| Summary: | Nanofilled polymer composites and lubricants have gained significant attention in fuelefficient vehicle designs due to the superior material properties and economic potentials with minimal filler loadings. However, mass-market applications are impeded by a lack of understanding of the complex rheological behavior arising from addition of nanofillers, especially in strong shear and extensional flows. In this thesis, these challenges are addressed through design of a rapid characterization protocol for the extensional rheology of such material systems, as well as a comprehensive rheological study of a prototypical graphene-derived nanocomposite with the development of a robust constitutive model framework to provide more insights into the microstructural variations that are induced through large deformations and strong flows during material processing and manufacturing operations.
In the first part of this thesis, an improved version of capillary breakup extensional rheometry (CaBER) is presented, with a special focus on quantifying the filament thinning dynamics which are governed by multiple contributions to the total tensile stress in the fluid. An Inelastic Rate-Thickening (IRT) constitutive model is proposed to characterize the weakly rate-dependent response of commercial synthetic motor oils. The evolution of the full-dimensional filament profiles is quantified through analytical and numerical calculations from which an explicit empirical expression is developed based on the magnitude of each stress contribution. Finally, a statistical strategy is proposed to select the best-fit model with regularized parameters on the basis of the Bayesian information criterion, paving the path for an automated industrial process to extract accurate and meaningful constitutive parameters from CaBER measurements.
The second part of this thesis focuses on the filament thinning dynamics of entangled polymer systems based on two modern tube models derived from reptation theory. One-dimensional numerical solutions of the governing equations are demonstrated to accurately capture a number of key observations reported in previous studies of concentrated polymer solutions, including rate-thinning behavior near filament breakup, and markedly different relaxation time constants in shear and extensional flows. An analytical expression for the ratio of these two relaxation times is obtained as a function of the polymer concentration and the number of entanglements, which shows excellent agreement with the experimental results from a number of polymer systems with no additional fitting parameters. As a case study, the material response predicted from the Rolie-Poly (Rouse-Linear-Polymer) model is used to interpret the rheology and dynamics of concentrated cellulose/ionic liquid systems, which are beginning to find application in fabric recycling and regeneration operations through a wet-spinning process. To obtain an accurate set of constitutive parameters, the material response in nonlinear shear and extensional flows are fitted to the model in order to obtain a universal set of constitutive parameters and scalings that can describe the rheology of these complex nanocomposite solutions as the concentration, temperature and degree of polymerization are varied.
The final part of this thesis presents a comprehensive study of the rheology of a graphene oxide (GO)/polyvinyl alcohol (PVA) system. Distinct features of the low-frequency dynamic moduli indicate the formation of a fractal nanofiller microstructure as the GO concentration is increased. A nonlinear fractional K-BKZ constitutive framework is used to develop a comprehensive rheological equation of state for this nanocomposite system in both the linear and non-linear regimes. In extensional flow the observed rheological behavior is similar to the prediction from the tube models due to the structural similarity of the materials, and the nanofiller orientation can be readily described in terms of the model parameters. The sensitivity of the nanofiller structural variations to the flow kinematics inspires the design of a new rheometric method to optimize nanofiller dispersion by using a periodic exponential shear flow. General principles for the design of the required flow profiles are provided and are justified via proof-of-concept experiments.
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