The Conductivity Of CB-filled Poly (Vinylidene Fluoride) And Its Blend

Over the past decades, there has been substantial progress in conductive polymer composite materials. An important category of conductive polymer composites is obtained by the dispersal of conductive particles such as carbon black or metal in the polymer matrix. The process of mixing conductive filler with a polymer matrix may result not only in conductivity but also in an anomalous increase electric resistivity over a temperature range around the melting point of the matrix. For the composites of some simi-crystalline polymer and carbon black, a PTC (positive temperature coefficient) effect producing a resistivity increase of several orders of magnitude over a 20℃ temperature range has been demonstrated. Some important applications of polymer PTC composites include over current protection devices, self-regulating heaters and switching materials. In theory, the mechanism of electric current conduction in the polymer materials have been studies for a long time, but there is still not a satisfactory explanation for PTC effect and conductivity and some of conductive mechanism still remain controversial. In the present paper, The positive temperature coefficient effect and conductivity of carbon black-filled poly(vinylidene fluoride) and blend were studied. The conclusion are summarized as follows: (1) PTC effect of carbon black filled poly(vinylidene fluoride) composite and changes in resistivity under different condition were studies. It was found that there were some coherence between the volume expansion and the crystalline melting of poly(vinylidene fluoride). It is believed that the homogenization diffusion of carbon black particles results in increasing resistivity during volume expansion and crystallite melting. At high temperature, the resistivity decrease is due to agglomeration of carbon black particles or aggregates, which results in a new carbon black particles distribution of better conductivity. On the base of the experimental evidence, it is concluded that the PTC effect of poly(vinylidene fluoride)/carbon black composite is dominated by the volume expansion of composite matrix, the diffusion of carbon black particles from amorphous region to melted crystalline region and the agglomeration of carbon black particles. (2) The conductivity mechanism of carbon black filled poly(vinylidene fluoride) composite was also investigated in this work. From the experimental results obtained, it can be seen that the relation between electrical current density (J) and applies voltage across the sample (V) coincides with Sinnos's equation (i.e. the electrical resistivity of the composite decreases with the applies voltage, especially at the critical voltage) .The minimum electrical resistivity occurs near the glass transition temperature (Tg) of poly(vinylidene fluoride). It can be concluded that electron tunneling is an important mechanism and a dominant transport process in the CB/PVDF composite .A new model of carbon black dispersion in the matrix was established, and the resistivity was calculated by using percolation and quantum theory. (3) (-ray radiation causes PTC intensity decreasing and the area of the resistivity(temperature curve loop increasing for the CB/PVDF composites irradiated in room temperature and high temperature. The resistity in room temperature increase and NTC effect decrease compared with the unirradated composite . These changes are attributed to macromolecular crosslinking in the polymer matrix under high energy ray. The blend of polyethylene (poly(vinylidene fluoride) (carbon black exhibits a double PTC effect. The NTC effect after PTC for polyethylene decreases with increasing dose, and no NTC effect was investigated at 300 KGY. Therefore, the PTC effect in a wide temperature range was established, which has an important practical significance for the applications in over current protection and self-regulating heating.