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School of Materials Science and Engineering

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https://hdl.handle.net/1853/70762

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College of Engineering

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Publication Search Results

Now showing 1 - 10 of 182

Optimization of mechanical properties and manufacturing techniques to enable shape-memory polymer processing

(Georgia Institute of Technology, 2009-11-20) Voit, Walter Everett

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.
Mesoporous silica/polymer nanocomposites

(Georgia Institute of Technology, 2009-11-13) Liu, Yi

New approaches through grafting initiators onto the surface of inner-wall of mesoporous silica to synthesize polymer inside the nano-channels to obtain mesoporous silica/polymer nanocomposites were developed and investigated. Using the newly developed approach, PMMA was successfully synthesized through free radical polymerization and nylon 6 though in situ anionic ring-opening polymerization inside the nano-channels. The spherical mesoporous silica/PMMA composites we obtained showed a higher degradation temperature and narrower degradation range than pure commercially available PMMA. Spherical PMMA capsules were obtained after the silica network was dissolved with hydrofluoric acid, these pure PMMA spheres had the same thermal properties and morphology as they had with in the composites. The BMS/nylon 6 nanocomposites were spheres with the same diameter as pure BMS. About 50 wt% of the composites was newly synthesized nylon 6. The synthesized nylon 6 was proven to contain both α-form crystallite and γ-form crystallites with covalent bonds with the surface of silica inside the nano-channels.
Mechanisms regulating osteoblast response to surface microtopography and vitamin D

(Georgia Institute of Technology, 2009-11-11) Bell, Bryan Frederick

A comprehensive understanding of the interactions between orthopaedic and dental implant surfaces with the surrounding host tissue is essential in the design of advanced biomaterials that better promote bone growth and osseointegration of implants. Dental implants with roughened surfaces and high surface energy are well known to promote osteoblast differentiation in vitro and promote increased bone-to-implant contact in vivo. In addition, increased surface roughness increases osteoblasts response to the vitamin D metabolite 1α,25(OH)2D3. However, the exact mechanisms mediating cell response to surface properties and 1α,25(OH)2D3 are still being elucidated. The central aim of the thesis is to investigate whether integrin signaling in response to rough surface microtopography enhances osteoblast differentiation and responsiveness to 1α,25(OH)2D3. The hypothesis is that the integrin α5β1 plays a role in osteoblast response to surface microtopography and that 1α,25(OH)2D3 acts through VDR-independent pathways involving caveolae to synergistically enhance osteoblast response to surface roughness and 1α,25(OH)2D3. To test this hypothesis the objectives of the studies performed in this thesis were: 1) to determine if α5β1 signaling is required for osteoblast response to surface microstructure; 2) to determine if increased responsiveness to 1α,25(OH)2D3 requires the vitamin D receptor, 3) to determine if rough titanium surfaces functionalized with the peptides targeting integrins (RGD) and transmembrane proteoglycans (KRSR) will enhance both osteoblast proliferation and differentiation, and 4) to determine whether caveolae, which are associated with integrin and 1α,25(OH)2D3 signaling, are required for enhance osteogenic response to surface microstructure and 1α,25(OH)2D3. The results demonstrate that integrins, VDR, and caveolae play important roles in mediating osteoblast response to surface properties and 1α,25(OH)2D3. Silencing of the β1 integrin in osteoblast-like MG63 cells significantly reduced osteogenic response to surface topography and 1α,25(OH)2D3. Silencing of the α5 subunit did not alter the response of MG63 cells to changing surface roughness or chemistry, although future work must confirm these results given similar cell surface α5 integrin expression observed in control and α5-silenced cells. Multifunctional RGD, KRSR, and KSSR coated surfaces show that RGD increased osteoblast proliferation and reduced differentiation, KRSR had no affect on osteoblast phenotype, and KSSR increased osteoblast differentiation. These results suggest that titanium surfaces can be modified to manipulate proliferation and differentiation and that RGD/KSSR functionalized surfaces could be further investigated for use as osteointegrative surfaces. The results using VDR deficient osteoblasts demonstrate that 1α,25(OH)2D3 acts via VDR-dependent mechanisms in cells cultured on titanium surfaces that support terminal differentiation. In caveolae deficient osteoblasts, 1α,25(OH)2D3 affected cell number, alkaline phosphatase activity, and TGF-β1 levels, although levels of osteocalcin and PGE2 were not affected. These results are consistent with the hypothesis that VDR is required for the actions of 1α,25(OH)2D3, but that caveolae-dependent membrane 1α,25(OH)2D3 signaling modulates traditional VDR signaling. The exact mechanisms for this interaction remain to be shown. Overall, these results are important in better understanding the role of β1 integrin partners in mediating osteoblast response to implant surfaces and in understanding how integrin signaling can alter osteoblast differentiation and responsiveness to 1α,25(OH)2D3 via genomic and non-genomic pathways.
The structure-property relation in nanocrystalline materials: a computational study on nanocrystalline copper by Monte Carlo and molecular dynamics simulations

(Georgia Institute of Technology, 2009-11-10) Xu, Tao

Nanocrystalline materials have been under extensive study in the past two decades. The reduction in grain size induces many abnormal behaviors in the properties of nanocrystalline materials, that have been investigated systematically and quantitatively. As one of the most fundamental relations in materials science, the structure-property relation should still apply on materials of nano-scale grain sizes. The characterization of grain boundaries (GBs) and related entities remains a big obstacle to understanding the structure-property relation in nanocrystalline materials. It is challenging experimentally to determine the topological properties of polycrystalline materials due to the complex and disordered grain boundary network presented in the nanocrystalline materials. The constantly improving computing power enables us to study the structure-property relation in nanocrystalline materials via Monte Carlo and molecular dynamic simulations. In this study, we will first propose a geometrical construction method based on inverse Monte Carlo simulation to generate digital microstructures with desired topological properties such as grain size, interface area, triple junction length as well as their statistical distributions. The influences on the grain shapes by different topological properties are studied. Two empirical geometrical laws are examined including the Lewis rule and Aboav-Weaire law. Secondly, defect free nanocrystalline Copper (nc-Cu) samples are generated by filling atoms into the Voronoi structure and then relaxed by molecular dynamics simulations. Atoms in the relaxed nc-Cu samples are then characterized into grain atoms, GB interface atoms, GB triple junction atoms and vertex atoms using a newly proposed method. Atoms in each GB entity can also be identified. Next, the topological properties of nc-Cu samples before and after relaxation are calculated and compared, indicating that there exists a physical limit in the number of atoms to form a stable grain boundary interface and triple junction in nanocrystalline materials. In addition, we are able to obtain the statistical averages of geometrical and thermal properties of atoms across each GB interfaces, the so-called GB profiles, and study the grain size, misorientation and temperature effects on the microstructures in nanocrystalline materials. Finally, nc-Cu samples with different topological properties are deformed under simple shear using MD simulation in an attempt to study the structure-property relation in nanocrystalline materials.
The role of the catalyst in the growth of one-dimensional nanostructures

(Georgia Institute of Technology, 2009-11-10) Kirkham, Melanie

Quasi one-dimensional (1D) nanostructures show great promise for many applications, including in solar cells, nanogenerators and chemical sensors, due to the high surface-to-volume ratio and unique properties of nanostructures. The growth of these nanostructures is commonly catalyzed by metal nanoparticles and generally attributed to the vapor-liquid-solid (VLS) mechanism. The purpose of this research is to better understand the role of the catalyst nanoparticles in the growth of 1D nanostructures, in order to allow improved control of the synthesis process. To this end, nanostructures were grown with a variety of compositions, including Au- and Sn-catalyzed ZnO, Au-catalyzed FexOy and Au-catalyzed Si nanostructures. The morphology of the nanostructures was characterized with electron microscopy, and the crystallographic orientation with X-ray texture analysis. The catalyst particles were further characterized with both in-situ and post-growth X-ray diffraction. The types of bonding in the source material and catalyst play a significant role in the diffusion path of the source material to the growth front and in the catalyst particle state during growth. Dissimilar bonding types in the source material and catalyst prevent bulk diffusion of the source material through the catalyst, thereby preventing eutectic melting of the catalyst. These results bring new insight into the catalyzed growth of 1D nanostructures and assist in the informed choice of appropriate catalyst materials, which may aid the utilization of 1D nanostructures in energy-related and other applications.
Microstructural optimization of solid-state sintered silicon carbide

(Georgia Institute of Technology, 2009-08-11) Vargas-Gonzalez, Lionel Ruben

In this work, the development of theoretically-dense, clean grain boundary, high hardness solid-state sintered silicon carbide (SiC) armor was pursued. Boron carbide and graphite (added as phenolic resin to ensure the carbon is finely dispersed throughout the microstructure) were used as sintering aids. SiC batches between 0.25-4.00 wt.% carbon were mixed and spray dried. Cylindrical pellets were pressed at 13.7 MPa, cold-isostatically pressed (CIP) at 344 MPa, sintered under varying sintering soaking temperatures and heating rates, and varying post hot-isostatic pressing (HIP) parameters. Carbon additive amounts between 2.0-2.5 wt.% (based on the resin source), a 0.36 wt.% B4C addition, and a 2050°C sintering soak yielded parts with high sintering densities (~95.5-96.5%) and a fine, equiaxed microstructure (d50 = 2.525 µm). A slow ramp rate (10°C/min) prevented any occurrence of abnormal grain growth. Post-HIPing at 1900°C removed the remaining closed porosity to yield a theoretically-dense part (3.175 g/cm3, according to rule of mixtures). These parts exhibited higher density and finer microstructure than a commercially-available sintered SiC from Saint-Gobain (Hexoloy Enhanced, 3.153 g/cm3 and d50 = 4.837 µm). Due to the optimized microstructure, Verco SiC parts exhibited the highest Vickers (2628.30 ± 44.13 kg/mm2) and Knoop (2098.50 ± 24.8 kg/mm2) hardness values of any SiC ceramic, and values equal to those of the "gold standard" hot-pressed boron carbide (PAD-B4C). While the fracture toughness of hot-pressed SiC materials (~4.5 MPa m1/2) are almost double that of Verco SiC (2.4 MPa m1/2), Verco SiC is a better performing ballistic product, implying that the higher hardness of the theoretically-dense, clean-grain boundary, fine-grained SiC is the defining mechanical property for optimization of ballistic behavior.
Learning and applying material-based sensing lessons from nature

(Georgia Institute of Technology, 2009-07-06) McConney, Michael Edward

The work presented in this dissertation was aimed at understanding biology's application of soft materials to enhance sensing abilities and initiate innovative bio-inspired material-based approaches for flow (fluidic and air) sensors and photo-thermal sensors. A key aim is to help strengthen this niche of functional materials science referred to, here, as bio-inspired materials in sensing roles. The work aspires to traverse the boundaries of the subject in order to provide a strong foundation for future scientific explorations of the subject. The studies presented here, include studies of flow sensing in fish and implementing a bio-mimetic approach to microfabricated flow sensors. The work also includes studies of material based signal filtering in spiders, as well as, bio-inspired photo-thermal transduction mechanisms. The capabilities of the methodology are demonstrated with successful engineering studies.
Microstructures and multifunctional microsystems based on highly crosslinked polymers

(Georgia Institute of Technology, 2009-07-02) Singamaneni, Srikanth

The work elucidates the novel physical and thermal properties of thin and ultra-thin films of crosslinked polymer and organized microstructures with a special emphasis on surface and interfacial effects and the structure-property relationships. Two major crosslinked polymer coatings have been thoroughly investigated: polymer microstructures fabricated by multi-laser interference lithography (IL), and plasma polymer coatings. We unveiled intriguing thermal properties of plasma polymer films originating from their physical state and exploiting the same for the design of ultrasensitve chemical sensors. A novel paradigm of surface coatings, single and bi-component periodic, porous crosslinked polymeric structures, has been introduced and thoroughly studied. Surface, interfacial, and mechanical properties of these novel class crosslinked polymer coatings clearly demonstrate the enormous potential of the IL microstructures as organized multicomponent polymer systems. When subjected to external or internal stresses the periodic porous structures can exhibit a sudden and dramatic pattern transformation resulting in remarkable change in the photonic, phononic and mechanical properties of these structures. Furthermore, the confinement of these instabilities to localized regions results in complex hierarchical structures. The two polymer coatings (plasma polymers and IL microstructures) with complementary attributes (such as periodic structure, vertical stratification, residual internal stresses, and high surface and interface tunability) enabled us to understand and design novel multifunctional polymer coatings.
Organic/inorganic nanostructured materials: towards synergistic mechanical and optical properties

(Georgia Institute of Technology, 2009-06-29) Gunawidjaja, Ray

Two designs of inorganic/organic hybrid micro-structures are discussed: (1) silver nanowire reinforced layer-by-layer (LbL) polyelectrolyte composite film and (2) bimetallic silver-gold core-shell nanoparticles. In this work, zero-dimensional spherical gold nanoparticles (AuNPs), one-dimensional silver nanowires (AgNWs), and two-dimensional silver nanoplates (AgNPls) represent the inorganic component. Three-arm star polymer and polyelectrolytes represent the organic component. In the first design, the one-dimensional AgNWs serves as a mechanical reinforcement for the fabrication of mechanically isotropic and anisotropic polyelectrolyte composite films. The composite film is mechanically isotropic when the AgNWs are randomly oriented, and it is anisotropic when the AgNWs are unidirectionally oriented within the LbL polyelectrolyte matrix. Furthermore, above the AgNWs percolation threshold, the AgNWs reinforced LbL composite film is electrically conductive. Therefore, it can find application in ultrathin LbL film-based sensor. In the second design, the zero-dmensional AuNPs were assembled onto one-dimensional AgNWs and two-dimensional AgNPls by means of star polymer linker, or alternatively using polyelectrolytes via electrostatics interaction. The unique feature of these bimetallic silver-gold core-shell nanoparticles is their ability to greatly enhance electric field, due to the silver-gold intra-particle interaction. This allows it to serve as a single-nanoparticle surface enhanced Raman scattering (SERS) substrate for chemical sensing.
Noble and transition metal aromatic frameworks: synthesis, properties, and stability

(Georgia Institute of Technology, 2009-05-14) Carson, Cantwell G.

In the first section, the electrical conductivity of rhodium phenylene-diisocyanide polymer is reported to be 3.4E-11 S/cm. However, the conductivity also exhibits an inverse exponential decay in air with t = 8 days. This change is attributed to the oxidation of the isocyanide functional group to an isocyanate, leading to degradation in the long-range metal-metal bonding, the dominant conductivity mechanism. Using a more stable carboxylate ligand, the Cu terephthalate (TPA) system is studied and compared against the Mg, Co, Ni, and Zn terephthalates. A synthesis in N,N-dimethylformamide (DMF) is developed and large quantities of the Cu(TPA)DMF can be synthesized in air. The crystal structure of the Cu(TPA) DMF is shown to be in the C2/m spacegroup. Upon desolvation, the Cu(TPA) is shown to have a large surface area of 625 m2/g. The magnetic susceptibility of the Cu(TPA) indicates anti-ferromagnetic coupling between adjacent Cu centers in the same dimer. The thermal stability of the Zn, Ni, Co, and Mg terephthalates is shown to increase with decreasing symmetric carboxylate stretch in the IR. The magnetic susceptibilities of the Co and Ni terephthalates have paramagnetic behavior, with a Weiss temperature of T = -12.9 K and T = 8.8 for Co(TPA) DMF and Ni(TPA)DMF respectively. A heterometallic Zn-Cu terephthalate is synthesized with Cu concentrations ranging from 0 to 100%. Upon the addition of Cu, Zn-rich frameworks increase in surface area, change in thermal stability, and increase their solvent retention from 16% to 25%. Zn is shown to couple with Cu in the same dimer at a high rate, changing the behavior of the dimer from anti-ferromagnetic to paramagnetic. The Weiss temperature suggests weak ferromagnetic interaction.