A. S. Vaughan

On the lamellar morphology of melt-crystallized isotactic polystyrene

Abstract The lamellar morphology of melt-crystallized isotactic polystyrene has been investigated by extending the technique of permanganic etching for electron microscopy. This paper reports on objects grown at 220°C which are comparatively uncomplicated, being aggregates of hexagonal lamellae, with smooth facets and distinguishable internal sectors, organized into axialites. To a first approximation only, hexagonal lamellae splay apart about a common diagonal presenting three characteristic projections in orthogonal directions. These are hexagonal, sheaflike and an array of approximately parallel lamellae. In reality the term axialite is an oversimplification. Splaying is not restricted to a single axis but occurs in three dimensions making these objects incipient spherulites. Polystyrene spherulites grown as low as 180°C the temperature of maximum growth rate are also constructed on the same principles as are spherulites of polyethylene and isotactic polypropylene. These are that a framework is established by individual dominant lamellae which branch and splay apart leaving interstices to be filled by later-crystallizing subsidiary lamellae. There is evidence that subsidiary lamellae contain shorter molecules on average than do dominant lamellae. The dominant/subsidiary construction is not what had been assumed in the Keith and Padden theory of spherulitic growth nor is there evidence of cellulation or local diffusion influencing development even though crystal sizes exceed the Keith and Padden parameter · by two orders of magnitude. We conclude that polymer spherulites can form independently of the mechanism proposed by Keith and Padden14. It is suggested that the cause of lamellae splaying apart is a mutual repulsion due to uncrystallized portions of molecules between dominant lamellae.

Structure-property relationships in polyethylene blends: the effect of morphology on electrical breakdown strength

Blends of linear and branched polyethylene were prepared covering the composition range 1–20% linear polyethylene, and three thermal treatments were subsequently chosen to produce a range of different morphologies. Isothermal crystallization at 124 °C gives rise to compact linear inclusions within a matrix of branched polyethylene, isothermal crystallization at 115 °C produces an open, banded spherulitic morphology and, finally, quenching leads to a continuous spherulitic texture. Ramp testing was then employed to investigate the effect of morphology on electrical strength. It was found that the electrical strength of the blend depends primarily on the morphology and that, by optimizing thermal treatment and linear polyethylene content, substantial improvements in properties can be obtained.

Dielectric properties of XLPE/Sio2 nanocomposites based on CIGRE WG D1.24 cooperative test results

A comprehensive experimental investigation of XLPE and its nanocomposite with fumed silica (SiO2) has been performed by CIGRE Working Group D1.24, in cooperative tests conducted by a number of members; covering materials characterization, real and imaginary permittivity, dc conductivity, space charge formation, dielectric breakdown strength, and partial discharge resistance. The research is unique, since all test samples were prepared by one source, and then evaluated by several expert members and their research organizations. The XLPE used for preparation of the nanocomposites was a standard commercial material used for extruded power cables. The improved XLPE samples, based on nanocomposite formulations with fumed silica, were prepared specifically for this study. Results of the different investigations are summarized in each section; conclusions are given. Overall, several important improvements over unfilled XLPE are confirmed, which augur well for future potential application in the field of extruded HV and EHV cables. Some differences/discrepancies in the data of participants are thought to be the result of instrumental and individual experimental technique differences.

On quantitative permanganic etching

Abstract Permanganic etching exposes lamellae within a variety of crystalline polymers. It is frequently observed that certain lamellar populations and other regions are removed preferentially by the etchant. This selectivity has been studied quantitatively by using 5 μm sections of a linear polyethylene that had been crystallized to form two populations, one of which contained lamellae roughly half as thick and molecules half as long as the other. Changes in the melting endotherm of sections with the time of etching and the related mass losses have been studied in relation to electron microscopy and molecular-mass data of polymer extracted from the sample with xylene. It was found that in sections cut at 20°C, the lower melting population suffered substantially greater deformation, but this could be limited by cutting at lower temperatures. The changes undergone by cold-cut sections were linear with etching time and revealed preferential removal of the thinner lamellae at a rate of 3.3 A s−1, compared to 1.6 A s−1 for the other population. It was also found that if sections were annealed at a temperature between the two melting peaks, a treatment giving similar populations to the original but with sectioning damage healed, then linear etching continued at a rate of 1.7 A s−1, but now with little or no discrimination between the two populations. It is concluded that when differential permanganic etching is observed it is not necessarily, at least in this case, an intrinsic property related to lamellar thickness or molecular mass, but reflects a secondary effect, namely the different responses of elements of physical texture to stresses imposed during sample preparation. Conversely these findings illustrate very well that the systematic spatial variation of texture imposed on a polymer by crystallization results in a corresponding systematic and local variation of properties.

Effect of additives on morphology and space charge accumulation in low density polyethylene

The effects of an antioxidant additive on polymer morphology and space charge formation have been investigated, as a function of temperature, in melt-quenched films of low-density polyethylene. On quenching, the additive-free polymer crystallizes to give extensive spherulitic structures; similar objects are not seen in the system containing the antioxidant. The pulsed electro-acoustic method was then used dynamically to follow both the formation of space charge at high voltage and its subsequent decay under short circuit conditions in both material systems. Data were obtained at room temperature, 50 and 70/spl deg/C. Comparing the additive free material with identical polymer containing a standard anti-oxidant package indicates that the addition of the antioxidant markedly changes both space charge formation and decay processes. At room temperature, both materials were found to behave similarly, suggesting that the changes in gross morphology described above are of secondary importance. At higher temperatures, the addition of the antioxidant enhances negative charge accumulation in the material, implying its association with relatively deep traps for the negative charges within the bulk.

On the structure and chemistry of electrical trees in polyethylene

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.

On the effects of morphology and molecular composition on the electrical strength of polyethylene blends

The effects of morphology and molecular composition on the electrical strength of blends of linear and branched polyethylenes were investigated. A range of blend systems were considered, in which both the molecular mass of the linear polymer and the comonomer in the branched component were varied. All the blends contained 10% linear polyethylene and 90% branched polymer and, in each system, three crystallization procedures were employed to modify the morphology. Isothermal crystallization at 124 °C generally resulted in compact linear inclusions within a branched matrix; isothermal crystallization at 115 °C produced a space-filling network of open, spherulitic structures; and quenching gave a banded spherulitic morphology. In these systems, the electrical strength, as measured by ramp testing, was dependent on the morphology of the material but was not influenced per se by significant changes in the molecular composition of the blend. The effect of crosslinking was also examined; the inclusion of a network did not, in itself, affect the breakdown strength or the morphology.

On the dielectric response of silica-based polyethylene nanocomposites

The dielectric response of silica-based polyethylene nanocomposites is studied by dielectric spectroscopy. The results indicate that nanocomposites absorb significantly more water than unfilled polyethylene, with the consequence that both permittivity and loss tangent increase with increasing duration of water immersion. However, appropriate surface treatment of nanosilica is found to reduce the water absorption effect and to modify the dielectric response of the nanocomposites compared with those containing untreated nanosilica. While water absorption may not be a technologically desirable characteristic, our results indicate that water molecules can act as effective dielectric probes of interfacial factors.

Structure property relationships in polyethylene/montmorillonite nanodielectrics

The influence of a montmorillonite (MMT) nanoclay, functionalized with dimethyl-di(hydrogenated tallow) quaternary amine, on structural evolution and electrical characteristics of a designed polyethylene system has been studied. Samples were prepared by mixing a polyethylene/MMT masterbatch into a matrix system containing 10% high density polyethylene and 90% low density polyethylene using an extruder; X-ray diffraction results suggest good dispersion and exfoliation, as no basal peak was observed. The introduction of this MMT system was found to result in little disruption of the polymer crystallization process and analysis of the crystallization kinetics of the matrix polymer suggests that it interacts only weakly with the incorporated MMT. This is in sharp contrast to our previous studies of a differently functionalized MMT system. Electrically, this combination of highly dispersed MMT within a weakly interacting polymer matrix results in a significant enhancement in short-term breakdown strength. However, this is accompanied by a massive increase in dielectric loss.

Nanodielectrics - How Much Do We Really Understand? [Feature Article]

This article aims to provide an introductory overview of the topic of nanocomposites, with a particular emphasis on their use in electrical applications in which their dielectric response is likely to be of importance. Composites are widely used engineering materials because of the enhancement in properties that can result from combining a number of distinct components. Reducing the size of the filler particles into the submicron range can overcome such problems and influence combinations of properties in a complex and often non-intuitive manner. It is this that has resulted in the rapid expansion of nanocomposites research in recent years.