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Functionalized nanoparticles have various applications, for which grafting of a chemical moiety onto the surface to induce/improve certain properties is needed. When incorporated in polymeric matrices, for instance, the modified nanoparticles can alter the interfacial characteristics leading to improvements ofthe macroscopic properties of the nanocomposites. The extent of these improvements is highly dependent on the thickness, morphology and conformity of the grafted layer. However, the common liquid-phase modification methods provide limited control over these factors. A novel gas-phase modification process was utilized, with 3-aminopropyltriethoxysilane (APTES) as precursor, to chemically deposit amino-terminated organic layers on fumed silica nanoparticles in a fluidized bed. A self-limiting surface saturation was achieved when the reaction was done at 200 °C. With this self-limiting feature, we were able to graft multiple layers of aminopropylsiloxane (APS) onto the silica nanoparticles using water as the coreactant. The feasibility of this process was analyzed using thermogravimetric analysis (TGA), diffuse reflectance IR Fourier transform spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), and elemental analysis (EA). By altering the number of APTES/water cycles, it was possible to control the thickness and conformity of the deposited aminopropylsiloxane layer. This novel approach allows to engineer the surface of nanoparticles, by introducing versatile functionalized layers in a controlled manner.

The effect of filler surface functionalization with 3-aminopropyltriethoxysilane (APTES) on the charge trapping and transport was studied in polypropylene (PP)/(ethylene-octene) copolymer (EOC)/silica nanodielectrics. Different reaction conditions were utilized for silica functionalization to alter the deposited layer morphology. This approach made it possible to engineer the filler–polymer interface to achieve optimized dielectric properties for the nanocomposites. The successful chemical modification of the silica surface was confirmed via thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Subsequently, the effect of the engineered filler–polymer interface on the nanocomposites’ crystallinity was analyzed with differential scanning calorimetry (DSC). Scanning electron microscopy (SEM) was utilized to observe the morphology of the nanocomposite as well as the silica dispersion. Finally, the effect of the silica functionalization on the dielectric properties of PP/EOC/silica nanocomposites was tested via thermally stimulated depolarization current (TSDC) and broadband dielectric spectroscopy (BDS). The results suggested that the presence of the amine functionality on the silica reduces interfacial losses in nanocomposites, and hinders further injection of space charge by introducing deep trap states at the filler–polymer interface. Under certain conditions, APTES can form an “island-like” morphology on the silica surface. These islands can facilitate nucleation, inducing transcrystallization at the filler–polymer interface. The island-like structures present on the silica would further contribute to the induction of deep traps at the filler–polymer interface resulting in the reduction of space charge injection. publishedVersion

New potential High Voltage Direct Current (HVDC) cable insulation materials based on nanocomposites are developed in this study. The nanocomposites are produced by blending of polypropylene (PP), propylene-ethylene copolymer (PP–HI) and a modified fumed silica (A-silica) in a concentration of 1 and 2 wt %. The A-silica is successfully modified with (3-aminopropyl)triethoxysilane (APTES) via a solvent-free method, as proven by infrared spectroscopy, thermogravimetry and transmission electron microscope mapping. A-silica in the polymer matrix acts as a nucleating agent resulting in an increase of the crystallization temperature of the polymers and a smaller crystal size. Moreover, the silica addition modified the crystals morphology of the unfilled PP/PP-HI blend. The composite containing A-silica with 2 wt% contains bigger-size silica clusters than the composite filled with 1 wt%. The composite with the higher A-silica concentration shows lower space charge accumulation and a lower charge current value. Besides, much deeper traps and lower trap density are observed in the composite with 2 wt% A-silica addition compared to the one with a lower concentration. Surprisingly, the presence of silica clusters with dimensions of more than 200 nm exhibit a positive effect on reducing the space charge accumulation. However, the real cause of this improvement might be due to change of the electron distribution stemming from the amine-amine hydrogen bond formation, or the change of the chain mobility due to the presence of occluded polymer macromolecules constrained inside the high structure silica clusters. Both phenomena may lead to a higher energetic barrier of charge de-trapping, thus increasing the depth of the charge traps. publishedVersion

This study compares different polymer-nanofiller blends concerning their suitability for application as insulating thermoplastic composites for High Voltage Direct Current (HVDC) cable application. Two polymer blends, PP/EOC (polypropylene/ethylene-octene copolymer) and PP/PP-HI (polypropylene/ propylene - ethylene copolymer) and their nanocomposites filled with 2 wt.% of fumed silica modified with 3-aminopropyltriethoxysilane were studied. Morphology, thermal stability, crystallization behavior dynamic relaxation, conductivity, charge trap distribution and space charge behavior were studied respectively. The results showed that the comprehensive performance of the PP/PP-HI composite is better than the one of the PP/EOC composite due to better polymer miscibility and flexibility, as well as lower charging current density and space charge accumulation. Nanosilica addition improves the thermal stability and dielectric properties of both polymer blends. The filler acts as nucleating agent increasing the crystallization temperature, but decreasing the degree of crystallinity. Dynamic mechanical analysis results revealed three polymer relaxation transitions: PP glass transition ( $\beta$ ), weak crystal reorientation ( $\alpha 1$ ) and melting ( $\alpha 2$ ). The nanosilica introduced deep traps in the polymer blends and suppressed space charge accumulation, but slightly increased the conductivity. A hypothesis for the correlation of charge trap distribution and polymer chain transition peaks is developed: In unfilled PP/EOC and PP/PP-HI matrices, charges are mostly located at the crystalline-amorphous interface, whereas in the filled PP/EOC/silica and PP/PP-HI /silica composites, charges are mostly located at the nanosilica-polymer interface. Overall, the PP/PP-HI (55/45) nanocomposite with 2 wt.% modified silica and 0.3 wt.% of antioxidants making it $a$ promising material for PP based HVDC cable insulation application with $a$ reduced space charge accumulation and good mechanical properties.

In this work five different kind of PE-based materials are presented and characterized: neat PE, PE with nano-silica particles, PE with AO in the amorphous phase, PE with AO-grafted nano-silica particles and the combination of the two latter. Initially, the chemical properties of the materials have been characterized by means of Oxidation Induction Time (OIT) and Fourier Transform InfraRed Spectroscopy (FTIR), in order to investigate their oxidability performances and their resulting chemical composition, respectively. Moreover, electrical performance of these materials has been analyzed through dielectric spectroscopy measurements in order to keep into account the actual suitability of the resulting material for electrical applications. In conclusion, materials with AO-grafted nanoparticles resulted to be convenient in terms of both chemical and electrical property evolution over time.

This study focuses on the influence of polar and unpolar surface functionalization of silica on the dielectric performance of PP/POE (polypropylene/poly(ethylene-co-octene)) nanocomposites. The silica fillers were surface-modified with unpoalr trimethylethoxysilane (TMES) or polar 3-aminopropyl triethoxysilane (APTES). Fourier-Transform Infrared Spectroscopy and Thermogravimetric analysis were performed and confirmed qualitatively and quantitatively the successful silica-silane modification. Silica/PP/POE nanocomposite films were prepared using a mini-extruder. Scanning Electron Microscope images showed good dispersion of the modified silica in the PP/POE matrix. However, the polar silica cluster size (300 nm) is slightly larger than the one of the unpolar silica (100 nm). Thermally Stimulated Depolarization Current (TSDC) characterization data showed that the polar silica introduced deeper charge traps than the unpolar one, which also showed higher trap density. The apparent conductivity measured during the TSDC poling phase indicated that the polar silica filled nanocomposites featured very fast polarization to reach the saturation state, while the unpolar silica had a rather slow polarization rate. From Pulsed Electroacoustic Analysis it was obvious that the space charge is suppressed by addition of the polar silica, but increased by the presence of the unpolar silica. acceptedVersion

A novel hybrid approach for surface modification of nano-silica for polyolefin-based dielectric composites is presented. A fumed nano-silica was modified with long oligomeric chains of a liquid rubber, connected via urethane bonds to the silica surface increasing the compatibility with the matrix. Additionally, a secondary amino-silane was grafted in order to alter the charge trapping properties of the silica. The long oligomeric chains provide good dispersibility of the nanosilica in polypropylene/poly(ethylene-co-octene) (PP/POE) blends, while the functional urethane group along with a secondary amino-silane affected the charge trapping properties of the nano-composites by reducing the charge traps density and simultaneously increasing their depth. Scanning Electron Microscopy along with Differential Scanning Calorimetry revealed that the silica is preferably located in the PP phase, most likely due to its lower viscosity in comparison to POE. The silica particles exhibit a nucleating effect visible in a noticeable increase of the crystallization temperature of the PP phase, especially when the silica surface treatment is performed. acceptedVersion

This paper focuses on novel insulation polypropylene/poly(ethylene-co-octene) (PP/POE) nanocomposites for High Voltage Direct Current (HVDC) cable application. The composites contain silica modified by a solvent-free method using silanes differing in polarity and functional moieties. Thermogravimetric Analysis and Fourier Transform Infrared Spectroscopy showed that the solvent-free method is an effective way to modify silica by silanes. Silica/PP/POE nanocomposites were prepared in a mini twin-screw compounder, and the effect of silica on crystallization, dispersibility and dielectric properties of the samples was investigated. Differential Scanning Calorimetry results showed that the unmodified and modified silicas acted as nucleation agents and increased the onset of the crystallization temperature of the polymeric matrix. Scanning Electron Microscopy images showed that the silica is mostly located in the PP phase matrix. For the PP/POE nanocomposites filled with unpolar silica, a higher trap density (measured by Thermally Stimulated Depolarization Current, TSDC) was found; this might be caused by the larger interfacial area due to a better dispersion of the unpolar silica in the polymeric matrix. Polar silicas introduce deeper traps than the unpolar ones, which is most likely due to the hetero-atom introduction. Nitrogen atoms were found to have the strongest effect on the charge trapping properties. According to these results, amine-modified silica is a promising candidate for PP/POE nanocomposites for HVDC cable applications. publishedVersion

This paper presents space charge accumulation characteristics of cable grade and capacitor grade nanostructured polypropylene (PP) materials, providing initial results regarding the electrical properties of nanofilled PP materials belonging to the European project GRIDABLE. This project has the aim to develop DC cable extruded insulation and MV and LV DC capacitor films having enhanced performance with respect to presently used materials. The paper shows that nanostructuration may be beneficial, especially at higher temperature, to improve the capacitor-grade material performance. On the other hand, this work validates the importance of surface functionalization of nanofillers, leading to lower space charge accumulation on DC cable-grade materials. acceptedVersion

The present study discusses that filler–filler mechanical engagement resulting from the grafted long-chain silanes on the silica surface is indeed a reinforcing mechanism in rubber composites, as already speculated by nonlinear viscoelastic properties in our previous study. The existence and severity of such a phenomenon are assessed purely by isolating the energetic contribution of reinforcement from interfering with filler mechanical engagement in the silica network formation and breakdown processes. In a novel approach, the driving force of fillers to flocculate energetically at elevated temperatures was defined using surface energy theories, and it was adjusted to be similar in two composites having silica treated by short- and long-chain silanes. Filler–filler mechanical engagement was monitored by tracking network formation (filler flocculation) in a matrix of styrene–butadiene rubber and also by conducting various dynamic viscoelastic experiments on liquid paraffin suspensions having short- and long-chain silica of similar surface energy. Results consistently confirmed the existence of mechanical engagement between silica particles having the long-chain silane in both rubber compounds and paraffin suspensions. The results may find applications in the rolling resistance of tires, for example, where stabilization of the filler network by displacing the peak energy dissipation of the network breakdown from applied service strains to larger values would be of technical importance.

This study presents a critical evaluation of the effect of two different sample manufacturing techniques on the morphological and dielectric properties of polypropylene (PP)-based nanocomposites, namely mini-scale injection molding (IM) vs. pilot-scale cast film extrusion. Polarized light microscopy revealed that the IM specimen morphology exhibited a layered "skin-core" type morphology, largely differing from the spherulitic morphology of the corresponding extruded cast films. Higher degree of crystallinity in the IM specimens was evidenced by calorimetric and X-ray diffraction methods. The processing-dependent morphological differences were found to affect the isothermal charging current (ICC) and thermally stimulated depolarization current (TSDC) characteristics due to differences in charge mobility and trapping, thus making direct comparison of IM and cast film specimens non-straightforward. Nevertheless, mini-scale injection molding can be seen as a resource-efficient sample manufacturing method for facilitating early-stage screening of the best-performing material candidates, given that the morphological features are carefully taken into account. I. INTRODUCTION The engineering dielectric properties of semi-crystalline polymers are known to be closely related with their crystalline structure and bulk morphological features. For instance, the processing-dependent morphology evolution at different scales of structures ranging from small (lamellar) to large (e.g. spherulitic) scale dictates the formation and dielectric properties of the crystalline-amorphous interface [1], thus making processing and specimen preparation non-trivial factors when evaluating and comparing dielectric performance of new materials [2]-[4]. PP based nanocomposite materials are currently being developed for high voltage direct current (HVDC) cable insulation applications in the EU project GRIDABLE, involving material optimization, compounding and sample manufacturing at several different batch sizes at VTT. Recently, mini-scale compounding and injection molding have been identified as a potential resource-efficient alternative for pilot-scale compounding and cast film extrusion to facilitate early-stage screening of the best-performing material candidates. This paper presents a critical comparison of the morphological and dielectric features of PP-based mini-injection molded and extruded cast film specimens, and evaluates the feasibility of the mini-injection molding technique for assessing the effect of nanosilica on the charge trapping and transport properties of PP based nanocomposites. acceptedVersion

Novel nanocomposites for dielectric applications-based polypropylene/poly(ethylene-co-octene) (PP/POE) blends filled with nano silica are developed in the framework of the European &lsquo, GRIDABLE&rsquo, project. A tailor-made low-pressure-plasma reactor was applied in this study for an organic surface modification of silica. Acetylene gas was used as the monomer for plasma polymerization in order to deposit a hydrocarbon layer onto the silica surface. The aim of this modification is to increase the compatibility between silica and the PP/POE blends matrix in order to improve the dispersion of the filler in the polymer matrix and to suppress the space charge accumulation by altering the charge trapping properties of these silica/PP/POE blends composites. The conditions for the deposition of the acetylene plasma-polymer onto the silica surface were optimized by analyzing the modification in terms of weight loss by thermogravimetry (TGA). X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray fluorescence spectroscopy (EDX) measurements confirmed the presence of hydrocarbon compounds on the silica surface after plasma modification. The acetylene plasma modified silica with the highest deposition level was selected to be incorporated into the PP/POE blends matrix. X-ray diffraction (XRD) showed that there is no new crystal phase formation in the PP/POE blends nanocomposites after addition of the acetylene plasma modified silica. Differential scanning calorimetry results (DSC) show two melting peaks and two crystallization peaks of the PP/POE blends nanocomposites corresponding to the PP and POE domains. The improved dispersion of the silica after acetylene plasma modification in the PP/POE blends matrix was shown by means of SEM&ndash, EDX mapping. Thermally stimulated depolarization current (TSDC) measurements confirm that addition of the acetylene plasma modified silica affects the charge trapping density and decreases the amount of injected charges into PP/POE blends nanocomposites. This work shows that acetylene plasma modification of the silica surface is a promising route to tune charge trapping properties of PP/POE blend-based nanocomposites.

A systematic study of the influence of surface modification of nanosilica with coupling agents having different polarities on the dielectric properties of a PP/POE/silica blend was performed. The main goal of this investigation was to tailor the chemical composition of the silica surface, in order to modify the charge trapping properties of the nanocomposites. For the modification of the silica surface, a “green” approach was utilized: a dry silanization method, which is performed without the need of a solvent. The results of the thermogravimetric analysis (TGA) indicate that the dry process is an effective method to perform silica surface modification using alkoxysilanes. The charge trapping properties were studied by Thermally Stimulated Depolarization Current (TSDC) measurements. The obtained TSDC results show significant differences in charge trapping properties of PP/POE composites filled with differently modified silicas. Polar functional groups attached to the surface of the silica appear to have a strong effect on the charge trapping properties: The trap depth distribution becomes deeper and the trap density decreases to significantly lower levels. All results show that incorporation of surface modified nanosilica into a PP/POE matrix is a promising approach to tailor its dielectric properties. Further development of these composites may lead to benefits for application in high-voltage cable and capacitor applications. Keywords: silica surface modification, dry silanization, charge trapping, PP, POE, TSDC, high voltage cable. publishedVersion

In this study an effective surface functionalization of fumed nano-silica for dielectric insulation composites is developed. A three step modification procedure was applied: 1. Silanization of the silica surface with (3-glycidoxypropyl)trimethoxysilane (GOPTMS); 2. Grafting of an acrylonitrile-butadiene telechelic oligomer terminated with two carboxylic acid groups (o-NBR); 3. Hydrogenation of the unsaturated polybutadiene segments for better compatibilization with the saturated polyolefin matrices and for higher dielectric performance.The deposition of the coupling system was monitored by thermogravimetric analysis (TGA) revealing 6.4 wt. % of GOPTMS and 13.6 wt. % of o-NBR bond to the silica, resulting in an exceptionally high total deposition (20.0 wt. %). The o-NBR grafting was analyzed qualitatively by Fourier transform infrared spectroscopy (FTIR) showing successful deposition of o-NBR and effective hydrogenation.The silica modified according to this procedure is a promising functional nano-filler for high-voltage insulation composites allowing improved dispersion and distribution in polyolefine matrices and therefore enhancing their electrical properties.

Various dielectric nanocomposite materials are studied in the frame of the European Commission funded project GRIDABLE. This project has the aim to develop DC cable extruded insulation and medium and low voltage DC capacitor films exhibiting enhanced performance with respect to presently used materials. The nanocomposites intended for cable applications are based on polypropylene (PP) blends filled with surface modified nano-silica particles. The surface modification is carried out via the state-of-the-art solution method using a polar silane as the modifying agent. Thermally Stimulated Depolarization Current (TSDC) measurements were carried out in order to study the charge trapping behavior of the nanocomposite samples. TSDC results indicate that the addition of the treated nano-silica, for most cases, reduces the density of the deep traps significantly. While more detailed studies are necessary, these results imply that the depth and the density of the deep trap states is profoundly influenced by the level of the silica modification i.e. the amount of the grafted silane on the silica surface. acceptedVersion

We evaluated the significance of mechanical engagement and energetic interaction between a polymer and a filler as two reinforcing mechanisms in SBR composites containing silica modified by short- and long-chain silanes. To exclude mechanical contributions of reinforcement from that of energetic contributions, surface energy of silica particles was systematically adjusted to prepare fillers of identical and diverse surface energies. Having analyzed interactions using a temperature sweep in a small-strain oscillatory test and a uniaxial tension test, results indicated that the chain length of the silane has remarkable influence on energetic filler–filler and filler–polymer interactions, but no detectable difference associated with filler–polymer mechanical engagement was observed from these experiments. However, dynamic strain sweep experiments showed that the rate of breakage of the filler network (Payne effect) is less for the composite having long-chain silane compared to that having short-chain silane. It was hypothesized that this behavior could be correlated to mechanical engagements of long-chain silanes existing on the filler structure.

Fumed silica-filled polypropylene (PP)-based nanodielectrics were studied in this work. To not only improve the dispersion of the silica but also introduce deep charge traps into the polymeric matrix, five types of modified silicas were manufactured with different surface modifications. The modified silica surfaces comprise an inner and a surface layer. The inner layer contains a polar urethane group for tailoring the charge trap properties of the PP/propylene–ethylene copolymer nanocomposites, whereas the surface layer consists of hydrocarbons (ethyl-, tert-butyl-, cyclopentyl-, phenyl-, or naphthalenyl moieties) in order to gain a good dispersion of the silica in the unpolar polymer blend. Scanning electron microscopic pictures proved that these tailored silicas show a much better dispersion than the unmodified one. Thermally stimulated depolarization current measurements revealed the ability of the silica to introduce deep charge traps with low trap density. The trap depth distribution depends on the type of the unpolar surface layer consisting of the different hydrocarbons. Among these five differently modified silicas, the introduction of the one with a surface layer consisting of tert-butyl moieties resulted in the lowest charge injection and the lowest charge current in the nanocomposite, proving good dielectric performance. Additionally, this silica exhibits good dispersion in the polymeric matrix, indicating a promising performance for nanodielectric application.