【作者】 薄淮聚；

【导师】 于文学；

【作者基本信息】 吉林大学 ， 材料物理与化学， 2004， 硕士

【摘要】 近几十年来，导电高分子复合材料的研究取得了重大进展。将炭黑或金属等导电粒子分散在聚合物基体中形成的复合材料是一类重要的导电高分子材料。将导电粒子填充到聚合物基体中不仅可以提高复合材料的导电性，而且可以在很宽的温度范围（室温到基体材料的熔化温度）提高电阻值。对于某些半晶聚合物和炭黑组成的PTC（正温度系数效应）复合材料, 在约20℃的温度范围内电阻率就可增加几个数量级。高分子PTC材料在电流限流器、电磁波屏蔽、自控温加热及高温保护等方面具有广阔的应用前景，是一种很有发展前途的新型功能材料。在理论方面，虽然聚合物基PTC材料的导电机理已被研究了很长时间，但是对PTC现象和导电性仍不能获得满意的解释，一些导电机理还存在相互矛盾的现象。因此，人们对聚合物基PTC复合材料导电性的研究尚需做更多的工作。 为了探索聚合物基PTC复合材料的导电性，我们将炭黑填充到聚偏氟乙烯及其共混物当中，利用广角X射线，热分析等实验手段，对复合材料在限制体积膨胀，高能射线等条件下的导电行为进行了观察和研究，并用Sinnons模型对该复合材料的电阻率进行了计算, 所得结果如下：对于聚偏氟乙烯/炭黑复合体系，将该材料被密封在陶瓷管里，在升温过程材料体积膨胀受限的状态下，发现聚偏<WP=78>氟乙烯的体积膨胀和材料的PTC特征存在某些相关性。辐射交联导致电阻率在高温下变化不敏感。 在相同炭黑含量时，结晶度高的材料比结晶度低的材料具有更高的PTC强度。对这些结果的分析表明，该复合体系的PTC效应并非单一导电机理起作用，而是受下面两个因素的共同控制。第一，在材料升温过程中，由于聚合物基体的体积膨胀导致炭黑粒子相互远离，引起导电通道不畅，电阻率随温度升高。 第二，炭黑粒子在升温过程中，电阻率的减小乃无定形区内的碳黑粒子向熔化的结晶区扩散和炭黑粒子附聚的结果。从聚偏氟乙烯/炭黑复合体系的导电行为来看，该材料的电流密度J和作用电压V之间的关系符合Sinnons模型（即复合物的电阻随作用电压减小， 特别是在临界电压时），电阻率的极小值出现在聚偏氟乙烯的玻璃化转变温度（Tg = -45℃）附近。 从实验结果中我们可以得出电子隧道效应是该复合物导电的一种重要机制。通过建立炭黑粒子在聚合物基体中的分散模型，用渝渗（percolation）理论和量子理论对该复合材料的电阻率随炭黑含量的变化进行了计算，表明理论值和实验值符合的很好。在室温或高温下对聚偏氟乙烯/炭黑复合物进行( 辐照，发现高能辐射引起聚偏氟乙烯/炭黑复合材料PTC强度降低、电阻率(温度环路曲线所包围的面积增大、室温电阻增加。高能辐射也可引起聚偏氟乙烯/炭黑复合材料在聚合物熔化后<WP=79>的NTC效应的减少或消除。这些变化主要归因于聚偏氟乙烯在辐照过程中产生的分子间的交联。将炭黑和聚乙烯、聚偏氟乙烯共混发现，该复合体系表现出双PTC效应。高能射线辐照使双PTC峰位移向低温。随辐照剂量的增加，双PTC峰间的NTC效应逐渐消除，使PTC双峰变成很宽的平台，这种现象对该材料在限流方面的应用具有重要的实际意义。此现象的物理原因归因于聚乙烯的辐射交链。更多还原

【Abstract】 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. <WP=81>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. <WP=82>(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. <WP=83>(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.更多还原