Date of Award

12-2006

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Advisor

Ogale, Amod A

Abstract

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 a shear-thinning region (Region I) with a slope of about -0.2 and a plateau (Region II) region. 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. Microstructural study on three orthogonal sections of the sheared samples, reported for the first time, indicates that the local maximum in shear stress was due to yielding of initial microstructure.
In addition to high-strain experiments, dynamic experiments were also performed in the linear viscoelastic region. 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 was found to be influenced not only by the textural size, but also by layer-plane orientation.
The flow-microstructural study was extended to the processing flow conditions by extruding AR-HP mesophase pitch through custom-made dies. Microstructural observations suggest that in the capillary, the orientation of the layer-plane was approximately radial near the wall and the orientation deviated from the radial orientation away from the wall. In the core, no preferred orientation of mesophase layer-planes was observed.
Simulation studies were performed using constitutive equations for discotic liquid-crystalline materials in simple shear flow, corresponding with the experimental studies. Two different initial conditions were considered 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.

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