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Nanoparticle transport via holographic photopolymerization

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Title: Nanoparticle transport via holographic photopolymerization
Author(s): Busbee, John D.
Director of Research: Braun, Paul V.
Doctoral Committee Chair(s): Braun, Paul V.
Doctoral Committee Member(s): Wiltzius, Pierre; Suslick, Kenneth S.; Shim, Moonsub; Vaia, Richard A.
Department / Program: Materials Science & Engineerng
Discipline: Materials Science & Engr
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Dissertation
Subject(s): nanoparticle holography holographic photopolymerization nanoparticle patterning nanoparticle assembly HPDLC holographic polymer dispersed liquid crystals silica nanoparticles thiol-ene
Abstract: This thesis studies methods of engineering the placement of nanoparticle with sub-micron resolution over macroscopic size scales. Such control holds great promise as an inexpensive methodology for manufacturing devices, such as optical elements, with enhanced functionality and properties. Specifically, we have examined the addition of silica nanoparticles into holographic photopolymerization systems for the purpose of controlling the location of the nanoparticle within the periodic structure based upon control of the surface chemistry of the nanoparticle. Herein we report the successful sequestration of methacrylate functionalized, silica nanoparticles into the polymeric domain of phase separated, liquid crystal—acrylate composite structures with regular, submicron periodicities. TEM analysis of the resultant Bragg gratings indicated that the nanoparticles were fully dispersed within the polymer. Optical characterization of the grating structures indicated that the particles did not negatively impact the optical properties of the grating, and enhanced the switching properties of the liquid crystal structure due to the roughly lamellar morphology of the liquid crystal droplets when the nanoparticle are present in the formulation. The successful functionalization of the nanoparticle surface was confirmed using NMR analysis. It was noted experimentally that at large, excess concentrations of the nanoparticle functionalization agent, a hybrid organic—siloxane corona was physisorbed onto the nanoparticle surface, imparting liquid-like properties to the nanoparticles without the presence of solvents. It was then demonstrated using SAXS analysis that one could achieve FCC structures with these highly viscous liquids containing up to 60% inorganic content. Because the corona is polymerizable, the structure can then be fixed in place. Excellent dispersion and isolation in the polymer domain was also demonstrated for high loadings (up to 20 wt%) of the same reactive silica nanoparticles in holographic syrups containing thiol-ene monomers. Because thiol-ene polymers have significant advantages over acrylates for these systems due to the step-growth polymerization mechanism, this system was chosen for further development as a method to create defined nanoparticle polymer hybrid structures. To guide future development efforts, real-time monitoring of grating development was conducted to increase knowledge of system variability. It was noted here that the addition of nanoparticles delays grating formation in a fashion linear with nanoparticle loading. In order to help understand the mechanism by which nanoparticles delay grating formation, the step-growth, holographic process was modeled using a reaction diffusion system that treats component diffusion as following Stokes-Einstein driven by pure concentration gradients. Ignoring the thermodynamic interactions proved to be an inadequate assumption, so the model was re-accomplished using Flory-Huggins Polymer—Solution Theory to account for free energy using an experimentally derived χ between the polymer and liquid crystal and by assuming that the nanoparticle could be treated as a polymer with a radius of gyration equal the nanoparticle radius. This model qualitatively matched the experimental observations and roughly predicted that the delay in grating formation was due to reactive incorporation of the nanoparticle into the polymer, which greatly slowed polymer diffusion. It was then speculated that this increased viscosity delayed phase separation, although phase separation was not treated directly with this model.
Issue Date: 2010-01-06
URI: http://hdl.handle.net/2142/14742
Rights Information: Copyright 2009 John D. Busbee
Date Available in IDEALS: 2010-01-06
2012-01-07
Date Deposited: December 2
 

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