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A resistive-type humidity sensor based on crosslinked polyelectrolyte prepared by UV irradiation
Sensors and Actuators B 135 (2009) 581–586
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A resistive-type humidity sensor based on crosslinked polyelectrolyte prepared by UV irradiation Xin Lv a,b , Yang Li a,b,∗ , Peng Li a,b , Mujie Yang a,b a b
Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Key Laboratory of Macromolecule Synthesis and Functionalization (Zhejiang University), Ministry of Education, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 22 May 2008 Received in revised form 8 October 2008 Accepted 9 October 2008 Available online 21 October 2008 Keywords: Humidity sensor Humidity sensing properties Crosslinked UV irradiation Polyelectrolyte
a b s t r a c t 2-(Dimethylamino) ethyl methacrylate was quaternized with n-butyl bromide and then copolymerized with 1,4-divinylbenzene to obtain a resistive-type crosslinked polyelectrolyte humidity sensitive material by UV irradiation at low temperature (≤60 ◦ C) in a short time (<1 h). The humidity sensor based on the crosslinked polyelectrolyte was aged by immersion in water for 5 min to remove water-soluble materials. The aged sensor exhibited a high sensitivity (impedance change from ∼107 to 103 in the humidity range of 22–97% RH), small hysteresis (∼1% RH) and fast response (t90% ∼9 and 32 s for adsorption and desorption between 33% RH and 97% RH, respectively). In addition, it showed good durability as indicated by slight impedance change after immersion in water. The effects of molar ratio of crosslinking agent to quaternary ammonium salt, UV irradiation time and environment temperature on its humidity sensing properties were investigated. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Humidity sensors have been widely used in the measurement and control of humidity in meteorology, process control, agricultural and industrial production, etc. Among the various types of humidity sensors developed, resistive-type polymeric humidity sensors, which are mainly based on polyelectrolyte, received much attention due to their advantages of high sensitivity, fast response, easy preparation, low cost, and so on [1–6]. However, the polymeric humidity sensitive materials are soluble in water, and their sensing properties soon deteriorated under high humidities. Several methods have been proposed to address the problem, including copolymerizing or grafting with hydrophobic monomers, applying water insoluble protective films, forming crosslinked or interpenetrating network structures in the sensitive films or forming organic/inorganic hybrids [7–12]. Among them, forming crosslinked structures in the sensitive films was proved to effectively improve their water durability and stability under humid environment. There have been a number of reports on the formation of crosslinked structure in resistive-type polymeric humidity sensi-
∗ Corresponding author at: Department of Polymer Science and Engineering, Zhejiang University, 38 Zhe Da Road, Hangzhou 310027, China. Tel.: +86 571 87952444; fax: +86 571 87952444. E-mail address: [email protected] (Y. Li). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.10.008
tive films [5,6,11–18]. Generally, the preparation involves heating at a high temperature (usually more than 100 ◦ C) and long time (over 10 h), suggesting a high power consumption and low productivity. Gong and coworkers prepared a reactive-amine containing polyelectrolyte humidity sensitive material, and the formation of crosslinked sensitive film was accomplished in a two step procedure: heating at 60 ◦ C for 2 h and 125 ◦ C for another 6 h [11]. They prepared another resistive-type humidity sensor based on mutually crosslinkable copolymer, and the crosslinking process was accomplished by heating at 60 ◦ C for 6 h and 120 ◦ C for 6 h [13]. Radiation curing is a first, energy saving and more efficient industrial process than heat-curable process (low temperature and shorter time), and it can easily lead to crosslinked network structure in polymer film [19]. There have been several reports on the preparation of crosslinked humidity sensitive film by irradiation. Smith and coworkers applied 60 Co irradiation to crosslink humidity sensitive film of poly(dimethyldiallylammonium chloride) [20]. Matsuguchi et al. obtained crosslinked poly(methyl methacrylate) by photoirradiation, and investigated the effect of crosslinking degree on their humidity sensitive properties [21]. In addition, UV treatment is not only used for crosslinking, but also for facile conversion of monomers into polymers. Kim and coworkers employed UV irradiation to polymerize several diacetylene monomers encapsulated in electrospun fiber mats, and obtained colorimetric sensors for volatile organic compounds [22,23]. In this study, UV irradiation approach was used to realize the polymerization and crosslinking of a quaternary ammonium
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salt and obtain a resistive-type humidity sensitive film with a crosslinked network structure. The whole procedure was accomplished at low temperature (<60 ◦ C) in a very short time (<1 h). The humidity sensitive characteristics of the resulting thin film humidity sensor were investigated, including its impedance response to humidity, hysteresis, response time and water durability.
2. Experimental 2.1. Materials 2-(Dimethylamino) ethyl methacrylate (DMAEMA), n-butyl bromide (BB), benzoin ethyl ether, ethanol, hydroquinone and acetone were purchased from domestic market and used as received. 1,4-Divinylbenzene (DVB) was purchased from Fluka and used as received. 2,2 -Azo-bis-iso-butyronitrile (AIBN) was purified by recrystallization from ethanol and dried under vacuum at room temperature for 12 h.
2.2. Synthesis of prepolymer 2.2.1. Synthesis of quaternary ammonium salt of DMAEMA 2-(Dimethylamino) ethyl methacrylate (40 g, 0.254 mol), nbutyl bromide (34.9 g, 0.254 mol) and hydroquinone (0.75 g, 6.8 mmol) were dissolved in acetone with stirring and put into a flask. The solution was heated at 40 ◦ C for 20 h under argon atmosphere. The resulting mixture was filtered and dried under vacuum at room temperature overnight to obtain a white powder, which is denoted as DMAEMA–BB (yield: 55%). M.P.: 113–114 ◦ C; 1 H NMR (D2 O, ppm) ı: 6.05 (m, 1H, vinyl-H), 5.67 (m, 1H vinyl-H), 4.52 (br, 2H, –OCH2 ), 3.65 (t, 2H, –NCH2 ), 3.31 (t, 2H, –NCH2 ), 3.05 (S, 3H, –NCH3 ), 1.83 (s, 3H, –CH3 ), 1.67 (m, 2H, –CH2 ), 1.27 (m, 2H, –CH2 ), and 0.84 (t, 3H, –CH3 ). FT-IR max (cm−1 ): 3416 (m), 3036 (w), 2966 (m), 1719 (s), 1632 (m), 1480 (m), 1320 (m), 1165 (s), 1044 (w), 965 (m), and 817 (m).
2.2.2. Synthesis of prepolymer of quaternary ammonium salt A typical procedure for the preparation of prepolymer of quaternary ammonium salt (PQAS) is described as follows: DMAEMA–BB (7.95 g, 27 mmol) and azodiisobutyronitrile (AIBN, 0.0040 g) were dissolved in absolute ethanol (15 mL), then added into a 50 mL three-necked flask. The mixture was flushed with argon for 0.5 h and magnetically stirred to form a homogeneous solution, then heated at 60 ◦ C for 10 min under argon atmosphere. FT-IR. max (cm−1 ): 3420 (m), 2961 (s), 2874 (m), 1723 (s), 1634 (w), 1469 (m), 1294 (m), 1162 (s), 956 (w), and 813 (w).
2.3. Fabrication of humidity sensors In a typical procedure, PQAS (1.23 g), DVB (0.27 g) and benzoin ethyl ether (0.059 g) were dissolved in absolute ethanol (13 mL) under magnetic stirring. The resulting mixture was then deposited on a clean interdigitated gold electrode with a ceramic substrate (6 mm × 5 mm × 0.5 mm) using an automatic dip-coating machine. The interdigital distances were 40 m. The as-prepared electrode was irradiated by UV light (110 W, = 375 nm) for 30 min to accomplish the curing procedure and obtain a resistive-type thin film humidity sensor based on polyelectrolyte with crosslinked structure. FT-IR of the UV irradiated polymer film max (cm−1 ): 3433 (m), 3089 (w), 2965 (m), 1720 (s), 1630 (m), 1430 (m), 989 (m), 907 (s), 801 (m), and 707 (m).
2.4. Measurements of humidity sensitive properties The impedance responses of the sensor to relative humidity (RH) were measured by using an in-house built electric circuit at 1 V and 1 kHz (waveform: sinus). Different humidities were obtained by controlling the ratio of dry and wet air and calibrated with a commercial hygrometer (Shinyei THT-N263A). The dry and wet air was prepared by bubbling air flow through a bottle containing silica gel and distilled water, respectively. The measurement of the response transients was performed by recording online the electrical responses of the sensor which was quickly transferred between chambers containing different saturated salt solutions in their equilibrium state (MgCl2 for 33% RH and K2 SO4 for 97% RH), and the laboratory humidity during the measurement was between 33% RH and 97% RH. All the measurements were carried out at 15 ◦ C unless noted otherwise.
3. Results and discussion Preparing sensitive films with crosslinked structure has been known to effectively improve the stability of polymeric resistivetype humidity sensors at high humidities and facilitate the practical applications [11–18]. Although most researchers employed heating at a high temperature for a long time to obtain crosslinked polymer sensitive films, UV curing can be an alternative method with great advantages. It is featured with formation of crosslinked polymer films at low temperature and short time, and is more suitable for batch production of humidity sensors with good water durability. In this study, UV technology was introduced to prepare a polyelectrolyte-based humidity sensitive film with a crosslinked structure. The whole procedure can be finished at a low temperature (≤60 ◦ C) in a short time (≤1 h), and has been illustrated in Scheme 1. From Scheme 1, it can be seen that the quaternary ammonium salt DMAEMA–BB was prepolymerized first, then reacted with DVB to form a polyelectrolyte humidity sensitive material with crosslinked structure via a photoinitiated radical polymerization under UV irradiation [24,25]. Judged from the structure of the sensitive film, it is seen that the ammonium salt unit contributes to the ionic conduction of the sensitive film at different humidities. While DVB, which is non-conductive, acts as the crosslinking agent for the formation of crosslinked structure and thus improves the water durability of the sensitive film. It is known that both the concentration of ions in the sensitive film and the crosslinking degree have a great effect on the humidity sensing properties of polyelectrolytebased humidity sensors [2,13,16,21]. Fig. 1 shows the effect of the feed mol ratio of DMAEMA–BB unit in PQAS to DVB on the impedance responses to humidity of the crosslinked polyelectrolytes during humidification and desiccation processes. It can be seen that the impedance of the polyelectrolytes changed for about three orders of magnitude over a wide humidity range (22–97% RH), exhibiting high sensitivity. When the ratio of DMAEMA–BB unit to DVB was increased from 1/1 to 4/1, the impedance of the resulting crosslinked polyelectrolytes was much decreased, which is proposed to result from higher ion concentration in the sensitive films. In addition, the hysteresis also decreased with increase in the ratio of DMAEMA–BB unit. When feed ratio of DMAEMA–BB unit to DVB is 4/1, the resulting polyelectrolyte showed the smallest hysteresis of ∼1% RH. And the ratio was adopted for preparation of humidity sensors in the following investigations. As described above, UV irradiation could lead to crosslinked sensitive films, and thus improve the water durability of the humidity sensors. We investigated the water durability of the polyelectrolyte sensitive film irradiated by UV light for 30 min by measuring the
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Scheme 1. Synthesis of the crosslinked polyelectrolyte.
impedance response of the humidity sensor which was soaked in water for different times, then dried in air before the measurement, and the results are shown in Fig. 2. It is seen that the impedance of the sensitive film increased obviously in the range of 34–97% RH after immersed in water for 5 min. However, the impedance responses changed little afterwards even after it was
immersed in water for another 25 min. It is proposed that the first 5-min immersion in water led to the removal of unreacted reactants and/or non-crosslinked water-soluble polyelectrolyte. The remaining crosslinked sensitive film exhibited good water durability and the impedance responses did not deteriorate even after immersion in water for a total of 30 min. Immersion in water for 5 min seemed to be an aging process and the sensitive film treated in this way could show high stability in humid environment judged from their good water durability. The experimental results also indicate that
Fig. 1. Hysteresis of the crosslinked polyelectrolytes prepared with different feed mol ratio of DMAEMA–BB unit to DVB. (A) 1/1 () desorption and () absorption; (B) 2/1 (䊉) desorption and () absorption; (C) 4/1 () desorption and () absorption.
Fig. 2. Impedance of the crosslinked polyelectrolyte versus time of immersion in water, measured at different humidities. () 22% RH; (䊉) 34% RH; () 43% RH; () 56% RH; () 75% RH; () 84% RH; () 97% RH.
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Fig. 3. Impedance responses to humidity of prepolymerized polyelectrolyte (A) before and (B) after immersion in water for 5 min, and polyelectrolyte without prepolymerization (C) before and (D) after immersion in water for 5 min.
UV irradiation is successful for the preparation of humidity sensors with high stability under humid environment. In Scheme 1, it is seen that the ammonium salt DMAEM–BB was prepolymerized before forming a crosslinked structure with DVB under UV irradiation. It is expected that prepolymerization promoted the complete conversion of the ammonium salt into polymer, and also facilitated its reaction with DVB to form a crosslinked structure under UV irradiation. Fig. 3 illustrates the effect of prepolymerization treatment and immersion in water on the humidity responses of the sensitive films which were irradiated under UV light for 30 min. It is clearly seen that the original curves of impedance versus humidity of the sensitive films with and without prepolymerization treatment are quite close to each other. However, a great difference in the humidity responses is observed for the sensitive films after immersion in water. The impedance of prepolymerized one increased by about one order of magnitude over the whole humidity range. In comparison, the one without prepolymerization showed a much higher impedance increase after treatment in water. Its impedance increased by more than three orders of magnitude, and the impedance at 75% RH is as high as 107 . At lower humidity, the impedance is too high to be measured and cannot be used as humidity sensors. The distinct difference in water durability of the sensitive films suggests that prepolymerization is crucial for the formation of highly crosslinked sensitive films and the improvement in their water durability. Apart from prepolymerization, the time of UV irradiation is also important for the humidity sensing properties of the polyelectrolyte. Prolonging irradiation time is expected to improve the crosslinking of the sensitive film and therefore the water durability, but exposure to UV light for a long time may also result in the polymer degradation. It is necessary to find a compromise between them. Fig. 4 depicts the effect of irradiation time on the humidity responses of the sensitive films after immersed in water for 5 min. It is seen that the impedance changes of the sensitive film after water treatment was greatly reduced when UV irradiation time was increased from 5 to 30 min, indicating a tendency of improved water durability. The impedance responses changed little when the irradiation time was prolonged to 60 min. Therefore UV irradia-
Fig. 4. The effect of irradiation time on the impedance response to humidity of the crosslinked polyelectrolyte after immersion in water for 5 min.
tion for 30 min was adopted for preparation of sensitive film in the present work since it could lead to good water durability (see Fig. 3) and minimize the film degradation by UV irradiation. The above analysis indicated that the sensitive film so prepared shows good water durability after an “aging” process of immersion in water for 5 min. The response time and hysteresis of the “aged” sensitive film are measured and illustrated in Figs. 5 and 6, respectively. The response times (t90% ) for absorption and desorption are estimated to be only ∼9 and ∼32 s from Fig. 5, respectively, indicating a quick response. Fig. 6 shows that the “aged” sensitive film also exhibited a very small hysteresis of only ∼1% RH. In addition, the impedance decreased from 107 to 103 over the whole tested humidity range (22–97% RH), exhibiting a high sensitivity. All these suggest the aged crosslinked polyelectrolyte prepared by UV irradiation can be a promising candidate for humidity sensitive material with good sensing properties. Temperature is known to have a great effect on conductivity of polyelectrolytes, and thus their humidity sensitive properties. Fig. 7 presents the humidity responses of the “aged” sensitive film
Fig. 5. Hysteresis of the crosslinked polyelectrolyte after immersed in water for 30 min () desorption and () absorption.
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temperature in a short time. The sensor so prepared exhibited good water durability after aging by immersion in water. In addition, it is featured with high sensitivity, quick response and very small hysteresis. It can be a good candidate for a high performance humidity sensor with good stability under high humidities. UV irradiation can be an effective alternative to heat treatment in introducing crosslinked structures into humidity sensitive films and improve their stability at high humidities. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (contract no. 50403020), Zhejiang Provincial Natural Science Foundation of China (grant no. M203093) and National “863” program of China (contract no. 2006AA10Z215). References
Fig. 6. Response time of the crosslinked polyelectrolyte after immersion in water for 30 min () desorption and (䊉) absorption.
Fig. 7. Effect of temperature on the humidity responses of the crosslinked polyelectrolyte after immersion in water for 5 min.
at different temperatures. It can be seen that the impedance of the sensitive film just decreased with temperature elevation as a result of enhanced ion mobility at higher temperature. The temperature coefficient is estimated to change from −0.36 to −0.42% RH/◦ C between 15 and 32 ◦ C, and the temperature dependence tendency was found to decrease with increasing humidity. In comparison, commercial sensor was reported to exhibit a temperature coefficient of ∼−0.6% RH/◦ C [16]. And it is proposed that temperature compensation may be needed for the practical application of the humidity sensor prepared in this work. 4. Conclusions A resistive-type humidity sensor based on crosslinked polyelectrolyte has been prepared by using UV irradiation at low
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Biographies Xin Lv is a Ph.D. student in the Department of Polymer Science and Engineering, Zhejiang University. His research interests include polymer materials for humidity sensors
Yang Li received his Ph.D. degree in polymer chemistry and physics from Zhejiang University in 2000. He has been working in Department of Polymer Science and Engineering, Zhejiang University since 2000 and was appointed as associate professor in polymer science in 2002. His research interests include polymer materials and organic/inorganic nanocomposites for chemical sensors. Peng Li is a postgraduate student in the Department of Polymer Science and Engineering, Zhejiang University, China. His research interests are polymer and composite materials for humidity sensors. Mujie Yang graduated from Zhongshan University, China in 1963. She has been working in Zhejiang University since 1963. She was promoted to full professor in polymer science in 1992. Her research interests are functional polymers with optical and electrical characteristics.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A resistive-type humidity sensor based on crosslinked polyelectrolyte prepared by UV irradiation Xin Lv a,b , Yang Li a,b,∗ , Peng Li a,b , Mujie Yang a,b a b
Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Key Laboratory of Macromolecule Synthesis and Functionalization (Zhejiang University), Ministry of Education, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 22 May 2008 Received in revised form 8 October 2008 Accepted 9 October 2008 Available online 21 October 2008 Keywords: Humidity sensor Humidity sensing properties Crosslinked UV irradiation Polyelectrolyte
a b s t r a c t 2-(Dimethylamino) ethyl methacrylate was quaternized with n-butyl bromide and then copolymerized with 1,4-divinylbenzene to obtain a resistive-type crosslinked polyelectrolyte humidity sensitive material by UV irradiation at low temperature (≤60 ◦ C) in a short time (<1 h). The humidity sensor based on the crosslinked polyelectrolyte was aged by immersion in water for 5 min to remove water-soluble materials. The aged sensor exhibited a high sensitivity (impedance change from ∼107 to 103 in the humidity range of 22–97% RH), small hysteresis (∼1% RH) and fast response (t90% ∼9 and 32 s for adsorption and desorption between 33% RH and 97% RH, respectively). In addition, it showed good durability as indicated by slight impedance change after immersion in water. The effects of molar ratio of crosslinking agent to quaternary ammonium salt, UV irradiation time and environment temperature on its humidity sensing properties were investigated. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Humidity sensors have been widely used in the measurement and control of humidity in meteorology, process control, agricultural and industrial production, etc. Among the various types of humidity sensors developed, resistive-type polymeric humidity sensors, which are mainly based on polyelectrolyte, received much attention due to their advantages of high sensitivity, fast response, easy preparation, low cost, and so on [1–6]. However, the polymeric humidity sensitive materials are soluble in water, and their sensing properties soon deteriorated under high humidities. Several methods have been proposed to address the problem, including copolymerizing or grafting with hydrophobic monomers, applying water insoluble protective films, forming crosslinked or interpenetrating network structures in the sensitive films or forming organic/inorganic hybrids [7–12]. Among them, forming crosslinked structures in the sensitive films was proved to effectively improve their water durability and stability under humid environment. There have been a number of reports on the formation of crosslinked structure in resistive-type polymeric humidity sensi-
∗ Corresponding author at: Department of Polymer Science and Engineering, Zhejiang University, 38 Zhe Da Road, Hangzhou 310027, China. Tel.: +86 571 87952444; fax: +86 571 87952444. E-mail address: [email protected] (Y. Li). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.10.008
tive films [5,6,11–18]. Generally, the preparation involves heating at a high temperature (usually more than 100 ◦ C) and long time (over 10 h), suggesting a high power consumption and low productivity. Gong and coworkers prepared a reactive-amine containing polyelectrolyte humidity sensitive material, and the formation of crosslinked sensitive film was accomplished in a two step procedure: heating at 60 ◦ C for 2 h and 125 ◦ C for another 6 h [11]. They prepared another resistive-type humidity sensor based on mutually crosslinkable copolymer, and the crosslinking process was accomplished by heating at 60 ◦ C for 6 h and 120 ◦ C for 6 h [13]. Radiation curing is a first, energy saving and more efficient industrial process than heat-curable process (low temperature and shorter time), and it can easily lead to crosslinked network structure in polymer film [19]. There have been several reports on the preparation of crosslinked humidity sensitive film by irradiation. Smith and coworkers applied 60 Co irradiation to crosslink humidity sensitive film of poly(dimethyldiallylammonium chloride) [20]. Matsuguchi et al. obtained crosslinked poly(methyl methacrylate) by photoirradiation, and investigated the effect of crosslinking degree on their humidity sensitive properties [21]. In addition, UV treatment is not only used for crosslinking, but also for facile conversion of monomers into polymers. Kim and coworkers employed UV irradiation to polymerize several diacetylene monomers encapsulated in electrospun fiber mats, and obtained colorimetric sensors for volatile organic compounds [22,23]. In this study, UV irradiation approach was used to realize the polymerization and crosslinking of a quaternary ammonium
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salt and obtain a resistive-type humidity sensitive film with a crosslinked network structure. The whole procedure was accomplished at low temperature (<60 ◦ C) in a very short time (<1 h). The humidity sensitive characteristics of the resulting thin film humidity sensor were investigated, including its impedance response to humidity, hysteresis, response time and water durability.
2. Experimental 2.1. Materials 2-(Dimethylamino) ethyl methacrylate (DMAEMA), n-butyl bromide (BB), benzoin ethyl ether, ethanol, hydroquinone and acetone were purchased from domestic market and used as received. 1,4-Divinylbenzene (DVB) was purchased from Fluka and used as received. 2,2 -Azo-bis-iso-butyronitrile (AIBN) was purified by recrystallization from ethanol and dried under vacuum at room temperature for 12 h.
2.2. Synthesis of prepolymer 2.2.1. Synthesis of quaternary ammonium salt of DMAEMA 2-(Dimethylamino) ethyl methacrylate (40 g, 0.254 mol), nbutyl bromide (34.9 g, 0.254 mol) and hydroquinone (0.75 g, 6.8 mmol) were dissolved in acetone with stirring and put into a flask. The solution was heated at 40 ◦ C for 20 h under argon atmosphere. The resulting mixture was filtered and dried under vacuum at room temperature overnight to obtain a white powder, which is denoted as DMAEMA–BB (yield: 55%). M.P.: 113–114 ◦ C; 1 H NMR (D2 O, ppm) ı: 6.05 (m, 1H, vinyl-H), 5.67 (m, 1H vinyl-H), 4.52 (br, 2H, –OCH2 ), 3.65 (t, 2H, –NCH2 ), 3.31 (t, 2H, –NCH2 ), 3.05 (S, 3H, –NCH3 ), 1.83 (s, 3H, –CH3 ), 1.67 (m, 2H, –CH2 ), 1.27 (m, 2H, –CH2 ), and 0.84 (t, 3H, –CH3 ). FT-IR max (cm−1 ): 3416 (m), 3036 (w), 2966 (m), 1719 (s), 1632 (m), 1480 (m), 1320 (m), 1165 (s), 1044 (w), 965 (m), and 817 (m).
2.2.2. Synthesis of prepolymer of quaternary ammonium salt A typical procedure for the preparation of prepolymer of quaternary ammonium salt (PQAS) is described as follows: DMAEMA–BB (7.95 g, 27 mmol) and azodiisobutyronitrile (AIBN, 0.0040 g) were dissolved in absolute ethanol (15 mL), then added into a 50 mL three-necked flask. The mixture was flushed with argon for 0.5 h and magnetically stirred to form a homogeneous solution, then heated at 60 ◦ C for 10 min under argon atmosphere. FT-IR. max (cm−1 ): 3420 (m), 2961 (s), 2874 (m), 1723 (s), 1634 (w), 1469 (m), 1294 (m), 1162 (s), 956 (w), and 813 (w).
2.3. Fabrication of humidity sensors In a typical procedure, PQAS (1.23 g), DVB (0.27 g) and benzoin ethyl ether (0.059 g) were dissolved in absolute ethanol (13 mL) under magnetic stirring. The resulting mixture was then deposited on a clean interdigitated gold electrode with a ceramic substrate (6 mm × 5 mm × 0.5 mm) using an automatic dip-coating machine. The interdigital distances were 40 m. The as-prepared electrode was irradiated by UV light (110 W, = 375 nm) for 30 min to accomplish the curing procedure and obtain a resistive-type thin film humidity sensor based on polyelectrolyte with crosslinked structure. FT-IR of the UV irradiated polymer film max (cm−1 ): 3433 (m), 3089 (w), 2965 (m), 1720 (s), 1630 (m), 1430 (m), 989 (m), 907 (s), 801 (m), and 707 (m).
2.4. Measurements of humidity sensitive properties The impedance responses of the sensor to relative humidity (RH) were measured by using an in-house built electric circuit at 1 V and 1 kHz (waveform: sinus). Different humidities were obtained by controlling the ratio of dry and wet air and calibrated with a commercial hygrometer (Shinyei THT-N263A). The dry and wet air was prepared by bubbling air flow through a bottle containing silica gel and distilled water, respectively. The measurement of the response transients was performed by recording online the electrical responses of the sensor which was quickly transferred between chambers containing different saturated salt solutions in their equilibrium state (MgCl2 for 33% RH and K2 SO4 for 97% RH), and the laboratory humidity during the measurement was between 33% RH and 97% RH. All the measurements were carried out at 15 ◦ C unless noted otherwise.
3. Results and discussion Preparing sensitive films with crosslinked structure has been known to effectively improve the stability of polymeric resistivetype humidity sensors at high humidities and facilitate the practical applications [11–18]. Although most researchers employed heating at a high temperature for a long time to obtain crosslinked polymer sensitive films, UV curing can be an alternative method with great advantages. It is featured with formation of crosslinked polymer films at low temperature and short time, and is more suitable for batch production of humidity sensors with good water durability. In this study, UV technology was introduced to prepare a polyelectrolyte-based humidity sensitive film with a crosslinked structure. The whole procedure can be finished at a low temperature (≤60 ◦ C) in a short time (≤1 h), and has been illustrated in Scheme 1. From Scheme 1, it can be seen that the quaternary ammonium salt DMAEMA–BB was prepolymerized first, then reacted with DVB to form a polyelectrolyte humidity sensitive material with crosslinked structure via a photoinitiated radical polymerization under UV irradiation [24,25]. Judged from the structure of the sensitive film, it is seen that the ammonium salt unit contributes to the ionic conduction of the sensitive film at different humidities. While DVB, which is non-conductive, acts as the crosslinking agent for the formation of crosslinked structure and thus improves the water durability of the sensitive film. It is known that both the concentration of ions in the sensitive film and the crosslinking degree have a great effect on the humidity sensing properties of polyelectrolytebased humidity sensors [2,13,16,21]. Fig. 1 shows the effect of the feed mol ratio of DMAEMA–BB unit in PQAS to DVB on the impedance responses to humidity of the crosslinked polyelectrolytes during humidification and desiccation processes. It can be seen that the impedance of the polyelectrolytes changed for about three orders of magnitude over a wide humidity range (22–97% RH), exhibiting high sensitivity. When the ratio of DMAEMA–BB unit to DVB was increased from 1/1 to 4/1, the impedance of the resulting crosslinked polyelectrolytes was much decreased, which is proposed to result from higher ion concentration in the sensitive films. In addition, the hysteresis also decreased with increase in the ratio of DMAEMA–BB unit. When feed ratio of DMAEMA–BB unit to DVB is 4/1, the resulting polyelectrolyte showed the smallest hysteresis of ∼1% RH. And the ratio was adopted for preparation of humidity sensors in the following investigations. As described above, UV irradiation could lead to crosslinked sensitive films, and thus improve the water durability of the humidity sensors. We investigated the water durability of the polyelectrolyte sensitive film irradiated by UV light for 30 min by measuring the
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Scheme 1. Synthesis of the crosslinked polyelectrolyte.
impedance response of the humidity sensor which was soaked in water for different times, then dried in air before the measurement, and the results are shown in Fig. 2. It is seen that the impedance of the sensitive film increased obviously in the range of 34–97% RH after immersed in water for 5 min. However, the impedance responses changed little afterwards even after it was
immersed in water for another 25 min. It is proposed that the first 5-min immersion in water led to the removal of unreacted reactants and/or non-crosslinked water-soluble polyelectrolyte. The remaining crosslinked sensitive film exhibited good water durability and the impedance responses did not deteriorate even after immersion in water for a total of 30 min. Immersion in water for 5 min seemed to be an aging process and the sensitive film treated in this way could show high stability in humid environment judged from their good water durability. The experimental results also indicate that
Fig. 1. Hysteresis of the crosslinked polyelectrolytes prepared with different feed mol ratio of DMAEMA–BB unit to DVB. (A) 1/1 () desorption and () absorption; (B) 2/1 (䊉) desorption and () absorption; (C) 4/1 () desorption and () absorption.
Fig. 2. Impedance of the crosslinked polyelectrolyte versus time of immersion in water, measured at different humidities. () 22% RH; (䊉) 34% RH; () 43% RH; () 56% RH; () 75% RH; () 84% RH; () 97% RH.
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Fig. 3. Impedance responses to humidity of prepolymerized polyelectrolyte (A) before and (B) after immersion in water for 5 min, and polyelectrolyte without prepolymerization (C) before and (D) after immersion in water for 5 min.
UV irradiation is successful for the preparation of humidity sensors with high stability under humid environment. In Scheme 1, it is seen that the ammonium salt DMAEM–BB was prepolymerized before forming a crosslinked structure with DVB under UV irradiation. It is expected that prepolymerization promoted the complete conversion of the ammonium salt into polymer, and also facilitated its reaction with DVB to form a crosslinked structure under UV irradiation. Fig. 3 illustrates the effect of prepolymerization treatment and immersion in water on the humidity responses of the sensitive films which were irradiated under UV light for 30 min. It is clearly seen that the original curves of impedance versus humidity of the sensitive films with and without prepolymerization treatment are quite close to each other. However, a great difference in the humidity responses is observed for the sensitive films after immersion in water. The impedance of prepolymerized one increased by about one order of magnitude over the whole humidity range. In comparison, the one without prepolymerization showed a much higher impedance increase after treatment in water. Its impedance increased by more than three orders of magnitude, and the impedance at 75% RH is as high as 107 . At lower humidity, the impedance is too high to be measured and cannot be used as humidity sensors. The distinct difference in water durability of the sensitive films suggests that prepolymerization is crucial for the formation of highly crosslinked sensitive films and the improvement in their water durability. Apart from prepolymerization, the time of UV irradiation is also important for the humidity sensing properties of the polyelectrolyte. Prolonging irradiation time is expected to improve the crosslinking of the sensitive film and therefore the water durability, but exposure to UV light for a long time may also result in the polymer degradation. It is necessary to find a compromise between them. Fig. 4 depicts the effect of irradiation time on the humidity responses of the sensitive films after immersed in water for 5 min. It is seen that the impedance changes of the sensitive film after water treatment was greatly reduced when UV irradiation time was increased from 5 to 30 min, indicating a tendency of improved water durability. The impedance responses changed little when the irradiation time was prolonged to 60 min. Therefore UV irradia-
Fig. 4. The effect of irradiation time on the impedance response to humidity of the crosslinked polyelectrolyte after immersion in water for 5 min.
tion for 30 min was adopted for preparation of sensitive film in the present work since it could lead to good water durability (see Fig. 3) and minimize the film degradation by UV irradiation. The above analysis indicated that the sensitive film so prepared shows good water durability after an “aging” process of immersion in water for 5 min. The response time and hysteresis of the “aged” sensitive film are measured and illustrated in Figs. 5 and 6, respectively. The response times (t90% ) for absorption and desorption are estimated to be only ∼9 and ∼32 s from Fig. 5, respectively, indicating a quick response. Fig. 6 shows that the “aged” sensitive film also exhibited a very small hysteresis of only ∼1% RH. In addition, the impedance decreased from 107 to 103 over the whole tested humidity range (22–97% RH), exhibiting a high sensitivity. All these suggest the aged crosslinked polyelectrolyte prepared by UV irradiation can be a promising candidate for humidity sensitive material with good sensing properties. Temperature is known to have a great effect on conductivity of polyelectrolytes, and thus their humidity sensitive properties. Fig. 7 presents the humidity responses of the “aged” sensitive film
Fig. 5. Hysteresis of the crosslinked polyelectrolyte after immersed in water for 30 min () desorption and () absorption.
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temperature in a short time. The sensor so prepared exhibited good water durability after aging by immersion in water. In addition, it is featured with high sensitivity, quick response and very small hysteresis. It can be a good candidate for a high performance humidity sensor with good stability under high humidities. UV irradiation can be an effective alternative to heat treatment in introducing crosslinked structures into humidity sensitive films and improve their stability at high humidities. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (contract no. 50403020), Zhejiang Provincial Natural Science Foundation of China (grant no. M203093) and National “863” program of China (contract no. 2006AA10Z215). References
Fig. 6. Response time of the crosslinked polyelectrolyte after immersion in water for 30 min () desorption and (䊉) absorption.
Fig. 7. Effect of temperature on the humidity responses of the crosslinked polyelectrolyte after immersion in water for 5 min.
at different temperatures. It can be seen that the impedance of the sensitive film just decreased with temperature elevation as a result of enhanced ion mobility at higher temperature. The temperature coefficient is estimated to change from −0.36 to −0.42% RH/◦ C between 15 and 32 ◦ C, and the temperature dependence tendency was found to decrease with increasing humidity. In comparison, commercial sensor was reported to exhibit a temperature coefficient of ∼−0.6% RH/◦ C [16]. And it is proposed that temperature compensation may be needed for the practical application of the humidity sensor prepared in this work. 4. Conclusions A resistive-type humidity sensor based on crosslinked polyelectrolyte has been prepared by using UV irradiation at low
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Biographies Xin Lv is a Ph.D. student in the Department of Polymer Science and Engineering, Zhejiang University. His research interests include polymer materials for humidity sensors
Yang Li received his Ph.D. degree in polymer chemistry and physics from Zhejiang University in 2000. He has been working in Department of Polymer Science and Engineering, Zhejiang University since 2000 and was appointed as associate professor in polymer science in 2002. His research interests include polymer materials and organic/inorganic nanocomposites for chemical sensors. Peng Li is a postgraduate student in the Department of Polymer Science and Engineering, Zhejiang University, China. His research interests are polymer and composite materials for humidity sensors. Mujie Yang graduated from Zhongshan University, China in 1963. She has been working in Zhejiang University since 1963. She was promoted to full professor in polymer science in 1992. Her research interests are functional polymers with optical and electrical characteristics.