The focus of this study was to explore process-structure-property relationships in biodegradable polymer nanocomposite films in order to eliminate the commonly used trial and error approach to materials design and to enable manufacturing of composites with tailored properties for targeted applications. The nanofiller type and concentration, manufacturing method and compounding technique, as well as processing conditions were systematically altered in order to study the process-structure-property relationships. Polylactic acid (PLA) was used as the polymer and exfoliated graphite nanoplatelets (GNP), carbon nanotubes (CNT), and cellulose nanocrystals (CNC) were used as reinforcement. The nanocomposite films were fabricated using three different methods: 1) melt compounding and melt fiber spinning followed by compression molding, 2) solution mixing and solvent casting, and 3) solution mixing and electrospinning followed by compression molding. Furthermore, the physical properties of the polymer, namely the crystallization characteristics were altered by using two different cooling rates during compression molding. The electrical response of the composite films was examined using impedance spectroscopy and it was shown that by altering the physical properties of the insulating polymer matrix, increasing degree of crystallinity, the percolation threshold of the GNP/PLA films is significantly reduced. Additionally, design of experiments was used to examine the influence of nanofiller type (CNT versus GNP), nanofiller content, and processing conditions (cooling rate during compression molding) on the elastic modulus of the composite films and it was concluded that the cooling rate is the primary factor influencing the elastic modulus of both melt compounded CNT/PLA and GNP/PLA films. Furthermore, the effect of nanofiller geometry and compounding method was examined and it was shown that the high nanofiller aspect ratio in the CNT/PLA films led to decreased percolation threshold compared to the GNP/PLA films. The melt compounded GNP/PLA films displayed a lower percolation threshold than the solution cast GNP/PLA films most likely due to the more homogeneous distribution and dispersion of GNP in the solution cast films. Fully biodegradable and biorenewable nanocomposite films were fabricated and examined through the incorporation of CNC in PLA. Through the addition of CNC, the degree of crystallinity of the matrix was significantly increased. Focusing the design space through investigation of process-structure-property relationships in PLA nanocomposites, can help facilitate nanocomposites with tailored properties for targeted applications.

Functionalization of nanoparticles often control the extent of their usage. With this in mind, I have explored methods of creating highly functionalized exfoliated graphite, by way of the Billups-Birch reduction, that can be used in the advancement of nanotechnology (i.e. biomedicine). The method explored the use of sodium as the source for the solvated electron. The results of this method produced exfoliated graphite the same size as graphene and with solubility similar to the substrates attached to it. It was further shown that functionalized graphite with a terminal carboxyl group allowed further synthesis to occur via an elimination-addition reaction after the acyl group was transformed into an acid chloride. This reaction makes it possible to create exfoliated graphite that contains any compound of interest as long as it has an oxygen or nitrogen group that is able to react with the acid chloride. Thus, these products have the potential to be used in biomedicine as drug delivery agents.

Ph.D. / This study explored the possible application of a relatively new electrode called exfoliated graphite electrode (EG electrode) in the electroanalysis of organic and inorganic water pollutants. This study also explored the applicability of this electrode in the removal of heavy metal pollutants in water. The EG material was successfully prepared by intercalating bisulphate ions into graphite flakes (>300 μm) followed by thermal shock at 800°C to obtain an exfoliated graphite. After compressing the EG at high pressure, the obtained circular sheets were used to fabricate electrodes. The electrochemical profiles of EG electrode and glassy carbon electrodes (GCE) were recorded and compared using cyclic voltammetryand square wave voltammetry in the presence of various supporting electrolytes and [Fe(CN)6]3-/4-, [Ru(NH3)6]2+/3+, ferrocene redox probes. In the supporting electrolytes (KCl, H2SO4, NaOH, tetrabutylammoniumtetraflouroborate, phosphate buffers), the potential windows of EG were found, in some cases, to be about 300 mV larger than that of GCE. The diffusion coefficients (cm2s-1) of the redox probes were calculated to be 3.638 x 10-6, 1.213 x 10-6 and 4.411 x 10-6 for [Fe(CN)6]3-/4-, [Ru(NH3)6]2+/3+ and ferrocene, respectively. These values are comparable to those obtained from GCE. Furthermore, EG was modified with various nanomaterials such as poly (propylene imine) dendrimer (PPI), gold nanoparticles, silver nanoparticles, dendrimer–gold nanoparticles composite, cobalt oxide and bismuth. The morphologies of the modified electrodes were studied using scanning electron microscopy and their electrochemical reactivities in the three redox probes were investigated. The current and the reversibility of redox probes were enhanced in the presence of modifiers in different degrees with dendrimer and gold nanoparticles having a favourable edge. The electrochemical determination of o-nitrophenol by square wave voltammetry using a nanocomposite of generation 2 poly(propyleneimine) dendrimer and gold nanoparticles modified GCEand EG electrode were compared. A characteristic reduction peak between -600 mV and -700 mV for o-nitrophenol was observed with enhanced current on both GCE and EG modified electrodes.

It is unclear whether all intercalation techniques truly lead to the insertion of atoms between the graphite layers, or also lead to other effects which contribute to expansion. The objective of this project is to better understand the effects caused by different intercalation methods. Three intercalation methods were explored to determine the method which incurs the least damage to the surface and microstructure of the graphite intercalated compounds, yet achieves the best intercalation and therefore expansion. All the main findings are summarised below:  The gas phase sample had virtually no mass loss at the point where expansion took place. Therefore the intercalation was very efficient, producing large expansion without significant mass loss.  The mass loss that only occurs at the sublimation of iron chloride (320 ºC) indicates the excessive "un-intercalated" or residual iron chloride.  After oxidation, before purification, the gas phase sample has 25 % residual mass; this also proves the presence of impurities and residual iron chloride in the exfoliated sample. For the Hummers and electrochemical samples, expansion and mass loss occur over a wide temperature range, this indicates that graphite oxide was formed rather than the theoretically expected "insertion of atoms between the sheets".  The mass losses before 200 ˚C of the samples of the Hummers and electrochemical methods are more evidence that graphite oxide and graphite surface complexes with oxygen were produced.  The Hummers and electrochemical intercalation methods show similar expansion and mass loss curves, therefore it can be concluded that the reaction mechanism for both these methods is alike.  The gas phase method yields the best expansion of 250 % using the TMA, whereas both the other methods deliver approximately 220 %.  Using microwave expansion the electrochemical intercalation method yields the best bulk volume expansion of 1500 %, with the gas phase sample delivering a volume expansion of 1450 %. The Hummers samples are extremely damaged. This is clear from the several and deep oxidation pits visible throughout the basal plane of these samples. The basal plane and the edges are even eroded before purification and oxidation. This intercalation technique employs oxidisers in the preparation method which additionally oxidises the samples. This explains why the Hummers method renders the most damage. The residual material on the gas phase sample acts as catalysts making the sample very reactive and consequently damaging the surface during oxidation. The partially oxidised purified gas phase sample visibly shows the pits and roughened edges. There are two “types” of intercalation. The first intercalation “type” is the actual insertion of atoms or molecules between the graphite layers, whereas the other “type” of intercalation is the production of graphite oxide. The compound comprises carbon, oxygen and hydrogen, obtained by treating graphite with strong oxidisers. The functional groups usually found in graphite oxide are carbonyl (C=O), hydroxyl (-OH), phenol amongst others and also some impurities of sulphur when sulphuric acid is used. Both these intercalation types lead to expansion. It is recommended that a more efficient method for removal of residual material in the gas phase samples be explored. It is also recommended that more research be done to determine the reaction mechanisms during the three different intercalation methods. The graphite surface complexes of the intercalated compounds and the evolved gases during expansion should be analysed. / Dissertation (MEng)--University of Pretoria, 2015. / tm2015 / Chemical Engineering / MEng / Unrestricted

No description available.

L'impact des émissions fugitives sur l'environnement est un enjeu capital pour les industries. Ainsi, de nombreuses réglementations et normes sont établies afin de prévenir des risques et limiter leurs impacts. L'une des sources majeures de ces émissions provient de la robinetterie industrielle. Dans ce domaine, l'un des systèmes les plus communs est le presse-garniture. Dans le secteur de l'Energie, l'étanchéité peut être garantie sur les éléments mobiles par un empilement de bagues en Graphite Expansé Matricé (GEM); matériau relativement souple, plébiscité pour ses caractéristiques et ayant une structure macroscopique particulière. Ces travaux de thèse s'intéressent à la compréhension du comportement mécanique complexe des solutions d'étanchéité de type GEM. L'objectif est de déterminer le comportement mécanique de ces garnitures afin de prédire leur étanchéité. Dans le but de caractériser ce comportement, diverses campagnes d'essais sont menées sur des joints en situation. Ainsi, sur la base d'un montage existant et d'un dispositif développé spécifiquement pour cette étude, la caractérisation de ce type de joints est effectuée via des essais de compression charge-décharge et cyclique. La base de données qui en résulte permet le développement d'un modèle numérique apte à décrire le comportement complexe de ces systèmes presse-garnitures. Ce modèle phénoménologique de type Hyperélasto-Hystérésis (HH) correspond à une loi additive en contrainte avec une contribution hyperélastique et une contribution hystérétique. La comparaison des réponses expérimentales et numériques du joint, dans le cadre de l'étude d'un seul joint mais également pour des empilements, est discutée. / The impact of fugitive emissions leakage rate on the environment is an essential issue for industries. Therefore, many regulations and standards are created to prevent risks and to limit their impacts. One of these major sources of fugitive emissions comes from industrial valves. In the Valve Industrie, the tightness of stem are often ensured by stuffing box seal. In the energy sector, the tightness could be ensure with compression packing which is manufactured in exfoliated graphite; which is a relatively flexible material, endorsed for its characteristics and having a particular macroscopic structure. The aim of this study is to improve the knowledge of the complex mechanical behaviour of compression packing in exfoliated graphite. The aim is to determinate the mechanical behaviour of these packings in order to predict their tightness. To characterize this behaviour, various test campaigns are conducted on seals in situation. Thus, based on an existing apparatus and a device developed specifically for this study, the characterization of this kind of packing is performed through loading-unloading and cyclic compression tests. A numerical model which is able to describe a complexe mechanical behaviour of compression packing, is performed thanks to resulting database. This Hyperelasto-Hysteresis phenomenological model corresponds to an additive decomposition in stress with an hyperelastic contribution and an hysteretic contribution. The comparison of the experimental and numerical responses of the seal, as part of the study a single seal but also of a packing, is discussed.

One of the major directions of research in the area of materials science is to impart multifunctionalities to materials. Carbon stands on the top of the list to provide various multifunctional materials. It exists in all dimensions, zero (fullerene), one (carbon nanotube, CNT), two (graphene) and three (graphite) dimensions are very well-known for their versatility in various studies. They are also used in various applications in nanoelectronics, polymer composites, hydrogen production and storage, intercalation materials, drug delivery, sensing, catalysis, photovoltaics etc. Electrical conductivity of carbon can be tuned from insulator (diamond) to semiconductor (graphene) to conductor (graphite) with varying band gap. The main reason for this versatility and varied properties is that carbon can be involved in different hybridizations. Graphene, a single layer of graphite has fascinated the world during the last several years culminating in a Nobel prize for Physics in 2010. The present study is an attempt to understand the physicochemical and electrochemical properties of graphite oxide and its reduced form. Graphene oxide (GO) possesses oxygen containing functional groups such as carbonyl, carboxyl and epoxy groups distributed very randomly in the extended graphene sheet which makes it ionically conducting and electrically insulating. The AFM images of single layer of graphite (graphene) obtained from micromechanical cleavage method and that of EGO are shown in figure 1. EGO is a layered material similar to graphite and can form very stable aqueous colloids over a wide pH range of 2-11. The stability of the colloid is due to electrostatic repulsive interactions between the functional groups. EGO behaves like a molecule due to its thickness (~1 nm) and like a particle due to its two dimensional nature (lateral size can vary from nm to few microns). It behaves as amphiphilic molecule having both hydrophilic and hydrophobic nature. Figure 1d shows the STM image of EGO which clearly indicates oxidized and unoxidized regions which will impart hydrophilic and hydrophobic regions respectively. Figure 1: AFM image of (a) graphene (b) EGO. STM image of (c) HOPG and (d) EGO. The present work is related to exploring EGO as a multifunctional material. Both hydrophilic and amphiphilic nature is explored for various studies. Reduced GO (rGO) is synthesized from EGO by assembling at different interfaces (solid-liquid and liquid-air) followed by reduction. Since EGO is hydrophilic, it is brought to the air-water interface with the help of a surfactant (CTAB) through electrostatic interactions. It is reduced chemically by hydrazine vapour to rGO and electrochemically by assembling EGO on gold through electrostatic interactions between EGO and amine groups of cystamine (figure 2). The reduction process is followed by AFM, UV-Visible and in-situ Raman spectroelectrochemistry. Figure 2: Schematic of EGO self assembly, cyclic voltammogram showing electrochemical reduction and schematic for in-situ Raman spectroelectrochemistry. The next section deals with composites of EGO and polymers. EGO/polyaniline (PANI) composite is formed by electrochemical polymerization under applied surface pressure. The in-situ electrochemical polymerization of aniline in the sub-phase of Langmuir-Blodgett trough under applied surface pressure in presence of EGO at the air-water interface leads to preferential orientation of PANI in the polaronic form. This is followed by electrochemistry and Raman spectroscopy. Figure 3 shows differential pulse voltammograms of EGO/PANI obtained under two different conditions. Externally polymerized sample shows three redox peaks at 0.086/0.064 V (A/A‟), 0.390/0.430 V (B/B‟) and 0.520/0.560 mV (C/C‟) which correspond to leucoemaraldine/emaraldine, quinone/hydroquinone and emaraldine/pernigraniline redox states respectively. The peak at C/C‟ vanishes when aniline is polymerized in-trough under applied surface pressure. This implies that oxidation of emaraldine to pernigraniline becomes difficult when sample is prepared in-trough. The Raman spectroscopy clearly reveals the preferential orientation of PANI in planar polaronic structure. Figure 3. Differential pulse voltammograms for EGO/PANI complex obtained through external polymerization (black) and in-trough polymerization (red). In the next part, EGO is used as a proton conducting material for polymer electrolyte membrane fuel cell (PEMFC). EGO possesses hydrophilic and hydrophobic regions similar to nafion (sulfonated tetrafluoroethylene based fluoropolymer-copolymer) and hence it can act as a good ionically conducting membrane. EGO is incorporated in poly(vinyl alcohol) (PVA) matrix and used in the present studies. The ionic conductivity increases from 10 μS cm-1 to 370 μS cm-1 when EGO content is increased from 1wt% to 7wt% in PVA matrix. Power densities of 25 and 90 mW cm-2 are obtained for PVA and PVA/EGO membranes in H2-O2 fuel cell at 40 0C respectively. In the next section, EGO is used as receptor for simultaneous electrochemical detection of heavy metal ions such as Cd, Pb, Cu and Hg with detection limit of 5 μM, 1 pM, 5 μM and 5 μM respectively. During the process it is observed that the EGO/PbO composite can give rise to detection limit of 10 nM for arsenic. Along with detection, EGO can also be used as an effective adsorbent for inorganics (metal ions) as well as organics (dye molecules). EGO behaves as good adsorbent for heavy metal ions and cationic dyes and rGO adsorbs anionic dyes effectively. Spectroscopic techniques are used to understand the interactions between adsorbent and adsorbates. The thesis is presented as follows: Chapter 1 gives general introduction about graphene and graphite oxide with particular emphasis on the latter one. Chapter 2 gives details on the experimental methods followed, along with schematics for various adsorption processes. Chapter 3 focuses on assembling EGO at interfaces (solid-liquid and liquid-air) followed by reduction with chemical and electrochemical methods. Chapter 4 explores EGO as an amphiphilic material where EGO is assembled at air-water interface with anilinium and subsequent electropolymerization to EGO/PANI composites. EGO/PVA composite is used as electrolyte for PEMFC. Chapter 5 explores EGO as receptor for heavy metal ion detection (Cd, Pb, Cu and Hg). Chapter 6 deals with EGO as adsorbent for adsorption of inorganics (metal ions) as well as organics (dye molecules). This is followed by summary and conclusions. The appendix section gives details on the studies on preparation of exfoliated graphite with various metal ion intercalation. The covalent functionalization of EGO with metal phthalocyanines and its assembly at air-water interface forms second part of the appendix. (For figures pl see the abstract pdf file)

La plupart des équipements électroniques et électriques sont enrobés ou encapsulés par de la résine epoxy, choisie pour ses qualités physiques, chimiques et surtout diélectriques. Cependant, ce matériau présente un inconvénient majeur : sa faible conductivité thermique (0.2 W/mK). Dans ce contexte, nous avons élaboré et caractérisé des composites epoxy / carbone dans le but d’améliorer la conductivité thermique de ce type de résine tout en conservant ses propriétés diélectriques. Nous avons ainsi testé le potentiel d’une large gamme de charges carbonées, de structures, formes et tailles variées (sphères, tubes et plaquettes), telles que des micro-sphères de carbone et des nanotubes multi-parois synthétisées par CVD et PECVD, mais aussi des charges industrielles : nano-plaquettes de graphite (graphite exfolié), du coke de pétrole, du graphite synthétique et naturel. Des échantillons de matériaux composites massifs (50 x 50 x 4 mm) ont été préparés à partir d’une résine industrielle DGEBA de viscosité élevée 8.5-15 Pa.s, en faisant varier le taux de charge. Les propriétés thermiques des composites ont été mesurées à partir de la technique du hot disk (source plane instationnaire). Les meilleurs résultats ont été obtenus à partir des nano-plaquettes de graphite : les conductivités thermiques des composites ont atteint (0.55 W/mK) pour une charge admissible maximale de 2.67 vol.%. L’accroissement relatif de conductivité thermique a été de 66 % pour 1 vol.%. Cet accroissement est particulièrement élevé dans la mesure où les meilleurs résultats reportés sont de 20 % / vol.% dans le cas de résines à viscosité plus faible de type DGEBF (2.5 - 4.5 Pa.s). La concentration de charge admissible (1.3 vol.%) pour conserver une résistivité électrique suffisamment élevée (> 105 ohm.m) nous a permis d’atteindre une conductivité thermique de 0.37 W/mK (soit une augmentation de 100% par rapport à la résine initiale). Ces résultats sont interprétés en termes de transport des phonons acoustiques dans un système composite bi-phasique. Les nano-plaquettes de graphite sont caractérisées par une morpholigie anisotrope, d’ une surface d’environ 26 x 26 microns dont l’épaisseur est de l’ordre de 6 nm. Elles combinent une structure lamellaire périodique bien ordonnée dans les plans de graphène (caractérisation par XPS, EDX et DRX), et des rapports d’acicularité élevés ( 4300), estimés à partir de différentes techniques : TEM, SEM et BET. Nous montrons qu’accroitre l’acicularité des nano-plaquettes de graphite par exfoliation, en préservant une grande surface des plans de graphène, et sans générer de défauts de structure, constitue un défi. Cette morphologie 2D particulière permet d’une part de conserver voire augmenter la conductivité intrinsèque des charges, favorisée dans les plans de graphène, et d’autre part, en raison de leur grande surface spécifique, de garantir après leur dispersion dans la résine, un meilleur transport des phonons acoustiques dans le composite. / Most electronic and electrical equipment are coated or encapsulated by epoxy resin due to its physical, chemical and dielectric properties. However, this material has a major drawback: its low thermal conductivity ( 0.2 W / mK). In this context, we have developed and characterized epoxy / carbon composites in order to improve the thermal conductivity of this type of resin while maintaining its dielectric properties. We have tested the potential of a wide range of carbonaceous fillers, structures, shapes and sizes (spheres, tubes and plates), such as carbon micro-spheres and multi-walled carbon nanotubes synthesized by CVD and PECVD, but also industrial fillers: graphite nano-platelets (exfoliated graphite), petroleum coke, synthetic and natural graphite. Large composite samples (50 x 50 x 4 mm) were prepared from a DGEBA engineering resin of high viscosity 8.5-15 Pa.s, by varying the charge vol%. The thermal properties of the composites were measured from the transient plane source technique (hot disk). The best results are obtained from graphite nano-platelets: the thermal conductivity reach (0.55 W / mK) for a maximum load of 2.67 vol%.. The relative increase of thermal conductivity is 66% to 1 vol.%. This increase is particularly high to the extent that the best results reported so far is 20% / vol% for resins with lower viscosity, type DGEBF (2.5 - 4.5 Pa.s). The allowable concentration (1.3 vol.%) to maintain a sufficiently high electrical resistivity (> 105 ohm.m) permits to increase of the thermal conductivity to 100% (0.37 W / mK) compared to the initial resin. These results are interpreted in terms of transport of acoustic phonons in the composite two-phase system. Graphite nano-platelets are characterized by anisotropic shapes with a surface of about 26 x 26 microns whose thickness is of the order of 6 nm. They combine an ordered periodic structure in graphene planes (characterization by XPS, EDX and XRD), and a high aspect ratio ( 4300), estimated using various techniques: TEM, SEM and BET. We show that graphite exfoliation permit to increase the aspect ratio of graphite nanoplatelets, maintaining large micronic graphene surface, and without generating structural defects is a challenge. This peculiar 2D morphology allows on one hand, to retain or even increase the intrinsic filler conductivity, favored in the graphene planes, and on another hand, due to their high surface area, to ensure after their dispersion in the resin, a better transport of acoustic phonons through the composite.

Στην παρούσα εργασία αναπτύχθηκαν και μελετήθηκαν πειραματικά σειρές σύνθετων υλικών πολυμερικής μήτρας, με παράμετρο τον τύπο και την περιεκτικότητα σε ενισχυτική φάση. Ως μήτρα χρησιμοποιήθηκε εποξειδική ρητίνη υψηλών προδιαγραφών. Ως ενισχυτική φάση χρησιμοποιηθήκαν μικροσωματίδια, νανοσωματίδια τιτανικού βαρίου και αποφλοιωμένα γραφιτικά νανοεπίπεδα (exfoliated graphite nanoplatelets). Η επιλογή των υλικών είχε ως στόχο να εκμεταλλευτούν σε κοινό σύνθετο σύστημα οι «θετικές» ιδιότητες των συστατικών του, όπως η θερμο-μηχανική σταθερότητα της μήτρας, η υψηλή διαπερατότητα και η σιδηροηλεκτρική συμπεριφορά του τιτανικού βαρίου και οι καλές μηχανικές ιδιότητες μαζί με την υψηλή ειδική αγωγιμότητα των αποφλοιωμένων γραφιτικών νανοεπιπέδων. Παρασκευάστηκαν και μελετήθηκαν τα παρακάτω συστήματα σύνθετων υλικών, για διάφορες περιεκτικότητες σε ενισχυτική φάση: (α) σύστημα μικροσωματιδίων τιτανικού βαρίου/εποξειδικής ρητίνης, (β) σύστημα νανοσωματιδίων τιτανικού βαρίου/εποξειδικής ρητίνης, (γ) σύστημα αποφλοιωμένων γραφιτικών νανοεπιπέδων/εποξειδικής ρητίνης, (δ) υβριδικό σύστημα μικροσωματιδίων τιτανικού βαρίου/νανοσωματιδίων τιτανικού βαρίου/εποξειδικής ρητίνης, (ε) υβριδικό σύστημα αποφλοιωμένων γραφιτικών νανοεπιπέδων/ νανοσωματιδίων τιτανικού βαρίου/εποξειδικής ρητίνης. Την παρασκευή των δοκιμίων ακολούθησε πολύπλευρος χαρακτηρισμός τους. Για λόγους αναφοράς παρασκευάστηκε και μελετήθηκε και δοκίμιο μη ενισχυμένης ρητίνης. Η μορφολογία τους διερευνήθηκε με την τεχνική της ηλεκτρονικής μικροσκοπίας σάρωσης (scanning electron microscopy) και την τεχνική σκέδασης ακτίνων-Χ (x-ray diffraction scattering). Διαπιστώθηκε η επιτυχής διασπορά των νανο-εγκλεισμάτων αλλά και η ύπαρξη μικρών συσσωματωμάτων. Τα φάσματα σκέδασης ακτίνων-Χ πιστοποίησαν την παρουσία των πληρωτικών μέσων που χρησιμοποιήθηκαν σε κάθε κατηγορία σύνθετου συστήματος. Ακολούθησε θερμικός χαρακτηρισμός των σύνθετων υλικών, με στόχο τον προσδιορισμό της θερμοκρασίας υαλώδους μετάπτωσής τους. Η μελέτη της μηχανικής συμπεριφοράς των συνθέτων έγινε υπό στατικές και δυναμικές συνθήκες. Η στατική συμπεριφορά εξετάστηκε με την τεχνική κάμψης τριών σημείων σε θερμοκρασία περιβάλλοντος. Διαπιστώθηκε αύξηση του μέτρου ελαστικότητας με την περιεκτικότητα σε ενισχυτική φάση, σε όλες τις κατηγορίες σύνθετων συστημάτων. Παράλληλα, διαπιστώθηκε μείωση της μηχανικής αντοχής με τη συγκέντρωση πληρωτικού μέσου σε όλες τις κατηγορίες σύνθετων υλικών που μελετήθηκαν. Η δυναμική μηχανική απόκριση μελετήθηκε με την τεχνική της δυναμικής θερμικής ανάλυσης (dynamic mechanical thermal analysis) σε ευρύ φάσμα θερμοκρασιών. Τα ενισχυμένα συστήματα παρουσιάζουν αυξημένες τιμές του μέτρου αποθήκευσης, ενώ οι κορυφές της εφαπτομένης απωλειών επιτρέπουν τον προσδιορισμό της θερμοκρασίας υαλώδους μετάπτωσης (Tg). Η Tg φαίνεται να διαφοροποιείται ελαφρά με την περιεκτικότητα σε ενισχυτική φάση, άλλοτε προς μεγαλύτερες και άλλοτε προς μικρότερες τιμές. Οι διαφοροποιήσεις αυτές εκφράζουν τις αλληλεπιδράσεις μεταξύ των φάσεων και ίσως την πλήρη ή μη διαβροχή των εγκλεισμάτων από τη μήτρα. Η ηλεκτρική απόκριση των σύνθετων συστημάτων εξετάστηκε με τη μέθοδο της διηλεκτρικής φασματοσκοπίας ευρέως φάσματος, σε μεγάλο εύρος συχνοτήτων και θερμοκρασιών. Η ανάλυση των πειραματικών δεδομένων έγινε μέσω των φορμαλισμών της ηλεκτρικής διαπερατότητας, του ηλεκτρικού μέτρου και της ειδικής αγωγιμότητας εναλλασσομένου. Η χρήση και των τριών φορμαλισμών προσφέρει τη δυνατότητα εξαγωγής περισσότερων πληροφοριών για τις φυσικές διεργασίες που λαμβάνουν χώρα στο εσωτερικό των συνθέτων. Διαπιστώθηκε η παρουσία δύο διηλεκτρικών χαλαρώσεων που σχετίζονται με την πολυμερική μήτρα. Αυτές αποδίδονται, στη μετάπτωση από την υαλώδη στην ελαστομερική φάση της εποξειδικής ρητίνης (α-χαλάρωση) και στην επαναδιευθέτηση πλευρικών πολικών ομάδων (β-χαλάρωση). Η παρουσία των εγκλεισμάτων στο εσωτερικό της μήτρας εισάγει ηλεκτρική ετερογένεια με αποτέλεσμα την εμφάνιση του φαινομένου διεπιφανειακής πόλωσης (interfacial polarization). Μη δέσμια φορτία συσσωρεύονται στη διεπιφάνεια των φάσεων, όπου σχηματίζουν μεγάλα δίπολα που παρουσιάζουν αδράνεια ως προς τον προσανατολισμό τους, παράλληλα του εφαρμοζόμενου πεδίου. Η διεπιφανειακή πόλωση είναι η πλέον αργή διεργασία και παρατηρείται σε χαμηλές συχνότητες και υψηλές θερμοκρασίες. Το πραγματικό μέρος της ηλεκτρικής διαπερατότητας, όπως και η ειδική αγωγιμότητα παρουσίασαν αύξηση με την περιεκτικότητα σε ενισχυτική φάση, ιδιαίτερα στην περίπτωση των συστημάτων με γραφιτικά νανοεπίπεδα. Η δυνατότητα αποθήκευσης ενέργειας στα συστήματα διερευνήθηκε με χρήση της πυκνότητας ενέργειας υπό σταθερό ηλεκτρικό πεδίο. Διαπιστώθηκε αύξηση της αποθηκευόμενης ενέργειας με αύξηση της περιεκτικότητας σε ενισχυτική φάση. Τη βέλτιστη συμπεριφορά επέδειξε το σύστημα με τη μέγιστη περιεκτικότητα σε γραφιτικά νανοεπίπεδα. Η δυναμική των χαλαρώσεων μελετήθηκε μέσω διαγραμμάτων Arrhenius, από τα οποία προέκυψαν και οι τιμές της ενέργειας ενεργοποίησης. Η θερμοκρασιακή γειτνίαση των διεργασιών της α-χαλάρωσης και της διεπιφανειακής πόλωσης οδήγησε σε αλληλοεπικάλυψη των διεργασιών. Από τις ενέργειες ενεργοποίησης που υπολογίστηκαν φαίνεται πως στο δοκίμια της μη ενισχυμένης ρητίνης επικρατεί η συνεισφορά της α-χαλάρωσης, ενώ στα σύνθετα συστήματα επικρατεί η συνεισφορά της διεπιφανειακής πόλωσης. Τα σωματίδια του τιτανικού βαρίου υφίστανται δομικό μετασχηματισμό από την πολική τετραγωνική δομή (σιδηροηλεκτρική φάση) στην μη-πολική κυβική δομή (παραηλεκτρική φάση) σε μία κρίσιμη θερμοκρασία, πλησίον των 130οC. Η μετάβαση αποδείχθηκε μέσω των φασμάτων ακτίνων-Χ και είναι περισσότερο έντονη στην περίπτωση των μικροσωματιδίων. Η λειτουργική συμπεριφορά των συστημάτων σχετίζεται με τη θερμικά διεγειρόμενη δομική μετάβαση από τη σιδηροηλεκτρική στην παραηλεκτρική φάση των εγκλεισμάτων τιτανικού βαρίου, τη μεταβολή του προσήμου του θερμοκρασιακού συντελεστή ειδικής αγωγιμότητας και τη δυνατότητα αποθήκευσης ενέργειας. Η συνύπαρξη σε κοντινές θερμοκρασίες των διεργασιών α-χαλάρωσης και διεπιφανειακής πόλωσης μαζί με την κρίσιμη θερμοκρασία μετάβασης των σιδηροηλεκτρικών εγκλεισμάτων, δυσχεραίνει πολύ την διάκρισή τους. Με την εισαγωγή της διηλεκτρικής συνάρτησης ενίσχυσης (dielectric reinforcing function) έγινε δυνατός ο διαχωρισμός των φαινομένων. Επιπλέον, η συνάρτηση διηλεκτρικής ενίσχυσης προσφέρει τη δυνατότητα εξέτασης της λειτουργικής συμπεριφοράς και της δυνατότητας αποθήκευσης ενέργειας, ανεξάρτητα των γεωμετρικών διαστάσεων του υλικού. Τέλος, το σύνολο των αποτελεσμάτων έγινε αντικείμενο συγκρίσεων και συζήτησης. / In this study, series of polymer matrix composite materials were developed and experimentally studied, varying the reinforcing phase content. The employed matrix was a high tech epoxy resin, while reinforcing phase was micro- and/or nano-barium titanate particles, as well as exfoliated graphite nanoplatelets. The choice of the materials was targeting to take advantage in a common composite system of the thermo-mechanical stability of the matrix, the high dielectric permittivity and the ferroelectric behaviour of barium titanate and the enhanced mechanical properties in tandem with the high conductivity of the exfoliated graphite nanoplatelets. The following composite materials systems were fabricated and studied, for various filler contents: (a) barium titante micro-particles/epoxy resin composite system, (b) barium titante nano-particles/epoxy resin composite system, (c) exfoliated graphite nanoplatelets/epoxy resin composite system, (d) barium titante micro-particles/barium titante nano-particles /epoxy resin hybrid composite system, (e) exfoliated graphite nanoplatelets /barium titante nano-partcles /epoxy resin hybrid composite system. The fabrication of the composites was followed by a multiple characterization of the produced specimens. For reference reasons pure resin was also prepared and studied. Systems’ morphology was investigated by means of scanning electronic microscopy and x-ray diffraction scattering. It was ascertained the existence of fine nanodispersions, as well as of small clusters, within the composites. XRD spectra verified the presence of filler in each category of composite systems. Thermal characterization was conducted via differential scanning calorimetry aiming to determine the glass to rubber transition temperature of all studied systems. Mechanical behaviour was investigated under static and dynamic conditions. Static behaviour was determined via three point bending tests at ambient temperature. It was found that modulus of elasticity increases with filler content in all composite systems categories. On the other hand, mechanical strength decreases with filler content. Dynamic response was studied by means of dynamic mechanical thermal analysis in a wide temperature range. Reinforced systems exhibit higher values of storage modulus, while the loss tangent peaks allow the determination of the glass transition temperature Tg. Tg slightly varies with reinforcing phase content, to higher or lower values depending on the type and the amount of filler concentration. These variations express the interactions between the phases of the composites and possibly the uncompleted wetting of the inclusions in some cases. The electrical response of the composite systems was examined by means of broadband dielectric spectroscopy in a wide frequency and temperature range. The analysis of the experimental data was carried out via the dielectric permittivity, electric modulus, and ac conductivity formalisms. The usage of all three formalisms provides the opportunity to extract more information concerning the physical mechanisms occurring within the composites. It was found that two dielectric processes are related to the polymer matrix. These are attributed to the glass to rubber transition of epoxy resin (α-relaxation) and to the re-arrangement of polar side groups of the main polymer chain (β-relaxation). The presence of inclusions within the matrix introduces electrical heterogeneity resulting in the occurrence of interfacial polarization. Unbounded charges accumulate at the interface of the phases, forming large dipoles, which exhibit inertia in orienting themselves parallel to the applied field. Interfacial polarization is the slowest process in the systems and thus it is observed at low frequencies and high temperatures. The real part of dielectric permittivity, as well as, the conductivity increase with reinforcing phase content, especially in the case of the systems with graphite nanoplatelets. The energy storage efficiency was investigated via the density of energy, at constant electric field. It was found that the energy storage capability increases with filler content. Optimum behaviour is displayed by the system with maximum content in graphite nanoplatelets. The dynamics of the relaxations was studied via Arrhenius graphs, from which the values of activation energy were calculated. Interfacial polarization and α-relaxation appear in adjacent temperature ranges, leading in a superposition of both processes. From the calculated values of activation energy it is concluded that in the pure resin specimen the dominating contribution is related to the α-relaxation, while in the composite systems the contribution of interfacial polarization seems to prevail. Barium titanate particles undergo a structural transition from the polar tetragonal structure (ferroelectric phase) to the non-polar cubic structure (paraelectric phase) at a critical temperature closed to 130oC. This transition was proved via XRD spectra and is more intense in the case of barium titanate microparticles. Systems’ functional behaviour is related to the thermally stimulated structural transition from the ferroelectric to the paraelectric phase of barium titanate inclusions, to the change of sign of the temperature coefficient of conductivity, and their ability for energy storage. The coexistence at adjacent temperatures ranges of α-relaxation and interfacial polarization, as well as the critical transition temperature of ferroelectric inclusions, hampers the discrimination of the effects. By introducing the dielectric reinforcing function the discrimination of the processes became possible. Furthermore, the dielectric reinforcing function provides the possibility to examine the functional behaviour and the energy storage efficiency of the systems, neglecting the materials’ geometrical characteristics influence. Finally, experimental results and analysis are compared and discussed.