The effect of the structural order of isotactic polypropylene containing magnetically aligned nickel particles on its electrical resistivity
Introduction
In recent years there has been a growing interest in conducting particle-filled composites, since, with the development of new electrical devices, the requirements for composite materials have diversified. These applications include electromagnetic interference shielding, transparent electrodes, solder replacement in surface mount technology and electrical interconnections [1], [2], [3], [4], [5]. Connections between the conducting particles are necessary in order that polymer composites become conducting. Generally, a particle volume fraction of more than 10% needs to be added in order to form percolating structures within a matrix [4], [6], [7], leading to an increase in the opacity, increase in weight, and deterioration in the mechanical properties.
In order to reduce the percolation threshold, some methods, such as the use of phase separated structures [8], [9], the addition of multiple particles [10], and so on, have been considered. Another method is magnetic field processing [2], [3], [5], [11], [12]. When a magnetic material such as Ni in particle form is introduced into the composite, the particles form columnar structures parallel to an applied magnetic field [13], resulting in the composite becoming conductive. In this case, only a small amount of particles need to be added, and it has been reported that the increase in weight and deterioration in the mechanical properties of such composites are suppressed, providing properties that enable them to be applied as more flexible transparent electrodes [2].
Many composite materials are based on crystalline polymers that have special electrical properties such as the positive-temperature-coefficient (PTC) effect on electrical resistivity [7], [11], [14], [15]. With the PTC effect the resistivity increases drastically with rising temperature. The effect takes place near the melting point of the base polymer and depends strongly on the chemical structure of the polymer matrix [7]. Therefore, it was believed that the PTC effect resulted from melting of the polymer matrix. However, the electrical conductivity of a composite with a crystalline polymer matrix depends on the crystallinity [16] and the PTC effect has also been observed below the melting point of the base polymer [16], [17], [18].
The details of the mechanism for the PTC effect have not yet been clarified from the perspective of the correlation between the high order structure of the polymer matrix and the electrical conduction phenomenon. One of the reasons it is difficult to understand the electrical conduction phenomenon is that the addition of many particles to make the composite material conducting prevents the structure from being examined. With a smaller amount of particles, examination of the structure is possible, even with an optical microscope [2], [5], [6].
In this paper, we report on the relationship between the high order structure of a polymer matrix composite containing magnetically aligned Ni particles and its electrical properties. The conductivity of this composite can be significantly increased with the addition of just a small amount of particles and we claim that controlling the high order structure of the polymer matrix is important in order to obtain a composite material with excellent electrical properties.
Section snippets
Materials
NOVATEC-PP MA03, provided by Japan Polypropylene Co. (Tokyo, Japan), was used as the composite matrix. This is an isotactic polypropylene (PP) and is a commonly used crystalline polymer. Its melt flow rate listed in the catalog is 25 g/10 min. The melting point of the as-received sample was 156 °C measured by DSC. Ni particles, with an average particle size of 2.5 μm in diameter, density of 8.91 g cm−3, and resistivity of 7 × 10−5 Ω cm, manufactured by Inco Ltd. (Toronto, Canada), were used as
Effect of the growth of PP spherulites on the conducting path
Fig. 2 shows polarized optical microscope images for the original PP and the PP composite containing Ni particles (PP/Ni). Clear shaped spherulites can be seen in the original PP. With increasing Ni content, the shape of the PP spherulites becomes unclear. The black areas in the images indicate aggregations of Ni particles and these increase with increasing Ni content. It seems that short conducting paths have formed around the PP spherulites; however, these paths were insufficient to make the
Conclusion
Differences in the crystallization process due to the addition of a nucleating agent resulted in changes to the columnar structure formed in Ni containing composites as a result of heating while in a magnetic field. This gave rise to large changes in the resistivity of the composite material, even though the columnar structures formed in the PP melt should be identical. Controlling the high order structure of the polymer matrix including the morphology is very important in order to be able to
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