Stimulus Response And Mechanistic Studies Of Flexible Conductive Thermoplastic Polyurethane Nanocomposites

In recent years,the conductive polymer composites(CPCs)have become the hotspot in the function of the polymer composites.Due to the significant electrical signal caused by the re-arrangement of the conductive network constructed by conductive fillers when exposed to the external stimulus(stress,thermal,organic vapors,et al.),CPCs have shown wide application prospects in the fabrication of novel high performance smart sensors.The important topics for the research and development of CPCs based sensors are the reduction of the percolation threshold of CPCs and the realization of the high response speed,large responsivity,stable and adjustable response behaviors through the regulation of the microstructure and conductive network morphology of CPCs.Here,two kinds of flexible CPCs with different structure were obtained by adding the conductive filler(graphene(G),carbon nanotube(CNT)or G&CNT)into the insulating polymer of thermalplastic polyurethane(TPU):(1)Flexible G/TPU and CNT/G/TPU CPCs sheets were fabricated by using co-coagulation plus compression molding technique.The physical and electrical properties and the tensile strain sensing behavior of the two different systems were studied systematically;The regulation mechanism of synergistic effect on the conductive network and tensile strain sensing behaviors were disclosed by comparing the conductive percolation behaviors and tensile strain sensing behaviors of CPCs with different conductive filler;the relationship between the microstructure of G/TPU and vapor sensing behavior was also discussed.(2)Flexible porous CNT/TPU and G/TPU CPCs were fabricated by using the thermal induced phase separation technique.The effect of the structure of conductive filler on the cell structure,morphology and the electrical properties of CPCs was studied,and the piezoresistive sensing behavior and sensing mechanism of the two different systems were also analyzed.Based on the investigation above,the relations between the sensing performances and the conductive network structures and their evolutions were established.A large number of results with vital significance to the theoretical study and the practical applications of CPCs based sensor were acquired.The principal research achievements are listed as follows:1 Flexible G/TPU CPCs sheet with ultralow percolation threshold of 0.05vol%(0.1wt%)was obtained by using the co-coagulation plus compression molding technique.The two dimensional graphene and the flexible TPU matrix imbued the CPCs with excellent tensile strain sensing performances,and the CPCs exhibited wide range of strain sensitivity(GF=0.78~17.7)and good recoverability and reproducibility.The sensing mechanism is mainly based on the change of the resistance caused by the change in the number of conductive pathways and tunneling distance under external strain.2 The synergistic effect between the conductive fillers with different morphology could regulate the conductive network and tensile strain sensing behavior of CPCs.The results showed that the separation and connection between graphene and CNT could enhance the dispersibility of conductive filler and reduce the percolation threshold of CPCs effectively,and simple and stable conductive network was also constructed.Compared with the dual-peak response pattern of the CNT/TPU based strain sensors,the synergistic effect endowed the CNT/G/TPU with stable single-peak response patterns under small strain(5%);Due to the destruction of the stable conductive network under larger strain(15% and 30%),prestraining was adopted to regulate the conductive network and stable single-peak response patterns were also obtained.3 Flexible G/TPU CPCs sheet exhibited special negative vapor coefficient(NVC)effect for organic vapors with different polarities,and the sensing sensing mechanism was based on the re-arrangement of conductive network induced by the swelling of TPU matrix under organic vapor environment,causing the change of the resistance of CPCs.According to the rule of ‘like dissolves like',the non-/low-polar vapors mainly caused the swelling of the soft segment rich microphase,its good flexibility could lead the recovery of conductive network after the desorption of organic vapor molecular,so the CPCs showed good recoverability and productivity;Polar organic vapors mainly swell polar hard segments,but the conductive network cannot recover to its initial state due to rigidity of hard segments,and the residual resistance was observed after the desorption of organic vapor molecular.In addition,the vapor sensing responsivity increased with increasing the temperature due to higher absorption activation energy at higher temperature.4 Flexible porous CNT/TPU interconnected cellular structure was fabricated by using the thermally induced phase separation technique.The density and the porosity were 0.1 g/cm3 and 90 % respectively;the addition of CNT was beneficial for the formation stronger cell structure to ensure the output of stable response behavior;Because of the contact(separation)of the CNT exposed on the surface of cell wall during the compression(recover)process,the resistance of CPCs decreased(increased),the porous CPCs exhibited stable monotonous piezoresistive behavior;Due to the good elasticity of TPU and the high porosity,the porous CPCs exhibited fast sensing capacity and good piezoresistive recoverability and reproducibility over a wide strain range up to 90%.5 Flexible porous G/TPU with interconnected cellular structure was fabricated by using the thermally induced phase separation technique.The density and the porosity were 0.11 g/cm3 and 90 % respectively;the addition of graphene was also beneficial for the enhancement of the stability of the porous CPCs,but some small holes still existed in the cell wall,which may be due to the good flexibility of graphene that enables it move together with TPU molecular chain during the contraction process.So the resistance of the porous CPCs first decreased and then increased during the compression(recover)process.Due to the good elasticity of TPU and the high porosity,the porous CPCs exhibited stable fast sensing capacity and good piezoresistive recoverability and reproducibility over a wide strain range up to 90%.So the type of conductive filler played an important role for the porous structure and piezoresistive sensing behavior of porous CPCs.