Humidity Sensors Based On Porous Organic Polymers

Humidity is a common physical quantity in the natural environment, and the humidity sensor is able to transform the environmental humidity into the corresponding electrical signals. The basic principle of the humidity sensor is based on the interaction between the humidity sensing materials and water molecules, which causes the change of the conductivity of the material. Therefore, the sensitive material is the key to determine the performance of the humidity sensor. Amphiphilic polymers applied in humidity sensors are usually formed by hydrophobic main chains and hydrophilic branched chains with polar groups, and the water molecules through the attraction of polar groups can be ionized to free movement of conductive ions. When the environmental humidity changes, the amount of water molecules adsorbed in the sensitive films changes accordingly, which can lead to changes in the concentration of conductive particles, hence the environmental humidity can be monitored by measuring the electrical conductivity of the sensitive films. Because of the strong interaction between hydrophilic groups and water molecules, the aggregation of hydrophilic groups can easily be caused by the long-term work of amphiphilic polymer materials at high relative humidity(> 80% RH). Therefore, the adsorption of water molecules is more likely to cause the partial dissolution and loss of sensitive substances, which have a very serious impact on the stability of humidity sensors. In this paper, new-type porous polymer materials are developed to improve the stability of humidity sensors, which porous polymer composite humidity sensitive materials and amphiphilic porous polymer materials are prepared through two different methods of physical admixture and chemical modification. Futhermore, a series of humidity sensors with excellent properties are successfully fabricated. The main contents are as follows:(1)Macroporous polymer P3 HBA is prepared by Friedel-Crafts alkylation reaction, and Li Cl/P3 HBA composites with excellent humidity sensitive properties are fabricated by physical blending method, which the humidity sensitive substance Li Cl is loaded to the pores of P3 HBA. The differences of humidity sensing properties of materials after the loading of Li Cl are discussed, and the influence of Li Cl on the humidity sensitive mechanism is researched. Firstly, the introduction of Li Cl enhances the hydrophilicity of the composite material, which can adsorb a certain amount of water molecules at low humidity environment leading to the change of the impedance of humidity sensor. Secondly, in the high humidity environment, Li Cl can be ionized in the adsorption of the liquid layer to participate in the conduction of ions, so that the impedance of the humidity sensor is further reduced.(2)The microporous polymer POF is synthesized by the solvent thermal method, and Li Cl/POF composites with good humidity sensing properties are prepared by the method of physical blending method, which the humidity sensitive substance Li Cl is loaded to the pores of POF. Compared with Li Cl/P3 HBA sensor, the change of the impedance of Li Cl/POF sensor reduces one order of magnitude, and the response/ recovery time get longer. As the size of internal pores in POF are too small(0.6 nm), and it is not benefit for the transmission and absorption of water molecules, hence microporous polymers as humidity sensing materials have certain limitations.(3)Hierarchical porous polymer microspheres HPPMs with different diameter are prepared by changing the concentration of reactant monomer, and HPPMs-1 with the largest specific surface area is chose for researching humidity sensing properties. Li Cl/HPPMs-1 composites with humidity sensing properties are prepared by the method of physical blending method, which the humidity sensitive substance Li Cl is loaded to the pores of HPPMs-1. Compared with the Li Cl/P3 HBA and Li Cl/POF sensors discussed above, the variation of the impedance of Li Cl/HPPMs-1 sensor is the largest, and the response/recovery time is shorter than that of Li Cl/POF sensor. The mesopores of HPPMs-1 contribute to the transmission of water molecules in the interior of the materials and the micropores in HPPMs-1 promote the adsorption of water molecules, hence hierarchical porous polymers have great potential application in the field of humidity sensor.(4)The introduction of lithium in porous crosslinked framework using a chemical modification method to synthetise Li P3 HBA and PLBTO. Firstly, we synthesize porous polymer P3 HBA with hydroxyl and carboxyl functional groups, then Li P3 HBA is obtained by lithium-modification of the hydroxyl and carboxyl. The preparation PLBTO is first modifying the monomer phloroglucinol with lithium, and then the lithium containing monomer is through polymerization reaction to obtain PLBTO. Li P3 HBA and PLBTO sensor both have excellent humidity sensing properties. The hydrophilicity of the materials are enhanced through chemical modification method of the lithium into polymeric porous framework, moreover, the crosslinked porous structure ensures the stability of the materials.In this paper, novel porous polymer materials are developed for humidity sensor, and they have a good structural stability, which can resolve the current problem of humidity sensor with poor stability at high humidity environment. Expanding the new application field of the porous polymer materials and the new method of the design of the porous polymer materials, and the new method of improving the stability at high humidity of the polymeric humidity sensor is put forward.