No description available.

No / We have studied the tensile behaviour of polycarbonate at large strains below the glass transition temperature. Experiments have been carried out at a series of constant temperatures and also under conditions of falling temperature. The specimens neck with a natural draw ratio of ~2, and the study focuses mainly on the necked material. Isothermal experiments reveal an elastic mechanism that initiates beyond the natural draw ratio. A model consisting of an Eyring process and two Gaussian elastic mechanisms is shown to be applicable to both the isothermal and anisothermal stress relaxation and stress-strain results. The same model also produces reasonable estimates of the stresses generated during the necking process. In addition, a simple relationship between isothermal and anisothermal stress relaxation is demonstrated.

No description available.

This dissertation introduces a new approach to measure the mechanical properties of soft tissues. A laser-induced microbubble, created by focusing a single nanosecond laser pulse with a custom-made objective lens, was created at desired locations inside a tissue sample. An acoustic radiation force was generated by a low frequency transducer to displace the microbubble. A custom-built high pulse repetition frequency (PRF) ultrasound system, consisting of two 25 MHz single element transducers, was used to track the dynamics of the microbubble. Reconstruction of the mechanical properties at the specific location in a tissue sample was performed using a theoretical model, which calculated the dynamics of a microbubble under an externally applied force in a viscoelastic medium. The theoretical model and the high PRF ultrasound system were successfully validated in both gelatin phantoms and ex vivo bovine crystalline lenses. Age-related sclerosis of the crystalline lenses from bovine was clearly detected, which might be linked to changes in the crystalline. Location-dependent variation explained that the outer cortex and the inner nucleus had different mechanical properties. In the old and young porcine vitreous humors, age-related changes were not found. However, local variations of the mechanical properties were discovered, which may coincide with the different distributions of the molecular compositions. The laser-induced microbubble approach shows potential for future research into the origin of physiological phenomena and the development of inherent disorders in the eye. I hope that further studies – in the development of a more suitable theoretical model for the microbubble dynamics, in extension to in vivo applications, and in defining the relationship of the mechanical properties to molecular components in the eye – may provide a plan for the therapeutic treatment of eye-related diseases. / text

Soft particle glasses like microgels and compressed emulsions are densely packed, disordered suspensions of deformable particles. Quantitative relationships among the constituent properties and the macroscopic properties of the suspension are determined for their customized design as rheological additives. The microscopic origin of their macroscopic properties is also determined. Advanced characterization techniques like Large Amplitude Oscillatory Shear (LAOS) and microrheology are studied to use them efficiently to characterize these materials. Their microstructure and rheology are investigated through theory, simulations and experiments. Soft particle glasses are used as rheological additives in many applications including coatings, solid inks and textured food and cosmetic products but their formulation is largely empirical. A quantitative connection between their formulation and rheology is critical to enable their rational design. Their microstructure will lead to the microscopic origin of some unique properties in common with other soft crowded materials like intracellular cytoplasm and clays. These are complex fluids and require novel techniques to characterize them. A study of these techniques is essential to efficiently interpret the observations in terms of their macroscopic properties and the microscopic dynamics involved. Particle scale simulations of steady and oscillatory shear flow are developed to predict the nonlinear rheology and microstructure of these glasses. The origin of yielding is determined as escape of particles from their cages giving rise to a shear induced diffusion. Microrheology is studied by developing simulations of a probe particle being pulled at a constant force and the rheological information from microrheology is quantitatively connected to that from bulk rheological measurements. Soft particle glasses develop internal stresses when quenched to a solid state by flow cessation during processing. Experiments are performed to characterize and a priori predict these stresses. Simulations are used to determine the particle scale mechanisms involved in the stress relaxation on flow cessation and the microstructural origin of internal stresses. A pairwise interaction theory is developed for quiescent glasses to quantitatively predict their microstructure and elastic properties. The theory is then extended to sheared glasses to quantitatively predict their nonlinear rheology. The implementation of the pairwise theories is computationally much faster than the full three-dimensional simulations. / text

The traditional method of modifying the properties of polymeric materials is their filling with mineral particles. The use of synthetic nano-sized fillers in comparison with coarse-dispersed natural ones is very promising due to the large specific surface of nanomaterials. The study of such compositions by rheometric methods makes it possible to assess the structural changes that occur when nanoparticles are introduced into the polymer solution. In this study, studies were carried out in the mode of harmonic oscillations with solutions of cellulose acetate DAC in DMSO in the concentration range of 5-20% by weight. polymer containing 3 and 5% of the mass. amorphous silica. The experiments were carried out on a Haake MARS rotary rheoscope with a working cone-plane unit in the voltage range of 0.1-100 Pa and frequencies of 0.1-100 Hz at temperatures of 298, 318 and 338 K. Measurements of the frequency dependences of the complex viscosity, elastic modulus and loss modulus were performed at a constant voltage of 10 Pa. It was found that the DAC solutions in DMSO are non-Newtonian fluids that exhibit elastic properties with a DAC content of more than 10% by weight. It is shown that the concentration of AC, at which the solution begins to show elastic properties, decreases with increasing concentration of aerosil. For the DAC-DMSO-Aerosil system, there is a temperature-concentration range in which the elastic modulus is greater than the loss modulus, that is, the introduction of aerosil leads to a significant change in the system behavior during deformation. According to the data obtained, the concentration dependences of the enthalpies of activation of the flow of DAC solutions in DMSO and DAC-DMSO-aerosil systems were calculated. / Традиционным методом модификации свойств полимерных материалов является их наполнение минеральными частицами. Применение синтетических наноразмерных наполнителей по сравнению с грубодисперсными природными является очень перспективным из-за большой удельной поверхности наноматериалов. Исследование таких композиций реометрическими методами дает возможность оценить структурные изменения, происходящие при введении наночастиц в раствор полимера. В данной работе исследования проводили в режиме гармонических колебаний с растворами ацетата целлюлозы ДАЦ в ДМСО в диапазоне концентраций 5-20% масс. полимера, содержащих 3 и 5 % масс. аморфного диоксида кремния. Опыты проводили на ротационном реоскопе Haake MARS с рабочим узлом типа конус-плоскость в диапазоне напряжений 0,1 -100 Па и частот 0.1-100 Гц при температурах 298, 318 и 338 К. Измерения частотных зависимостей комплексной вязкости, модуля упругости и модуля потерь проводили при постоянном напряжении 10 Па. Обнаружено, что растворы ДАЦ в ДМСО являются неньютоновскими жидкостями, проявляющими упругие свойства при содержании ДАЦ более 10%масс. Показано, что величина концентрации АЦ, при которой раствор начинает проявлять упругие свойства, уменьшается с увеличением концентрации аэросила. Для системы ДАЦ-ДМСО-аэросил существует температурно- концентрационный диапазон, в котором модуль упругости больше модуля потерь, то есть введение аэросила приводит к существенному изменению поведения системы при деформировании. По полученным данным рассчитаны концентрационные зависимости величин энтальпий активации течения растворов ДАЦ в ДМСО и систем ДАЦ-ДМСО-аэросил.

Aromatic polyimides have found widespread applicability which can be partially attributed to their thermal stability, chemical resistance, and high glass transition temperature. However, deficiencies in their processability, solubility, transparency, and relatively high dielectric constants do not always provide the optimum properties for many specialty microelectronics applications. The incorporation of aliphatic segments to form partially aliphatic polyimides, has been used to counteract these shortcomings. Many of the potential uses of partially aliphatic polyimides require them to adhere to ceramic substrates, a main topic of this research. Polyimides and copolyimides that varied in chemical composition by their aliphatic content were characterized by their molecular weight, glass transition temperature, thermal stability, coefficient of thermal expansion, refractive index, dielectric behavior, and mechanical properties. Structure-property relationships were established. The gamma and beta sub-Tg viscoelastic relaxations were investigated to understand their molecular origins. The adhesion performance of a selected series of partially aliphatic polyimides to SiO2/Si was examined using a shaft loaded blister test, which was designed and instrumented for use in a dynamic mechanical analysis instrument. The adhesion was studied at high and low percent relative humidities and for several temperatures to examine if adhesion strength is influenced by polymer chemical composition. The adhesion energy could not be quantified for the entire series of polyimides. It was possible to interpret the quantitative adhesive fracture energies along with the qualitative adhesion strength behaviors, the failure surface analyses, and to offer an understanding of the adhesive chemical structure-physical property relationships. These understandings provide a conclusion that the incorporation of aliphatic segments into the polyimide chemical structure improves the durability of the adhesive bond to SiO2/Si under high percent relative humidities. / Ph. D.

The spherulitic growth rate data for a molecular weight series of isotactic polystyrene are analyzed in context of the Lauritzen-Hoffman kinetic theory of polymer crystallization. The primary objectives of the study are to critically test the Lauritzen-Hoffinan theory under conditions not rigorously investigated before and to gain a better understanding of the molecular weight dependence of crystal growth rate for isotactic polystyrene. The analyses yield values for fundamental kinetic and thermodynamic quantities associated with polymer crystallization. The physical meaning of the resulting parameters is assessed by comparing these results to those obtained from methods independent of crystal growth rate or crystallization theory altogether. This study differs from others reported in the literature in a number of ways, such as, the narrow molecular weight distribution and the molecular weight range of polystyrenes investigated. Also, growth rate measurements were extended to higher temperatures and a more appropriate kinetic equation for crystal growth rate analysis was applied. The majority of published studies that have used the Lauritzen-Hoffman theory applied an approximated form of the kinetic equation which does not fully describe the temperature dependence of polymer crystallization. The results of the study show that a transition from molecular weight dependent to independent crystal growth rate occurs at a molecular weight of about 250,000 g/mole for isotactic polystyrene. Also, comparison of viscoelastic and crystal growth rate data indicate that the Vogel form of the transport term in the Lauritzen Hoffman kinetic growth rate equation correctly describes the temperature dependence of molecular transport for the crystallization process of isotactic polystyrene. Furthermore, the study suggests that the equilibrium melting temperature for the polymer is significantly higher than the value that has been generally accepted for the past 25 years. The study also provided the opportunity to investigate various other factors and theories associated with polymer crystallization. For example, the theoretical relationship between the crystal's lateral surface free energy, σ, and the characteristic ratio, C_∞, was evaluated. Also, the spherulitic morphology as a function of molecular weight and temperature was examined by scanning electron microscopy, SEM. / Ph. D.

Fiber hybridization involves the blending of high and low performance fibers in a common matrix to yield a composite with a balance of properties that cannot be achieved by using either fiber alone. In this study, the random wetlay process was used as a compounding method to investigate the effects of fiber hybridization on the mechanical, viscoelastic, and sorption characteristics of steam-exploded wood (SEW) and lyocell (high performance regenerated cellulose) fiber-reinforced polypropylene (PP) composites. The two fiber types were blended in varying proportions within a fixed total fiber content of 50 wt. % and compared with non-hybrid lyocell- and SEW-PP controls. Using PP matrix as basis, it was observed that moduli of all composites generally increased with increasing lyocell concentration, ranging from a minimum 66 % for SP 50 (SEW/PP control) to a maximum 233 % for LP 50 (lyocell/PP control). Ultimate strengths on the other hand, declined for SP 50 but increased with the inclusion of lyocell fibers. Comparisons of hybrid (having 5 - 20 wt % lyocell) with non-hybrid (having 25 - 50 wt. % lyocell) composites revealed a surprisingly greater strength and modulus-building efficiency (by as much as 2.6 times) in the hybrid composites. This observation indicated possible synergism between lyocell and SEW. Analyses of composite property gains as a function of fiber cost also showed greater cost benefits (highest for tensile modulus) in favor of hybridization. The advantages of fiber hybridization on composite properties were again evident under dynamic mechanical analysis where no significant differences in the storage moduli were found between a hybrid composite with 20 wt. % lyocell and a non-hybrid composite with 50 wt. % lyocell loading. Application of the time-temperature superposition principle (TTSP) made it possible to predict storage moduli over extended frequencies for PP and its composites. Comparison of shift factor versus temperature plots revealed decreasing relaxation times of PP with increasing lyocell concentration, which indicated that PP interacted better with lyocell than with SEW fibers. Finally, it was observed from sorption tests that hybrid composites absorbed less moisture than non-hybrid counterparts of either fiber type. The reasons for this observation were not apparent. It is however possible that moisture transport mechanisms within the composites may have been modified as a result of hybridization. / Master of Science

Polypropylene (PP) and ethylene-co-octene copolymer (EOC) blends were prepared at various component ratios and reinforced with silica nanoparticles (SiO2). Strategies to improve filler dispersion involved the grafting of a silane coupling agent on the PP matrix, the addition of a maleated PP (PP-g-MA) as a compatibilizer and the use of hydrophobic silica nanoparticles. These approaches resulted in a fine dispersion of the nanoparticles within the PP phase and induced a reduction of the size of the EOC domains, due to a barrier effect. Tensile and flexural properties were significantly increased, whereas ductility and impact properties were not affected. These enhancements are attributed to the favourable microstructure of the blends, featuring a segregated microstructure, and to the improved interfacial adhesion between the functionalized polymer matrix and the surface of the nanoparticles. The microstructure and rheology of model melt compounded EOC-based nanocomposites were investigated. Functionalization of the polyolefin matrix was accomplished through silane grafting, or addition of a maleated EOC (EOC-g-MA) compatibilizer. Various grades of unmodified SiO2 having different specific surface areas (SSA), as well as a surface-modified grade were added to the EOC matrix at various loadings. The formation of covalent and hydrogen bonds between the silanol groups and the functionalized polymer generated strong polymer/filler (P/F) interactions, resulting in improved filler dispersion. Bound polymer characterization revealed that in the compatibilized materials, the amount of polymer physically attached to the nanoparticles was higher than in the non-compatibilized samples. In the absence of a compatibilizer, larger SiO2 aggregates formed upon increasing SSA because of increased probability of hydrogen bonding between the particles. The increased propensity for aggregation was revealed by time sweeps as well as by the increased strain sensitivity in stress sweeps. On the contrary, the compatibilized composites exhibited a stable response and a higher critical strain for the onset of non-linearity, indicative of stronger adhesion between the fillers and the matrix. Superposition of oscillatory and creep/recovery experiments revealed that the viscoelastic properties in the terminal region were influenced substantially by the state of dispersion of the nanoparticles. In the absence of a compatibilizer, substantial enhancements in the linear viscoelastic (LVE) functions were noted and an increasing SSA resulted in more significant deviations from terminal flow. On the contrary, the SSA of the particles had no effect on the viscoelastic and mechanical properties of the compatibilized composites. / Thesis (Ph.D, Chemical Engineering) -- Queen's University, 2011-04-18 15:17:52.471