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ItemThe chemical and mechanical behaviors of polymer / reactive metal systems under high strain rates(Georgia Institute of Technology, 2012-08-27) Shen, YubinAs one category of energetic materials, impact-initiated reactive materials are able to release a high amount of stored chemical energy under high strain rate impact loading, and are used extensively in civil and military applications. In general, polymers are introduced as binder materials to trap the reactive metal powders inside, and also act as an oxidizing agent for the metal ingredient. Since critical attention has been paid on the metal / metal reaction, only a few types of polymer / reactive metal interactions have been studied in the literature. With the higher requirement of materials resistant to different thermal and mechanical environments, the understanding and characterization of polymer / reactive metal interactions are in great demand. In this study, PTFE (Polytetrafluoroethylene) 7A / Ti (Titanium) composites were studied under high strain rates by utilizing the Taylor impact and SHPB tests. Taylor impact tests with different impact velocities, sample dimensions and sample configurations were conducted on the composite, equipped with a high-speed camera for tracking transient images during the sudden process. SHPB and Instron tests were carried out to obtain the stress vs. strain curves of the composite under a wide range of strain rates, the result of which were also utilized for fitting the constitutive relations of the composite based on the modified Johnson-Cook strength model. Thermal analyses by DTA tests under different flow rates accompanied with XRD identification were conducted to study the reaction mechanism between PTFE 7A and Ti when only heat was provided. Numerical simulations on Taylor impact tests and microstructural deformations were also performed to validate the constitutive model built for the composite system, and to investigate the possible reaction mechanism between two components. The results obtained from the high strain rate tests, thermal analyses and numerical simulations were combined to provide a systematic study on the reaction mechanism between PTFE and Ti in the composite systems, which will be instructive for future energetic studies on other polymer / reactive metal systems.
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ItemChemical and mechanical characterization of fully degradable double-network hydrogels based on PEG and PAA(Georgia Institute of Technology, 2012-05-18) Worrell, KevinBiodegradable hydrogels have become very promising materials for a number of biomedical applications, including tissue engineering and drug delivery. For optimal tissue engineering design, the mechanical properties of hydrogels should match those of native tissues as closely as possible because these properties are known to affect the behavior and function of cells seeded in the hydrogels. At the same time, high water-contents, large mesh sizes and well-tuned degradation rates are favorable for the controlled release of growth factors and for adequate transport of nutrients through the hydrogel during tissue regeneration. With these factors in mind, the goal of this research was to develop and investigate the behavior of injectable, biodegradable hydrogels with enhanced stiffness properties that persist even at high degrees of swelling. In order to do this, degradable functionalities were incorporated into photo-crosslinkable poly(ethylene glycol) and poly(acrylic acid) hydrogels, and these two components were used to make a series of double-network hydrogels. Synthesis of the precursor macromers, photopolymerization of the hydrogels, and structural parameters of the hydrogels were analyzed. The composition and the molecular weight between crosslinks (Mc) of the hydrogel components were varied, and the degradation, swelling, thermal and mechanical properties of the hydrogels were characterized over various time scales. These properties were compared to corresponding properties of the component single-network hydrogels.
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ItemAtomistic modeling of environmental aging of epoxy resins(Georgia Institute of Technology, 2012-03-29) Li, YaoIn this work, epoxy resins were modeled using all atom representations in nanoscale simulation boxes. Tetrafunctional epoxy and corresponding multifunctional amine were chosen as model materials. Algorithms of constructing interconnected network structures were invented developed to properly account for the chemical structures and computational cost. Monomers were generated in diamond lattice and crosslinked to model complex epoxy multifunctional network. The initial configurations were relaxed and equilibrated using molecular dynamics and suitable force field. Physical, thermal and mechanical properties resulting from equilibrated simulation box are in good agreement with experimental results. Possible impact of chemical degradation was studied by adopting oxidation and hydrolysis algorithms. Mechanism of degradation was based on bonds reaction probability and chemical structures of epoxies. Both oxidation and hydrolysis were found to decrease materials performances by reducing number of crosslinking points. Elastic modulus of materials was directly related to crosslinking density. Interfaces between two types of epoxies were constructed to study interactions at interfaces. Covalent bonds linking two components play an important role in interfacial strength. Free volume calculation helps to identify and monitor nucleation of crazes and voids within materials. It was found voids and cracks prefer to initiate and grow at 2 interfaces and lead to failures. Additional compatibilizer layers can improve overall composite performances by preventing void growth at interfaces. Diffusion pattern of water in epoxy resins was studied by tracking displacement of single molecules during certain time intervals. The characteristic of water diffusion in epoxies was interpreted by free volume theory. Reactive force field was introduced to study thermal degradation behavior of epoxy resins. Number of molecules and variation of different types of covalent bonds during heating processes were tracked and analyzed to uncover the degradation mechanism of epoxy resins.
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ItemSynthesis and characterization of shape memory poly (epsilon-caprolactone) polyurethane-ureas(Georgia Institute of Technology, 2012-01-17) Ren, HongfengA series of segmented poly (epsilon-caprolactone) polyurethane-ureas (PCLUUs) were prepared from poly (epsilon-caprolactone) (PCL) diol, different dissociates and chain extenders to improve the recovery stress of shape memory polymers. NMR and FT-IR were used to identify the structure of the synthesized shape memory polyurethane-ureas. Parameters such as soft segment content (molecular weight and content), chain extender and the rigidity of the main chain were investigated to understand the structure-property relationships of the shape memory polymer systems through DSC, DMA, physical property test, etc. Cyclic thermal mechanic tests were applied to measure the shape memory properties which showed that the recovery stress can be improved above 200% simply by modifying the chain extender. Meanwhile, the synthesis process was optimized to be similar to that of Spandex /LYCRA®. Continuous fibers were made from a wet spinning process, which indicated excellent spinnability of the polymer solution. Small angle neutron scattering (SANS) was used to study the morphology of the hard segment at different temperatures and stretch rates and found that the monodisperse rigid cylinder model fit the SANS data quite well. From the cylinder model, the radius of the cylinder increases with the increasing hard segment content. The SANS results revealed phase separation of hard and soft segments into nano scale domains.
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ItemCarbon-based magnetic nanohybrid materials for polymer composites and electrochemical energy storage and conversion(Georgia Institute of Technology, 2011-11-01) Kim, Il TaeThe role of nanohybrid materials in the fields of polymer composites and electrochemical energy systems is significant since they affect the enhanced physical properties and improved electrochemical performance, respectively. As basic nanomaterials, carbon nanotubes and graphene were utilized due to their outstanding physical properties. With these materials, hybrid nanostructures were generated through a novel synthesis method, modified sol-gel process; namely, carbon nanotubes (CNTs)-maghemite and reduced graphene oxide (rGO)-maghemite nanohybrid materials were developed. In the study on polymer composities, developed CNTs-maghemite (magnetic carbon nanotbues (m-CNTs)) were readily aligned under an externally applied magnetic field, and due to the aligned features of m-CNTs in polymer matrices, it showed much enhanced anisotropic electrical and mechanical properties. In the study on electrochemical energy system (Li-ion batteries), rGO-maghemite were used as anode materials; as a result, they showed improved electrochemical performance for Li-ion batteries due to their specific morphology and characteristics.