Thermally/Mechanically Responsive Polymeric Composites with Shape Memory/Self-Healing Properties

Smart materials with the ability to respond to external stimulus have attracted tremendous attention in both academic institutions and industry. Such unique aspect of these materials make them great candidates to address current challenges in the world of Materials Science including those that no human intervention is a necessity. The objective of this dissertation is to employ different techniques to develop novel thermally and mechanically responsive polymeric composites for several industrial applications. In Chapter 2, the curing kinetics and polymerization induced phase separation (PIPS) of an epoxy matrix with and without a semicrystalline thermoplastic was investigated. Same PIPS technique studied in this chapter was utilized in the following chapters to develop composites featuring triple shape memory and self-healing properties as we now describe. In Chapter 3, we report an investigation into the preparation and characterization of shape memory assisted self-healing (SMASH) coatings utilizing shape memory (SM) response of a glassy amorphous epoxy matrix and rebonding the crack by a low melt-viscosity thermoplastic. Note that same materials and polymerization induced phase separation (PIPS) technique as in Chapter 2 was employed in this chapter. Seeking to develop a simpler method to fabricate composites featuring SMASH, we successfully designed a setup for dual-electrospinning two immiscible polymers in Chapter 5. Specifically Poly(vinyl acetate) (PVAc) and poly(ε-caprolactone) (PCL) solutions were dual-electrospun to fabricate composites featuring shape memory assisted self-healing (SMASH) and SM properties. The resulting material was capable of restoring its shape and mechanical properties with a simple thermal trigger. Continuing on the subject of Self-Healing , fiber reinforced composites (FRCs) in which the healing agent was encapsulated in polymeric fibers and released upon fracture were developed and studied in Chapter 4. Unique core-sheath fibers featuring stiff polyacrylonitrile (PAN) in the sheath and epoxy based self-healing agents in the core were fabricated by coaxial electrospinning. Upon damage, fibers break and the healing agent would flow to the damage site and polymerize to restore the mechanical properties of the composites. The proposed FRCs will lead to a cost-effective and much more durable composite structure capable of withstanding loads that would otherwise fail due to lack of proper reinforcement. Chapter 6 then represents a novel strategy exploiting organic based layered double hydroxides (LDHs) to enhance mechanical and barrier properties of tire rubber-LDH composites. Pneumatic tires are composite structures inflated with pressurized gas to provide weight support, shock absorbance and traction transmission for an automotive. Therefore, developing tires that can hold inflation pressure for an extensive period of time is of great interest in auto industry. This was achieved by nano-exfoliation of the organic based LDHs in tire rubber. The relationship between microstructure and mechanical properties of such composites were investigated in this chapter. In the case of Shape Memory , Chapter 7 will focus on fabrication of triple shape memory composites featuring a semicrystalline thermoplastic and an amorphous epoxy. Such composites utilize polymerization induced phase separation (PIPS) (introduced in Chapter 2) and exhibit two distinct transition temperatures required for triple shape memory behavior. This study explores the relationships between the morphology of triple shape memory polymers and their shape memory characteristics. Finally, Chapter 8 explores for the first time design, preparation, and characterization of triple shape memory polymeric foams that is open cell in nature and features a two phase, crosslinked SMP with a glass transition temperature of one phase at a temperature lower than a melting transition of the second phase. The soft materials were observed to feature high fidelity, repeatable triple shape behavior, characterized in compression and demonstrated for complex deployment by fixing a combination of foam compression and bending. We further explored the wettability of the foams, revealing composition-dependent behavior favorable for future work in biomedical investigations. It is noteworthy that all the aforementioned materials and methods exhibit great potential for industrial applications considering their simplicity and low manufacturing costs. Beside, future work is required for each project some of which are listed at the end of each chapter

Soft Shape-memory Polymers As A Platform For Biomedical Applications

The overall objective of this work was to expand on the previous efforts carried out by other researchers to develop series of smart or stimulus-responsive shape-memory polymers for biomaterials applications. For this purpose, novel shape-memory polymers were fabricated and their macrostructure and microstructure were studied to understand their effects on overall shape-memory characteristics and mechanical properties of these materials. The advent of shape-memory polymers has significantly influenced the development and rapid growth of various functional polymers. Shape-memory polymers are used where the dynamic functions of polymers under an applied stimulus are required, and they find applications as catheters, sutures, drug delivery systems, and scaffolds in tissue regeneration, as well as aerospace applications. Each of these applications demands materials with unique chemical, physical, and mechanical properties to provide efficient functions. Consequently, a wide range of shape-memory polymers have been developed and investigated for these applications, but more research is required to optimize the overall property and function of these polymers. Furthermore, recent advances in the field of polymer science and shape-memory polymers, coupled with the novel characterization methods, necessitate the development of novel functional polymers for specific applications. This dissertation highlights various polymeric materials currently investigated for use in applications requiring shape-memory polymers. Chapter 1 gives an overview of biomaterials along with a background on shape-memory polymers. In the first case described in Chapter 2, epoxy-based triple shape-memory composites (TSMCs) were investigated, and the poly(ε-caprolactone) (PCL) compositional effect on triple shape-memory behavior was explored using heat and water stimuli. The TSMCs were developed using PIPS to achieve particle/matrix morphology. In Chapter 3, two newly TSMCs, one featuring a semicrystalline epoxy and the other featuring an amorphous epoxy, were explored, and the relationships between the morphology of TSMCs and their shape-memory characteristics were studied. Chapter 4 focuses on studying the effect of morphology on shape-memory behavior. This study reveals the effect of particle/matrix and co-continuous fiber/matrix morphology on triple shape-memory behavior of polymers with similar compositions. The knowledge, which was built upon the results of Chapter 2 through 4, would help in optimization of design strategy used for fabrication of triple shape memory polymers with enhanced shape fixing and recovery. In Chapter 5, an innovative smart anisotropic polymeric hydrogel was introduced which can be activated using hydration. The developed anisotropic hydrogel forms helicoids in response to hydration; and the dependence of the radius of curvature and the pitch of the formed helicoids on fiber angle orientation and thickness of hydrogel composites was evident. In Chapters 6 and Chapter 7, another class of shape memory polymers was investigated: liquid crystalline elastomer. Chapter 6 focuses on a new design strategy for fabrication of a hydrogel-forming liquid crystalline elastomer that exhibited soft shape memory properties in response to thermal and water stimuli. This approach involved incorporating liquid crystalline mesogens into the polymer networks to fabricate unique materials. The effect of liquid crystalline mesogens on thermal, mechanical, and shape memory performance of these unique materials was studied. Chapter 7 focuses on epoxy based liquid crystalline elastomers. Similar to Chapter 6, it was confirmed that unique and responsive smart materials can be fabricated using liquid crystalline mesogens. In this study, a better route to synthesize LCEs, based on epoxy chemistry with amenability to open air and catalyst-free synthesis, was introduced. Finally in Chapter 8, another class of responsive smart polymers: near infrared fluorescence shape memory web containing indocyanine green dye, was investigated. This approach presents a new design strategy, for incorporate dyes uniformly without manipulating their properties, to fabricate polymers with unique imaging and shape memory characteristics. For this study, incorporation of ICG dye into PVAc polymer was achieved using the electrospinning technique, yielding near infrared fluorescence polymeric material with high fluorescence intensity and uniform dye incorporation. All the aforementioned polymeric materials have great potentials for different applications and can significantly influence the growth and development of new biomaterials and medical devices. Chapter 9 discusses the conclusions and provides recommendations for future research and development for each chapter of the dissertation

On the thermoelastic analysis of solar cell arrays and related material properties

Accurate prediction of failure of solar cell arrays requires accuracy in the computation of thermally induced stresses. This was accomplished by using the finite element technique. Improved procedures for stress calculation were introduced together with failure criteria capable of describing a wide range of ductile and brittle material behavior. The stress distribution and associated failure mechanisms in the N-interconnect junction of two solar cell designs were then studied. In such stress and failure analysis, it is essential to know the thermomechanical properties of the materials involved. Measurements were made of properties of materials suitable for the design of lightweight arrays: microsheet-0211 glass material for the solar cell filter, and Kapton-H, Kapton F, Teflon, Tedlar, and Mica Ply PG-402 for lightweight substrates. The temperature-dependence of the thermal coefficient of expansion for these materials was determined together with other properties such as the elastic moduli, Poisson's ratio, and the stress-strain behavior up to failure

Multiscale viscoplastic-viscodamage analysis of shape memory polymer fibers with application to self healing smart materials

Self-healing smart material systems have been introduced into the research arena and they have already been deployed into industrial applications. The Close-Then-Heal (CTH) healing mechanism for polymeric self-healing systems is addressed herein and then a new generation of Shape Memory Polymer (SMP) based self-healing system is proposed in this work. This system incorporates SMP fibers to close the cracks while the embedded Thermoplastic Particles (TPs) are diffused into the crack surfaces upon heating and provide a molecular level of healing. The SMP fiber manufacturing procedure is briefly addressed in this work in which the bobbin of SMP fibers are heat treated in a specific procedure and then they are wound to produce SMP fibers. The performance of the proposed healing system is highly dependent on mechanical responses of SMP fibers. The polyurethane SMP fibers are categorized as semicrystalline polymeric material systems. These semicrystalline SMP fibers are then constituted from two distinguishable phases, which are amorphous and crystalline polymers. Such a multiphase system can be evaluated through a multiscale analysis within the micromechanics framework in which the macroscopic mechanical responses are evolved through averaging the microscale mechanical fields. Then in this research the constitutive relation for each of the micro-constituents are utilized to compute the microscale mechanical fields and then these fields are correlated to the macroscopic field through the micromechanics framework. The cyclic viscoplastic and viscodamage of these fibers are of utmost importance for designing self-healing systems in which repeatability of the healing process and the healing efficiency for subsequent healing cycles are highly dependent on cyclic responses of these fibers. A new approach in measurement of cyclic damage of SMP fibers is proposed in this work in which the reduction in recoverable stress after each cyclic stress recovery is correlated to the damage. In this approach the damage is interpreted as failure of the polymeric bonds to recover their original shape (SM effect). In general the proposed self-healing scheme establishes a new generation of self-healing systems while the developed theoretical multiscale analysis provides a well-structured method to investigate the cyclic viscoplastic and viscodamage of the SMP fibers

Smart hybrid nanomaterials for biomimetic membranes

This thesis focuses on the preparation of nanomaterials made of proteins and polymers. Even though the technology has advanced in the last decades to design new devices at the atomic scale, researchers are still inspired by what Nature has produced and optimized for millions of years. Following this concept, this work uses proteins forming water-filled channels, called porins, which regulate the flow of ions and biomolecules in cellular life. Two proteins were studied: Omp2a and VDAC36. The first part of the dissertation is the thermomechanical properties study of the latest hybrid membrane developed by the IMEM group: an thin nanoperforated poly(lactic acid) (PLA) film with the Omp2a porin immobilized onto the surface . For this purpose, a new equipment based on the microcantilever technology was used. The SCAnning LAser analyzer (SCALA) characterizes the coated cantilevers which allows the following of the cantilever bending induced by the compression/expansion of the sample coating (i.e. proteins or polymers). In this study, the intermolecular reorganization of Omp2a aggregates was evidenced as well as the protein secondary structure stability against temperature. The same method was employed to study the impact of nanofeatures (perforations and drugs domains) on films of PLA. They affected the glass transition and the cold crystallization temperatures. The changes were dependent on the size and abundance of the nanofeatures, which can modulate the properties of future materials. Moreover, this work established a protocol for the study of biomolecules and polymers attached to microcantilevers, allowing an accurate study of the thermomechanical properties using very low amounts of sample. The second part of the thesis is the development of new hybrid nanomaterials composed of VDAC36, PLA and poly(3,4-ethylenedioxythiophene) (PEDOT). An efficient protocol was established for the production of VDAC36 and its subsequent refolding was achieved. The beta-barrel nature of the protein was revealed and its tendency to form oligomers was demonstrated. Finally, the size of the protein inner channel could be determined. The VDAC36 was added to the polymer material made of three alternating layers of PLA and PEDOT. The electrical properties of the material were modified by the addition of the protein: the overall resistance was reduced and the supercapacitive behaviour was enhanced. The description of the electrical equivalent circuit also revealed that the protein induced the diffusion of ions. To improve the material, the number of layers was increased and the conducting polymer was modified by incorporating a monomer bearing a dodecyl chain. The modifications were proved useful as the protein content and the electrical properties increased. Finally, the new hybrid material could provide an adaptive electrical response according to the concentration of biomolecules.Esta tesis se centra en la preparación de nanomateriales basados en proteínas y polímeros. A pesar de los avances realizados en las últimas décadas en el diseño de nuevos dispositivos a escala nanométrica, los investigadores aún se inspiran en lo que la Naturaleza ha producido y ha optimizado durante millones de años. A partir de esta premisa, en este trabajo se han usado proteínas, que constituyen canales de agua y cuya función es regular el paso de iones y biomoléculas en organismos celulares. Las proteínas involucradas son Omp2a y VDAC36. La primera parte de esta disertación se centra en el estudio de las propiedades termo-mecánicas de los componentes una novedosa membrana híbrida desarrollada per el grupo IMEM: una película ultra-delgada de ácido poli(láctico) (PLA) nano-perforada y funcionalizada en la superficie con moléculas de Omp2a. Para su caracterización se usó un nuevo equipo basado en la tecnología de micro-palancas. Un analizador laser de barrido (SCALA, el acrónimo de dicho aparato en inglés) permite caracterizar palancas recubiertas de muestra polimérica mediante la reflexión de un rayo de luz láser sobre la superficie del soporte revestido. Mediante su acoplado a una cámara termo-controlada, SCALA permite seguir la deformación del soporte inducida per la compresión/expansión de la muestra en forma de recubrimiento (ya sean polímeros como proteínas). Mediante esta técnica se evidenció la reorganización intermolecular en agregados de la proteína Omp2a, así como la alta estabilidad de su estructura secundaria en frente de la temperatura. El mismo método fue usado para estudiar el impacto de las nano-características sobre las películas de PLA. Nano-poros, nano-perforaciones y nano-dominios fueron añadidos a los films de PLA. Dichas modificaciones afectan tanto a su transición vítrea como a la cristalización en frío de dichas películas. Los cambios observados dependen del tamaño y la abundancia de las nano-modificaciones, lo cual va a permitir modular las propiedades de futuros nano-materiales. Más aún, este trabajo ha establecido las bases para un protocolo general de uso de micro-palancas para estudiar proteínas y polímeros unidos a ellas, permitiendo la caracterización de sus propiedades termo-mecánicas usando cantidades ínfimas de material. Se pudo establecer un protocolo eficiente para la producción de VDAC36 i su subsecuente re-naturalización por medio de una combinación de detergentes y alcoholes. Per medio de experimentos de dicroísmo circular se puso de manifiesto su naturaleza de barril beta y se mostró su tendencia a formar oligómeros mediante entrecruzamientos químicos. El tamaño del poro se pudo determinar mediante ensayos de hinchado. A continuación, VDAC36 se incorporó al material polimérico constituido por tres capas de polímero, alternando PLA y PEDOT. Las propiedades eléctricas de este material quedaron visiblemente modificadas por la adición de la proteína sobre los films de polímero: se redujo su resistancia mientras que su comportamiento como supercondensador, consecuencia la presencia de PEDOT, aumentó. La descripción del circuito eléctrico equivalente reveló a su vez que la proteína inducía la difusión de iones. Para mejorar la retención de proteínas y la integridad mecánica del material, las capas de polímero de la membrana se aumentaron hasta cinco. A su vez, el monómero de EDOT se modificó para incorporar una cadena de dodecilo y poder así imitar una membrana celular. Estas últimas modificaciones se mostraron de gran utilidad puesto que el contenido en proteína aumentó y los cambios eléctricos se hicieron más pronunciados. Finalmente, este nuevo material híbrido fue capaz de proporcionar una respuesta eléctrica adaptativa como respuesta a cambios en la concentración de biomoléculas.Postprint (published version

Thermo-visco-elastic modelling of photovoltaic laminates: Advanced shear-lag theory and model order reduction techniques

During lamination, residual thermo-mechanical stresses are induced in the encapsulated solar cells composing photovoltaic (PV) modules. Depending on the material and geometrical configuration of the layers of the laminate, this residual stress field can be beneficial since it may lead to a compressive stress state in Silicon and therefore crack closure effects in the presence of cracks, with a recovery of electrical conductivity in cracked solar cells. It is therefore important to investigate the distribution of thermo-mechanical stresses within the PV laminate with a view to optimizing the coupling between the electrical response and elastic deformation in the operation of PV modules. A promising approach proposed in the present thesis regards the prediction of residual stresses in composite laminates by using a shear-lag theory to model the epoxy-vinil-acetate polymeric layers, accounting for their thermo-visco-elastic response. Moreover, it will be shown that thermomechanical formulations for stress analysis of a PV laminate lead to a system of higher order ordinary differential equations or partial differential equations in which the exact solutions may be impossible to be determined in closed form and hence numerical schemes become desirable. However, the computational cost associated with the implementation of the numerical scheme may be significantly expensive. Therefore, a method to reduce the computational complexity is expected to be very important. To this aim, Model Order Reduction (MOR) techniques are applied hierarchically, first to the thermal system of a PV module in service, and then extended to coupled thermo-mechanical problems. A combination of proper orthogonal decomposition (POD) and discrete empirical interpolation method (DEIM) with a modified formulation is proposed for the first-order thermal equations of photovoltaic system during service and a new coupled second-order Krylov based formulation is developed for model order reduction of the coupled thermo-mechanical model of the photovoltaic module. The results of these reduction schemes show a huge computational gain in the reduced system solutions and a high accuracy of the reduced system outputs

Structure-Property Relationships in Novel Electrospun Composites for Advanced Applications

Active polymeric materials that alter shape in response to an external stimulus offer unique avenues for the design and study of dynamic structures. This research focused on developing elastomeric polymer composites with multiple functionalities by exploring the design and properties for various applications including controlled drug delivery and shape memory. The first part of this dissertation describes the fabrication and characterization of a soft, elastomeric polymeric composite with inherent shape memory properties capable of localized, long-term tunable drug release. In Chapter 2, the fibers were loaded with a hydrophilic drug model, Rhodamine B, and embedded within a siloxane-based elastomeric matrix to form a composite, which is critical to regulating water transport from the environment to the fibers to liberate the drug. In vitro drug release studies were conducted in PBS under physiological conditions to evaluate the effect of drug concentration, fiber size, fiber crystallinity, drug loading and the addition of the crosslinked siloxane. The effect of the microstructural properties of the fibrous phase on drug release were explored and tuned through thermal treatment of the composite. The findings from Chapter 2 were then applied to Chapter 3 for the development of a vascular graft with controlled and sustained nitric oxide (NO) releasing capabilities and suitable mechanical properties for the prevention of restenosis. To avoid the unwanted systemic side effects associated with a free radical such as NO, our approach delivered NO locally by supplying it from the vascular graft material. It was found that reducing the tin catalyst used for crosslinking the silicone constituent significantly improved cell viability, however, the NO interacted with the catalyst activity, affecting the silicone crosslinking reaction. The NO-releasing composite was demonstrated to be a strong chemottractant to endothelial cells. The next part of this research focused on the development of a shape memory elastomeric composite featuring thermoplastic fibers imbibed by polyanhydride-based elastomer. It was determined in Chapter 4 that the polyanhydride elastomer is capable of dynamic covalent exchange reactions at elevated temperatures among the network chains that allowed near-complete reconfiguration of the permanent shape in the solid state. Together, these features were combined to create a shape memory elastomer capable of arbitrary programming of both temporary and permanent shapes. The degradation properties of this composite were then studied in Chapter 5 under in vitro conditions, where it was revealed that the degradation rate of the PAH matrix was strongly influenced by the selection of the composition of polymeric fibrous phase. The degradation of this composite was found to occur as a modified surface to bulk degradation, although the PAH by itself erodes heterogeneously. A hydrophilic model drug was incorporated in the fibrous phases and used to study the in vitro controlled release properties of these composites, where drug release correlated with the matrix degradation. The shape memory properties of these polyanhdride-based compositions were also examined. Lastly, Chapter 6 investigated the design, fabrication, and characterization of a polymeric composite composed of oriented semicrystalline polymeric fibers embedded within a crosslinked epoxy matrix. This anisotropy enabled the construction of complex three dimensional geometries featuring latent mechanical programming. Rather than relying on specific molds to manipulate a new shape, this system capitalized on strain conditioning to influence a new structure. Additionally, we found that by compressing the oriented fibers of each ply during composite cure, the composites constructed from such plies exhibited actuation. The shape memory composites studied in this dissertation demonstrated the potential be broadly applicable from drug releasing implants, tailorable degradability, and the self-assembly of complex shapes. Chapter 7 provides some recommendations for future directions

Phase-segregated copolyetherimides. Optimization of the structure and composition for gas separation applications.

Se ha realizado el estudio de sistemas de copolieter-imidas, que tienen la capacidad de segregar en fases para su aplicación en el campo de la separación de gases, concretamente en la separación CO2/N2. Se realizó el análisis de la influencia en las propiedades de los copolímeros de: su composición (utilizando diferentes dianhidridos, diaminas y poliéteres); y su estructura (variando la longitud y cantidad de polieter); y por último se ha desarrollado un modelo predictivo de propiedades de permeación. La permabilidad al CO2 se vió favorecida con una mejora en la segregación de fases, y esto ocurría cuando aumentabamos la rigidez de los monómeros, la longitud o el contenido en poliéter; sin efecto en la selectividad . A porcentajes elevados de polieter, la fase aromática no tenía influencia en las propiedades de separación. El modelo EMT predijo todas estas situaciones sin necesidad de conocer detalles estructurales de los copolimeros sintetizados.Departamento de Física Aplicad

Development of Dual-Cure Hybrid Polybenzoxazine Thermosets

Polybenzoxazines are potential high performance thermoset replacements for traditional phenolic resins that can undergo an autocatalytic, thermally initiated ring - opening polymerization, and possess superior processing advantages including excellent shelf-life stability, zero volatile loss and limited volumetric shrinkage. The simplistic monomer synthesis and availability of a wide variety of inexpensive starting materials allows enormous molecular design flexibility for accessing a wide range of tailorable material properties for targeted applications. Despite the fact, once fully cured, benzoxazines are difficult to handle due to their inherent brittleness, leaving a very little scope for any modifications. The motivation of this dissertation is directed towards addressing the common limitations of polybenzoxazines and to enable tailor made material properties for expanding the scope of future applications. In this work, a unique approach has been demonstrated incorporating a dually polymerizable bifunctional benzoxazine based monomer; designed to form a sequentially addressable intermediate B-staged network, followed by the formation of a final hybrid network via thermal curing of benzoxazines. This strategy offers a systematic route to study the formation of glassy polymeric materials in discrete, orthogonal steps, and a handle to access a broad range of material properties within the same system. The dissertation study is focused on manipulating the monomer design, to study different cure chemistries, in conjunction with benzoxazines. These cure chemistries included - rapid UV curable thiol-ene click chemistry, thermally curable ring-opening metathesis polymerization of norbornene, and free radical photo-polymerization of meth(acrylate) functionalities. A strong fundamental understanding of structure-property relationships with respect to network structure, kinetics, processing control and material properties of the hybrid networks was established