The work deals mainly with preparation protocol of nanocomposites. The task of this work is to study structural and procedural parameteres that control the dispersion of nanoparticles in polymer solution to be able to prepare desired spatial organization of nanoparticles. The work resolves the effect of various components such as polymer matrices, nanoparticles and solvent, in which matrices and nanoparticles are blended. Used components control final dispersion state of nanoparticles and it influences also properties of investigated materials such as glass transition temperature, stiffness and rheological properties.

No description available.

<p>U ovom radu pripremljeni su različiti polimerni i keramički nanostrukturni materijali, u cilju ispitivanja uticaja vrste i udela nanopunila (čađ, fuleren, silicijum(IV)oksid, aluminijum(III)oksid i titanijum(IV)oksid) na strukturu, reolo&scaron;ko i toplotno pona&scaron;anje kompozita, dobijenih na osnovu stirena, metilmetakrilata i akrilamida, primenom različitih metoda sinteze. Ispitivan je uticaj prisustva različitog udela nanočestica (1, 3 i 5 %<em> m/m</em>) na kinetiku polimerizacije stirena, i na oblast prelaska u staklasto stanje polistirenskih hibridnih materijala. Sintetisana je i serija nanokompozita polimerizacijom metilmetakrilata u prisustvu čestica (silicijum(IV)oksid, aluminijum (III)oksid i titanijum(IV)oksid) različitih dimenzija i hidrofilnosti, ali istog zapreminskog udela (1 %<em> v/v</em>). Na osnovu primene izotermne diferencijalno skanirajude metode (DSC), razvijen je kinetički model za opisivanje dve razičite reakcije tokom polimerizacije vinilnih monomera (reakciju prvog reda i samoubrzanje), i izračunata je debljina međufaznog sloja polimera na čestici u cilju određivanja njegovog uticaja na temperaturu prelaska u staklasto stanje hibridnih materijala. Za ispitivanje strukture i morfologije polistiren/silicijum(IV)oksid nanokompozita dobijenih metodom isparavanja rastvarača, kori&scaron;dene su infracrvena spektroskopija sa Furijeovom transformacijom (FT-IR) i skanirajuda elektronska mikroskopija (SEM). Radi utvrđivanja uticaja udela hidrofobnog silicijum(IV)oksida (2, 5, 10, 15 i 30 %<em> m/m</em>) na toplotnu postojanost polistirenskih materijala, primenjene su istovremena termogravimetrijska i diferencijalno skanirajuda analiza (TG-DSC). Takođe, određen je uticaj veličine čestice fulerena C60 i submikronske čestice čađi na reolo&scaron;ka svojstva polistirenskih kompozita sintetisanih taloženjem polimera iz rastvora. Ispitivanjem reolo&scaron;kog pona&scaron;anja hibridnih materijala, proučavan je uticaj veličine čestica, molekulske mase polimera i indeksa polidisperznosti na viskoznost polistirenskih kompozita. Na osnovu<em> in-situ</em> reolo&scaron;kih analiza polimerizacije u toku želiranja suspenzije nanočestica aluminijum(III)oksida u vodenom rastvoru monomera metakrilamida i N,N&rsquo;-metilenbisakrilamida, utvrđena je jaka katalitička aktivnost povr&scaron;ine aluminijum(III)oksida na nastajanje slobodnih radikala. Radi nalaženja veze između željenih svojstava keramičkih proizvoda i načina njihovog dobijanja, proučavan je uticaj uslova vođenja polimerizacije na slaganje čestica u dobijenom odlivku i na gustinu krajnjeg sinterovanog proizvoda.</p> / <p>In this work, polymeric and ceramic nanostructured materials were prepared using different methods, in order to investigate the influence of nanofiller content and its type (carbon black, fullerene, silica, alumina and titania) on the structure, rheological and thermal behavior of composites, based on styrene, methylmethacrylate and acrylamide. The effect of particles content (1, 3 and 5 wt. %) on the kinetics of styrene radical polymerization and on the glass transition temperature of polystyrene/silica composites was investigated. A series of polymethylmethacrylate nanocomposites containing 1 vol. % of silica, alumina or titania particles (differing in dimensions and surface properties) was obtained. On the basis of isothermal differential scanning calorimetry (DSC), the kinetic model for describing two reactions during vinyl monomer polymerization (first order and autoacceleration) was developed, and the thickness of interfacial layer formed on nanoparticle surface was determined, in order to investigate its influence on the glass transition temperature of polymethylmethacrylate hybrid materials. The structure and the morphology of polystyrene/silica nanocomposites prepared by solvent evaporation were investigated using Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The influence of&nbsp; hydrophobic silica content (2, 5, 10, 15 and 30 wt. %) on polystyrene thermal stability was studied by simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC). The effect of fullerene C60 and submicron carbon black particle size on rheological properties of polystyrene composites prepared by the rapid coprecipitation was determined. Following the rheology of hybrid melts, the influence of filler size, molecular weight and polydispersity of polymer matrix on the viscosity of polystyrene composites was studied. On the basis of in-situ rheology analysis of polymerization process during the gelation of alumina nanoparticles suspension in aqueous solution of methacrylamide and N,N&rsquo;- methylene bisacrylamide monomers, the strong catalytic activity of alumina surface on the free radicals formation was determined. In order to find the correlation between desired properties of ceramic products and their preparation procedure route, the influence of polymerization conditions on the green body structure and sintered body density was studied in details.</p>

The thesis describes the study of slow dynamics of confined polymers and soft colloids. We study the finite size effect on the dynamics of glassy polymers using newly developed interfacial microrheology technique. Systematic measurement have been performed to address the issue of reduction of glass transition under confinements. Slow and heterogeneous dynamics are the underlined observed behavior for dynamics in confined glassy polymers. The slow relaxation dynamics and dynamical heterogeneity in polymer grafted nanoparticles (PGNPs) systems were studied using advanced X - ray photon correlation spectroscopy (XPCS) techniques. Our studies presented in this thesis on dynamics of polymer grafted nanoparticle systems in melts and solution are the first attempt to study them experimentally. Thus our work shed the light about new technique to study confined system more accurately and explore new soft colloidal system to study fascinating dynamics and interesting phase behavior. 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 present our newly developed technique (interfacial microrhelogy) and its consequences to study the complex fluids at interface. Chapter 4 discusses the concentration and temperature dependent glassy dynamics in confined glassy polymers. In Chapter 5 we provide the structural and dynamical study of polymer grafted nanoparticles in melts and solutions. We provide the summary of our result and the future prospective of the work in Chapter 6. Chapter-1 provides the ground work and theoretical aspects for understanding the results presented in this thesis. It starts with the discussion about the slow dynamics of complex fluids and transit to dynamic behavior of polymer in confinement, glassy dynamics in confinements . This also discusses the basic aspects of studying viscoelastic properties using rheology, interface rheology, microrheology, interface microrheology techinques. In continuation it discusses structure and dynamics of different soft colloids investigated for last decade and then theoretical aspects of XPCS is discussed. Towards the end of this Chapter, we discuss the procedure to explain and understand systems dynamical heterogeneity near glass like phase transition. Chapter-2 contains the details of the experimental techniques which has been used for the study of confined polymers and soft colloids. Brief introduction to basic principles of the measurements followed by details of the material and methods have been provided. Chapter-3 we discuss the interafacial microrheology of different complex fluids and advantages of the techniques is discussed in Chapter 3. This includes discussion about the technique sensitivity at the surface using quantum dots (QDs) as a probe and about the configuration of the QDs at/on monolayer. Later on establishment of the technique has been demonstrated through easurements on arachidic acid, poly(methylmethacrylate) (PMMA), poly(vinylacetate) (PVAc), poly(methylacrylate) (PMA) monolayers. The extracted subdiffusive nature of QDs in on monolayers through mean square displacement has been explained using fractional Brownian motion model. Towards the end of the chapter we discuss about the extraction of real and imaginary elastic modulus from mean square displacement data using generalized Stokes-Einstein relation for the quasi two dimensional systems and explains about the possible viscoelastic transition in the different monolayers. The concentration and temperature dependent glassy dynamics of confined polymers (PMMA) are discussed in Chapter-4. We demonstrate the microscopic nature of spatio-temporal variation of dynamics of glassy polymers confined to a monolayer of 2 3 nm thickness as a function of surface density and temperature. It illustrates the systems dynamical heterogeneity and explain the observed large reduction of glass transition temperature in confined system through finite size effect. In Chapter 5 we discuss the result based on systematic studies of dynamics of PGNPs in melts and solutions. In addition it also illustrates the structural anisotropy and anomalous dynamical transitions in binary mixture of PGNPs and homopolymers in good solvent condition. It provides temperature and wave vector dependent XPCS measurements on polymer grafted nanoparticles with the variation of functionality. The functionality ( f ) dependent nonmonotonic relaxation in melts of PGNPs and solvent quality dependent non monotonic relaxation of PGNPs system have been elaborated in the continuation. We present possible phase behavior of PGNPs system in good solvent with addition of homopolymer of two different molecular weight. Chapter 6 contains the summary and the future perspective of the work presented.

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.

The advancement in smart technologies for organic conducting polymers as flexible substrates in LEDs, PVs and solid state lighting necessitates the development of ultra-high barrier films to protect the devices from moisture and oxygen. The current encapsulation methodology of using layers of plastics and inorganic oxides has several deficiencies. Alternatively, the use of single layer of polymer nanocomposites is a promising substitute for these inorganic based encapsulation layers. The use of polymer materials have the advantage of flexibility, active electrodes printability and easy to make the devices for large area applications. The nano-fillers with high aspect ratio as nanocomposites ingredient in polymers reinforces its mechanical strength and also acts as a scavenging material for moisture and increases the residence time and/or for the penetrating moisture in the film. Chapter 1 gives the basic overview in the field of barrier technology films and coatings from polymers and inorganic oxide as either mono/multi layer hermetic encapsulation methods. The understanding of both chemistry and physics behind the moisture permeation and its interaction with the film material was discussed. The inclusion of functional nano-fillers as moisture trapping agents in the film provide better device protection achieved. The methods and instruments to measure such ultra-low permeation within the films are discussed. Finally, the advantage of polymer based nanocomposites for low-permeable films with existing materials are briefly discussed in this chapter. In this thesis, we employed both thermoplastic and thermoset polymer nanocomposites as encapsulation layer for device sealing. The use of ion-containing polymers (ionomers) as a sealant layer was also studied. Chapter 2 presents the detailed experimental procedures with materials and methods used in this thesis along with the synthesis methodologies to make films from the polymer. In chapter 3, we used cyclic olefin copolymer COC (copolymer of ethylene and norbornene) as an encapsulation layer with silica and layered silicate nano-fillers. The compatibility between hydrophilic silica and hydrophobic COC was achieved by maleic anhydride grafted PE with anchoring on COC as a compatibilizer and then silica filler was added to make the nanocomposite films. FTIR spectroscopy confirms the bond formation of silica with COC/MA-g-PE. The mechanical (tensile and DMA) and thermal studies (DSC) suggested that there is an improvement observed when adding silica/silicate layers in the polymer matrix with increased tensile strength, storage modulus and Tg. The calcium degradation test show enhanced performance towards moisture impermeation in the film. Chapter 4 deals with the synthesis of PVB based nanocomposite film with silica/layered silicate as nanofillers in the base matrix with varying degree of acetalization in the film. The FTIR and NMR spectroscopy show the evidence for acetal link formation in the in-situ synthesized PVB with silica/silicate nanofillers with three different acetyl contents. The tensile and DMA studies show the observed improvement in mechanical strength (increased tensile strength, storage modulus) were due to the intercalation of clay galleries during PVB formation and the interaction of silica particles interactive bond formation with –OH groups of PVA in PVB. The higher clay/silica particles show agglomerated nature and reduction in film strength. Thermal studies (DSC) show that there is an improvement observed in Tg when adding silica/silicate layers in the polymer matrix with moderate to low acetal content. The calcium degradation test show enhanced performance towards moisture impermeation in the film. Chapter 5 describes the inclusion of ionic groups (ionomers) in PVB and its effects on moisture permeation and mechanical properties. PVB ionomer was synthesized using formyl benzene 2-sulfonic acid sodium salt and 2-carboxy benzaldehyde (both sulfonic and carboxylic acid sources) as co-aldehyde with butyraldehyde and PVA. These acid groups were neutralized with potassium, magnesium and zinc ions. The level of acid content in the films was maintained between 6 to 28 mol percent. The sulfonic acid films with zinc and magnesium ions of 14 mol% exhibit good mechanical strength and low moisture permeation. Chapter 6 deals with the epoxy terminated silicone polymer nanocomposites as moisture barrier coatings for device encapsulation. Both silica and clay silicate layers were used to reinforce the silicone matrix. The silica nanoparticles were grafted with amino-silane groups, this would help in better mixing of silica particles in the silicone matrix due to the amine groups interaction in curing with epoxy groups. The calcium degradation test was used to determine the WVTR of the nanocomposites and device encapsulation was employed to estimate the degradation after exposure to ambient environment. Chapter 7 presents the concluding remarks of the results presented. The benefits as well as limitations of the polymer nanocomposite film and the future developmental work to be carried out are discussed in this chapter.

Philosophiae Doctor - PhD / Tuberculosis (TB) is an infectious disease that, despite all efforts devoted towards its eradication, remains a threat to many countries including South Africa. Current diagnostic assays do offer better performance than the conventional sputum smear microscopy and tuberculin skin tests. However, these assays have been proven to be affected by various factors including the condition of an individual's immune system and vaccination history. By far, electrochemical biosensors are amongst the currently investigated techniques to address the shortcomings associated with these diagnostics. / 2020-08-31

The growing incidences of orthopedic problems globally have created a huge demand for strong bioactive materials for bone tissue engineering. Over the years, studies have shown chemical, physical, and mechanical properties of biomaterials influence the cellular interactions at the material-tissue interface, which subsequently controls biological response to materials. Strong biomaterials with surface properties that actively direct cellular response hold the key for engineering the next generation orthopedic implants. With its unique properties graphene can be used to reinforce poly (ε-caprolactone) (PCL) to prepare strong and bioactive polymer nanocomposites for bone tissue regeneration. The thesis entitled ―Engineering bioactive and multifunctional graphene polymer composites for bone tissue regeneration” systematically studies the effect of different chemically functionalized and metal-graphene hybrid nanoparticles in PCL composites for bone tissue engineering. The thesis comprises of seven chapters. Chapter 1 is an outline review on the impact of graphene and graphene derived particles to prepare supporting substrates for tissue regeneration and the associated cell response to multifunctional graphene substrate. This chapter discusses how cells interact with different graphene based particles and the interplay between cells performance and multifunctional properties of graphene based substrates. Chapter 2 describes the role, if any, of the functionalization of graphene on mechanical properties, stem cell response and bacterial biofilm formation. PCL composites of graphene oxide (GO), reduced GO (RGO) and amine-functionalized GO (AGO) were prepared at different filler contents (1%, 3% and 5%). Although the addition of the nanoparticles to PCL markedly increased the storage modulus, this increase was higher for GO and AGO than with RGO. In vitro cell studies revealed that the AGO and GO particles significantly increased human mesenchymal stem cell (hMSC) proliferation. AGO was most effective in augmenting stem cell osteogenesis leading to mineralization. Bacterial studies revealed that interaction with functionalized GO induced bacterial cell death due to membrane damage which was further accentuated by amine groups in AGO. The synergistic effect of oxygen containing functional groups and amine groups on AGO-reinforced composites renders the optimal combination of improved modulus, favorable stem cell response and biofilm inhibition desired for orthopaedic applications. In Chapter 3, toward preparing strong multi-biofunctional materials, poly(ethylenimine) (PEI) conjugated graphene oxide (GO_PEI) was synthesized using poly(acrylic acid) (PAA) as spacer and incorporated in PCL at different fractions. GO_PEI significantly promoted proliferation and formation of focal adhesions in hMSCs on PCL. GO_PEI was highly potent in inducing stem cell osteogenesis leading to 90% increase in alkaline phosphatase activity and mineralization over neat PCL with 5% filler content and was 50% better than GO. Remarkably, 5% GO_PEI was as potent as soluble osteo-inductive factors. Increased adsorption of osteogenic factors due to the amine and oxygen containing functional groups on GO_PEI augment stem cell differentiation. GO_PEI was also highly efficient in imparting bactericidal activity with 85% reduction in counts of E. coli colonies compared to neat PCL at 5% filler content and was more than twice as efficient as GO. This may be attributed to the synergistic effect of the sharp edges of the particles along with the presence of the different chemical moieties. Thus, in contrast to using labile biomolecules, GO_PEI based polymer composites can be utilized to prepare bioactive resorbable biomaterials for fabricating orthopedic devices for fracture fixation and tissue engineering. Chapter 4 describes the preparation of hybrid nanoparticles of graphene sheets decorated with strontium metallic nanoparticles and its advantages in bone tissue engineering. Strontium-decorated reduced graphene oxide (RGO_Sr) nanoparticles were synthesized by facile reduction of graphene oxide and strontium nitrate. X-ray diffraction, transmission electron microscopy, and atomic force microscopy revealed that the hybrid particles were composed of RGO sheets decorated with 200 – 300 nm metallic strontium particles. Thermal gravimetric analysis further confirmed the composition of the hybrid particles as 22 wt% of strontium. Macroporous tissue scaffolds were prepared incorporating RGO_Sr particles in PCL. The PCL/RGO_Sr scaffolds were found to elute strontium ions in aqueous medium. Osteoblast proliferation and differentiation was significantly higher in the PCL scaffolds containing the RGO_Sr particles in contrast to neat PCL and PCL/RGO scaffolds. The increased biological activity can be attributed to the release of strontium ions from the hybrid nanoparticles. This study demonstrates that composites prepared using hybrid nanoparticles that elute strontium ions can be used to prepare scaffolds with osteoinductive property. These findings have important implications for designing the next generation of biomaterials for use in tissue regeneration. Chapter 5 discusses the use of hybrid graphene-silver particles (RGO_Ag) to reinforce PCL and compared with PCL/RGO and PCL/Ag composites containing RGO and silver nanoparticles (AgNPs), respectively. RGO_Ag hybrid particles were well dispersed in the PCL matrix unlike the RGO and AgNPs due to enhanced exfoliation. RGO_Ag led to 77 % increase in the modulus of PCL and provided a conductive network for electron transfer. Electrical conductivity increased four orders of magnitude from 10-11 S/cm to 10-7 S/cm at 5 wt % filler that greatly exceeded the improvements with the use of RGO and AgNP in PCL. RGO_Ag particles reinforced in PCL showed sustained release of silver ions from the PCL matrix unlike the burst release from PCL/Ag. PCL/RGO_Ag and PCL/RGO composites were non-toxic to hMSCs and supported osteogenic differentiation unlike the PCL/Ag composites which were highly toxic at ≥3% filler content. The PCL/RGO_Ag composites exhibited good antibacterial effect due to a combination of silver ion release from the AgNPs and the mechanical rupture induced by the RGO in the hybrid nanoparticles. Thus, the synergistic effect of Ag and RGO in the PCL matrix uniquely yielded a multifunctional material for use in implantable biomedical devices and tissue engineering. Chapter 6 presents investigation of potential differences in the biological response to graphene in polymer composites in the form of 2D substrates and 3D scaffolds. Results showed that osteoblast response to graphene in polymer nanocomposites is markedly altered between 2D substrates and 3D scaffold due to the roughness induced by the sharp edges of graphene at the surface in 3D but not in 2D. Osteoblast organized into aggregates in 3D scaffolds in contrast to more well spread and randomly distributed cells on 2D discs due to the macro-porous architecture of the scaffolds. Increased cell-cell contact and altered cellular morphology led to significantly higher mineralization in 3D scaffolds compared to 2D. This study demonstrates that the cellular response to nanoparticles in composites can change markedly by varying the processing route. Chapter 7 summarizes the important results and future directions of the work. This chapter provides general conclusions arising from this study, and makes suggestions for future work designed to provide a greater understanding of the in vivo response in terms of bio-distribution of the released functionalized graphene from the scaffold or substrate must be assessed with special attention on their accumulation or excretion.

Cette étude porte sur le développement de modèles et de méthodes numériques pour prédire les propriétés électriques et mécaniques des nanocomposites polymères/graphènes.Dans une première partie, un modèle nonlinéaire de conduction électrique prenant en compte l’effet tunnel est introduit pour déterminer la conductivité effective de ces nanocomposites au travers d’une procédure d’homogénéisation numérique. Celle-ci, basée sur une formulation éléments finis a mis en évidence l’influence des paramètres microstructuraux sur la conductivité effective au travers d’une étude statistique.Ensuite, un modèle atomistique de l’interface polymère/graphène a été proposé pour valuer les propriétés de l'interface et de l'interphase. Les champs de contrainte et de déplacement ont été identifiés par une extension de la procédure d'Hardy-Murdoch à partir des simulations de mécanique moléculaire. À l'aide de ces champs, un modèle élastique continue avec des interfaces imparfaites a été identifié et comparé aux résultats des simulations de mécanique moléculaire. Finalement, le modèle atomistique a permis d’identifier un modèle de zone cohésive nonlinéaire pour modéliser la décohésion à l’interface polymère/graphène. Une procédure d’homogénéisation numérique par la méthode des éléments finis a été introduite pour estimer les propriétés mécaniques effectives dans le cadre des transformations finies. Les microstructures déformées ont été utilisées dans le modèle électrocinétique pour déterminer l’impact de la décohésion interfaciale sur la conductivité effective. / This work contributes to developing numerical methodologies for predicting the electrical and mechanical properties of graphene/polymer nanocomposites, which can provide a better view for the design of new materials.First, a nonlinear electrical conduction model taking into account the tunneling effect is introduced to determine the effective conductivity of the graphene/polymer nanocomposites through a numerical homogenization procedure. The influences of barrier height and microstructural parameters on the conductivity were demonstrated.Then, to characterize the properties of interphases and interfaces, we employed the Murdoch-Hardy procedure combined with the molecular dynamics method to study the mechanical properties of the graphene/polymer nanocomposites. The stiffness tensor components of the interphase, interface andnbulk polymer region are identified. Based on these fields, a continuous elastic model with imperfect interface has been identified and compared with the results of molecular dynamics simulations.Finally, the atomistic model was used to identify a nonlinear cohesive zone model to simulate the decohesion at the interface of polymer and graphene. A numerical homogenization procedure by finite element method was introduced to estimate the effective mechanical properties in the framework of the finite strains. The proposed mechanical modeling is finally extended to the finite strain problem to predict the evolution of percolation threshold under tension within the proposed electrical model.

Tato dizertační práce se zabývá přípravou a charakterizací nanokompozitních polymerních pěn se zaměřením na strukturu materiálu a aplikaci v 3D tisku. Cílem práce je studium materiálu s vysoce organizovanou hierarchickou strukturou – od nanoměřítka, přes mikroskopickou strukturu po makroskopická tělesa. V první části práce byly řešeny strukturní vlastnosti nanokompozitů připravených z polymerních skel roztokovou metodou. Byl hledán obecně platný trend, pomocí kterého by bylo možné předpovídat disperzi nanočástic v kompozitu. Ukázalo se, že řídícím faktorem může být závislost na rozdílu parametrů rozpustnosti polymeru a rozpouštědla. Tento poznatek byl ověřen na systémech obsahujících různé nanočástice, polymery a rozpouštědla. Se znalostí principů pro řízení struktury nanokompozitů byly připraveny nanokompozity impaktního polystyrenu plněného nanosilikou. Tyto nanokompozity posloužily jako základ pro přípravu polymerních nakompozitních pěn. Porézní struktury bylo dosaženo pomocí termálního chemického nadouvadla azodikarbonamidu. Z těchto materiálů byly extrudovány filamenty, které byly následně zpracovány pomocí 3D tisku do požadovaných tvarů a vypěněny. Výsledkem byla hierarchická struktura s organizací struktury od nano (organizace nanočástic), přes mikro (struktura dvoukomponentní polymerní směsi a struktura pěny) po makroměřítko (struktura pěny a design 3D tisku). Byl pozorován vliv nanočástic na strukturu a termální a mechanické vlastnosti polymerních pěn. Nanočástice fungují při tvorbě pěny jako nukleační činidlo, na jejich povrchu snadno dochází k tvorbě pórů, takže s obsahem nanočástic v materiálu bylo vytvořeno více menších pórů, což napomohlo k homogenitě pěnové struktury. Přítomnost nanočástic změnila povrchovou energii zrn nadouvadla, díky čemuž docházelo k jeho rozkladu za nižích teplot a pěnění bylo i rychlejší. Nanočástice mají zároveň potenciál vyztužit stěny pěny a zlepšit tak mechanické vlastnosti. 3D tisk je oblíbená a hojně rozšířená technika, díky své jednoduchosti je v mnoha laboratořích a zkušebnách, proto roste poptávka po filamentech se speciálními vlastnostmi. Materiál vyvinutý v této dizertační práci je v podstatě hotovým a charakterizovaným produktem, který by mohl přispět k uspokojení této pohledávky.