Conducting-Insulating Polymer Composites: Selectively Sensing

Chapter 7

Conducting-Insulating Polymer Composites: Selectively Sensing Materials for Humidity and CO

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K. Ogura and H . Shiigi

Downloaded by PURDUE UNIV on March 17, 2016 | http://pubs.acs.org Publication Date: October 10, 2002 | doi: 10.1021/bk-2003-0832.ch007

Department of Applied Chemistry, Yamaguchi University, Ube 755-8611, Japan

Conducting/insulating polymer composites which respond selectively to humidity and CO at room temperature have been prepared, and their sensing schemes disclosed. The logarithmic conductivity of a composite of emeraldine salt-type polyaniline (ES-PAn) with poly(vinyl alcohol)(PVA) is proportional to the humidity in a wide range (10 to 10 Scm ). The movement of protonic acid, that is, the shift of the salt-base equilibrium of the conducting polymer, occurring in the desiccating and moisturizing processes, causes the change in conductivity of the composite. On the other hand, the composite consisting of emeraldine base-type ΡAn (EB-PAn) and PVA makes no response to water because this type of polymer has no dopant anion to be released. In the presence of CO however, the carbonate ion formed by the hydrolysis of CO can be incorporated into EB-PAn to generate ES-PAn, resulting in an increase in conductivity of the composite film. This is the basis for sensing CO with the EB-PAn/PVA composite. 2

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© 2003 American Chemical Society Rubinson and Mark; Conducting Polymers and Polymer Electrolytes ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


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Extensive investigations on polyaniline (PAn) and its derivatives have been carried out (i) since they possess a moderate conductivity upon doping with protonic acid and an excellent stability under ambient conditions (2,5). PAn Is simply prepared by the chemical and electrochemical oxidation of aniline or its derivatives in aqueous solution. In general, however, the chemical and electrochemical polymerization of aniline monomer lead merely to an insoluble powder and a thin brittle film, respectively. Hence, it is veiy difficult to process PAn for a practical use. In order to deal well with this problem, the improvement of processability of PAn has been studied by preparing polymer composites (4) and soluble PAn (5,6) and using plasma polymerization (7) and postsulfonation of PAn (8,9). Another approach to the preparation of processible PAn is to apply a precursor polymer, e.g., PAn can be produced by the thermal treatment of poly(anthranilic acid) (PANA) (10). This method is particularly useful for the preparation of processible PAn or its composites with other insulating polymers since it does not use external dopants that often cause an inconvenient situation associated with a practical use of the conducting polymer. In the present study, PAn derived from PANA was characterized by thermogravimetric/mass (TG/MS) and Fourier transform infrared (FUR) spectroscopies. The composites of PAn with insulating polymers have been found to be selectively sensing to humidity and C 0 (U-Î4), and the application of PAn derivedfromPANA to humidity and C 0 sensors is described here in detail. 2

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Experimental The preparation of PANA was performed by the following procedure. An aqueous solution of 0.2 M (NH4)S20g was slowly poured with stirring into a 0.1 M H S0 solution containing 50 mM anthranilic acid, and the polymerization was done at room temperature for 48 h. The precipitate was filtered, washedfirstwith methanol and then with a 0.1 M H S0 solution, and dried under vacuum. The powdery PANA obtained was dark brown. The conversion of PANA to PAn was achieved by the pyrolytic elimination of CO2 at elevated temperature. The chemical characterization of PANA and PAn was determined by a Jeol 220 TG/MS and a Shimadzu FTIR (Type 8100M). The TG/MS analyses were carried out under a helium atmosphere, and temperature was changedfrom25 °C to 500 °C at the programmed heating rate of 5 °Cmin \ The sample for this experiment was prepared by casting a dimethyl sulfoxide (DMSO) solution of the sample on a polyethylene substrate, evaporating the solvent in a vacuum, and 2

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Rubinson and Mark; Conducting Polymers and Polymer Electrolytes ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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peeling the resultingfilmofffromthe substrate. FTIR spectra were obtained in the transmission mode as KBr tablets. The PANA which was thus prepared was found to be in the form of emeraldine-salt polyaniline (ES-PAn). The solubility of PANA in DMSO was 32 gL' , and the electrical conductivity 3.5*10" Son . The composite film consisting of PANA and poly(vinyl alcohol) (PVA) was very sensitive to humidity: the conductivity of this composite changed from conducting level to insulating upon the variation of relative humidity. The electrical conductivity was measured with a comb-shaped microelectrode that was made by depositing a thin platinumfilmin a comb shape on a glass substrate in a vacuum (11). A DMSO solution containing given quantities of PANA and PVA was cast on a microelectrode, and the solvent was removed under vacuum. The thickness of the applied composite was always 100 nm, which was estimated with a scanning electron microscope (SEM, Hitachi S-2300). The film-cast microelectrode was set in a measuring cell connected to a vacuum line permitting us to introduce a controlled pressure of water vapor at 25 °C. The conductivity was monitored by the two-probe direct current technique after the pressure was restored to 760 Torr by introducing nitrogen gas. On the other hand, the composite consisting of emeraldine-base PAn (EB-PAn) and PVA did not respond to relative humidity but to C0 . The EB-PAn was prepared by die pyrolytic eUmination of C 0 from PANA. The pyrolysis was carried out at various elevated temperatures in a helium atmosphere. The electrical conductivity of the EB-PAn/PVA composite under varied conditions of humidity and CO2 concentration was measured with a comb-shaped microelectrode in the same manner as that described above.


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Results and Discussion Characterization of PANA and the preparation of EB-PAn FTIR spectra of PANA and heat-treated PANAs were measured in the transmission mode as KBr tablets. The results are exhibited in Figure 1, where the heat-treatment was done at 150,200, 250 and 280 °C for 2k TTie absorption bands at 1690 and 1450 cm , which are seen in the spectrum of PANA without heat-treatment (curve a), are attributable respectively to the C=0 and C-0 stretching vibration of the carboxyl group (15). The bands at 1570 and 1380 cm" are assignable respectively to the antisymmetric and symmetric stretching vibration of the ionized carboxyl group (15). Hence, both -COOH and -COO" groups are confirmed as being present in PANA without heat-treatment On the other hand, the absorption intensities at 1690 and 1450 cm" are observed to decrease with elevating the treatment temperature, -1

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Figure 1. FTIR spectra of the PANA powder treated at different temperatures: (a) without heat-treatment, (b) 150, (c) 200, (d) 250, (e) 280 °C.


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while the bands at 1570 and 1380 cm" are almost invariable in magnitude up to 200 °C. It is therefore indicated that the carboxyl groups are readily detached from the PANA backbone with increasing the temperature while the ionized carboxyl group is thermally stable at least until 200 °C. The ionized form can be stabilized because this group acts as a self-dopant on PANA, i.e., the positive nitrogen atom interacts with the ionized carboxyl group (16). The spectrum of the PANA treated at the highest temperature (280 °C) showed the disappearance of the bands belonging both to the carboxyl and ionized carboxyl groups, but distinct absorption bands are seen around 1600,1510 and 1250 cm" instead. These three bands are attributed to the stretching vibration of the benzenoid ring, quinoid ring and CN, respectively. The features of this spectrum is in good agreement with those of the spectrum for an emeraldine base of PAn prepared by the chemical oxidation of aniline monomer (17). TTius, EB-PAn is obtainable by the pyrolysis of PANA. The TG curve of PANA and the corresponding MS spectra are shown in Figures 2 and 3, respectively. The TG curve demonstrates three major stages (I, Π and HI) of weight loss. A comparison between the two figures indicates that the weight loss observed at 85 °C (stage I) is due to H 0 (m/z==18) desorbed from the polymer. The weight loss at stage Π is caused by the decarboxylation (m/z=44). Many MS peaks including the peak at m/z=93 attributable to aniline occur at stage ΠΙ, resulting from the pyrolysis of the PANA backbone. In Figure 4, the MS chromatogram for C 0 (m/z=44) and the electrical conductivity of PANA are shown as a function of the temperature at which PANA was heat-treated. The chromatographic intensity commences to increasefrom100 °C, and reaches a maximum around 200 °C with a shoulder at about 170 °C. It is therefore indicated that the thermal decomposition of the carboxyl group to C 0 occurs via two steps, i.e., one decomposes around 170 °C, and the other at a temperature higher than 200 °C. This may support the existence of both -COOH and -COO" groups intimated from the FTIR results. The decrease in MS intensity of heat-treated PANA beyond 200 °C is probably caused by the diminution in concentration of the remaining carboxyl groups. On the other hand, the electrical conductivity of PANA (1.7* 10~ Son ) heat-treated at 150 °C is comparable to that (3.5* 10" Scm") without heat-treatment (25 °C). The conductivity of PANA became much lower as the polymer was treated at a temperature above 150 °C, and it reached 3.8X10" Scm" at 250 °C. Hence, it follows that the electrical conductivity of PANA is not related to the carboxyl group decomposed around 150 °C but to the ionized carboxyl group decomposed at more elevated temeperature. The stabilization of the latter carboxyl group is brought about by the interaction with the positive nitrogen atom as noted above.


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100

100

^

b

-44 (Ο

c φ •4—»

ç

ω ω

ο

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.

(D Œ

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50

Τ » ι • ι • ι 100 150 200 250

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m/z Figure 3. MS spectra ofPANA taken at different temperatures : (a) 85, (b) and (c) 500 "C.



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ΙΟ"

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Temperature / °C Figure 4. Relationship between the electrical conductivity ofPANA and the treatment temperature.


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From these results, the following scheme (Scheme I) can be given for the pyrolysis of PANA in which the decarboxylation occurs in two stages from -COOH and -COO' groups around 170 °C and 250 °C, respectively.


PANA/PVA composites as a humidity sensor We have found that a composite film consisting of a conducting polymer and an insulating polymer is useful for a humidity sensor (11). The PANA as prepared and the PAÏNA doped with surfuric acid (PANA-SA) were both used as the conducting polymer in the present study. The PANA-SA was prepared as follows. PANA wasfirstheat-treated at 250 °C, and then the PANA was externally doped with surfuric acid. The electrical conductivity of PANA-SA was 6.2 Scm". This value is rather higher than that (5 Scm") (18) reported for the PAn prepared by the electrochemical and chemical polymerization of aniline monomer. In general, PAn is insoluble in DMSO, but PANA-SA is soluble to some extent (15 gL' ). The solubility of PANA-SA enables us to make a completely mixed composite of PANA-SA and PVA, which is very important for die preparation of a good humidity sensor with such a composite. The electrical conductivities of the PANA/PVA (a) and PANA-SA/PVA (b) composites measured under a constant humidity are shown in Figure 5. In both composites, die logarithmic conductivity is linearly related to the relative humidity, and there is no hysteresis in the measurements at the moistening and desiccating stages. The linearity is valid covering over 5 orders and 4 orders of magnitude for the PANA-SA/PVA (b) and PANA/PVA (a) composites, respectively. The humidity-dependence of the PANA-SA/PVA composite can be explained on the basis of a salt-base transition of the conducting polymer (13). In the desiccating process, the protonic acid incorporated in the PANA-SA during its preparation is expelled and moves into the strongly bound water in PVA as shown in Scheme Π. Hence, the conductivity of the composite decreases with a decrease of environmental humidity, andfinallythe composite becomes insulating. Conversely, in the moistening process, the external moisture is weakly rebound to the composite, and the protonic acid is likely to disperse over the composite, resulting in the recovery of the PANA-SA to the conducting level. 1

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EB-PAn/PVA composite as a C0 sensor 2

As mentioned above, there is a good linear relationship between the log of electrical conductivity and the relative humidity for the PANA/PVA composite, while the conductivity of the EB-PAn/PVA composite was about 3*10" Scm" and was independent of the relative humidity. This signifies that PANA is completely converted to the base-type PAn and there is no dopant anion, (i.e., 5


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Scheme Ι.


25 °C PANA COOH

COOH

H

H

170t

-C0

300*0

2

jQr^UQTXX,

EB-PAn

H



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Humidity / %RH Figure 5. Dependence ofelectrical conductivity ofa PANA/PVA (a) and a PANA-SA/PVA (b) composite on the relative humidity at the moistening (ο, Δ.) and desiccating (· A ) stages.

Scheme II.

+ PVA PANA-SA

Ν + A

moistening

Ν + A-

desiccating

Η EB-PAn



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I I I II

ι

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10

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10

C0

I—ι

I 3

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I

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concentration / ppm

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Figure 6. Dependence of electrical conductivity ofa EB-PAn(14 wt%)/PVA (86 wt%) composite on the CO2 concentration at the humidity of 30 %RH at 25 °C, The measurements were performed by increasing (o) and decreasing (·) the C0 concentration. 2

absence of self-dopant, -COO') in the polymer. In the presence of CO2 and humidity, however, the electrical conductivity of the EB-PAn/PVA composite was linearly related with the C 0 concentration. In Figure 6, the logarithmic conductivity of EB-PAn (14 wt%) / PVA (86 wt%) is plotted versus the C 0 concentration at the humidity of 30 %RH at 25 °C. The conductivity changes from 5*10" Scm' to 8>

Conducting-Insulating Polymer Composites: Selectively Sensing

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