The Molecular Design,Performance And Application Of Hyperbranched Polymer-based Humidity Sensitive Materials

Advances in digital technology have driven the digital transformation of the healthcare industry,shifting healthcare from treatment to real-time personalized monitoring and remote diagnostic treatment.The application of non-contact medical devices and wearable healthcare devices can improve the healthcare environment and experience of medical treatment,realize the prevention and management of chronic diseases,which has become the development direction of digital health tools.Sensors,which are devices that detect changes in physiological parameters of the human body and convert them into electrical signals,have received a lot of attention.Water is an important part of the human body,and many physiological activities cause changes in humidity,carrying a wealth of information.Humidity sensing provides a more convenient,non-contact method of information acquisition for wearable digital healthcare.As a core component of humidity sensors,the performance of humidity-sensitive materials directly affects the performance of humidity sensors.As a widely used humidity-sensitive material,polyelectrolyte possesses the advantages of low cost,simple preparation process,and ease in molecular design.However,long response/recovery times,poor cyclic and long-term stability,and mechanical rigidity limit its further development.Hyperbranched polymers have the advantages of more reactive groups,less chain entanglement,low crystallinity,simple synthesis,large free volume,controllable properties,and low viscosity,and have been widely used in various fields.The large free volume facilitates the penetration and detachment of water molecules and promotes the transport of carriers;the abundant functional groups can improve the hydrophilicity and ionization ability of the material.Therefore,the utilization of hyperbranched polymers as humidity-sensitive materials is expected to achieve fast response and recovery of humidity and enhance sensor sensitivity.Based on the advantages and characteristics of hyperbranched polymers,aiming to obtain high-performance and durable polymer-based humidity sensors,in this thesis,by using the functional groups in the polymers with different strengths of forces on water molecules,and the interactions between the modifier and the polymer chains in the humidity-responsive process,sulfonated hyperbranched polysiloxane,hyperbranched polymer with zwitterionic resonance structures,and hyperbranched poly（ionic liquid）s were designed and prepared,respectively.By investigating the structure,humidity sensing properties,humidity response mechanism,and applications of the materials,the resulting polymer-based humidity-sensitive materials were demonstrated to have excellent properties such as fast response/recovery,good low humidity conductivity,high linearity,and wide humidity detection range.To address the problem of slow response rate for polymer materials in humidity change sensing,and also to simplify the synthesis steps as well as to save costs,polymers with hyperbranched structures were prepared by one-step synthesis,and the polymer membranes were modified by organic-inorganic blending.Lithium Chloride（LiCl）is widely employed in the field of organic-inorganic modification of polymer-based humidity sensitive materials due to its strong hygroscopicity and ionization property.However,when LiCl is combined with polymer matrix by simple physical blending method,the problem of LiCl loss occurs,resulting in poor cycle stability and long-term stability of the composite membrane,and losing the significance of modification.Therefore,in Chapter 2,sulfonate-functionalized hyperbranched polysiloxanes（HPS）with terminal carboxyl groups were successfully prepared using a one-step polycondensation reaction.By combining the weak interaction between the terminal carboxyl group and the water molecule with the stronger force of the sulfonate group on the water molecule,the affinity and humidity sensitivity of the material to the water molecule were enhanced,accelerating the humidity response process.The conductivity of the HPS material at low humidity was improved by utilizing the promotion of Li⁺migration with the flexible polysiloxane chains.In high humidity environments,hygroscopic LiCl,together with strong hydrophilic groups,constructed a continuous layer of water in the humidity-sensitive membrane,accelerating the movement of conductive ions and lowering the impedance of the sensor.The humidity detection range of the HPS sensor was extended on the basis of the enhanced low humidity response.In addition,the large number of oxygen atoms in the polymer confined Li⁺to the polymer matrix,optimizing the cyclic and long-term stability of the material.The recovery time of the sensor was reduced from 177 s to 31 s when the LiCl content reached 2 wt%.The HPS/LiCl-4%sensor featured optimal linearity（R²=0.9844）,small humidity hysteresis（3%RH）,fast humidity response（1 s）,and high response at low humidity（11%-33%RH）.Hence,different breathing patterns,varying respiratory rates,coughs,and various voice commands were accurately detected by the HPS/LiCl-4%sensors.The humidity signal of a non-contact finger was measured with a humidity sensor,and the sensor impedance was in good linear relationship with the distance of the finger.The combination of sensors and control circuits produced a portable device that allowed wireless transmission of humidity data and real-time display to a cell phone program.Although LiCl is an effective material to enhance the water absorption of polymer-based humidity sensors,accelerate the migration of conductive ions,and shorten the humidity response time.But,in the course of the study,it was found that the polyelectrolyte had a slow dehydration rate due to the presence of a large number of hydrophilic groups in its structure.Manufacturing a humidity sensor with a polyelectrolyte will inevitably affect the impedance recovery performance of the humidity sensor,resulting in a reduction of the lifetime for the sensor.Moreover,polyelectrolyte materials fail to remain stable for long periods of time in high humidity environments created by human exhaled gases,restricting their further application and development.In contrast to the polyelectrolyte with strong affinity for water molecules,zwitterionic polymers with a unique zwitterionic resonance structure enable the formation of the appropriate ion-dipole interactions with gas molecules,very suitable for gas detection.Thus,in order to improve the performance of humidity sensors,in Chapter 3 of this paper,from the viewpoint of polymer molecular structure design and organic-inorganic blending modification,a hyperbranched polymer containing zwitterionic resonance structure capable of immobilizing LiCl was prepared by a simple synthetic method,effectively solving the problem of poor recovery performance for humidity sensors due to the strong hydrophilicity with functional groups of polymers,as well as the problem of LiCl loss.By utilizing the high affinity of the carbonyl groups for Li⁺in the squaraine moieties on the polymer chain,the carbonyl groups acted as active sites in the humidity response process,effectively improving the conductivity and stability of the PTS/LiCl humidity sensors and accelerating the response rate.At the same time,the rate of adsorption and desorption for water molecules was balanced by the appropriate ion-dipole force of the zwitterionic groups in the polymers,without irreversible binding of the water molecules,which shortened the humidity response/recovery time of the sensors and improved the repeatability of the sensor response.Under high humidity conditions,water molecules penetrated into the pores of the PTS and formed a continuous water layer,and the mobility of conductive ions（H⁺,H₃O⁺,Li⁺,Cl^-）increased.Meanwhile,the unique layered/porous structure of the PTS material facilitated the penetration and detachment of water molecules along with the diffusion of conductive ions,further enhancing the conductivity and lowering the impedance of the humidity sensor.The PTS sensor exhibited a fast and well-balanced response/recovery（3 s/3 s）under the rational regulation of forces on water molecules.The humidity response rate of the sensor was optimized by using the hygroscopicity and ionization ability of LiCl.The response time of the PTS/LiCl sensor with a content of 6 wt%was only 0.2 s.The impedance change of humidity sensors containing LiCl was improved by 2-4 orders of magnitude compared to pure PTS.In addition,PTS/LiCl sensors possessed good immunity to interference and long-term stability.The PTS/LiCl sensor could sense changes in breathing rate and non-contact finger distance.The humidity sensor array prepared based on this property dynamically recognized different non-contact gestures and sent the gesture information to a microcontroller.After analysis and processing,non-contact digital inputs on the computer are implemented.Humidity sensors based on rigid ceramic substrates fail to conform well to the human body in wearable applications,and even fall off,which not only affects the user experience,but also causes problems such as poor stability and low accuracy of test results.Nevertheless,flexible materials produced by incorporating conductive fillers into a flexible matrix are susceptible to filler leakage and disruption of the conductive network under the influence of external mechanical forces.Therefore,poly（ionic liquid）s with excellent electrochemical properties and mechanical flexibility are introduced in Chapter 4 for the purpose of improving the mechanical properties of humidity sensors.Based on the work in the previous two chapters,the combination of both ionic liquid groups and hyperbranched polymer structures in Chapter 4 not only modulated the polymer molecular structure and functional groups while improving the humidity sensitivity and electrical conductivity of the materials,but was also of great importance in improving the mechanical properties for humidity sensitive membranes.Poly（ionic liquid）s with hyperbranched structures containing imidazolium-salt backbone were prepared by haloalkylation and thiol-ene click reactions.And the transparent flexible polymer humidity sensitive membrane HPIL was prepared by introducing counter anions with different hydrophobicity through a simple ion exchange method.The humidity sensors constructed with flexible HPIL films solved the problem of poor mechanical properties for the sensors due to the rigid substrate in the previous two chapters.By utilizing the large number of ionic liquid groups and flexible chain segments in the polymer chains as well as the adhesion of the humidity sensitive film to the substrate,the proton conduction of the material during deformation was ensured and the tolerance to mechanical deformation was improved.The ionic groups in HPIL can provide abundant proton transfer sites and water bonding sites under anhydrous conditions,facilitating proton transport and effectively improving the electrical conductivity at low humidity with the assistance of the large free volume generated by the hyperbranched polymer chains.At high humidity,a continuous water layer was formed on the surface of the HPIL film,and conductive ions（H⁺,H₃O⁺,TFSI^-）moved rapidly through the continuous transport channels,reducing the impedance of the sensor.The low impedance characteristic of the HPIL-TFSI sensor at very low humidity enabled it to operate in low humidity environments and was suitable for detection over a wide humidity range（6%-98%RH）.The electrostatic interaction of the HPIL polycationic backbone with the anion prevented anion leakage and enhanced the long-term stability of the sensor.In addition,the HPIL-TFSI sensor exhibited a fast response time of 4 s along with a good humidity response during bending deformation.In wearable tests,the HPIL-TFSI sensor can distinguish between different respiratory rates and monitor skin moisture content,validating the high humidity response performance and mechanical properties of the flexible humidity sensor.