Browsing by Subject "polymers"
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Item A continuous impingement mixing process for effective dispersion of nanoparticles in polymers(Texas A&M University, 2006-10-30) Ganapathy Subramanian, Santhana GopinathMixing refers to any process that increases the uniformity of composition and is an integral part of polymer processing. The effective mixing of nanoparticles into polymers continues to be one of the leading problems that limit large scale production of polymer nanocomposites. Impingement mixing is a novel, relatively simple, continuous flow mixing process wherein mixing is accomplished by immersing a high velocity jet in a slower co-flowing stream. The resulting recirculating flow produces an energy cascade that provides a wide range of length scales for efficient mixing. An impingement mixing process was developed and studied through experiments and simulations. Numerical simulations were conducted using FLUENT to understand better the mechanism of operation of the mixer. The formation of a recirculation zone was found to affect the dispersion of nanoparticles. Results of the simulations were compared with experimental data obtained under similar conditions. While this process may be used for any polymernanoparticle combination, the primary focus of this study was the dispersion of Single Walled Carbon Nanotubes (SWNTs) in an epoxy matrix. The dispersion of SWNTs was evaluated by analyzing SEM images of the composites. The image analysis technique used the concept of Shannon Entropy to obtain an index of dispersion that was representative of the degree of mixing. This method of obtaining a dispersion index can be applied to any image analysis technique in which the two components that make up the mixture can be clearly distinguished. The mixing process was also used to disperse SWNTs into a limited number of other polymers. The mixing process is an "enabling" process that may be employed for virtually any polymer-nanoparticle combination. This mixing process was shown to be an effective and efficient means of quickly dispersing nanoparticles in polymers.Item Dendritic and linear polymers for separations(Texas A&M University, 2005-02-17) Gonzalez, Sergio OmarMost new fields in chemistry usually began as a curiosity by the researchers, followed by an intrinsic interest in basic biological, physical and chemical properties of reactions, interactions, structural features, and response to external stimuli by chemical elements and/or chemical compounds. If the ?curiosity? has appealing bio-physico-chemical properties this trend is followed by studies on the possible applications of such new fields. As a result, is it expected that these curiosities develop or give insights into new technologies. The development of the field of dendrimer chemistry is no different. In fact, dendrimer chemistry illustrates this trend fittingly. The research in this dissertation follows a similar trend. First, the synthesis of a melamine-based dendrimer is achieved. The synthesis illustrates the concept of using triazines as building blocks in dendrimer synthesis. The characterization of this molecule was followed by a basic inquiry of the properties that were unique relative to its composition. This dendrimer is compared against a small library of similar dendrimers in a structure-activity relationship (SAR) study. From the basic concept of an SAR, we moved toward more applied studies of these molecules. The grafting of organic molecules onto inorganic supports has had influences in the fields of catalysis, separations, and sensors. We developed protocols for the grafting of melamine-based molecules onto hydroxyl rich surfaces. After extensive characterization using solution and surface analyses, we tested the sequestration abilities of these new materials toward the separation of molecules of environmental importance from water. Following the data collected in these experiments, we moved toward a different type of applied technology. The use of linear polymers for separations instead of dendrimers is more attractive from an engineering perspective. We then used what was learned from the study of the separations performed by dendrimers and applied it to the design of linear polymers. We take advantage of a latent solid phase response to external stimuli to remove the herbicide atrazine from aqueous solution to the limit of detection.Item Fundamental Scratch Behavior of Styrene-Acrylonitrile Random Copolymers(2011-10-21) Browning, Robert LeeThe present study employs a standardized progressive load scratch test (ASTM D7027/ISO 19252) to investigate the fundamental physical and mechanistic origins of scratch deformation in styrene-acrylonitrile (SAN) random copolymers. Previous findings from numerical simulation using finite element methods are used to establish correlation between mechanical properties and key scratch deformation mechanisms of the SAN model systems. For SAN, the acrylonitrile (AN) content and molecular weight (MW) can be changed to alter mechanical properties such as tensile strength and ductility. The key scratch deformation mechanisms are identified as: scratch groove formation, scratch visibility, periodic micro-cracking and plowing. Groove formation has been correlated to the secant modulus at the compressive yield point while micro-cracking and plowing are related to the tensile strength of the material. The fundamentals and physical origins of scratch visibility are discussed. It is explained how unbiased evaluation is accomplished by means of an automatic digital image analysis software package (ASV?). Frictional behavior and the effects of scratch speed and moisture absorption are also addressed. Increasing the AN content and/or the MW of the SAN random copolymers generally enhances the scratch resistance of the material with regard to the onset of the key deformation mechanisms. Increasing the scratch speed increases the brittleness of the material, resulting in failure at lower applied loads. Moisture absorption increases with AN content and imparts a degree of plasticization as the moisture diffuses into the sub-surface. This plasticization initially results in a degradation of scratch resistance with respect to the key deformation mechanisms, but then, after saturation, the moisture on the surface provides lubrication and improves the scratch resistance. It is important to note that polymers are fundamentally different in nature, but the findings of this study serve as an important stepping stone down the path to a deeper understanding of polymer scratch behavior.Item Scratch behavior of polymers(Texas A&M University, 2005-11-01) Lim, Goy TeckThis dissertation work is focused on the analytical and numerical examination of the mechanical response of polypropylene (PP) under scratch deformation by a semispherical indenter. The finite element (FE) method is employed as the analysis technique and ABAQUS??, a commercial FE package is adopted to perform the analysis. Important physical and computational considerations on the implementation of FE analyses for the scratch problem are reviewed. It is shown through the discussion of the generated results that a good understanding can be gained on how different scratch conditions can affect scratch behavior of PP. A phenomenological deduction of the scratch damage process and mechanisms is also established. Considering the two main damage modes of polymers, shear yielding and crazing, it is shown that the two damage modes not only exist in the scratch deformation, and moreover, that they may compete against each other for dominance. A parametric study is also performed to assess the influence of material and surface properties on scratch response of material. A secondary research effort is also made to investigate the material constitutive modeling of polymers. Focusing on elastomeric or rubbery materials, a new mixed network model between the Gaussian and eight-chain non-Gaussian models is proposed. This mixed model inherently preserves the good predictive power of these two models and yields better predictions over a wider range of deformation than that of the rubber model adopted by ABAQUS??.Item The effect of branch density polyoxymethylene copolymers(2009-05-15) Ilg, Andrea DianeToday, there is a great need for polymers made from biorenewable resources due to the increasing price and diminishing supplies of petroleum and the overabundance of plastic waste in landfills. Polyoxymethylene can be produced from biorenewable feedstocks, depolymerized to formaldehyde through chemical recycling, and may be a viable alternative to many polyolefins. However, there has been limited research on varying the thermomechanical properties of polyoxymethylene so that it can be used in a wider variety of applications. Our approach employs the cationic copolymerization of trioxane with various amounts of 1,2-epoxyalkanes and 4-alkyl-1,3-dioxolanes to arrive at polyoxymethylene derivatives with controlled branching and morphology. Branching content has been measured by nuclear magnetic resonance (NMR) spectroscopy and correlates well with the comonomer feed fraction. The melting temperatures of the copolymers, determined from differential scanning calorimetry (DSC), are depressed predictably with increasing amounts of comonomer incorporation. The copolymerizations behaved the same regardless of whether the comonomer was an alkyldioxolane or epoxyalkane. 1,2-Epoxybutane/trioxane copolymers and 4-ethyl-1,3-dioxolane/trioxane copolymers gave the best melting point and % crystallinity results using boron trifluoride diethyl etherate as the cationic initiator.