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Sulphonic acids doped poly(N-ethyl aniline): a material for humidity sensing application.

INTRODUCTION

In the past few decades, inherently conducting polymer (ICP) polyaniline, (Pani) has been paid considerable attention because of its high electrical conductivity [1], thermal stability [2], ease of preparation method, and good environmental stability [3] and is found to be the most promising candidate for technological applications [4, 5], especially in light weight batteries and sensors [6, 7]. However, the great potential of this compound is masked by serious disadvantages such as its insolubility in common organic solvents, infusibility, and hence nonprecessibility [8]. The conducting form of polyaniline (emeraldine salt) powder synthesized in aqueous HCI solution is insoluble in most common organic solvents including 1-methyl 2-pyrrolidone (NMP). Because of strong interchain interactions, these compounds are not soluble and do not melt, thus they can be neither solutions nor thermally processed. The implementation of practical applications has been facilitated by the improvement of the processibility of the conducting polymers. For these reasons, in the past few years, significant research efforts have been directed toward the synthesis of processable forms of electroactive polymers [9, 10]. Among these areas of progress, the approach of chemical synthesis method and the use of functional dopants in the polymer matrix as an inherent part of the doping are more attractive, as it eliminates the use of volatile dopants. Recently, new types of protonic acids have been used, such as polyacids and functionalized acids, to dope chemically synthesized polyaniline to improve the selected properties, in particular, solubility, processibility, and electrical conductivity [11, 12].

Incorporation of polar functional groups or long and flexible alkyl chains in the polymer backbone is a common technique to prepare Pani-type polymers, which are soluble in water and/or organic solvents. For example, substituted polyanilines, such as polytoluidines, polyanisidines, or poly(N-methyl) or poly(N-ethyl anilines), are more soluble in common organic solvents than the unsubstituted polyaniline, but render the low conductivity. Figure 1 shows the structures of poly(N-ethyl aniline) (a), its reduced (b), and oxidized (c) units.

An accurate and reliable estimate of water vapor in different environments is an important prerequisite for a variety of processes such as agriculture, weather control, drying technology, food processing, textile technology, etc. Measurement and control of the humidity of the environment is also important for domestic comfort (e.g., air conditioning) and also for the working of several instruments useful for industrial-controlled systems. Therefore, humidity sensors have found wide applications in industry productions, process control, environmental monitoring, storage, electrical applications, etc. [13], and the research, devoted to the development of new materials for sensor device, is gaining more and more attention. Different types of materials such as electrolytes, porous ceramics, and organic polymers [14] are currently being used for making humidity sensors. Conducting polymers or [pi]-conjugated polymers possess special electrical, electrooptical properties, and have been widely employed in the construction of various types of sensors. Recently, they were also investigated as humidity-sensitive materials and some encouraging results have been obtained [15, 16]. We have taken up a systematic investigation of conducting polymers [17-22] and polymers doped with suitable molecules for the development of humidity sensors [23, 24].

[FIGURE 1 OMITTED]

Therefore, the aim of this work is to synthesize the poly(N-ethyl aniline) (PNEA) by using camphor sulphonic acid (CSA) and p-TSA as dopants and to obtain the structural information by studying the physicochemical properties of this polymer using various analytical techniques such as UV-vis and FTIR spectroscopy, SEM, TGA, DSC, and conductivity measurements. The synthesized polymers were then successfully utilized as humidity sensors for a broad range of humidity ranging between 20 and 100% RH.

EXPERIMENTAL

All chemicals used were of analytical reagent (AR) grade. The solutions were prepared in doubly distilled water. The polymerization of the monomer, N-ethyl aniline (0.69 ml), was initiated by the drop wise addition of the oxidizing agent, (N[H.sub.4])[.sub.2][S.sub.2][O.sub.8] (1.26 g), under constant stirring at low temperature between 0 and 5[degrees]C in an acidified solution (10 ml) containing 3.08 g of CSA or 1.90 g of p-TSA. The monomer to oxidizing agent ratio was kept as 1:1. After the complete addition of oxidizing agent, the reaction mixture was kept under constant stirring for 24 h. The greenish-black precipitate of the polymer was then isolated by filtration and conditioned by washing with double-distilled water (to remove the unreacted monomer and acid) and drying in a hot air over oven at 80[degrees]C for 24 h at ordinary pressure. UV--vis spectra of polymer solution in m-cresol were recorded using Hitachi-U3210 spectrophotometer in the range of 300-900 nm.

FTIR spectra of the polymer were taken on a Perkin-Elmer-Spectrum 2000 spectrophotometer between 400 and 4000 [cm.sup.-1]. The samples were prepared in the pellet form using spectroscopic grade KBr powder. Morphological studies were performed with the help of Philips XL-30 scanning electron microscope. Thermogram of the polymer samples was recorded using Mettler-Toledo 851 thermogravimetric analyzer in the presence of [N.sub.2] atmosphere from RT to 900[degrees]C with a heating rate of 10[degrees]C/min.

The room temperature conductivity (surface resistivity) was measured using two-probe technique. Dry powdered samples were made into pellets using a steel die having 1.5 cm diameter in a hydraulic press under a pressure of 7 tons. Temperature-dependent electrical conductivity of the polymer samples was measured using a laboratory made setup. The electrical contacts were made by using platinum foils. The controlled heating of the sample was carried out by the heater placed near the sample (i.e., the pellet of the material is sandwiched between two platinum foils and kept on a metal plate provided with the heater). The change in resistance was recorded with the increase in temperature, and the temperature was controlled by the controller. The conductivity values were calculated directly from the measured resistance and sample dimensions.

Figure 2 shows the schematic diagram of the dynamic humidity chamber used for varying and measuring the humidity by two-temperature method. The humidity system used consists of a closed flask (total volume 500 ml) with two necks for inserting thermometers and the sensor. The flask is partially filled with water and kept in a thermocole container. External container is filled with ice to the equal level of water (which is present in the flask). The temperature of the system is adjusted by mixing ice and salt as required (thereby the temperature of the water inside the flask is lowered). Thus, the water inside the flask can be kept at the required temperature ([T.sub.1]). The sensor {i.e., pellets of poly(N-ethyl aniline)} was mounted inside the flask at height of 6 cm from the surface of the water, and the temperature of the sample ([T.sub.2]) is measured with a thermometer, which is placed at an equal height of the sample to be studied.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The humidity inside the chamber is calculated by taking the ratio of the saturated water vapor pressure at water temperature ([T.sub.1]) and the sample temperature ([T.sub.2]). The values of the saturated vapor pressure are obtained from the CRC Handbook of Chemistry and Physics. It is to be noted that the temperature of the sample changes by 3-6[degrees]C during the experiment. The % RH inside the flask is given by

% RH = [[[E.sub.w]([T.sub.1])]/[[E.sub.w]([T.sub.2])]] x 100

where [E.sub.w] ([T.sub.1]) is the saturated water vapor pressure at the temperature of water and [E.sub.w] ([T.sub.2]) the saturated water vapour pressure at the temperature of the sensor element. Different % RH values are obtained by adjusting the temperature of the water inside the flask, with ice, and water mixture from room temperature to 0[degrees]C. The system equilibrium time is quite small and stable readings were obtained within 10 min [24].

RESULTS AND DISCUSSION

UV-vis spectroscopy is a very sensitive tool for the studies of protonation as well as for the elucidation of the interactions between the solvent, the dopant, and the polymer chains. Figure 3 shows the optical absorption spectra of the poly(N-ethyl aniline) doped with CSA and p-TSA recorded by using m-cresol as a solvent. In m-cresol, poly(N-ethyl aniline) exhibits two peaks at 320 and 420 nm and increasing absorption at ~820 nm. The peak at 320 nm is very sharp and intense, which corresponds to the [pi]-[pi]* transition of the benzenoid rings, while a small peak at 420 nm can be attributed to the localized polarons, which are the characteristics of the protonated polyaniline. The increasing absorption at 820 nm showing free carrier tail, characteristic of extended coil conformation, is assigned to the conducting emeraldine salt phase of the polymer [17].

The spectral features observed in Fig. 3 reveal the enhanced solubility of poly(N-ethyl aniline) doped with CSA and p-TSA, in m-cresol as a solvent. In comparison of the two spectra, it is observed that the shoulder at 660 nm representing insulating pernigraniline phase of the polymer is observed clearly as a small hump in CSA-doped polymer, whereas it is absent in the p-TSA-doped PNEA. This indicates that there is no or very small formation of pernigraniline phase of the polymer synthesized using p-TSA as a dopant.

Figure 4 represents the FTIR spectra of the poly(N-ethyl aniline) doped with CSA and p-TSA, and the peak positions related to the corresponding chemical bonds are listed in Table 1. From the figure and Table 1, it is observed that the structure of the poly(N-ethyl aniline) is similar to that of the polyaniline, with characteristics modes of the polyaniline backbone with a very small variation. The presence of the two bands in the vicinity of 1500 and 1600 [cm.sup.-1] is assigned to the nonsymmetric [C.sub.6] ring stretching modes. The higher frequency vibration at 1600 [cm.sup.-1] has a major contribution from the quinoid rings, while the lower frequency mode at 1500 [cm.sup.-1] depicts the presence of benzenoid ring units. The presence of these two bands clearly shows that the polymer is composed of the amine and imine units. Further, this also supports our UV-vis characterization, discussed earlier, where the different phases are observed in the spectrum. Also, the presence of well-defined peaks at 1732 and 1032 [cm.sup.-1] related to the C=0 and S[O.sub.3.sup.-] groups, respectively, show that the incorporation of CSA and p-TSA into the polymer backbone [25]. The presence of vibration bands of the dopant ion and other characteristic bands confirm the presence of conducting emeraldine salt phase in the polymer. On careful observation, it is noticed that the peak at 1732 [cm.sup.-1] related to the C=O group of CSA is very sharp and intense in CSA-doped PNEA, showing the efficient doping of the polymer. On the other hand, the peak at 1032 [cm.sup.-1] representing S[O.sub.3.sup.-] group of p-TSA is not that much strong but appears as a short and broad peak showing less effective doping of the polymer. This is further supported by our conductivity measurements, where the conductivity of CSA-doped PNEA is higher than that doped with p-TSA. The peak at 2924 [cm.sup.-1], which appears due to C-H stretching vibration of the substituent ethyl group, present on the nitrogen atom is observed exclusively in the poly(N-ethyl aniline) and are absent in the polyaniline [26].

[FIGURE 4 OMITTED]

Morphology of the polymer was also found to be dependant upon the acid used for doping purpose. Figures 5 and 6 show the micrographs of poly(N-ethyl aniline) doped with CSA and p-TSA at different magnifications, respectively. The morphology of the polymer synthesized using CSA exhibits exclusively a spongelike structure at 1000x and 2000x (Fig. 5a and b). Whereas at higher magnifications of 4000x (Fig. 5c), the clarity of the morphology is found to be further enhanced and it is observed that the spongelike structure is because of aggregation of big oval and circular-shaped uniform-sized granules arranged in a compact manner with less voids. The appearance of granules in poly(N-ethyl aniline) can be explained by considering the steric contribution of the substituent ethyl group present on the nitrogen atom. It leads to the distortion in the polymer chains, which, in turn, results in a break down of the polymer chain into the granular-shaped fragments, instead of a long chain or fibers, appearing as granules at higher magnification in the micrograph.

The p-TSA-doped PNEA exhibits similar type of morphology showing spongelike structure at lower magnifications of 1000x and 2000x (Fig. 6a and b). Whereas at higher magnifications of 4000x (Fig. 6c), one can clearly see that the spongelike structure is composed of small fibers throughout the surface of the polymer with a netlike appearance. But the structure is not so compact as in CSA-doped PNEA. From the SEM studies, we can conclude that CSA-doped material shows spongelike morphology, while p-TSA-doped material offers fibrillar morphology (with small fibers, showing netlike appearance) to the polymer. Thus, depending upon the type of the dopant, the morphology is found to be varying from spongelike structure to fibrillar in nature.

Figure 7 displays the thermal profile (TG/SDTA) of the poly(N-ethyl aniline) doped with CSA and p-TSA. From the figure, it is observed that the polymer exhibits a three-step decomposition pattern similar to that of unsubstituted polyaniline. The first step in the decomposition pattern from RT-100[degrees]C is obviously due to the removal of free water molecules/moisture present in the polymer matrix. The second step loss starting from 220 to 300[degrees]C is mainly because of the loss of the dopant ion from the polymer chains (thermal dedoping). Whereas, the third-step loss starting from 350[degrees]C onwards is accounted for the degradation and decomposition of the skeletal polymer backbone after the elimination of the dopant ion [17].

The first derivative plot also reveals the similar weight loss pattern. The first step of weight loss responsible for the loss of water molecules or moisture showing small peak in SDTA completes at ~100[degrees]C in both the polymers. The second-step weight loss, due to the dedoping observed as a sharp peak, which starts at 220[degrees]C and completes at 350[degrees]C. The degradation of the polymer backbone appears as a sharp peak starting from 350[degrees]C and completes at 450[degrees]C in both the polymers. From the careful observation of the thermograms, it is noticed that the third step loss is split into a small step and a plateau in CSA-doped polymer (Fig. 7a); on the other hand, a gradual weight loss with respect to temperature is observed in p-TSA-doped PNEA.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

In both the polymers, rapid weight loss is observed in the third step of decomposition over a small temperature range of 350-500[degrees]C. This is mainly because of the presence of the alkyl substituent present on the nitrogen atom of the polymer backbone. Because of the greater extent of steric effect rendered by this flexible ethyl group, loosening of the polymer skeleton requires the less amount of energy to undergo a complete breakdown of the polymer chains. Thus, we can say that the incorporation of substituent on the nitrogen atom reduces the thermal stability of the resulting polymer because of the formation of the defects in the polymer chains, which favors the early degradation and decomposition of the polymer matrix. Major weight loss is observed in 250-450[degrees]C in both the polymers. On comparison of the weight losses in the third step of decomposition, it is observed that the 50% of the original weight is stable upto 440[degrees]C in CSA-doped PNEA, whereas in p-TSA-doped PNMA it is stable only up to 390[degrees]C. Thus, CSA-doped PNEA is thermally more stable than that doped with p-TSA.

Figure 8 shows the DSC thermogram of the CSA and p-TSA-doped poly(N-ethyl aniline) and exhibits only one endotherm, which completes at 120[degrees]C, which is attributed to the expulsion of water molecules present in the polymer matrix [27]. The second endothermic peak, at 210[degrees]C, is due to the thermal dedoping of the CSA from the polymer chains. The exothermic peak at about 370[degrees]C suggests the interchain crosslinking and thermally effected morphological changes [28-30]. The thermogram also confirms the absence of any glass transition ([T.sub.g]) and melting ([T.sub.m]) temperature for the polyaniline salt system. These results are well in agreement with our thermogravimetric analysis results described in the previous section. Similar trends were observed in both the polymers.

[FIGURE 8 OMITTED]

The RT solid-state conductivities were measured on pressed pellets having a diameter of 1.5 cm using two-probe technique. Table 2 gives the values for the same. The conductivity of CSA-doped poly(N-ethyl aniline) is higher than that doped with p-TSA. This strongly supports our FTIR characterization where efficient doping is observed in CSA-doped PNEA. The lower conductivity relative to polyaniline in both the polymers may be explained by an increase of the interchain distance and diluting effect of the charge carriers caused by the presence of bulky methyl group present on the nitrogen atom in the polymer. Consequently, the torsion angle between the repeat units is greater in substituted polyaniline. This explains the noticed decrease in the conductivity.

Figure 9 shows the temperature-dependent conductivity of poly(N-ethyl aniline) doped with CSA and p-TSA. From the figure, it is observed that the conductivity is found to increase with temperature for both the samples. In CSA-doped PNEA, there is a continuous increase in the conductivity as a straight line over the entire temperature range studied, while in p-TSA-doped polymer slow, steady increase in the conductivity is observed up to 100[degrees]C. After 100-120[degrees]C, there is a sudden increase in conductivity. The increase in conductivity with increase in temperature is the characteristic of "thermal activated behavior." The increase in conductivity is due to the increase of efficiency of charge transfer between the polymer chains and the dopant with increase in the temperature [31, 32]. It can also be suggested that the thermal curing affects the chain alignment of the polymer, which leads to the increase of conjugation length and that brings about the increase of conductivity. Also, there had to be molecular rearrangement on heating, which made the molecular conformation favorable for electron delocalization [33, 34].

[FIGURE 9 OMITTED]

Figure 10 shows the characteristic response of poly(N-ethyl aniline) doped with CSA and p-TSA as a function of relative humidity (% RH). From the figure, it can be noted that the resistance varied almost linearly from 20 to 100% RH and found to decrease from low humidity (dry state) to high humidity (wet state).

The decrease in the resistance or increase in the conductivity with increasing humidity can be attributed to the mobility of the dopant ion, which is loosely attached to the polymer chain by weak van der Walls forces of attraction. At low humidity, the mobility of the dopant ion is restricted because under dry conditions the polymer chains would tend to curl up into compact coil form. On the contrary, at high humidity, the polymer absorbs water molecules, and the polymer chains get hydrated. As a result, swelling up of the polymer chains takes place, followed by the uncurling of the compact-coil form into straight chains that are aligned with respect to each other. This geometry of the polymer is favorable for enhanced mobility of the dopant ion or the charge transfer across the polymer chains and hence the conductivity.

Furthermore, the adsorbed water molecules or water content plays an important role in the transport properties [35]. Also, it has been reported that the conductivity of conducting polymer increases when the sample absorbs the moisture. The water molecules get dissociated into [H.sub.3][O.sup.+] (O[H.sup.-]) kind of species and the respective ions behave like oxidizing or reducing agent, which changes the effective doping level of the polyaniline. Decrease of resistance with increase in the humidity proves the adsorption of the water molecules, which makes the polymer more p-type in nature, i.e., the whole concentration is increased by the donation of the lone pair from the conducting complex toward the dopant water molecules. Thus, the partial charge-transfer process of conducting species with that of water molecules results into the decrease in the sheet resistivity. At higher humidity level, the mechanism may be different [36]. According to the Matveeva [37] and Ogura et al. [6], the reaction of polyaniline with residual water occurs as shown in Scheme 1. The amine nitrogen centers act as the acceptor of protons, and the imine ones act as the donor of protons. According to Scheme 1, water dissociates at the imine center (HOH [left and right arrow] [H.sup.+] + O[H.sup.-]) and the proton incorporates into the polymer chain and [pi]-conjugation of aromatic rings, which promotes easier electron transfer. This effect is comparable to that caused by acid doping of polyaniline. Earlier it was shown by Lubentsov et al. [16] that the interaction of water vapor with an emeraldine form of polyaniline influences the crystal structure of polyaniline and changes its conductivity. It has also been proposed that the [H.sub.2]O molecules may be bound either by two hydrogen bonds--in which case they are fixed--or by a single hydrogen bond, in which case they can rotate. In that case, they are attached either to the polymer or to a fixed [H.sub.2]O molecule [35]. Two possibilities can be represented as shown in Fig. 11.

[FIGURE 10 OMITTED]

The almost linear variation with respect to % relative humidity (% RH) can be used in an amplifier circuit for converting the measured values into measurable % RH values. On careful observation of Figure 10, it is clearly seen that poly(N-ethyl aniline) doped with CSA shows a linear response from 20 to 100% RH. On the other hand, in p-TSA-doped polymer, the resistance is found to drop down from 20 up to 50% RH rapidly in a linear fashion, while after 50% RH slightly saturation is observed with a very small decrease in the resistance up to 80% RH. In other words, we can say that poly(N-ethyl aniline) doped with p-TSA shows two-step sensing response. Thus, poly(N-ethyl aniline) doped with CSA shows better sensing properties and exhibits good linearity in sensing response curve than that doped with p-TSA. Further detailed investigations of response time, hysteresis, and stability are in progress in our laboratory.

CONCLUSIONS

Poly(N-ethyl aniline) doped with CSA and p-toluene sulphonic acid (p-TSA) was synthesized by in situ chemical polymerization method using ammonium per sulphate as an oxidizing agent. This is a single-step polymerization process for the synthesis of directly conducting emeraldine salt phase of the polymer. Formation of mixed phases of polymer together with conducting emeraldine salt phase is confirmed by spectroscopic techniques. SEM studies revealed that PNEA doped with CSA shows spongelike morphology with oval and circular shaped big granules, while small size fibers were observed in p-TSA-doped PNEA resulting in a netlike structure. Thermal analysis shows that poly(N-ethyl aniline) has three-stage decomposition pattern similar to polyaniline. The less conductivity in poly(N-ethyl aniline) compared to polyaniline is due to the cumulative steric as well as electronic effects of the bulky methyl substituent present on the nitrogen atom. High temperature conductivity measurements show the "thermal activated behavior." The almost linear response of poly(N-ethyl aniline) doped with CSA to the broad range of humidity together with good electrical conductivity proves to be a competent material for humidity sensor.

[GRAPHIC OMITTED]

[FIGURE 11 OMITTED]

ACKNOWLEDGMENTS

Authors are thankful to Dr. B. K. Das, Executive Director, C-MET, Pune, for his constant encouragement.

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Milind V. Kulkarni, Annamraju Kasi Viswanath

Photonics and Advanced Materials Laboratory, Centre for Materials for Electronics Technology, Panchawati, Pune 411 008, India

Correspondence to: A.K. Viswanath; e-mail: v_kasi@hotmail.com
TABLE 1. Characteristic frequencies of chemically synthesized CSA and
p-TSA doped poly(N-ethyl aniline).

Wavenumber ([cm.sup.-1])
Poly(N-ethyl Poly(N-ethyl
aniline)-CSA aniline)-p-TSA Band characteristics

 579.85 557.98 C-H out of plane bending vibration
 814.49 806.48 Paradisubstituted aromatic rings
 indicating polymer formation
1030.92 1032.34 Because of S[O.sub.3.sup.-] group of the
 CSA and p-TSA
1115.71 1110.77 C-H in plane bending vibration
1312.05 1311.02 Aromatic C-N stretching indicating
 secondary aromatic amine group
1496.97 1495.22 C-N stretching of benzenoid rings
1577.22 1570.06 C-N stretching of quinoid rings
1732.40 - >C=O group of CSA
2924.16 2924.04 C-H stretching frequencies of
 characteristic of N-C[H.sub.3] groups
3231.23 3231.13 The aromatic C-H stretching
3431.93 3431.88 >N-H stretching vibration

TABLE 2. Room temperature conductivity values of poly(N-ethyl aniline)
doped with CSA and p-TSA.

Polymer Conductivity (S/cm)

Poly(N-ethyl aniline) -- CSA 1.91 x [10.sup.-4]
Poly(N-ethyl aniline) -- p-TSA 3.04 x [10.sup.-5]
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