| With the rapid development of society,electronic humidity sensors have been widely used in industrial production,cargo storage,meteorological monitoring,agricultural production and home life.Resistive humidity sensor has been developing rapidly at present,with the advantages of high sensitivity to humidity detection,easy measurement of signals and so on.Now,with the development of wearable electronic devices,there is a growing trend towards developing portable and low-cost humidity sensors.Most traditional humidity sensors are prepared based on a hard ceramic substrate with interdigitated electrodes,which is unable to meet the requirements of flexibility and wearability.Nowadays,some flexible materials such as polyethylene terephthalate(PET)have been applied as the substrate material.However,the traditional method of preparing a humidity-sensing coating cannot be replicated directly to prepare a flexible humidity-sensing coating owing to the requirement of high temperature and organic solvents in traditional preparation methods,which will damage the flexible substrate and the electrodes on the substrate surface.Therefore,it is necessary to develop novel kinds of humidity sensitive coating materials which is suitable for flexible organic substrates.In this work,a series of environmentally-friendly humidity-sensing materials whose preparation does not involve high-temperature curing,such as carbon nanotube composites and polyelectrolytes,were developed.These humidity-sensing materials were coated on the PET substrate with carbon interdigitated electrodes by suitable methods to prepare flexible resistive humidity-sensing coatings,which provides new ideas for the preparation of flexible humidity sensors.1.Preparation and properties of humidity-sensing coating based on tannic acid-polyethyleneimine modified carbon nanotubesTannic acid was adsorbed on the surface of carbon nanotubes through ?-? interaction which cross-linked with polyethyleneimine(PEI)through Michael addition and Schiff base reaction to realize the surface modification of the carbon nanotubes.The modified carbon nanotubes(CNT-TA-PEI)can not only be stably dispersed in water,but also have good film-forming properties owing to the successful modification of TA-PEI.CNT-TA-PEI was dispersesd in an aqueous solution and then deposited on the flexible interdigitated electrode by drop casting.The effect of the feeding ratio of TA/PEI/CNTs as well as drop volume on the humidity-sensing response of the coating was studied.As the carbon nanotube content of the coating increased,and the resistance and response decreased.When the TA content increased,the sensitivity of the humidity-sensing coating increased.And when the PEI content increased,the sensitivity of the humidity-sensing coating increased and then decreased.When the mass ratio of TA to CNTs was 2: 1 and the mass ratio of PEI to CNTs was 1: 1,the as-prepared humidity sensor exhibited the best sensing perofrmance.The detection range of the humidity-sensing coating was 6%-91%RH,and the hysteresis was 14%RH.A bending test on the coating was carried,and the results showed that the humidity-sensing coating has good flexibility.2.Preparation and properties of UV-cured carbon nanotube-based humidity-sensing coatingsCarbon nanotubes were dispersed in a UV-curable resin,and a flexible humidity-sensing coating was prepared by a one-step UV-curing method.Firstly,the CNTs and photoinitiator were dispersed in UV-curable resin by ball-milling.The UV-curable resin was composed of a hydrophobic epoxy soybean oil acrylate(AESO)and hydrophilic components with hydrophilic groups.Then the mixture was spin-coated on the flexible PET substrate and cured by UV curing to prepare humidity-sensing coating.The effect of CNTs content,the structure of the hydrophilic component and the ratio of the hydrophilic component to hydrophobic component on the humidity-sensing properties was studied.As the content of carbon nanotubes increased,the response of the coating increased and then decreased.And when the carbon nanotube content was 3.3 wt%,the response of the humidity-sensing coating was the best.It was found that when the hydrophilic component was polyethylene glycol methoxyacrylate(PEGA)with a long hydrophilic chain,the coating not only exhibited higher response but also better recover property.With the increasing PEGA content,the response increased.When the content was 43 wt%,the coating exhibited the highest response to humidity in the range of 6%-84%RH with good repeatability and long-term stability,and the hysteresis was about 7%RH.The bending test showed that the humidity-sensing coating has good flexibility,which could be applied to real-time monitoring of humidity in some electronic devices.3.Preparation and performance of UV-cured polyelectrolyte-based humidity-sensing coatingsA quaternary ammonium salt with double bond as a humidity-sensing monomer,methyl methacrylate as a hydrophobic monomer,and hexanediol diacrylate as a cross-linking agent were UV cured to prepare flexible polyelectrolyte-based coating.The influence of the introduction of hydrophobic monomer,crosslinking agent,and the ratio of hydrophilic quaternary ammonium salt monomer to the hydrophobic monomer on the humidity-sensing performance was studied.In addition,the humidity-sensing mechanism of the humidity sensor was studied by AC impedance spectroscopy.As a result,the hydrophobic monomer could improve the repeatability of the coating at high relative humidity and the introduction of the crosslinking agent could improve the stability in further.As the content of the hydrophobic MMA increased,the response of the humidity-sensing coating increased but the detection range became narrowed.When the content of MMA was 75 wt%,the prepared coating showed optimized sensing performance,whose detection range was 22%-91%RH and hysteresis was 2%RH.Also,the humidity-sensing coating exhibited a short response time(5 s)and recovery time(76 s).The bending test of the humidity-sensing coating showed that the coating has good flexibility.In addition,the entire preparation method is simple,fast,efficient,and environmentally friendly,which is expected to be applied to some flexible electronic devices to monitor humidity in real time. |
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