Synthesis and Characterization of Diblock Copolymer Templated Iron Oxide Nanoparticles

Templating ordered assemblies of magnetic oxide nanoparticles within self-assembled diblock copolymers of varying morphologies is an important problem with a wide applicability such as in electromagnetics, optical devices, metal catalysts, medicine and biology. In this thesis, the effects of different polymer structures on particle ordering and resultant magnetic properties have been investigated using various microstructure and magnetic characterization tools. Ring-opening metathesis polymerization (ROMP) of norbornene and functionalized norbornene monomers has been used to synthesize diblock copolymers of narrow polydispersities using Grubbs' catalyst. These block copolymers can be used as templates to form inorganic nanoparticles. In this research, the structural and physical understanding of the inorganic-copolymer system was studied by small-angle neutron and x-ray scattering techniques and transmission electron microscopy. Synthesis of $\gamma$-Fe$_2$O$_3$ nanoparticles has been achieved within novel block copolymers of (norbornene)-b-(deuterated norbornene dicarboxylic) acid and (norbornene methanol)-(norbornene dicarboxylic acid). The polymer morphologies were controlled by varying the volume fractions of the constituent blocks. The pure norbornene based diblock copolymer morphologies were demonstrated by electron microscopy for the first time. Spherical, cylindrical and lamellar morphologies of these novel diblock copolymers were reported. The block ratios of the synthesized polymers were determined using gel permeation chromatography - light scattering, elemental analysis and UV-VIS spectroscopy. Solution phase doping and submersion of thin films in metal salt solutions were employed as metal doping methods and the observed nanoparticle structures were compared to those of the undoped copolymer morphologies. This project reports on the types of templating structures and dispersion of the nanoparticles. The effects of particle interactions on the microphase separation and magnetic properties were also investigated. The knowledge gained from understanding the templating mechanism in block copolymer / iron oxide nanocomposites can be applied to other similar systems for a variety of biological and catalyst applications

Alignment-Assisted Networks of Polyelectrolyte-Grafted Cellulose Nanocrystals

This study aims to understand the role of polyelectrolyte grafting on the dispersed cellulose nanocrystal (CNC) rods in water through measuring transport coefficients using depolarized and polarized dynamic light scattering and by measuring the viscoelastic properties using rheometer. Rotational and translational diffusivities are found to slow down with poly(acrylic acid) (PAA)-grafted chains compared to bare CNCs. Translational diffusion is shown to remain constant between pH 3 and 9, indicating the good dispersion and stability of PAA-grafted CNC suspensions. At the overlap solution concentration, chains play a significant role in bridging the CNC and form a network, as measured with the viscoelastic properties of neutral chains. When chains are ionized by altering the pH, the higher viscosity is measured because of the hydrogen bonding between ionized and un-ionized carboxylic groups, as previously demonstrated with PAA-grafted spherical nanoparticles. We further measured the viscoelastic response of PAA-grafted CNC after applying large steady shear. The results show that CNCs with long grafts presented enhanced viscoelastic moduli, and their critical strain value decreased after large shear flow application. Short grafts, in contrast to the long grafts, did not show any changes in the viscoelastic response under shear. These results indicate that the alignment-assisted networks of PAA-grafted CNC enable better entanglements between long grafted chains at the neutral state

Assembly of Polymer-Grafted Magnetic Nanoparticles in Polymer Melts

Hydrophobic iron oxide nanoparticles grafted with hydrophobic polymer chains of varying molecular weights and graft densities are synthesized to underpin the role of brush entanglement and dipolar forces on creating nanostructures. Grafting density on magnetic nanoparticles is controlled in grafting-to method by changing the concentration of functionalized polymer in solution. The grafting density and brush length have varied systemically to observe the changes in nanostructures. Bridging between grafted chains and dipolar forces become effective only at low grafting density and result in long chains of particles. We demonstrate experimentally that structural transition of magnetic nanoparticles is controlled with the balance between grafted chain entanglements and dipolar forces

An Interface-Driven Stiffening Mechanism in Polymer Nanocomposites

Dynamic mechanical response in responsive and adaptive composites can be achieved either through the responsive polymer; with the chemical regulators affecting the bonding between fillers or through reversible covalent bonding. Tuning the interfaces between fillers and polymer matrix potentially plays a critical role in all these systems to enhance their adaptive responses. Here, we present that the bonding–debonding of chains on nanoparticles can be modulated under extensive periodic strains. Mechanical response of an attractive model polymer composite, poly­(methyl methacrylate) filled with silica nanoparticles, is monitored in a series of deformation–resting experiments allowing us to tune the interfacial strength of polymer. Chains that are desorbed from the surface with the oscillatory shear entangle with the free chains during the rest time. We show that periodic deformation process results in unusual stiffening of composites. Mechanical response during the recovery reveals this behavior arising from the enhancement in the entanglement of chains at interfaces. The interfacial hardening can be used in designing polymer composites with stress-sensitive interfaces to achieve new repair mechanisms for biomedical applications, and also in energy absorbing reinforced systems

Reversible Thermal Stiffening in Polymer Nanocomposites

Miscible polymer blends with different glass transition temperatures (<i>T</i>_g) are known to create confined interphases between glassy and mobile chains. Here, we show that nanoparticles adsorbed with a high-<i>T</i>_g polymer, poly­(methyl methacrylate), and dispersed in a low-<i>T</i>_g matrix polymer, poly­(ethylene oxide), exhibit a liquid-to-solid transition at temperatures above <i>T</i>_g’s of both polymers. The mechanical adaptivity of nanocomposites to temperature underlies the existence of dynamically asymmetric bound layers on nanoparticles and more importantly reveals their impact on macroscopic mechanical response of composites. The unusual reversible stiffening behavior sets these materials apart from conventional polymer composites that soften upon heating. The presented stiffening mechanism in polymer nanocomposites can be used in applications for flexible electronics or mechanically induced actuators responding to environmental changes like temperature or magnetic fields

Effect of Ionic Groups on Polymer-Grafted Magnetic Nanoparticle Assemblies

Conductivity in ionomer melts is governed by the density of conducting ions and ionic aggregation within low dielectric polymers. New material design strategies are needed to direct ion aggregation by utilizing low ion densities that will improve ion conductivity in polymer composite films. Here, we report the dispersion of ionomer-grafted magnetic nanoparticles (NPs) in polymers to explore their potential in energy applications. Iron oxide NPs coated with a uniform silane layer are grafted with polystyrene (PS) chains and are randomly sulfonated to various extents. We examine the interplay between ionic interactions and chain repulsion by varying the ion concentration and length of grafted chains. Transmission electron microscopy and small-angle X-ray scattering results show that ion-containing polymer-grafted NPs form highly ordered chain-like structures below 3 mol % sulfonation in bulk at two particle loadings (5 and 15 wt %). Moreover, increasing grafted chain length leads to long-range spacing correlations between sulfonated strings. This strategy to create discrete and connected highly ordered string nanostructures can be used as a means of controlling the ion aggregation and transport in polymer nanocomposites

Design of Ion-Containing Polymer-Grafted Nanoparticles for Conductive Membranes

While sulfonated polymers are commonly used in membranes for fuel cells and water filtration applications, challenges of controlling ionic aggregation and understanding morphology effects on conductivity and transport still remain. In this work, we investigate the aggregation of copolymer-grafted nanoparticles that are designed to form conductive structures with low sulfonation amounts of chains. We demonstrate that long grafts of polystyrene chains with sulfonated end groups form side-by-side aggregated strings and retain their structures in ionic liquid, 1-hexyl-3-methylimidazolium bis­(trifluoro­methyl­sulfonyl)­imide, [HMIM]­[TFSI]. Transmission electron tomography results revealed that these aggregates are monolayers of particles at low sulfonations and planar-like networks at 3 mol % sulfonation in the ionic liquid. Organization of magnetic nanoparticles with the polymer grafting approach is shown, for the first time, to enhance conductivity upon incorporation of an ionic liquid