Le contrôle de la mouillabilité de surface est un enjeu majeur pour le développement de matériaux innovants liés aux nano, bio et smart technologies. La mouillabilité est fonction de deux paramètres majeurs : l’énergie de surface du matériau et sa morphologie. La combinaison de ces deux paramètres est à la base de phénomènes tels que la super/parahydrophobie ou la superoléophobie. Ces capacités extrêmes à repousser les liquides avec soit une forte ou faible adhésion sont des propriétés de surface très intéressantes pour de multiples applications industrielles. La présente thèse propose l’étude d’une série de dérivés du polypyrrole élaborés par électrodéposition permettant d’influencer les paramètres régissant la mouillabilité de surface. Par cette approche, il a été possible d’élaborer des surfaces aux morphologies diverses avec une gamme de mouillabilité complète. Les différentes fonctionnalisations par des groupements hydrophobes greffés sur différentes positions préférentielles du monomère ont conduit à l’élaboration de surfaces para et superhydrophobes mettant en évidence l’impact de l’énergie de surface et de la morphologie sur la mouillabilité. Des études préliminaires ont mis en évidence la possibilité d’obtenir des morphologies variées allant de sphères jusqu’à des fibres à l’échelle du micro/nanomètre. Finalement, ces travaux contribuent à un contrôle en amont de la mouillabilité et de la morphologie de surface pour de nombreuses applications potentielles comme les matériaux collecteurs d’eau, les membranes séparatrices de liquide ou bien les revêtements auto nettoyant. / The control of the surface wettability is a key point for the development of innovative materials in several domains such as nano-, bio- and smart-technologies. The wettability is a function of two main parameters of the materials, such as the surface energy and the surface morphology. The combination of these two parameters allows to observe wetting phenomena as super/parahydrophobicity and superoleophobicity. These extreme abilities to repel liquids with different adhesion behaviors are very interesting properties for several industrial applications. This work presents a series of polypyrrole derivatives elaborated by electrodeposition allowing to influence the parameters driving the surface wettability. Following this approach, it was possible to develop surfaces with several types of morphology and different wetting behaviors from a low to high wettability. The different functionalizations using hydrophobic compounds grafted on various preferential positions on the monomer core yielded to para and superhydrophobic surfaces showing the impact of the surface energy and morphology on the wettability. Thanks to preliminary studies, it was showed the possibility to obtain several morphologies from spherical aggregates to fibers at the micro/nano scale. Finally, this work contributes to an upstream control of the surface wettability and morphologies for many potential applications such as water harvesting, separation membranes and self-cleaning coatings.

Among the many active materials in use today, piezoelectric composite patches have enabled notable advances in emerging technologies such as disturbance sensing, control of flexible structures, and energy harvesting. The macro fiber composite (MFC), in particular, is well known for its outstanding performance. Multiscale models are typically required for smart-structure design with MFCs. This is due to the need for predicting the macroscopic response (such as tip deflection under a transverse load or applied voltage) while accounting for the fact that the MFC has microscale details. Current multiscale models of the MFC exclusively focus on predicting the macroscopic response with homogenized material properties. There are a limited number of homogenized properties available from physical experiments and various aspects of existing homogenization techniques for the MFC are shown here to be inadequate. Thus, new homogenized models of the MFC are proposed to improve smart-structure predictions and therefore improve device design. It is notable that current multiscale modeling efforts for MFCs are incomplete since, after homogenization, the local fields such as stresses and electric fields have not been recovered. Existing methods for obtaining local fields are not applicable since the electrodes of the MFC are embedded among passive layers. Therefore, another objective of this work was to find the local fields of the MFC without having the computational burden of fully modeling the microscopic features of the MFC over a macroscale area. This should enable smart-structure designs with improved reliability because failure studies of MFCs will be enabled. Large-scale 3D finite element (FE) models that included microscale features were constructed throughout this work to verify the multiscale methodologies. Note that after creating a free account on cdmhub.org, many files used to create the results in this work can be downloaded from https://cdmhub.org/projects/ernestocamarena.<br><br>First, the Mechanics of Structure Genome (MSG) was extended to provide a rigorous analytical homogenization method. The MFC was idealized to consist of a stack of homogeneous layers where some of the layers were homogenized with existing rules of mixtures. For the analytical model, the electrical behavior caused by the interdigitated electrodes (IDEs) was approximated with uniform poling and uniform electrodes. All other assumptions on the field variables were avoided; thus an exact solution for a stack of homogeneous layers was found with MSG. In doing so, it was proved that in any such multi-layered composite, the in-plane strains and the transverse stresses are equal in each layer and the in-plane electric fields and transverse electric displacement are constant between the electrodes. Using this knowledge, a hybrid rule of mixtures was developed to homogenize the entire MFC layup so as to obtain the complete set of effective device properties. Since various assumptions were avoided and since the property set is now complete, it is expected that greater energy equivalence between reality and the homogenized model has been made possible. The derivation clarified what the electrical behavior of a homogenized solid with internal electrodes should be—an issue that has not been well understood. The behavior was verified by large-scale FE models of an isolated MFC patch.<br> <br>Increased geometrical fidelity for homogenization was achieved with an FE-based RVE analysis that accounted for finite-thickness effects. The presented theory also rectifies numerous issues in the literature with the use of the periodic boundary conditions. The procedure was first developed without regard to the internal electrodes (ie a homogenization of the active layer). At this level, the boundary conditions were shown to satisfy a piezoelectric macrohomogeneity condition. The methodology was then applied to the full MFC layup, and modifications were implemented so that both types of MFC electrodes would be accounted for. The IDE case considered nonuniform poling and electric fields, but fully poled material was assumed. The inherent challenges associated with these nonuniformities are explored, and a solution is proposed. Based on the homogenization boundary conditions, a dehomogenization procedure was proposed that enables the recovery of local fields. The RVE analysis results for the effective properties revealed that the homogenization procedure yields an unsymmetric constitutive relation; which suggests that the MFC cannot be homogenized as rigorously as expected. Nonetheless, the obtained properties were verified to yield favorable results when compared to a large-scale 3D FE model.<br> <br>As a final test of the obtained effective properties, large-scale 3D FE models of MFCs acting in a static unimorph configuration were considered. The most critical case to test was the smallest MFC available. Since none of the homogenized models account for the passive MFC regions that surround the piezoelectric fiber array, some of the test models were constructed with and without the passive regions. Studying the deflection of the host substrate revealed that ignoring the passive area in smaller MFCs can overpredict the response by up to 20%. Satisfactory agreement between the homogenized models and a direct numerical simulation were obtained with a larger MFC (about a 5% difference for the tip deflection). Furthermore, the uniform polarization assumption (in the analytical model) for the IDE case was found to be inadequate. Lastly, the recovery of the local fields was found to need improvement.<br><br><br>

Inventée en 1983, comme procédé de prototypage rapide, la fabrication additive (FA) est aujourd’hui considérée comme un procédé de fabrication quasiment au même titre que les procédés conventionnels. On trouve par exemple des pièces obtenues par FA dans des structures d’aéronef. Cette évolution de la FA est due principalement à la liberté de forme permise par le procédé. Le développement de diverses techniques sur le principe de fabrication couche par couche et l’amélioration en quantité et en qualité de la palette de matériaux pouvant ainsi être mis en forme, ont été les moteurs de cette évolution. De nombreuses autres techniques et matériaux de FA continuent de voir le jour. Dans le sillage de la FA (communément appelée impression 3D) a émergé un autre mode de fabrication : l’impression 4D (I4D). L’I4D consiste à explorer l’interaction matériaux intelligents (MIs) – FA. Les MIs sont des matériaux dont l’état change en fonction d’un stimulus ; c’est le cas par exemple des matériaux thermochromiques dont la couleur change en réponse à la chaleur ou des hydrogels qui peuvent se contracter en fonction du pH d’un milieu aqueux ou de la lumière. Les objets ainsi obtenus ont – en plus d’une forme initiale (3D) – la capacité de changer d’état (en fonction des stimuli auxquels sont sensibles les MIs dont ils sont faits) d’où la 4e dimension (temps). L’I4D fait – à juste titre – l’objet d’intenses recherches concernant l’aspect fabrication (exploration de nouveaux procédés et matériaux, caractérisation, etc.). Cependant très peu de travaux sont entrepris pour accompagner les concepteurs (qui, a priori, ne sont ni experts FA ni des experts de MIs) à l’utiliser dans leurs concepts. Cette nouvelle interaction procédé-matériau requiert en effet des modèles, des méthodologies et outils de conception adaptés. Cette thèse sur la conception pour l’impression 4D a pour but de combler ce vide méthodologique. Une méthodologie de conception pour la FA a été proposée. Cette méthodologie intègre les libertés (forme, matériaux, etc.) et les contraintes (support, résolution, etc.) spécifiques à la FA et permet aussi bien la conception de pièces que celle d’assemblages. En particulier, la liberté de forme a été prise en compte en permettant la génération d’une géométrie minimaliste basée sur les flux fonctionnels (matière, énergie, signal) de la pièce. Par ailleurs, les contributions de cette thèse ont porté sur la conception avec les matériaux intelligents. Parce que les MIs jouent plus un rôle fonctionnel que structurel, les préoccupations portant sur ces matériaux doivent être menées en amont du processus de conception. En outre, contrairement aux matériaux conventionnels (pour lesquels quelques valeurs de paramètres peuvent suffire comme information au concepteur), les MIs requièrent d’être décrits plus en détails (stimulus, réponse, fonctions, etc.). Pour ces raisons un système d’informations orientées conception sur les MIs a été mis au point. Ce système permet, entre autre, d’informer les concepteurs sur les capacités des MIs et aussi de déterminer des MIs candidats pour un concept. Le système a été matérialisé par une application web. Enfin un cadre de modélisation permettant de modéliser et de simuler rapidement un objet fait de MIs a été proposé. Ce cadre est basé sur la modélisation par voxel (pixel volumique). En plus de la simulation des MIs, le cadre théorique proposé permet également le calcul d’une distribution fonctionnelle de MIs et matériau conventionnel ; distribution qui, compte tenu d’un stimulus, permet de déformer une forme initiale vers une forme finale désirée. Un outil – basé sur Grasshopper, un plug-in du logiciel de CAO Rhinoceros® – matérialisant ce cadre méthodologique a également été développé. / Invented in 1983, as a rapid prototyping process, additive manufacturing (AM) is nowadays considered as a manufacturing process almost in the same way as conventional processes. For example, parts obtained by AM are found in aircraft structures. This AM evolution is mainly due to the shape complexity allowed by the process. The driving forces behind this evolution include: the development of various techniques on the layer-wise manufacturing principle and the improvement both in quantity and quality of the range of materials that can be processed. Many other AM techniques and materials continue to emerge. In the wake of the AM (usually referred to as 3D printing) another mode of manufacturing did emerge: 4D printing (4DP). 4DP consists of exploring the smart materials (SM) – AM interaction. SMs are materials whose state changes according to a stimulus; this is the case, for example, with thermochromic materials whose color changes in response to heat or hydrogels which can shrink as a function of an aqueous medium’s pH or of light. The objects thus obtained have – in addition to an initial form (3D) – the capacity to shift state (according to the stimuli to which the SMs of which they are made are sensitive) hence the 4th dimension (time). 4DP is – rightly – the subject of intense research concerning the manufacturing aspect (exploration of new processes and materials, characterization, etc.). However, very little work is done to support the designers (who, in principle, are neither AM experts nor experts of SMs) to use it in their concepts. This new process-material interaction requires adapted models, methodologies and design tools. This PhD on design for 4D printing aims at filling this methodological gap. A design methodology for AM (DFAM) has been proposed. This methodology integrates the freedoms (shape, materials, etc.) and the constraints (support, resolution, etc.) peculiar to the AM and allows both the design of parts and assemblies. Particularly, freedom of form has been taken into account by allowing the generation of a minimalist geometry based on the functional flows (material, energy, and signal) of the part. In addition, the contributions of this PhD focused on designing with smart materials (DwSM). Because SMs play a functional rather than a structural role, concerns about these materials need to be addressed in advance of the design process (typically in conceptual design phase). In addition, unlike conventional materials (for which a few parameter values may suffice as information to the designer), SMs need to be described in more detail (stimulus, response, functions, etc.). For these reasons a design-oriented information system on SMs has been developed. This system makes it possible, among other things, to inform designers about the capabilities of SMs and also to determine SMs candidates for a concept. The system has been materialized by a web application. Finally, a modeling framework allowing quickly modeling and simulating an object made of SMs has been proposed. This framework is based on voxel modeling (volumetric pixel). In addition to the simulation of SMs behaviors, the proposed theoretical framework also allows the computation of a functional distribution of SMs and conventional material; distribution which, given a stimulus, makes it possible to deform an initial form towards a desired final form. A tool – based on Grasshopper, a plug-in of the CAD software Rhinoceros® – materializing this methodological framework has also been developed.

Doctor of Philosophy (PhD) / With improvements in actuation technology and sensory systems, it is becoming increasingly feasible to create powered exoskeletal garments that can assist with the movement of human limbs. This class of robotics referred to as human-machine interfaces will one day be used for the rehabilitation of paralysed, damaged or weak upper and lower extremities. The focus of this project was the development of an exoskeletal interface for the rehabilitation of the hands. A novel sensor was designed for use in such a device. The sensor uses simple optical mechanisms centred on a spring to measure force and position simultaneously. In addition, the sensor introduces an elastic element between the actuator and its corresponding hand joint. This will allow series elastic actuation (SEA) to improve control and safely of the system. The Hand Rehabilitation Device requires multiple actuators. To stay within volume and weight constraints, it is therefore imperative to reduce the size, mass and efficiency of each actuator without losing power. A method was devised that allows small efficient actuating subunits to work together and produce a combined collective output. This work summation method was successfully implemented with Shape Memory Alloy (SMA) based actuators. The actuation, sensory, control system and human-machine interface concepts proposed were evaluated together using a single-joint electromechanical harness. This experimental setup was used with volunteer subjects to assess the potentials of a full-hand device to be used for therapy, assessment and function of the hand. The Rehabilitation Glove aims to bring significant new benefits for improving hand function, an important aspect of human independence. Furthermore, the developments in this project may one day be used for other parts of the body helping bring human-machine interface technology into the fields of rehabilitation and therapy.

This research investigates the synthesis and manufacture of shape-memory polymer (SMP) systems for use in biomedical and commodity applications. The research centers on improving the mechanical properties of thermoset acrylate copolymers with memory properties at reasonable cost through various design and manufacturing techniques: high-strain polymer synthesis and radiation crosslinking. The research assesses combinations of linear monomers and a low density of crosslinker to characterize new functional materials and optimize emerging mechanical properties such as the glass transition temperature (Tg) and rubbery modulus (ER). Exploring materials with large recoverable strains, a model copolymer of photo-polymerized methyl acrylate (MA), isobornyl acrylate and crosslinker bisphenol A ethoxylate dimethacrylate was shown to strain above 800%, twice the previously published value for SMPs, and recover fully. In the quest to maximize fully recoverable strains, a new hybrid molecule nicknamed Xini, which serves as both an initiator and a crosslinker, was also theorized, synthesized, polymerized into SMP networks and characterized. In the past, thermoset SMPs were made into complex shapes using expensive top-down techniques. A block of polymer was made and custom machining was required to craft complex parts. This prohibited devices in cost-competitive commodity application spaces. This research has proposed and validated a new method for accurately tuning the thermomechanical properties of network acrylates with shape-memory properties: Mnemosynation, eponymously named for the Greek goddess of memory. This novel manufacturing process imparts long term 'memory' on an otherwise amorphous thermoplastic material utilizing radiation-induced covalent crosslinking, and can be likened to Vulcanization, which imparts strength on natural rubber utilizing sulfur crosslinks. Adjustment of ER in the range from below 1 MPa to above 13 MPa has been demonstrated. ER was tailored by varying both radiation dose between 5 and 300 kGy and crosslinker concentration between 1.00 and 25.0 wt%. Tg manipulation was demonstrated between 23 ˚C and 70 ˚C. Mnemosynation combines advances in radiation grafting and acrylic SMP synthesis to enable both traditional plastics processing (blow molding, injection molding, etc.) and control of thermoset shape-memory properties. Combining advances in both high strain polymer synthesis and radiation crosslinking, a new paradigm in SMP composites manufacture-namely, that materials can be designed to enhance strain capacity at moderate stress, rather than maximum strength-was established. Various fibers with very different mechanical properties were impregnated with SMPs and thermo-mechanically assessed to develop an understanding of the technical parameters necessary to craft self-adjusting, multi-actuated, SMP-fiber composite orthopedic casts. This exploration syncs with the overarching aim of the research, which is to understand the fundamental scientific drivers necessary to enable new devices mass-manufactured from acrylate copolymers and optimize their emerging mechanical properties.

Stoichiometric polyelectrolyte-surfactant complexes represent a type of comb-shaped polymers, in which every polymer chain unit has an electrostatically bound &amp;quot;side chain&amp;quot;. These complexes are water-insoluble. In the solid state they assemble spontaneously into mesogenic structures. The [X,Y]-ionenes ([(CH2)XN+(CH3)2(CH2)YN+(CH3)2]nBr-2n) investigated formed stoichiometric complexes with alkyl sulfates. The ionene alkyl sulfate complexes display mesogenicity, i.e. optically isotropic dry complexes underwent lyotropic and thermotropic phase transitions to the optically anisotropic phase (and vice versa) under controlled relative humidity. The optically anisotropic phases exhibited hexagonal textures as revealed by polarizing microscopy. A new feature is the lyotropic transition brought about by the uptake of water through the gas phase. The complexes were all sensitive to both humidity and temperature. In principle, the effects can be applied to measure humidity.

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.

This thesis investigates the performance of composite frame structures with smart partially-restrained (PR) concrete filled tube (CFT) column connections through simplified 2D and advanced 3D computational simulations. It also provides a design methodology for new types of innovative connections based on achieving a beam hinging mechanism. These types of connections intend to utilize the recentering properties of super-elastic SMA tension bars, the energy dissipation capacity of low-carbon steel bars, and the robustness of CFT columns. In the first part of this study, three different PR-CFT connection prototypes were designed based on a hierarchy of strength models for each connection component. Numerical simulations with refined three dimensional (3D) solid elements were conducted on full scale PR-CFT connection models in order to verify the strength models and evaluate the system performance under static loading. Based on system information obtained from these analyses, simplified connection models were formulated by replacing the individual connection components with spring elements and condensing their contributions. Connection behavior under cyclic loads was extrapolated and then compared with the monotonic behavior. In the second part of this study, the application of these connections to low-rise composite frames was illustrated by designing both 2D and 3D, 4 and 6 story buildings for the Los Angeles region. A total of 36 frames were studied. Pushover curves plotted as the normalized shear force versus inter story drift ratio (ISDR) showed significant transition points: elastic range or proportional limit, full yielding of the cross-section, strength hardening, ultimate strength, and strength degradation or stability limit. Based on the transition points in the monotonic pushover curves, three performance levels were defined: Design Point, Yield Point, and Ultimate Point. All frames were stable up to the yield point level. For all fames, after reaching the ultimate point, plastic rotation increased significantly and concentrated on the lower levels. These observations were quantified through the use of elastic strength ratios and inelastic curvature ductility ratios. The composite frames showed superior performance over traditional welded ones in terms of ductility and stability, and validated the premises of this research.

The objective of this research is to thoroughly investigate the shape memory effect in polymers, characterize, and optimize these polymers for applications in information storage systems. Previous research effort in this field concentrated on shape memory metals for biomedical applications such as stents. Minimal work has been done on shape memory poly- mers; and the available work on shape memory polymers has not characterized the behaviors of this category of polymers fully. Copolymer shape memory materials based on diethylene glycol dimethacrylate (DEGDMA) crosslinker, and tert butyl acrylate (tBA) monomer are designed. The design encompasses a careful control of the backbone chemistry of the materials. Characterization methods such as dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC); and novel nanoscale techniques such as atomic force microscopy (AFM), and nanoindentation are applied to this system of materials. Designed experiments are conducted on the materials to optimize spin coating conditions for thin films. Furthermore, the recovery, a key for the use of these polymeric materials for information storage, is examined in detail with respect to temperature. In sum, the overarching objectives of the proposed research are to: (i) design shape memory polymers based on polyethylene glycol dimethacrylate (PEGDMA) and diethylene glycol dimethacrylate (DEGDMA) crosslinkers, 2-hydroxyethyl methacrylate (HEMA) and tert-butyl acrylate monomer (tBA). (ii) utilize dynamic mechanical analysis (DMA) to comprehend the thermomechanical properties of shape memory polymers based on DEGDMA and tBA. (iii) utilize nanoindentation and atomic force microscopy (AFM) to understand the nanoscale behavior of these SMPs, and explore the strain storage and recovery of the polymers from a deformed state. (iv) study spin coating conditions on thin film quality with designed experiments. (iv) apply neural networks and genetic algorithms to optimize these systems.

Doctor of Philosophy (PhD) / With improvements in actuation technology and sensory systems, it is becoming increasingly feasible to create powered exoskeletal garments that can assist with the movement of human limbs. This class of robotics referred to as human-machine interfaces will one day be used for the rehabilitation of paralysed, damaged or weak upper and lower extremities. The focus of this project was the development of an exoskeletal interface for the rehabilitation of the hands. A novel sensor was designed for use in such a device. The sensor uses simple optical mechanisms centred on a spring to measure force and position simultaneously. In addition, the sensor introduces an elastic element between the actuator and its corresponding hand joint. This will allow series elastic actuation (SEA) to improve control and safely of the system. The Hand Rehabilitation Device requires multiple actuators. To stay within volume and weight constraints, it is therefore imperative to reduce the size, mass and efficiency of each actuator without losing power. A method was devised that allows small efficient actuating subunits to work together and produce a combined collective output. This work summation method was successfully implemented with Shape Memory Alloy (SMA) based actuators. The actuation, sensory, control system and human-machine interface concepts proposed were evaluated together using a single-joint electromechanical harness. This experimental setup was used with volunteer subjects to assess the potentials of a full-hand device to be used for therapy, assessment and function of the hand. The Rehabilitation Glove aims to bring significant new benefits for improving hand function, an important aspect of human independence. Furthermore, the developments in this project may one day be used for other parts of the body helping bring human-machine interface technology into the fields of rehabilitation and therapy.