Preparation Of Wet-spun Elastic Composite Fibers And Their Strain Sensing Properties

Nowadays,smart wearables have more and more applications,including human motions monitoring,sports behavior tracking,medical health recovery,user sensing recognition and so on,which provide great convenience for human lives.One of core devices of smart wearables is the strain sensor with properties of short reaction time,high sensitivity and good stability.Traditional metal-based strain sensors are limited in the area of smart wearables due to its high rigidity and low stretchability.However,fiber-based flexible strain sensor is considered to be an ideal material for preparing a new generation of flexible wearable electronic products thanks to its softness,lightness,weaving,and fitting the human body to the greatest extent.At present,fiber-based flexible strain sensors based on different flexible substrates have the performances of wide strain response range,good stability and repeatability,high sensitivity,but many advantages are difficult to be obtained at the same time,especially high sensitivity and high stretchability.This paper fabricated elastic conductive composite fibers with high stretchability,high sensitivity and good stability via wet-spinning method,which utilized poly（styrene-butadiene-styrene）（SBS）as the polymer matrix,and multi-walled carbon nanotubes（MWCNTs）as conductive fillers.（1）Explore the spinnability of the matrix material SBS of the conductive composite fibers.Explore the effects of tetrahydrofuran（THF）,cyclohexane（CYH）and ethyl acetate（EAC）on SBS spinning,and select THF as the solvent for further spinning.The spinning process parameters of SBS fibers were explored by changing the concentration of spinning solution,spinning speed,and drafting ratio.The 25%SBS fiber has the highest elongation at break and excellent elastic recovery performance.The fiber at a spinning speed of 5mm/h has the highest tensile strength and elongation at break.For the increase of the drafting ratio,the tensile breaking strength of the fiber increases but the elongation at break decreases,and the elastic recovery performance of the fiber after drafting gets worse.（2）Explore the spinnability of SBS/MWCNTs conductive composite fibers.Explore the effect on spinning with MWCNTs,carboxylated carbon nanotubes（MWCNTs-COOH）,aminated carbon nanotubes（MWCNTs-NH₂）and hydroxylated carbon nanotubes（MWCNTs-OH）,and finally select MWCNTs-COOH as the conductive fillers to prepare fibers.The composite fibers with more than 7%MWCNTs-COOH is conductive,and the electrical conductivity is 1.26×10^-4 S/cm.Meanwhile,the surface of the composite fibers is rough with particle aggregates in the cross section,which results in the lower mechanical properties than that of SBS fibers.（3）With the structure improved,the core-sheath composite fibers were designed to reduce the amount of MWCNTs-COOH,with SBS/MWCNTs-COOH as the skin layer and SBS as the core layer.The core-sheath structure can be observed by scanning electron microscopy.The tensile strength and elongation at break of the fiber decreased with the increase of MWCNTs-COOH concentration,and the elastic recovery rate was greater than84%.In addition,the fiber exhibits extremely high sensitivity and high stretchability,with exploring the low strain signal（0.5%）and high response range（44%）.The stability of the fibers is good,which can explore more than 4000 times stretching-releasing cycles.Most importantly,through the thermo gravimetric analysis,it is found that the design of the skin-core structure makes the actual consumption of MWCNTs-COOH of 0.77%,1.59%,2.63%and 3.62%,respectively,which are 88.30%,81.27%,72.97 and 64.99%,lower than the MWCNTs-COOH consumption in SBS/MWCNTs conductive composite fibers.（4）The core-sheath composite fiber as the weft yarns was inserted into the weft-inserted fabric to monitor and simulate various strain signals in simulated bending,compression,and torsion motions.For the three major strains,the relationship between the resistance（R）and the angle（α）change can be summarized:For tensile strain,R=215.6α³-18093.5α²+406773.5α+11669700.0（Goodness of Fit（r²）is 0.996）;for compressive strain,R=341903.6α²-6.8α+9663280.0（r² is 0.990）;for torsional strain,R=9538.8α³-864829.7α²+24072200.0α+168514000.0（r² is 0.998）.These results indicate that the core-sheath conductive composite fiber can be used in the area of human motion detection and capture.