Indian Journal of Chemical Technology Vol. 18, November 2011, pp. 446-450 Synthesis and characterization of polypyrrole polyvinyl alcohol composite film with various organic acids dopants and their gas sensing behavior D B Dupare 1, M D Shirsat 2 & A S Aswar 1* 1 Department of Chemistry, Sant Gadge Baba Amravati University, Amravati 444 602, India 2 Department of Physics, Dr. Babasaheb Ambedkar Marathwda University, Aurangabad 431 004, India Received 13 September 2010; accepted 10 October 2011 Polypyrrole polyvinyl alcohol (PPY-PVA ) composite films with various organic acids such as camphor sulphonic acid (CSA), dodecyl benzene sulphonic acid (DBSA) and p-toluene sulphonic acid (p-tsa), have been synthesized in situ by chemical oxidative polymerization method. These synthesized films are characterized by using electrochemical technique, conductivity measurement, UV-visible spectroscopy, FTIR and SEM. It is observed that the PPY-PVA composite films provide good electrochemical properties, conductivity as well as polymer matrix with uniform and porous surface morphology which can be used for the immobilization of biocomponent. The current-voltage characterization reveals that these films are conducting in nature with ohmic behavior. The PPY-PVA doped composites thin films are used for different concentrations (ppm) of trimethyl ammonia and ammonia gas for their sensitivity at room temperature (304K). Results reveal that with organic acids dopant electrical property and environmental stability of PPY-PVA composites film are enhanced. Keywords: Conducting polymer, Polypyrrol, Thin films, TMA gas sensor Chemical sensors are being widely developed for environmental monitoring, industrial hazards detection and sensing of chemical warfare agents. The development of reliable and selective solid-state gas sensors has great importance both in the field of atmospheric pollution and emission control from combustion plants. The determination of hazardous gases is one of the most popular and well-known applications of gas sensors. During the past two decades, numerous efforts have been made to develop gas sensor with fast, selective and sensitive response towards various gases. Conducting polymers are being widely used in gas sensor applications because they provide stable and porous matrix for the gas component and also facilitates the electron transfer process. Recently, many efforts have been aimed to improve the physical properties of polypyrrole, such as processability, stability or mechanical integrity. Such composite formation is being optimized in order to prepare processable material which can be used and processed like common polymers. The poly-conjugated conducting polymers composites have recently been used for gas sensing applications *Corresponding author. E-mail: anandaswar@yahoo.com due to several useful features like direct and easy deposition on the sensor electrode, control of thickness and redox conductivity of polyelectrolyte characteristics 1. These possibilities as well as the improvements in atmospheric stability of polypyrrole composites make them serious candidates for the use in specific technological applications 2. The advantages of conducting polymer composites applicability to gas sensors are the low cost and its affinity for various substrates 3,4. Polypyrrole (PPY) is an inherently conductive polymer due to the presence of excluded π conjugation of electrons which is stabilized by the heterocyclic group. PPy is an especially promising conductive polymer for commercial applications, owing to its high conductivity, good environmental stability and ease of synthesis. It is easy to prepare by standard electrochemical techniques and its surface charge characteristics can easily be modified by changing the dopant anion (X - ) that is incorporated into the material during synthesis. Conducting polymers have extensively been used in biosensors 5, EMI shielding 6, light weight batteries 7, electrochromic display devices 8, electronic devices 9, actuators 10 and electrochromic materials 11. Considering the importance of this compound, the present study was undertaken to synthesize and characterize
DUPARE et al.: SYNTHESIS & CHARACTERIZATION OF PPY-PVA COMPOSITE FILM 447 polypyrrole polyvinyl alcohol (PPY-PVA) films doped with various organic acids such as camphor sulphonic acid (CSA), dodecyl benzene sulphonic acid (DBSA) and p-toluene sulphonic acid (p-tsa). The gas sensitivity of these is investigated at room temperature towards trimethyl ammonia (TMA) and ammonia gases in the concentration range 5 800 ppm and above level. Experimental Procedure The pyrrole monomer (Rankhem Ranbaxy, New Delhi, India) was distilled twice before use. The dopants camphor sulphonic acid (CSA) (Merck, India), dodecyl benzene sulphonic acid (DBSA) and p-toluene sulphonic acid (p-tsa) (Loba Chemie), anhydrous ferric chloride (Spectrochem, India) and hydrochloric acids (Qualigen Fine Chem India) were used as received. All reactions were carried out in double distilled water. Synthesis of composite films The synthesized PPY-PVA composites thin films were doped with organic acids like CSA, DBSA and p-tsa at room temperature on glass substrate by using chemical oxidative polymerization method. Double distilled pyrrole (monomer) was used. Initially the molar concentrations of monomer (pyrrole), primary dopant (HCl), polymer additive matrix (PVA), secondary dopant organic acids and oxidant (FeCl 3 ) were optimized. The polyvinyl alcohol was dissolved in conductivity water with continuous stirring, then an appropriate molar concentration of pyrrole (monomer 0.5 M), primary dopant (HCL 1M), PVA additive (25 mg), organic acids (CSA, DBSA and p-tsa) and oxidant (FeCl 3 0.5M) was added. This resulting reaction mixture along with glass substrate was kept for 24 h in closed vessel at room temperature to get uniform PPY-PVA composite thin films. A suitable combination which shows good response to ammonia and TMA gases has been selected for further synthesis and characterization. Characterization The structural and morphological characterizations of doped PPY-PVA composites thin films have been studied. The UV-visible and FTIR spectra of all polymer samples were recorded at room temperature using dimethylsulphoxide (DMSO) as solvent. The surface morphology was characterized by using scanning electron microscopy (SEM) at different magnification range by using JEOL-JSM-6360A scanning electron microscope. Gas sensitivity of synthesized PPY-PVA doped organic acids films was investigated towards ammonia and TMA gases at room temperature by using indigenously developed computer controlled gas sensor system. The electrical conductivity (I-V characteristics) of the films was recorded using four probe-method with computer control system. Results and Discussion UV-Visible spectra The UV-Visible absorption spectra of the polymer films were recorded by dissolving the polymer film in dimethyl sulfoxide (DMSO) and the absorption spectra of PPY- PVA organic acids doped composites films are shown in a Fig. 1. The band observed at 262-280 nm corresponds to π -π* transition of PPY-PVA organic acids composites films. The band in the 380-395 nm region may be due to n-π * transition in PPY-PVA film owing to the presence of lone pair on nitrogen in pyrrole ring, which is inter-charge transfer band associated with change of benzenoid to quinonoid ring. The transitions of quinone-imine groups, together with the extending tail, is observed at 940-1100 nm. The conducting emeraldine salt (E S) phase in the polymer is identified by observation of broad peak at 950 nm. Thus, from the above measurements, it is confirmed that the polymer is composed of mixed phase, i.e. conducting and insulating, of the polymer. The UV Visible spectral data suggest that synthesized films of PPY-PVA doped with organic acids are composite in nature. FTIR analysis The molecular structure of synthesized PPY-PVA composites films was characterized by FTIR spectroscopy. The infrared spectra of all PPY-PVA Fig.1 UV visible spectra of organic acid doped PPY-PVA films
448 INDIAN J. CHEM. TECHNOL., NOVEMBER 2011 composites films appear similar when dissolved in DMSO solvent. The FTIR spectra of synthesized PPY and PPY-PVA blend thin film are shown in Fig. 2. The bands appear in the region 3300-3500 cm -1 due to N-H stretching frequency of an aromatic amine 12. A broad band observed at 3471 cm -1 in the spectrum of PPY-PVA film and NH region also shows dependence of the doping anion of organic acids. Anion which typically forms hydrogen bond with amine group shows variations in the intensity and shape of the NH band, thus indicating that the doping is higher in the sample. The bands at 2900 and 3000 cm -1 are due to CH 3 and CH 2 (C-H stretching). The two bands observed in the range 1410-1440 cm -1 are due the stretching of C N frequency of the benzonic and quinonic rings respectively because of the conducting state of the polymer 13,14. The bands corresponding to quinoid (N=Q=N) and benzenoid (N B N) ring stretching modes are observed at 1666 cm -1 for (C=N) stretching and 1439 cm -1 for (C-N) respectively. The shoulder observed at 1033 cm -1 is due to the symmetric and asymmetric C-O stretching vibrations of polyvinyl group, and the peaks observed at 707 cm -1 are due to C-H bending. All these observations support the presence of conducting emeraldine salt phase of of PPY-PVA composites material 15,16. Morphology film The surface morphology of the synthesized PPY-PVA organic acids doped composites films are shown in Fig. 3. This shows the relationship between adjacent particles and small group of particles. The figure exhibits better porous, granular and globular surface morphology with very good uniformity and adhesiveness for synthesized film samples, indicating their suitability for sensor applications. Current-voltage characterization The current-voltage (I-V) characterization of the PPY-PVA film was measured by four- probe method using indigenously developed computer controlled I-V measurement system at room temperature. The current voltage characteristics of the synthesized PPY-PVA films doped with CSA, DBSA and p-tsa thin films were studied to ensure an ohmic behavior of the films. A linear relationship of the I-V characteristics is observed, which shows the ohmic behavior of doped PPY-PVA films 17 (Fig. 4). Gas sensing characteristics The TMA and ammonia gas sensing characteristic of the synthesized PPY-PVA films has been analysed at room temperature using the four-probe technique for resistivity measurements. The synthesized PPY-PVA composites films were exposed to TMA and ammonia Fig. 2 FTIR spectra of composite of films
DUPARE et al.: SYNTHESIS & CHARACTERIZATION OF PPY-PVA COMPOSITE FILM 449 Fig. 3 SEM images (a) CSA-doped PPY-PVA, (b) DBSA doped PPY-PVA films and (c) p-tsa doped PPY-PVA films gas for 5 min response time. The recovery time was measured by exposing the film to air for 5 min. The change in resistivity of the film was measured at an interval of 10 s. All the samples show response to the Fig. 4 Relationship of the I-V characteristics of (a) CSA doped PPY-PVA, (b)dbsa doped PPY-PVA, and (c) p-tsa doped PPY-PVA ammonia and TMA gas vapors. The ammonia and TMA gas sensing curves of PPY-PVA at different concentrations of ammonia gas (5-800 ppm) have been analysed. It is observed that the resistivity of the PPY-PVA doped thin film increases in the presence of ammonia and TMA gases and after a few minutes becomes saturated. The resistivity decreases steadily to a minimum value, when the ammonia and TMA gas are removed. However, a drift from its original value was observed 18,19. The relationship between change in resistivity and exposure time of the synthesized PPY-PVA doped composites films to different concentrations of TMA and ammonia gas are shown in Fig. 5.
450 INDIAN J. CHEM. TECHNOL., NOVEMBER 2011 Fig. 5 Relationships between resistivity and exposure time of PPY-PVA organic acids doped composites films of ammonia gas and TMA gas sensing response from 5-800ppm (a) PPY-PVA p-tsa film, (b) PPY-PVA CSA film, and (c) PPY-PVA DBSA film Conclusion In the present investigation, the PPY-PVA-organic acids doped composite films on glass substrate have been synthesized for their comparative study. The synthesis of polymer films of PVA with pyrrole is done in the presence of three different supporting organic acids. It is observed that polymer composites doped with p-tsa result in better quality film. Results also show different morphologies for different organic acids. Conducting and thermally more stable composites are obtained in comparison to that with polypyrrole. Thus, it may be concluded that synthesized PPY-PVA organic acids doped composites films are applicable for monitoring gaseous pollution in environment. Acknowledgement One of the authors (DBD) is grateful to the University Grant Commission, New Delhi, for providing financial support to carry out research work. References 1 Gaikwad P D, Savale P A, Shirale D J, Kharat H J, Kakde K P, Gade V K & Shirsat M D, Microwaves and Optoelectronics, edited by M D Shirsat (Anshan Tunbridge Wells U K), 2005, 450. 2 Malinauskas A, Rev Synth Met,107(1999)75-83. 3 Maksymiuk K, Electroanalysis, 18 (2006)1537-1551. 4 Eftekhari A, Synth Met, 145 (2004) 211. 5 Roth S & Graupher W, Synth Met, 57 (1993) 3623. 6 Bartlett P N & Birkin P R, Anal Chem, 65 (1993) 1118. 7 Depaoli M A, Gasalboremiceli G, Girotto E M, Gazotti W A & Macdiarmid A G, Electrochim Acta, 44 (1999) 2983. 8 Delccuw O M, Simenan M M, Brown A R & Einerchand R E F, Synth Met, 87 (1997) 53. 9 Kaneto K, Kaneko M, Min Y & Macdiarmid A G, Synth Met, 71 (1998) 2211. 10 Chen S A & Cho C J, Synth Met, 79 (1996) 93. 11 Skotheim T A, Elsenbsumsr R L & Reynolds, J R, Handbook of Conducting Polymers (EdII) (Marcel Dekker, New York), 1998. 12 Ameer Q & Adeloju S B, Sensor & Actuators B, 106 (2005) 541-552. 13 Ghosh M, Barman A, De S K & Chatterjee S, Solid State Commun, 103 (1997) 629. 14 Bai H & Shi G, Gas sensors based on conducting polymers, Rev Sensors, 7(2007) 267. 15 Mabrook M F, Pearson C & Petty M C, Sensor Actuators B, 115 (2006) 547. 16 Shirale D J, Gade V K, Gaikwad P D, Kharat H J, Kakde K P, Savale P A, Hussaini S S, Dhumane N R & Shirsat M D, Mater Lett, 60 (2006) 1407. 17 Gangopadhyay R & De A, Sensor Actuators B, 77 (2001) 326. 18 Kharat H J, Kakde K P, Savale P A, Ghosh P, Datta K & Shirsat M D, Polym Adv Technol, 18 (2007)1. 19 Dupare D B, Aswar A S & Shrisat M D, Sensor Transducer, 6 (2009) 94.