Preparation And Strain Sensing Properties Of PEDOT?PSS-based Conducting Polymer Hydrogels

People's demand for efficient and convenient personalized medical services continues to grow with the development of the era of big data.Wearable strain sensors made of flexible soft material can monitor various large and small strain signals generated by human activities in real time,and convert them into electrical signal output to reflect health status in time.Therefore,it has shown great development prospects in the fields of personalized medical treatment and human movement health monitoring.The key problem in the development of strain sensor is still to realize the synchronous improvement of the strain sensing performance such as stretchability,high sensitivity and stability.From the material itself,it is still urgent to develop the high-performance strain sensing materials.In recent years,conductive polymer hydrogels,as one of the best material choices for the development of wearable electronic devices,display many advantages including the high conductivity of conductive polymers,excellent stretchability,viscoelasticity and biocompatibility of hydrogels,so it shows great application potential in the field of wearable strain sensors.Herein,this dissertation uses poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)as the ion-electron dual conductive network,which interacts with the viscoelastic polymer mechanical support network.A variety of PEDOT:PSS based conductive polymer hydrogels with bi-continuous microphase separation or semi-microphase separation molecular network structure were prepared,and their morphological structure,mechanical,electrical and other properties were systematically explored.At the same time,the influence of hydrogel concentration,crosslinking method,microstructure and configuration on the sensing performance of the sensor is discussed.Finally,the application of high sensitivity and wide range monitoring of human motion health is effectively realized.The specific research contents and results are as follows:1.PEDOT:PSS-PVA hydrogels with excellent fatigue stability were prepared by designing the physically cross-linked PEDOT:PSS-PVA semi-microphase separation molecular network structure.After freeze-dried,the PEDOT:PSS was re-dispersed in a binary mixed solvent to form a conductive nanofiber network with microphase separation of PEDOT and PSS.Cyclic freeze-thawing or dry-annealing method forms a highly flexible PVA three-dimensional cross-linked network containing a large number of hydrogen-bonded crystal domains.The interaction between two phases of the network made the PSS microphase in the PEDOT:PSS partially dissolve with the PVA phase,and finally formed a semi-micro phase separation molecular network structure,which is beneficial to improving the electrical conductivity of the prepared PEDOT:PSS-PVA hydrogel and the mechanical and electrical stability under cyclic tensile.Based on 3D printing technology,two kinds of physical crosslinking methods were used to crosslink PEDOT:PSS-PVA hydrogels,and their morphology,structure,water content,mechanics,electricity and strain sensing properties were studied in detail.Subsequently,they were used as strain sensing layers to assemble tensile strain sensors.It successfully realizes the effective monitoring of human movement,such as finger bending and eye rotation.The study found that the prepared PEDOT:PSS-PVA hydrogel has good fatigue resistance,and the output stress change remains stable after200 cycles of cyclic stretching.The crosslinking method has an effect on the properties of the prepared hydrogel,for example,the PEDOT:PSS-PVA hydrogel prepared by dry-annealing method has higher conductivity(75 wt%PEDOT:PSS-PVA up to 4.88 S/cm)and better strain sensing sensitivity(up to 3.36)due to its less internal water content and more dense conductive network.The PEDOT:PSS-PVA hydrogel prepared by cyclic freeze-thawing method has lower elastic modulus(0.1 ? 0.4 MPa),which is more consistent with the mechanical properties of human skin,and the stability of strain sensing is better.Finally,the mechanical,electrical and strain sensing properties of PEDOT:PSS-PVA hydrogels under the two crosslinking methods were simultaneously improved by adjusting the mass ratio of PEDOT:PSS and PVA phases and doping appropriate ratio of carboxyl carbon nanotubes aqueous dispersion(c-SWCNTs).2.PEDOT:PSS-PU hydrogels with high electrical conductivity and high strength were prepared by designing the physically cross-linked PEDOT:PSS-PU dual continuous microphase separation molecular network structure.The intramolecular or intermolecular polar groups of PU are cross-linked by hydrogen bonds to form a high-strength PU network.When PU network interacts with PEDOT:PSS high conductivity nanofiber network,due to the bad solvent effect,nano scale microphase separation occurs between PEDOT:PSS and PU.The conductive network and mechanical stretching network are independent and immiscible,which can independently play the greatest advantage of single-phase network.As result,the prepared PEDOT:PSS-PU hydrogel has high strength and excellent electrical conductivity.Based on this,3D-printing solution was prepared,and PEDOT:PSS-PU hydrogel was prepared by 3D printing technology and natural drying crosslink method.Then a wearable strain sensor was assembled,which could identify different expression changes through the difference of output electrical signal intensity and shape,and successfully realized the monitoring of human microexpression changes.It was found that the dual continuous microphase separation structure made the electrical conductivity of the prepared PEDOT:PSS-PU hydrogel much higher than that of PEDOT:PSS-PVA hydrogel(30 wt% PEDOT:PSS-PU,i.e.,up to 27 S/cm),and the mechanical tensile property reached 300% ? 800%,the Young's modulus reached 1 ? 2 MPa.However,due to the unconstrained conductive network,the stability of resistance variation under repeated stretching cycles is poor.Then,by changing the configuration of the hydrogel into a "sandwich" structure,that is,the orthogonal printing PEDOT:PSS-PU hydrogel is used as the upper and lower two substrates with carboxylated SWCNTs coating in the middle,which effectively improves the resistance stability of the monolayer PEDOT:PSS-PU hydrogel under cyclic stretching and raises the linearity of the cyclic loading-unloading response.The sensitivity was about 1.69 in the whole strain range and the hysteresis is effectively reduced.3.PEDOT:PSS-PAM hydrogels with excellent strain sensing performance were prepared by designing the chemical crosslinked PEDOT:PSS-PAM semi-microphase separation molecular network structure.The PEDOT:PSS-PAM hydrogel with semi-microphase separation network structure was obtained by thermal initiation of free radical polymerization of acrylamide monomer in PEDOT:PSS nanofiber network,and the PAM network formed by free radical polymerization was partially soluble with PSS microphase.The molecular chains of PAM network formed by free radical polymerization are more stable with covalent bonds,which is beneficial to improve the tensile properties,the resistance change rate and stability under mechanical tensile,and improve the comprehensive strain sensing performance of hydrogels.The results showed that the prepared PEDOT:PSS-PAM hydrogel exhibited the highest conductivity of 0.58 S/m,the remarkable stretch rate(300% ?600%),and the low Young's modulus(5 ? 2 KPa),which were better matched with the mechanical properties of human skin tissue.The PEDOT:PSS-PAM hydrogel strain sensor exhibited a high linearity response with a linearity of 0.99,sensitivity of2 at 0% ? 100% strain,and 4 of 100% ? 500%,and very low hysteresis.The assembled strain sensor can respond timely and effectively to the movement of the human leg under large strain and the movement of the wrist under small strain,and is responsive to the direction of movement,which provides an idea for the development of multifunctional wearable strain sensors.