Ces travaux de thèse ont porté sur des matériaux constitués de dispersions de particules colloïdales nanométriques, issues d'un matériau ferroélectrique, dans un cristal liquide chiral à phase smectique ferroélectrique. Ils ont pour but d'étudier les effets occasionnés par ces dispersions sur les propriétés du nanocolloïde, notamment celles liées à leur ferroélectricité. Cette étude a montré que les comportements mésomorphes et ferroélectriques de ces matériaux sont conservés. Une baisse de polarisation spontanée ainsi qu'un recul des températures des transitions ont été mis en évidence pour des faibles concentrations en NPs. Une "transition" de ces comportements a été observée pour une concentration critique au-delà de laquelle les particules s'agrègent pour former des amas au sein du milieu cristal liquide. Nous nous sommes intéressés ensuite à deux modes de relaxation diélectriques. Le premier lié aux mouvements de distorsions de l'hélice dans la phase ferroélectrique, le second aux mouvements de compression des couches smectiques de part et d'autre de la transition ferroélectrique-paraélectrique. Les comportements observés semblent être gouvernés par les modifications des propriétés visco-élastiques des nanocolloïdes, occasionnés par l'intercalation des nanoparticules entre les couches smectiques. / The present thesis work concerns materials made of dispersions of nanometric colloidal particles, from a bulk ferroelectric material, dispersed within a chiral smectic phase of a ferroelectric liquid crystal. The goal of this work is to study the effect of the dispersed nanoparticles over the nanocolloïd properties, specially the ones related to ferroelectricity. This study showed no change over mesomorphic and ferroelectric behavior of the materials. A decrease in spontaneous polarization and phase transition temperatures was found for low nanoparticle concentrations. A "transition" of these behaviors was observed for a critical concentration, beyond which, nanoparticles aggregate and form clusters inside the liquid crystal matrix. Afterwards, we have studied two dielectric relaxation modes. The first one related to distorsions of the helix in the ferroelectric phase and the second one to the compression movements of the smectic layers around the ferroelectric-paralectric transition. The observed behaviors seem to be due to modifications of the visco-elastic properties of nanocolloids, produced by intercalation of nanoparticles between the smectic layers.

Τα δομικά χαρακτηριστικά των μαγνητικών κολλοειδών με βάση το οξείδιο του σιδήρου είναι ιδιαίτερα σημαντικά κατά την χρήση τους σε βιοϊατρικές εφαρμογές όπως η απεικόνιση σε μαγνητικό τομογράφο (MRI), η μεταφορά φαρμάκων, η μαγνητική υπερθερμία και στόχευση. Ιδιαίτερα για την μαγνητική στόχευση, η αύξηση της μαγνήτισης κορεσμού των κολλοειδών είναι ιδιαίτερης σημασίας. Συνεπώς, η μελέτη της σχέσης δομής-ιδιοτήτων είναι απαραίτητη για την περαιτέρω βελτίωση της απόδοσης των εν λόγω συστημάτων στις προαναφερθείσες εφαρμογές. Με στόχο τη βελτίωση της μαγνήτισης κορεσμού αυτών των υλικών έχει μελετηθεί στο παρελθόν μια στρατηγική αντικατάστασης ενός ποσοστού ιόντων σιδήρου στη δομή του νανοκρυστάλλου με άλλα μεταλλικά ιόντα. Οι μελέτες αυτές έχουν δείξει πως αναλόγως της συνθετικής πορείας άλλοτε επιτυγχάνεται το επιθυμητό αποτέλεσμα και άλλοτε όχι. Σε αυτά τα πλαίσια, σκοπός της παρούσας εργασίας ήταν η εφαρμογή αυτής της στρατηγικής σε μία συνθετική πορεία ανάπτυξης μαγνητικών νανοκολλοειδών στην οποία δεν έχει εκτιμηθεί μέχρι τώρα η επιτυχία της. Το ενδιαφέρον έγκειται στο γεγονός ότι η συγκεκριμένη συνθετική πορεία οδηγεί σε νανοκρυσταλλίτες οξειδίου του σιδήρου με αυξημένες μαγνητικές ιδιότητες σε σχέση με τη βιβλιογραφία, οι οποίες θα μπορούσαν (πιθανώς) να βελτιωθούν ακόμα περισσότερο με μερική ιοντική αντικατάσταση. Τα νανοκολλοειδή παρασκευάστηκαν με την μέθοδο της υδρολυτικής αλκαλικής καταβύθισης από μία πρόδρομη ένωση σιδήρου (FeCl2 ή FeSO4), με προσμίξεις διαφόρων αναλογιών με Zn(Cl2/SO4) ή Mn(Cl2/SO4). Η χημική τροποποίηση της επιφάνειας των μαγνητικών νανοκολλοειδών έγινε με στοχευμένη προσθήκη του φυσικού βιοπολυμερούς του αλγινικού νατρίου κατά τη διαδικασία κρυστάλλωσης του ανόργανου μαγνητικού πυρήνα. Η μελέτη των φυσικοχημικών χαρακτηριστικών και της δομής των νανοφορέων πραγματοποιήθηκε με την χρήση των παρακάτω αναλυτικών τεχνικών: Ηλεκτρονική Μικροσκοπία Σάρωσης (SEM) σε συνδυασμό με Φασματομετρία Ενεργειακής Διασποράς Ακτινοβολίας-X (EDS), Περίθλαση Ακτινοβολίας-X (XRD), Θερμοσταθμική Ανάλυση (TGA), Δυναμική Σκέδαση Φωτός (DLS), Ηλεκτροκινητικές Μετρήσεις, Μαγνητομετρία Δονούμενου Δείγματος (VSM), Μαγνητοφόρηση και Μαγνητική Υπερθερμία Δείγματος. / Structural characteristics of magnetic ferrite nanocolloids are particularly important in biomedical applications such as magnetic resonance imaging (MRI), drug delivery, magnetic hyperthermia and targeting. Particularly about magnetic targeting, increasing saturation magnetization is crucial. Therefore, studying the structure-properties relation of colloids is necessary, in order to improve further the performance of these systems in the above applications. In order to enhance the saturation magnetization of those materials, substitution of a percentage of iron ions in the structure of the nanocrystal with other metal ions has been previously studied. Results have shown that the desired properties are obtained under certain circumstances, depending on the synthetic route. Within this frame, the goal of the present work is to test this strategy on a synthetic route which has not been so far evaluated. The interest lies in the fact that this synthetic route leads to iron oxide nanocrystallites with increased magnetic properties compared to the literature, which could (possibly) be further improved with partial ionic replacement. Nanocolloids were synthesized by hydrolytic alkaline precipitation from a single iron molecular precursor (FeCl2 or FeSO4), doped at different ratios with Zn(Cl2/SO4) or Mn(Cl2/SO4). The surface modification of the magnetic nanocolloids was performed by in-situ grafting of the natural biopolymer of sodium alginate, during the crystallization process of the inorganic magnetic core. The evaluation of the structural, magnetic and physicochemical characteristics of the nanocarriers was performed with the use of the following analytical techniques: Scanning Electron Microscopy (SEM) in conjunction with Energy-Dispersive X-Ray Spectrometry (EDS), X-Ray Diffraction (XRD), Thermal Gravimetric Analysis (TGA), Dynamic Light Scattering (DLS), Electrokinetic Measurements, Vibrating Sample Magnetometer (VSM), Magnetophoresis and Magnetic Hyperthermia of the Sample.

Les cristaux liquides sont des matériaux organiques utilisés pour réaliser des dispositifs électroniques ; avant de les intégrer dans des applications, il est nécessaire d'étudier leurs propriétés physico-chimiques et diélectriques pour optimiser leurs performances. Ce travail de thèse est consacré aux nanocolloïdes obtenus par dispersion de nanoparticules ferroélectriques dans un cristal nématique ; il s'agit d'étudier l'influence des inclusions sur les transitions de phases et sur les propriétés diélectriques de la matrice. L'étude des transitions de phases à l'aide de l'Analyse Enthalpique Différentielle (AED) a mis en évidence l'influence des nanoparticules ferroélectriques ; ceci résulte de deux principaux effets ; la polarisation spontanée des nanoparticules et l'ancrage entre les molécules du cristal liquide et les inclusions. La caractérisation diélectrique a révélé le couplage entre la polarisation macroscopique des inclusions et le champ électrique ; ce couplage se manifeste par une augmentation des températures de transitions de phases par rapport à celles déterminées par l'AED. La compétition entre les effets de la polarisation sous champ électrique et de l'ancrage induit une modification des permittivités (parallèle et perpendiculaire) et de l'anisotropie diélectriques. L'utilisation des nanoparticules fortement polaires sélectionnées a confirmé l'importance de la polarisation macroscopique des nanoparticules pour améliorer les propriétés des nanocolloïdes étudiés ; en effet, de très faibles quantités de ces nanoparticules donnent lieu à des améliorations plus significatives que celles obtenues par les nanoparticules brutes. / Liquid crystals are organic materials used to make electronic devices ; before using this material in applications, it is necessary to study their physical-chemical and dielectric properties in order to optimize their performance. This study is devoted to the nanocolloids obtained by dispersing ferroelectric nanoparticles in a nematic liquid crystal ; it means an inclusion influences the phase transitions temperatures and the dielectric properties of the host. The phase transitions measured by using Differential scanning calorimetry (DSC) evidenced the ferroelectric nanoparticles influence ; which is attributed two effects : the nanoparticles spontaneous polarization and the anchoring effect between nanoparticles and liquid crystal. The dielectric characterisation revealed the coupling between the macroscopic polarization of the inclusions and the electric field ; this coupling is manafested by an increase of phase transition temperatures compared to those determined by DSC. The competition between the polarization effect under an electric field and the anchoring effect induces a modification of the permittivities (parallel and perpendicular) and the dielectric anisotropy. Using harvested nanoparticles, the study confirmed the importance of the nanoparticles polarization to increase the properties of the studied nanocolloids. In fact, very small quantity of the harvested nanoparticles presents more significant improvements than those obtained with the non-harvested nanoparticles.

Binary mixtures of polymers and soft nanocolloids, also called as polymer nanocomposites are well known and studied for their enormous potentials on various technological fronts. In this thesis blends of polystyrene grafted gold nanoparticles (PGNPs) and polystyrene (PS) are studied experimentally, both in bulk and in thin films. This thesis comprises three parts; 1) evolution of microscopic dynamics in the bulk(chapter-3),2) dispersion behavior of PGNPs in thin and ultra thin polymer matrices (chapter-4) 3) effect of dispersion on the glass transition behavior (chapter-5). In first part, the state of art technique, x-ray photon correlation spectroscopy is used to study the temperature and wave vector dependent microscopic dy¬namics of PGNPs and PGNP-PS mixtures. Structural similarities between PGNPs and star polymers (SPs) are shown using small angle x-ray scatter¬ing and scaling relations. We find unexpected (when compared with SPs) non-monotonic dependence of the structural relaxation time of the nanoparticles with functionality (number of arms attached to the surface). Role of core-core attractions in PGNPs is shown and discussed to be the cause of anomalous behavior in dynamics. In PGNP-PS mixtures, we find evidence of melting of the dynamically arrested state of the PGNPs with addition of PS followed by a reentrant slowing down of the dynamics with further increase in polymer frac¬tion, depending on the size ratio(δ)of PS and PGNPs. For higher δ the reen¬trant behavior is not observed with polymer densities explored here. Possible explanation of the observed dynamics in terms of the presence of double-glass phase is provided. The correlation between structure and reentrant vitrifica¬tion in both pristine PGNPs and blends are derived rather qualitatively. In the second part, the focus is shifted to miscibility between PGNPs and polymers under confinement i.e., in thin films. This chapter provide a compre¬hensive study on the different parameters affecting dispersion viz., annealing conditions, fraction of the added particles, polymer-particle interface and more importantly the thickness of the films. Changes in the dispersion behavior with annealing is shown and the need for annealing the films at temperatures higher than the glass transition temperature of the matrix polymers is clearly elucidated. Irrespective of the thickness of the films( 20 and 65 nm) studied, immiscible particle-polymer blends unequivocally prove the presence of gradi¬ent in dynamics along the depth of the films. To our knowledge for the first time, we report results on confinement induced enhancement in the dispersion of the nanoparticles in thin polymer films. The enhanced dispersion is argued to be facilitated by the increased free volume in the polymer due to confinement as shown by others. Based on these results we have proposed a phase diagram for dispersibility of the nanoparticles in polymer films. The phase diagram for ultra thin films highlights an important point: In ultra thin films the particles are dispersed even with grafting molecular weight less than matrix molecular weight. In the third part, we have studied the glass transition of the thin films whose structure has been studied earlier in the earlier part. Non-monotonic variation in glass transition with the fraction of particles in thin films has increased our belief on the gradient in the dynamics of thin polymer films. En¬hanced dispersion with confinement is captured with the enhanced deviation in glass transition temperature of ultra thin films. Effect of miscibility param¬eter on Tgis studied and the results are explained with the subtle interplay of polymer-particle interface and confinement.

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.