| The microstructure development within mesophase pitch-based carbon materials depends on the flow history that the pitch is subjected to. Therefore, a fundamental understanding of flow and its influence on the microstructure is required to obtain carbon materials with desired properties. The objective of this research was to investigate the flow and microstructural behavior of a synthetic mesophase pitch (AR-HP) in rheometric and processing flow conditions. In addition, simulation studies were performed to establish a frame work for modeling the flow behavior of this complex material in different flow situations.;The steady-shear viscosities obtained from a cone-plate rheometer during increasing rate-sweep experiments exhibited shear-thinning (Region I) and plateau (Region II) responses. However, the slope of the shear-thinning region was only about -0.2, much lower than -0.5 observed in some pitches and liquid-crystalline polymers. This difference could arise from the different molecular constituents of pitches. At higher shear rates, as measured from capillary rheometers, the viscosity values remained almost constant. The transient shear stress responses, as measured from cone-plate rheometer, exhibited nonmonotonic behavior as a function of applied strain at all shear rates and temperatures tested. After rheological experiments, the samples were collected by developing a new experimental protocol for preservation of the sample for microstructural analysis. Microstructural observations obtained from three orthogonal sections, reported for the first time in the literature, indicate that the local maximum in shear stress was due to yielding of initial microstructure. The microstructure became flow oriented with further shearing, and the structure size decreased with increasing shear rates.;In addition to high-strain experiments, dynamic experiments were also performed in the linear viscoelastic region where no significant deformation of fluid takes place. The elastic response was found to be strongly dependent on the microstructure, and a lower slope of 0.8 for the elastic modulus in the low-frequency terminal region was observed as compared to 2 observed for flexible chain polymers. Relaxation of microstructure resulted in an increase of storage moduli. However, the relaxation time did not follow the scaling argument, tau ∼ etaa2/ K. It is postulated that the relaxation process is influenced not only by the textural size, but also by layer-plane orientation.;The flow-microstructural study was extended to the processing flow conditions and in this case AR-HP mesophase pitch was extruded through custom-made dies using a single-screw extruder. Due to changing dimensions of these dies, the mesophase pitch was subjected to varying shear rates. Microstructural observations suggest that in the capillary region of these dies, the orientation of the layer-plane was approximately radial near the wall. Away from the wall, the deviation of orientation of the layer-planes from the radial direction was significant and some layer-planes were oriented tangentially. In the core, the microstructure was coarse and no preferred orientation of mesophase layerplanes was observed.;Simulation studies were performed using constitutive equations for discotic liquid-crystalline materials in simple shear flow, corresponding with the experimental studies. The simulation studies were performed for two different initial conditions that resemble the experimental results. At steady state, the bulk of the discs were found to be oriented at a flow-aligned angle of -64.1°, which is consistent with the theoretical predictions. Although, the simulation studies could not capture the complex microstructure observed experimentally, similarities in flow-aligned domains were observed. This study establishes a frame work to simulate the flow dynamics of complex mesophase pitch in multiscale-mulidimensional problems using the computational facility and expertise of the Center for Advanced Engineering Fibers and Films (CAEFF). |
