Preparation And Characterization Of Polyacrylamide Nanocomposite Conductive Hydrogel

Flexible,stretchable,and wearable electronic devices are playing an important role in human activity monitoring and personal health management because of their comprehensive features of wearability,comfort,remote operation,and timely feedback,and are gradually replacing traditional hard materials to become a new research direction.As an important component of wearable devices,wearable sensors can be attached to textiles by tape or even directly mounted on human skin to monitor physical signals for disease diagnosis and health monitoring,and thus are of increasing interest.As a core component of the sensor,the conductor will directly affect the performance of the sensor.The conductor materials should not only have good flexibility,high stretchability and high sensitivity,but also excellent self-healing,fatigue resistance and biocompatibility that closely matches the curved surface.Hydrogel is a biological material-like"soft material"with high ionic conductivity.Its high flexibility,excellent stretchability and good biocompatibility make it ideal for flexible sensor conductor materials.In this paper,polyacrylamide?PAAM?is used as the hydrogel matrix material,TEMPO-oxidized cellulose nanofibers?TOCNs?are used as nano-reinforcing phase,and carbon nanotube?CNT?and lithium chloride?LiCl?are used as conductive substance,respectively,to form a cellulose-enhanced nanocomposite conductive hydrogel by free radical polymerization of acrylamide?AM?monomers on TOCNs.Due to its nanometer size,large specific surface area and strong surface tension,CNTs are highly agglomerated in aqueous solution,thus greatly affecting their dispersion in hydrogels.Therefore,in addition to the nano-strengthening and toughening effect in hydrogels,TOCNs have also played a role in assisting the dispersion of CNTs and the construction of a continuous and stable conductive CNT network in the gel matrix.After the incorporation of LiCl,the conductive hydrogels show higher conductivity and high transmittance and sensitivity.The as-prepared hydrogels exhibit excellent mechanical properties,high toughness,good self-recovery property,perfect fatigue resistance and ideal sensing performance.The preparation of nanocomposite conductive hydrogels and the effect of TOCNs and CNT content on the properties of hydrogels,as well as the assembly and sensing performance of PDMS encapsulated TOCN-LiCl/PAAM hydrogel-based strain sensors with high transparency and waterproof and water retention were studied in this paper.The morphology of the hydrogel was characterized by scanning electron microscopy and transmission electron microscopy.The mechanical strength,toughness,self-recovery and fatigue resistance of the hydrogel were tested by rheological,compressive and tensile test.The functional group change and transmittance of the hydrogel were analyzed by Fourier transform infrared test and transmittance test.The conductivity of the conductive hydrogel and the sensing performance of assembled flexible strain sensor were performed by electrochemical workstation.The specific results are as follows:?1?When the content of TOCNs was 3.0 wt%,the transmittance of the composite hydrogel in the visible light range was as high as 84.8%,the G'_max was about 51.11 kPa,the compressive stress at 80%strain was about 2.43 MPa,and the energy dissipation was about 9.68 MJ/m³?60%strain?,which was 7,27 and 14 times that of the pure PAAM under the same conditions,respectively.The maximum tensile stress?⁰.07 MPa?and elongation at break?²77%?of 3%TOCN/PAAM were nearly 3.5 and 2 times higher than that of pure PAAM,respectively,and the toughness was as high as 12.21 MJ/m³.After stretching to 150%strain,the self-recovery rate of mechanical properties after resting for 30 min exceeded 93.2%,and the residual strain was only2%.It exhibited high fatigue resistance during both compression and stretching cycles,and also showed shape memory performance under the function of Fe³⁺.?2?When the content of TOCNs was 3.0 wt%and CNT was 2.0 wt%,the G'_max of the conductive composite hydrogel was about 79.36 kPa,the compressive stress at 60%strain was about 2.55 MPa,and the energy dissipation was about 29.8.MJ/m³,which were 11.4,85 and 27.1times higher then that of pure PAAM,and 1.6,2.7 and 5.1 folds of TOCN/PAAM under the same conditions,respectively.The maximum tensile stress?⁰.15 MPa?,elongation at break?²26%?and toughness?¹9.51 MJ/m³?of TOCN-2.0%CNT/PAAM increased by nearly 7.5,1.4 and 9.7times compared with pure PAAM.After 150%tensile strain,the self-recovery rate of mechanical properties reached 92.2%after resting for 30 min,and the residual strain was only 9.7%.It exhibited high fatigue resistance during cyclic loading.The conductivity reached 2.42±0.03 S/m.The GF value reached a high strain sensitivity of about 11.8 in the strain range of 100%-200%,and can be used as a strain sensor to monitor the movement of human fingers or joints.?3?The strain sensor assembled with silane-modified TOCN-LiCl/PAAM ion-conducting hydrogel(S-PAAM_C-Li)and silane-modified PDMS?S-PDMS?had an average transmittance of89.5%,only about 2%of water loss,a tensile strength of⁰.88 MPa,and a conductivity of5.21±0.15 S/m.The sensor could achieve a high GF value of 3.8 in the low strain range,and exhibited sensing stability and excellent mechanical durability after about 250 cycles of stretching.Even under mechanical deformation such as bending,stretching and pressing,it still showed excellent strain sensing performance and high sensitivity,which could be further used to monitor large-scale or small movements of the human body in real time,including finger bending,arm joint movement,writing or smiling,frowning,swallowing and other micro-expression monitoring.