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Humidity sensitive properties of substituted polyacetylenes
Synthetic Metals 129 (2002) 285–290
Humidity sensitive properties of substituted polyacetylenes Yang Li, Mujie Yang* Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Received 6 February 2002; received in revised form 22 April 2002; accepted 22 April 2002
Abstract Thin film resistive humidity sensors have been prepared based on a series of homogeneously doped soluble substituted polyacetylenes synthesized using palladium acetylide complex catalyst. The humidity sensitive properties of the polymers (poly(propiolic acid) (PPA); poly(propiolic acid-co-ethynylbenzene) (PA-co-EB); poly(propiolic acid-co-p-diethynylbenzene) (PA-co-DEB); poly(propiolic acid-copropargyl alcohol) (PA-co-OHP)) doped with FeCl3, H2SO4 and HClO4 have been investigated and compared. The chemical structures of polymers and the doping agents have great influence on the sensing properties of the sensors. A sensor based on PA-co-OHP doped with HClO4 shows the best response. Its logarithm of impedance varies linearly with relative humidity (RH) for four orders of magnitude (107– 103 O) over a wide range of 30–95%RH, and the response time is <6 and <15 s for absorption and desorption, respectively. Furthermore, the effect of temperature on the sensing behaviour of the polymers has also been described. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Humidity sensor; Substituted polyacetylenes; Humidity sensitive property
1. Introduction Polymers have been widely used as sensing materials in preparation of a great variety of chemical sensors, including humidity sensor [1–4]. The most often used polymeric humidity sensitive materials are saturated polyelectrolytes and hydrophobic polymers, which can be used in the preparation of resistive and capacitive humidity sensors, respectively [4,5]. In recent years, conjugated polymers have also been found to be sensitive to water and other gases, and a number of reports have been given [6–11]. Polyacetylenes are one of the most important conjugated polymers, and they were also investigated as humidity sensitive materials for preparation of resistive, capacitive and surface acoustic wave (SAW) sensors since the early 1990s. They exhibit high sensitivity and relatively fast response, and have attracted considerable interest [11–17]. However, polyacetylenes usually exhibit poor solubility after doping and it is difficult to use them to prepare thin film sensors, which has greatly hindered their further research and applications. Furthermore, there are few reports discussing the relationship between chemical structures of these polymers and their humidity sensitive properties. In the past several years, we have synthesized a series of soluble substituted polyacetylenes with aliphatic or aromatic substituents, and used them * Corresponding author. Tel./fax: þ86-571-8795-2444. E-mail address: [email protected] (M. Yang).
to prepare novel resistive and/or capacitive humidity sensors [13,15,18–21]. By using impedance complex analysis, sensing mechanism of these polyacetylenes has also been investigated, and modified Onsager equation was developed to explain the sensing behaviour [13,19–21]. Recently, by using a series of novel transition metal acetylide catalysts, we have successfully realized the homopolymerization and copolymerization of polar with non-polar acetylenes, obtaining soluble polymers. In this paper, these soluble polymers are homogeneously doped with salts and acids to prepare thin film humidity sensors. Correlation of chemical structures of the polymers with their sensing behaviour has been attempted, and the effect of doping agents, temperature on humi-sensitive properties has also been investigated.
2. Experimental 2.1. Materials Only analytical grade quality chemicals were used. p-Diethynylbenzene (p-DEB) was prepared by literature method [22] and purified by sublimation before use. Propargyl alcohol (OHP) was distilled under nitrogen at reduced pressure. Propiolic acid (PA) was prepared by modifying reported procedure [23]. Ethynylbenzene was purchased from Aldrich Chemical (USA). All the solvents used were dried with activated alumina.
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2.2. Preparation of Pd(PPh3)2(CBCC(CH3)2OH)2 (PPB) Into a suspension of Pd(PPh3)2Cl2 (263 mg, 0.336 mmol) and CuI (5 mg) in diethylamine (HNEt2) (15 ml), 2-methyl3-butyn-2-ol (HCBCC(CH3)2OH) (0.287 ml, 2.77 mmol) was slowly dropped with magnetic stirring. The mixture was stirred at room temperature for about 30 min, then diluted with 10 ml HNEt2, filtered, washed with HNEt2 and ethanol, and dried in vacuum for 18 h to give a white powder with a yield of 60%. mp, 159–160 8C; UV–Vis lmax (CHCl3, nm): (241, 277). FT-IR (KBr, cm1): 3588 (s); 2110 (m); 1600 (m); 1435 (s); 1321 (m); 1099 (s); 1028 (w); 1000 (w); 891 (m); 746 (s); 694 (s); 559 (w); 522 (s); 497 (m); 459 (w); 432 (w). Anal. Calcd. for PdO2P2C46H44: C, 69.30%; H, 5.52%. Found: C, 70.03%; H, 5.76%. 2.3. Synthesis of substituted polyacetylenes The homopolymer poly(propiolic acid) (PPA) and copolymers poly(propiolic acid-co-ethynylbenzene) (PA-co-EB), poly(propiolic acid-co-p-diethynylbenzene) (PA-co-DEB) and poly(propiolic acid-co-propargyl alcohol) (PA-co-OHP) were synthesized by homogeneous polymerization using palladium acetylide complex catalyst. The typical polymerization procedure was as follows: a solution of PA (420 mg, 6 mmol), PPB (48 mg, 0.06 mmol) in 1.6 ml chloroform/ methanol mixed solvents (volume ratio ¼ 3=1) was kept at 60 8C under N2 for 16 h. A dark orange red solution was obtained, then precipitated in petroleum ether (30–60 8C fraction), filtered, washed with petroleum ether, and dried under dynamic vacuum for 16 h, giving a brown powder with about 60% yield. 2.4. Doping The polymers so obtained were homogeneously doped with solutions of FeCl3, H2SO4 and HClO4 in tetrahydrofuran (THF) respectively, and then aged at 30 8C for 24 h. For doping with FeCl3, 15 mg of polymer was mixed with 52.5 mg FeCl3 anhydrous in 3 ml THF; for doping with H2SO4, 5 mg of polymer was mixed with H2SO4 (1.64 M, polymer:acid ¼ 80%, w/w) in 1 ml THF solvent; for doping with HClO4, 5 mg of polymer was mixed with HClO4 (1.68 M, polymer:acid ¼ 80%, w/w) in 1 ml THF solvent. 2.5. Sensor fabrication The above-mentioned doped polymers solutions were deposited on glass ceramics substrate (4 mm 6 mm 0:5 mm), where an interdigital array of gold electrodes had been previously evaporated and photolithographically defined, to prepare thin film humidity sensors. The thickness and width of electrode are 2 and 40 m, respectively, and the scheme of the sensor is shown in Fig. 1.
Fig. 1. Scheme of humidity sensitive device.
2.6. Measurements IR spectra were recorded on a Bruker Vector model 22 spectrometer as KBr pellets. UV–Vis spectra were obtained on a Cary 100 Bio UV–Vis spectrophotometer. Melting point was determined on a Yanaco MP-500 melting point apparatus. Elemental analysis was carried out on a Carlo Erba Model 1106 elemental analyzer. Measurements of humidity response of the sensors were carried out by two methods: (a) the sensors were placed in a cell in which humidity and temperature can be controlled (Shinyei SRH-3MC135ADR) and the impedance of sensors were measured with a digital multimeter (UT-51, Guangdong); (b) the sensors were placed in a home-made cell equipped with a commercial humidity sensor (Eliwell mod. EWHS 31). Different relative humidity values (RH%) were obtained by bubbling dry argon in water, and the impedance measurements were done with a Solartron 1255 frequency response analyzer coupled with a 1286 Solartron electrochemical interface at constant temperature (22 8C).
3. Results and discussion The chemical structures of the four polymers: PPA, PA-co-EB, PA-co-DEB and PA-co-OHP used in the present study are shown in Fig. 2. The detailed description on their synthesis and characterization will be published elsewhere. All the polymers contain PA unit, which is different from the commonly used polyacetylene-type humidity sensitive materials. Polyacetylenes, especially doped polyacetylenes, usually exhibit poor solubility [13,19]. But the introduction of PA unit in the main chain results in their homogeneous doping with salts and acids, obtaining soluble doped polymers for preparation of thin film humidity sensors. Furthermore, the polymers containing PA unit bear –COOH group, which can dissociate to give protons for charge transport with the help of absorbed moisture. However, the conductivity is still too low without doping, and it is necessary to dope the polymers with salts or acids to increase conductivity and widen the sensing region.
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Fig. 2. Chemical structures of substituted polyacetylenes.
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The impedance changes 3–4 orders of magnitude (103– 107 O) from 97 to 10%RH, showing much higher sensitivity and better linearity. Furthermore, a lower impedance and better linearity is observed for F-PA-co-OHP. The difference in sensing response may be explained by the different chemical structures of the polymers. It was proposed that conductivity of the doped polyacetylenes mainly depend on the ion transport in presence of humidity [13,19,20]. In the four doped polymers, the polar groups –COOH and –OH may be the site where the interactions with water molecules take place, and the conjugated p–p double bonds in the polyacetylene chain may also involve in the charge transport [18]. In the presence of humidity, the doping agent FeCl3 dissociates according to the following equation [24]:
3.1. Humidity response of polymers doped with FeCl3 Fig. 3 shows the humidity response of thin film sensors based on the polyacetylenes doped with FeCl3. All the four polymers are sensitive to humidity variation, but their response curves are very different. The impedance of the two copolymers PA-co-EB and PA-co-DEB doped with FeCl3 (F-PA-co-EB and F-PA-co-DEB), which are composed of both polar and non-polar units, changes only 1–2 orders of magnitude over the investigated humidity range (10–90%RH), showing low sensitivity. Moreover, the response of F-PA-co-DEB is similar to that of homopolymer of DEB [18] and exhibits a sigmoidal trend with decreased sensitivity at high humidities. In contrast, PPA and PA-coOHP doped with FeCl3 (F-PPA and F-PA-co-OHP), which are composed of polar units, exhibit much better response.
where [FeCl4] is stable and forms complex with polymer. However, [FeCl2]þ is unstable and can be easily reduced through charge-transfer reaction as depicted below: ½FeCl2 þ þ R þ nH2 O ! Rþ þ FeCl2 nH2 O where R ¼ PPA, PA-co-EB, PA-co-DEB and PA-co-OHP. The ability to form complex and interact with the doping agents through charge-transfer reaction and absorb water molecules in environment is different for the polymers with different chemical structures. The polar groups –COOH and –OH in the polymers may easily form complexes with doping agents, and also absorb more water molecules to assist in the dissociation of ions. Therefore, the polymers composed of polar units, PPA and PA-co-OHP, show lower impedance, especially at higher humidities. All the four doped polymers exhibit hysteresis between absorption and desorption process, and larger hysteresis is found for polymers with higher polarity. Fig. 4 shows the hysteresis of F-PPA and F-PA-co-DEB. At the same RH, the impedance in desorption process is much lower than that in absorption process for PPA, and a large hysteresis is observed, while the difference in sensing curves recorded for PA-co-OHP is much smaller, suggesting a small hysteresis. This may be due to the different interactions between water and doped polymers. In PPA, –COOH group has stronger interaction with water molecules (for example, the formation of hydrogen bonds), and water cannot be easily removed from polymer chain in desorption process, resulting in large hysteresis. In contrast, non-polar DEB unit has weak interaction with water molecules in PA-co-DEB, thus desorption of water molecules is easier, and the hysteresis is greatly decreased. 3.2. Humidity response of polymers doped with H2SO4
Fig. 3. Humidity response of polymers doped with FeCl3.
Previously, poly(propargyl alcohol) (POHP) and poly(p-diethynylbenzene) (PDEB) were doped with H2SO4 to
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Fig. 5. Humidity response of polymers doped with H2SO4.
with FeCl3. This may indicate doping with H2SO4 is not so successful. 3.3. Humidity response of polymers doped with HClO4 HClO4 is another acid frequently used in the doping of polyacetylenes, and the responses of thin films of PA-co-EB, PA-co-DEB and PA-co-OHP doped with HClO4 (C-PA-coEB, C-PA-co-DEB and C-PA-co-OHP) are shown in Fig. 6a–c. It can be seen clearly that all the three polymers exhibit good sensing response irrespective of their chemical structures. The impedance of C-PA-co-EB changes linearly for two orders of magnitude (105–107 O) from 95 to 10%RH in semi-logarithmic scale, showing better linearity and higher sensitivity with respect to that of polymer doped with FeCl3 and H2SO4. C-PA-co-DEB exhibits impedance change of 4 orders of magnitude from 30 to 75%RH, which is much
Fig. 4. Hysteresis of: (a) PPA; (b) PA-co-DEB doped with FeCl3.
obtain insoluble products, which were used to prepare humidity sensors with good sensing properties in the form of pellets [13,19]. Here the polymers are soluble after doped with acids, and used to prepare thin film sensors. Fig. 5 shows the humidity responses of PPA, PA-co-DEB and PAco-OHP doped with H2SO4 (S-PPA, S-PA-co-DEB and SPA-co-OHP). Of the three doped polymers, S-PA-co-OHP and S-PA-co-DEB exhibit the best and worst response in terms of sensitivity and linearity, respectively. This may suggest it is more difficult to dope the polymers bearing non-polar groups with acid. In general, polymers doped with H2SO4 show low sensitivity and poor linearity, and their responses are worse than those of polymers doped
Fig. 6. Humidity response of polymers doped with HClO4: (a) PA-co-OHP; (b) PA-co-EB; (c) PA-co-DEB.
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Fig. 8. Effect of temperature on humidity response of PA-co-OHP doped with FeCl3.
strongly affect their sensing properties, and the best sensing response can be obtained by choosing suitable combination of polymerization monomers, and HClO4 is a better doping agent with respect to H2SO4 and FeCl3. 3.4. Effect of temperature on humidity response
Fig. 7. Response time of PA-co-OHP doped with HClO4: (a) from 33–97– 33%RH; (b) absorption and desorption isotherm.
improved over the response of F-PA-co-DEB and S-PA-coDEB. Again, PA-co-OHP doped with HClO4 gives the best response. The impedance changes from 107 to 103 O over the range of 30–95%RH, showing very high sensitivity, and a good linearity is observed. In addition, C-PA-co-OHP exhibits very quick response in both absorption and desorption process, as illustrated in Fig. 7. The response time are calculated to be 6 and 15 s for absorption and desorption process, respectively. This quick response can be explained as follows: there are two functional groups, –COOH and –OH, with different polarity in PA-co-OHP. The OH group has weaker interaction with water molecules compared with COOH group, and the absorbed water molecules can be removed more easily in desorption process, thus the response time will be greatly shortened, and the hysteresis will be lower. This is similar to the case of decreased hysteresis in a polymer chain containing both strong and weak acid groups as reported by Huang [25]. In addition, compared with copolymers bearing both polar and non-polar group, such as PA-co-EB and PA-co-DEB, PA-co-OHP exhibits higher ability to absorb water molecules and form complexes with the doping agents, therefore, it shows high sensitivity over the range of tested humidity. All these results indicate that the chemical structures of the polymers can
As mentioned above, the conductivity of the doped polyacetylenes are ionic in nature in presence of humidity. Therefore, temperature has some effect on the humidity response. Fig. 8 shows the humidity response of PA-co-OHP doped with FeCl3 at different temperatures. When the temperature increases from 15 to 40 8C, the impedance decreases slightly in the range of low to middle humidity (30–70%RH). Only at high humidities, a great decrease in impedance is observed. The negative temperature coefficient may result from the following two reasons: (1) the ion movement is enhanced at higher temperatures; (2) at the same RH, the water content at higher temperature increases and more water molecules are absorbed. The great change in impedance observed only at high humidity suggests that the second reason contribute more to the impedance change with temperature.
4. Conclusion A series of soluble substituted polyacetylenes: PPA, PAco-EB, PA-co-DEB, PA-co-OHP synthesized with palladium acetylide catalysts can be doped with salt and acids to prepare thin film humidity sensors. The sensing behaviour of the polymers depends on their chemical structures. Among all the polymers, PA-co-OHP, which bears both strong and weak polar groups, shows the best response in terms of sensitivity, linearity and response time. Of the three doping agents used, HClO4 is better than H2SO4 and FeCl3.
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The polymers exhibit negative temperature coefficient over the tested humidity range.
Acknowledgements This work was supported by the Scientific and Technological Agreement People’s Republic of China–Italy. The authors wish to thank Dr. G. Casalbore-Miceli of FRAECNR, Italy for kind help in measurements of humidity sensitive properties.
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Humidity sensitive properties of substituted polyacetylenes Yang Li, Mujie Yang* Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Received 6 February 2002; received in revised form 22 April 2002; accepted 22 April 2002
Abstract Thin film resistive humidity sensors have been prepared based on a series of homogeneously doped soluble substituted polyacetylenes synthesized using palladium acetylide complex catalyst. The humidity sensitive properties of the polymers (poly(propiolic acid) (PPA); poly(propiolic acid-co-ethynylbenzene) (PA-co-EB); poly(propiolic acid-co-p-diethynylbenzene) (PA-co-DEB); poly(propiolic acid-copropargyl alcohol) (PA-co-OHP)) doped with FeCl3, H2SO4 and HClO4 have been investigated and compared. The chemical structures of polymers and the doping agents have great influence on the sensing properties of the sensors. A sensor based on PA-co-OHP doped with HClO4 shows the best response. Its logarithm of impedance varies linearly with relative humidity (RH) for four orders of magnitude (107– 103 O) over a wide range of 30–95%RH, and the response time is <6 and <15 s for absorption and desorption, respectively. Furthermore, the effect of temperature on the sensing behaviour of the polymers has also been described. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Humidity sensor; Substituted polyacetylenes; Humidity sensitive property
1. Introduction Polymers have been widely used as sensing materials in preparation of a great variety of chemical sensors, including humidity sensor [1–4]. The most often used polymeric humidity sensitive materials are saturated polyelectrolytes and hydrophobic polymers, which can be used in the preparation of resistive and capacitive humidity sensors, respectively [4,5]. In recent years, conjugated polymers have also been found to be sensitive to water and other gases, and a number of reports have been given [6–11]. Polyacetylenes are one of the most important conjugated polymers, and they were also investigated as humidity sensitive materials for preparation of resistive, capacitive and surface acoustic wave (SAW) sensors since the early 1990s. They exhibit high sensitivity and relatively fast response, and have attracted considerable interest [11–17]. However, polyacetylenes usually exhibit poor solubility after doping and it is difficult to use them to prepare thin film sensors, which has greatly hindered their further research and applications. Furthermore, there are few reports discussing the relationship between chemical structures of these polymers and their humidity sensitive properties. In the past several years, we have synthesized a series of soluble substituted polyacetylenes with aliphatic or aromatic substituents, and used them * Corresponding author. Tel./fax: þ86-571-8795-2444. E-mail address: [email protected] (M. Yang).
to prepare novel resistive and/or capacitive humidity sensors [13,15,18–21]. By using impedance complex analysis, sensing mechanism of these polyacetylenes has also been investigated, and modified Onsager equation was developed to explain the sensing behaviour [13,19–21]. Recently, by using a series of novel transition metal acetylide catalysts, we have successfully realized the homopolymerization and copolymerization of polar with non-polar acetylenes, obtaining soluble polymers. In this paper, these soluble polymers are homogeneously doped with salts and acids to prepare thin film humidity sensors. Correlation of chemical structures of the polymers with their sensing behaviour has been attempted, and the effect of doping agents, temperature on humi-sensitive properties has also been investigated.
2. Experimental 2.1. Materials Only analytical grade quality chemicals were used. p-Diethynylbenzene (p-DEB) was prepared by literature method [22] and purified by sublimation before use. Propargyl alcohol (OHP) was distilled under nitrogen at reduced pressure. Propiolic acid (PA) was prepared by modifying reported procedure [23]. Ethynylbenzene was purchased from Aldrich Chemical (USA). All the solvents used were dried with activated alumina.
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2.2. Preparation of Pd(PPh3)2(CBCC(CH3)2OH)2 (PPB) Into a suspension of Pd(PPh3)2Cl2 (263 mg, 0.336 mmol) and CuI (5 mg) in diethylamine (HNEt2) (15 ml), 2-methyl3-butyn-2-ol (HCBCC(CH3)2OH) (0.287 ml, 2.77 mmol) was slowly dropped with magnetic stirring. The mixture was stirred at room temperature for about 30 min, then diluted with 10 ml HNEt2, filtered, washed with HNEt2 and ethanol, and dried in vacuum for 18 h to give a white powder with a yield of 60%. mp, 159–160 8C; UV–Vis lmax (CHCl3, nm): (241, 277). FT-IR (KBr, cm1): 3588 (s); 2110 (m); 1600 (m); 1435 (s); 1321 (m); 1099 (s); 1028 (w); 1000 (w); 891 (m); 746 (s); 694 (s); 559 (w); 522 (s); 497 (m); 459 (w); 432 (w). Anal. Calcd. for PdO2P2C46H44: C, 69.30%; H, 5.52%. Found: C, 70.03%; H, 5.76%. 2.3. Synthesis of substituted polyacetylenes The homopolymer poly(propiolic acid) (PPA) and copolymers poly(propiolic acid-co-ethynylbenzene) (PA-co-EB), poly(propiolic acid-co-p-diethynylbenzene) (PA-co-DEB) and poly(propiolic acid-co-propargyl alcohol) (PA-co-OHP) were synthesized by homogeneous polymerization using palladium acetylide complex catalyst. The typical polymerization procedure was as follows: a solution of PA (420 mg, 6 mmol), PPB (48 mg, 0.06 mmol) in 1.6 ml chloroform/ methanol mixed solvents (volume ratio ¼ 3=1) was kept at 60 8C under N2 for 16 h. A dark orange red solution was obtained, then precipitated in petroleum ether (30–60 8C fraction), filtered, washed with petroleum ether, and dried under dynamic vacuum for 16 h, giving a brown powder with about 60% yield. 2.4. Doping The polymers so obtained were homogeneously doped with solutions of FeCl3, H2SO4 and HClO4 in tetrahydrofuran (THF) respectively, and then aged at 30 8C for 24 h. For doping with FeCl3, 15 mg of polymer was mixed with 52.5 mg FeCl3 anhydrous in 3 ml THF; for doping with H2SO4, 5 mg of polymer was mixed with H2SO4 (1.64 M, polymer:acid ¼ 80%, w/w) in 1 ml THF solvent; for doping with HClO4, 5 mg of polymer was mixed with HClO4 (1.68 M, polymer:acid ¼ 80%, w/w) in 1 ml THF solvent. 2.5. Sensor fabrication The above-mentioned doped polymers solutions were deposited on glass ceramics substrate (4 mm 6 mm 0:5 mm), where an interdigital array of gold electrodes had been previously evaporated and photolithographically defined, to prepare thin film humidity sensors. The thickness and width of electrode are 2 and 40 m, respectively, and the scheme of the sensor is shown in Fig. 1.
Fig. 1. Scheme of humidity sensitive device.
2.6. Measurements IR spectra were recorded on a Bruker Vector model 22 spectrometer as KBr pellets. UV–Vis spectra were obtained on a Cary 100 Bio UV–Vis spectrophotometer. Melting point was determined on a Yanaco MP-500 melting point apparatus. Elemental analysis was carried out on a Carlo Erba Model 1106 elemental analyzer. Measurements of humidity response of the sensors were carried out by two methods: (a) the sensors were placed in a cell in which humidity and temperature can be controlled (Shinyei SRH-3MC135ADR) and the impedance of sensors were measured with a digital multimeter (UT-51, Guangdong); (b) the sensors were placed in a home-made cell equipped with a commercial humidity sensor (Eliwell mod. EWHS 31). Different relative humidity values (RH%) were obtained by bubbling dry argon in water, and the impedance measurements were done with a Solartron 1255 frequency response analyzer coupled with a 1286 Solartron electrochemical interface at constant temperature (22 8C).
3. Results and discussion The chemical structures of the four polymers: PPA, PA-co-EB, PA-co-DEB and PA-co-OHP used in the present study are shown in Fig. 2. The detailed description on their synthesis and characterization will be published elsewhere. All the polymers contain PA unit, which is different from the commonly used polyacetylene-type humidity sensitive materials. Polyacetylenes, especially doped polyacetylenes, usually exhibit poor solubility [13,19]. But the introduction of PA unit in the main chain results in their homogeneous doping with salts and acids, obtaining soluble doped polymers for preparation of thin film humidity sensors. Furthermore, the polymers containing PA unit bear –COOH group, which can dissociate to give protons for charge transport with the help of absorbed moisture. However, the conductivity is still too low without doping, and it is necessary to dope the polymers with salts or acids to increase conductivity and widen the sensing region.
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Fig. 2. Chemical structures of substituted polyacetylenes.
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The impedance changes 3–4 orders of magnitude (103– 107 O) from 97 to 10%RH, showing much higher sensitivity and better linearity. Furthermore, a lower impedance and better linearity is observed for F-PA-co-OHP. The difference in sensing response may be explained by the different chemical structures of the polymers. It was proposed that conductivity of the doped polyacetylenes mainly depend on the ion transport in presence of humidity [13,19,20]. In the four doped polymers, the polar groups –COOH and –OH may be the site where the interactions with water molecules take place, and the conjugated p–p double bonds in the polyacetylene chain may also involve in the charge transport [18]. In the presence of humidity, the doping agent FeCl3 dissociates according to the following equation [24]:
3.1. Humidity response of polymers doped with FeCl3 Fig. 3 shows the humidity response of thin film sensors based on the polyacetylenes doped with FeCl3. All the four polymers are sensitive to humidity variation, but their response curves are very different. The impedance of the two copolymers PA-co-EB and PA-co-DEB doped with FeCl3 (F-PA-co-EB and F-PA-co-DEB), which are composed of both polar and non-polar units, changes only 1–2 orders of magnitude over the investigated humidity range (10–90%RH), showing low sensitivity. Moreover, the response of F-PA-co-DEB is similar to that of homopolymer of DEB [18] and exhibits a sigmoidal trend with decreased sensitivity at high humidities. In contrast, PPA and PA-coOHP doped with FeCl3 (F-PPA and F-PA-co-OHP), which are composed of polar units, exhibit much better response.
where [FeCl4] is stable and forms complex with polymer. However, [FeCl2]þ is unstable and can be easily reduced through charge-transfer reaction as depicted below: ½FeCl2 þ þ R þ nH2 O ! Rþ þ FeCl2 nH2 O where R ¼ PPA, PA-co-EB, PA-co-DEB and PA-co-OHP. The ability to form complex and interact with the doping agents through charge-transfer reaction and absorb water molecules in environment is different for the polymers with different chemical structures. The polar groups –COOH and –OH in the polymers may easily form complexes with doping agents, and also absorb more water molecules to assist in the dissociation of ions. Therefore, the polymers composed of polar units, PPA and PA-co-OHP, show lower impedance, especially at higher humidities. All the four doped polymers exhibit hysteresis between absorption and desorption process, and larger hysteresis is found for polymers with higher polarity. Fig. 4 shows the hysteresis of F-PPA and F-PA-co-DEB. At the same RH, the impedance in desorption process is much lower than that in absorption process for PPA, and a large hysteresis is observed, while the difference in sensing curves recorded for PA-co-OHP is much smaller, suggesting a small hysteresis. This may be due to the different interactions between water and doped polymers. In PPA, –COOH group has stronger interaction with water molecules (for example, the formation of hydrogen bonds), and water cannot be easily removed from polymer chain in desorption process, resulting in large hysteresis. In contrast, non-polar DEB unit has weak interaction with water molecules in PA-co-DEB, thus desorption of water molecules is easier, and the hysteresis is greatly decreased. 3.2. Humidity response of polymers doped with H2SO4
Fig. 3. Humidity response of polymers doped with FeCl3.
Previously, poly(propargyl alcohol) (POHP) and poly(p-diethynylbenzene) (PDEB) were doped with H2SO4 to
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Fig. 5. Humidity response of polymers doped with H2SO4.
with FeCl3. This may indicate doping with H2SO4 is not so successful. 3.3. Humidity response of polymers doped with HClO4 HClO4 is another acid frequently used in the doping of polyacetylenes, and the responses of thin films of PA-co-EB, PA-co-DEB and PA-co-OHP doped with HClO4 (C-PA-coEB, C-PA-co-DEB and C-PA-co-OHP) are shown in Fig. 6a–c. It can be seen clearly that all the three polymers exhibit good sensing response irrespective of their chemical structures. The impedance of C-PA-co-EB changes linearly for two orders of magnitude (105–107 O) from 95 to 10%RH in semi-logarithmic scale, showing better linearity and higher sensitivity with respect to that of polymer doped with FeCl3 and H2SO4. C-PA-co-DEB exhibits impedance change of 4 orders of magnitude from 30 to 75%RH, which is much
Fig. 4. Hysteresis of: (a) PPA; (b) PA-co-DEB doped with FeCl3.
obtain insoluble products, which were used to prepare humidity sensors with good sensing properties in the form of pellets [13,19]. Here the polymers are soluble after doped with acids, and used to prepare thin film sensors. Fig. 5 shows the humidity responses of PPA, PA-co-DEB and PAco-OHP doped with H2SO4 (S-PPA, S-PA-co-DEB and SPA-co-OHP). Of the three doped polymers, S-PA-co-OHP and S-PA-co-DEB exhibit the best and worst response in terms of sensitivity and linearity, respectively. This may suggest it is more difficult to dope the polymers bearing non-polar groups with acid. In general, polymers doped with H2SO4 show low sensitivity and poor linearity, and their responses are worse than those of polymers doped
Fig. 6. Humidity response of polymers doped with HClO4: (a) PA-co-OHP; (b) PA-co-EB; (c) PA-co-DEB.
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Fig. 8. Effect of temperature on humidity response of PA-co-OHP doped with FeCl3.
strongly affect their sensing properties, and the best sensing response can be obtained by choosing suitable combination of polymerization monomers, and HClO4 is a better doping agent with respect to H2SO4 and FeCl3. 3.4. Effect of temperature on humidity response
Fig. 7. Response time of PA-co-OHP doped with HClO4: (a) from 33–97– 33%RH; (b) absorption and desorption isotherm.
improved over the response of F-PA-co-DEB and S-PA-coDEB. Again, PA-co-OHP doped with HClO4 gives the best response. The impedance changes from 107 to 103 O over the range of 30–95%RH, showing very high sensitivity, and a good linearity is observed. In addition, C-PA-co-OHP exhibits very quick response in both absorption and desorption process, as illustrated in Fig. 7. The response time are calculated to be 6 and 15 s for absorption and desorption process, respectively. This quick response can be explained as follows: there are two functional groups, –COOH and –OH, with different polarity in PA-co-OHP. The OH group has weaker interaction with water molecules compared with COOH group, and the absorbed water molecules can be removed more easily in desorption process, thus the response time will be greatly shortened, and the hysteresis will be lower. This is similar to the case of decreased hysteresis in a polymer chain containing both strong and weak acid groups as reported by Huang [25]. In addition, compared with copolymers bearing both polar and non-polar group, such as PA-co-EB and PA-co-DEB, PA-co-OHP exhibits higher ability to absorb water molecules and form complexes with the doping agents, therefore, it shows high sensitivity over the range of tested humidity. All these results indicate that the chemical structures of the polymers can
As mentioned above, the conductivity of the doped polyacetylenes are ionic in nature in presence of humidity. Therefore, temperature has some effect on the humidity response. Fig. 8 shows the humidity response of PA-co-OHP doped with FeCl3 at different temperatures. When the temperature increases from 15 to 40 8C, the impedance decreases slightly in the range of low to middle humidity (30–70%RH). Only at high humidities, a great decrease in impedance is observed. The negative temperature coefficient may result from the following two reasons: (1) the ion movement is enhanced at higher temperatures; (2) at the same RH, the water content at higher temperature increases and more water molecules are absorbed. The great change in impedance observed only at high humidity suggests that the second reason contribute more to the impedance change with temperature.
4. Conclusion A series of soluble substituted polyacetylenes: PPA, PAco-EB, PA-co-DEB, PA-co-OHP synthesized with palladium acetylide catalysts can be doped with salt and acids to prepare thin film humidity sensors. The sensing behaviour of the polymers depends on their chemical structures. Among all the polymers, PA-co-OHP, which bears both strong and weak polar groups, shows the best response in terms of sensitivity, linearity and response time. Of the three doping agents used, HClO4 is better than H2SO4 and FeCl3.
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The polymers exhibit negative temperature coefficient over the tested humidity range.
Acknowledgements This work was supported by the Scientific and Technological Agreement People’s Republic of China–Italy. The authors wish to thank Dr. G. Casalbore-Miceli of FRAECNR, Italy for kind help in measurements of humidity sensitive properties.
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