Title

Fabrication of Nanoporous Polymer Films from Self-Assembling Liquid Crystalline Mesogens

Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Environmental Engineering (ENAS)

First Advisor

Osuji, Chinedum

Abstract

Self-assembled liquid crystalline mesophases are attractive material platforms for various current and emerging applications. These materials provide nearly mono-disperse, periodic geometric features such as cylinders, lamellae and spheres of sizes as small as 1-2 nanometers. The preparation of thin films of such nanostructures is a critical step in a diverse array of applications ranging from photonics to separation science. In particular, advances in thin-film fabrication methods are sought to harness the emerging potential of self-assembled liquid-crystalline materials as next-generation membranes. In the first part of this work, we show that nanometer-scale control over the thickness of self-assembled mesophases can be enacted by directional photopolymerization in the presence of highly photo-attenuating molecular species. Metrology reveals average film growth rates below ten nanometers per second, indicating that high-resolution fabrication is possible with this approach. Simulations connect the experimentally observed nanometer-scale control of film growth to the photo-attenuating nature of the mesophase. Measured water permeabilities for the fabricated thin films compare favorably with current state-of-the-art nanofiltration and reverse osmosis membranes. In the second part of this work, we demonstrate a novel approach that relies on transport in a water continuous medium of a nanostructured polymer templated from a direct lyotropic hexagonal cylinder mesophase, removing the need for a pore alignment step during membrane fabrication. The high-fidelity nano-structure retention, confirmed by TEM and AFM imaging, and the mechanical robustness of the resulting material, rely on a dual-crosslinker strategy that results in a solid film consisting of internally and externally crosslinked nanofibrils embedded in a continuous aqueous medium. Fabricated membranes show strong size selectivity at the 1-2 nm length scale and high water permeabilities of ~ 1-2 L m-2 h-1 bar-1 µm. In the last part of this work, we introduce a simplified mesogen design for the fabrication of gyroid mesophases, which do not require pore alignment but are typically formed from structurally complex mesogens and exhibit narrow stability windows. The simplified mesogen design is combined with mesogen blending to modify the pore dimensions of the gyroid nanochannel. Diffusive transport in the gyroid mesophase is characterized via ion conductivity and solute adsorption tests in fabricated bulk films. Convective transport is characterized via pressure driven filtration through the fabricated gyroid films. Water permeabilities are found to be significantly lower than those of the previously described cylinder-mesophase membranes, possibly indicating that the gyroid network structure may not afford significant advantage as a membrane geometry over the direct hexagonal cylinder network due to the increased pore tortuosity of the former. Overall, this dissertation advances engineering knowledge on the fabrication of nanoporous polymer membranes derived from liquid crystalline materials. Three distinct mesophase systems with different pore geometries were studied. Engineering challenges unique to each system were resolved, followed by preliminary estimation of membrane performance.

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