Study On Preparation And Properties Of Thermoplastic Polyurethane Based Flexible Foam With Aligned Structure

Conductive polymer composites(CPCs)are the polymer composite materials with conductive function fabricated by dispersing one or several kinds of conductive fillers into polymers.Due to its low cost,easy processing and the performance which can be controlled in a wide range,the CPCs have drawn widespread concerns in the world.The resistance of CPCs can regularly change with the external stimulus(strain,temperature,organic vapors,et al.).As a consequence,CPCs have a widespread application prospect in flexible sensor field.Nevertheless,the strain sensors,especially the compression strain sensor,prepared by solid CPCs have some weaknesses such as its instability of the output signal,small compression strain and so on,which restrict the application of CPCs.With the development of research and the innovation of technology,the porous CPCs strain sensors with large strain have been prepared.However,the mechanical properties of the material are weak and the stability and recovery are poor,which makes the application face enormous challenges.Therefore,it is a hot research to adjust the microstructure of the materials for CPCs based strain sensors with large strain,high mechanical properties,excellent stability and reversibility.Due to the superior elasticity and flexibility,thermoplastic polyurethane(TPU)was used as the polymer matrix to fabricate CPCs with highly flexibility.One dimensional carbon nanotubes(CNTs)was chosen as the electrical conductive filler arising from its excellent mechanical properties,high electrical conductivities and large aspect ratio.Directional freezing and freeze-drying methods were used to fabricate aligned CNTs/TPU foam with lightweight,flexible properties.Besides,the structure and properties of aligned CNTs/TPU foam were also researched.The principal achievements are briefly introduced as follows:Firstly,the lightweight and flexible CNTs/TPU conductive foam with an aligned porous structure was successfully prepared by directional-freeze and freeze-drying technology for the first time.The results of scanning electron microscope(SEM)reveals that regular aligned structure was evident,and the diameter of foam is about 10 ?m.The CNTs were homogeneous anchored in the skeleton of the foam.The conductivity,density and mechanical properties of aligned CNTs/TPU porous composite were studued.Owing to the homodisperse and excellent interaction of CNTs and TPU matrix,an ultralow percolation threshold(0.0023 vol.%)has been achieved.What is more,the density of the aligned porous materials was as low as 0.13 g·cm-3 because of the ultrahigh porosity.As a consequence,our sample can be placed in the petal without bending the pistil.In addition,the mechanical properties along with the orientation direction are enhanced compared with disordered foam.Secondly,the compression sensing behaviors of aligned CNTs/TPU porous composite under different amplitudes were researched.The resistance of composites monotonically decreased with the compression and recovered to the original value when the strain released.During the compression process,more CNTs contact with each other,leading to the decline of the resistance;when released,the new formed conductive path broke again and the resistance raised.It is worth noting that the resistance of the composites can recover to its original value owing to the aligned porous structure.The excellent linear characteristic can maintain until to the strain of 77%,which is much better than other materials.In addition,the sample still has excellent stability and recoverability in mutil-cyclic compress-release process due to the aligned structure and enhanced mechanical propertied.The aligned foam can maintain the favourable stability and recoverability even after 2000 cycles,indicating the long-term use of our pressure sensor.Simultaneously,the human motion detections such as walk,jump,squat etc.are demonstrated to be detected by the pressure sensors,suggesting the capabilities of the aligned CNTs/TPU foam in human motion detection.Finally,the bend sensing and stretch sensing behaviors of aligned CNTs/TPU porous composite under different amplitudes were explored.The resistance declined in the bending process and raised when the strain released.Different from the compression,the resistance of composites cannot recover to its original value due to the permanent breakup of some porous cell in the cyclic bend strain.The pretreatment of 20 cycles cyclic bend almost have no influence to the stability.And then,the sample was suffered 20 cycles cyclic bend and recovery time of 24 h as pretreatment,the stability of the foam has been improved greatly with the strain lower than the pretreatment strain.Therefore,the pretreatment of material has an important influence on its stability.As for the tensile property,the CNTs/TPU aligned foam can be stretched to 170% owing to the flexibility of TPU matrix and the aligned structure,which is much higher than other porous materials.The resistance raised in the tensile process and declined when the strain released.The resistance cannot recover to its original value due to the permanent damage of porous cell.The multi-cycle test shows poor stability.Inspired by the experimental result of bend pretreatment,the stability and recoverability also improved much after pretreatment.