Date of Award
Fall 2024
Document Type
Open Access Dissertation
Department
Chemical Engineering
First Advisor
Monirosadat Sadati
Abstract
Over time, environmental pressures driving evolution have promoted the propagation of biomaterials boasting properties that elude replication using modern techniques. Such disparities are often predicated on Nature’s ability to optimize mechanical or optical performance by deftly constructing intricate architectures with nanoscale precision. For instance, organisms exhibiting vibrant, iridescent structural colors achieve this unique characteristic through periodic nanostructures that selectively reflect visible light via Bragg-like reflection, with the reflected wavelength depending on the spacing of the structure. Using constructive wave interference as a physical basis, this phenomenon presents an opportunity to access the entire visible light spectrum in a single material. However, to realize such a feat in viable products, manufacturing methods facilitating material design across length scales spanning several orders of magnitude must be developed. Liquid crystal systems based on hydroxypropyl cellulose (HPC) partially address the burden of nanoscale design, as the chiral nematic phase, which spontaneously self-assembles at sufficient HPC concentrations, assumes a helicoidal structure analogous to that observed in naturally derived photonic materials. Here, the structural color is a function of the helical pitch, defined as the distance required for a single layer of HPC molecules to complete a full 2π rotation. By employing HPC-based inks in extrusion-based 3D printing, the intrinsic deformation process can be leveraged to implement bottom-up design of printed constructs, enabling fabrication of intricate macroscale geometries with programmable embedded nanoscale functionality analogous to structurally colored materials observed in nature.
In chapter two, aqueous liquid crystal HPC formulations were initially designed with polyethylene glycol (PEG) to modulate the solid-state helical pitch by inducing kinetic arrest during the drying process. Increasing PEG concentrations led to larger pitch values, and enabled access to structural colors spanning the visible spectrum. Rheological characterization of the viscoelastic inks was used to define shear rates inducing a pseudo-nematic flow state (23 s-1), where subsequent self-assembly resulted in chiral nematic domains whose helical axis was oriented normal to the printing substrate, giving optimal reflection intensities. At higher shear rates (100 s-1), the domain axis was commonly directed along the print direction to produce filaments whose observed color varied with viewing angle. Printing parameters corresponding to the optimal shear rate were implemented to construct complex, structurally colored designs at high resolutions.
In chapter three, photocurable HPC inks composed of dimethyl sulfoxide (DMSO) and hydroxyethyl acrylate (HEA) were developed to enable multi-layer 3D printing using in-situ photopolymerization. In these high viscosity solutions, rheo-optical characterization revealed complex flow and relaxation dynamics that were heavily dependent on the applied shear rate. Here, shear rates (2 s-1) sufficient to induce order on the chiral nematic domains without achieving pseudo-nematic flow proved to be optimal for preserving structural color in the polymerized filaments. Unlike the HPC/PEG system, shear rates associated with the pseudo-nematic state cause a significant elastic response upon flow cessation, which disordered the orientation of HPC molecules and impeded self-assembly of the desired structure.
Chapter four explores the implementation of this liquid crystal system in more exotic applications, including emulsions displaying static or dynamic structural color, anti-counterfeiting, and the design of novel photonic morphologies through shear-induced flow instabilities. Challenges impeding the development of practical products are discussed, as well as a brief outline of future work that can provide solutions.
Rights
© 2025, Kyle George
Recommended Citation
George, K.(2024). Shear-Induced Nanoscale Morphologies in 3D Printed Liquid Crystal Polymers for Programmable Structural Color. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/8129