The Design And Performance Study Of Polymeric Humidity Sensors Based On Sulfonated Poly(Ether Ether Ketone)s

Environmental humidity is closely related with normal production and living of human beings.Being able to accurately and reliably measure environmental humidity is an important prerequisite for industrial,production and manufacturing processes.Humidity sensors can automatically provide real-time monitoring the levels of water vapor,which is used in various fields of human activities,and its design and manufacturing has become one of the most active research fields.The humidity sensing material is the core component of humidity sensor by using its absorption and desorption effects on water molecules to detect changes of humidity in the environment.Since the humidity sensor is directly exposed to the environment and work for a long time,and the working conditions are complex and difficult to measure because of high temperature and humidity.Therefore,to explore and develop humidity sensing materials with high sensitivity,high accuracy,good reliability and long-term stability is the key to meet the needs of next-generation novel humidity sensors.Polymer polyelectrolyte materials have the advantages of designable structure,wide measurement range,fast response,small hysteresis,low cost,and a wide range of sources.So,they have become the commonly used humidity sensing materials and applied in resistive humidity sensors.Based on the properties and mechanism of polyelectrolyte humidity sensing materials,it is found that there are two main disadvantages of humidity sensing polyelectrolyte in humidity sensor applications:First,the polar groups in the material will adsorb a large amount of water molecules at high humidity,which causes swelling or degradation.It seriously reduces the performance of humidity sensor and shortens its service life.Second,due to the high ionization degree of polyelectrolyte,its conductivity reaches very high even in low relative humidity environments,resulting in low sensitivity of humidity sensor.Therefore,it is necessary to find a novel polymer material which can not only ensure its excellent comprehensive performance,but also carry strong polar ionic groups,which will have good application potential and practical significance in the field of humidity sensors.Poly?ether ether ketone??PEEK?,as an excellent special engineering material,has a series of excellent properties such as high thermal stability and good corrosion resistance.Among them,sulfonated poly?ether ether ketone??SPEEK?becomes a kind of polyelectrolyte by introducing sulfonated functional groups through chemical reaction,which has the advantages of excellent hydrophilic property and good dimensional stability.At the same time,the sulfonic acid group in SPEEK is a strong polar group,which has a strong ability to adsorb water molecules.It can adsorb different numbers of water molecules under different environmental humidity,and then show the change of proton conductivity.Based on the advantages and conduction mechanism of SPEEK,with the goal of developing a novel type of humidity sensing material in the field of resistive humidity sensors,SPEEK,which has a hydrophobic main chain and strong polar sulfonic acid groups,has been synthesized in this dissertation.This research work was carried out from the structural characteristics,humidity sensitivity characteristics and sensing mechanism of the materials,respectively.Post-sulfonated PEEK and main chain sulfonated PEEK were synthesized and modified through a series of modification methods.Polymeric humidity sensors with excellent properties such as wide working range,good response linearity,high sensitivity,and fast response/recovery were obtained.The main contents are listed as follows:1.First,sulfonated poly ether ether ketone?SPEEK?was prepared by a post-sulfonation method,and SPEEK-Ca²⁺and SPEEK-Cu²⁺were obtained through metal-ion substitution.The comprehensive properties of SPEEK polymers before and after metal-ion substitution were researched.The results showed that the substitution of Ca²⁺would interact with the sulfonic acid groups to enhance the thermal stability.Although the water absorption of SPEEK decreases after metal-ion substitution,Ca²⁺substitution improves ability to combine with water molecules and changes the location of water molecules in the materials.At the same time,the results of the humidity sensitive characteristics showed that the humidity sensor based on SPEEK-Ca²⁺exhibited the best comprehensive performance,including excellent linearity,small hysteresis,good repeatability,fast response and long-term stability,which proves SPEEK can be used as a novel humidity sensing material,offering insight for the development of humidity sensors.2.Since humidity sensing properties of polyelectrolyte-based resistance type humidity sensors depend on the chemical structure of the polymer,in order to obtain SPEEK with precise and higher Ds,a series of SPEEK-x sulfonated polymers with different Ds values were prepared by direct aromatic nucleophilic substitution of sulfonated monomers.Combining with SAXS results and water adsorption behaviors,it is proved that the interaction between water molecules adsorbed and polymers main chain with the increase of Ds under high humidity environment changes.The higher the degree of sulfonation is,the easier it is to aggregate and form water molecule clusters under high humidity,showing obvious hydrophilic and hydrophobic phase separation structure.The size of the ion clusters increases with the increase of Ds.The SPEEK-x sulfonated polymer was prepared as humidity sensors,and sensing characteristics were studied.The experimental results show that with the increase of Ds,that is,the proportion of ionic monomers in the polymer chain segments,hysteresis of humidity sensors first decreases and then increases,while humidity sensor based on SPEEK-6polymer with medium sulfonation has the best comprehensive performance.The main chain sulfonated poly ether ether ketone was applied to humidity sensor for the first time,and the direct synthesis method can accurately control and increase the sulfonation degree of SPEEK,thus expanding the relative humidity test interval of SPEEK humidity sensing materials and improving its comprehensive performance.3.In chapter 3,it was proved that the humidity sensor based on the main chain type SPEEK has excellent comprehensive performance,but the hysteresis of the humidity sensor decreases first and then increases because of the strong water absorption of the sulfonic acid groups.Since the addition of hydrophilic sites in the system can reduce the hysteresis effect of polyelectrolyte,polyethylene glycol?PEG?flexible segment which is nonionic hydrophilic compound,is introduced into the polymer system.In order to ensure the compatibility of PEG and SPEEK host polymer,PEEK-co-PEG copolymer was synthesized by copolymerization to prepare humidity sensors based on SPEEK/PEEK-co-PEG composite material.The optimal PEEK-co-PEG blend ratio was selected through the comparison of hysteresis test,which proved that the introduction of PEG flexible chain segment could significantly reduce the hysteretic effect of the humidity sensor.Furthermore,LiCl,which is widely used in the field of humidity sensing materials,was selected for doping,and the influence of different LiCl loading content on the overall performance of the humidity sensor was studied.Combining with the analysis of electrochemical impedance spectroscopy,it is speculated that the humidity sensor based on SPEEK/PEEK-co-PEG/LiCl complex has both polyelectrolyte SPEEK ionized ion conductivity and PEG-Li⁺complexation conduction mechanism.The introduction of PEG and Li⁺can significantly improve the hygroscopic property of the humidity sensor.4.On the basis of the work in the previous chapters,SPEEK with low Ds was selected to be doped with CaCl2 of different proportions to form polymer-inorganic salt complex?SPEEK/CaCl2?as a dopant,and also as ionic cross-linking agent.Ionic cross-linking is formed between Ca²⁺and sulfonic acid groups nearby the aromatic ring in the main chain of SPEEK,and ionic cross-linking structure of Ca²⁺-SPEEK was designed.The SAXS results showed that the ionic region size increased with the increase of CaCl2doping amount,and the degradation temperature of sulfonic acid groups in the doped SPEEK polymer increases,proving the formation of an ionic crosslinking network in the system.Compared with non-doped pure SPEEK,the appropriate ratio of SPEEK/CaCl2 humidity sensor had the best sensing performance such as high sensitivity,small hysteresis,and fast response.It was proved that CaCl2 not only significantly improves the sensitivity of the humidity sensor as a dopant,but also reduces the hysteresis of the humidity sensor as a cross-linking agent.At the same time,through the analysis of electrochemical impedance spectroscopy,it is speculated that there are three types of conduction modes in the conduction process of SPEEK after doping:vehicle mechanism,hopping mechanism,and ion diffusion.