DISSERTATION OUTLINE

While the effect of concentration and ionic strength on the viscosity of PBDT solutions has been reported using capillary viscometry, a thorough understanding of the rheological behavior of PBDT is lacking. In Chapter 2, we present the first rheological measurements of fully nematic PBDT solutions in water. Under steady shear, we observe anomalous shear thickening behavior, followed by a hesitation in the viscosity accompanied by an extremely narrow range of negative first normal stress difference. The Peclet number (𝑃𝑒, shear rate normalized by rod rotational diffusivity) for the onset of shear thickening is in agreement with predictions from previous, high-resolution numerical simulations of the Doi-Edwards-Hess kinetic theory. We interrogate these dynamic responses through shear step-down experiments, revealing a complex evolution of transient responses. Detailed analysis of the stress transients provides compelling evidence that the nematic director undergoes a cascade of transitions and coexistence of periodic states known as kayaking, tumbling, and wagging, before transitioning to steady flow alignment above a critical shear rate. Our results on nematic PBDT solutions reveal the nature of periodic director states and introduce a new model system to study the complex rheology of LCPs.

In Chapter 3, we extend our rheological characterization of PBDT to both the isotropic and nematic phases and employ small-angle neutron scattering to quantify the orientational order

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under shear. The concentration dependence of the zero-shear viscosity, longest relaxation time, and terminal modulus of quiescently isotropic solutions are compared to the Doi-Edwards theory for hard rods. Within the concentrated isotropic regime, we find a non-monotonic shear rate dependence of the first normal stress difference, reminiscent of a sheared nematic phase. In the fully nematic phase, we investigate the steady-state and transient nonlinear rheological measurements to characterize the flow behavior. The steady-state viscosity as a function of shear rate is characterized by anomalous shear thickening behavior, which we assign as originating from the kayaking response of the nematic director. Utilizing transient shear step-down experiments, we characterize the response of the nematic director by constructing dynamic stress paths, where the shear stress is plotted as a function of the first normal stress difference. The rotation direction of the stress path indicates the relative importance of the viscous and elastic contributions to the stress tensor, which we find undergoes a transition from elastic to viscous at the onset of shear thickening. We conclude that the observed shear thickening behavior of nematic PBDT solutions arises from viscous stress contributions due to director kayaking, rather than an elastic stress contribution due to broadening of the molecular ODF under shear.

In Chapter 4, we report on the structure and rheology of nematic PBDT solutions at high concentrations, far above the concentration for fully nematic phase behavior. At rest, the liquid crystalline solutions are kinetically stable against gelation and exhibit low viscosity and shear- thinning behavior. Under steady shear at, or above, a critical shear rate, a physically crosslinked, nematic gel network progressively forms due to linear growth and branching of the rods. The time scale of gelation can be tuned from hours to nearly instantaneously by varying the shear rate above a critical shear rate and solution concentration. The shear-activated gels are distinct in their structure and rheological properties from thermoreversible gels, maintaining their local

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positional correlations and nematic orientational ordering. At a fixed concentration, the induction time prior to gelation decreases exponentially with the shear rate, reminiscent of a thermally activated Arrhenius process. This result indicates that shear-activated thermalization of the electrostatically stabilized rods overcomes the energy barrier for rod-rod contact, enabling rod fusion and subsequent irreversible network formation. To our knowledge, this is the first example of irreversible shear-activated gelation in a liquid crystalline polymer, possibly providing a novel route towards a unique microstructure of physically crosslinked rodlike polymer.

In Chapter 5, we investigate the structure and mechanical reinforcement of solid-state PBDT nanocomposites reinforced with LCGO. Solution mixing of the nematic phase of PBDT with LCGO results in a homogeneous hybrid liquid crystalline phase. We fabricated nanocomposite films by shear-casting and subsequent in-situ thermal reduction of LCGO to reduced GO (rGO). Wide-angle X-ray scattering of nanocomposite films reveals an increase in PBDT polymer alignment both in-plane and along the casting direction. Scanning electron microscopy and modulus mapping using atomic force microscopy of nanocomposite cross-sections reveals a stratified morphology and enhanced local stiffness of polymer in the vicinity of LCGO sheets. We observed a comprehensive improvement in tensile properties, with an increase in Young’s modulus from 16 to 37 GPa, and tensile strength from 210 to 678 MPa at 1.8 vol.% rGO. Dynamic mechanical thermal analysis and time-temperature superposition measurements revealed an increased activation energy of a local polymer relaxation due to the presence of LCGO. Our high-performance nanocomposites exhibit a 26 GPa storage modulus with 1.8 vol.% rGO at 400 °C. These findings demonstrate that extreme mechanical reinforcement of liquid crystalline polymers is achieved by utilizing LCGO through hybrid liquid crystalline dispersions.

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In Chapter 6, we present a structure-property relationship study of conductive PBDT-IL composites. In the presence of IL, PBDT forms glassy and high aspect ratio hierarchical nanofibrils, which enables fabrication of polymer electrolyte membranes (PEMs) with both high volume fractions of IL and high elastic modulus. We report direct evidence for nanofibrillar networks of PBDT that serves as a matrix for dispersed IL using atomic force microscopy and small- and wide-angle X-ray scattering. These supramolecular nanofibrils form through myriad non-covalent interactions to produce a physically crosslinked glassy network, which boasts the best combination of room temperature modulus (0.1–2 GPa) and ion conductivity (4–8 mS cm−1₎

of any polymer-IL electrolyte reported to date. The high thermo-mechanical properties of our PBDT-IL composites, i.e., 𝐸 ≈ 1 GPa, at temperatures up to 200 °C, enable a wide device operation window with stable mechanical properties. Together, the high-performance nature of sulfo-aramids in concert with the inherent properties of ILs impart PBDT-IL composites with thermal stability up to 350 °C. Thus, nanofibrillar ionic networks based on sulfo-aramids and ILs represent a new design paradigm enabling PEMs with exceptionally high moduli at exceedingly low polymer concentrations.

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CHAPTER 2: OBSERVATION OF TRANSITION CASCADES IN A SHEARED LIQUID