| Conductive polymer composites(CPCs) are prepared by embedding one or several kinds of conductive filler into the insulating polymer matrix. Since CPCs do not only have the advantage of low cost, easy to shape and easy to mass production, but also exhibit unique features due to the abrupt changes in electrical conductivity caused by thermal, organic liquid or external stress field, leading to wide applications in the fields of self-regulating heaters, overtemperature protection devices, sensors and so on. However, CPCs fabricated by traditional methods usually have high percolation threshold, which will result in some disadvantages for the composites, such as poor processability and poor mechanical properties. Hence, developing new methods to prepare CPCs with nice conductivity, low percolation threshold, fine processability and fine mechanical properties have currently drawn a quite wide attention.In the present work, electrically conductive carbon black(CB)/polyamide 6(PA6)/ high density polyethylene(HDPE) composite with a CB particles coated electrospun PA6 superfine microfibrils network was prepared by the application of electrospinning technology and hot compression. The percolation behavior and the variations of the resistivity of the CPCs to different thermal fields and organic solvents were studied in detail. The mechanisms of the interesting results were also explained by considering evolutions of the microstructures. The main results are as follows: 1. The fabrication of CB/PA6/HDPE composite and conductive properties of the CPC(1) Firstly, the PA6 membrane was prepared by electrospining. Then the conductive CB/PA6 network was fabricated by localizing CB particles on the surfaces of the electrospun PA6 microfibrils membrane through a dip-coating method. Finally, CB/PA6/HDPE CPC was prepared by embedding conductive CB/PA6 network into the HDPE matrix.(2) Based on the morphological control and the CB distribution design, a low percolation threshold 4.3 vol% was achieved. This value is much lower than that of CB/HDPE CPC which was prepared through the common method(ca. 8.5%).(3) Based on the general percolation law, the dimension of the conductive network, that is, the t value, can be estimated. For CB/PA6/HDPE composite in the present work, the t value was about 2.57, which indicated a deviation from the classical percolation theory. This strange phenomenon can be ascribed to the complex geometry of the conductive network and the tunneling conduction between adjacent fragmentized CB/PA6 networks. 2. Temperature-resistivity behaviors of CB/PA6/HDPE composite(1) Through tuning the isothermal treatment(IT) time, a strong positive temperature coefficient(PTC) intensity(IPTC) has been achieved. At the same time, the negative temperature coefficient(NTC) intensity(INTC) of the CPC has also become weak. Not only the large size of the conductive CB/PA6 networks, but also the position evolution of the scattered CB in the polymer matrix led to this interesting phenomenon.(2) The hightest IPTC occurrence temperature increased gradually with increasing the heating rate. This was mainly ascribed to the influence of heating rate on the melting point and the volume expansion of the polymer matrix. While for the IPTC, the value increased with the increasing of the heating rate first, it reached the biggest value at the heating rate of 2 °C/min, showing a peak, but then decreased gradually. This anomalous phenomenon of the CPC is strongly related to the competition between the thermal expansion and the thermal residual stresses of the polymer matrix.(3) When the top test temperature was set as 140 oC, the room-temperature resistivity after one heating-cooling run was more than 30 times higher than the initial room-temperature resistivity. For the top test temperature of 180 oC, room-temperature resistivity at the end of the cooling almost returned to the original value. As for 150 oC, the top test temperature between 140 oC and 180 oC, the final room-temperature resistivity was slightly higher than the original value. Differences in the evolution of the conductive network structure which resulted from the difference of the heating-cooling run and the viscosity of the CPC were responsible for the interesting phenomenon.(4) In conclusion, the temperature-resistivity behaviors of the CPC are dependent on the structure of conductive network and the test conditions. As for this study, different temperature-changing processes lead to different evolutions of the conductive network, and finally result in different temperature-resistivity behaviors. So this study is of guiding significance for the applications of the PTC materials, overcurrent, self-regulating heaters, etc. 3. Liquid sensing behaviors of CB/PA6/HDPE composite(1) Liquid sensing properties of the CPC towards xylene, cyclohexane, dichlormethane and ethylacetate during ten immersion-drying runs were investigated by detecting the variations of the electrical resistance. It was observed that the CPC had a good sensing selectivity. In addition, compared with the other three kinds of oganic liquids, the composite showed a high sensing responsivity in xylene. This is attributed to the Flory-Huggins interaction parameter12c.(2) After 72 h immersion treatment, the sensing responsivity to the four kinds of liquid as mentioned above was lower than before. This is because more stable conductive network was formed after long-time immersion.(3) In order to study the influence of temperature on liquid sensing behaviors, the responsivity versus time of the CPC at different temperatures in cyclohexane was studied. It is observed that the electrical resistance of the CPC increased with increasing the temperature. This is because solubility and diffusivity coefficients which affect the liquid sensing behaviors of the composite become larger at a higher temperature.(4) According to this study, it can be concluded that for CB/PA6/HDPE composite, the nature of the organic liquid, the length of immersion and the ambient temperature have an important influence on the liquid sensing behaviors. |
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