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Humidity sensors using chemically modified polymeric materials
&n.sors and Actuators B, 13-14 (1993) 82-85
82
Humidity sensors using chemically modified polymeric materials Yoshiro Sakai Departmentof Applied Chemiwy, Faculfy of Engineering, Ehime Universily,Matsuyama-shi790 (Japan)
Abstract Various chemical modifications of polymers are proposed to meet the requirement for use in two types of humidity sensors, i.e. the resistive type and the capacitive type.. Hydrophilic polymers are adequate to prepare the resistive type sensor, but they must be modified in order to be insoluble in water. Three methods, i.e. graft polymerization, cross-linking and IPN formation, have been proposed. For the capacitive type sensor, the crosslinking of hydrophobic polymers has been proposed to modify them so they will not deform in the vapors of organic solvents.
1. Introduction
Various kinds of polymers have been used to prepare humidity sensors. From the view point of their basic principles, they are classified into two categories. The first one is based on the change in electrical properties of the material due to the sorption of water vapor and the second one is based on the gravimetric change in the material. The latter utilizes a quartz crystal oscillator. The first group is divided into two types, that is, the electrical resiitive type and the capacitive type. For the resistive type humidity sensors, hydrophilic polymers are used, while hydrophobic polymers are preferable for capacitive type sensors. There are several requirements for practical humidity sensors, i.e. high sensitivity, fast response, little hysteresis, small temperature coefficient, long term stability especially at high humidities, resistance to dewing, durable in various gases and organic vapors, etc. In order to meet these requirements, one must modify the polymeric materials using chemical reactions. In this paper, I review some chemical modification methods which were found to be successful in our 1aboratoIy when preparing the materials for humidity sensors.
Resistive type humidity sensors The polymers which we have found to be useful for the resistive type humidity sensors have a basic or acidic group such as a quaternary ammonium or sulfonate group which sorbs moisture. The amount of sorbed water in the polymers correlates to environmental humidity. Since the electrical resistance of the polymer varies with the amount of sorbed water, the humidity
0925-4005/93/$6.00
can be determined by measuring its resistance or impedance. However, these hydrophilic polymers have a serious shortcoming in that they are soluble in water, so they cannot be used in high humidities. In order to solve this problem we have proposed three methods by which the polymers can be modified to make them insoluble in water. These methods are (i) preparation of a graft copolymer composed of a hydrophobic trunk polymer and a hydrophilic branch polymer, (ii) crosslinking of a hydrophilic poIymer, and (iii) preparation of an interpenetrating polymer network (IPN) between a cross-linked hydrophilic polymer and a cross-linked hydrophobic polymer. All the materials prepared in our laboratory for the resistive type humidity sensors are durable in water. Details of these three methods are presented here.
Graji copolymer film senson When a hydrophilic polymer is chemically bonded on a hydrophobic polymer film by the action of rrays or catalysts, the resultant graft polymer film sorbs water vapor but is totally insoluble in water. Such graft polymer films have ideal properties for humidity sensors. In our study, a polytetrafluoroethylene (PTFE) or microporous polyethylene (PE) ti was used as the hydrophobic film. As shown in Fig. 1, styrene or vinylpyridine was graft polymerized onto the PTFE film. The PTFE-graftpolystyrene was then sulfonated to form F’lTE-graftpolystyrene sulfonic acid [l]. The PTFE-graft-polyvinylpyridine was quaternized to form the quatemized pyridine branch [2, 31. A pair of gold electrodes was deposited on the graft films to fabricate either a surface type or sandwich type sensor as shown in Fig. 2. In place of the FTFE film, a microporous polyethylene
Q 1993 - Elsevier Sequoia. All rights reserved
83
t
4’
0
I
50
100
%W Fig. 3. Humidity dependence of impedance of microporous polyethylene -graft -poly-2-hydroxy-3-methacxyloxypropyltrimethylammonium chloride. Grafting percentages are indicated on the Fig.
I +*
I z
FF2 7F2
_ WY
i;CH24-C”.r-
t
7F2
7F2
‘72
Rx,
R = alkyl x = halogeo Fig. 1. Reaction scheme of synthesis of FTFE-graft-polystyrene sulfonic acid and FTFE-graft-quateruized polyvinylpyridine.
surface type
sandwich type
Fig. 2. Humidity sensor composed of graft film: (a) surface type, (b) sandwich type.
film can be used as the trunk polymer &I. In this case a hydrophilic monomer such as 2-acrylamidoZmethylpropane sulfonate (AMPS) [4] or 2-hydroxy3-methacxyloxypropyl-trimethylammonium chloride (HMFTAC) [S, 61 was used as the branch polymer. The branch polymers are grafted on the surface of the pore walls. The humidity dependence of the impedance of this graft film is shown in Fig. 3.
Cross-linked polymer film sensors When a hydrophilic polymer is cross-linked by an appropriate reaction, the polymer becomes insoluble in water [2, 71. In order to prepare a cross-linked polymer film, a solution of the hydrophilic polymer and the cross-linking reagent was dip coated on the surface of the sintered alumina substrate having a pair of interdigitated gold electrodes. The sample was then heated to accelerate the cross-linking reaction. After the reaction, the unreacted materials were washed out with solvents. Examples of this reaction are illustrated in Fig. 4. The sensors thus prepared proved to be durable in water for at least a couple of hours. Their electrical resistance varies with humidity. They have the same degree of sensitivity as the corresponding non-cross-linked homopolymers. IPN film semors An interpenetrating polymer network (IPN) technique has recently been found to be an excellent method for preparing homogeneous polymer alloys. We have applied this technique to, the preparation of a polymer layer composed of an interpenetrated, cross-linked hydrophilic polymer and hydrophobic polymer [g, 91. As shown in Fig. 5, both of the polymer networks are intermixed together so that it is insoluble in water or in any solvent. We have chosen the cross-linked HMPTAC polymer as the hydrophilic polymer network and three different IPN sensors were prepared using (i) melamine resin or (ii) poly(Zhydroxyethy1 methacrylate (HEMA) cross-linked with diisocyanate or (iii) cross-linked ethylene glyc~l dimethacrylate (EGDMA) as the hydrophobic network. The procedure for preparing this sensor composed of the IPN film of HMITAC and EGDMA is as follows. The HMPTAC polymer was dip coated on an alumina substrate having inter-
84 8
F’ (F),
t
-
((iHz)n
(FL
-CH,-CH-CH2-CH-
-CH,-CH-CH,-CH-
(4 F”z
--+CH2-_::,,
FHJ
c=o
+CHr+-
A
F’O P
+2
p
yH-CH2
Cl-
-kW ?H-cH2 P c=o
Cl
+
--N(CHJ)~
OH
6” +
l;lco R ’ NC0
P” CH-CHZ
R -
CH2
P CH-CH?
Cl-
CH, I -
9
Fig. 4. Cross-linking polymer.
hydrophobic
lliH
F=O -t$CH&
t -N(CH+ Cl-
of (a) polyvinyl pyridine, (b) HMFTAC
netwwk hydrophilic
Fig. 6. Impedance of the sensors as a function of humidity: (0, 0) non-cross-linked HMPTAC film sensor; (A, A) crosslinked HMPTAC tilm sensor; (0, W) IPN of HMPTAC and EGDMA film sensor.
network
\
digitated gold electrodes. A solution of hexamethylenediisocyanate (HMDI) was then penetrated into the polymer film. The HMPTAC polymer was cross-linked when the film was heated at 60 “C. The hydrophobic monomer ethylene glycol dimethacrylate (EGDMA) and azobisisobutyronitrile (AIBN) was penetrated into the cross-linked HMPTAC film. When the film was heated at 80 “C, EGDMA cross-linked itself and the IPN film was formed. Figure 6 shows the impedance of the sensor composed of an IPN film of HMPTAC and EGDMA as a function of relative humidity. The impedance of the non-cross-linked HMPTAC and the cross-linked HMPTAC are also plotted in the same Figure. The plots track on the same curve. The result shows that the formation of IPN does not affect the impedance of the film. The IPN sensors were found to be durable in water for more than 24 h.
Capacitive type humidity sensors
Fig. 5. Interpenetrating polymer network of a hydrophilic and a hydrophobic polymer.
In contrast to the resistive type humidity sensors already mentioned, for the capacitive type humidity sensor a thin film of a hydrophobic polymer is sandwiched between the electrodes. During the early stage of this research, cellulose derivatives were used, then various types of polyimides were tested by many researchers. The dielectric constants of these hydrophobic polymers are usually very low (c. 3) when compared to that of water (c. 80). When a small amount of water is sorbed by these polymers, the apparent dielectric constants increase, resulting in a linear increase in capacitance with relative humidity except for the case
85
humiditysensor does not dissolvein water.The polymers are also modified to be durable in organic vapors for the capacitivetype humiditysensor. In both cases,crosslinking or formation of an interpenetrating polymer network is a promising method for preparing a long term stable sensor. References
%RH Fig. 7. Capacitance of the sensor composed of cross-linked PMMA as a function of humidity.
when the sorbed water molecules form clusters. In addition, when the sorbed water forms clusters, hysteresis is observed. Consequtntly, it is desirable to choose polymers in which the sorbed water does not form clusters. It is also required that the apparent dielectric constant is not affected by organic solvent vapors. Although the hydrophobic polymers are insoluble in water they have some affinityfor organicsolvents. Recently, we have prepared a capacitivetype humidity sensor composed of polymethylmethacrylate (PMMA) cross-linked with divinyl monomers such as ethylene glycol dimethacrylate [lo]. Figure 7 shows the capacitance of the sensor composed of cross-linkedPMMA as a function of humidity. The cross-linked PMMA shows little hysteresis since it sorbs water much less than the cellulosederivatives.This sensor is also durable in organic solvent vapors.
Conclusions
Humidity sensors are improved by chemical modification of the polymers such as graft polymerization, cross&king or IPN formation so that the resistivetype
1 Y. Sakai, Y. Sadaoka and K. Ikeucbi, Humidity sensors composed of graft copolymers, Sensors andActuators, 9 (1986) 125-131. 2 Y. Sakai, Y. Sadaoka and H. Fukumoto, Humidity sensitive and water resistive polymeric materials, Sensors anddchrafors, 13 (1988) 243-250. 3 Y. Sakai, Y. Sadaoka, M. Matsugucbi, Y. Kanakura and M. Tamura, A humidity sensor using polytetral?uoroetbylenegraft-quatemized-polyvinylpyridine, I. EMrochem Sot., 138 (1991) 2474-2478. 4 Y. Sakai, V. L. Rao, Y. Sadaoka and M. Matsugucbi, Humidity sensor, composed of a microporous film of polyetbylene-graftpoly-(2-acrylamido-2-methylpropane sulfonate), Polyp. Bull, 18 (1987) 501-506. 5 Y. Sakai, Y. Sadaoka, M. Matsugucbi and V. L. Rao, Humidity sensor using microporous ftbn of polyethylene-graft-poly-(2hydroxy-3-metbacryloxypropyl-trimetbylammonium chloride, J. Malt-r. Sci., 24 (1989) 432-438. 6 Y. Sakai, Y. Sadaoka, M. Matsugucbi, V. L. Rao and M. Kamigakii A humidity sensor using graft copolymer with polyelectrolyte branches, Polymer 30 (1989) 106b1071. 7 Y. Sakai, Y. Sadaoka and M. Matsugucbi, A humidity sensor using cross-linked quatemized polyvinylpyridine, I. Elecirothem Sot., 136 (1989) 171-174. 8 Y. Sakai, Y. Sadaoka, M. Matsugucbi, I. Hiramatsu and K. Hirayama, Humidity sensor composed of interpenetrating polymer networks of hydrophilic and hydrophobic polymers, Proc. 3rd Int. Meet. Chemical Sensors, Clewkanri,OH, USA, Sept. 24-26, 1990, pp. 273-276. 9 Y. Sakai, Y. Sadaoka, M. Matsugucbi and K. Hirayama, Water resistive humidity sensor composed of interpenetrating polymer networks of hydrophilic and hydrophobic metbacrylate, LXgesst Tech. Papers, Inr. Cor$ SoWsrare SensorsondAciuators, San Fmncisco, CA, USA, June 24-27, 1991, pp. 562-565. 10 M. Matsugucbi, Y. Sadaoka, Y. Sakai, T. Kuroiwa and A. Ito, A capacitive-type humidity sensor using cross-linked poIy(metby1 methacrylate) thin tilms,J. Ekctmchem. Sot., I38 (1991) 2474-2478.
82
Humidity sensors using chemically modified polymeric materials Yoshiro Sakai Departmentof Applied Chemiwy, Faculfy of Engineering, Ehime Universily,Matsuyama-shi790 (Japan)
Abstract Various chemical modifications of polymers are proposed to meet the requirement for use in two types of humidity sensors, i.e. the resistive type and the capacitive type.. Hydrophilic polymers are adequate to prepare the resistive type sensor, but they must be modified in order to be insoluble in water. Three methods, i.e. graft polymerization, cross-linking and IPN formation, have been proposed. For the capacitive type sensor, the crosslinking of hydrophobic polymers has been proposed to modify them so they will not deform in the vapors of organic solvents.
1. Introduction
Various kinds of polymers have been used to prepare humidity sensors. From the view point of their basic principles, they are classified into two categories. The first one is based on the change in electrical properties of the material due to the sorption of water vapor and the second one is based on the gravimetric change in the material. The latter utilizes a quartz crystal oscillator. The first group is divided into two types, that is, the electrical resiitive type and the capacitive type. For the resistive type humidity sensors, hydrophilic polymers are used, while hydrophobic polymers are preferable for capacitive type sensors. There are several requirements for practical humidity sensors, i.e. high sensitivity, fast response, little hysteresis, small temperature coefficient, long term stability especially at high humidities, resistance to dewing, durable in various gases and organic vapors, etc. In order to meet these requirements, one must modify the polymeric materials using chemical reactions. In this paper, I review some chemical modification methods which were found to be successful in our 1aboratoIy when preparing the materials for humidity sensors.
Resistive type humidity sensors The polymers which we have found to be useful for the resistive type humidity sensors have a basic or acidic group such as a quaternary ammonium or sulfonate group which sorbs moisture. The amount of sorbed water in the polymers correlates to environmental humidity. Since the electrical resistance of the polymer varies with the amount of sorbed water, the humidity
0925-4005/93/$6.00
can be determined by measuring its resistance or impedance. However, these hydrophilic polymers have a serious shortcoming in that they are soluble in water, so they cannot be used in high humidities. In order to solve this problem we have proposed three methods by which the polymers can be modified to make them insoluble in water. These methods are (i) preparation of a graft copolymer composed of a hydrophobic trunk polymer and a hydrophilic branch polymer, (ii) crosslinking of a hydrophilic poIymer, and (iii) preparation of an interpenetrating polymer network (IPN) between a cross-linked hydrophilic polymer and a cross-linked hydrophobic polymer. All the materials prepared in our laboratory for the resistive type humidity sensors are durable in water. Details of these three methods are presented here.
Graji copolymer film senson When a hydrophilic polymer is chemically bonded on a hydrophobic polymer film by the action of rrays or catalysts, the resultant graft polymer film sorbs water vapor but is totally insoluble in water. Such graft polymer films have ideal properties for humidity sensors. In our study, a polytetrafluoroethylene (PTFE) or microporous polyethylene (PE) ti was used as the hydrophobic film. As shown in Fig. 1, styrene or vinylpyridine was graft polymerized onto the PTFE film. The PTFE-graftpolystyrene was then sulfonated to form F’lTE-graftpolystyrene sulfonic acid [l]. The PTFE-graft-polyvinylpyridine was quaternized to form the quatemized pyridine branch [2, 31. A pair of gold electrodes was deposited on the graft films to fabricate either a surface type or sandwich type sensor as shown in Fig. 2. In place of the FTFE film, a microporous polyethylene
Q 1993 - Elsevier Sequoia. All rights reserved
83
t
4’
0
I
50
100
%W Fig. 3. Humidity dependence of impedance of microporous polyethylene -graft -poly-2-hydroxy-3-methacxyloxypropyltrimethylammonium chloride. Grafting percentages are indicated on the Fig.
I +*
I z
FF2 7F2
_ WY
i;CH24-C”.r-
t
7F2
7F2
‘72
Rx,
R = alkyl x = halogeo Fig. 1. Reaction scheme of synthesis of FTFE-graft-polystyrene sulfonic acid and FTFE-graft-quateruized polyvinylpyridine.
surface type
sandwich type
Fig. 2. Humidity sensor composed of graft film: (a) surface type, (b) sandwich type.
film can be used as the trunk polymer &I. In this case a hydrophilic monomer such as 2-acrylamidoZmethylpropane sulfonate (AMPS) [4] or 2-hydroxy3-methacxyloxypropyl-trimethylammonium chloride (HMFTAC) [S, 61 was used as the branch polymer. The branch polymers are grafted on the surface of the pore walls. The humidity dependence of the impedance of this graft film is shown in Fig. 3.
Cross-linked polymer film sensors When a hydrophilic polymer is cross-linked by an appropriate reaction, the polymer becomes insoluble in water [2, 71. In order to prepare a cross-linked polymer film, a solution of the hydrophilic polymer and the cross-linking reagent was dip coated on the surface of the sintered alumina substrate having a pair of interdigitated gold electrodes. The sample was then heated to accelerate the cross-linking reaction. After the reaction, the unreacted materials were washed out with solvents. Examples of this reaction are illustrated in Fig. 4. The sensors thus prepared proved to be durable in water for at least a couple of hours. Their electrical resistance varies with humidity. They have the same degree of sensitivity as the corresponding non-cross-linked homopolymers. IPN film semors An interpenetrating polymer network (IPN) technique has recently been found to be an excellent method for preparing homogeneous polymer alloys. We have applied this technique to, the preparation of a polymer layer composed of an interpenetrated, cross-linked hydrophilic polymer and hydrophobic polymer [g, 91. As shown in Fig. 5, both of the polymer networks are intermixed together so that it is insoluble in water or in any solvent. We have chosen the cross-linked HMPTAC polymer as the hydrophilic polymer network and three different IPN sensors were prepared using (i) melamine resin or (ii) poly(Zhydroxyethy1 methacrylate (HEMA) cross-linked with diisocyanate or (iii) cross-linked ethylene glyc~l dimethacrylate (EGDMA) as the hydrophobic network. The procedure for preparing this sensor composed of the IPN film of HMITAC and EGDMA is as follows. The HMPTAC polymer was dip coated on an alumina substrate having inter-
84 8
F’ (F),
t
-
((iHz)n
(FL
-CH,-CH-CH2-CH-
-CH,-CH-CH,-CH-
(4 F”z
--+CH2-_::,,
FHJ
c=o
+CHr+-
A
F’O P
+2
p
yH-CH2
Cl-
-kW ?H-cH2 P c=o
Cl
+
--N(CHJ)~
OH
6” +
l;lco R ’ NC0
P” CH-CHZ
R -
CH2
P CH-CH?
Cl-
CH, I -
9
Fig. 4. Cross-linking polymer.
hydrophobic
lliH
F=O -t$CH&
t -N(CH+ Cl-
of (a) polyvinyl pyridine, (b) HMFTAC
netwwk hydrophilic
Fig. 6. Impedance of the sensors as a function of humidity: (0, 0) non-cross-linked HMPTAC film sensor; (A, A) crosslinked HMPTAC tilm sensor; (0, W) IPN of HMPTAC and EGDMA film sensor.
network
\
digitated gold electrodes. A solution of hexamethylenediisocyanate (HMDI) was then penetrated into the polymer film. The HMPTAC polymer was cross-linked when the film was heated at 60 “C. The hydrophobic monomer ethylene glycol dimethacrylate (EGDMA) and azobisisobutyronitrile (AIBN) was penetrated into the cross-linked HMPTAC film. When the film was heated at 80 “C, EGDMA cross-linked itself and the IPN film was formed. Figure 6 shows the impedance of the sensor composed of an IPN film of HMPTAC and EGDMA as a function of relative humidity. The impedance of the non-cross-linked HMPTAC and the cross-linked HMPTAC are also plotted in the same Figure. The plots track on the same curve. The result shows that the formation of IPN does not affect the impedance of the film. The IPN sensors were found to be durable in water for more than 24 h.
Capacitive type humidity sensors
Fig. 5. Interpenetrating polymer network of a hydrophilic and a hydrophobic polymer.
In contrast to the resistive type humidity sensors already mentioned, for the capacitive type humidity sensor a thin film of a hydrophobic polymer is sandwiched between the electrodes. During the early stage of this research, cellulose derivatives were used, then various types of polyimides were tested by many researchers. The dielectric constants of these hydrophobic polymers are usually very low (c. 3) when compared to that of water (c. 80). When a small amount of water is sorbed by these polymers, the apparent dielectric constants increase, resulting in a linear increase in capacitance with relative humidity except for the case
85
humiditysensor does not dissolvein water.The polymers are also modified to be durable in organic vapors for the capacitivetype humiditysensor. In both cases,crosslinking or formation of an interpenetrating polymer network is a promising method for preparing a long term stable sensor. References
%RH Fig. 7. Capacitance of the sensor composed of cross-linked PMMA as a function of humidity.
when the sorbed water molecules form clusters. In addition, when the sorbed water forms clusters, hysteresis is observed. Consequtntly, it is desirable to choose polymers in which the sorbed water does not form clusters. It is also required that the apparent dielectric constant is not affected by organic solvent vapors. Although the hydrophobic polymers are insoluble in water they have some affinityfor organicsolvents. Recently, we have prepared a capacitivetype humidity sensor composed of polymethylmethacrylate (PMMA) cross-linked with divinyl monomers such as ethylene glycol dimethacrylate [lo]. Figure 7 shows the capacitance of the sensor composed of cross-linkedPMMA as a function of humidity. The cross-linked PMMA shows little hysteresis since it sorbs water much less than the cellulosederivatives.This sensor is also durable in organic solvent vapors.
Conclusions
Humidity sensors are improved by chemical modification of the polymers such as graft polymerization, cross&king or IPN formation so that the resistivetype
1 Y. Sakai, Y. Sadaoka and K. Ikeucbi, Humidity sensors composed of graft copolymers, Sensors andActuators, 9 (1986) 125-131. 2 Y. Sakai, Y. Sadaoka and H. Fukumoto, Humidity sensitive and water resistive polymeric materials, Sensors anddchrafors, 13 (1988) 243-250. 3 Y. Sakai, Y. Sadaoka, M. Matsugucbi, Y. Kanakura and M. Tamura, A humidity sensor using polytetral?uoroetbylenegraft-quatemized-polyvinylpyridine, I. EMrochem Sot., 138 (1991) 2474-2478. 4 Y. Sakai, V. L. Rao, Y. Sadaoka and M. Matsugucbi, Humidity sensor, composed of a microporous film of polyetbylene-graftpoly-(2-acrylamido-2-methylpropane sulfonate), Polyp. Bull, 18 (1987) 501-506. 5 Y. Sakai, Y. Sadaoka, M. Matsugucbi and V. L. Rao, Humidity sensor using microporous ftbn of polyethylene-graft-poly-(2hydroxy-3-metbacryloxypropyl-trimetbylammonium chloride, J. Malt-r. Sci., 24 (1989) 432-438. 6 Y. Sakai, Y. Sadaoka, M. Matsugucbi, V. L. Rao and M. Kamigakii A humidity sensor using graft copolymer with polyelectrolyte branches, Polymer 30 (1989) 106b1071. 7 Y. Sakai, Y. Sadaoka and M. Matsugucbi, A humidity sensor using cross-linked quatemized polyvinylpyridine, I. Elecirothem Sot., 136 (1989) 171-174. 8 Y. Sakai, Y. Sadaoka, M. Matsugucbi, I. Hiramatsu and K. Hirayama, Humidity sensor composed of interpenetrating polymer networks of hydrophilic and hydrophobic polymers, Proc. 3rd Int. Meet. Chemical Sensors, Clewkanri,OH, USA, Sept. 24-26, 1990, pp. 273-276. 9 Y. Sakai, Y. Sadaoka, M. Matsugucbi and K. Hirayama, Water resistive humidity sensor composed of interpenetrating polymer networks of hydrophilic and hydrophobic metbacrylate, LXgesst Tech. Papers, Inr. Cor$ SoWsrare SensorsondAciuators, San Fmncisco, CA, USA, June 24-27, 1991, pp. 562-565. 10 M. Matsugucbi, Y. Sadaoka, Y. Sakai, T. Kuroiwa and A. Ito, A capacitive-type humidity sensor using cross-linked poIy(metby1 methacrylate) thin tilms,J. Ekctmchem. Sot., I38 (1991) 2474-2478.