Construction And Strain Sensing Performance Of Flexible Hierarchical Fibers With Hollow-Monolith Structure

Conductive polymer composite（CPC）,an electrically conductive material via incorporating conductive fillers into a single-phase or multi-phase polymer matrix,exhibits unique physical response behaviors under external stimuli,indicating its tremendous potential applications in the field of flexible wearable sensors.In this paper,carbon nanotube（CNT）and thermoplastic polyurethane（TPU）were chosen as conductive fillers and matrix,respectively.Afterwards,the flexible conductive composite fibers,with skin-core,hollow and monolith structure,were fabricated through coaxial wet spinning technology successfully.The structure and strain sensing properties of the fibers were investigated in detail,and all the results were as follows:（1）Morphological characterization showed that the fibers consisted of two parts:the conductive outer layer（sensitive zone）composed of the TPU porous skeleton and the conductive network of CNT;the insulative inner layer（elastic zone）composed of the TPU porous skeleton.By changing experimental parameters,the structure and size of the fibers were tuned.With the increase of CNT loading in the outer layer,the initial decomposition temperature and elastic modulus of the fibers increased significantly,while both the tensile strength and elongation at break first increased and then decreased.The fibers obtained an ultra-low percolation threshold（0.17 wt.%）,favorable ohmic properties and excellent durability.（2）Tensile sensing test revealed that the responsivity（R/R₀）of the fiber increased exponentially with strain,and the fiber with lower CNT loading showed higher sensitivity.TPU-8CNT@TPU（8 wt.%CNT in outer layer）,which successfully achieved a perfect combination of light weight（0.46 g/cm³）,wide strain detection range（>350%）and high sensitivity（gauge factor=166.7 at 350%strain）,was selected for subsequent testing.The phenomenon of shoulder peaks,residual resistance,and the initial drop of the response peaks for TPU-8CNT@TPU had been observed during the multi-cycle tensile test.Besides,TPU-8CNT@TPU displayed consistent response behaviors and fine ohmic characteristics under different strains.The cyclic response peaks of TPU-8CNT@TPU slightly shifted up with the increase of the tensile rate.TPU-8CNT@TPU showed the tensile response time of 167 ms,excellent long-term stability and repeatability during 10,000 stretching/releasing cycles.（3）Bending sensing test was performed to TPU-8CNT@TPU with 100%pre-strain.It was found that as the chord length decreased,the R/R₀ of the bending sensor gradually increased;when the chord length was 13.28 mm（arc length was 60 mm）,R/R₀ reached a maximum value;R/R₀ dropped with further decreasing the chord length.The bending response sensitivity of bend sensors was 0.16 rad^-1（chord length was 40mm）.The phenomenon of shoulder peaks and initial drop of response peaks in cyclic bending response behaviors were observed.（4）In compression sensing test,we found that R/R₀ change of TPU-8CNT@TPU was divided into two stages:loading force（F）≤0.025 N,the R/R₀ decreased with increasing F;the R/R₀ increased sharply with increasing F（F>0.025 N）.When F>0.025 N,the R/R₀ of TPU-8CNT@TPU exhibited a perfect linear dependence with F and the compression response sensitivity was S_P=0.34%N^-1.The compression response time was 115 ms.The phenomenon of the shoulder peaks,initial drop of response peaks and residual resistance in cyclic compression response behaviors were observed.A series of human characterizations based on TPU-8CNT@TPU demonstrated its potential application in the field of human health and sports monitoring.（5）The response behaviors of the fiber were analysised based on the tunnel theory,and the intrinsic response mechanisms of the fiber in the external stimuli of stretching,bending and compression were consistent,which provided us further understanding of the strain-sensitive behaviors of the fiber.