e-Polymers 2007, no. 042. http://www.e-polymers.orgISSN 1618-7229
Gas sensing property of a conducting copolymer
Metin Ak,1 Basak Yigitsoy,1 Yusuf Yagci,2 Levent Toppare1*
1 Middle East Technical University, Department of Chemistry, 06531 Ankara, Turkey;
Fax: +903122103200; toppare@metu.edu.tr
2 Istanbul Technical University, Department of Chemistry, Maslak, Istanbul 80626,
Turkey; Fax:+90 212 285 6169; yusuf@itu.edu.tr (Received: 5 September, 2006; published: 2 May, 2007)
Abstract: A bifunctional amido thiophene, namely hexamethylene (bis-3-thiophene
acetamide) (HMTA) was synthesized and its copolymer with pyrrole was electrochemically prepared in water/PTSA solvent/electrolyte medium. Sensory behavior of this copolymer against ammonia, methanol, dichloromethane (DCM) and n-hexane vapors was investigated. The methanol vapor, which can form hydrogen bonds with the polar groups on the copolymer surface, showed the maximum response. DCM, which is less polar, yielded relatively lower response than methanol. Hexane, being the non-polar vapor, showed the lowest response. Introduction
Conducting polymers (CPs) contain π-electron backbones which is responsible for their unusual electronic properties such as electrical conductivity, low energy optical transitions, low ionization potential and high electron affinity. This extended π-conjugated system of the CPs has single and double bonds alternating along the polymer chain. The higher values of the electrical conductivity obtained in such organic polymers have led to the name “synthetic metals”. Many applications of CPs are well known [1].
The importance of environmental gas monitoring is well understood and much research has focused on the development of suitable gas-sensitive materials. Recently, there has been considerable interest in exploiting organic substances such as phthalocyanides [2, 3], porphyrin [4] and doped conductive polymers (CPs) [5-10] for that matter. Previous reports revealed that the CPs have great advantages of higher sensitivity towards toxic gases such as NH3, a lower detectable limit in the
range of a few tens of parts per millions, and the potential to operate at or near room temperature. This mechanism is attributed to the π-conjugated system in the conducting polymer chain. The interaction between the organic material and gas molecules result in an increase/decrease of polaron and/or bipolaron densities on the band gap of the polymer. Therefore, their modification implies both electrical and optical properties change for the conducting polymer [11].
CPs can also quantitatively detect chemical solvent vapors. Exposure of the conducting polymers to chemical solvent vapors causes polymer swelling, resulting in a dramatic increase in the electrical resistivity. The degree of swelling and the corresponding fractional increase in resistance are found to be proportional to the concentration of the analyte vapor. Therefore, by measuring the size of response, a quantitative vapor sensor can be made using conducting polymers [12].
One of the most interesting CPs is polypyrrole (PPy), which has environmental stability, good mechanical properties, high conductivity, and easy synthesis. These properties are promising for gas sensing applications. However, it brings about a self-limitation of its use. Due to its sensitivity to various gases and vapors the selectivity is low, which is the most serious problem in the use of PPy for gas or vapor detection. Due to the fact that copolymers generally exhibit physical and mechanical properties far different from those of blends of the same individual homopolymers, sensitivity and selectivity of the PPy to selected vapors and gases can be enhanced by copolymerization.
In this study we synthesized hexamethylene (bis-3-thiophene acetamide) (HMTA) [13] and its copolymer with pyrrole in aqueous media. Sensory behavior of this copolymer against NH3, methanol, dichloromethane and n-hexane vapors was
investigated.
Results and discussion FTIR Spectra
The IR spectral characteristics of the copolymer were discussed in comparison to that of the monomer. HMTA showed a characteristic, intense peak at 1640 cm-1, which belongs to C=O stretching vibrations. Two peaks at 2948 cm-1 and 2867 cm-1 correspond to aliphatic methylene stretchings.The peak at 1251 cm-1 indicates C−O– C ester group vibrations while the peak at 3265 cm-1 is attributed to N-H streching vibration. Also, the peak at 739 cm-1 is the result of aromatic C−Hα stretching of
thiophene units.
Poly(HMTA-co-Py) shows a new shoulder occurring at around 1620 cm-1 indicating conjugation. The disappearance of peak at 739 cm-1 was also evidence of the polymerization from the 2,5 position of thiophene ring [15]. In addition, the characteristic monomer peaks, especially carbonyl peak at 1640 cm-1, are also observed in the spectrum of the copolymer. These results prove the presence of HMTA moieties in the resultant polymers.
Conductivity Measurements
The four probe technique was utilized in order to measure the electrical conductivities of the copolymer film. Solution and electrode side conductivities were found as 2x10-3
and 4x10-3 S/cm, respectively. The same order of magnitude in the conductivities of both the electrode and the solution sides indicates the homogeneity of the films.
Morphology of the Films
As seen in Figure 1 surface morphologies of the polypyrrole and poly(HMTA-co-Py) (solution sides of the film) are completely different. Thus, besides other characterization techniques SEM also reflected the formation of copolymer and due to the different surface morphology, it is expected that sensing properties of the films will be also quite different.
Response of poly(HMTA-co-Py) Film to NH3 and Various Organic Vapors
The response of poly(HMTA-co-Py) when exposed to ammonia (%1) appears to be reversible as given in Figure 2.
0 20 40 60 80 100 120 140 off off off on on on 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 R ela ti v e resi st anc e Time (min)
Fig. 2.Reponse of the P(HMTA-co-Py) film to NH3 vapor.
0 5 10 15 20 0 5 10 15 20 Rel ati ve re si sta nce Time (min) hexane DCM methanol
Fig. 3. Response of the P(HMTA-co-Py) film to methanol, dichloromethane and hexane vapors.
The conductivity decreased rapidly by exposing the sensor to NH3 gas, and
recovered original value after reverting to N2 gas. The sensing mechanism is
explained by the compensation effect [14]. The decrease in the conductivity of the copolymer upon exposure to ammonia gas can be understood from the fact that the nitrogen contains a lone pair of electrons which can be donated to the initially oxidized polymer. This will neutralize the polymer cation and, therefore, the number of carriers will decrease, resulting in the decrease of the conductivity. With N2,
however desorption of the NH3 will the lead to reformation of polymer cation, and
therefore, the conductivity increases.
It was observed from our measurements that each solvent has its own impact on the variation of resistance, which is good for the detection and identification of gases. Results for methanol, dichloromethane and hexane vapors are presented in Figure 3. Methanol showed a large increase in resistance whereas hexane yielded a low response.
Swelling of the polymer matrix due to absorption of organic vapors may also increase the volume and thus, increase the distance between polymer chains, thereby increasing the contact resistance. The extent of swelling, and hence the electrical response, depends on the interactions between polymer and vapor. The methanol vapor, which can form hydrogen bonds with the polar groups on the copolymer surface, revealed the maximum response. Apparently, DCM exhibited lower response than methanol because of its less polar nature. Expectedly, non-polar hexane has a considerably low response.
We have also studied the reversibility of the sensor by exposing them at regular times. Figures 4, 5 and 6 show the response of the gas sensor for a periodic exposure to the DCM, hexane and methanol, respectively. As can be seen from the figures, methanol not only showed large increase in resistance but also good reversibility in the response of the copolymer film.
-10 0 10 20 30 40 50 60 70 80 0 1 2 3 4 5 6 7 off off off on on on Relat iv e Res ist ance Time (min.)
-10 0 10 20 30 40 50 60 70 off off off on on on 1.0 0.8 0.6 0.2 0.4 0.0 Re lat ive Re sis ta n ce Time (min)
Fig. 5. Reversibility in the response of sensory film to hexane vapor.
0 10 20 30 40 50 60 -2 0 2 4 6 8 10 12 14 16
18 off off off
on on on R el ativ e Re sistan ce Time (min.)
Fig. 6. Reversibility in the response of sensory film to methanol vapor. Experimental part
Materials and Reagents
3-Thiophene acetic acid, hexamethylene diamine, thionyl chloride, p-toluene sulphonic acid (PTSA), methanol, dichloromethane and n-hexane were purchased from Aldrich and used without further purification.
Equipments
Electrochemical synthesis of the copolymer was performed by using Wenking POS 73 potentiostat. Nicolet 510 FTIR Spectrophotometer and JEOL JSM-6400 were
used for FTIR and scanning electron microscopy (SEM) studies, respectively. Resistivity measurements were performed by using Keithley 614 electrometer.
Synthesis of Hexamethylene (bis-3-thiophene acetamide)
A mixture of 3-thiophene acetic acid (0.80 g 5.6 mmol) and thionyl chloride (0.77 g, 6.5 mmol) was placed in a 25 mL round bottom flask carrying a drying tube and refluxed for 1 h. A water aspirator vacuum was applied to remove the excess thionyl chloride. The liquid product was dropwise added to hexamethylene diamine (0.32 g, 2.81 mmol) in 20 mL dry THF in an ice bath under rapid stirring. The mixture was stirred for about 30 min. The product was collected by suction filtration and washed with THF to remove unreacted hexamethylene diamine and 3-thiophene acetic acid. A light-brownish solid product was obtained in 45% yield [13]. Scheme 1 shows synthesis route for monomer.
Synthesis of Copolymer of HMTA with Pyrrole [Poly(HMTA-co-Py)]
Electrochemical polymerization was carried out in an H-shaped cell. Platinum foils (1 cm2) were used as the working (WE) and the counter electrodes (CE). Ag/Ag+ was utilized as the reference electrode.
S C O NH (CH2)6 NH C O S pyrrole PTSA water H2N NH2 + S SOCl2 1.1V HMTA P(HMTA-co-Py) N H NH (H2C)6 O O S S N H N H N H N H x y HMTA O OH
Scheme 1. Synthesis route of the HMTA and poly(HMTA-co-Py).
HMTA was coated on the working electrode surface from its dichloromethane solution. The electrolyses were carried out in water with PTSA (5×10-2 M) and pyrrole
(8×10-2 M) at 1.1 V vs Ag/Ag+ at room temperature. Free standing black films (30 μm)
were formed in 1h (Scheme 1).
Measurement of the Gas and Volatile Organic Vapor Sensing Behavior
Gas sensing properties of the electrochemically prepared P(HMTA-co-Py) films were studied by exposing them to NH3 (gas). Schematic representation of the experimental
configuration employed for examining the gas sensing characteristics of the copolymer is shown Figure 7. The copolymer films were fixed within a home made closed stainless steel chamber, attached to a gas flow controller. Dried NH3 gas was
purged from the flow controller, using dry N2 as the diluent as well as the carrier gas.
N
2NH
3 Multimeter MM l i FlowmetersSensor chamber
Fig. 7. Schematic representation of the experimental configuration employed for examining the gas sensing characteristics of the copolymer.
The electrical response of the copolymer to various volatile organic vapors was measured by the experimental configuration schematically shown in Figure 8. An organic vapor diluted with dry nitrogen gas was passed through the chamber containing the sensor. The concentration of the vapors was adjusted by a flowmeter.
Fig. 8. Schematic representation of the experimental configuration employed for examining the volatile organic vapor sensing characteristics of the copolymer.
For evaluating sensory capability, the relative change of the sensor resistance of the copolymer to a certain gas or vapor (relative resistance) was calculated from
ΔR/R=[(Rgas-R0)/Rgas] where R0 is the steady electrical resistance of the sensor in
nitrogen; and Rgas is the electrical resistance upon exposure to a test gas.
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