Martin Wåhlander

Novel Nanocomposites of Poly(lauryl methacrylate)-Grafted Al2O3 Nanoparticles in LDPE.

Aluminum oxide nanoparticles (NPs) were surface-modified by poly(lauryl methacrylate) (PLMA) using surface-initiated atom-transfer radical polymerization (SI-ATRP) of lauryl methacrylate. Nanocomposites were obtained by mixing the grafted NPs in a low-density polyethylene (LDPE) matrix in different ratios. First, the NPs were silanized with different aminosilanes, (3-aminopropyl)triethoxysilane, and 3-aminopropyl(diethoxy)methylsilane (APDMS). Subsequently, α-BiB, an initiator for SI-ATRP, was attached to the amino groups, showing higher immobilization ratios for APDMS and confirming that fewer self-condensation reactions between silanes took place. In a third step SI-ATRP of LMA at different times was performed to render PLMA-grafted NPs (NP-PLMAs), showing good control of the polymerization. Reactions were conducted for 20 to 60 min, obtaining a range of molecular weights between 23 000 and 83 000 g/mol, as confirmed by size-exclusion chromoatography of the cleaved grafts. Nanocomposites of NP-PLMAs at low loadings in LDPE were prepared by extrusion. At low loadings, 0.5 wt % of inorganic content, the second yield point, storage, and loss moduli increased significantly, suggesting an improved interphase as an effect of the PLMA grafts. These observations were also confirmed by an increase in transparency of the nanocomposite films. At higher loadings, 1 wt % of inorganics, the increasing amount of PLMA gave rise to the formation of small aggregates, which may explain the loss of mechanical properties. Finally, dielectric measurements were performed, showing a decrease in tan δ values for LDPE-NP-PLMAs, as compared to the nanocomposites containing unmodified NP, thus indicating an improved interphase between the NPs and LDPE.

Superhydrophobicity: Cavity growth and wetting transition

We show by using AFM colloidal probe microscopy (combinations of hydrophobic/superhydrophobic as probe/surface) that superhydrophobicity displays a set of specific events when compared with hydrophobicity. Both attraction (due to capillary and wetting forces) and repulsion (most likely due to repelling air/vapor layers or micro-/nanobubbles) occur upon approach and when surfaces are pulled apart both shorter range (50-100 nm or more) and longer range (several micrometers) attractive forces are displayed. The interaction is explained by forces generated through the formation of air and water vapor cavities, in the shorter-range (>50 nm) case maintaining a constant volume of the cavity, in agreement with calculation of capillary forces, and in the longer-range (>1 μm) case through access of air to the cavity, in agreement with thermodynamics of cavity growth. An added sodium dodecyl sulphate surfactant gave a partially reversible wetting transition and reduced the longer-range interaction to shorter-range, suggesting a transfer from the Cassie-Baxter to the Wenzel wetting regime. The findings would be of interest in development of practical applications, such as for anti-soiling, anti-icing, protection of electrical components and for extreme water-repellency in paper and textiles.

Tailoring dielectric properties using designed polymer-grafted ZnO nanoparticles in silicone rubber

Polymer grafts were used to tailor the interphases between ZnO nanoparticles (NPs) and silicone matrices. The final electrical properties of the nanocomposites were tuned by the grafted interphases, by controlling the inter-particle distance and the NP-morphology. The nanocomposites can be used in electrical applications where control of the resistivity is desired. Hansens solubility parameters were used to select a semi-compatible polymer for grafting to obtain anisotropic NP morphologies in silicone, and the grafted NPs self-assembled into various morphologies inside the silicone matrices. The morphologies in the semi-compatible nanocomposites could be tuned by steering the graft length of poly(n-butyl methacrylate) via entropic matrix-graft wetting using surface-initiated atom-transfer radical polymerization. Image analysis models were developed to calculate the radius of primary NPs, the fraction of aggregates, the dispersion, and the face-to-face distance of NPs. The dielectric properties of the nanocomposites were related to the morphology and the face-to-face distance of the NPs. The dielectric losses, above 100 Hz, for nanocomposites with grafted NPs were approximately one decade lower than those of pristine NPs. The isotropic nanocomposites increased the resistivity up to 100 times compared to that of neat silicone rubber, due to the trapping of charge carriers by the interphase of dispersed NPs and nanoclusters. On the other hand, the resistivity of anisotropic nanocomposites decreased 10–100 times when the inter-particle distance in continuous agglomerates was close to the hopping distance of charge carriers. The electrical breakdown strength increased for compatible isotropic nanocomposites, and the temperature dependence of the resistivity and the activation energy were ∼50% lower in the nanocomposites with grafted NPs. These flexible dielectric nanocomposites are promising candidates for low-loss high-voltage transmission cable accessories, mobile electronic devices, wearables and sensors.

Dielectric and physico-chemical properties of epoxy-mica insulation during thermoelectric aging

Three stator bars were subjected to 1142 hours of thermoelectric aging. At intervals during this time, the aging of the insulation was investigated by capacitance (C) and dissipation factor (DF) measurements and by the physicochemical techniques of infrared spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC). Results indicated different degradation paths under discharge conditions, where the volume of voids within the insulation played a major role. Postcuring reactions were predominant during the first aging cycles. Such reactions were detected as reductions in capacitance and dissipation factor parameters, as a depletion of the chemical group epoxide at the absorption band 909 cm-1 as measured by FTIR, and as an increase of the glass transition temperature (Tg) of the epoxy resin. Since the postcuring reactions occur globally in the insulation, they were detected by both dielectric and physicochemical techniques. With the advance of the aging program the epoxy resin underwent structural changes, seen as modifications of functional groups CH2, CH3 and C=O. The air atmosphere where partial discharges occurred also promoted the formation of reactive species. When the air volume was high enough, the findings of FTIR measurements suggested a local degradation mechanism of mica involving cation exchange reactions between partial discharge byproducts and potassium layers in the mica.

Reduced and Surface‐Modified Graphene Oxide with Nonlinear Resistivity

Field-grading materials (FGMs) are used to reduce the probability for electrical breakdowns in critical regions of electrical components and are therefore of great importance. Usually, FGMs are heavily filled (40 vol.%) with semi-conducting or conducting particles. Here, polymer-grafted reduced graphene oxide (rGO) is used as a filler to accomplish percolated networks at very low filling ratios (<2 vol.%) in a semi-crystalline polymer matrix: poly(ethylene-co-butyl acrylate) (EBA). Various simulation models are used to predict the percolation threshold and the flake-to-flake distances, to complement the experimental results. A substantial increase in thermal stability of rGO is observed after surface modification, either by silanization or subsequent polymerizations. The non-linear DC resistivity of neat and silanized rGO and its trapping of charge-carriers in semi-crystalline EBA are demonstrated for the first time. It is shown that the polymer-grafted rGO improve the dispersibility in the EBA-matrix and that the graft length controls the inter-flake distances (i.e. charge-carrier hopping distances). By the appropriate selection of graft lengths, both highly resistive materials at 10 kV mm-1 and FGMs with a large and distinct drop in resistivity (six decades) are obtained, followed by saturation. The nonlinear drop in resistivity is attributed to narrow inter-flake distance distributions of grafted rGO.

Comparison of oil-impregnated papers with SiO 2 and ZnO nanoparticles or high lignin content, for the effect of superimposed impulse voltage on AC surface PD

Surface discharge behavior of modified oil-impregnated paper (OIP) with nanoparticles (NPs), has been investigated under AC voltage with superimposed impulses. Surface Partial Discharges (PD) can develop at an oil-paper interface and lead to its degradation. Modified paper, made from fibers with adsorbed nanoparticles, can affect the partial discharge behavior of a paper in combination with oil at the interface between oil and fibers. Papers with two different concentrations (2 wt% and 6 wt%) of silica (SiO2), and paper with silanized zinc oxide (ZnO) nanoparticles (1 wt%) have been studied. Papers with SiO2 NPs showed lower impulse-induced surface PD activity. However, thorough purification during the production of SiO2 filled papers was necessary to achieve a good performance. With less purification, paper with 2 wt% of SiO2 did not show such significant improvements. Paper with 6 wt% of SiO2 NPs showed a large number of AC surface PDs, but low influence of impulse voltage on subsequent PD. Papers containing 1 wt% of silanized ZnO showed reduced relative permittivity, but no significant difference in surface PD behavior. The effect of high lignin content in Kraft paper has also been studied. Paper with higher lignin content showed better surface PD characteristics under the impulse. Paper with low concentrations of pure SiO2 NPs, and paper with high lignin content thus appear good candidates for further studies to improve the surface PD behavior of OIP.