A polymer sensitive to humidity and its electrical properties analysis

Abstract

A humidity sensor is a device which can covert the ambient moisture variation into an electrical signal (resistance or capacitance) variation, so it can be easily installed in electronic equipment to detect or control the ambient humidity. The humidity sensors based on polymers can be classified as resistive-type and capacitivetype. Because it has a simple fabrication technique and small hysteresis, is easily decorated and does not need heat cleaning, the former type is more interesting. But for all humidity sensors based on polymers, a problem still remains that they cannot be used in high humidity or at a place where dew (condensation water) may occur. Otherwise, the sensitive film will gradually change due to a very slow irreversible reaction with water vapor in atmosphere. The investigation for the problem is focused on improving water-resistive properties of the material. The main methods to solve the problem are interpenetrated polymer network (INP), graft polymerization and modification of substrate and/or surface [1, 2]. In general, the molecular structure of polymers sensitive to moisture has either hydrophilic group or hydrophobic group. We made hydrophilic monomer graft polymerized with hydrophobic monomer and obtained copolymer. The ratio of the two monomers in the reaction is an important parameter to effect the characteristics of the humidity sensor. The humidity sensor based on the polymer is water-resistive, even when put in water atmosphere for 48 min, its characteristic has no change. We utilize the hydrophilic monomer graft polymerization reaction with hydrophobic monomer. The hydrophilic monomer is hydroxyethylate methylacrylate trimethyl ammonium chloride (Qb), the hydrophobic monomer is butyl methylacrylate (Bu). Fig. 1 gives the structures of the two monomers. From many experiments, we found the proportion of two monomers, reaction path and reaction time is important to material properties. The best result shows that Bu : Qb= 7 : 3 is a suitable ratio. The reaction path is better, when Qb is dipped into the reaction container after Bu has been heated for a period of time under 351 K (absolute temperature). The infrared spectrograph (IR) gives main absorption peaks as shown in Fig. 2. The N–C bond vibration peaks appear at 3087.48 cm−1 and 952.06 cm−1. We believe polymerization really took place because homo-copolymer of Bu and Bu itself are both dissolvable in water, and the special peaks still exist after the material obtained dissolves in water. The substrate is a ceramic plate with gold interdigital electrodes made by screen-printing. The ceramic plate scale is 10 mm× 5 mm, electrode dimension is 4 mm× 4 mm, and the wire width and span of electrode are both 0.5 mm, as shown in Fig. 3. The spin-coating technique is employed to fabricate the sensitive film. In order to improve water-resistive properties of the sensor further, another polymer (cellulose) film is coated on the surface of the sensing film. We called the film “protecting film”, which can allow water molecules to