Browsing by Subject "Structure-Property Relationship"
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Item Mechano-Activated Electronic and Molecular Structures(2011-02-22) Wang, KeFor centuries, researchers have been trying to achieve precise control and tailor materials properties. Several approaches, i.e., thermo-activation, electro-activation, and photo-activation, have been widely utilized. As an alternate and fundamentally different approach, mechano-activation is still relatively less-known. In particular, understanding the roles of mechano-activated electronic and molecular structures is yet to be achieved. This research contributes the fundamental understanding in mechanisms of mechano-activation and its effects on materials properties. Experimental investigation and theoretical analysis were involved in the present research. A methodology was developed to introduce the mechnao-activation and to study its subsequent effects. There are three major areas of investigation involved. First, the means to introduce mechanoactivation, such as energetic particle collision or a bending deformation (tensile force); Second, in-situ and ex-situ characterization using AFM, FTIR, UV-Vis, and XPS etc. techniques; Third, theoretical analysis through modified Lennard-Jones potentials in order to explain the behavior of materials under mechano-activation. In the present research, experiments on a Diamond-Like Carbon (DLC) film, a Polyvinylidene Fluoride (PVDF) film, and the Silver-Crown Ether nanochains (Ag-NCs) were carried out. For DLC, the collision-induced transformation between hybridization states of carbon was confirmed, which also dominated the friction behavior of the film. For PVDF, results show that the applied tensile force induced the transformation of [alpha], [beta], and [upsilon] crystalline phase. In addition, the transformation observed was time and direction dependent. For Ag-NCs, a new approach based on the mechanism of mechano-activation was developed for nanochain structure synthesis. Molecular dynamics simulation and experimental results revealed that the formation of Ag-NCs is a synergetic physicalchemical procedure. Experimental results from DLC and PVDF were further used to validate the proposed potential, which brought new insight into the activation process. The current research achieves a precise control on engineering materials properties. The force-activated materials have wide applications in many areas, such as functional coating, sensing, and catalysis. In this study selected experiments have demonstrated the effects of mechanoactivation in different material systems (ceramic, polymer, metallic nano structure) and at different length scales. For the first time, a modified potential was proposed to explain the observed mechano-activation phenomena from the energy point of view. It was validated by experimental results of DLC and PVDF. The current research brings new understanding in mechano-activation and opens potential for its applications in tailoring materials properties.Item Structure-Property Relationships in Carbon Nanotube-Polymer Systems: Influence of Noncovalent Stabilization Techniques(2010-01-20) Liu, LeiA variety of experiments were carried out to study the dispersion and microstructure of carbon nanotubes in aqueous suspensions and polymer composites with the goal to improve the electrical conductivity of the composites containing nanotubes. Epoxy composites containing covalently and noncovalently functionalized nanotubes were compared in terms of electrical and mechanical behavior. Covalent functionalization of nanotubes is based on chemical attachments of polyethylenimine (PEI) whereas noncovalent functionalization takes place through physical mixing of nanotubes and PEI. The electrical conductivity is reduced in composites containing covalently functionalized nanotubes due to damage of the tube?s conjugated surface that reduces intrinsic conductivity. Conversely, the mechanical properties are always better for epoxy composites containing covalently functionalized nanotubes. Clay particles were used as a rigid dispersing aid for nanotubes in aqueous suspensions and epoxy composites. When both nanotubes and clay were introduced into water by sonication, the suspension is stable for weeks, whereas the nanotubes precipitate almost instantly for the suspension without clay. In epoxy composites, nanotubes form separated clusters of aggregation, whereas a continuous threedimensional nanotube network is achieved when clay is introduced. Electrical conductivity of the epoxy composite is shown to significantly improve with a small addition of clay and the percolation threshold is simultaneously decreased (from 0.05 wt% nanotubes, when there is no clay, to 0.01 wt% when 2 wt% clay is introduced). The addition of clay can also improve the mechanical properties of the composites, especially at higher clay concentration. Weak polyelectrolytes (i.e., pH-responsive polymers) were also studied for their interaction with nanotubes and the electrical properties of the dried composite films. When dispersed by sonication, Nanotubes show pH-dependent dispersion and stability in poly(acrylic acid) water solution, as evidenced by changes in suspension viscosity and cryo-TEM images. The nanotube suspensions were then dried under ambient conditions and the composite films exhibit tailorable nanotube dispersion as a function of pH. The percolation threshold and maximum electrical conductivity are reduced when the pH is changed from low to high. Some other pH-responsive polymers were also studied, but their pH-dependent viscosity and conductivity were not as large or reversible as poly(acrylic acid).