Structure Formation And Piezoresistive Sensing Mechanism Analyses Of TPU-based Microcellular Nanocomposites

Thermoplastic polyurethane（TPU）-based microcellular nanocomposites can be used in the field of flexible pressure sensors due to their good flexibility and high resilience.The properties of nanocomposites are closely related to the microstructure,and the properties can be tuned by changing the microstructure.In this dissertation,TPU/carbon fillers nanocomposites are prepared via melt mixing,and the nanocomposites are foamed in autoclave with supercritical carbon dioxide（scCO₂）as physical foaming agent.The rheological behavior of the nanocomposites is investigated,and the effects of processing parameters on the microstructure of the nanocomposites and the cellular structure of the foamed samples are analyzed.The influence of cell structure on the sensing performance of flexible pressure sensor is investigated,which provides theoretical guidance for the preparation of high-performance porous flexible piezoresistive sensor.The effect of the fillers（contents of 1,2,and 3 wt%）,shear rate,and temperature are investigated on the rheological behavior of the thermoplastic polyurethane（TPU）/multi-wall carbon nanotube（MWCNT）,TPU/graphene nanoplates（GNP）,and TPU/MWCNT/GNP nanocomposites.The results demonstrate that the TPU and TPU-based nanocomposites do not fit the time-temperature superposition principle.Anomalous rheological behavior is found in the nanocomposites with 2 and 3 wt%MWCNTs（2CNT and 3CNT）and 3 wt%MWCNTs/GNPs（3Hybrid）,that is,their complex viscosities increase with rising temperature.This can be attributed to the strong interface interaction in the nanocomposite.To further explore the applicability of superposition principle,the effects of temperature,pressure,and scCO₂ concentration are investigated on the rheological behavior of poly（methyl methacrylate）/expanded graphite（PMMA/EG）composites with 1,4 and 7 wt%EG.The time-temperature superposition principle holds for the PMMA/EG composites with 1 and 4 wt%EG but fails for that with 7 wt%EG.In addition,the time-pressure-scCO₂ concentration superposition principle is valid for all the prepared PMMA/EG composite samples.This is because the dissolved scCO₂ at the PMMA matrix-EG layer interfaces reduces the interfacial interaction between the PMMA chains and EG layers and weakens the effect of the EG layers on the chains’relaxation.TPU/GNP,TPU/MWCNT and TPU/GNP/MWCNT nanocomposites are foamed.Compared to the foamed TPU/MWCNT and TPU/GNP/MWCNT samples,the mean cell diameter of the foamed TPU/GNP samples is smaller and the cell density is higher,because the lamellar structure of GNP limits the growth of cells.The cellular structure of foamed TPU/MWCNT samples is changed by varying the foaming temperature and saturation pressure.The foamed samples are assembled as sensors and the effect of cellular structure is investigated on the piezoresistive behavior of sensors.The foamed samples with thin cell walls exhibit positive piezoresistive phenomenon at large strains.For sensing substrates with moderately thick cell walls,conductive fillers are easy to contact each other when pressed,which is conducive to improving the sensitivity of the sensor.The deformation of the cellular structure of foamed samples with thick and thin cell walls during compression and recovery is observed online,it is found that the cell walls of foamed samples with thick cell walls are wrinkled under large strain,while cell walls of foamed samples with thin cell walls are bent under large strain.In order to improve the detection range and durability of TPU-based foams as piezoresistive sensors,TPU/polypropylene（PP）/MWCNT blend nanocomposites are prepared via melt mixing.The addition of PP improves the mechanical properties and electrical conductivity of the nanocomposites.The prepared nanocomposites are foamed,when the PP content reaches 10 and 15 wt%,part of the open-celled structure is formed in the foamed samples due to the fact that the densely distribute and high-strength PP droplets pierce the cell walls,and the open-cell content increase with the increasing the foaming temperature.Compared with other foam samples,the sensor assembled by the foamed TPU/PP/MWCNT sample with PP content of 10 wt%and MWCNTs content of 2 wt%prepared at 100℃（90/10/2CNT-100）exhibits good sensing performance with the high sensitivity（GF=12.9）,detection range of 35%,low hysteresis（DH<6%）,and good stability.The TPU/PP/MWCNT blend nanocomposites with fibrous structure PP dispersed phase are prepared.The cyclic compression test results show that along the fiber length,the nanocomposite samples exhibit higher compression modulus but larger energy loss.After foaming,the elongated cellular structure is formed in the nanocomposite with PP fibers.When PP dispersed phase is distributed in the cell wall in the form of fiber,the sensitivity of the sensor can be improved（GF=15.96）and the detection range can be widened（>50%）.