Four sets of random propylene-based copolymers with 1−10 mol % of ethylene, 1-butene, 1-hexene, or 1-octene as co-units, synthesized with the same metallocene catalyst, were investigated by differential scanning calorimetry and wide-angle X-ray scattering following rapid and isothermal crystallization. Parameters related to defect concentration, defect type, and microstructure and thermodynamic and kinetic factors were evaluated as to their role in developing the γ polymorph. The effect of the comonomer in enhancing the fractional content of the γ polymorph is akin to the role of defects in the homo-poly(propylene) chain. However, differences in the partitioning of the comonomer between the crystalline and noncrystalline regions leads to contents of the γ phase that differ among the copolymers at any given crystallization temperature. Qualitatively, these differences can be used to assess the degree to which a counit participates in the crystallite. The experimental results suggest that there is no discrimination of the defects that enter the crystal lattice (stereo, regio, ethylene, or butylene units) between the α or γ crystallites. The results with copolymers establish that the bases that lead to the formation of the γ polymorph are the same for homo-poly(propylene) and its copolymers.

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Space charge occurs in a dielectric material when the rate of charge accumulation is different from the rate of removal, which arises due to moving or trapped charges. Inevitably, the local electric field is increased at some point within the material, which then leads to faster degradation and premature failure. The determination of space charge behavior has seen wide implementation in characterizing novel dielectric materials, especially in connection with the newly emerging field of nanocomposites. In this paper, we report on an investigation into space charge dynamics in silica-based polyethylene nanocomposites. The various systems differed with respect to the amount of filler and its surface chemistry; the pulsed electro-acoustic (PEA) technique was used to evaluate the space charge distribution in each. Experimental results indicate that the incorporation of nanosilica into polyethylene results in a significant amount of homocharge development near both electrodes. With appropriate surface treatment of the nanofiller, homocharge formation was successfully suppressed, indicating less severe space charge development in the nanocomposite materials. The mechanisms leading to the observed space charge development and direct current (DC) breakdown properties of the nanocomposites are discussed.

The structure and chemistry of two electrical trees (designated Tree A and Tree B) grown in low density polyethylene have been studied by a combination of confocal Raman microprobe spectroscopy, optical microscopy and scanning electron microscopy. Despite being grown under similar conditions (A, 30 °C and 13.5 kV; B, 20 °C and 13.5 kV), these two trees exhibit very different structures. Tree A exhibits a branched structure while Tree B is more bush-like. In Tree A, the very tips of the structure are made up of hollow tubules, which exhibit just the Raman signature of polyethylene. On moving towards the high voltage needle electrode, fluorescent decomposition products are first detected which, subsequently, are replaced by disordered graphitic carbon. From the relative intensity of the graphitic sp2 G and D Raman bands, the constituent graphitic domains are estimated to be ∼4 nm in size, which leads to a local tree channel resistance per unit length of 1–10 Ω µm−1. These structures are therefore sufficiently conducting to prevent local electrical discharge activity. In Tree B, the observed fluorescence increases continuously from the growth tips to the needle. Here, the tree channels are not sufficiently conducting to prevent electrical discharge activity within the body of the tree. These results are discussed in terms of mechanisms of tree growth and, in particular, the chemical processes involved.

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Abstract-A series of polyethylene-based nanocomposites was prepared, utilizing silicon nitride or silicon dioxide (silica) nanopowders, and the effect of filler loading and conditioning (i.e. water content) on their morphology and electrical properties was examined. The addition of nano-silicon nitride led to systems that were free of obvious nanoparticle aggregates, whereas the nanosilica based systems showed evidence of aggregation up to the micrometer-scale. While the nano-silicon nitride composites remained essentially dry under ambient conditions, the nanosilica-based composites absorbed appreciable quantities of water from the ambient environment, indicating that interactions with water are dependent on the nanoparticle surface chemistry. Dielectric spectroscopy showed a broad relaxation peak due to adsorbed water at nanoparticle surfaces, which shifted to higher frequencies with increased water content. Similarly, the electrical conductivity was found to be highly sensitive to the presence of absorbed water, particularly for systems containing well dispersed nanoparticles. We conclude that, in nanodielectric applications, nanoparticle surface chemistry is important in determining macroscopic properties, and not just as a means of compatibilizing the filler and the matrix. Additional factors can be critical, here, as exemplified by interactions with water.

The effect of moisture content on the dielectric properties of polymer/nano-silica blends was investigated. It was found that the DC breakdown strength, electrical conductivity and complex permittivity were all strongly influenced by absorbed water. However, a control sample without nano-silica was largely unaffected by changes in moisture content. This has important implications for researchers and cable designers.