Date of Award
Open Access Dissertation
Chemistry and Biochemistry
College of Arts and Sciences
The surface modification of solid inorganic substrates for thermodynamic compatibility with organic polymer media has become an important area of study for the development of advanced functional composite materials. Polymer ligands covalently bound to the substrate surface have been an effective means to achieve said compatibility. Current investigations focus on multifunctional ligand engineering where multiple chemically distinct species, each with specific functionality, can be implemented on the filler surface simultaneously. A combination of matrix compatible polymer brushes and conjugated charge trapping moieties attached to filler surfaces are investigated for enhancements in dielectric properties. Presented herein are the synthesis and characterization of multifunctional ligand grafted nanoparticles as well as the properties afforded by them.
Click chemistry has proven to be a mild and efficient strategy for the functionalization of silica surfaces. Presented is a one pot synthesis using click chemistry for simultaneous attachment of two chemically distinct species. RAFT polymerization was used to synthesize PGMA polymer chains for compatibility with an epoxy resin. Short conjugated species (terthiophene or ferrocene) were used to add charge trapping functionality. The ligands were synthesized to contain alkyne terminal groups while the silica substrate was modified to contain azide functionality. Assembly of the bimodal functionalized nanoparticles was successful using the Huisgen cycloaddition click reaction. Analysis of the resultant composites showed improvements in dielectric breakdown strength over neat epoxy, and epoxy filled with unmodified and monomodal PGMA modified silica.
With the efficacy of multifunctional grafted silica established, further investigation was then performed into the parameters that govern dielectric breakdown in composites. Using a grafting-from methodology and sequential addition of ligands, multiple graft densities of PGMA chains were attached to silica ultimately achieving multiple states of filler dispersion. The effect of dispersion was explored for both bimodal and monomodal surface modified nanoparticles on dielectric breakdown strength. A diminishing return was observed for improvements in DBS with increasing dispersion. The effect of short ligand chemistry was also explored at well-dispersed states. Excellent correlation was observed for calculated values of ionization energy + electron affinity (IE+EA) of short conjugated species and experimentally obtained DBS of the composite. Concentration of conjugated ligand groups present at the filler surface was also explored by implementing bimodal brush grafted nanoparticles.
Focus was then shifted to the modification of metal oxide nanoparticles. Ligands were designed to contain phosphate or phosphonic acid moieties for robust covalent attachment. Grafting-to strategies were again used for surface functionalization. A bimodal population of PDMS chains was used to induce compatibility of 5 nm TiO2 nanoparticles with a silicone matrix. Two species of anthracene ligands were synthesized both containing phosphonic acid moieties for surface attachment. The anthracene ligands differed in substitution at the 10 position to study the effect of adding an electron withdrawing group on composite dielectric breakdown strength. A novel phosphate containing RAFT agent also was synthesized in an attempt to perform surface initiated RAFT polymerization on the surface of ZrO2 nanoparticles.
Finally, surface modification of silica microparticles for enhanced fracking proppants was explored. An activated free radical azo initiator was synthesized and subsequently attached to the silica particle. Free radical polymerization was then used to polymerize acrylamide from the particles surface. The particles were studied for their ability to remain suspended in aqueous solution. It was found that surface modified particles had the ability to remain suspended longer in aqueous solutions than unmodified particles. Correlation was observed between the wt% of polymer present on the particle surface and suspension time of the particles.
Bell, M. H.(2016). Ligand Engineering For Advanced Functional Composite Materials. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/3818