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Synthesis and humidity sensitive property of cross-linked water-resistant polymer electrolytes
Sensors and Actuators B 208 (2015) 277–282
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Synthesis and humidity sensitive property of cross-linked water-resistant polymer electrolytes Teng Fei, Hongran Zhao, Kai Jiang, Tong Zhang ∗ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 20 August 2014 Received in revised form 30 October 2014 Accepted 8 November 2014 Available online 15 November 2014 Keywords: Humidity sensor Water-resistant polymer Cross-linking Polymer electrolyte Solvothermal
a b s t r a c t Defined structure and water-resistance are two important requirements for polymeric humidity sensors. Most of polymer electrolytes applied in humidity sensors do not bear both features at the same time. Herein, a novel method was used to develop stable humidity sensitive polymer electrolytes with defined structures. Nanoporous polymers based on 1,4-divinylbenzene (DVB) and methacrylatoethyl trimethyl ammonium chloride (DMC) were synthesized by free radical polymerization with a solvothermal route. The cross-linked structure could afford good stability of the polymers even at high humidity levels. The impedance module of the optimized sensor changed more than three orders of magnitude over the whole humidity range, with little humidity hysteresis and rapid response. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Humidity sensors have attracted the attention of many researchers in recent years [1,2]. Many different materials were developed for humidity sensing, including ceramics [3–5], organic polymers [1,2,6,7] and composites [8–10]. Resistive-type organic polymeric humidity sensors are most attractive because of their long-term stability, easy solution-processing and low cost. Sensitive materials for polymeric humidity sensors are usually amphiphilic polymers, however, the stability of polymeric humidity sensors is not satisfactory because the polar groups may result in dissolution of the polymers under high relative humidity (RH) ambience for long term operation, especially for dew condenses. In order to solve the above problem, the chemical stability of the sensitive polymers should be improved. The cross-linking of the hydrophilic polymers coated on the sensor substrate has been known to enhance the stability of polymeric resistive-type humidity sensors under high humidities [11–13]. The cross-linking method is effective, however, most of the works on humidity sensors were based on a solid phase reaction of the polymers, thus leading to uncontrollable structures of the cross-linked films and un-reproducible sensors. Therefore, water-resistant polymer electrolytes with definite chemical structures may be the ideal model for humidity sensors.
∗ Corresponding author. Tel.: +86 431 85168385; fax: +86 431 85168270. E-mail address: [email protected] (T. Zhang). http://dx.doi.org/10.1016/j.snb.2014.11.044 0925-4005/© 2014 Elsevier B.V. All rights reserved.
Recently, we have used hydrophobic and porous polymer (PDVB) in association with humidity sensitive component (LiCl) as a composite for the function of humidity sensing, and PDVB was used as host material with almost no humidity sensitive property itself [14]. Herein, novel organic porous polymer electrolytes were synthesized by free radical polymerization with different ratios of monomers through a solvothermal route. The polymers possess good stability because of their cross-linked network structures, which could be adjusted by feed ratios and the structures of monomers. Sensors with high sensitivity among the whole humidity range were realized based on the obtained polymers, and the sensing mechanism of the optimized sensor was researched.
2. Experimental 2.1. Synthesis of polymers The polymers were synthesized by free radical polymerization through a solvothermal route. The detailed procedure was as follows (PDVB/DMC-33 as an example): 1.5 g of divinylbenzene (DVB), 3.0 g of methacrylatoethyl trimethyl ammonium chloride (DMC), 38.0 mg of azodiisobutyronitrile (AIBN) and 30 mL of N,Ndimethylmethanamide (DMF) were put in an autoclave and treated at 100 ◦ C for 24 h. The autoclave was then cooled down to room temperature and a gel column was obtained. The gel was heated at 60 ◦ C under vacuum to remove the solvent, and loose solid (4.0 g) was obtained.
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2.2. Sensor preparation The polymers were mixed with n-butanol to form a paste, then the paste was dip-coated on a ceramic substrate (6 mm × 3 mm, 0.5 mm thick) with five pairs of Ag–Pd interdigitated electrodes (electrodes width: 0.15 mm; distance: 0.15 mm) to form a sensing film with the thickness of ∼100 m, as shown in Fig. 1a. The films were dried under air at ∼20 ◦ C for 12 h. Finally, the humidity sensors were obtained after aging at 95% RH with an alternating current (AC) of 1 V, 100 Hz for 24 h to improve their stability and durability. 2.3. Measurements The IR spectra of polymers were obtained on a WQF-510A FTIR spectrometer. Pore width distribution profiles of polymers were obtained at 77 K on a Micromeritics Tri-star 3000 analyzer using nitrogen. The morphologies of polymers were performed on a JEOL JSM-6700F scanning electron microscopy (SEM). The electrical responses of the sensors were measured at 1 V AC under different RH with a ZL-5 intelligent LCR analyzer at ∼20 ◦ C, as shown in Fig. 1b. In order to measure the impedances of the sensors, at least 10 sensors for each polymer were fabricated under the same condition and their sensing properties were defined by repeatedly results. The atmosphere of RH was produced by different saturated salt solutions in their equilibrium states including LiCl for 11% RH, MgCl2 for 33% RH, Mg(NO3 )2 for 54% RH, NaCl for 75% RH, KCl for 85% RH, and KNO3 for 95% RH. 3. Results and discussion Polymer electrolytes based on DVB and DMC were synthesized by free radical polymerization with a solvothermal route. The synthetic routes to polymers are shown in Fig. 2. DVB and
DMC are hydrophobic and hydrophilic monomers, respectively, which could be used for constructing polymer electrolytes. The two ethylenic bonds in single DVB molecule could afford hydrophobic cross-linked aromatic skeleton, which would be modified with hydrophilic alkyl chains for stable polymer electrolytes. DMF was chosen as solvent because it could dissolve both monomers, which afforded a homogeneous polymerization system. The feed ratio of monomers could be used to tune the structures of the polymers, and three polymers (PDVB/DMC-31, PDVB/DMC-33 and PDVB/DMC34) with different feed ratios (the molar ratio of DVB to DMC is 3:1, 3:3 and 3:4 for PDVB/DMC-31, PDVB/DMC-33 and PDVB/DMC34, respectively) were synthesized under similar polymerization condition. It should be noticed that a gel column was obtained after each polymerization, thus indicating a homogeneous polymerization of the vinyl monomers. The solvent (DMF) was removed by heating under vacuum from the gel columns, and loose powers were then obtained. Therefore, by the cross-linking reaction in homogeneous solutions, structure-defined polymer electrolytes were synthesized. In addition, this kind of polymerization by solvothermal method could also construct nanoporous polymers [15]. A polymer with porosity is beneficial for humidity sensing, since it is beneficial for the transport of water molecules in both adsorption and desorption processes. The FT-IR spectra of the obtained polymers are shown in Fig. 3. The absorptions at ∼3021, 797 and 710 cm−1 are assigned to the stretching of C H in the benzene ring. The absorptions at ∼1635 and 1479 cm−1 come from the aromatic C C stretching. And the stretching of alkyl chain shows a strong vibrational peak at ∼2938 cm−1 . It should be noted that the strong absorptions at ∼1728 and 1157 cm−1 contributed to the saturated ester and the absorption at ∼950 cm−1 from the unique C N stretching of quaternary ammonium also appear in the FT-IR spectra. Because both of the saturated ester and quaternary ammonium come from
Fig. 1. (a) A schematic diagram of the humidity sensor and (b) the schematic image of the equipment for measurement.
Fig. 2. Synthetic routes to polymers PDVB/DMC-31, PDVB/DMC-33 and PDVB/DMC-34.
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Fig. 5. The impedance modules of PDVB/DMC-31, PDVB/DMC-33 and PDVB/DMC34 sensors at different RH (1 V AC, 100 Hz).
Fig. 3. The FT-IR spectra of the obtained polymers.
the hydrophilic monomer DMC, the corresponding absorptions for PDVB/DMC-31 (with lower content of DMC) are much weaker. These results demonstrate the desired polymers were obtained. Pore width distribution profiles of polymers are shown in Fig. 4(a)–(c). The pore diameter is 4.5 nm, 4.3 nm and 5.0 nm for PDVB/DMC31, PDVB/DMC33 and PDVB/DMC34, respectively. The SEM images of resultant polymers are shown in Fig. 4(d) and (e). As can be seen, the obtained polymers are stacked by microparticles, and the packing between particles becomes loose with the increasing DMC content in the polymer. The porous polymers could provide channels for the transport of water molecules, combining with a stable cross-linked aromatic skeleton. It is considered that the interaction between hydrophilic quaternary ammonium salt and water molecules could be amplified by this kind of structure.
Organic porous polymers have been applied in catalyst [16,17], gas adsorption and storage [18,19], organic electronics [20], chemosensors [21], etc. The application of porous linear polymers on optical humidity sensors has also been reported [22,23]. These polymers show different absorption properties to visible light at different RH conditions because of the polymers’ swelling behavior. Different from above works based on linear polymers, the resultant materials in our work are cross-linked polymer electrolytes with better stability. Humidity sensors based on the obtained polymers were fabricated and their humidity sensing curves under different RH are shown in Fig. 5. The impedance modules (|Z|) of all sensors decrease with the increasing RH. And for different polymers, the impedance modules increase with the content of hydrophobic monomer at certain RH, because less water molecules are adsorbed in the polymer with higher content of hydrophobic structure. What is more, it is interesting to notice that the impedance modules of the three sensors all show a straight line semi-log fit to RH, among which
Fig. 4. Pore width distribution profiles of (a) PDVB/DMC-31, (b) PDVB/DMC-33 and (c) PDVB/DMC-34; SEM images of (d) PDVB/DMC-31, (e) PDVB/DMC-33 and (f) PDVB/DMC34.
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Fig. 6. The impedance modules of PDVB/DMC-33 sensor at different RH under different frequencies (1 V AC).
PDVB/DMC-33 sensor is the best. In addition, the impedance modules changed about three orders of magnitude from 11% to 95% RH for sensors based on PDVB/DMC-33 and PDVB/DMC-34, while the change is much smaller for PDVB/DMC-31. Taking semi-log fit of impedance module to RH and impedance module change of the sensors into account, PDVB/DMC-33 sensor shows the best properties, so it was used for further investigation in the following experiments. To determine the optimal working parameters, the impedance modules of PDVB/DMC-33 sensor under different frequencies at ∼20 ◦ C were measured and the results are shown in Fig. 6. The impedance module decreases with the increasing RH for each frequency, and under a certain RH it decreases with increasing working frequency. It is worthy to note that there are obvious impedance module changes for different operating frequencies under a certain RH when RH is low, but at higher RH, the curves almost overlap together, indicating the effect of frequencies on the impedance module is small in this range. The impedance module of 100 Hz shows a straight line semi-log fit to RH, so the operation condition was kept at AC 1 V, 100 Hz for following experiments. Humidity hysteresis is an important characteristic of humidity sensors, which reflects the interaction between water molecules and the sensitive materials, especially the hydrophilic groups. Since the applied polymers for humidity sensors were constructed with hydrophilic groups modified aromatic skeleton, it was considered that the interaction between the hydrophilic groups could be restrained, so the aggregation of the hydrophilic groups could be restricted, which would be useful for little humidity hysteresis. It is necessary to note that the structure character of the obtained polymer is different from linear amphiphilic polymers, among which polar groups are easy to aggregate due to the polymers’ flexible structures, and result in strong interaction with water molecules. PDVB/DMC-33 sensor was studied as an example. Fig. 7 shows the humidity hysteresis characteristic of PDVB/DMC-33 sensor by keeping the applied voltage at 1 V and the frequency at 100 Hz. The impedance of the sensor was first measured from 11% to 95% RH (corresponding to the adsorption process), and then in the opposite direction (corresponding to the desorption process). The equilibration time of the sensor at each RH atmosphere was 5 min. The maximum humidity hysteresis PDVB/DMC-33 sensor is around 4% RH under 50% RH, although the molar ratio of hydrophilic monomer for synthesizing the polymer PDVB/DMC-33 is as high as 50%. So a balanced adsorption–desorption water molecules was controlled by adjusting the content of hydrophilic groups in the polymer frameworks and a small humidity resistance was realized, which is beneficial for the stability of the sensor.
Fig. 7. Humidity hysteresis curve of PDVB/DMC-33 sensor.
Response and recovery time is an important parameter for polymeric humidity sensors. Fig. 8 shows the response and recovery property of PDVB/DMC-33 sensor corresponding to several continuous water adsorption and desorption processes, and the sensor showed good repeatability in the continuous measurements. The time taken by a sensor to achieve 90% of the total impedance module change is defined as the response or recovery time [24]. For PDVB/DMC-33 sensor, the response time was 3 s when RH increased from 11% to 95% RH, and the recovery time was 75 s when RH decreased from 95% to 11% RH. The rapid response of PDVB/DMC-33 comes from the porosity characteristic of the resultant sensitive material, which is beneficial for the transport of water molecules in adsorption and desorption processes. In order to research the sensing mechanism of PDVB/DMC-33 sensor, the complex impedances of the sensor were measured at 1 V with the operating frequency from 20 to 100 kHz and the RH from 11% to 95% RH at ∼20 ◦ C. The real part and imaginary part were magnified with same times (marked under the curves) to compare several complex impedance plots conveniently. As shown in Fig. 9a, the curve is a part of a semicircle at 11% RH. Because the polymer adsorbs few water molecules at such a low RH, H+ hopping conduction is the main contributor to the conduction of the film [25]. The complex impedance at 11% RH could be described by an equivalent circuit composed of a resistor and a capacitor in parallel (Fig. 9b). A short straight line appears after the semicircle in the low frequency range at 33% RH, and the straight line increases while the semicircle decreases as RH increased, which represents a
Fig. 8. Response and recovery curve of PDVB/DMC-33 sensor between 11% and 95% RH.
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Fig. 9. (a) The complex impedances of PDVB/DMC-33 sensor under different RH. ImZ, imaginary part; ReZ, real part. The equivalent circuits of complex impedances under (b) 11% RH and (c) 33%, 54%, 75%, 85% and 95% RH.
typical Warburg impedance due to the diffusion of ionic species at the sensing material/electrode interface [26,27]. The straight line demonstrates the conduction of Cl− ions. Because continuous water layers form on the polymer, which could make the polymer ionize, a part of Cl− ions are ionized out and free from the interaction with the opposite charges part. The corresponding equivalent circuit is shown in Fig. 9c. The complex impedance plots demonstrate the conductance of the polymer PDVB/DMC-33 is the totality of the protons and ions contribution. The H+ hopping conduction is dominant at lower RH, and the ions make a significant contribution for higher RH levels. It is worthy to note that the obvious ionic contribution appears at a relatively low RH (33%) for PDVB/DMC-33 sensor compared with some reported amphiphilic polymers or inorganic electrolytes loaded composites [14,27–29]. This phenomenon is attributed to the high content hydrophilic groups in the polymer PDVB/DMC-33, which would interact with water molecules strongly under a RH as low as 33%. On the other hand, the sensor showed high sensitivity, little hysteresis and rapid response. The excellent humidity sensitive properties of PDVB/DMC-33 sensor come from the special structure of the polymer, including the cross-linked rigid hydrophobic frameworks and large quantities of hydrophilic groups on the frameworks. In addition, from the complex impedances (Fig. 9) and frequency properties (Fig. 6) of the PDVB/DMC-33 sensor, it could be concluded that ionic response drops with increasing frequency. First of all, the concentration of ions increases with the RH, and the
complex impedance results demonstrate ions make a significant contribution for increasing RH levels. As shown in Fig. 9, the Warburg impedance from the diffusion of Cl− at the polymer/electrode interface is an important contribution at RH higher than 33%. What is more, the ions could not follow the rapid change of the electric field under high frequency, so there is very little frequency dependence at high RH (Fig. 6). Therefore, choosing a proper operating frequency is necessary for the humidity sensors’ performances. Long-term stability is very important for the practical application of humidity sensors. The impedance module of PDVB/DMC-33 sensor was tested every five days for a month and the results are shown in Fig. 10a. There is an acceptable change in the impedance modules measured at different times, demonstrating good stability of the obtained sensor. The stability of the PDVB/DMC-33 sensor is attributed to the water-resistant property and the crosslinked aromatic skeleton of the polymer. In addition, the surface of PDVB/DMC-33 film is hydrophobic because of its surface roughness [30,31]. As shown in Fig. 10b, when a pure water droplet was allowed to contact the surface of the PDVB/DMC-33 sample, the polymer PDVB/DMC-33 showed superhydrophobic (defined by a contact angle of higher than 150◦ ) property with a contact angle of 155◦ [31]. It is known that the superhydrophobic surface could be created from an amphiphilic polymer [32]. The hydrophobic property of polymer PDVB/DMC-33 is contributed to its rough surface, while its chemical structure is amphiphilic. Therefore, the polymer could react with water vapor molecules by polar group modified on
Fig. 10. (a) The impedance modules of PDVB/DMC-33 sensor measured under different RH conditions for a month (the measurements were repeated every five days). (b) The photo of a water droplet on a tablet of PDVB/DMC-33 sample.
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the skeleton, and on the other hand, the superhydrophobic rough surface of the polymer is beneficial for the long-term stability of the sensor. 4. Conclusion Water-resistant cross-linked nanoporous polymer electrolytes were designed to develop stable resistance-type humidity sensors. The polymers were synthesized by a solvothermal method and the structures could be controlled by feeding ratios. Humidity sensors based on the resultant polymers were fabricated and researched, and the sensors with excellent humidity sensing properties, including little hysteresis and rapid response were obtained, demonstrating the good stability of cross-linked polymers is beneficial for humidity sensors. Our work provided a new method for developing stable humidity sensitive polymers. Acknowledgements This work was supported by the Natural Science Foundation Committee (NSFC, No. 51103053), Postdoctoral Science Foundation of China (PSFC, Nos. 2011M500608, 2013T60324), Doctoral Fund of Ministry of Education of China (No. 20110061120053) and Program from Changjiang Scholars and Innovativation Research Team in University (No. IRT13018). References [1] Y. Sakai, Y. Sadaoka, M. Matsuguchi, Humidity sensors based on polymer thin films, Sens. Actuators B 35 (1996) 85–90. [2] Y. Sakai, Y. Sadaoka, M. Matsuguchi, Y. Kanakura, M. Tamura, A humidity sensor using polytetrafluoroethylene-graft-quaternized-polyvinylpyridine, J. Electrochem. Soc. 138 (1991) 2474–2478. [3] J. Wang, F.Q. Wu, Humidity sensitivity of composite material of lanthanum ferrite/polymer quaternary acrylic resin, Sens. Actuators B 99 (2004) 586–591. [4] Q. Qi, T. Zhang, Q.J. Yu, R. Wang, Y. Zeng, L. Liu, H.B. Yang, Properties of humidity sensing ZnO nanorods-base sensor fabricated by screen-printing, Sens. Actuators B 133 (2008) 638–643. [5] M. Bayhan, N. Kavasoglu, A study on the humidity sensing properties of ZnCr2 O4 –K2 CrO4 ionic conductive ceramic sensor, Sens. Actuators B 117 (2006) 261–265. [6] P.G. Su, S.-C. Huang, Humidity sensing and electrical properties of a composite material of SiO2 and poly-[3-(methacrylamino)propyl] trimethyl ammonium chloride, Sens. Actuators B 105 (2005) 170–175. [7] Y. Li, M.J. Yang, Y. She, Humidity sensitive properties of crosslinked and quaternized poly(4-vinylpyridine-co-butyl methacrylate), Sens. Actuators B 107 (2005) 252–257. [8] P.G. Su, Y.L. Sun, C.C. Lin, Humidity sensor based on PMMA simultaneously doped with two different salts, Sens. Actuators B 113 (2006) 883–886. [9] Q. Qi, T. Zhang, L.J. Wang, Improved and excellent humidity sensitivities based on KCl-doped TiO2 electrospun nanofibers, Appl. Phys. Lett. 93 (2008) 3, 023105. [10] Y. Li, L.J. Hong, Y.S. Chen, H.C. Wang, X. Lu, M.J. Yang, Poly(4vinylpyridine)/carbon black composite as a humidity sensor, Sens. Actuators B 123 (2007) 554–559. [11] Y. Sakai, M. Matsuguchi, T. Hurukawa, Humidity sensor using cross-linked poly (chloromethyl styrene), Sens. Actuators B 66 (2000) 135–138. [12] Y. Li, L. Hong, M. Yang, Crosslinked and quaternized poly(4-vinylpyridine)/ polypyrrole composite as a potential candidate for the detection of low humidity, Talanta 75 (2008) 412–417. [13] J.-R. Cha, M.S. Gong, Preparation of epoxy/polyelectrolyte IPNs for flexible polyimide-based humidity sensors and their properties, Sens. Actuators B 178 (2013) 656–662. [14] T. Fei, K. Jiang, S. Liu, T. Zhang, Humidity sensors based on Li-loaded nanoporous polymers, Sens. Actuators B 190 (2014) 523–528. [15] Y. Zhang, S. Wei, F. Liu, Y. Du, S. Liu, Y. Ji, T. Yokoi, T. Tasumi, F.-S. Xiao, Superhydrophobic nanoporous polymers as efficient adsorbents for organic compounds, Nano Today 4 (2009) 135–142.
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Biographies Teng Fei received his B.S. degree in 2005 in chemical engineering and technology and Ph.D. degree in 2010 in polymer chemistry and physics from Jilin University, China. He is currently an associate professor in the College of Electronic Science and Engineering, Jilin University. His research interests include sensing functional materials and devices. Hongran Zhao received his B.S. degree from the College of Electronic Science and Engineering, Jilin University, China in 2013. As a M.S. student, his research interest is functional sensing materials and devices. Kai Jiang received his B.S. degree from the College of Electronic Science and Engineering, Jilin University, China in 2011. As a Ph.D. student, his research interest is humidity sensors based on sensing functional materials. Tong Zhang completed her M.S. degree in semiconductor materials in 1992 and her Ph.D. in the field of microelectronics and solid-state electronics in 2001 from Jilin University. She was appointed as a full-time professor in the College of Electronic Science and Engineering, Jilin University in 2001. Her research interests are sensing functional materials, gas sensors and humidity sensors.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Synthesis and humidity sensitive property of cross-linked water-resistant polymer electrolytes Teng Fei, Hongran Zhao, Kai Jiang, Tong Zhang ∗ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 20 August 2014 Received in revised form 30 October 2014 Accepted 8 November 2014 Available online 15 November 2014 Keywords: Humidity sensor Water-resistant polymer Cross-linking Polymer electrolyte Solvothermal
a b s t r a c t Defined structure and water-resistance are two important requirements for polymeric humidity sensors. Most of polymer electrolytes applied in humidity sensors do not bear both features at the same time. Herein, a novel method was used to develop stable humidity sensitive polymer electrolytes with defined structures. Nanoporous polymers based on 1,4-divinylbenzene (DVB) and methacrylatoethyl trimethyl ammonium chloride (DMC) were synthesized by free radical polymerization with a solvothermal route. The cross-linked structure could afford good stability of the polymers even at high humidity levels. The impedance module of the optimized sensor changed more than three orders of magnitude over the whole humidity range, with little humidity hysteresis and rapid response. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Humidity sensors have attracted the attention of many researchers in recent years [1,2]. Many different materials were developed for humidity sensing, including ceramics [3–5], organic polymers [1,2,6,7] and composites [8–10]. Resistive-type organic polymeric humidity sensors are most attractive because of their long-term stability, easy solution-processing and low cost. Sensitive materials for polymeric humidity sensors are usually amphiphilic polymers, however, the stability of polymeric humidity sensors is not satisfactory because the polar groups may result in dissolution of the polymers under high relative humidity (RH) ambience for long term operation, especially for dew condenses. In order to solve the above problem, the chemical stability of the sensitive polymers should be improved. The cross-linking of the hydrophilic polymers coated on the sensor substrate has been known to enhance the stability of polymeric resistive-type humidity sensors under high humidities [11–13]. The cross-linking method is effective, however, most of the works on humidity sensors were based on a solid phase reaction of the polymers, thus leading to uncontrollable structures of the cross-linked films and un-reproducible sensors. Therefore, water-resistant polymer electrolytes with definite chemical structures may be the ideal model for humidity sensors.
∗ Corresponding author. Tel.: +86 431 85168385; fax: +86 431 85168270. E-mail address: [email protected] (T. Zhang). http://dx.doi.org/10.1016/j.snb.2014.11.044 0925-4005/© 2014 Elsevier B.V. All rights reserved.
Recently, we have used hydrophobic and porous polymer (PDVB) in association with humidity sensitive component (LiCl) as a composite for the function of humidity sensing, and PDVB was used as host material with almost no humidity sensitive property itself [14]. Herein, novel organic porous polymer electrolytes were synthesized by free radical polymerization with different ratios of monomers through a solvothermal route. The polymers possess good stability because of their cross-linked network structures, which could be adjusted by feed ratios and the structures of monomers. Sensors with high sensitivity among the whole humidity range were realized based on the obtained polymers, and the sensing mechanism of the optimized sensor was researched.
2. Experimental 2.1. Synthesis of polymers The polymers were synthesized by free radical polymerization through a solvothermal route. The detailed procedure was as follows (PDVB/DMC-33 as an example): 1.5 g of divinylbenzene (DVB), 3.0 g of methacrylatoethyl trimethyl ammonium chloride (DMC), 38.0 mg of azodiisobutyronitrile (AIBN) and 30 mL of N,Ndimethylmethanamide (DMF) were put in an autoclave and treated at 100 ◦ C for 24 h. The autoclave was then cooled down to room temperature and a gel column was obtained. The gel was heated at 60 ◦ C under vacuum to remove the solvent, and loose solid (4.0 g) was obtained.
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2.2. Sensor preparation The polymers were mixed with n-butanol to form a paste, then the paste was dip-coated on a ceramic substrate (6 mm × 3 mm, 0.5 mm thick) with five pairs of Ag–Pd interdigitated electrodes (electrodes width: 0.15 mm; distance: 0.15 mm) to form a sensing film with the thickness of ∼100 m, as shown in Fig. 1a. The films were dried under air at ∼20 ◦ C for 12 h. Finally, the humidity sensors were obtained after aging at 95% RH with an alternating current (AC) of 1 V, 100 Hz for 24 h to improve their stability and durability. 2.3. Measurements The IR spectra of polymers were obtained on a WQF-510A FTIR spectrometer. Pore width distribution profiles of polymers were obtained at 77 K on a Micromeritics Tri-star 3000 analyzer using nitrogen. The morphologies of polymers were performed on a JEOL JSM-6700F scanning electron microscopy (SEM). The electrical responses of the sensors were measured at 1 V AC under different RH with a ZL-5 intelligent LCR analyzer at ∼20 ◦ C, as shown in Fig. 1b. In order to measure the impedances of the sensors, at least 10 sensors for each polymer were fabricated under the same condition and their sensing properties were defined by repeatedly results. The atmosphere of RH was produced by different saturated salt solutions in their equilibrium states including LiCl for 11% RH, MgCl2 for 33% RH, Mg(NO3 )2 for 54% RH, NaCl for 75% RH, KCl for 85% RH, and KNO3 for 95% RH. 3. Results and discussion Polymer electrolytes based on DVB and DMC were synthesized by free radical polymerization with a solvothermal route. The synthetic routes to polymers are shown in Fig. 2. DVB and
DMC are hydrophobic and hydrophilic monomers, respectively, which could be used for constructing polymer electrolytes. The two ethylenic bonds in single DVB molecule could afford hydrophobic cross-linked aromatic skeleton, which would be modified with hydrophilic alkyl chains for stable polymer electrolytes. DMF was chosen as solvent because it could dissolve both monomers, which afforded a homogeneous polymerization system. The feed ratio of monomers could be used to tune the structures of the polymers, and three polymers (PDVB/DMC-31, PDVB/DMC-33 and PDVB/DMC34) with different feed ratios (the molar ratio of DVB to DMC is 3:1, 3:3 and 3:4 for PDVB/DMC-31, PDVB/DMC-33 and PDVB/DMC34, respectively) were synthesized under similar polymerization condition. It should be noticed that a gel column was obtained after each polymerization, thus indicating a homogeneous polymerization of the vinyl monomers. The solvent (DMF) was removed by heating under vacuum from the gel columns, and loose powers were then obtained. Therefore, by the cross-linking reaction in homogeneous solutions, structure-defined polymer electrolytes were synthesized. In addition, this kind of polymerization by solvothermal method could also construct nanoporous polymers [15]. A polymer with porosity is beneficial for humidity sensing, since it is beneficial for the transport of water molecules in both adsorption and desorption processes. The FT-IR spectra of the obtained polymers are shown in Fig. 3. The absorptions at ∼3021, 797 and 710 cm−1 are assigned to the stretching of C H in the benzene ring. The absorptions at ∼1635 and 1479 cm−1 come from the aromatic C C stretching. And the stretching of alkyl chain shows a strong vibrational peak at ∼2938 cm−1 . It should be noted that the strong absorptions at ∼1728 and 1157 cm−1 contributed to the saturated ester and the absorption at ∼950 cm−1 from the unique C N stretching of quaternary ammonium also appear in the FT-IR spectra. Because both of the saturated ester and quaternary ammonium come from
Fig. 1. (a) A schematic diagram of the humidity sensor and (b) the schematic image of the equipment for measurement.
Fig. 2. Synthetic routes to polymers PDVB/DMC-31, PDVB/DMC-33 and PDVB/DMC-34.
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Fig. 5. The impedance modules of PDVB/DMC-31, PDVB/DMC-33 and PDVB/DMC34 sensors at different RH (1 V AC, 100 Hz).
Fig. 3. The FT-IR spectra of the obtained polymers.
the hydrophilic monomer DMC, the corresponding absorptions for PDVB/DMC-31 (with lower content of DMC) are much weaker. These results demonstrate the desired polymers were obtained. Pore width distribution profiles of polymers are shown in Fig. 4(a)–(c). The pore diameter is 4.5 nm, 4.3 nm and 5.0 nm for PDVB/DMC31, PDVB/DMC33 and PDVB/DMC34, respectively. The SEM images of resultant polymers are shown in Fig. 4(d) and (e). As can be seen, the obtained polymers are stacked by microparticles, and the packing between particles becomes loose with the increasing DMC content in the polymer. The porous polymers could provide channels for the transport of water molecules, combining with a stable cross-linked aromatic skeleton. It is considered that the interaction between hydrophilic quaternary ammonium salt and water molecules could be amplified by this kind of structure.
Organic porous polymers have been applied in catalyst [16,17], gas adsorption and storage [18,19], organic electronics [20], chemosensors [21], etc. The application of porous linear polymers on optical humidity sensors has also been reported [22,23]. These polymers show different absorption properties to visible light at different RH conditions because of the polymers’ swelling behavior. Different from above works based on linear polymers, the resultant materials in our work are cross-linked polymer electrolytes with better stability. Humidity sensors based on the obtained polymers were fabricated and their humidity sensing curves under different RH are shown in Fig. 5. The impedance modules (|Z|) of all sensors decrease with the increasing RH. And for different polymers, the impedance modules increase with the content of hydrophobic monomer at certain RH, because less water molecules are adsorbed in the polymer with higher content of hydrophobic structure. What is more, it is interesting to notice that the impedance modules of the three sensors all show a straight line semi-log fit to RH, among which
Fig. 4. Pore width distribution profiles of (a) PDVB/DMC-31, (b) PDVB/DMC-33 and (c) PDVB/DMC-34; SEM images of (d) PDVB/DMC-31, (e) PDVB/DMC-33 and (f) PDVB/DMC34.
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Fig. 6. The impedance modules of PDVB/DMC-33 sensor at different RH under different frequencies (1 V AC).
PDVB/DMC-33 sensor is the best. In addition, the impedance modules changed about three orders of magnitude from 11% to 95% RH for sensors based on PDVB/DMC-33 and PDVB/DMC-34, while the change is much smaller for PDVB/DMC-31. Taking semi-log fit of impedance module to RH and impedance module change of the sensors into account, PDVB/DMC-33 sensor shows the best properties, so it was used for further investigation in the following experiments. To determine the optimal working parameters, the impedance modules of PDVB/DMC-33 sensor under different frequencies at ∼20 ◦ C were measured and the results are shown in Fig. 6. The impedance module decreases with the increasing RH for each frequency, and under a certain RH it decreases with increasing working frequency. It is worthy to note that there are obvious impedance module changes for different operating frequencies under a certain RH when RH is low, but at higher RH, the curves almost overlap together, indicating the effect of frequencies on the impedance module is small in this range. The impedance module of 100 Hz shows a straight line semi-log fit to RH, so the operation condition was kept at AC 1 V, 100 Hz for following experiments. Humidity hysteresis is an important characteristic of humidity sensors, which reflects the interaction between water molecules and the sensitive materials, especially the hydrophilic groups. Since the applied polymers for humidity sensors were constructed with hydrophilic groups modified aromatic skeleton, it was considered that the interaction between the hydrophilic groups could be restrained, so the aggregation of the hydrophilic groups could be restricted, which would be useful for little humidity hysteresis. It is necessary to note that the structure character of the obtained polymer is different from linear amphiphilic polymers, among which polar groups are easy to aggregate due to the polymers’ flexible structures, and result in strong interaction with water molecules. PDVB/DMC-33 sensor was studied as an example. Fig. 7 shows the humidity hysteresis characteristic of PDVB/DMC-33 sensor by keeping the applied voltage at 1 V and the frequency at 100 Hz. The impedance of the sensor was first measured from 11% to 95% RH (corresponding to the adsorption process), and then in the opposite direction (corresponding to the desorption process). The equilibration time of the sensor at each RH atmosphere was 5 min. The maximum humidity hysteresis PDVB/DMC-33 sensor is around 4% RH under 50% RH, although the molar ratio of hydrophilic monomer for synthesizing the polymer PDVB/DMC-33 is as high as 50%. So a balanced adsorption–desorption water molecules was controlled by adjusting the content of hydrophilic groups in the polymer frameworks and a small humidity resistance was realized, which is beneficial for the stability of the sensor.
Fig. 7. Humidity hysteresis curve of PDVB/DMC-33 sensor.
Response and recovery time is an important parameter for polymeric humidity sensors. Fig. 8 shows the response and recovery property of PDVB/DMC-33 sensor corresponding to several continuous water adsorption and desorption processes, and the sensor showed good repeatability in the continuous measurements. The time taken by a sensor to achieve 90% of the total impedance module change is defined as the response or recovery time [24]. For PDVB/DMC-33 sensor, the response time was 3 s when RH increased from 11% to 95% RH, and the recovery time was 75 s when RH decreased from 95% to 11% RH. The rapid response of PDVB/DMC-33 comes from the porosity characteristic of the resultant sensitive material, which is beneficial for the transport of water molecules in adsorption and desorption processes. In order to research the sensing mechanism of PDVB/DMC-33 sensor, the complex impedances of the sensor were measured at 1 V with the operating frequency from 20 to 100 kHz and the RH from 11% to 95% RH at ∼20 ◦ C. The real part and imaginary part were magnified with same times (marked under the curves) to compare several complex impedance plots conveniently. As shown in Fig. 9a, the curve is a part of a semicircle at 11% RH. Because the polymer adsorbs few water molecules at such a low RH, H+ hopping conduction is the main contributor to the conduction of the film [25]. The complex impedance at 11% RH could be described by an equivalent circuit composed of a resistor and a capacitor in parallel (Fig. 9b). A short straight line appears after the semicircle in the low frequency range at 33% RH, and the straight line increases while the semicircle decreases as RH increased, which represents a
Fig. 8. Response and recovery curve of PDVB/DMC-33 sensor between 11% and 95% RH.
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Fig. 9. (a) The complex impedances of PDVB/DMC-33 sensor under different RH. ImZ, imaginary part; ReZ, real part. The equivalent circuits of complex impedances under (b) 11% RH and (c) 33%, 54%, 75%, 85% and 95% RH.
typical Warburg impedance due to the diffusion of ionic species at the sensing material/electrode interface [26,27]. The straight line demonstrates the conduction of Cl− ions. Because continuous water layers form on the polymer, which could make the polymer ionize, a part of Cl− ions are ionized out and free from the interaction with the opposite charges part. The corresponding equivalent circuit is shown in Fig. 9c. The complex impedance plots demonstrate the conductance of the polymer PDVB/DMC-33 is the totality of the protons and ions contribution. The H+ hopping conduction is dominant at lower RH, and the ions make a significant contribution for higher RH levels. It is worthy to note that the obvious ionic contribution appears at a relatively low RH (33%) for PDVB/DMC-33 sensor compared with some reported amphiphilic polymers or inorganic electrolytes loaded composites [14,27–29]. This phenomenon is attributed to the high content hydrophilic groups in the polymer PDVB/DMC-33, which would interact with water molecules strongly under a RH as low as 33%. On the other hand, the sensor showed high sensitivity, little hysteresis and rapid response. The excellent humidity sensitive properties of PDVB/DMC-33 sensor come from the special structure of the polymer, including the cross-linked rigid hydrophobic frameworks and large quantities of hydrophilic groups on the frameworks. In addition, from the complex impedances (Fig. 9) and frequency properties (Fig. 6) of the PDVB/DMC-33 sensor, it could be concluded that ionic response drops with increasing frequency. First of all, the concentration of ions increases with the RH, and the
complex impedance results demonstrate ions make a significant contribution for increasing RH levels. As shown in Fig. 9, the Warburg impedance from the diffusion of Cl− at the polymer/electrode interface is an important contribution at RH higher than 33%. What is more, the ions could not follow the rapid change of the electric field under high frequency, so there is very little frequency dependence at high RH (Fig. 6). Therefore, choosing a proper operating frequency is necessary for the humidity sensors’ performances. Long-term stability is very important for the practical application of humidity sensors. The impedance module of PDVB/DMC-33 sensor was tested every five days for a month and the results are shown in Fig. 10a. There is an acceptable change in the impedance modules measured at different times, demonstrating good stability of the obtained sensor. The stability of the PDVB/DMC-33 sensor is attributed to the water-resistant property and the crosslinked aromatic skeleton of the polymer. In addition, the surface of PDVB/DMC-33 film is hydrophobic because of its surface roughness [30,31]. As shown in Fig. 10b, when a pure water droplet was allowed to contact the surface of the PDVB/DMC-33 sample, the polymer PDVB/DMC-33 showed superhydrophobic (defined by a contact angle of higher than 150◦ ) property with a contact angle of 155◦ [31]. It is known that the superhydrophobic surface could be created from an amphiphilic polymer [32]. The hydrophobic property of polymer PDVB/DMC-33 is contributed to its rough surface, while its chemical structure is amphiphilic. Therefore, the polymer could react with water vapor molecules by polar group modified on
Fig. 10. (a) The impedance modules of PDVB/DMC-33 sensor measured under different RH conditions for a month (the measurements were repeated every five days). (b) The photo of a water droplet on a tablet of PDVB/DMC-33 sample.
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the skeleton, and on the other hand, the superhydrophobic rough surface of the polymer is beneficial for the long-term stability of the sensor. 4. Conclusion Water-resistant cross-linked nanoporous polymer electrolytes were designed to develop stable resistance-type humidity sensors. The polymers were synthesized by a solvothermal method and the structures could be controlled by feeding ratios. Humidity sensors based on the resultant polymers were fabricated and researched, and the sensors with excellent humidity sensing properties, including little hysteresis and rapid response were obtained, demonstrating the good stability of cross-linked polymers is beneficial for humidity sensors. Our work provided a new method for developing stable humidity sensitive polymers. Acknowledgements This work was supported by the Natural Science Foundation Committee (NSFC, No. 51103053), Postdoctoral Science Foundation of China (PSFC, Nos. 2011M500608, 2013T60324), Doctoral Fund of Ministry of Education of China (No. 20110061120053) and Program from Changjiang Scholars and Innovativation Research Team in University (No. IRT13018). References [1] Y. Sakai, Y. Sadaoka, M. Matsuguchi, Humidity sensors based on polymer thin films, Sens. Actuators B 35 (1996) 85–90. [2] Y. Sakai, Y. Sadaoka, M. Matsuguchi, Y. Kanakura, M. Tamura, A humidity sensor using polytetrafluoroethylene-graft-quaternized-polyvinylpyridine, J. Electrochem. Soc. 138 (1991) 2474–2478. [3] J. Wang, F.Q. Wu, Humidity sensitivity of composite material of lanthanum ferrite/polymer quaternary acrylic resin, Sens. Actuators B 99 (2004) 586–591. [4] Q. Qi, T. Zhang, Q.J. Yu, R. Wang, Y. Zeng, L. Liu, H.B. Yang, Properties of humidity sensing ZnO nanorods-base sensor fabricated by screen-printing, Sens. Actuators B 133 (2008) 638–643. [5] M. Bayhan, N. Kavasoglu, A study on the humidity sensing properties of ZnCr2 O4 –K2 CrO4 ionic conductive ceramic sensor, Sens. Actuators B 117 (2006) 261–265. [6] P.G. Su, S.-C. Huang, Humidity sensing and electrical properties of a composite material of SiO2 and poly-[3-(methacrylamino)propyl] trimethyl ammonium chloride, Sens. Actuators B 105 (2005) 170–175. [7] Y. Li, M.J. Yang, Y. She, Humidity sensitive properties of crosslinked and quaternized poly(4-vinylpyridine-co-butyl methacrylate), Sens. Actuators B 107 (2005) 252–257. [8] P.G. Su, Y.L. Sun, C.C. Lin, Humidity sensor based on PMMA simultaneously doped with two different salts, Sens. Actuators B 113 (2006) 883–886. [9] Q. Qi, T. Zhang, L.J. Wang, Improved and excellent humidity sensitivities based on KCl-doped TiO2 electrospun nanofibers, Appl. Phys. Lett. 93 (2008) 3, 023105. [10] Y. Li, L.J. Hong, Y.S. Chen, H.C. Wang, X. Lu, M.J. Yang, Poly(4vinylpyridine)/carbon black composite as a humidity sensor, Sens. Actuators B 123 (2007) 554–559. [11] Y. Sakai, M. Matsuguchi, T. Hurukawa, Humidity sensor using cross-linked poly (chloromethyl styrene), Sens. Actuators B 66 (2000) 135–138. [12] Y. Li, L. Hong, M. Yang, Crosslinked and quaternized poly(4-vinylpyridine)/ polypyrrole composite as a potential candidate for the detection of low humidity, Talanta 75 (2008) 412–417. [13] J.-R. Cha, M.S. Gong, Preparation of epoxy/polyelectrolyte IPNs for flexible polyimide-based humidity sensors and their properties, Sens. Actuators B 178 (2013) 656–662. [14] T. Fei, K. Jiang, S. Liu, T. Zhang, Humidity sensors based on Li-loaded nanoporous polymers, Sens. Actuators B 190 (2014) 523–528. [15] Y. Zhang, S. Wei, F. Liu, Y. Du, S. Liu, Y. Ji, T. Yokoi, T. Tasumi, F.-S. Xiao, Superhydrophobic nanoporous polymers as efficient adsorbents for organic compounds, Nano Today 4 (2009) 135–142.
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Biographies Teng Fei received his B.S. degree in 2005 in chemical engineering and technology and Ph.D. degree in 2010 in polymer chemistry and physics from Jilin University, China. He is currently an associate professor in the College of Electronic Science and Engineering, Jilin University. His research interests include sensing functional materials and devices. Hongran Zhao received his B.S. degree from the College of Electronic Science and Engineering, Jilin University, China in 2013. As a M.S. student, his research interest is functional sensing materials and devices. Kai Jiang received his B.S. degree from the College of Electronic Science and Engineering, Jilin University, China in 2011. As a Ph.D. student, his research interest is humidity sensors based on sensing functional materials. Tong Zhang completed her M.S. degree in semiconductor materials in 1992 and her Ph.D. in the field of microelectronics and solid-state electronics in 2001 from Jilin University. She was appointed as a full-time professor in the College of Electronic Science and Engineering, Jilin University in 2001. Her research interests are sensing functional materials, gas sensors and humidity sensors.