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Browsing by Author "Norris, Ann Marie Walstrom"

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    Dielectric studies of novel polymeric systems
    Norris, Ann Marie Walstrom (Virginia Polytechnic Institute and State University, 1987)
    This work combines many characterization techniques in an effort to enhance understanding of molecular motions of polymers and. how they are influenced by' structure. The primary characterization method was dielectric spectroscopy which utilizes an AC electric field as the stress field. A variety of new, well controlled polymeric systems were studied. The first series included a number of radial starblock copolymers, styrene/isoprene, t-butyl- styrene/isoprene, and t-butyl-styrene/butadiene. These ABA copolymers consisted of hard and soft blocks, with the soft block comprising 75% by weight. The effect of microstructure of the soft block, casting solvent, hydrogenation, and chemical composition of the hard block were some of the variables studied. The amount of phase separation and the molecular motions occurring will be influenced by these parameters. Hydrogenation of the soft block increased the phase separation. Another system investigated included some stereospecific poly(alkyl methacrylates) which were synthesized anionically. In this series the alkyl group was systematically changed in order to study the effects of the bulkiness of the substituent and the tacticity on the a and ß transitions. The ß transition associated with side chain rotations was only observed in the case of the methyl and ethyl substituents. The Havriliak-Negami data analysis was used to evaluate the breadth and the skewness of the distribution of relaxation times. Finally, some high temperature thermoplastic polymers were evaluated with dielectric spectroscopy. The effect of the backbone composition, moisture, and fillers on the β transition was looked at. These studies showed that moisture and fillers play an important role on the magnitude and temperature of the observed β transition.
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    Performance Evaluation and Durability Studies of Adhesive Bonds
    Ranade, Shantanu Rajendra (Virginia Tech, 2014-10-06)
    In this dissertation, four test approaches were developed to characterize the adhesion performance and durability of adhesive bonds for specific applications in areas spanning from structural adhesive joints to popular confectionaries such as chewing gum. In the first chapter, a double cantilever beam (DCB) specimen geometry is proposed for combinatorial fracture studies of structural adhesive bonds. This specimen geometry enabled the characterization of fracture energy vs. bondline thickness trends through fewer tests than those required during a conventional "one at a time" characterization approach, potentially offering a significant reduction in characterization times. The second chapter investigates the adhesive fracture resistance and crack path selection in adhesive joints containing patterns of discreet localized weak interfaces created using physical vapor deposition of copper. In a DCB specimen tested under mode-I conditions, fracture energy within the patterned regions scaled according to a simple rule of mixture, while reverse R-curve and R-curve type trends were observed in the regions surrounding weak interface patterns. Under mixed mode conditions such that bonding surface with patterns is subjected to axial tension, fracture energy did not show R-curve type trends while it was observed that a crack could be made to avoid exceptionally weak interfaces when loaded such that bonding surface with defects is subjected to axial compression. In the third chapter, an adaptation of the probe tack test is proposed to characterize the adhesion behavior of gum cuds. This test method allowed the introduction of substrates with well-defined surface energies and topologies to study their effects on gum cud adhesion. This approach and reported insights could potentially be useful in developing chewing gum formulations that facilitate easy removal of improperly discarded gum cuds from adhering surfaces. In the fourth chapter we highlight a procedure to obtain insights into the long-term performance of silicone sealants designed for load-bearing applications such as solar panel support sealants. Using small strain constitutive tests and time-temperature-superposition principle, thermal shift factors were obtained and successfully used to characterize the creep rupture master curves for specific joint configurations, leading to insights into delayed failures corresponding to three years through experiments carried out in one month.