Synthesis of Highly Conductive Polypyrrole Nanoparticles via Microemulsion Polymerization

Transcription

1 Journal of Metals, Materials and Minerals, Vol.18 No.2 pp.27-31, 2008 Amparat REUNG-U-RAI, Artita PROM-JUN, Walaiporn PRISSANAROON-OUAJAI *, and Sirisart OUAJAI Department of Industrial Chemistry, Faculty of Applied Science, King Mongkut s University of Technology North Bangkok, Abstract Received Nov. 13, 2008 Accepted Jan. 30, 2009 Polypyrrole (PPy) nanoparticles were chemically synthesized via microemulsion polymerization of pyrrole and ammonium peroxydisulfate (APS) in the presence of surfactant dopants. Influences of polymerization temperatures, stabilizers and dopant natures on size and conductivity of PPy nanoparticles were investigated. Spherical PPy nanoparticles of nm in diameter with the specific conductivity in the range of S/cm have been achieved, depending upon polymerization conditions. It was obviously found that low temperature-polymerization resulted in small particle size and high specific conductivity. In addition, n-amyl alcohol, acting as a stabilizer, induced well-conjugation of PPy chains leading to the highly conducting PPy nanoparticles. This result could not be observed for poly(vinyl alcohol) stabilizer. The PPy doped with sodium dodecyl sulfate (SDS) exhibited higher conductivity that doped with dodecylbenzene sulfonic acid (DBSA). UV-VIS spectra exhibited a good support with the conductivity results. Simple synthesis of highly conductive PPy nanoparticles offers a potential in using as conducting filler in making conducting polymer composites. Key words : Polypyrrole, Nanoparticles, Micro-emulsion polymerization. Introduction Polypyrrole (PPy), one of the most extensively investigated conducting polymers, has attracted a great deal of interest because of its good electrical conductivity, environmental stability and easy synthesis. (21) PPy has been performed in a number of applications, such as batteries, supercapacitors, sensors, (16, 7, microwave shielding and corrosion protection. 8, 2) In addition, PPy can been used as conducting filler in insulating polymer matrices in preparation of electrically conducting composites. These composites offer potentials in EMI shields, electronic packaging, display devices and electrodes. (12, 22, 9) Distribution of PPy particles in polymer matrices, however, becomes an issue. It has been reported that PPy nanoparticles can be effectively dispersed due to large surface area for reactions and highly porous in sol form (Jang, 2006 and Suri, et al. 2003), then they provide good conductivity. Microemulsion polymerization has been developed to prepare polymer nanostructures. This technique allows particles to transfer into spherical aggregates through the surfactant template. Some research groups have prepared conducting polymer nanoparticles using microemulsion polymerization processes and reported particle sizes in the range of nm. (6, 10) There are two reasons for using the surfactant in the polymerization. One is the creation of a micro-reactor vessel via micelle formation, where monomer is restricted in a localized environment originated from encapsulation by the surfactant. The second reason is to improve the physical properties of polymers such as conductivity, stability, solubility in organic solvents and processibility. (20) Yan, et al have prepared PPy nanoparticles with the size of nm in oil water microemulsion.(23) Their studies confirmed that, compared with the aqueous solution and conventional emulsion polymerization, the microemulsion polymerization of PPy increased the yield of the resultant PPy, the extent of the p-conjugation along the polymer backbone, and the ordered arrangement of the macromolecular chains. Song, et al have reported the synthesis of PPy nanoparticles doped with a variety of alkylbenzenesulfonic acid (ABSA). (18) PPy doped with short alkyl chain showed higher conductivity than long alkyl chain. Liu, et al have reported that addition of Tel ext.4808, Fax ext pwalaip@hotmail.com

2 28 REUNG-U-RAI, A. et al. alcohol during microemulsion polymerization at room temperature (25 C) affected the conductivity of PPy nanoparticles. (11) They revealed that n-amyl alcohol has led PPy with the conductivity of 9.6 S/cm. The n-amyl alcohol, however, has not been performed at low temperature. In the present research, we report the preparation of the PPy nanoparticles by using microemulsion polymerization. Influences of polymerization temperatures and stabilizers including n-amyl alcohol and Poly(vinyl alcohol) (PVAL) on particle size and conductivity of PPy particles have been examined. SDS and DBSA were selected to use as surfactant dopants since they are not too expensive and commercially available. FT-IR and UV-VIS were also carried out to investigate the characteristics of PPy nanoparticles. Materials and Experimental Procedures Materials Pyrrole (Fluka) was purified by double distillation under reduced pressure and stored in a refrigerator before use. The oxidant, ammonium peroxydisulfate (APS, Sigma-Aldrich) and the dopants including SDS (Ajax Finechem) and DBSA (Fluka) were used as received. PVAL (Mw , BDH) and n-amyl alcohol were used as stabilizer. Distilled water was used as solvent for all solutions. Preparation of PPy Nanoparticles A typical synthesis was done as follows: an aqueous solution of dopant (0.08 M) was stirred vigorously for 20 min at the certain temperature. Pyrrole was subsequently added into the above solution and further stirred for 30 min, followed by adding of APS. The initial molar ratio of monomer/oxidant/dopant was kept at 1:1:1. After 24 h, large excessive methanol was pour into the solution to terminate the reaction. The resulting PPy precipitate was centrifuged, washed with methanol, distilled water and acetone for several times. Finally, the PPy precipitate was vacuumfiltered, dried and kept in a desiccator. containing dopant and pyrrole monomer before adding APS solution. Characterization FTIR analysis was carried out on a Spectrum Transmission spectra were obtained by forming a thin KBr pellet containing PPy nanoparticles. UV-Vis absorption spectra were measured in transmission by a Lambda 2S UV-Vis spectrometer. Scanning electron spectroscopy (SEM) of PPy nanoparticles covered with a thin layer of sputtered gold, was recorded on a JEOL-JSM-6480LV instrument. The average particle size was measured using a nanosizer (Malvern Instrument). Electrical conductivity of the compressed pellets of PPy was measures by a four-point probe method with a 2400 Keithley source-meter under laboratory condition. Results and Discussion Morphology As seen in Figure 1. SEM measurement reveals that the resulting PPy particles have spherical morphology with nm in average diameter. The PPy nanoparticles were obtained at a high yield (>107%) because anionic surfactants are incorporated into the PPy backbones (He, et al. 2003) However, the PPy nanoparticles tended to form an aggregation since the average particle size, measured by using a nanosizer, was found to be in the range of nm. This value was much larger than the particle size observed from SEM images. The degree of aggregation depended on the polymerization conditions. For example, when the SDS-doped PPy was synthesized at room temperature without a stabilizer, the average particle size was found to be 1300 nm, indicating high degree of aggregation. The particle size of 180 nm was observed for the PPy obtained at low temperature. (3) At relatively low temperature, the mobility of surfactant is restricted, leading to the decrease in the inner volume of micelles that encapsulate the monomer and the oxidant. Reduced micelle volume results in reduced nanoparticle size. To investigate the influence of stabilizer on the properties of PPy nanoparticles, either n-amyl alcohol or PVAL was mixed with the solution

3 29 Conductivity Conductivity of the PPy nanoparticles was strongly influenced by polymerization temperature, stabilizer and dopant as presented in Table 1. The conductivity of the PPy nanoparticles was improved obviously by lowering temperature of polymerization; the conductivity changes from 0.8 S/cm (Sample 1) to 47.2 S/cm (Sample 2). At low temperature, the polymerization rate is relatively low resulting in the formation of well conjugation and then allowing easy transport of electrons along PPy backbones. (3) (a) Figure 1. Typical SEM image of PPy nanoparticles. FT-IR analysis FT-IR spectra of the PPy nanoparticles synthesized at different polymerization conditions indicated the typical characteristics of PPy which were consistent with literatures. (18, 15, 1) Figure 2 shows a typical FT-IR spectrum the PPy nanoparticles in fingerprint band. The peaks at 1550 and 1475 cm -1 correspond to the stretching vibration of C=C and C-N of PPy, respectively. The peak at 1190 cm -1 is attributed to breathing vibration of the pyrrole ring. The bands of C-H and N-H in-plane deformation vibration are located at 1037 cm -1 while the band of C-H out-of-plane deformation vibration is found at 901 cm -1. The expected peak of the S=O stretching vibration of SO 3 - at 1183 cm -1 could not be clearly observed due to overlapping with the pyrrole ring vibration at 1190 cm %T Wavenumber (cm -1 ) Figure 2. Typical FT-IR spectrum PPy nanoparticles in the fingerprint band Table 1. Conductivity of PPy nanoparticles synthesized at different polymerization conditions Sample Dopant Temp.( C) Stabilizer Conductivity (S/cm) 1 SDS 30 none 0.8±0.0 2 SDS 0 none 47.2±8.5 3 SDS 0 n-c 5 H 11 OH 61.9±5.9 4 SDS 0 PVAL 4.1±0.0 5 DBSA 0 n-c 5 H 11 OH 2.9±0.2 By introduction of n-amyl alcohol into the low temperature-polymerization system as a stabilizer (Sample 3), the conductivity of the PPy nanoparticles increases to 61.9 S/cm. Alcohol is assumed to act as effective co-surfactant to optimize the structure of PPy chains via an interaction between hydroxyl group of alcohols and nitrogen atom of pyrrole. (24) However, highly conductive PPy could not be observed when PVAL was used as a stabilizer (Sample 4). It is important to note that PVAL is an insulating polymer and possibly forms polymer composite when interacting with PPy nanoparticles. (12) Compared to SDS, the PPy nanoparticles doped with DBSA show much lower conductivity (Sample 5). In microemulsion polymerization, the dopant plays a dual function; dopant and surfactant, leading to the incorporation of the dopant anions to the PPy backbones. The dopant containing bulky structure like DBSA can be a steric barrier for the charge transport in the PPy chains, resulting in low conductivity. (15)

4 30 REUNG-U-RAI, A. et al. UV-Visible analysis UV-Visible spectra of the PPy nanoparticles correspond well with the conductivity results. Fig. 3a shows the UV-Visible spectrum of PPy nanoparticles having low conductivity (Sample 1). The spectrum showed a weaker absorption at around 460 nm (2.7 ev) and stronger absorption at around 900 nm (1.4 ev). The first absorption band is associated with the π-π* transition while the second is assigned to the bipolaron state of PPy, respectively. (5) Similar peaks are observed for the PPy with high conductivity (Sample 2-5) as shown in Figure 3b. The peak at 460 nm, due to π-π* transition band is more intense, compared to Figure 3a. Significantly, a broad free-carrier-tail band at around 900 nm (1.4 ev) is observed. This characteristic is due to the extended coil of PPy chains indicating a well conjugation, hence high degree of doping and conductivity. (17) (a) stabilizer. The PPy doped with SDS exhibited higher conductivity that doped with DBSA. UV- VIS spectra exhibited a good support with the conductivity results. Simple synthesis of highly conductive PPy nanoparticles offers a potential in using as conducting filler for preparation of conducting polymer composites. Acknowledgment The authors gratefully acknowledge the financial support from King Mongkut s University of Technology North Bangkok. References 1. Acik, M., Baristiran, C. and Sonmez, G Mater. Sci. 41: Goodwin, J.W., Markham, G.M. and Vinent, B. J Phys. Chem. 101: He, C., Yang, C. and Li, Y Chemical synthesis of coral-like nanowires and nanowire networks of conducting polypyrrole. Synth. Met. 139 : Absorbance 0.8 (b) Jang, J Adv. Polym. Sci. 199 : Jang, K.S., Lee, H. and Moon, B Synthesis and characterization of water soluble polypyrrole doped with functional dopants. Synth. Met. 143 : Wavelength (nm) Figure 3. UV-vis spectra of PPy nanoparticles (a) Sample 1 and (b) Sample 2, 3, 4 and 5 Conclusions PPy nanoparticles were chemically synthesized via microemulsion polymerization of pyrrole and APS in the presence of surfactant dopant. Spherical PPy nanoparticles of nm with the specific conductivity in the range of S/cm have been achieved, depending upon polymerization conditions. It was obviously found that low temperature-polymerization resulted in the decrease in particle size and increase in specific conductivity. Addition of n-amyl alcohol as a stabilizer induced well-conjugation of PPy chains, leading to the highly conducting PPy nanoparticles whereas this result could not be seen for PVAL 6. Jang, J. and Oh, J.H Chem. Commun Jurewicz, K., Delpeux, S., Bertagna, V., Beguin, F. and Frackowiak, E Supercapacitors from nanotubes/polypyrrole composites. Chem. Phys. Lett. 347 : Lin, C.W., Hwang, B.J. and Lee, C.R Methanol sensors based on the conductive polymer composites from polypyrrole and poly (vinyl alcohol). Mater. Chem. Phys. 55 : Lin, C.W., Hwang, B.J. and Lee, C.R Sensing behaviors of the electrochemically co-deposited polypyrrole-poly (vinyl alcohol) thin film exposed to ammonia gas. Mater. Chem. Phys. 58 :

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