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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.

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 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.

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Tuning of the dielectric properties of PDMS–ZnO nanocomposites by the control of inter-particle distances and self-assembled NP-morphologies via polymer grafting.

We demonstrate a novel route to synthesise hydrophobic matrix-free composites of polymer-grafted graphene oxide (GO) showing isotropic or nematic alignment and shape-memory effects. For the first time, a cationic macroinitiator (MI) has been immobilised on anionic GO and subsequently grafted with hydrophobic polymer grafts. Dense grafts of PBA, PBMA and PMMA with a wide range of average graft lengths (MW: 1-440 kDa) were polymerised by surface-initiated controlled radical precipitation polymerisation from the statistical MI. The surface modification is designed similarly to bimodal graft systems, where the cationic MI generates nanoparticle repulsion, similar to dense short grafts, while the long grafts offer miscibility in non-polar environments and cohesion. The state-of-the-art dispersions of grafted GO were in the isotropic state. Transparent and translucent matrix-free GO-composites could be melt-processed directly using only grafted GO. After processing, birefringence due to nematic alignment of grafted GO was observed as a single giant Maltese cross, 3.4 cm across. Permeability models for composites containing aligned 2D-fillers were developed, which were compared with the experimental oxygen permeability data and found to be consistent with isotropic or nematic states. The storage modulus of the matrix-free GO-composites increased with GO content (50% increase at 0.67 wt%), while the significant increases in the thermal stability (up to 130 °C) and the glass transition temperature (up to 17 °C) were dependent on graft length. The tuneable matrix-free GO-composites with rapid thermo-responsive shape-memory effects are promising candidates for a vast range of applications, especially selective membranes and sensors.

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