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Preparation of water-durable humidity sensor by attachment of polyelectrolyte membrane to electrode substrate by photochemical crosslinking reaction
Sensors and Actuators B 147 (2010) 539–547
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Preparation of water-durable humidity sensor by attachment of polyelectrolyte membrane to electrode substrate by photochemical crosslinking reaction Myoung-Seon Gong a,∗ , Jae-Uk Kim b , Jong-Gyu Kim b a b
Department of Nanobiomedical Science and WCU Research Center of Nanobiomedical Science, Dankook University, Cheonan, Chungnam 330-714, Republic of Korea Department of Chemistry, Dankook University, Cheonan, Chungnam 330-714, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 December 2009 Received in revised form 27 March 2010 Accepted 12 April 2010 Available online 18 April 2010 Keywords: Humidity sensor Benzophenone Photoinitiator Photo-crosslinking Water durability
a b s t r a c t New photo-crosslinkable benzophenone-containing polyelectrolytes were prepared by copolymerization of quaternary ammonium salt-containing monomer [2-(acryloyloxy)ethyl] dimethyl propyl ammonium bromide (AEPAB), styrene (ST), 2-hydroxyethylmethacrylate (HEMA), and 4-acryloyloxybenzophenone (BPA). These polyelectrolytes are self-crosslinkable copolymers for humidity-sensitive polyelectrolytes composed of copolymers with different contents of AEPAB/ST/HEMA/BPA = 70/25/3/2, 70/25/2/3, 70/23/5/2, and 70/23/4/3. Pretreatment of the alumina substrate with 3-glycidoxypropyltrimethoxysilane (GPTMS) containing silane-coupling agent was performed to anchor the humidity-sensitive membrane to the substrate through covalent bonds. The sensors were irradiated with UV light, causing the electronically excited benzophenone molecules to form a crosslinking reaction. The resistance ranged from 107 to 103 between 20% relative humidity (RH) and 95% RH, the humidity range required for a sensor operating at ambient humidity. Their water durability, long-term stability under various environments, hysteresis, and response and recovery times were measured and evaluated as a humidity-sensing membrane. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Humidity sensors made from polyelectrolytes possess poor durability against water or dew, as they are generally soluble in water. Several methods have been proposed to solve the problem, including introducing hydrophobic groups by copolymerization [1–5] and grafting [6,7], applying protective films [8], obtaining interpenetrating network structures [9–11], or forming organic/inorganic hybrids [12–16]. Among them, forming crosslink structures in the sensitive film was proven to effectively improve their water-resistance and environmental stability [17–25]. However, the stability of these sensors is still not sufficient to be used under severe conditions, such as high temperature, high humidity, or conditions with excessive dew. Therefore, there is a significant need for the development of resistance-type humidity sensors that are effective under such severe conditions. The covalent attachment of ultra thin films of organic/inorganic hybrid materials to a solid substrate is often desirable in order to enhance the stability of the films against solvents and displacement reagents (e.g., water for hydrophilic surface). As a consequence of this need, numerous organic/inorganic hybrid protocols have been developed [26–29]. The most common procedures
∗ Corresponding author. Tel.: +82 41 550 1476; fax: +82 41 550 3431. E-mail address: [email protected] (M.-S. Gong). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.04.017
use silane-coupling agents that are suitable for binding to the inorganic surface and organic polymer. Typical coupling agents, such as 3-glycidoxypropyltrimethoxysilane, consist of a difunctional molecule, having both the glycidyl group, which reacts with the photo-curable polyelectrolyte after hydrolysis, and the alkoxysilyl group, which interacts with silica or alumina. Treatment with these agents could improve the adhesion between incompatible inorganic surfaces and organic polymers through covalent bonds [30–33]. Photopolymerization science and technology have drawn significant attention in recent years for industrial applications in regards to coating of various materials, adhesives, printing inks, photoresist, and biomaterials [34–37]. In photopolymerization, a typical formulation consists of a vinyl monomer and a photoinitiator. This technology is based on the use of a self-crosslinkable polyelectrolyte, containing a photoinitiator system suited to absorb a light radiation of the appropriate wavelength and to produce primary radical species in order to convert a polyelectrolyte into a crosslinked network [38]. We have been interested in the synthesis of a new family of photocurable polyelectrolytes, in which the polyelectrolyte chains are simply crosslinked by irradiation of UV light after fabrication of the self-crosslinkable humidity-sensitive membrane on the aluminum electrode [39]. This paper describes the use of the silane-coupling agent, its immobilization on an alumina electrode surface, and the
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subsequent attachment of photo-curable polyelectrolytes by irradiation of UV light of dip-coating layers of this copolymer on a dihydroxy-modified surface. Copoly(AEPAB/PS’HEMA/BPA)s were chosen because it represents examples of self-curable and attachable polyelectrolytes to the substrate hydroxyl group, and therefore, demonstrates the water-durable humidity sensor system. 2. Experimental 2.1. Chemicals and instruments 4-Acryloxybenzophenone (BPA) was prepared by the method previously reported [37]. [2-(Acryloyloxy)ethyl] trimethyl ammonium chloride (AEPAB) was synthesized by quaternization reaction of 2-(N,N-dimethylamino)ethyl acrylate with propyl bromide [40]. Styrene (ST), 2-hydroxyethylmethacrylate (HEMA), and azobisisobutyronitrile (AIBN, Aldrich Chem. Co.) were used as received. 2-Methoxyethanol was purified by conventional purification methods. 3-Glycidoxypropyltrimethoxysilane (GPTMS, Aldrich Chem. Co.) was used as received. The humidity and temperature controller (Jeio Tech Korea, Model: TM-NFM-L; 20–95% RH) was used for the measurement of relative humidity at constant temperature. The resistance of the sensors was measured with a LCR meter (ED-Lab Korea, Model EDC-1630, 0.1–20 M). UV exposure of the humidity-sensitive membrane was performed using a commercial bench-top high power UV curing system (Hg, MTL 1000 W, 1 KW (80 W/CM), Sei Myung Vactron Co. Ltd.). 2.2. Sensor electrode Tooth-comb gold electrode (width: 0.22 mm; thickness of electrode: 8–10 m) was silkscreen-printed on the alumina substrate (10 mm × 5.0 mm × 0.635 mm) as shown in Fig. 1. The surface resistance of the gold electrode was approximately 0.04 . The sensor chips were rinsed in 0.1N NaOH and 0.1N HCl for 3 h and washed with distilled water before use. The alumina substrates were pretreated with glycidoxy-containing silane-coupling reagent. A solution of HCl (1 wt%) and GPTMS (2 wt%) in methanol/water (v/v = 95/5) was spread on substrates, and the substrates were dried at 130 ◦ C for 1 h. 2.3. Representative preparation of self-crosslinkable humidity-sensitive copolymers A mixture of the humidity-sensitive monomer AEPAB (3.73 g, 14 mmol), comonomer styrene (0.48 g, 4.6 mmol), HEMA (0.13 g, 1.0 mmol), BPA (0.1 g, 0.4 mmol), and AIBN (0.03 g, 0.2 mmol), dissolved in anhydrous 2-methoxyethanol (10.4 g), was placed in a glass ampoule. The solution was degassed by freeze-thaw method. The sealed glass ampoule was heated at 60 ◦ C and maintained for 24 h. The polymerized mixture was precipitated into a large amount of anhydrous ethyl ether. The product was purified by dissolution in dry 2-methoxyethanol followed by reprecipitation in n-hexane. The copolymer (AEPAB/ST/HEMA/BPA = 70/23/5/2) was dried under vacuum at 50 ◦ C for 12 h. Other copolymers with different contents of AEPAB/ST/HEMA/BPA = 70/25/2/3, 70/25/3/2 and 70/23/4/3 were prepared by similar methods described above. 2.4. Fabrication of humidity-sensitive membrane Humidity sensors were prepared from the copolymers, AEPAB/ST/HEMA/BPA = 70/25/3/2, 70/25/2/3, 70/23/5/2, and 70/25/4/3 summarized in Table 1, by following procedures. The copolymer (0.65 g) was dissolved in anhydrous DMSO (5.0 g) at
Fig. 1. Schematic view of sensor electrode.
room temperature. The mixture was fabricated on the alumina electrode by dipping. The sensor chips were then heated at 60 ◦ C for 2 h and subsequently at 100 ◦ C for 1 h. The photochemical crosslinking reaction of the sensor chips was induced by irradiating UV light for 5 min at room temperature. 2.5. Measurements of resistance characteristics When we measured the resistance characteristics in order to estimate the temperature dependence, frequency dependence, hysteresis, water durability and long-term stability, we extracted capacitance components from the electrical signals. The value of the parallel resistance, Rp , was estimated by extrapolating the semicircle or the pile to the real axis based on the assumption of an Table 1 Results of radical polymerizations of AEPAB, ST, HEMA and BPA in 2-methoxyethanol at 65 ◦ C for 24 h. Entry no.
1 2 3 4 a b c d e
Monomers feed (mole ratio) AEPABa
STb
HEMAc
BPAd
70 70 70 70
25 25 23 23
3 2 5 4
2 3 2 3
inh e
Yield (%)
0.71 0.68 0.67 0.62
93 92 91 95
[2-(acryloyloxy)ethyl] dimethyl propyl ammonium bromide. Styrene. 2-Hydroxyethyl methacrylate. 4-Acryloyloxybenzophenone. Inherent viscosities were measured in 2-methoxyethanol in 1 g/dL at 25 ◦ C.
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was immersed in water for 3 min, 1 h, 10 h, and 24 h, and then dried in air. 3. Results and discussion 3.1. Preparation of photo-curable polyelectrolytes
Scheme 1.
equivalent circuit consisting of a parallel combination of the resistance Rp and the capacitance Cp for the impedance spectroscopy and activation energy. Resistance versus relative humidity characteristics of the sensors were measured for an absorption process at 20% RH → 95% RH, and for a desorption process at 95% RH → 20% RH at 1 V, 1 kHz, and 25 ◦ C. Temperature dependence was measured at temperatures of 15 ◦ C, 25 ◦ C and 35 ◦ C at 1 V and 1 kHz. Frequency dependence was obtained by changing frequencies to 100 Hz, 1 kHz and 10 kHz at 1 V and 25 ◦ C. Response time was determined over saturated salt solutions of KNO3 for 94% RH and MgCl2 ·6H2 O for 33% RH at its equilibrium state. The long-term stability at high temperature and high humidity was evaluated at 80% RH at 80 ◦ C. The durability of the humidity sensor in water was tested. The sensor
Photochemically crosslinkable polyelectrolytes were prepared by free radical copolymerization of AEPAB, ST, HEMA, and BPA using AIBN as the initiator in 2-methoxyethanol at 60 ◦ C as shown in Scheme 1 and Table 1. The ratios of AEPAB/ST/HEMA/BPA were determined as 70/25/2/3, 70/25/3/2, 70/23/5/2, and 70/23/4/3 in order to optimize the degree of crosslinking for the waterdurability. Radical polymerization gave polymers of moderate molecular weights, judging from the viscosities. The polymers obtained from the addition-polymerization possessed inherent viscosities of 0.71 dL/g, 0.68 dL/g, 0.67 dL/g, and 0.62 dL/g, respectively. The copoly(AEPAB/ST/HEMA/BPA)s were hygroscopic and very sensitive to light as a result of the copolymer crosslinking. The copolymers were soluble in common organic solvents, such as ethanol, 2-methoxyethanol, DMSO, and N-methylpyrrolidinone. The ST comonomer was adopted for the control of humiditysensitive characteristics, specifically the dependence of resistance on the content of hydrophobic comonomer in the polymer chain. The HEMA comonomer was incorporated into the polymer backbone as an effective hydrogen donor for the photo-reduction of the benzophenone group of the same polymer chain. The BPA comonomer, based on benzophenone, was used for the photoinitiator. 3.2. Pretreatment of substrate with hydroxyl-containing silane-coupling agent 3-Glycidoxypropyltrimethoxysilane (GPTMS) was used as a silane-coupling agent between the humidity-sensitive membrane and the alumina substrate, in order to improve adhesion properties and to achieve increased water stability of the sensors. After the trimethoxysilyl group was converted into a hydrophilic functional silanol by hydrolysis, the silanol group reacted with the free hydroxyl group of the substrate [38]. On the aluminum oxide
Scheme 2.
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Scheme 3.
layer, the hydrolyzed silane initially formed an adsorbed basic Hbonded silane, and following a curing process, a strong covalent bond that formed a thin dihydroxy layer after hydrolysis as shown in Scheme 2.
undergo several possible competing reactions, including hydrogen abstraction and coupling reaction, to form a crosslinked polymer. In this process, the crosslinking of a polyelectrolyte and the anchoring of a polyelectrolyte into the alumina substrate were resulted, through the photo-reaction of the hydroxyl group of the pretreated silane coupling agent simultaneously as shown in Scheme 4.
3.3. Photo-crosslinking mechanism The photochemical crosslinking reaction is illustrated in Scheme 3. It is now well established that exposing benzophenone and a suitable reductant such as hydroxyethyl in HEMA to UV light will result in the reduction of electronically excited benzophenone molecules to form hydroxydiphenylmethyl radicals and the concomitant oxidation of the reductant [38]. The possible photocrosslinking sites were: the hydroxyl group in HEMA, the benzyl proton in ST, and the tertiary proton in AEPAB in the copolymer chain. The resulting hydroxydiphenylmethyl radicals can then
3.4. Humidity-sensitive characteristics Resistance versus relative humidity characteristics of the sensors were measured at 1 V and 1 kHz. The typical resistance characteristic curves of the photo-crosslinked copolymers, AEPAB, ST, HEMA, and BPA irradiated UV light at a temperature of 25 ◦ C as shown in Fig. 2. The resistance of humidity sensors prepared on the pretreated substrate from AEPAB/ST/HEMA/BPA = 70/25/3/2, 70/25/2/3, 70/23/5/2, and 70/23/4/2 was measured between
Scheme 4.
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Fig. 2. Dependence of resistance on the relative humidity for the humidity sensor obtained from AEPAB/ST/HEMA/BPA = (a) 70/25/3/2, (b) 70/25/2/3, (c) 70/23/4/3 and (d) 70/23/5/2 at 25 ◦ C, 1 kHz and 1 V.
20% RH and 95% RH and yielded results of 3.84 M–0.624 k, 5.4 M–0.732 k, 6.50 M–0.820 k, and 8.90 M–1.05 k, respectively. When the solution of the copolymer was fabricated on the electrode, the adhesion properties to the alumina electrode were very efficient. When 5 mol% of HEMA and 2 mol% of BPA was used, the resulting photo-crosslinked polyelectrolyte was stable enough to endure high humidity or a dew point. Higher BPA and HEMA contents in the precursor should lead to a greater extent of covalent crosslinking of the polyelectrolyte chain. Humidity sensors, prepared from the copolymer AEPAB/ST/HEMA/BPA = 70/23/5/2 on the untreated substrate, displayed a slightly lower resistance than the sensor prepared on the pretreated substrate, because there was no cover layer of silane-coupling agent. In addition the humidity sensor prepared on the pretreated substrate without crosslinking also showed a lower resistance than those of the crosslinked humidity sensors. The characteristic resistance of the device fabricated with the copolymer AEPAB/ST/HEMA/PAB = 70/23/5/2 and crosslinked by UV irradiation, decreased by four orders of magnitude with increasing humidity from 20% RH to 95% RH as shown in Fig. 3. The semi-logarithmic response curve exhibited moderate linearity over all humidity regions.
Fig. 3. Dependence of resistance on the relative humidity for the humidity sensors obtained from AEPAB/ST/HEMA/PAB = 70/23/5/2 at 25 ◦ C, 1 kHz and 1 V.
Fig. 4. Dependence of resistance on the relative humidity for the humidity sensor obtained from AEPAB/ST/HEMA/BPA = (a) 70/25/3/2 and 70/25/4/3 (silane-treated) and (b) 70/23/5/2 and 70/25/2/3 (silane-treated and untreated) before and after soaking in water for 1 h at 25 ◦ C.
3.5. Water-resistant properties The sensors prepared on the pretreated substrates were subjected to the water stability test. The pretreatment with 3glycidoxypropyltrimethoxysilane (GPTMS) greatly improved the adhesion and the water stability of the sensors, whereas untreated substrate had virtually no effect. The sensors prepared on the pretreated substrates with GPTMS did not show any peeling-off of the humidity-sensitive membrane after soaking in water for 24 h at 20 ◦ C. Figs. 4 and 5 show the resistance for the sensor obtained from AEPAB/ST/HEMA/BPA = 70/25/3/2, 70/25/2/3, 70/23/5/2, and 70/23/4/3 before and after the soaking, indicating little to no changes in the humidity characteristics of the sensors. A comparison between the differences of resistance in Fig. 4(a) and (b) indicates the improved water stability for the sensor from AEPAB/ST/HEMA/BPA = 70/23/5/2 and 70/23/4/3, as compared to sensors from other polyelectrolytes with different compositions or the untreated substrate. The best stability for copolymers seems to be AEPAB/ST/HEMA/BPA = 70/23/5/2 due to the effective anchoring and photo-crosslinking of the copolymer chain. Durability against high temperature was tested by soaking the sensors in hot water at 50 ◦ C or 80 ◦ C for 5 h (Fig. 5(a) and (b)). The non-crosslinked sensor samples were nearly broken and showed extremely large increases in resistance. After soaking in hot water at 50 ◦ C and 80 ◦ C for more than 5 h, a slight increase in resistance was observed. Resistance increased by 20–30%, indicating the signifi-
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Fig. 6. Dependence of resistance on the relative humidity and hysteresis for the humidity sensor obtained from AEPAB/Styrene/HEMA/BPA = 70/23/5/2; (hollow symbol), desorption process and (solid symbol), absorption process at 25 ◦ C, 1 kHz and 1 V.
3.7. Temperature dependence
Fig. 5. Dependence of resistance on the relative humidity for the humidity sensor obtained from AEPAB/Styrene/HEMA/BPA = 70/23/5/2 before and after soaking in water at (a) 20 ◦ C and (b) 50 ◦ C and 80 ◦ C.
cant degradation of the sensor in hot water. On the other hand, the AEPAB/ST/HEMA/BPA = 70/23/5/2 sensor did not show any changes in regards to sensor characteristics even at 25 ◦ C for 24 h. We have used AEPAB/ST/HEMA/BPA = 70/23/5/2 polyelectrolyte on surface untreated electrode as a model humidity sensor and compared with surface pretreated humidity sensors. We could not prepare the other commercial humidity sensors as a model sensor. But it was reported that the sensor samples obtained from poly(2-hydroxy-3methacryloxypropyl-trimethylammonium chloride) crosslinked with hexamethoxymethylol melamine showed a significant increase of resistance within 100 h [15]. And we have obtained a sample from the Shinyei Corporation and SY High Tech. But the all the sensor membrane samples were soluble in water within 5 min [41].
To determine the effects of temperature, the humidity sensor element was measured at three different ambient temperatures: 15 ◦ C, 25 ◦ C, and 35 ◦ C. Fig. 7 shows the corresponding humidity-resistance characteristics of the sensor element. The sensor resistances across the full humidity range generally decrease with an increase of the ambient temperature, due to improved mobility of the carrier ion. The temperature dependence was observed in order to relate to the temperature and humidity range. At a high humidity range and low temperature, the temperature coefficients became similar to one another. The temperature dependence coefficient between 15 ◦ C and 35 ◦ C is −0.5 to −0.6% RH/◦ C. The temperature dependency of the commercial sensors has been previously reported as −0.6% RH/◦ C [41]. The influence of the temperature on the sensor resistance humidity characteristics can be compensated for by integrating an NTC resistor for the application of a humidity sensor. 3.8. Impedance spectroscopy Fig. 8(a) shows that the impedance plot obtained at 25 ◦ C was semi-circular for the crosslinked AEPAB/ST/HEMA/BPA = 70/23/5/2
3.6. Hysteresis The hysteresis of humidity sensors with AEPAB/ST/ HEMA/BPA = 70/23/5/2, defined as changes between the absorption process and the desorption process, was also measured between 20% RH and 95% RH as shown in Fig. 6. It is worth noting that the sensor composed of the copolymer exhibited highly reversible sensitive properties, and the sensing curves for adsorption and desorption process overlapped, showing a very small amount of hysteresis. The resistance on desiccation is slightly lower than that on humidification, and the corresponding hysteresis falls within −0.2% RH. This fact demonstrates that the rate of humidification and desiccation during the desiccation process of the absorbed water was slower than that during the humidification process.
Fig. 7. The resistance dependence on relative humidity of humidity sensor obtained from AEPAB/Styrene/HEMA/BPA = 70/23/5/2 at () 15 ◦ C, (䊉) 25 ◦ C and () 35 ◦ C at 1 kHz and 1 V.
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Fig. 9. Resistance vs. temperature variation for a AEPAB/Styrene/HEMA/ BPA = 70/23/5/2 sensor at 30% RH, 50% RH, 70% RH and 90% RH.
Fig. 8. The complex impedance plots of a AEPAB/Styrene/HEMA/BPA = 70/23/5/2 sensor at (a) 30–50% RH and (b) 60–90% RH at 25 ◦ C.
at 30% RH due to film impedance, whereas a straight line was observed at high humidity range as shown in Fig. 7(b). From their impedance spectra, it seems that the straight line for the film reflects the fact that the response of the sensor to relative humidity was mostly caused by the change in resistance of the bulk film at high humidity [42–45]. However, the inclined semicircle can be modeled by an equivalent parallel circuit consisting of a resister and a capacitor as proposed in previous reports [46–48]. The conductivity paths through the crosslinked films are formed by the tunneling barrier between crosslinked matrix coming into contact with each other in the AEPAB/ST/HEMA/BPA = 70/23/5/2. These results suggest that the electrical conduction occurs by essentially the same mechanism for the polyelectrolyte containing quaternary ammonium halide films over 20–95% RH ranges. The transport of bromide or chloride ions plays a predominant role in the electrical conduction at high humidity [9]. The sorbed water can facilitate the dissociation of bromide ions from the quaternary ammonium ions due to the high dielectric constant of water. The generation of protons from the sorbed water and their transport must be taken into consideration. The effect of the sorbed water on the migration of ionic carrier is significant especially at high humidity. In the bromide- and chloride-containing polyelectrolytes, the majority carrier ion is not always the same for the entire humidity range. At low humidities, the majority carrier ion is bromide anion, and at high humidities, proton becomes the majority carrier [49].
activation energy was obtained. The resistance of the sensor followed the Arrhenius equation, with straight line plots for the semi-log of the resistance against 1/T. It was observed that the slopes of these straight lines increased as the relative humidity decreased. From the slopes of the Arrhenius plot of the resistance, the activation energy, Ea , was calculated according to the formula Rp = Ro exp(Ea /kT) in the range of 30–90% RH. The activation energy decreases monotonously from 0.86 eV to 0.21 eV with an increase from 30% RH to 90% RH as shown in Fig. 9. This behavior seems to reflect that the conductivity is contributed by bromide ions, not by proton. At the present time, we can estimate the humidity-sensitive film properly. 3.10. Frequency dependence The electrical characteristics of the polymeric film humidity sensor were measured in the a.c. fields, as the sensor became unstable under d.c. fields due to the electrolysis of the polyelectrolyte. The conducting mechanism of this sensor is ionic, with protons as the main charge carrier. The resistance of the sensor is affected by the frequency of the applied voltage. Fig. 10 shows the frequency dependence of the resistance of the sensor at 100 Hz, 1 kHz and 10 kHz. The frequency characteristics of the sensor are also
3.9. Activation energy In order to understand the potential barrier to conduction of the photo-crosslinked AEPAB/ST/HEMA/BPA = 70/23/5/2 film, its
Fig. 10. Dependence of resistance on the applied frequency of () 100 Hz, (䊉) 1 kHz and () 10 kHz (AEPAB/Styrene/HEMA/BPA = 70/23/5/2) at 25 ◦ C and 1 V.
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Fig. 13. Long-term stability of the sensor obtained from the photo-crosslinked copolymer AEPAB/Styrene/HEMA/BPA = 70/23/5/2 kept at 70% RH at 60 ◦ C. Fig. 11. Response time of the humidity sensor obtained from AEPAB/Styrene/ HEMA/BPA = 70/23/5/2 at 25 ◦ C.
dependent on the humidity range. Specifically, the resistance at a frequency of 10 kHz is more affected at low relative humidity, while the resistance at a frequency of 100 Hz is more affected at high humidity. 3.11. Response and recovery time The response times of the sensors were measured when the sensors were connected to a voltage at 1 kHz. This sensor was moved very quickly from a humidity level of 94% RH (with the sensor being kept in a closed bottle saturated with water vapor) to another bottle adjusted to a humidity level of 33% RH, and vice versa. Fig. 11 shows the humidity response curve corresponding to water adsorption and desorption. A relatively long period of time seems to be required to desorb the water vapor. The typical response times were approximately 75 s for the adsorption process and 85 s for the desorption processes. A substantial rise in the first 50 s was followed by a more gradual rise until the sensor equilibrated at 94% RH. The response time of commercial humidity sensors was 1–3.5 min with or without wind speed [13,41]. 3.12. Stability at high temperature and humidity In general, humidity sensors tend to drift significantly when used at high temperature and high humidity. In order to ana-
lyze the drift of the samples, they were maintained at 90% RH as well as 90% RH and 80 ◦ C, and measured at 1 kHz at 1 V. The sensors AEPAB/ST/HEMA/BPA = 70/23/5/2 appear to be very stable over 240 h. When the device is kept at 80% RH or 90% RH at 80 ◦ C, resistances show little change as shown in Fig. 12. The variation of the humidity detection output shall be within ±0.15% RH against the initial value. The photo-crosslinked polyelectrolyte anchored to alumina substrate was stable when operated at high humidity and high temperature at the applied 1 kHz at 1 V. 3.13. Long-term stability When the humidity sensors were used over a long period of time, sensing characteristic was deteriorated significantly. In order to determine the drift of the sample, they have been exposed to 70% RH at 60 ◦ C for 180 days. The sensors appear to be very stable over 225 days. When the device is kept in an ambient atmosphere and temperature, the resistance at 70% RH displayed a little change showing 0.06% RH difference of the resistance as shown in Fig. 13. 4. Conclusion A new family of photo-curable copolymer polyelectrolyte, containing the benzophenone sensitizer group, and potentially useful in humidity-sensitive materials, was synthesized and fabricated on the aluminum electrode pretreated with a 3-glycidoxypropyltrimethoxysilane (GPTMS) silane-coupling reagent. Humidity sensors, using AEPAB, ST, HEMA, and BPA copolymers, demonstrated resistance values ranging from 103 to 107 in the humidity range between 20% RH and 95% RH. The temperature coefficient between 15–35 ◦ C is −0.50 to −0.60% RH/◦ C, while the response and recovery time is 75–85 s between 23% RH and 94% RH. The photo-crosslinking techniques and anchoring of the humidity-sensitive membrane through the silane-coupling agent are very efficient in improving water durability. This type of humidity sensor displays long-term stability at high temperature and high humidity and is considered potentially valuable as a humidity sensor. Acknowledgements
Fig. 12. Stability of the sensor obtained from the photo-crosslinked copolymer AEPAB/Styrene/HEMA/BPA = 70/23/5/2 under 80% RH and 90% RH at 80 ◦ C.
This research was supported by WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R3110069).
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Biographies Myoung-Seon Gong obtained a B.S. degree in chemical technology in 1978 from Seoul National University, South Korea, and a Ph.D. degree in 1983 from Korea Advanced Institute Science and Technology, South Korea, for work on Polymer Chemistry. From 1983 to 2008 he was employed in the department of chemistry Dankook University (Cheonan, South Korea) as a professor. In 2009 he has moved to Department of Nanobiomedical Science at Dankook University, South Korea, where he holds a faculty position. His main field of current scientific interest is thermally stable polymers, humidity sensors and OLEDs materials. Jae-Uk Kim received his B.S. degrees on chemistry from Dankook University in 2009, South Korea. He is working for his Master degree at Dankook University for work on Physical Chemistry. His main areas of interest are polyelectrolytes and its application for sensor techniques. Jong-Gyu Kim obtained a B.S. degree in chemical technology in 1988 from Dankook University, South Korea, and a Ph.D. degree in 1996 from Dankook University, South Korea, for work on Physical Chemistry. Since 2005 he has been employed in the department of chemistry, Dankook University (Cheonan, South Korea) as a professor. His main field of current scientific interest is thermochromic dye and humidity sensors.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Preparation of water-durable humidity sensor by attachment of polyelectrolyte membrane to electrode substrate by photochemical crosslinking reaction Myoung-Seon Gong a,∗ , Jae-Uk Kim b , Jong-Gyu Kim b a b
Department of Nanobiomedical Science and WCU Research Center of Nanobiomedical Science, Dankook University, Cheonan, Chungnam 330-714, Republic of Korea Department of Chemistry, Dankook University, Cheonan, Chungnam 330-714, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 December 2009 Received in revised form 27 March 2010 Accepted 12 April 2010 Available online 18 April 2010 Keywords: Humidity sensor Benzophenone Photoinitiator Photo-crosslinking Water durability
a b s t r a c t New photo-crosslinkable benzophenone-containing polyelectrolytes were prepared by copolymerization of quaternary ammonium salt-containing monomer [2-(acryloyloxy)ethyl] dimethyl propyl ammonium bromide (AEPAB), styrene (ST), 2-hydroxyethylmethacrylate (HEMA), and 4-acryloyloxybenzophenone (BPA). These polyelectrolytes are self-crosslinkable copolymers for humidity-sensitive polyelectrolytes composed of copolymers with different contents of AEPAB/ST/HEMA/BPA = 70/25/3/2, 70/25/2/3, 70/23/5/2, and 70/23/4/3. Pretreatment of the alumina substrate with 3-glycidoxypropyltrimethoxysilane (GPTMS) containing silane-coupling agent was performed to anchor the humidity-sensitive membrane to the substrate through covalent bonds. The sensors were irradiated with UV light, causing the electronically excited benzophenone molecules to form a crosslinking reaction. The resistance ranged from 107 to 103 between 20% relative humidity (RH) and 95% RH, the humidity range required for a sensor operating at ambient humidity. Their water durability, long-term stability under various environments, hysteresis, and response and recovery times were measured and evaluated as a humidity-sensing membrane. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Humidity sensors made from polyelectrolytes possess poor durability against water or dew, as they are generally soluble in water. Several methods have been proposed to solve the problem, including introducing hydrophobic groups by copolymerization [1–5] and grafting [6,7], applying protective films [8], obtaining interpenetrating network structures [9–11], or forming organic/inorganic hybrids [12–16]. Among them, forming crosslink structures in the sensitive film was proven to effectively improve their water-resistance and environmental stability [17–25]. However, the stability of these sensors is still not sufficient to be used under severe conditions, such as high temperature, high humidity, or conditions with excessive dew. Therefore, there is a significant need for the development of resistance-type humidity sensors that are effective under such severe conditions. The covalent attachment of ultra thin films of organic/inorganic hybrid materials to a solid substrate is often desirable in order to enhance the stability of the films against solvents and displacement reagents (e.g., water for hydrophilic surface). As a consequence of this need, numerous organic/inorganic hybrid protocols have been developed [26–29]. The most common procedures
∗ Corresponding author. Tel.: +82 41 550 1476; fax: +82 41 550 3431. E-mail address: [email protected] (M.-S. Gong). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.04.017
use silane-coupling agents that are suitable for binding to the inorganic surface and organic polymer. Typical coupling agents, such as 3-glycidoxypropyltrimethoxysilane, consist of a difunctional molecule, having both the glycidyl group, which reacts with the photo-curable polyelectrolyte after hydrolysis, and the alkoxysilyl group, which interacts with silica or alumina. Treatment with these agents could improve the adhesion between incompatible inorganic surfaces and organic polymers through covalent bonds [30–33]. Photopolymerization science and technology have drawn significant attention in recent years for industrial applications in regards to coating of various materials, adhesives, printing inks, photoresist, and biomaterials [34–37]. In photopolymerization, a typical formulation consists of a vinyl monomer and a photoinitiator. This technology is based on the use of a self-crosslinkable polyelectrolyte, containing a photoinitiator system suited to absorb a light radiation of the appropriate wavelength and to produce primary radical species in order to convert a polyelectrolyte into a crosslinked network [38]. We have been interested in the synthesis of a new family of photocurable polyelectrolytes, in which the polyelectrolyte chains are simply crosslinked by irradiation of UV light after fabrication of the self-crosslinkable humidity-sensitive membrane on the aluminum electrode [39]. This paper describes the use of the silane-coupling agent, its immobilization on an alumina electrode surface, and the
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subsequent attachment of photo-curable polyelectrolytes by irradiation of UV light of dip-coating layers of this copolymer on a dihydroxy-modified surface. Copoly(AEPAB/PS’HEMA/BPA)s were chosen because it represents examples of self-curable and attachable polyelectrolytes to the substrate hydroxyl group, and therefore, demonstrates the water-durable humidity sensor system. 2. Experimental 2.1. Chemicals and instruments 4-Acryloxybenzophenone (BPA) was prepared by the method previously reported [37]. [2-(Acryloyloxy)ethyl] trimethyl ammonium chloride (AEPAB) was synthesized by quaternization reaction of 2-(N,N-dimethylamino)ethyl acrylate with propyl bromide [40]. Styrene (ST), 2-hydroxyethylmethacrylate (HEMA), and azobisisobutyronitrile (AIBN, Aldrich Chem. Co.) were used as received. 2-Methoxyethanol was purified by conventional purification methods. 3-Glycidoxypropyltrimethoxysilane (GPTMS, Aldrich Chem. Co.) was used as received. The humidity and temperature controller (Jeio Tech Korea, Model: TM-NFM-L; 20–95% RH) was used for the measurement of relative humidity at constant temperature. The resistance of the sensors was measured with a LCR meter (ED-Lab Korea, Model EDC-1630, 0.1–20 M). UV exposure of the humidity-sensitive membrane was performed using a commercial bench-top high power UV curing system (Hg, MTL 1000 W, 1 KW (80 W/CM), Sei Myung Vactron Co. Ltd.). 2.2. Sensor electrode Tooth-comb gold electrode (width: 0.22 mm; thickness of electrode: 8–10 m) was silkscreen-printed on the alumina substrate (10 mm × 5.0 mm × 0.635 mm) as shown in Fig. 1. The surface resistance of the gold electrode was approximately 0.04 . The sensor chips were rinsed in 0.1N NaOH and 0.1N HCl for 3 h and washed with distilled water before use. The alumina substrates were pretreated with glycidoxy-containing silane-coupling reagent. A solution of HCl (1 wt%) and GPTMS (2 wt%) in methanol/water (v/v = 95/5) was spread on substrates, and the substrates were dried at 130 ◦ C for 1 h. 2.3. Representative preparation of self-crosslinkable humidity-sensitive copolymers A mixture of the humidity-sensitive monomer AEPAB (3.73 g, 14 mmol), comonomer styrene (0.48 g, 4.6 mmol), HEMA (0.13 g, 1.0 mmol), BPA (0.1 g, 0.4 mmol), and AIBN (0.03 g, 0.2 mmol), dissolved in anhydrous 2-methoxyethanol (10.4 g), was placed in a glass ampoule. The solution was degassed by freeze-thaw method. The sealed glass ampoule was heated at 60 ◦ C and maintained for 24 h. The polymerized mixture was precipitated into a large amount of anhydrous ethyl ether. The product was purified by dissolution in dry 2-methoxyethanol followed by reprecipitation in n-hexane. The copolymer (AEPAB/ST/HEMA/BPA = 70/23/5/2) was dried under vacuum at 50 ◦ C for 12 h. Other copolymers with different contents of AEPAB/ST/HEMA/BPA = 70/25/2/3, 70/25/3/2 and 70/23/4/3 were prepared by similar methods described above. 2.4. Fabrication of humidity-sensitive membrane Humidity sensors were prepared from the copolymers, AEPAB/ST/HEMA/BPA = 70/25/3/2, 70/25/2/3, 70/23/5/2, and 70/25/4/3 summarized in Table 1, by following procedures. The copolymer (0.65 g) was dissolved in anhydrous DMSO (5.0 g) at
Fig. 1. Schematic view of sensor electrode.
room temperature. The mixture was fabricated on the alumina electrode by dipping. The sensor chips were then heated at 60 ◦ C for 2 h and subsequently at 100 ◦ C for 1 h. The photochemical crosslinking reaction of the sensor chips was induced by irradiating UV light for 5 min at room temperature. 2.5. Measurements of resistance characteristics When we measured the resistance characteristics in order to estimate the temperature dependence, frequency dependence, hysteresis, water durability and long-term stability, we extracted capacitance components from the electrical signals. The value of the parallel resistance, Rp , was estimated by extrapolating the semicircle or the pile to the real axis based on the assumption of an Table 1 Results of radical polymerizations of AEPAB, ST, HEMA and BPA in 2-methoxyethanol at 65 ◦ C for 24 h. Entry no.
1 2 3 4 a b c d e
Monomers feed (mole ratio) AEPABa
STb
HEMAc
BPAd
70 70 70 70
25 25 23 23
3 2 5 4
2 3 2 3
inh e
Yield (%)
0.71 0.68 0.67 0.62
93 92 91 95
[2-(acryloyloxy)ethyl] dimethyl propyl ammonium bromide. Styrene. 2-Hydroxyethyl methacrylate. 4-Acryloyloxybenzophenone. Inherent viscosities were measured in 2-methoxyethanol in 1 g/dL at 25 ◦ C.
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was immersed in water for 3 min, 1 h, 10 h, and 24 h, and then dried in air. 3. Results and discussion 3.1. Preparation of photo-curable polyelectrolytes
Scheme 1.
equivalent circuit consisting of a parallel combination of the resistance Rp and the capacitance Cp for the impedance spectroscopy and activation energy. Resistance versus relative humidity characteristics of the sensors were measured for an absorption process at 20% RH → 95% RH, and for a desorption process at 95% RH → 20% RH at 1 V, 1 kHz, and 25 ◦ C. Temperature dependence was measured at temperatures of 15 ◦ C, 25 ◦ C and 35 ◦ C at 1 V and 1 kHz. Frequency dependence was obtained by changing frequencies to 100 Hz, 1 kHz and 10 kHz at 1 V and 25 ◦ C. Response time was determined over saturated salt solutions of KNO3 for 94% RH and MgCl2 ·6H2 O for 33% RH at its equilibrium state. The long-term stability at high temperature and high humidity was evaluated at 80% RH at 80 ◦ C. The durability of the humidity sensor in water was tested. The sensor
Photochemically crosslinkable polyelectrolytes were prepared by free radical copolymerization of AEPAB, ST, HEMA, and BPA using AIBN as the initiator in 2-methoxyethanol at 60 ◦ C as shown in Scheme 1 and Table 1. The ratios of AEPAB/ST/HEMA/BPA were determined as 70/25/2/3, 70/25/3/2, 70/23/5/2, and 70/23/4/3 in order to optimize the degree of crosslinking for the waterdurability. Radical polymerization gave polymers of moderate molecular weights, judging from the viscosities. The polymers obtained from the addition-polymerization possessed inherent viscosities of 0.71 dL/g, 0.68 dL/g, 0.67 dL/g, and 0.62 dL/g, respectively. The copoly(AEPAB/ST/HEMA/BPA)s were hygroscopic and very sensitive to light as a result of the copolymer crosslinking. The copolymers were soluble in common organic solvents, such as ethanol, 2-methoxyethanol, DMSO, and N-methylpyrrolidinone. The ST comonomer was adopted for the control of humiditysensitive characteristics, specifically the dependence of resistance on the content of hydrophobic comonomer in the polymer chain. The HEMA comonomer was incorporated into the polymer backbone as an effective hydrogen donor for the photo-reduction of the benzophenone group of the same polymer chain. The BPA comonomer, based on benzophenone, was used for the photoinitiator. 3.2. Pretreatment of substrate with hydroxyl-containing silane-coupling agent 3-Glycidoxypropyltrimethoxysilane (GPTMS) was used as a silane-coupling agent between the humidity-sensitive membrane and the alumina substrate, in order to improve adhesion properties and to achieve increased water stability of the sensors. After the trimethoxysilyl group was converted into a hydrophilic functional silanol by hydrolysis, the silanol group reacted with the free hydroxyl group of the substrate [38]. On the aluminum oxide
Scheme 2.
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Scheme 3.
layer, the hydrolyzed silane initially formed an adsorbed basic Hbonded silane, and following a curing process, a strong covalent bond that formed a thin dihydroxy layer after hydrolysis as shown in Scheme 2.
undergo several possible competing reactions, including hydrogen abstraction and coupling reaction, to form a crosslinked polymer. In this process, the crosslinking of a polyelectrolyte and the anchoring of a polyelectrolyte into the alumina substrate were resulted, through the photo-reaction of the hydroxyl group of the pretreated silane coupling agent simultaneously as shown in Scheme 4.
3.3. Photo-crosslinking mechanism The photochemical crosslinking reaction is illustrated in Scheme 3. It is now well established that exposing benzophenone and a suitable reductant such as hydroxyethyl in HEMA to UV light will result in the reduction of electronically excited benzophenone molecules to form hydroxydiphenylmethyl radicals and the concomitant oxidation of the reductant [38]. The possible photocrosslinking sites were: the hydroxyl group in HEMA, the benzyl proton in ST, and the tertiary proton in AEPAB in the copolymer chain. The resulting hydroxydiphenylmethyl radicals can then
3.4. Humidity-sensitive characteristics Resistance versus relative humidity characteristics of the sensors were measured at 1 V and 1 kHz. The typical resistance characteristic curves of the photo-crosslinked copolymers, AEPAB, ST, HEMA, and BPA irradiated UV light at a temperature of 25 ◦ C as shown in Fig. 2. The resistance of humidity sensors prepared on the pretreated substrate from AEPAB/ST/HEMA/BPA = 70/25/3/2, 70/25/2/3, 70/23/5/2, and 70/23/4/2 was measured between
Scheme 4.
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Fig. 2. Dependence of resistance on the relative humidity for the humidity sensor obtained from AEPAB/ST/HEMA/BPA = (a) 70/25/3/2, (b) 70/25/2/3, (c) 70/23/4/3 and (d) 70/23/5/2 at 25 ◦ C, 1 kHz and 1 V.
20% RH and 95% RH and yielded results of 3.84 M–0.624 k, 5.4 M–0.732 k, 6.50 M–0.820 k, and 8.90 M–1.05 k, respectively. When the solution of the copolymer was fabricated on the electrode, the adhesion properties to the alumina electrode were very efficient. When 5 mol% of HEMA and 2 mol% of BPA was used, the resulting photo-crosslinked polyelectrolyte was stable enough to endure high humidity or a dew point. Higher BPA and HEMA contents in the precursor should lead to a greater extent of covalent crosslinking of the polyelectrolyte chain. Humidity sensors, prepared from the copolymer AEPAB/ST/HEMA/BPA = 70/23/5/2 on the untreated substrate, displayed a slightly lower resistance than the sensor prepared on the pretreated substrate, because there was no cover layer of silane-coupling agent. In addition the humidity sensor prepared on the pretreated substrate without crosslinking also showed a lower resistance than those of the crosslinked humidity sensors. The characteristic resistance of the device fabricated with the copolymer AEPAB/ST/HEMA/PAB = 70/23/5/2 and crosslinked by UV irradiation, decreased by four orders of magnitude with increasing humidity from 20% RH to 95% RH as shown in Fig. 3. The semi-logarithmic response curve exhibited moderate linearity over all humidity regions.
Fig. 3. Dependence of resistance on the relative humidity for the humidity sensors obtained from AEPAB/ST/HEMA/PAB = 70/23/5/2 at 25 ◦ C, 1 kHz and 1 V.
Fig. 4. Dependence of resistance on the relative humidity for the humidity sensor obtained from AEPAB/ST/HEMA/BPA = (a) 70/25/3/2 and 70/25/4/3 (silane-treated) and (b) 70/23/5/2 and 70/25/2/3 (silane-treated and untreated) before and after soaking in water for 1 h at 25 ◦ C.
3.5. Water-resistant properties The sensors prepared on the pretreated substrates were subjected to the water stability test. The pretreatment with 3glycidoxypropyltrimethoxysilane (GPTMS) greatly improved the adhesion and the water stability of the sensors, whereas untreated substrate had virtually no effect. The sensors prepared on the pretreated substrates with GPTMS did not show any peeling-off of the humidity-sensitive membrane after soaking in water for 24 h at 20 ◦ C. Figs. 4 and 5 show the resistance for the sensor obtained from AEPAB/ST/HEMA/BPA = 70/25/3/2, 70/25/2/3, 70/23/5/2, and 70/23/4/3 before and after the soaking, indicating little to no changes in the humidity characteristics of the sensors. A comparison between the differences of resistance in Fig. 4(a) and (b) indicates the improved water stability for the sensor from AEPAB/ST/HEMA/BPA = 70/23/5/2 and 70/23/4/3, as compared to sensors from other polyelectrolytes with different compositions or the untreated substrate. The best stability for copolymers seems to be AEPAB/ST/HEMA/BPA = 70/23/5/2 due to the effective anchoring and photo-crosslinking of the copolymer chain. Durability against high temperature was tested by soaking the sensors in hot water at 50 ◦ C or 80 ◦ C for 5 h (Fig. 5(a) and (b)). The non-crosslinked sensor samples were nearly broken and showed extremely large increases in resistance. After soaking in hot water at 50 ◦ C and 80 ◦ C for more than 5 h, a slight increase in resistance was observed. Resistance increased by 20–30%, indicating the signifi-
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Fig. 6. Dependence of resistance on the relative humidity and hysteresis for the humidity sensor obtained from AEPAB/Styrene/HEMA/BPA = 70/23/5/2; (hollow symbol), desorption process and (solid symbol), absorption process at 25 ◦ C, 1 kHz and 1 V.
3.7. Temperature dependence
Fig. 5. Dependence of resistance on the relative humidity for the humidity sensor obtained from AEPAB/Styrene/HEMA/BPA = 70/23/5/2 before and after soaking in water at (a) 20 ◦ C and (b) 50 ◦ C and 80 ◦ C.
cant degradation of the sensor in hot water. On the other hand, the AEPAB/ST/HEMA/BPA = 70/23/5/2 sensor did not show any changes in regards to sensor characteristics even at 25 ◦ C for 24 h. We have used AEPAB/ST/HEMA/BPA = 70/23/5/2 polyelectrolyte on surface untreated electrode as a model humidity sensor and compared with surface pretreated humidity sensors. We could not prepare the other commercial humidity sensors as a model sensor. But it was reported that the sensor samples obtained from poly(2-hydroxy-3methacryloxypropyl-trimethylammonium chloride) crosslinked with hexamethoxymethylol melamine showed a significant increase of resistance within 100 h [15]. And we have obtained a sample from the Shinyei Corporation and SY High Tech. But the all the sensor membrane samples were soluble in water within 5 min [41].
To determine the effects of temperature, the humidity sensor element was measured at three different ambient temperatures: 15 ◦ C, 25 ◦ C, and 35 ◦ C. Fig. 7 shows the corresponding humidity-resistance characteristics of the sensor element. The sensor resistances across the full humidity range generally decrease with an increase of the ambient temperature, due to improved mobility of the carrier ion. The temperature dependence was observed in order to relate to the temperature and humidity range. At a high humidity range and low temperature, the temperature coefficients became similar to one another. The temperature dependence coefficient between 15 ◦ C and 35 ◦ C is −0.5 to −0.6% RH/◦ C. The temperature dependency of the commercial sensors has been previously reported as −0.6% RH/◦ C [41]. The influence of the temperature on the sensor resistance humidity characteristics can be compensated for by integrating an NTC resistor for the application of a humidity sensor. 3.8. Impedance spectroscopy Fig. 8(a) shows that the impedance plot obtained at 25 ◦ C was semi-circular for the crosslinked AEPAB/ST/HEMA/BPA = 70/23/5/2
3.6. Hysteresis The hysteresis of humidity sensors with AEPAB/ST/ HEMA/BPA = 70/23/5/2, defined as changes between the absorption process and the desorption process, was also measured between 20% RH and 95% RH as shown in Fig. 6. It is worth noting that the sensor composed of the copolymer exhibited highly reversible sensitive properties, and the sensing curves for adsorption and desorption process overlapped, showing a very small amount of hysteresis. The resistance on desiccation is slightly lower than that on humidification, and the corresponding hysteresis falls within −0.2% RH. This fact demonstrates that the rate of humidification and desiccation during the desiccation process of the absorbed water was slower than that during the humidification process.
Fig. 7. The resistance dependence on relative humidity of humidity sensor obtained from AEPAB/Styrene/HEMA/BPA = 70/23/5/2 at () 15 ◦ C, (䊉) 25 ◦ C and () 35 ◦ C at 1 kHz and 1 V.
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Fig. 9. Resistance vs. temperature variation for a AEPAB/Styrene/HEMA/ BPA = 70/23/5/2 sensor at 30% RH, 50% RH, 70% RH and 90% RH.
Fig. 8. The complex impedance plots of a AEPAB/Styrene/HEMA/BPA = 70/23/5/2 sensor at (a) 30–50% RH and (b) 60–90% RH at 25 ◦ C.
at 30% RH due to film impedance, whereas a straight line was observed at high humidity range as shown in Fig. 7(b). From their impedance spectra, it seems that the straight line for the film reflects the fact that the response of the sensor to relative humidity was mostly caused by the change in resistance of the bulk film at high humidity [42–45]. However, the inclined semicircle can be modeled by an equivalent parallel circuit consisting of a resister and a capacitor as proposed in previous reports [46–48]. The conductivity paths through the crosslinked films are formed by the tunneling barrier between crosslinked matrix coming into contact with each other in the AEPAB/ST/HEMA/BPA = 70/23/5/2. These results suggest that the electrical conduction occurs by essentially the same mechanism for the polyelectrolyte containing quaternary ammonium halide films over 20–95% RH ranges. The transport of bromide or chloride ions plays a predominant role in the electrical conduction at high humidity [9]. The sorbed water can facilitate the dissociation of bromide ions from the quaternary ammonium ions due to the high dielectric constant of water. The generation of protons from the sorbed water and their transport must be taken into consideration. The effect of the sorbed water on the migration of ionic carrier is significant especially at high humidity. In the bromide- and chloride-containing polyelectrolytes, the majority carrier ion is not always the same for the entire humidity range. At low humidities, the majority carrier ion is bromide anion, and at high humidities, proton becomes the majority carrier [49].
activation energy was obtained. The resistance of the sensor followed the Arrhenius equation, with straight line plots for the semi-log of the resistance against 1/T. It was observed that the slopes of these straight lines increased as the relative humidity decreased. From the slopes of the Arrhenius plot of the resistance, the activation energy, Ea , was calculated according to the formula Rp = Ro exp(Ea /kT) in the range of 30–90% RH. The activation energy decreases monotonously from 0.86 eV to 0.21 eV with an increase from 30% RH to 90% RH as shown in Fig. 9. This behavior seems to reflect that the conductivity is contributed by bromide ions, not by proton. At the present time, we can estimate the humidity-sensitive film properly. 3.10. Frequency dependence The electrical characteristics of the polymeric film humidity sensor were measured in the a.c. fields, as the sensor became unstable under d.c. fields due to the electrolysis of the polyelectrolyte. The conducting mechanism of this sensor is ionic, with protons as the main charge carrier. The resistance of the sensor is affected by the frequency of the applied voltage. Fig. 10 shows the frequency dependence of the resistance of the sensor at 100 Hz, 1 kHz and 10 kHz. The frequency characteristics of the sensor are also
3.9. Activation energy In order to understand the potential barrier to conduction of the photo-crosslinked AEPAB/ST/HEMA/BPA = 70/23/5/2 film, its
Fig. 10. Dependence of resistance on the applied frequency of () 100 Hz, (䊉) 1 kHz and () 10 kHz (AEPAB/Styrene/HEMA/BPA = 70/23/5/2) at 25 ◦ C and 1 V.
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Fig. 13. Long-term stability of the sensor obtained from the photo-crosslinked copolymer AEPAB/Styrene/HEMA/BPA = 70/23/5/2 kept at 70% RH at 60 ◦ C. Fig. 11. Response time of the humidity sensor obtained from AEPAB/Styrene/ HEMA/BPA = 70/23/5/2 at 25 ◦ C.
dependent on the humidity range. Specifically, the resistance at a frequency of 10 kHz is more affected at low relative humidity, while the resistance at a frequency of 100 Hz is more affected at high humidity. 3.11. Response and recovery time The response times of the sensors were measured when the sensors were connected to a voltage at 1 kHz. This sensor was moved very quickly from a humidity level of 94% RH (with the sensor being kept in a closed bottle saturated with water vapor) to another bottle adjusted to a humidity level of 33% RH, and vice versa. Fig. 11 shows the humidity response curve corresponding to water adsorption and desorption. A relatively long period of time seems to be required to desorb the water vapor. The typical response times were approximately 75 s for the adsorption process and 85 s for the desorption processes. A substantial rise in the first 50 s was followed by a more gradual rise until the sensor equilibrated at 94% RH. The response time of commercial humidity sensors was 1–3.5 min with or without wind speed [13,41]. 3.12. Stability at high temperature and humidity In general, humidity sensors tend to drift significantly when used at high temperature and high humidity. In order to ana-
lyze the drift of the samples, they were maintained at 90% RH as well as 90% RH and 80 ◦ C, and measured at 1 kHz at 1 V. The sensors AEPAB/ST/HEMA/BPA = 70/23/5/2 appear to be very stable over 240 h. When the device is kept at 80% RH or 90% RH at 80 ◦ C, resistances show little change as shown in Fig. 12. The variation of the humidity detection output shall be within ±0.15% RH against the initial value. The photo-crosslinked polyelectrolyte anchored to alumina substrate was stable when operated at high humidity and high temperature at the applied 1 kHz at 1 V. 3.13. Long-term stability When the humidity sensors were used over a long period of time, sensing characteristic was deteriorated significantly. In order to determine the drift of the sample, they have been exposed to 70% RH at 60 ◦ C for 180 days. The sensors appear to be very stable over 225 days. When the device is kept in an ambient atmosphere and temperature, the resistance at 70% RH displayed a little change showing 0.06% RH difference of the resistance as shown in Fig. 13. 4. Conclusion A new family of photo-curable copolymer polyelectrolyte, containing the benzophenone sensitizer group, and potentially useful in humidity-sensitive materials, was synthesized and fabricated on the aluminum electrode pretreated with a 3-glycidoxypropyltrimethoxysilane (GPTMS) silane-coupling reagent. Humidity sensors, using AEPAB, ST, HEMA, and BPA copolymers, demonstrated resistance values ranging from 103 to 107 in the humidity range between 20% RH and 95% RH. The temperature coefficient between 15–35 ◦ C is −0.50 to −0.60% RH/◦ C, while the response and recovery time is 75–85 s between 23% RH and 94% RH. The photo-crosslinking techniques and anchoring of the humidity-sensitive membrane through the silane-coupling agent are very efficient in improving water durability. This type of humidity sensor displays long-term stability at high temperature and high humidity and is considered potentially valuable as a humidity sensor. Acknowledgements
Fig. 12. Stability of the sensor obtained from the photo-crosslinked copolymer AEPAB/Styrene/HEMA/BPA = 70/23/5/2 under 80% RH and 90% RH at 80 ◦ C.
This research was supported by WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R3110069).
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Biographies Myoung-Seon Gong obtained a B.S. degree in chemical technology in 1978 from Seoul National University, South Korea, and a Ph.D. degree in 1983 from Korea Advanced Institute Science and Technology, South Korea, for work on Polymer Chemistry. From 1983 to 2008 he was employed in the department of chemistry Dankook University (Cheonan, South Korea) as a professor. In 2009 he has moved to Department of Nanobiomedical Science at Dankook University, South Korea, where he holds a faculty position. His main field of current scientific interest is thermally stable polymers, humidity sensors and OLEDs materials. Jae-Uk Kim received his B.S. degrees on chemistry from Dankook University in 2009, South Korea. He is working for his Master degree at Dankook University for work on Physical Chemistry. His main areas of interest are polyelectrolytes and its application for sensor techniques. Jong-Gyu Kim obtained a B.S. degree in chemical technology in 1988 from Dankook University, South Korea, and a Ph.D. degree in 1996 from Dankook University, South Korea, for work on Physical Chemistry. Since 2005 he has been employed in the department of chemistry, Dankook University (Cheonan, South Korea) as a professor. His main field of current scientific interest is thermochromic dye and humidity sensors.