Les nanocomposites polymères diélectriques ayant une constante diélectrique élevée et une faible perte diélectrique ont reçu un grand intérêt pour une utilisation dans le domaine du condensateur électrostatique. De manière générale, les performances diélectriques améliorées des nanocomposites sont déterminées par le type et la nature des polymères et des nanocharges sélectionnés, ainsi que par l'effet de couplage interfacial entre les matrices et les nanocharges. Parmi ces facteurs, les propriétés physiques, les géométries et les structures des composants des nanocharges jouent un rôle essentiel dans la détermination des performances diélectriques des nanocomposites. Selon les conductivités des nanocharges, les nanocomposites polymères diélectriques peuvent être classés en deux types: les nanocomposites polymères diélectriques conducteurs (CDPN) et les nanocomposites polymères diélectriques-diélectriques (DDPN). Cependant, la perte diélectrique élevée accompagnée au voisinage du seuil de percolation pour les CDPN et la charge élevée de nanocharges en céramique entravent le développement de nanocomposites polymères diélectriques à haute performance.Tout d'abord, des nanocomposites ternaires BNNS/CNT/PVDF ont été fabriqués. L'incorporation de BNNS dans les nanocomposites binaires CNT/PVDF a amélioré la dispersion des NTC et optimisé le réseau conducteur. La connexion directe entre les CNT pourrait être entravée en augmentant le contenu de BNNS.Deuxièmement, des hybrides CNT@AC à structure cœur-coquille ont été préparés par méthode CVD. La couche de carbone amorphe entrave non seulement le contact direct des NTC, mais améliore également la dispersibilité des NTC dans la matrice de PVDF. Le seuil de percolation augmente avec la prolongation du temps de dépôt du carbone. Plus important encore, la perte diélectrique a subi une forte diminution après le processus de revêtement. Troisièmement, les hybrides BNNSs@C avec des teneurs en carbone différentes ont été synthétisés par la méthode CVD. La fraction de carbone dans les hybrides BNNSs@C pourrait être ajustée avec précision en contrôlant le temps de dépôt de carbone. Les propriétés diélectriques des nanocomposites BNNSs@C/PVDF pourraient être ajustées avec précision en ajustant la teneur en carbone. Les polarisations interfaciales améliorées des interfaces BNNS/C et C/PVDF ont doté les nanocomposites de performances diélectriques améliorées.Quatrièmement, les hybrides TiO2@C NW structurés en noyau et en coquille ont été synthétisés par une combinaison d'une réaction hydrothermale et du procédé CVD. L'épaisseur de la couche de carbone dans les hybrides TiO2@C NW obtenus pourrait être précisément ajustée en contrôlant le temps de dépôt du carbone. De plus, les propriétés diélectriques des nanocomposites TiO2@C NWs/PVDF pourraient être ajustées avec précision en ajustant l'épaisseur de la coque en carbone. Les polarisations interfaciales améliorées des interfaces TiO2/C et C/PVDF ont doté les nanocomposites d'excellentes performances diélectriques.Enfin, des nanofils structurés de TiO2@C@SiO2 structurés à double coques ont été synthétisés par une combinaison de réactions hydrothermales modifiées, de CVD et de réactions sol-gel. L'introduction de carbone comme enveloppe interne entre le noyau de TiO2 et l'enveloppe externe de SiO2 a induit deux types supplémentaires de polarisation interfaciale. Les nanocomposites de PVDF obtenus avec TiO2@C@SiO2 NWs présentaient simultanément une constante diélectrique améliorée et des caractéristiques de perte diélectrique supprimées. La constante diélectrique et la perte de nanocomposites ont augmenté avec l'augmentation de l'épaisseur de la couche interne de carbone et ont diminué avec l'augmentation de l'épaisseur de la couche externe de SiO2. La relation entre la perte diélectrique et l'épaisseur de l'enveloppe extérieure en SiO2 a été démontrée par les résultats de la simulation finie. / Dielectric polymer nanocomposites with a high dielectric constant and low dielectric loss have received broad interest for use in the field of the electrostatic capacitor and they are usually composed of dielectric polymers as matrix and inorganic or organic nanofillers as the reinforcement. Generally, the improved dielectric performance of nanocomposites is decided by the type and nature of selected polymers and nanofillers as well as interfacial coupling effect between matrices and nanofillers. Among these factors, the physical properties, geometries, component structures of nanofillers play a critical role in deciding the dielectric performance of nanocomposites. According to the conductivities of nanofillers, the dielectric polymer nanocomposites can be classified into two types: conductive-dielectric polymer nanocomposites (CDPNs) and dielectric-dielectric polymer nanocomposites (DDPNs). However, the accompanied high dielectric loss in the vicinity of the percolation threshold for CDPNs and high loading of ceramic nanofillers hinders the development of high performance dielectric polymer nanocomposites.Firstly, ternary BNNSs/CNTs/PVDF nanocomposites were fabricated. The incorporation of BNNSs into the binary CNTs/PVDF nanocomposites improved the dispersion of CNTs and optimized the conductive network, which contributed to the enhanced dielectric constant. The direct connection between CNTs could be hindered by increasing the content of BNNS.Secondly, core-shell structured CNTs@AC hybrids were prepared by CVD method. The amorphous carbon layer not only hindered the direct contact of CNTs but also improved the dispersibility of CNTs in the PVDF matrix. The percolation threshold increased with the prolongation of carbon deposition time. More importantly, the dielectric loss underwent a sharp decrease after the coating process, which was attributed to the decrease in leakage current. The results suggested that the influence of AC interlayer on the final dielectric performance after percolation was much more obvious than that before percolation.Thirdly, BNNSs@C hybrids with different carbon contents were synthesized by the CVD method. The carbon fraction in the BNNSs@C hybrids could be accurately adjusted through controlling the carbon deposition time. The dielectric properties of BNNSs@C/PVDF nanocomposites could be accurately tuned by adjusting the carbon content. The improved interfacial polarizations of BNNSs/C and C/PVDF interfaces endowed the nanocomposites with enhanced dielectric performance.Fourthly, core-shell structured TiO2@C NW hybrids were synthesized by a combination of a hydrothermal reaction and the CVD method. The carbon shell thickness in the obtained TiO2@C NW hybrids could be precisely tuned by controlling the carbon deposition time. The TiO2@C NWs/PVDF nanocomposites exhibited a percolative dielectric behavior. Moreover, the dielectric properties of the TiO2@C NWs/PVDF nanocomposites could be accurately adjusted by tuning the carbon shell thickness. The enhanced interfacial polarizations of the TiO2/C and C/PVDF interfaces endowed the nanocomposites with excellent dielectric performance.Lastly, core@double-shells structured TiO2@C@SiO2 nanowires were synthesized by a combination of modified hydrothermal reaction, CVD, and sol-gel reaction. The introducing of carbon as an inner shell between the TiO2 core and SiO2 outer shell induced two additional types of interfacial polarization. The obtained PVDF nanocomposites with TiO2@C@SiO2 NWs exhibited simultaneously enhanced dielectric constant and suppressed dielectric loss characteristics. The dielectric constant and loss of nanocomposites increased with the increase of carbon inner shell thickness and decreased with the increasing of SiO2 outer shell thickness. The relationship between the dielectric loss and SiO2 outer shell thickness was further demonstrated by the finite simulation results.

Les nanocomposites polymères ont fait l'objet de recherches académiques et industrielles au cours des dernières décennies, du fait de leurs remarquables propriétés mécaniques et rhéologiques comparés aux polymères purs. En particulier, ils présentent du renforcement pour des fractions volumiques modérées, et des effets non linéaires pour des déformations relativement faibles. Malgré des décennies de recherche, la relation entre la rhéologie et la structure des nanocomposites est loin d'être comprise. Les simulations atomistiques peuvent donner une vision détaillée de l'interaction entre la dynamique des chaînes polymères et les charges renforçantes à une échelle locale. Cependant, il est difficile d'aborder les propriétés émergentes à une échelle mésoscopique, par exemple, simuler un grand nombre d'agrégats dans une matrice polymère enchevêtrée reste toujours hors de portée. Dans ce travail, nous proposons un modèle mésoscopique pour simuler la rhéologie des nanocomposites avec un fluide simple ou une matrice polymère enchevetrée, en utilisant la dynamique brownienne et la dynamique généralisée de Langevin, respectivement. Dans les deux dynamiques, le mouvement des chaines de polymère n'est pas décrit de façon explicite et son effet sur la dynamique de la charge est «moyenné». En utilisant ce modèle, nous étudions l'influence du type de charge, de leur taille, morphologie, et fraction volumique sur la rhéologie du composite modèle, ainsi que la morphologie des charges dans les simulations. Un cas particulièrement intéressant est celui d'agrégats quasi-fractals, qui peuvent être flexibles ou bien rigides. Nous démontrons que les systèmes avec agrégats présentent un renforcement significatif, qui augmente avec la taille des agrégats, leur rigidité, leur fraction volumique et leur polydispersité en taille. Une relaxation lente est également mise en évidence, et nous montrons qu'elle est liée à la rotation lente des agrégats. L'effet Payne, associé à la réponse non linéaire des modules dynamiques avec l'amplitude de déformation de cisaillement, est également observé pour nos modèles de composites. Nous faisons le lien entre l'arrangement microscopique des charges sous cisaillement et les propriétés macroscopiques du composite / Polymer nanocomposites have drawn a lot of attention both from the academic and industrial research in the last decades, thanks to their remarkable mechanical and rheological properties as compared to pure polymers. In particular, they may display reinforcement for moderate volume fractions, and several non linear effects that appear for small deformation amplitudes. In spite of decades of research, the relation between nanocomposites structure and rheology is far from being understood. Atomistic simulations can give a detailed view of the interplay between polymer chains dynamics and fillers at a local scale. However, it is much more difficult to address the properties emerging at a mesoscopic scale, for instance, to simulate a large number of aggregates in an entangled polymeric matrix remains out of reach. In this work, we build a mesoscopic model to simulate the rheology of polymer nanocomposites with a simple fluid and an entangled polymer matrix, by using the Brownian dynamics and the generalized Langevin dynamics, respectively. In both cases, the motion of the polymer chains is not explicitly described and its effect on the filler dynamics is "averaged out". Using this model, we quantitatively determine the influences of the filler type, the filler volume fraction, size and morphology on the rheology of the model composite. Of particular interest is the case of fractal-like aggregates, which may be flexible or rigid. We demonstrate that model aggregates display significant reinforcement, which increases with the aggregate size, aggregate rigidity, filler volume fraction and polydispersity. Long relaxation times are also evidenced, which are related to the slow rotation of the aggregates. The well-known Payne effect, associated to the nonlinear response of the dynamic moduli with the shear deformation amplitude, is also seen in our model composites. We relate the behavior of microscopic filler to the macroscopic properties of the composite

<p>This work reports the chemical synthesis, characterisation and electrochemical application of titanium dioxide (TiO2) and iron oxide (Fe2O3) nanoparticles in the determination of phthalates. The other part of this work involved electrochemical polymerization of aniline doped with titanium and iron oxide nanoparticles for the sensor platform in the electrolytic determination of phthalates. The TiO2 and Fe2O3 nanoparticles were prepared by sol gel and hydrothermal methods respectively. Particle sizes of 20 nm (TiO2) and 50 nm (Fe2O3) were estimated from transmission electron microscopy (TEM) The other technical methods used in this study for the characterization of the TiO2 and iron oxide Fe2O3 NPs were SEM, XRD and UV- visible spectroscopy. Cyclic voltammetry, square wave voltammetry and electrochemical impedance spectroscopy (EIS) were used to study the electrochemical properties of the nanoparticles. These electrochemical studies of the nanoparticles were performed with a Fe2O3 or TiO2/nafion/glassy carbon membrane electrode in 0.1 M phosphate buffer (pH 7.0) and 0.1 M lithium perchlorate (pH 6.8) under an aerobic condition.</p>

The thesis describes the study of structure and dynamics of polymers in confined geometry. We study the finite size effect on the dynamics of non glassy and glassy polymers. Systematic measurement have been performed to address the issue of the possibility of entanglement and hence reptation dynamics of the polymer segments in confinement. The confinement effect on the glassy dynamics has been studied for Langmuir monolayers as well as for polymer nanoparticle hybrid systems. Slow and heterogeneous dynamics are the underlined observed behavior for dynamics in hybrid systems. The available theories explains the slowing down of the dynamics as the system is cooled from the liquid state in terms of increasing cooperative motion of the molecules. The size of the cooperative region is predicted to grow with reducing temperature. Experiments, theories and simulation in confined dimensions have been motivated to detect this length scale of the cooperatively rearranging region. The surface and interface effects on glass transition were studied using measurements based on modulated differential scanning calorimetry and small angle X ray scattering techniques. The dynamical heterogeneity in glassy polymers were studied using advanced X ray photon correlation spectroscopy techniques. Our studies presented in this thesis are also an small step to contribute to the existing experimental results on studying the surface, interface and finite size effects on the morphology and dynamics of confined systems. These effects were studied for, firstly ultra thin Langmuir monolayers and secondly polymer nanoparticle hybrid systems. In Chapter 1, we provide the theoretical background along with brief review of the literature for understanding the results presented in this thesis. The details of the experimental set up and their operating principle along with the details of the experimental conditions are provided in Chapter 2. In Chapter 3 we presents our experimental results on surface morphology and surface dynamics in ultra thin Langmuir monolayer of polymers. Chapter 4 and Chapter 5 discusses the result based on polymer nanoparticle hybrid systems. We provide the summary of our result and the future prospective of the work in Chapter 6. In appendix we have shown the complete derivation of the equation used in Chapter 3 for understanding the surface morphology of Langmuir monoalyers on water surface. Chapter 1 provides in detail the introduction to several aspects related with the dynamics of both glassy and non glassy polymers in confinement. It starts with brief introduction to structure and dynamics of polymers in bulk. In the next section we discuss the macroscopic viscoelastic behavior of materials followed by a very brief discussion on the common techniques used for such measurement. Further it discusses the theory and several available models present in literature to understand the dynamics of glass transition. This section is followed by discussion on surface and interface effects on structure and dynamics of such systems in confinement. Towards the end of this chapter we discuss the universal behavior of slow dynamic observed in soft glassy materials. Chapter 2 contains the details of the experimental techniques which has been used for the study. Brief introduction to basic principles of the measurements followed by details of the material and methods have been provided. The surface morphology and dynamics of Langmuir monolayer of polymers confined at air water interface, under compressive mechanical strain has been discussed in Chapter 3. The results presented for surface morphology are based on the studies using the combination of in situ grazing angle incidence small angle X ray scattering and ex situ atomic force microscopy measurements on monolayers transfered on silicon substrate. The issue of the presence of reptation motion in confinement has been addressed by performing systematic measurements as a function of surface concentration and molecular weight at fixed temperature. The glassy dynamical behavior has been studied on different glassy polymer layer as a function of surface concentration and temperature. In Chapter 4 we show the glass transition behavior of polymer nanoparticle (PMMA gold) hybrid system based on thermal measurements. This chapter discusses the role of the existence of a length scale in deciding the dynamics of the glass transition temperature of polymers. The confinement effect was tuned by the variation of the inter particle spacing between the nanoparticles in the polymer matrix. It also discusses the model to understand the observed behavior of the glass transition temperature in terms of the tunability of the polymer particle interface and the effect of the interface morphology on the dynamics of glass transition temperature. Chapter 5 is about the study of dynamics of polymer nanocomposites near glass transition as a function of temperature, wave vector and volume fraction of gold nanoparticles using X ray photon correlation spectroscopy. Based on our experimental results , we provide a phase diagram for dynamics in 2D space of temperature, wave vector and volume fraction for our PMMA gold nanoparticle hybrid samples. Chapter 6 contains the summary and the future perspective of the work presented.

Small concentrations of graphene can significantly alter the phase behavior and the mechanical and electrical characteristics of polymeric materials. In this Masters thesis, we present results from a hierarchical simulation methodology that leads to the prediction of the thermodynamic, conformational, structural, dynamic and mechanical properties of polymer nanocomposites. As a model system, we have chosen syndiotactic poly(methyl methacrylate) or sPMMA reinforced with uniformly dispersed graphene sheets. How graphene functionalization affects the elastic constants of the resulting nanocomposite is also examined. The simulation strategy entails three steps: 1) Generation of an initial structure which is subjected to potential energy minimization and detailed molecular dynamics (MD) simulations at T=500K and P=1atm, to obtain well relaxed melt configurations of the nanocomposite and to extract any interested properties. Furthermore, for the sPMMA/graphene nanocomposite: 2) Gradual cooling of selected configurations down to room temperature to obtain a good number of structures representative of its glassy phase, and 3) Molecular mechanics (MM) calculations of its mechanical properties following the method originally proposed by Theodorou and Suter. The MD simulations have been executed with the LAMMPS code using the all-atom DREIDING force-field. By analyzing MD trajectories under constant temperature and pressure, all nanocomposite systems were found to exhibit slower terminal and segmental dynamics than the unfilled ones. The addition of a small fraction of graphene sheets in the polymer matrix led to the enhancement of its elastic constants especially when functionalized graphene sheets were used. / Μικρές συγκεντρώσεις γραφενίου μπορούν να τροποποιήσουν σημαντικά τη φασική συμπεριφορά και τα μηχανικά και ηλεκτρικά χαρακτηριστικά των πολυμερικών υλικών. Στη παρούσα εργασία παρουσιάζουμε αποτελέσματα από μία ιεραρχική μεθοδολογία προσομοίωσης που οδηγεί στη πρόβλεψη των θερμοδυναμικών, δομικών, δυναμικών και μηχανικών ιδιοτήτων πολυμερικών νανοσύνθετων υλικών. Σαν σύστημα μοντελοποίησης, επιλέξαμε τον συνδιοτακτικό πολυμεθακρυλικό μεθυλεστέρα, syndiotactic poly(methyl methacrylate), ή sPMMA, ενισχυμένο με ομοιόμορφα διεσπαρμένα φύλλα γραφενίου. Επίσης εξετάζεται και το πώς η χημική τροποποίηση του γραφενίου επηρεάζει τις ελαστικές ιδιότητες του νανοσύνθετου υλικού. Η στρατηγική της προσομοίωσης των συστημάτων συνοψίζεται σε τρία βήματα: 1) Δημιουργία αρχικών απεικονίσεων οι οποίες υποβάλλονται σε ελαχιστοποίηση της δυναμικής τους ενέργειας και στη συνέχεια σε λεπτομερείς προσομοιώσεις Μοριακής Δυναμικής (MD) σε T=500K και P=1atm, ώστε να εξαγάγουμε πλήρως χαλαρωμένες διαμορφώσεις τήγματος του νανοσύνθετου υλικού και να υπολογίσουμε ιδιότητες που μας ενδιαφέρουν. Επιπλέον για τα νανοσύνθετα υλικά sPMMA/γραφενίου συνεχίζουμε με 2) Σταδιακή ψύξη επιλεγμένων ατομιστικών διαμορφώσεων σε θερμοκρασία δωματίου με σκοπό την εξαγωγή ενός ικανοποιητικού αριθμού δομών, αντιπροσωπευτικών της υαλώδους φάσης τους, και 3) Εφαρμογή της μεθόδου της Μοριακής Μηχανικής (MM) για τον υπολογισμό των μηχανικών ιδιοτήτων τους ακολουθώντας τη μέθοδο που προτάθηκε από τους Θεοδώρου και Suter. Οι προσομοιώσεις της Μοριακής Δυναμικής πραγματοποιήθηκαν με τη χρήση του κώδικα LAMMPS, εφαρμόζοντας το DREIDING πεδίο-δυνάμεων και υιοθετόντας το μοντέλο των διακριτών ατόμων για την περιγραφή των ατομιστικών αλληλεπιδράσεων των συστημάτων. Αναλύοντας τις τροχιές των ατόμων από τις προσομοιώσεις της Μοριακής Δυναμικής υπό σταθερή θερμοκρασία και πίεση, τα υπό μελέτη νανοσύνθετα συστήματα βρέθηκαν να παρουσιάζουν βραδύτερη ολική και τοπική δυναμική σε σχέση με το καθαρό πολυμερές. Η προσθήκη μικρών κλασμάτων φύλλων γραφενίου στην πολυμερική μήτρα οδήγησε στην ενίσχυση των ελαστικών ιδιοτήτων της και σε μία περαιτέρω βελτίωση αυτών, όταν χρησιμοποιήθηκαν χημικώς τροποιημένα φύλλα γραφενίου.

<p>This work reports the chemical synthesis, characterisation and electrochemical application of titanium dioxide (TiO2) and iron oxide (Fe2O3) nanoparticles in the determination of phthalates. The other part of this work involved electrochemical polymerization of aniline doped with titanium and iron oxide nanoparticles for the sensor platform in the electrolytic determination of phthalates. The TiO2 and Fe2O3 nanoparticles were prepared by sol gel and hydrothermal methods respectively. Particle sizes of 20 nm (TiO2) and 50 nm (Fe2O3) were estimated from transmission electron microscopy (TEM) The other technical methods used in this study for the characterization of the TiO2 and iron oxide Fe2O3 NPs were SEM, XRD and UV- visible spectroscopy. Cyclic voltammetry, square wave voltammetry and electrochemical impedance spectroscopy (EIS) were used to study the electrochemical properties of the nanoparticles. These electrochemical studies of the nanoparticles were performed with a Fe2O3 or TiO2/nafion/glassy carbon membrane electrode in 0.1 M phosphate buffer (pH 7.0) and 0.1 M lithium perchlorate (pH 6.8) under an aerobic condition.</p>

Magister Scientiae - MSc / This work reports the chemical synthesis, characterisation and electrochemical application of titanium dioxide (TiO2) and iron oxide (Fe2O3) nanoparticles in the determination of phthalates. The other part of this work involved electrochemical polymerization of aniline doped with titanium and iron oxide nanoparticles for the sensor platform in the electrolytic determination of phthalates. The TiO2 and Fe2O3 nanoparticles were prepared by sol gel and hydrothermal methods respectively. Particle sizes of 20 nm (TiO2) and 50 nm (Fe2O3) were estimated from transmission electron microscopy (TEM) The other technical methods used in this study for the characterization of the TiO2 and iron oxide Fe2O3 NPs were SEM, XRD and UV- visible spectroscopy. Cyclic voltammetry, square wave voltammetry and electrochemical impedance spectroscopy (EIS) were used to study the electrochemical properties of the nanoparticles. These electrochemical studies of the nanoparticles were performed with a Fe2O3 or TiO2/nafion/glassy carbon membrane electrode in 0.1 M phosphate buffer (pH 7.0) and 0.1 M lithium perchlorate (pH 6.8) under an aerobic condition. / South Africa

Polymers are widely used as insulating materials in high voltage power apparatus because of their excellent electrical insulating properties and good thermomechanical behavior. However, under high electrical stress, polymeric materials can get deteriorated which can eventually lead to the failure of the insulation and thereby the power apparatus. Electrical treeing is one such phenomena whereby dendritic paths progressively grow from a region of high electrical stress and branch into conducting channels in a solid dielectric. The propagation of electrical trees is of particular interest for the power industry as it is one of the major causes of failure of high voltage insulation especially in high voltage cables, cast resin transformers as well as rotating machines. To improve the life time of the electrical insulation systems there is a need to improve the electrical treeing resistance of the insulating material for high voltage application. With the development of nanotechnology, polymer nanocomposites containing nano sized particles have drawn much attention as these materials are found to exhibit unique combinations of physical, mechanical and thermal properties that are advantageous as compared to the traditional polymers or their composites. Literature reveals that significant progress has been made with respect to the mechanical, optical, electronic and photonic properties of these functional materials. Some efforts have also been directed towards the study of dielectric/electrical insulation properties of these new types of materials. Considering the above facts, the present research work focuses on utilizing these new opportunities which have been opened up by the advent of nanocomposites to develop tree resistant insulating materials for high voltage power applications. Electrical treeing is a common failure mechanism in most of the polymeric insulation systems and hence electrical treeing studies have been carried out on two types of polymers (viz. polyethylene used in high voltage cable and epoxy used in rotating machines and resin cast transformers) along with three different types of nano-fillers, viz. Al2O3, SiO2 and MgO and with different filler loadings (0.1, 1, 3, 5 wt%). Furthermore, considering the fact that electrical treeing is a discharge phenomenon, the partial discharge characteristics during electrical tree growth in polymer nanocomposites was studied. As morphological changes in the polymer influence the electrical tree growth, the influence of nano-particle induced morphological changes on the electrical treeing has also been studied. Above all, an attempt has also been made to characterize and analyze the interaction dynamics at the interface regions in the polymer nanocomposite and the influence of these interface regions on the tree growth phenomena in polymer nanocomposites. A laboratory based nanocomposite processing method has been successfully designed and adopted to prepare the samples for treeing studies. Treeing experimental results show that there is a significant improvement in tree initiation time as well as tree inception voltage with nano-filler loading in polymer nanocomposites. It is observed that even with the addition of a small amount (0.1 and 1 % by weight) of nano-particles to epoxy results in the improvement of electrical treeing resistance as compared to the unfilled epoxy. In fact, different tree growth patterns were observed for the unfilled epoxy and epoxy nanocomposites. Surprisingly, even though there is not much improvement in tree inception time, a saturation tendency in tree growth with time was observed at higher filler loadings. To understand the influence of nano-particles on electrical treeing, the interaction dynamics in the epoxy nanocomposites were studied and it was shown that the nature of the bonding at the interface play an important role on the electrical tree growth in epoxy nanocomposites. The results of electrical treeing experiments in polyethylene nanocomposites obtained in this study also reveal some interesting findings. An improved performance of polyethylene against electrical treeing with the inclusion of nano-fillers is observed. It is observed that there is a significant improvement in the tree inception voltage even with low nano-filler loadings in polyethylene. Other interesting results such as change in tree growth pattern from branch to bush as well as slower tree growth with increase in filler loading were also observed. Another peculiar observation is that tree inception voltage increased with increase in filler loading upto a certain filler loadings (3 % by weight) and then decreased in its value at high filler loading. The morphology of polyethylene nanocomposites was studied and a good correlation between morphological changes and treeing results was observed. Effect of cross-linking on electrical treeing has also been studied and a better performance of cross-linking of nano-filled polyethylene samples as compared to the polyethylene samples without cross-linking was observed. The partial discharge (PD) activity during electrical tree growth was monitored and different PD characteristics for unfilled and nano-filled polyethylene samples were observed. Interestingly, a decrease in PD magnitude as well as the number of PD pulses with electrical tree growth in polyethylene nanocomposites was observed. It is known that PD activity depends on the tree channel conductivity, charge trapping and gas pressure inside the tree channel. The ingress of nano-particles into the tree channel influences the above known phenomena and affects the PD activity during electrical tree growth. The observed decrease in PD magnitude with increase in filler loading leads to the slow propagation of electrical trees in polyethylene nanocomposites. In summary, it can be concluded that polymer nanocomposites performed better against electrical treeing as compared to the unfilled and the conventional micron sized filled polymer composites. Even with low filler loading an improved electrical treeing resistance was observed in polymer nanocomposites. An optimum filler loading and a suitable filler to inhibit electrical treeing in the polymers studied are proposed. This work also establishes the fact that the characteristics of the interface region and the induced morphological changes have a strong influence on the electrical treeing behaviors of nanocomposites. These encouraging results showed that epoxy and polyethylene nanocomposites can be used as tree resistant insulating materials for high voltage applications. These results also contribute to widen the scope of applications of polymer nanocomposites in electrical power sector as well as development of multifunctional insulation systems.

The uniform dispersion of the nano fillers without agglomeration in a polymeric matrix is widely adapted for the purpose of mechanical properties enhancement. In the context to biomedical applications, the type and amount of nanoparticles can potentially influence the biocompatibility. In order to address these issues, High Density Polyethylene (HDPE) based composites reinforced with graphene oxide (GO) were prepared by melt mixing followed by compression moulding. In an attempt to tailor the dispersion and to improve the interfacial adhesion, polyethylene (PE) was immobilized onto GO sheets by nucleophilic addition-elimination reaction. A good combination of yield strength (ca. 20 MPa), elastic modulus (ca. 600 MPa) and an outstanding elongation at failure (ca. 70 %) were recorded with 3 wt % polyethylene grafted graphene oxide (PE-g-GO) reinforced HDPE composites. Considering the relevance of protein adsorption as a biophysical precursor to cell adhesion, the protein adsorption isotherms of bovine serum albumin (BSA) were determined to realize three times higher equilibrium constant (Keq) for PE-g-GO reinforced HDPE composites as compared to GO reinforced composites. In order to assess the cytocompatibility, osteoblast cells (MC3T3) were grown on HDPE/GO and HDPE/PE-g-GO composites, in vitro. The statistically significant increase in metabolically active cell was observed, irrespective of the substrate composition. Such observation indicated that HDPE with GO or PE-g-GO addition (upto 3 wt %) can be used as cell growth substrate. The extensive proliferation of cells with oriented growth pattern also supported the fact that tailored GO addition can support cellular functionality, in vitro. Taken together, the experimental results suggest that the PE-g-GO in HDPE can effectively be utilized to enhance both mechanical and cytocompatibility properties and can further be explored for potential biomedical applications.

Outdoor electrical insulator is one of the important components of a power system which directly influences the system reliability. Traditionally ceramic insulators have been used for close to a century in both transmission and distribution lines. In the last few decades, polymer based outdoor insulators are being increasingly used in the above application. Polymeric insulators offer attractive advantages such as light weight, resistance to vandalism and they also outperform conventional ceramic insulators under contaminated wet conditions at least in the initial stages of their usage. However, there are certain disadvantages with polymeric insulators which have made the utilities hesitant to replace readily the ceramic insulators with polymeric insulators. One of the major concerns with the polymeric insulators is the aging w.r.t time due to the presence of multiple environmental stresses (fog, humidity, temperature, rain as well as contamination due to industrial, sea and agricultural pollution) along with electrical stress. The manifestations of the aging of insulators include tracking or/and erosion of the weathersheds. Polymers in pure form (unfilled) can not perform satisfactorily all the required functions (electrical, mechanical, thermal etc.) of an insulator used in such high voltage transmission lines. Polymers have inherently poor thermal stability. Thermal stability directly influences the tracking and erosion resistance of the weathershed. Without adequate tracking and erosion resistance, polymeric insulators can not perform satisfactorily under contaminated wet conditions. Hence the common practice to improve the tracking and erosion resistance (and other properties such as mechanical, thermal) is by filling the base polymer with large loadings (> 30 wt %) of micron sized fillers. This makes the processing of the polymer composite difficult as the viscosity of the material rises substantially at such large loadings. Due to the large filler loadings beyond a certain limit, the flexibility of the end product also suffers. Though tracking and erosion resistance of the polymer has been improved substantially at these large filler loadings, the recent failures in the field suggest the need for an alternate material with higher tracking and erosion resistance than what is achieved at these large loadings of micron sized fillers. Of late nanocomposites are emerging as promising alternatives which can offer the above mentioned functionalities at low filler loadings itself without sacrificing the flexibility in the end product as well as ease of processing. There are even indications suggesting that the tracking and erosion resistance performance is better than what is obtained using micronsized fillers. As the development of nanocomposite dielectrics/insulation is still at its infancy, it is required to investigate their specific properties needed for outdoor applications and to understand the various mechanisms responsible for the interesting behaviour of the nanocomposites. Also, it is known that dc pollution performance of ceramic insulators is much inferior to the performance under ac stress. With the introduction of higher ac/dc transmission voltages in many countries including India, it is required to design insulators with better performing materials so as to get a reliable performance under polluted wet conditions. Due to the hydrophobic nature of the polymers, it is believed that polymers especially silicone rubber insulators can perform better as compared to the ceramic insulators under polluted conditions under ac and dc. As the dc tracking and erosion (T&E) resistance of polymer is poor compared to the ac tracking and erosion resistance, it is required to investigate the T&E resistance characteristics of the nanocomposites under dc stress. In addition, due to the enhanced electric fields at the line end of the insulators in extra and ultra high voltage transmission lines, there is always a possibility of corona generation on the hardware at the metal-sheath junction and at the water droplet tips on the weathersheds of the polymeric insulators especially under foul weather conditions. It is reported that the long-term exposure to such corona has the potential to degrade the polymeric material. The effects include reduction of the hydrophobicity, surface oxidation of the weathersheds and development of microcracks on the surface of the polymeric material. These cracks (corona cutting) can worsen the wet pollution performance of the insulator. If the cracks grow deeper, then FRP rod would get exposed to the atmospheric conditions leading to brittle fracture of the FRP rod and finally resulting in the line drop. Hence, the corona aging resistance of nanocomposites has also been studied especially at low filler concentrations to see its performance under the above mentioned adverse conditions. Therefore, the research work presented here deals with three aspects of the aging (1) Study the ac and dc tracking and erosion resistance performance of silicone rubber nanocomposites with low concentrations of fillers and their suitability for outdoor applications (2) Study the corona aging performance of silicone rubber nanocomposites with low concentrations of fillers and (3) To develop a model to explain the unusual behaviour of nanocomposites observed in the above studies. The thesis also reports results of the accelerated multistress weathering studies conducted on normal polymeric outdoor insulators under prolonged dry conditions. The major challenge in case of the polymer nanocomposite processing is getting uniform distribution of the fillers. A protocol has been standardised for the processing which comprises high shear mechanical mixing followed by sonication to get good dispersion of the fillers. Room Temperature Vulcanised (RTV) silicone rubber was successfully processed with different micron and nanosized fillers and with different weight (wt.) percentages in the present work. For carrying out the T & E resistance, corona aging and multistress aging studies, facilities (such as Inclined Plane T & E Resistance Test Apparatus in line with IEC/ASTM standards and aging chambers) have been designed and developed in house as a part of the thesis work. The ac tracking and erosion resistance performance of the unfilled, microcomposite (filled with alumina trihydrate filler of 5, 10, 15, 20 and 30 % by wt) and nanocomposite (filled with alumina, silica and magnesium hydroxide fillers of 2.5 and 4 % by wt) have been compared in inclined plane (IP) tracking and erosion resistance test facility specifically developed for the work. It was very interesting to observe that nanocomposites at 4 % performed on par with the microcomposites at 30 % filler loadings. Leakage current was also measured during the IP test and it was found that the form factor (ratio of r.m.s to average leakage current) was in good agreement with the variation in the erosion resistance of the silicone rubber composites and hence it can be used as a diagnostic tool for assessing the aging state of the polymeric materials. It was also observed that the performance under positive dc stress was much inferior to the performance under ac stress. The dissipation of power under dc stress was estimated by measuring the leakage current through the sample and is found to be about four times (towards the end of the test) higher as compared to the power dissipation under ac stress. Intense electrolytic corrosion has been observed (under positive dc) on the grounded electrode and on the sample and chemical studies of the same have been carried out. The poor performance under dc is due to the absence of the voltage zero crossing, more accumulation of the contaminant (scaling) and electrolytic corrosion. It was also observed that to get the same tracking and erosion resistance under dc as in the case of ac during IP test, dc stress levels have to be reduced to about 60 % of the ac stress. This information would be helpful to the design engineer of the outdoor insulators for the HVDC transmission lines. To understand the different mechanisms responsible in improving the tracking and erosion resistance of the micro and nanocomposites, thermal, SEM and FTIR studies have been carried out. Thermal stability of the samples was measured using thermogravimetric analysis (TGA) and differential thermo gravimetric (DTG) studies. It was observed that thermal stability of nanocomposites even at low filler loadings (4 wt %) was comparable with the microcomposites at higher filler loadings (30 wt %). SEM studies indicate that the barrier resistance (against discharges) offered by the fillers in the nanocomposites even at low filler loadings (4 %) could be comparable with the microcomposites at higher filler loadings (30 %). The interaction between the fillers and the host matrix has been studied using various techniques. SEM studies done on the eroded regions of the composites revealed that a honey comb type formation had taken place on the nanocomposites during the IP test which was believed to be due to the interaction of the filler and the polymer. This honey comb structure formation at the eroded site in the nanocomposites greatly helps to protect the sample from further damage due to the discharges. The interaction at the interface between the polymer and fillers could also lead to further improvement in the thermal stability of the nanocomposite. A model was proposed which considers barrier resistance and a single-layer interaction around the fillers to explain the improvements offered by the nanocomposites. Corona aging studies have been carried out on unfilled silicone rubber, micro and nanocomposites for 25 h and 50 h of aging using a needle-plane electrode arrangement. Different parameters such as hydrophobicity, surface roughness, microcracks width on the aged surface, FTIR and SEM studies were carried out to study the corona aging resistance of the new and aged samples. The studies indicate that silicone rubber samples containing nanofillers at 3 wt % are able to impart significantly enough corona resistance compared to the unfilled and microcomposite samples. It is known that the discharge resistance offered by the fillers and the interaction/bonding between the fillers and polymers directly influences the corona aging resistance. Hence, the model proposed (discussed above) is valid for understanding the corona aging performance of the nanocomposites which is better than the unfilled and ATH filled silicone rubber. In addition to the tracking and erosion resistance and corona aging studies, multistress aging of commercially available polymeric insulators containing micron sized fillers has been carried out. The aging behaviour of the polymeric insulators under tropical and subtropical conditions (in the absence of discharges under wet conditions) has not been explored. Further, the long-term influence of the UV radiation on silicone rubber in the presence of temperature and electric stress is also not explored. Hence, to understand the aging phenomena (weathering characteristics) under multistress (electric, thermal and UV), distribution class composite polymeric insulators were aged for 30,000 h in a multistress aging chamber developed specifically for the studies. Insulators were continuously subjected to the accelerated electric and thermal stresses as well as UV radiation. Different studies like leakage current, SEM, hydrophobicity, surface roughness and low molecular weight (LMW) molecules content in the samples before and after the aging have been investigated. It is interesting to observe that even in the absence of electrical discharges on the surface of the material, significant monotonous reduction in LMW molecules has been observed w.r.t weathering time. Appreciable increase in the surface roughness (at least 200 % as that of the new material) as well as increased oxygen levels on the surface has also been observed. The results indicate that surface hydrophobicity is dynamic in nature and may not reflect the slow and permanent changes taking place in the bulk of the material. The results obtained for the nanocomposites enable us to design a better material with improved tracking, erosion and corona resistance without sacrificing the flexibility in the end product as well as ease of processing. The silicone rubber nanocomposites also open up the possibility for economically designing a smart material possibly with a higher reliability for outdoor insulator application.