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Shape morphing in aerospace structures has the potential to reduce noise, improve efficiency, and increase the adaptability of aircraft. Among the many challenges in developing morphing technologies is finding suitable wing skin materials that can be both stiff to support the structural loads, while being elastic and compliant to support this shape morphing an minimize actuation energy. This remains an open challenge, but many possible solutions have been found in smart materials, namely shape memory alloys and polymers. Of these, shape memory polymers have received more attention for wing skins due to their low density and cost, and high elastic limits in excess of 100% strain, but they suffer from generally low overall moduli. Shape memory polymer composites have been considered to address this, typically in the form of particulate/nanoscale reinforcements or by using them as matrix materials in laminate composites. While these can serve to increase the stiffness of the composite, there is still a present need for reinforcement strategies that can also maintain the large changes in stiffness of shape memory polymers. An alternative shape memory composite relies on honeycomb materials with shape memory polymer infills. Previous research has shown that polymer filled honeycombs exhibit greater in-plane moduli greater than the infill or honeycomb alone, but there has been little research focused on understanding this behavior. Moreover, while most engineered cellular structures are comprised of symmetric and periodic cells, cellular structures in nature are commonly spatially varying, asymmetric networks, which have not been considered in these composites. Motivated by these challenges in designing materials for shape morphing, this work seeks to explore the use of shape memory polymer-filled honeycomb composites for use as variable stiffness materials. First, the interaction between infill and the honeycomb, and the relationship between the honeycomb geometry and the effective composite properties is not well understood. This research first investigates the mechanisms of stiffening in these composites through both unit cell finite element models and through experimental characterization. Parametric studies are completed for selected honeycomb geometry design variables, and three key mechanisms of stiffening are identified. Next, these mechanisms are further supported by experimental studies, and comparisons are made showing the limitations of the few existing analytic models. With the knowledge gained from these studies, shape memory polymer infills are considered to create variable stiffness composites. In the first study, sizing design variables are selected to parametric the honeycomb cell geometry, with the designs constrained to be symmetric in-plane. A constrained multiobjective design optimization is completed for two chosen performance objectives, and corresponding local sensitivity studies are completed as well. The results predict that these composites meet and exceed the current bounds of both shape memory polymers and their composites, but also variable stiffness materials in general. A great degree of tailorability is demonstrated, and the model predictions are validated against experimental results from fabricated honeycomb composite samples. Next, generally asymmetric cell geometries are considered by defining shape design variables for the cell geometry. These cells are constrained to be periodic but not symmetric, allowing for the possible benefits of asymmetric to be investigated. Additionally, interconnected and spatially varying multicell unit cells are considered, further allowing for the study of spatially varying cell geometries. Multiobjective optimizations are completed for two unit cell cases, and Pareto fronts are identified. The results are compared to both those from the sizing optimization study and to the current state of the art, and are similarly found to demonstrate high performance and a great degree of tailorability in effective properties. / Doctor of Philosophy / Vehicle shape morphing, the smooth, continuous change of an aircraft's external shape, can greatly improve the efficiency and reduce noise in modern and future vehicles. Among the is challenges in this field is finding suitable skin materials that can be both stiff to support the forces exerted on an aircraft, while being soft and compliant to support this shape morphing. Smart materials, namely shape memory polymers, present many attractive options for this need, but generally need to have a higher stiffness to be suitable for large scale applications. To address this, adding reinforcements to shape memory polymers has been of interest, and current work has largely been focused on using long fiber composites or particulate and nano-reinforcements. As an alternative to these strategies, inspiration can be found in nature where polygon cells are a common means of reinforcement in both plants and animals. Motivated by the current state of the art and the promise of shape morphing structures, this work seeks to investigate cellular structures in the form of hexagonal honeycombs as a means of increasing the stiffness of shape memory polymer infills. This is done by first improving the understanding of more general polymer-filled honeycomb, which exhibit effective stiffnesses greater than the honeycomb or polymer alone. With a working understanding of how the honeycomb stiffens the infill and how the cell geometry influences this behavior, variable modulus infills are next considered. First, sizing design variables (i.e. the lengths and thicknesses of the honeycomb geometry) are selected to describe cell geometries. Design optimization problems are considered and used to estimate the bounds of possible performance for these composites. Relationships between the design variables and the composite performance are investigated, and an improved understanding of these composites is developed. Next, shape design variables are selected to allow for the asymmetry and spatial variation found in natural cellular structures, and similar design optimizations are completed. The results of this work are experimentally validated, and demonstrate that these composites allow for combinations of stiffness and stiffness change that meet and exceed the current state of the art. Furthermore, tailoring the cell geometry allows for an easy means of changing the behavior of the composite. This work represents a great improvement and an important step in overcoming the challenges in developing shape morphing systems.

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This dissertation demonstrates the design of reversibly self-morphing novel liquid crystalline elastomer (LCE) antennas that can dynamically change electromagnetic performance in response to temperature. This change in performance can be achieved by programming the shape change of stimuli-responsive (i.e., temperature-responsive) LCEs, and using these materials as substrates for reconfigurable antennas. Existing reconfigurable antennas rely on external circuitry such as Micro-Electro-Mechanical-Systems (MEMS) switches, pin diodes, and shape memory alloys (SMAs) to reconfigure their performance. Antennas using MEMS or diodes exhibit low efficiency due to the losses from these components. Also, antennas based on SMAs can change their performance only once as SMAs response to the stimuli and is not reversible. Flexible electronics are capable of morphing from one shape to another using various techniques, such as liquid metals, hydrogels, and shape memory polymers. LCE antennas can reconfigure their electromagnetic performance, (e.g., frequency of operation, polarization, and radiation pattern) and enable passive (i.e., battery-less) temperature sensing and monitoring applications, such as passive radio frequency identification device (RFID) sensing tags. Limited previous work has been performed on shape-changing antenna structures based on LCEs. To date, self-morphing flexible electronics, including antennas, which rely on stimuli-responsive LCEs that reversibly change shape in response to temperature changes, have not been previously explored. Here, LCE antennas will be studied and developed. Also, the metallization of LCEs with different metal conductors and their fabrication process, by either electron beam (E-Beam) evaporation or optical gluing of the metal film will be observed. The LCE material can have a significant impact on sensing applications due to its reversible actuation that can enable a sensor to work repeatedly. This interdisciplinary research (material polymer science and electrical engineering) is expected to contribute to the development of morphing electronics, including sensors, passive antennas, arrays, and frequency selective surfaces (FSS).

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Smart materials are intelligent materials that change their structural, chemical, mechanical, or thermal properties in response to an external stimulus such as heat, light, and magnetic and electric fields. With the increase in usage of smart materials in many sensitive applications, the need for a remote, wireless, efficient, and biologically safe stimulus has become crucial. This dissertation addresses this requirement by using high intensity focused ultrasound (HIFU) as the external trigger. HIFU has a unique capability of maintaining both spatial and temporal control and propagating over long distances with reduced losses, to achieve the desired response of the smart material. Two categories of smart materials are investigated in this research; shape memory polymers (SMPs) and piezoelectric materials. SMPs have the ability to store a temporary shape and returning to their permanent or original shape when subjected to an external trigger. On the other hand, piezoelectric materials have the ability to convert mechanical energy to electrical energy and vice versa. Due to these extraordinary properties, these materials are being used in several industries including biomedical, robotic, noise-control, and aerospace. This work introduces two novel concepts: First, HIFU actuation of SMP-based drug delivery capsules as an alternative way of achieving controlled drug delivery. This concept exploits the pre-determined shape changing capabilities of SMPs under localized HIFU exposure to achieve the desired drug delivery rate. Second, solving the existing challenge of low efficiency by focusing the acoustic energy on piezoelectric receivers to transfer power wirelessly. The fundamental physics underlying these two concepts is explored by developing comprehensive mathematical models that provide an in-depth analysis of individual parameters affecting the HIFU-smart material systems, for the first time in literature. Many physical factors such as acoustic, material and dynamical nonlinearities, acoustic standing waves, and mechanical behavior of materials are explored to increase the developed models' accuracy. These mathematical frameworks are designed with the aim of serving as a basic groundwork for building more complex smart material-based systems under HIFU exposure. / Doctor of Philosophy / Smart materials are a type of intelligent materials that have the ability to respond to external stimuli such as heat, light, and magnetic fields. When these materials respond, they can change their structural, thermodynamical, mechanical or chemical nature. Due to this extraordinary property, smart materials are being used in many applications including biomedical, robotic, space, microelectronics, and automobile industry. However, due to increased sensitivity and need for safety in many applications, a biologically safe, wireless, and efficient trigger is required to actuate these materials. In this dissertation, sound is used as an external trigger to actuate two types of smart materials: shape memory polymers (SMPs) and piezoelectric materials. SMPs have an ability to store a temporary (arbitrarily deformed) shape and return to their permanent shape when exposed to a trigger. In this dissertation, focused sound induced thermal energy acts as a trigger for these polymers. A novel concept of focused ultrasound actuation of SMP-based drug delivery capsules is proposed as a means to solve some of the challenges being faced in the field of controlled drug delivery. Piezoelectric materials have an ability to generate electric power when an external mechanical force is applied and vice versa. In this study, sound pressure waves supply the external force required to produce electric current in piezoelectric disks, as a method for achieving power transfer wirelessly. This study aims to solve the current problem of low efficiency in acoustic power transfer systems by focusing sound waves. This dissertation addresses the fundamental physics of high intensity focused ultrasound actuation of smart materials by developing comprehensive mathematical models and systematic experimental investigations, that have not been performed till now. The developed models enable an in-depth analysis of individual parameters including nonlinear material behavior, acoustic nonlinearity and resonance phenomena that affect the functioning of these smart systems. These mathematical frameworks also serve as groundwork for developing more complex systems.

Surface-assisted directed assembly allows ultrasoft and replusive functional polymeric “colloids” to assemble into the organized supramolecular ultrathin films on a monomolecular level. This study aims at achieving a fundamental understanding of molecular morphology and responsive behavior of major classes of branched star-shaped polymers (star amphiphilic block copolymers and star polyelectrolytes) and their aggregation into precisely engineered functional ultrathin nanofilms. Thus, we focus on elucidating the role of molecular architecture, chemical composition, and intra/intermolecular interactions on the assembly behavior of highly-branched entities under variable environmental and confined interfacial conditions. The inherent molecular complexity of branched architectures facilitates rich molecular conformations and phase states from the combination of responsive dynamics of flexible polymer chains (amphiphilic, ionizable arms, multiple segments, and free chain ends) and extened molecular design parameters (number of arms, arm length, and segment composition/sequence). These marcromolecular building components can be affected by external conditions (pH, salinity, solvent polarity, concentration, surface pressure, and substrate nature) and transformed into a variety of complex nanostructures, such as two-dimensional circular micelles, core/shell unimicelles, nanogel particles, pancake & brush micelles, Janus-like nanoparticles, and highly nanoporous fractal networks. The fine balance between repulsive mulitarm interactions and surface energetic effects in the various confined surfaces and interfaces enables the ability to fabricate and tailor well-organized ultrathin nanofilms. The most critical findings in this study include: (1) densely packed circular unimicelle monolayers from amphiphilic and amphoteric multiblock stars controlled by arm number, end blocks, and pH/pressure induced aggregation, (2) monolayer polymer-metal nanocomposites by in-situ nanoparticle growth at confined interfaces, (3) on-demand control of exponentially or linearly grown heterogeneous stratified multilayers from self-diffusive pH-sensitive star polyelectrolyte nanogels, (4) core/shell umimicelle based microcapsules with a fractal nanoporous multidomain shell morphology, and (5) preferential binding and ordering of Janus-like unimicelles on chemically heterogeneous graphene oxide surfaces for biphasic hybrid assembly. The advanced branched molecular design coupled with stimuli responsive conformational and compositional behavior presents an opportunity to control the lateral diffusion and phase segregation of branched compact supermolecules on the surface resulting in the generation of well-controllable monolayers with tunable ordering and complex morphology, as well as to tailor their stratified layered nanostructures with switchable morphological heterogeneity and multicompartmental architectures. These surface-driven star polymer supramolecular assemblies and interfaces will enable the design of multifunctional nanofilms as hierarchical responsive polymer materials.

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