2, while the tensile modulus reduced from 6:26  105 to 3:94  105 kg/cm2. However, a uniform composite film with 603 kg/cm2 of tensile strength, 4:36  105 kg/cm2 of tensile modulus and 80% of ultimate elongation could be fabricated when the content of PANI–b-NSA was 16%. This result indicates that the processability of PANI–b-NSA nanostructures could be resolved by using blending with some soluble polymers as the matrix.

4. Conclusions Composite films of PANI–b-NSA nanostructures (e.g. nanotubes or nanorods) synthesized by a template-free method were fabricated by blending with water-soluble PVA as a matrix. It was found that the electrical, thermal and mechanical properties of the composite films are affected by the loading of PANI–b-NSA in the PVA matrix. A uniform composite film containing 16% of PANI–b-NSA was successfully prepared in this study, which has a conductivity of 102 S/cm, a tensile strength of 603 kg/cm2, a tensile modulus of 4:36  105 kg/cm2 and an ultimate elongation of 80%.

Acknowledgements

PVA

PANI–b-NSA/ PVA (16%)

PANI–b-NSA/ PVA (24%)

802 6.26 123

603 4.36 80

482 3.94 5.4

This project was supported by 973 program of China (No. G1999064504), National Natural Science Foundation (No. 29974037 and No. 50133010) and Center for Molecular Sciences Institute of Chemistry, Chinese Academy of Sciences (No. CMC-CX 2001).

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