Masanobu Matsuguchi

Humidity sensors based on polymer thin films

Studies on humidity sensors fabricated with organic polymers for the last 10 years are reviewed. Several useful methods for improving the characteristics of humidity sensors based on polymers are proposed. In the case of a resistive-type sensor, cross-linking of hydrophilic polymers or formation of interpenetrated polymer networks with a hydrophobic polymer makes the hydrophilic polymers durable at high humidities. Graft polymerization is another method of preparing water-resistive humidity sensors. In the case of capacitive-type sensors, cross-linking is also useful to modify the hydrophobic polymer to produce a sensor with small hysteresis, high selectivity and high sensitivity.

Humidity sensor using cross-linked poly(chloromethyl styrene)

Abstract A resistive-type humidity sensor was prepared using a poly(chloromethyl styrene) (PCMS) film which was simultaneously cross-linked and quaternized. The impedance dependence on humidity for samples with various reaction degrees was measured. The sorption isotherm curves of water vapor in the cross-linked films were obtained using the quartz crystal microbalance method for the various samples. It was found that the degree of hysteresis depends on the density of the quaternary ammonium group, which affects the diffusion coefficient of the water molecules in the film.

Effect of humidity on NH3 gas sensitivity of polyaniline blend films

Abstract The effect of humidity on the NH 3 detection capability of polyaniline (PANI)-insulating matrix polymer blend films was investigated. Poly(methyl methacrylate) (PMMA) and polystyrene (PSt) were used as the matrix polymers. The presence of water vapor, functioning as the interference gas, led to the lowering of the NH 3 response of the sensor in a humid atmosphere. Although PMMA had more hydrophilic character than PSt (PMMA sorbed more than eight times as much water as PSt), the influence of humidity on the sensor response was not as large as expected. This unexpected finding was due to the characteristic morphology of the “PANI–PMMA blend film”, that is, the blend film consisted of porous structures which resulted in a composition of the coexistence of the PANI–PMMA blend with the PANI and/or PMMA single species. In such cases, water molecules were able to interact with doped PANI directly, and the water sorbing characteristic of PMMA exerted less influence on the conductivity.

Humidity sensor based on alkali salts of poly(2-acrylamido-2-methylpropane sulfonic acid)

Abstract Poly(2-acrylamido-2-methylpropane sulfonic acid), AMPS, and its alkali salts were prepared. The impedances of these polymers were measured at various humidities and temperatures in the frequency range of 100–1 MHz. The results suggest that in the films of the alkali salts of poly(AMPS) containing 1–2 water molecules per AMPS monomer unit in the low and medium humidity ranges, the majority carrier is the alkali cations. For practical use, a humidity sensor, which is completely water-resistive, was prepared by forming an interpenetrating polymer network composed of a copolymer containing AMPS and hydrophobic poly(ethylene glycol dimethacrylate).

Characterization of polymers for a capacitive-type humidity sensor based on water sorption behavior

Abstract The water sorption behavior of polymers for a capacitive-type humidity sensor was examined using three different techniques. The sorbed water in hydrophilic polymers with a pyridine or hydroxyl group was highly bound and that in less hydrophilic polymers with a carbonyl or ether group was loosely bound to the polymer as determined from the sorption analysis. Among less hydrophilic polymers, the tendency to form the clustered sorbed water molecules was higher for PEG and EC, which have an ether oxygen as the hydrogen bonding basicity. The results of this study proved that cluster formation of sorbed water of a sensing polymer was closely related to the sensors characteristics. The polymers with a weak hydrophilic site, which have a low hydrogen bond propensity, are suitable for the capacitive-type humidity sensor.

Stability and reliability of capacitive-type relative humidity sensors using crosslinked polyimide films

Abstract Newly developed crosslinked polyimide and fluorinated polyimide-based humidity sensors were fabricated for office building and factory applications. Both sensors showed good initial sensor performance. The stability and the reliability of these sensors were evaluated by exposing the sensor elements to various test conditions. The crosslinked polyimide sensor was resistant to chemicals and the crosslinked fluorinated polyimide one exhibited long-term stability in harsh environments. By using these sensors properly, their sensor performance is superior to that of most commercially available capacitive-type sensors.

A thin-film polysulfone-based capacitive-type relative-humidity sensor

Abstract A capacitive type relative humidity sensor utilizing newly developed cross-linked fluorinated polyimide has been fabricated using thin film technology and evaluated for commercial building and factory applications. The sensor showed a good initial performance with less than 1% r.h. hysteresis and a 0 ± 0.1% r.h./°C temperature coefficient of capacitance along with good stability especially under high temperature and high humidity conditions. Also, because of the inherent chemical resistance of the polyimide, good sensor stability when subjected to volatile environments has been demonstrated.

Humidity sensor durable at high humidity using simultaneously crosslinked and quaternized poly(chloromethyl styrene)

Abstract In order to prepare a resistive-type humidity sensor that is durable at high humidities or even in dewing, poly(chloromethyl styrene) is simultaneously crosslinked and quaternized (that is, a quaternary ammonium group is added to the polymer) by the reaction with N, N, N′, N′-tetramethyl-1,6-hexanediamine on the surface of an alumina substrate on which a pair or gold electrodes is previously deposited. The sensor is durable even after soaking in water for two hours. The response time is shorter in samples with a higher degree of quaternization. An interpenetrating polymer network (IPN) film is also formed on a similar substrate using the crosslinked quaternized poly(chloromethyl styrene) and ethyleneglycol dimethylacrylate. A sensor based on the IPN film has a shorter response time than a sensor based on the crosslinked quaternized poly(chloromethyl styrene).

A Capacitive‐Type Humidity Sensor Using Cross‐Linked Poly(methyl methacrylate) Thin Films

A capacitive‐type humidity sensor was prepared using cross‐linked poly(methyl methacrylate) (PMMA). The electrical capacitance changed linearly with the relative humidity over the range of 10–90% RH and the sensitivity (the ratio of capacitance at 90% RH to that at 10% RH) was about 1.13. The amount of sorbed water on PMMA at 90% RH was 25 mg/g which was one‐third of that on cellulose acetate butyrate. This low hygroscopicity of PMMA was effective in reducing any hysteresis. The decrement of temperature coefficient and high durability for acetone vapor were achieved using the cross‐linked PMMA film with divinyl benzene or ethylene glycol dimethacrylate as the cross‐linking agent.

Humidity sensors based on organopolysiloxanes having hydrophilic groups

Abstract Two types of humidity sensors were prepared from organosilicon compounds having hydrophilic groups such as NH2, N + (CH3)3Cl− and SO3H. The first is a pressed silica gel disk grafted with hydrophilic organosilicon compounds. The second type is a sintered alumina plate on which a cross-linked organopolysiloxane film having a hydrophilic group was formed. The impedance was measured in the humidity range 0 – 100%. The impedance varies with the hydrophilic group in the order SO3H N + (CH3)3Cl− N + (CH3)3Cl− > SO3H > NH2 > OH. The grafted silica gel device is quite resistant to water. The humidity-sensing characteristics of the alumina-plate type sensor do not change even after soaking in water for several hours.