Preparation Of Conductive TPU Foam Composites By ScCO₂ And Its Piezoresistive Properties

In recent years,research on the preparation of flexible sensors based on the strain-resistance properties of conductive polymer composites?CPCs?has been extensive.CPCs mainly achieve the purpose of conducting electricity by forming a unique conductive network inside the polymer.The CPCs are deformed when subjected to external force,and the internal conductive network also undergoes deformation,resulting in changes in the electrical properties of the material,thus realizing the conversion from the deformation signal to electrical signal.In this paper,thermoplastic polyurethane?TPU?was used as a polymer matrix,and multi-walled carbon nanotubes?MWCNTs?were used as conductive particles.Supercritical carbon dioxide?ScCO₂?was used for batch foaming.Two series of flexible pressure sensitive sensors were prepared by different methods:?1?Conductive TPU/MWCNTs nanocomposites were prepared by solution blending method in DMF,and samples were foamed with good piezoresistance cycling performance;?2?cellulose/MWCNTs hybrid areogel was firstly prepared and consequently blended with TPU solution,and the as-prepared foamed samples exhibited excellent piezoresistance performance.Firstly,cellulose was dissolved by NaOH/thiourea system,carbon nanotubes were used as conductive particles,cellulose/MWCNTs hybrid aerogel was prepared by freeze-dried method.Then,the conductive aerogel/TPU composite was prepared by solution blending method.Finally,samples were foamed to obtain a light weight,high strength,high sensitive piezoresistance composite.Compared with two different foam composites,the influence of foaming conditions on the cell structure,the cyclic compression performance,the electrical conductivity,the piezoresistive performance and the simulation of human motion monitoring were studied.The main research results were as follows:The TPU/MWCNTs nanocomposite with a carbon nanotube content of 5.0 wt%?TPUC5.0?had a tensile strength of 19.8 MPa,and an elongation at break of 660.3%.300%proof stress of 9.93 MPa was the highest of all samples.The compressive strength and compressive modulus of the foamed composite TPUC5.0 foam prepared under saturated conditions of 100?,13.8 MPa,and 12 h were 1.05 MPa and 3.79MPa,respectively,which was 1362.5%and 1856.4%higher than that of the pure TPU foamed sample.The the energy loss coefficient was stability below 0.210 in the late stage of multiple cycle compression.The density of TPUC1.5 foam with MWCNTs content of 1.5 wt%was as low as 0.427 g/cm³,and the volume expansion ratio can reach 2.97 times.The conductivity of the material increases with the increase of the filler.The conductivity of the foamed composite TPUC10.0 foam with MWCNTs content of 10 wt%was 1.6×10^-44 S/m,which was 4.5 orders of magnitude higher than that of the pure TPU foamed material.After foaming,the percolation threshold of the material was reduced from 0.90 vol%to 0.31 vol%.TPUC5.0 foam had a GF value of1.88 in the range of 35%deformation,and had obvious linear ohmic characteristics.The piezoresistive behavior was not affected by the strain rate,and it exhibits obvious high repetitive strain-electric conversion behavior in 100 cycles.With the character of high sensitivity,wide strain range,and stable resistance,the application possibilities for real-time monitoring during the movement of the human foot had been confirmed.The cellulose aerogel C₅C₁₀ with MWCNTs content of 10 wt%?volume fraction of 0.66 vol%?had a density of 0.139 g/cm³,a porosity of 90.93%,a conductivity of2.04 S/m,and a percolation threshold as low as 0.070 vol%.The aerogel/TPU composite prepared by solution blended had good dispersibility and mechanical strength.The storage modulus of C₅C₁₀/TPU composite can reach 2314.3 MPa at-60?,which was 28.5%higher than that of pure TPU.The foamed composite C₅C₁₀/TPU foam prepared under the conditions of 100?,13.8 MPa,and 12 h had a cell diameter of 8.39?m and a cell density of 1.50×10⁹ cells/cm³.Its compressive strength at 50%deformation can reach 0.79 MPa,and it exhibited good mechanical stability at the later of 100 cycles of compression test,and the energy loss coefficient was less than 0.412.The weight ratio of MWCNTs in C₅C₁₀/TPU foam was 1.67 wt%,which conductivity was 1.08×10^-33 S/m,and the conductivity of solution blending foaming composite TPUCC-blend foam that of the same MWCNTs content was 4.7orders of magnitude lower.C₅C₁₀/TPU foam had extremely high sensing sensitivity in the low strain range?10%?and the GF value can reach 7.84.With a pronounced linear ohmic behavior,the piezoresistive behavior was unaffected by the strain rate.For the foamed composite C₅C₅/TPU foam with a low MWCNTs content?0.83 wt%?,the linear resistance range was wide,and the linear piezoresistive property was exhibited in the range of 50%,and the GF value was 1.60.Both exhibited high repetitive strain-electricity conversion behavior in 100 cycles.The application possibilities for real-time monitoring during elbow,knee and foot movements have been demonstrated.The innovation of this paper was that the conductive TPU/cellulose/MWCNTs foam composite which prepared by preparing cellulose/MWCNTs hybrid aerogel first to form a conductive network and blended with TPU.Compared with the nanocomposites prepared by traditional solution blending and foaming method,it had slightly lower mechanical properties and cycle stability,but greatly improve the electrical conductivity and piezoresistive sensitivity.Since the conductive particles are confined inside the aerogel skeleton,the effect of high conductivity at a low load can be achieved.