Investigation On VO₂ Induced PTC Effects Of The Polymer-based Composites

The polymer-based conductive composites are widely used in life because of their low processing cost and safety in the utilization.However,there are also many problems to be solved for conductive polymer composites,such as high room temperature resistivity and poor repeatability.They directly restrict the further application and popularization of conductive polymer composites.According to the current research status of conductive polymer composites,in this paper,low-density polyethylene（LDPE）,high-density polyethylene（HDPE）and polypropylene（PP）were selected as the matrices,and graphite powders（Gr）,vanadium dioxide（VO₂）and flake graphite（NFG）were used as main filler particles,the doped particles and contrastive filler particles,respectively.The conductive composites with positive temperature coefficient effect were prepared by melt blending.And then,the influences of the single and double matrices,types of the conductive particles and vanadium dioxide particles on the positive temperature coefficient behavior of the composites were investigated.The main works and conclusions were as follows:（1）The HDPE/Gr/VO₂ composites with HDPE as the matrix,Gr as the main filler particles and VO₂ as the second component were prepared by the melt blending and hot press forming methods.Differential scanning calorimetry（DSC）,resistance test,scanning electron microscopy（SEM）,polarized light microscopy（POM）and dynamic rheological testing were used to measure the properties of HDPE/Gr/VO₂ composites.The results showed that the room temperature resistivity of the system of HDPE/Gr/VO₂ decreased significantly with VO₂ increasing.The increase of VO₂ made the degree of concentration of Gr particles in the amorphous phase increase,which was beneficial to the formation of conductive chains.When VO₂ content was 6wt%,the temperature was close to the transition temperature of VO₂ and the melting temperature of HDPE,which were PTC turning points of HDPE/Gr/VO₂.This indicated that the addition of VO₂ could result in a double PTC effect in the absence of double matrices and lead to the largest PTC intensity.This suggested that the introduction of VO₂ was the direct cause of the dual PTC effect.（2）The LDPE/PP/Gr/VO₂ composites were explored by adjusting the content of the second component VO₂ in the composite system.The experimental results showed that the higher the content of VO₂ in LDPE/PP（4/1）/Gr/VO₂ composites was,the lower the room temperature resistance was.VO₂ was preferentially distributed on the interface between LDPE and PP through the SEM analysis.However,Gr was partially distributed into the LDPE phase with the increase of VO₂,and the concentration of Gr in the LDPE phase increased,resulting in the conductive chains increasing and the room temperature resistance decreasing.The representation of the EDS graphs confirmed the distribution of Gr and VO₂.And the harmonic mean equation and the geometric mean equation further confirmed the distribution of particles in the theory.The results obtained from the X-ray diffraction（XRD）spectra showed that the addition of Gr particles and VO₂ hindered the growth of PP crystals and inhibited the crystallization of PP,leading to weak crystal peaks in the XRD pattern.The crystal phase change of the polymer directly affected the PTC behavior of the whole system.On the other hand,the dynamic mechanical（DMA）test of the composite material showed that with the increase of VO₂ content,the storage modulus of the system increased and the loss factor decreased.This was because more VO₂ adsorption on the substrate enhanced the force between the polymer chains and the particles to pass with improving the system stiffness.And some of the polymer segments may be interspersed within the VO₂ crystals.Thereby,the movement of the polymer chains was limited,and storage modulus increased.When the VO₂ content was 8wt% in the LDPE/PP（4/1）/Gr/VO₂ system,the PTC intensity was largest.（3）The resistance test of the HDPE/PP（1/1）/NFG-Gr and HDPE/PP（1/1）/NFG composite system was performed.The results showed that the room temperature resistance of the two systems was very close,and wasn’t affected by flake graphite（NFG）content.The PTC phenomenon was more obvious and the trend was similar.But the PTC transition temperature of HDPE/PP（1/1）/ NFG was delayed.The composite viscosity of the system was measured by a dynamic rheometer.The results showed that the composite viscosity trend and viscosity value of the two composites were similar.This indicated that the change of NFG content was little effects on the composite viscosity of the materials.The dynamic rheological behavior of the composite systems was tested.The experimental data showed that the storage modulus of HDPE/PP（1/1）/NFG system was larger and the loss factor was smaller.This was due to the high content of NFG in the HDPE/PP（1/1）/ NFG system.The aspect ratio of NFG was larger compared with that of Gr and the stronger interaction between the NFG and the matrices.And the polymer chains were easy to tangle.So more the conductive chains established by the NFG were not easy to break.So the need for a higher temperature was required in order to appear PTC phenomenon under the same conditions,that is,PTC turning temperature rised.The morphology and crystallization of the particles on the matrices were observed by the polarized light microscopy（POM）.The results displayed the crystal brightness of NFG in the HDPE/PP（1/1）/NFG system was lower than that of HDPE/PP（1/1）/NFG-Gr.It was confirmed that the adhesion between NFG and the matrices was stronger than that between Gr and the matrices.This further confirmed that the PTC transition temperature was higher in HDPE/PP（1/1）/NFG compared with that in HDPE/PP（1/1）/NFG-Gr.