Monodispersed microgels composed of poly-acrylic acid (PAAc) and poly(N-isopropylacrylamide) (PNIPAM) interpenetrating networks were synthesized by 2-step method with first preparing PNIPAM microgel and then polymerizing acrylic acid that interpenetrates into the PNIPAM network. The semi-dilute aqueous solutions of the PNIPAM-PAAc IPN microgels exhibit an inverse thermo-reversible gelation. Furthermore, IPN microgels undergo the reversible volume phase transitions in response to both pH and temperature changes associated to PAAc and PNIPAM, respectively. Three applications based on this novel hydrogel system are presented: a rich phase diagram that opens a door for fundamental study of phase behavior of colloidal systems, a thermally induced viscosity change, and in situ hydrogel formation for controlled drug release. Clay-polymer hydrogel composites have been synthesized based on PNIPAM gels containing 0.25 to 4 wt% of the expandable smectic clay Na-montmorillonite layered silicates (Na-MLS). For Na-MLS concentrations ranging from 2.0 to 3.2 wt%, the composite gels have larger swelling ratio and stronger mechanical strength than those for a pure PNIPAM. The presence of Na-MLS does not affect the value of the lower critical solution temperature (LCST) of the PNIPAM. Surfactant-free hydroxypropyl cellulose (HPC) microgels have been synthesized in salt solution. In a narrow sodium chloride concentration range from 1.3 to 1.4 M, HPC chains can self-associate into colloidal particles at room temperature. The microgel particles were then obtained in situ by bonding self-associated HPC chains at 23 0C using divinyl sulfone as a cross-linker. The volume phase transition of the resultant HPC microgels has been studied as a function of temperature at various salt concentrations. A theoretical model based on Flory-Huggins free energy consideration has been used to explain the experimental results. Self-association behavior and conformation variation of long chain branched (LCB) poly (2-ethyloxazoline) (PEOx) with a CH3-(CH2)17 (C18) modified surface are investigated using light scattering techniques in various solvents. The polymer critical aggregation concentration (cac) strongly depends on solvent polarity, decreasing as the solvent becomes more hydrophobic.

Underfill material is a special colloidal dispersion system with silicon dioxide particles in the organic liquid. It is used to improve the reliability of integrated circuits (IC) packaging in the microelectronics. In order to successfully synthesize the nanocomposite underfill meeting the requirements of the chip package, it is necessary to have a fundamental understanding about the particle stability in the non-aqueous liquid and the relationship between materials properties and interphase structure in the composite. The results of this thesis contribute to the knowledge of colloidal dispersion of nanoparticles in organic liquid by systematically investigating the effects of particle size, particle surface chemistry and surface tension, and liquid medium polarity upon the rheological and thermal mechanical properties of underfill materials. The relaxation and dielectric properties studies indicate that the polymer molecular chain motion and polarization in the interphase region can strongly influence the material properties of nanocomposite, and so a good interaction between particle and polymer matrix is key. With this study, a potential nanocomposite underfill can be synthesized with low viscosity, low thermal expansion, and high glass transition temperature. The excellent transmittance of nanoparticles leads to further investigation of their ability as reinforcing filler in the photo-curable polymer.

The use of hydrogel nanoparticles, or nanogels, as targeted delivery vehicles to cancer cells was described. The nanogels were synthesized by free radical precipitation polymerization, with poly(N-isopropylmethacrylamide) as the main monomer, and have a core/shell architecture. The nanogels were near 50 nm in radius, contained fluorescein for visualization, and had an amine-containing shell for bioconjugation, making these particles ideal for delivery studies. The nanogels were conjugated with the YSA (YSAYPDSVPMMSC) peptide, which is an ephrin mimic, allowing for uptake by the EphA2 (erythropoietin-producing hepatocellular) receptor. We have delivered YSA-conjugated nanogels to Hey cells and BG-1 cells, as evidenced by fluorescence microscopy. We have shown that the nanogels can encapsulate siGLO Red Transfection Indicator (siGLO) and deliver the siGLO to Hey cells in vitro. After successful delivery of the non-targeting siGLO, we delivered siRNA for knockdown of epidermal growth factor receptor (EGFR). We have shown protein knockdown from 24-120 h after nanogel delivery, as well as knockdown with different siRNA concentrations delivered to the cells. Furthermore, addition of taxol following EGFR knockdown suggests that the chemosensitivity of the Hey cells is increased. Successful in vitro delivery of the nanogels prompted in vivo studies with the nanogels. The nanogels were used to encapsulate silver nanoclusters for potential bioimaging applications. Targeting of the nanogels to MatrigelTM plugs in mice suggest that the particles hold promise as in vivo delivery agents.

Hydrogels, which are polymeric cross-linked networks that swell in aqueous environments, are versatile materials that can contain a variety of chemical functionalities, mechanical properties, and topographical features. Microgels are the stable colloidal form of hydrogel materials that range in size from approximately 100 nm to a few microns in diameter. While they also can exhibit similar properties to those of macrogels, microgels can be used as building blocks in a bottom-up approach to assemble films of higher complexity. In this dissertation, work is focused on understanding the assembly and behavior of microgel thin films as non-fouling surfaces, centrifugally deposited materials, self-healing coatings, and degradable constructs. Non-fouling films were assembled using PEG cross-linked microgels to reduce non-specific protein adsorption and mitigate cellular adhesion. These constructs were assembled in a polyelectrolyte multi-layered fashion, of alternating anionic microgels and cationic linear polymer, to effectively block the substrate from the biological environment and consequently exhibited control over cellular adhesion with the surface. The utility and application of these non-fouling microgel coatings on functional implants was also explored. Centrifugal deposition was used to rapidly generate non-fouling microgel multi-layered interfaces on planar surfaces, and upon closer inspection of the microgel monolayers, it was found that the centrifugally deposited films contained closer-packed microgel assemblies with microgels of smaller footprint size, compared to microgels that are passively adsorbed to the surface. Microgels that are centrifugally deposited may adopt a higher energy chain conformation than passively adsorbed microgels, and this higher energy chain conformation may translate into the multi-layered materials. Nonetheless, the centrifugally deposited non-fouling microgel multi-layered films were found to effectively block macrophage adhesion. Films were also assembled in a polyelectrolyte fashion on soft substrates, and were observed to become significantly damaged under mechanical manipulation (poking, bending, or stretching), but then self-heal upon addition of water. By altering the building blocks of the polyelectrolyte multi-layered films, such as the molecular weight of the polycation between microgel layers or by using anionic rigid spheres as the particle in the assembly, changes in the observed film damage suggest that particle-linear polymer interpenetration and polyvalency likely play an important role in the strength and integrity of the microgel thin films. Fluorescently-labeled microgels were also used to interrogate how the films reorganize in the lateral direction, and these early studies suggest that the microgel multi-layered films reorganize when damaged and also possibly when they are undamaged and simply incubated in an aqueous environment. Additional studies were also conducted on microgels synthesized with a hydrolyzable cross-linker, and by supporting these degradable constructs on substrates, detailed single-particle morphological changes during erosion could be interrogated in complex media such as serum. This work, as a collection, demonstrates the ability to obtain information about microgel thin film assemblies and their behavior using microscopy techniques such as ambient and in liquid atomic force microscopy, brightfield optical microscopy, and fluorescence microscopy. The observations made here illustrate how microgels can be used to fabrication thin films that can be utilized in biological applications (non-fouling, self-healing, and erodable constructs), and how different deposition methods (centrifugal deposition and polyelectrolyte multi-layers) can dictate their behavior.

Liquid dispersions of colloidal particles play a big role in nature and as industrial products or intermediates. Their material properties are largely determined by the liquid-mediated particle-particle interaction. In water-based systems, electric charge is ubiquitous and electrostatic particle interaction often is the primary factor in stabilizing dispersions against decomposition by aggregation and sedimentation. Very nonpolar liquids, by contrast, are usually considered free of charge, because their low dielectric constant raises the electrostatic cost of separating opposite charges above the available thermal energy. Defying this conventional wisdom, nonpolar solutions of certain ionic surfactants do support mobile ions and surface charges. Even some nonionic surfactants have recently been found to raise the conductivity of nonpolar oils and promote surface charging of suspended particles, but this counter-intuitive behavior is not yet widely acknowledged, nor is the mechanism of charging understood. The present study provides the first characterization of the electrostatic particle interaction caused by nonionizable surfactants in nonpolar oils. The methods used in this study are video microscopy experiments where particle positions of equilibrium ensembles are obtained and translated into particle interactions. Experimentally, equilibrium particle positions are monitored by digital video microscopy, and subjected to liquid structure analysis in order to find the energy of interaction between two particles. The observed interaction energy profiles agree well with a screened-Coulomb potential, thus confirming the presence of both surface charge and mobile ions in solution. In contrast to recently reported electrostatic particle interactions induced by ionic surfactants in nonpolar solution, the present study finds evidence of charge screening both above and below the surfactant's critical micelle concentration, CMC. Fitted Debye screening lengths are much larger than in aqueous systems, but similar to the Debye length in nonpolar oils reported for micellar solutions of ionic surfactants cite{hsu_charge_2005}. Radial distribution functions obtained from experiments are compared to Monte-Carlo simulations with input potentials obtained from a fit to the interaction measurement. The measured electrostatic forces and fitted surface potentials are fairly substantial and easily capable of stabilizing colloidal dispersions. Although few in number, surface charges formed on polymer particle surfaces submerged in nonpolar solutions of nonionizable surfactants create surface potentials comparable to those in aqueous systems.

Many concentrated paste-like materials are composed of deformable particles randomly packed into a dense suspension. Examples of the constituent soft particles include polyelectrolyte microgels, emulsion droplets, polymer coated colloids, and star polymers. These materials share in common many properties such as yield stress, shear thinning, non-zero normal stresses, wall-slip, shear-banding, memory and aging (similar to that in structural, spin and polymer glasses). Their unique properties make soft particle pastes (SPPs) scientifically interesting and extremely useful in industrial applications (as rheological modifiers). In this dissertation particle simulations, theoretical models and experiments are used to study the flow dynamics and rheological behavior of SPPs near confining surfaces - wall-slip and shear flow, and in the bulk - elasticity at small stresses and the non-linear shear rheology. In the study of slip near smooth surfaces, rheological measurements are shown that identify the influence of the chemical nature of the shearing boundary on slip at the shearing boundary. A modified elastohydrodynamic model is presented that incorporates attractive and repulsive short range interactions between the paste particles and captures the corresponding suppression and promotion of slip at the wall. Further, fluorescence microscopy and particle tracking velocimetry is used to visualize slip and flow of pastes near smooth boundaries and study the sensitivity of the bulk flow profile to the nature of the shearing surface. In the study of elastic properties of pastes, SPPs are modeled as three-dimensional systems of randomly packed elastic spheres. Simulations are performed wherein the packing is subject to small deformations to compute the high- and low-frequency shear moduli. The simulation results are compared with the data from experiments on microgel pastes. This model is extended to study paste dynamics under simple shear with added pairwise elastohydrodynamic lubrication interactions between the densely packed soft particles. The shear and normal stress differences generated during simple hear flow are calculated that compare well with the experimental data. In addition, the pair distribution function of the initial isotropic configuration, the elastically deformed and the steady sheared configurations is investigated. A semi-empirical analysis of the microstructure and its evolution due to shearing is presented. / text

Traditionally, the experimental model of choice for studying the structure and dynamics of glasses or crystals are hard-sphere colloids. An analogy with molecular or atomic materials is often drawn, in which each colloidal particle represents an atom or a molecule. Making the individual particles deformable allows an even wider range of phenomena to be observed. In this thesis, I report the three-dimensional confocal microscopic study of the structure and dynamics of aqueous suspensions of fluorescently labeled poly(N-Isopropylacrylamide)-co-(Acrylic Acid), or p(NIPAm-co-AAc), microgel particles of hydrodynamic diameter 1.0 - 1.5 μm. Image analysis techniques and particle tracking algorithms are used to quantify the particle dynamics and the suspension structure. The phase behavior of the suspensions is dependent on a number of factors including pH, temperature, and concentration. By adjusting the pH, the interactions between the microgel particles can be tuned from purely repulsive near neutral pH, to weakly attractive at low pH. At low pH and low concentration, dynamic arrest results mainly from crystallization driven by the attraction between particles; crystal nucleation occurs homogeneously throughout the sample. The dynamics is nucleation limited where fast crystallization follows a delay time. At low pH and high concentration, relaxation of the suspension is constrained and it evolves only slightly to form disordered solid. At neutral pH, the dynamics are a function of the particle number concentration only; a high concentration leads to the formation of a disordered soft glassy solid. Additionally, the three-dimensional image stacks are studied to determine crystal structure by calculating pair correlation functions, g(r), bond order parameters, and structure factors, s(q). The results show that crystal structure is independent of concentration, charge, size, and stiffness of particles remaining FCC under all conditions. At low concentrations and low pH, the structures formed are polycrystalline solids. Moreover, the ability of the particles to compress enables the suspensions to maintain their crystal structure when subjected to external stress. The results help us better understand the relationship between dynamics and structure in soft colloidal suspensions, enhance our ability to use the colloids to model materials, and improve applications of the colloids in industrial products. / Physics

In this thesis, computer simulations are used to study the properties of new colloidal systems with structured interactions. These are pair interactions that include both attraction and repulsion. Structured colloids differ from conventional colloids in which the interactions between particles are either strictly attractive or strictly repulsive. It is anticipated that these novel interactions will give rise to new microscopic structure and dynamics and therefore new material properties. Three classes of structured interactions are considered: radially structured interactions with an energetic barrier to pair association, Janus surface patterns with two hemispheres of different surface charge, and striped surface patterns. New models are developed to capture the structured interactions of these novel colloid systems. Dynamical computer simulations of these models are performed to quantify the effects of structured interactions on colloid properties. The results show that structured interactions can lead to unexpected particle ordering and novel dynamics. For Janus and striped particles, the particle order can be captured with simpler isotropic coarse-grained models. This relates the static properties of these new colloids to conventional isotropically attractive colloids (e.g. depletion attracting colloids). In contrast, Janus and striped particles are found to have substantially slower dynamics than isotropically attractive colloids. This is explained by the observation of longer-duration reversible bonds between pairs of structured particles. Dynamical mapping methods are explored to relates the dynamics of these structured colloids to isotropically attractive colloids. These methods could also facilitate future nonequilibrium simulation of structured colloids with computationally efficient coarse-grained models.

Microgels are a class of colloids that are mechanically soft, and while in many cases can behave similarly to their hard-sphere counterparts, their interaction potentials are quite different. The softness of the interaction between microgels makes them capable of deformation and compression into more concentrated assemblies. This concentrated regime is interesting because little, if any, experimental work has been done to see how the bulk properties of soft-sphere assemblies deviate from those of hard-spheres at the point where their interaction potentials begin to diverge. In this thesis the effects on assembly phase behavior and dynamics of both particle compression and softness of the interaction potential are addressed. Poly(N-isopropylacrylamide) (pNIPAm) microgels are an excellent model system in which to study these effects. The thermoresponsivity of the polymer provides the experimentalist with a dial to tune the volume fraction of an assembly, while maintaining a constant particle number density in the system. Optical microscopy, particle tracking analysis and rheology have been used to investigate the effects of packing and particle structure on equilibrium phase behavior and localized perturbations to the phase of the assembly of this soft-sphere system. It has been elucidated from these experiments and others involving deswelling of large microgel particles in the presence of high concentrations of smaller microgels, that the soft, repulsive interaction between microgels is caused by a longer-range repulsion than was previously believed. The particles are acting on each other from a distance through the osmotic pressure of the assembly, which causes each particle to deswell without coming into direct contact with a neighboring particle.

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