Investigation On Microstructure,Properties And Flexible Conductor Of PTFE Based On Stretching Stress

Fluoroplastics is an irreplaceable polymer material in modern life.Especially,Polytetrafluoroethylene（PTFE）has an excellent resistance to high and low temperature and good chemical resistant property,which can be applied in advanced fields such as aerospace and automobile manufacturing as a kind of high-performance polymer material.However,due to its low molecular polarity and inferior force between the molecular chains,its products are prone to stress relaxation phenomenon under the action of long-term external forces.Hence,the researchers have been focused on the creep resistance enhancement and thermal stability optimization of PTFE composite.Filling modification as a conventional approach is the most widely applicable scope modification methods,but this method would make PTFE lose its original flexibility and reduce its application range as a sealing material.However,the previous experiments of our research group found that the flexibility of PTFE matrix composite is significantly enhanced after the pre-stretching process,which can not only maintain the inherent advantages of PTFE,but also significantly improve its creep resistance and other properties.Therefore,on the basis of previous studies and based on the response of PTFE to stretching stress,this dissertation intends to systematically investigate the structure of PTFE based on stretching stress,mainly from the microstructure,crystal structure and interfacial strength between packing and matrix.By analyzing the influence of different pre-stretching rates on the microstructure of PTFE material,it is found that the short-glass-fiber（SGF）would be debonded from the polymer matrix,and the material is introduced into the air phase,which reduced the rigidity and density of the composite,that is,PTFE is transformed into porous material.Moreover,the pre-stretching treatment can also induce the rearrangement of PTFE molecular chain and increase the lamellae thickness,and the number of through-hole increases with the increase of the stretching rate,so the heat diffusivity and thermal stability of the composite improved slightly.Despite decreased the Young’s modulus and increased the tensile strength of the composite by pre-stretching,the stretching rate needs to match the PTFE segments of relaxation time,which means excessively fast or slow stretching rate affects the arrangement of molecular chain.As a result,the tensile strength and Young’s modulus of composite tend to increase first and then decrease with the increase of stretching rate.It is concluded that PTFE/SGF composite has the best comprehensive property when the stretching rate has carried out 50 mm·min^-1.Moreover,the relationship between the packing and the microstructure or thermal properties of PTFE porous materials is also investigated via the influence on crystal structure by packings with different diameter.It is found that the decrease of the packing diameter would increase the number of packings per unit volume in material,which means the small-size packings disperse the compression-force from the polymer shrinkage during the cooling process,thus the desorption rate of resin from the packing surface increased,while the compression induced crystallization rate reduced.XRD results show that the lamella thickness of the composite decreases with the decreasing packing diameter,which indicates that crystal thickness around the large packing and the interface physical stability between the matrix and packing are higher than that of the small one.A greater probability of debonding from the matrix for small-diameter fillers under stretching stress,resulting in decrease of the thermal stability of the stretched PTFE with the decrease of the fiber diameter.However,a rapid decline in the number of hydroxyl groups on the surface of the small-diameter chopped glass fiber after coupling agent treatment,so the binding ability between packing and polymer matrix can be significantly improved.Even if the packings would debonding from matrix via pre-stretching process,but they are still connected by microfibrils.Hence,the silane coupling agent surface treatment can significantly increase the thermal stability of the stretched PTFE matrix composite filled with small-diameter packings.In addition,the coupling agent can also promote the crystallization behavior of the polymer matrix around the packing and enhance the phonon passability.Through the structure and properties of PTFE on the research above,a flexible PTFE conductor filled with graphene nanosheets（GNs）coated by ionic liquid（IL）can be designed to flexible conductive materials via pre-stretching process.The conclusion reveals that although the pre-stretching reduces the conductivity due to the destruction of the conductive pathway formed by GNs,but at the same time,the enhanced interfacial strength between GNs and matrix contribute to improve the conductivity via increasing the interlamellar spacing of GNs during pre-stretching process.The PTFE/GNs flexible conductor has a measured electrical conductivity at 5.5×10^-2 S·m^-1 and percolation threshold for 1.49 vol%.The experiment results show that the surface resistance of 3.0 vol%PTFE/GNs@ILs flexible conductor only increased by 0.30%,0.38%and 0.87%after 180°twisting,180°bending and 10%tensile strain for1000 cycles,respectively.Moreover,after 100 cycles of 10%tensile strain at 125°C,the instantaneous volume resistance of this flexible conductor at 10%tensile strain increases slightly,demonstrating a good conductivity with tensile deformation under room and elevated temperature.Hence,pre-stretching can reduce the hardness without the negative influence to conductivity of the flexible composite,making a potential application value for flexible strain sensing material.