Ο όρος “σύνθετα υλικά” αναφέρεται σε μείγματα δύο ή και περισσότερων υλικών, τα οποία διαφέρουν στη μορφή ή και στη σύνθεση, συνήθως δεν διαλύονται το ένα στο άλλο και είναι δυνατόν να εντοπισθεί διεπιφάνεια μεταξύ των συστατικών τους. Τα σύνθετα υλικά αναπτύχθηκαν με σκοπό τη δημιουργία προϊόντων με εξειδικευμένο συνδυασμό ιδιοτήτων και επιτρέπουν την εξοικονόμηση φυσικών πόρων. Τα σύνθετα υλικά αποτελούνται από τη μητρική και την ενισχυτική φάση. Πολύ σημαντικό ρόλο στη συμπεριφορά ενός σύνθετου παίζει η διεπιφάνεια, δηλαδή η κοινή επιφάνεια μεταξύ μήτρας και εγκλείσματος, καθώς και η περιοχή στα όρια αυτής της επιφάνειας. Σκοπός της παρούσας εργασίας είναι ο μορφολογικός, θερμικός και ηλεκτρικός χαρακτηρισμός των σύνθετων συστημάτων πολυμερικής μήτρας-κεραμικού TiO2. Ως μήτρα χρησιμοποιήθηκε μια εμπορικά διαθέσιμη, χαμηλού ιξώδους εποξειδική ρητίνη (Epoxol 2004, Neotex S.A., Athens, Greece). Η συγκεκριμένη ρητίνη είναι ένα τυπικό μονωτικό πολυμερές χαμηλού ιξώδους, της οποίας η διηλεκτρική σταθερά καθώς και η αγωγιμότητα δεν διαφέρουν σημαντικά από τις αντίστοιχες τιμές για το TiO2. Ως ενισχυτική φάση (έγκλεισμα) επιλέχθηκε το κεραμικό TiO2. Η μελέτη του TiO2 παρουσιάζει μεγάλο ενδιαφέρον λόγω των οπτικών και ηλεκτρονικών ιδιοτήτων του. Είναι ένας ημιαγωγός ευρέως χάσματος, διηλεκτρικά ανισότροπο υλικό, και μπορεί να κρυσταλλωθεί σε τρεις διαφορετικές δομές: ρουτίλιο, ανατάσιο και βρουκίτης (rutile, anatase και brookite). Στην παρούσα εργασία χρησιμοποιήθηκε TiO2 με δομή ανατασίου σε μορφή νανοεγκλεισμάτων με μέση διάμετρο μικρότερη από 25 nm. Παρασκευάστηκαν σύνθετα υλικά συγκέντρωσης 3, 5, 7, 10 και 12 phr σε TiO2 καθώς επίσης και δείγματα καθαρής ρητίνης τα οποία χρησιμοποιήθηκαν ως δείγματα αναφοράς. Από τον μορφολογικό χαρακτηρισμό των δειγμάτων που πραγματοποιήθηκε με Ηλεκτρονικό Μικροσκόπιο Σάρωσης (Scanning Electron Microscopy, SEM) προκύπτει ότι έχει επιτευχθεί καλή διασπορά των εγκλεισμάτων στην πολυμερική μήτρα. Επομένως η παρασκευή των δειγμάτων χαρακτηρίζεται επιτυχής. Ο θερμικός χαρακτηρισμός έγινε με διαφορική θερμιδομετρία σάρωσης (Differential Scanning Calorimetry, DSC). Στα θερμοδιαγράμματα που ελήφθησαν φάνηκε μικρή τάση αύξησης της θερμοκρασίας υαλώδους μετάβασης (Tg) στα σύνθετα δείγματα σε σχέση με το δείγματα αναφοράς. Για τον ηλεκτρικό χαρακτηρισμό των σύνθετων, εφαρμόστηκε η τεχνική της διηλεκτρικής φασματοσκοπίας ευρέως φάσματος (Broadband Dielectric Spectroscopy, BDS). Για κάθε εξεταζόμενο δείγμα έλαβαν χώρα ισόθερμες σαρώσεις συχνοτήτων σε εύρος από 0.1Hz ως 1MHz και σε περιοχές θερμοκρασιών από -100οC ως 150οC με βήμα 10οC. Όλη η διάταξη είναι συνδεδεμένη με ηλεκτρονικό υπολογιστή για ταυτόχρονο έλεγχο και αποθήκευση των δεδομένων. Παρατηρήθηκαν διεργασίες χαλάρωσης οι οποίες αποδίδονται στην πολυμερική μήτρα, οι οποίες εμφανίστηκαν τόσο στα σύνθετα δείγματα όσο και στα δείγματα αναφοράς. Οι διεργασίες αυτές είναι η α-χαλάρωση, που σχετίζεται με την υαλώδη μετάβαση της πολυμερικής μήτρας, η γ-χαλάρωση, που είναι ο πιο γρήγορος μηχανισμός και αποδίδεται σε περιορισμένες τοπικές κινήσεις μικρών τμημάτων της πολυμερικής αλυσίδας, και η β-χαλάρωση που προέρχεται από τον επαναπροσανατολισμό πλευρικών ομάδων της πολυμερικής αλυσίδας. Στην περιοχή των υψηλών συχνοτήτων και στα σύνθετα υψηλής συγκέντρωσης σε TiO2, παρατηρήθηκε η διεργασία IDE (Intermediate Dipolar Effect), η οποία αποδίδεται στην ενισχυτική φάση. Επιπλέον, στην περιοχή των χαμηλών συχνοτήτων και υψηλών θερμοκρασιών παρατηρήθηκε και το φαινόμενο της Διεπιφανειακής πόλωσης (IP) Σε όλες τις συγκεντρώσεις η β-χαλάρωση παρουσιάζει εξάρτηση από τη θερμοκρασία, η οποία ακολουθεί την εξίσωση Arrhenius, ενώ η εξάρτηση της α- χαλάρωσης από τη θερμοκρασία περιγράφεται από την εξίσωση Vogel-Fulcher-Tamann (VTF). Εξετάστηκε τέλος, η ικανότητα αποθήκευσης ενέργειας των σύνθετων δειγμάτων, η οποία φάνηκε να αυξάνεται αυξανομένης της συγκέντρωσης της ενισχυτικής φάσης. Από τη σύγκριση της ικανότητας αποθήκευσης ενέργειας σε νανοσύνθετα δείγματα και σε μικροσύνθετα δείγματα με εγκλείσματα TiO2, φάνηκε γενικά ότι τα νανοσύνθετα παρουσιάζουν πολλαπλάσια ικανότητα αποθήκευσης ενέργειας συγκριτικά με τα μικροσύνθετα ίδιας περιεκτικότητας. / The term “composite materials” refers to mixtures of two or more materials, which differ in shape or in composition, usually not dissolving in one another and it is possible to detect an interface between the components. The composite materials were developed with the purpose of creating products with specialized combination of properties allowing the preservation of natural resources. The composites consist of the matrix and the reinforcing phase. The interface, ie the common face between the matrix and the inclusions and the boundaries of this region, has a very important role in the behaviour of a composite. The purpose of this study is the morphological, thermal and electrical characterization of complex systems with polymer matrix and ceramic TiO2 inclusions. The matrix used was a commercially available low viscosity epoxy resin (Epoxol 2004, Neotex S.A., Athens, Greece). This resin is a typical insulating polymeric material of low viscosity, whose dielectric constant and conductivity does not differ significantly from the corresponding values for the TiO2. The ceramic TiO2 was chosen as the reinforcing phase. The study of TiO2 is of great interest because of its optical and electronic properties. It is a wide gap semiconductor, an anisotropic dielectric material and it crystallizes in three different structures: rutile, brookite and anatase. In the present work the TiO2 used was the anatase structure, in the form of nanoinclusions with mean diameter less than 25 nm. Nanocomposites were prepared, in five different concentrations 3, 5, 7, 10 and 12 phr in TiO2. Samples of pure resin were also prepared, which were used as control samples. From the morphological characterization of the samples, which was performed via scanning electron microscopy (Scanning Electron Microscopy, SEM), it can be concluded that a good dispersion of the inclusions in the polymeric matrix was achieved. Hence the sample preparation can be characterized as successful. The thermal characterization was conducted by differential scanning calorimetry (Differential Scanning Calorimetry, DSC). The thermograms obtained showed a tendency to increase the glass transition temperature (Tg) in the composite samples compared to the samples of pure resin. The technique of Broadband Dielectric Spectroscopy (BDS) was applied for the electrical characterization of the composites. Isothermal frequency scans were conducted for each of the examined specimens in the frequency range of 0.1Hz to 1MHz and temperature range of –100°C to 150°C with the step of 10°C. The whole measuring system device was computer controlled for simultaneous control and data storage. The relaxation processes observed, which appeared in both the composite samples and the pure epoxy samples, are attributed to the polymeric matrix. These processes are the α-relaxation, which is related to the glass transition of the polymeric matrix, the γ-relaxation, which is the faster mechanism, attributed to limited local movements of small parts of the polymer chain and the β-relaxation derived from reorientation of polar side groups of the polymer chain. In the high frequency region and in the samples with high concentration in TiO2, the process IDE (Intermediate Dipolar Effect) was observed, which is attributed to the reinforcing phase. Moreover, in the low frequencies and high temperatures the phenomenon of Interfacial Polarization (IP) was observed. At all concentrations the β-relaxation exhibits temperature dependence according to the Arrhenius equation, while the temperature dependence of the α-relaxation is described by the Vogel-Fulcher-Tamann (VTF) equation. Finally, the energy storage capacity of the composite samples was examined and seems to increase with increasing concentration of the reinforcing phase. A comparison of the energy storage capacity in nanocomposites and in samples with micro scale inclusions in TiO2, reveals that, in general, the nanocomposites exhibit higher energy storage capacity compared to microcomposite samples with the same concentration.

Κατά την διάρκεια αυτής της πτυχιακής εργασίας παρασκευάστηκαν δυο δείγματα νανοσύνθετων υλικών TiO2/αλλοϋσίτη, με διαφορετικές αναλογίες (60:40, 50:50) όπου μελετήθηκαν πειραματικά οι ιδιότητες τους, με σκοπό να χρησιμοποιηθούν σε επόμενο στάδιο στη φωτοκαταλυτική αποδόμηση αέριων ρύπων (π.χ. NOx) και οργανικών πτητικών ενώσεων (VOCs). Η μορφή του φωτοκαταλύτη (TiO2) που συντέθηκε ήταν ο ανατάσης. Στα τροποποιημένα πλέον δείγματα του αλλοϋσίτη, εφαρμόστηκαν διάφορες τεχνικές για τον χαρακτηρισμό των ιδιοτήτων τους, όπως: Περιθλασιμετρία Ακτίνων X, Υπέρυθρη Φασματοσκοπία Μετασχηματισμού κατά Fourier, Ηλεκτρονική Μικροσκοπία Σάρωσης και Προσδιορισμός Μεγέθους και Κατανομής Πόρων καθώς και Ειδικής Επιφάνειας. Ο φωτοκαταλύτης διασπείρεται σχετικά ομοιογενώς στις εξωτερικές επιφάνειες του αλλουσίτη, δίνοντας πολύ ενθαρρυντικά αποτελέσματα για την φωτοκαταλυτική του δράση που θα μελετηθεί σε επόμενο στάδιο. Τα νανοσύνθετα που παρασκευάστηκαν, εμφανίζουν τη δημιουργία μεσοπορώδους δομής με πόρους μεγέθους περίπου στα 5,8nm. Τα τροποποιημένα αργιλικά ορυκτά παρουσιάζουν ιδιότητες αντίστοιχες άλλων νανοσύνθετων TiO2-αλλουσίτη αποτελεσματικών για την καταπολέμηση των αέριων ανόργανων ρύπων (NOx) και των οργανικών πτητικών ενώσεων (VOCs). / During this thesis prepared two samples nanocomposite materials TiO2 / halloysite, with different ratios (60:40, 50:50) where experimentally studied their properties, to be used at a later stage in the photocatalytic degradation of gaseous pollutants (p. x. NOx) and volatile organic compounds (VOCs).

The current thrust towards the compaction of electrical power equipment, resulting in increased insulation electrical stress levels, necessitates new electrical insulating materials. In the last few decades, polymeric materials that exhibit light weight, excellent mechanical properties, low cost, and some with unique non-wetting surface characteristics, have surpassed the use of the conventional porcelain and glass insulating materials. Despite these advantages, polymeric materials are incapable of withstanding the high heat from surface arcing that is instigated by the synergism of pollution, moisture, and voltage. Surface arcing results in material loss due to heat ablation and/or the electrical tracking of polymeric materials. To overcome such issues, inorganic fillers are added to the base polymers to enhance their resistance to surface discharge activities and other performances. Since their addition can significantly reduce material costs, their use is compelling. Micron-sized fillers, here after defined as microfillers, have been used to acquire these desirable properties, but due to limitations in material processability, the further application of such fillers is limited. Consequently, nano-sized fillers, here after defined as nanofillers, have been viewed as replacements or assistant combinations to microfillers. Nanofillers are characterized by large surface areas, resulting in increased bond strengths that yield significant improvements in the various properties at fill levels well below that of microfillers. However, the primary problem of using nanofillers is their characteristic property of agglomeration due to their physical size and the forces between the fillers. Conventional mechanical mixing of nanofillers does not adequately separate the nanofillers, leading to behaviour similarly to that of microfillers. Therefore, the implementation of nanofillers is not completely effective. In chemical dispersion techniques, for example, the use of surfactants, are normally very elaborate and complicated. Due to the negative impact of agglomeration, the successful dispersion of nanofillers is pivotal in the further development of nanodielectrics for various insulation applications. In this thesis, electrospinning is proposed and realized as a new dispersal method for nanofillers in polymeric materials. This novel technique facilitates polymeric nanocomposites with improved properties due to the uniform distribution of fillers. Scanning electron microscopy (SEM) images and energy dispersive X-ray analysis (EDX) clearly indicate that electrospun nanocomposites demonstrate a better filler distribution than nanocomposites, produced by conventional mechanical mixing. Also electrospinning introduces the possibility of separating different nanofillers in different base polymers. The mechanical properties: tensile strength and hardness; the electrical properties: permittivity, tracking, and erosion resistance; and the thermal properties: thermal conductivity, thermal degradation, and heat erosion resistance of electrospun nanocomposites are compared to those of conventional nanocomposites for silicone rubber and cycloaliphatic epoxy-based polymers. All the experimental studies in this thesis confirm that electrospun nanocomposites exhibit better thermal performances than the conventional composites which are attributed to the improved distribution of the nanofillers by the newly developed electrospinning process. Also in this investigation, a two-dimensional thermal model is developed in COMSOL MultiphysicsTM by using the finite element method (FEM) to theoretically address the benefits of using nanofillers and the effects of filler dispersion. The model confirms that electrospun nanocomposites have much more uniform temperature distribution than conventional nanocomposites. This thesis presents the possible mechanisms by which nanofillers improve the heat and erosion resistance of silicone rubber nanocomposites, and also addresses the possible mechanism by which electrospinning improves nanofiller dispersion.

The world’s increased energy storage demand, as well as the environmental concerns related to the combustion of fossil fuels, has triggered a transition to intermittent renewable energy sources as well as to electrical and hybrid vehicles. Current day rechargeable batteries are, due to the invention and development of lithium ion batteries, technologically well positioned to answer to some of these demands. Conventional batteries, however, utilize inorganic materials of limited supply that require large amounts of energy during refining and processing. The materials also add a significant cost to the final product, making the rechargeable batteries less attractive for large scale applications. During the last decade, significant efforts have been made to find suitable organic matter based electrode materials that can replace the inorganic materials. One class of organic materials that can be used for electrical energy storage, or be included as components in organic matter based energy storage systems, is conducting polymers. The aim of this thesis was to investigate the possibilities and limitations of using the conducting polymer polypyrrole in energy storage applications. The polymer was synthesized onto cellulose extracted from the Cladophora sp. algae, and the result was a flexible composite material. Symmetrical energy storage devices constructed with the composite material were shown to exhibit a pseudocapacitive behavior. The resistance in the cells was investigated and was found to scale linearly with the separator thickness. Cells could be cycled for 4,000 cycles without significant capacitance loss and cells that were overcharged to 1.8 V cell potential, were found to be protected by a resistive potential drop. Devices were constructed as proof-of-concept and were used to power a remote control and a digital thermometer. The self-discharge in polypyrrole was studied extensively. It was found that oxygen was responsible for the oxidation of the reduced electrode, while the positive electrode self-discharged due to a faradaic reaction. Through spectroscopy and the temperature dependence of the self-discharge, it was suggested that the self-discharge of oxidized polypyrrole is linked to the degradation at high potentials, commonly referred to as overoxidation.

Current technology of polymer nanocomposites (PNC) emphasizes the need for fundamental understanding of the links between manufacturing method and macro-scale properties in order to engineer processing and performance of PNCs. The manufacturing method is one key variable that dramatically defines interfacial interactions on the nano-scale and thus the properties of polymer near the interface of nanomaterial/polymer or interphase, level of dispersion and the crystallization behavior of semi-crystalline PNCs. These factors in particular govern reinforcing mechanisms at the interface and consequently impart important properties to PNCs. The current approach to manufacturing PNCs involves trial and error with elaborate, costly and time consuming experimental characterization of PNCs. Therefore, a deep insight into the links among manufacturing method, interfacial interactions and bulk properties is essential in order to design and fabricate PNCs with engineered performance. The main goal of this study was to provide a better understanding of the effect of manufacturing methods on the macro-scale properties of PNCs, with a focus on the role of interfacial interactions, that can lead to fabrication of PNCs with multifunctional performance. The objectives of this research were to: i) determine the detail correlations among manufacturing method, nano- and microstructure and macro-scale properties of multifunctional exfoliated graphite nanoplatelets/polyamide 12 polymer nanocomposites with enhanced mechanical and electrical performance through systematic manufacturing and experimental methodologies, ii) understand correlations among nano-scale interfacial interactions, physical and structural properties of the polymer at the interface and macro-scale behavior of PNCs, and iii) evaluate effect of manufacturing method on electrical behavior of PNCs with directionally dependent performance. This study demonstrated key correlations among manufacturing techniques, interfacial interactions and macro-scale properties of PNCs. A methodology was introduced to understand and determine the characteristics of a complex constrained region produced at the interface of nanomaterials and polymer in semi-crystalline PNCs. Finally, the study illustrated superior electrical and morphological properties of selective laser sintering (SLS) processed parts over injection molded PNCs and thus confirmed the capability of SLS in the development of electrically conductive PNCs that exhibit multifunctional performance. In conclusion, the study provided an insight into the links among process, nano-scale interfacial interactions and microstructure to better understand effects of manufacturing technique on macro-scale properties of PNCs, which enables fabrication of conductive PNCs with multifunctional performance.

The focus of this research was to investigate the use of functionalized/non-functionalized multi walled carbon nanotubes (MWCNTs) as reinforcements for the Portland cement paste. The unique geometrical characteristics of the carbon nanotubes (CNTs), as well as its unique mechanical properties such as high strength, ductility and stiffness, were the vital motivation for this study. In this research, we combined this unique material (CNTs) with concrete which is the most used man-made material. When compared to other composite materials, a limited amount of research has been conducted on the CNTs/cement composites. In order to investigate how the aspect ratio of functionalized/non-functionalized MWCNTs affects the mechanical properties of cementitious composites, ten different mixes of the MWCNTs/cement composites were prepared and tested. The different batches had a fixed water/cement ratio of 0.4, and variations of MWCNTs length, concentration and surface treatment. The cement nanocomposites were cast in small-scale specimens (beams) for the three-point flexural testing. Four major mechanical properties were evaluated at ages of 7, 14, and 28 days from the casting day: the maximum flexural strength, ultimate strain capacity (ductility), modulus of elasticity, and modulus of toughness. The results for the different nanocomposite batches were compared with the plain cement (reference) batch. The mechanical testing results showed that at 28 days almost all of the MWCNTs composites increased the flexural strength of the cement nanocomposites. At 28 days, the long MWCNTs increased the flexural strength more than the short MWCNTs. In general, the ultimate strain (ductility) of the short MWCNTs nanocomposites was higher than the ultimate strain of the long MWCNTs nanocomposites. The flexural strength of short 0.2 percent MWNT and long 0.04 percent MWNT (OH) increased by 269 percent and 83 percent, respectively, compared to the plain cement sample at 28 days. The highest ductility at 28 days for the short 0.1 percent MWNT and the short 0.2 percent MWNT was 86 percent and 81 percent, respectively. Clear evidence was obtained from the SEM images for micro-crack bridging; many of the MWCNTs were stretching across the micro-cracks. In conclusion, CNTs as nano reinforcements, can effectively improve certain mechanical properties of the cement paste composites.

Doctor of Philosophy / The drastic improvements in stiffness and strength even with the addition of small percentage of clay to a polymer are commonly traded-off with significant reductions in fracture toughness. It is believed that the presence of a stiff nano-filler will restrict the mobility of the surrounding matrix chains, and thus limit its ability to undergo plastic deformation, thereby decreasing their fracture toughness. To understand the role of rigid nano-fillers, like clay and their constraint effect on the surrounding polymer matrix, the effects of preferentially organized polyamide 6 lamellae in the vicinity of organoclay layers on the toughening processes are studied and compared with polyamide 6 filled with an elastomeric additive (POE-g-MA). It is suggested that to impart high toughness to polymer/organoclay nanocomposites, full debonding at the polymer-organoclay interface is necessary so that shear yielding of large volumes of matrix material can be enhanced. However, due to the strong tethering junctions between the individual organoclay layers and the matrix, full-scale debonding at the polymer-organoclay interface is rarely observed under stress conditions indicating that the constraint on the polymer adjacent to the clay is not relieved. Therefore, this has led to the development of ternary nanocomposites by adding a soft elastomeric dispersed phase to polymer/clay systems to obtain well-balanced mechanical properties. Polyamide 66/SEBS-g-MA/organoclay nanocomposites are prepared with four different blending protocols to understand the effect of blending protocol on the microstructure, mechanical properties and fracture mechanisms of the ternary nanocomposites so as to obtain new insights for producing better toughened polymer nanocomposites. In general, it is found that the level of enhancement of fracture toughness of ternary nanocomposites depends on: (i) the location and extent of dispersion of organoclay and (ii) the internal cavitation of rubber particles leading to effective relief of crack-tip tri-axial constraint and thus activating the matrix plastic deformation. Based on the wear/scratch damage studies on different polymer nanocomposite systems, it is suggested that elastic modulus and toughness of polymer nanocomposites are not the predominant factors controlling the material removal or friction coefficient and cannot be the sole indicators to compare and rank candidate materials. It is also found that nano-fillers by themselves, even if uniformly dispersed with good interfacial interaction with the matrix, do not irrevocably improve the wear (and friction) properties. Although it is important to consider these factors, it is necessary to thoroughly understand all microstructural parameters and their response to wear/scratch damage. Other important factors that should be considered are the formation of a uniform and stable transfer film on the counterface slider and the role of excessive organic surfactants or other modifiers added to disperse nanoparticles in a polymer matrix. It is also emphasized that the mechanisms of removal of materials during the wearing/scratching process should be studied meticulously with the use of high resolution microscopic and other analytical tools as this knowledge is critical to understand the surface integrity of polymer nanocomposites.

Thesis (Ph. D.)--University of Massachusetts Amherst, 2009. / Includes bibliographical references (p. 126-135). Print copy also available.

Thesis (Ph. D.)--University of North Texas, Aug., 2009. / Title from title page display. Includes bibliographical references.

Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 51-53). Also available on the World Wide Web.