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Entropic effects, i.e. the translational entropy of the NPs and the matrix, the entropy of mixing of the grafts and the matrix, and the conformational entropy of the chains appear to thus play a second order effect even in the context of these model systems. Each of these insights provides details around controlling the organization and assembly of NPs in polymers for the purpose of improving their mechanical properties, all while changing the way in which the material is designed.
Controlled dispersion and distribution of functional nanoparticles (NPs) in polymer matrix is prerequisite for improved properties of the composite materials. How to control the distribution of NPs in a facile manner remains to be a recurring challenge in the applications of polymer nanocomposites (PNCs). Surface functionalization of NPs with polymer brushes has emerged as an effective and versatile platform of tuning the interactions between the nanoparticles and the polymer hosts, allowing their integration into polymer nanocomposites. The current work aims to understand the phase behaviors of polymer-grafted nanoparticles (PGNPs) in polymer thin films and further control the spatial distribution of PGNPs through the interactions between the grafted and matrix polymer chains. In particular, polystyrene-grafted titanium dioxide nanoparticles (PS-TiO2) embedded in polystyrene (PS) thin film matrices having an initial film thickness h0 » 90 nm were investigated, where fluctuations in the grafting brush layer enables the formation of self-assembled PGNP clustering structures. Nanoimprinting directed lateral organization of the PGNP clusters in polymer thin films via topographically soft-pattern confinement was demonstrated. The PGNP clusters segregate to thicker film regions where they are less confined during thermal annealing. The partitioning of the PGNP clusters to the patterned regions was quantified by introducing the cluster partition coefficient Kc. It shows that the highly selective segregation of the clusters was driven by entropic driving forces while the film surface homogenization and shape transition of the clusters were induced by geometrical confinement of the nanopatterning. Simultaneously, the stability of the low molecular weight PS thin films is greatly enhanced against dewetting by the addition of PGNPs. The extent of the dewetting suppression depends on the PGNP concentration and can also be altered by nanopatterning. This form of soft pattern-directed self-assembly may boost colligative properties and provide enhanced and anisotropic optical such as UV-Vis, electronic and other material properties associated with organized NP clusters into precise large-scale patterns. With better understanding of the chemically identical blend systems, we further extend our model study to other PGNP/polymer blends where enthalpic interactions also participate in the phase behavior. The hybrid blend system composed of polystyrene-grafted silica nanoparticles in a poly (vinyl methyl ether) (PS-SiO2/PVME) blend thin film (≈100 nm) was studied where the brush and matrix polymers exhibit LCST type of phase behavior. Phase separation between the polymer-grafted nanoparticles (PGNPs) and matrix polymer occurs at a temperature ≈ 40° C lower than the LCST of classic binary linear PS/PVME polymer blends. Spatially organized PGNP domain structures on submicrometer scale were illustrated by introducing the symmetry-breaking soft elastomer pattern. Selective partition of the nanoparticles in both one-phase and two-phase regions can be obtained via nanoimprinting. Thermal cycling of the composite film through the critical temperature allows for thermodynamically reversible formation and dissolution of PGNP-rich domain structures. This nanoimprinting guided assembly of PGNPs in polymer nanocomposites would open pathways of novel hybrid materials for many technological applications such as responsive materials.
Mixed homopolymer brush-grafted nanoparticles, in which two distinct homopolymers randomly or alternatively immobilized by one end on the curved surface of particles, represent a new class of environmentally responsive hybrid materials. The mixed end-tethered polymers can self-assemble into intriguing nanostructures in response to environmental variations. This Ph. D. study focuses on the investigation of nanostructures formed by densely grafted poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) mixed brushes on 67 nm silica nanoparticles in various environmental conditions. As a powerful characterization method, transmission electron microscopy (TEM) was utilized in this research to achieve detailed, three dimensional (3D) nanostructures of polymer brushes. A deeper understanding of the self-assembly of polymer brush-grafted nanoparticles would benefit potential applications of these responsive particles in the nanotechnology field. Chapter 1 introduces the background of this research. Chapter 2 focuses on the 3D hierarchical nanostructures of mixed PtBA/PS brush-grafted 67 nm silica nanoparticles cast from a nonselective good solvent by electron tomography (3D TEM). The interparticle interactions and size distribution of silica cores were found to play a key role in the self-assembly of mixed PtBA/PS brushes in a dense monolayer film of hairy silica nanoparticles. Lacking of interparticle interactions, the individual particles showed unique 3D assembly nanostructures. Chapter 3 focuses on the responsive self-assembly of mixed PtBA/PS brush-grafted 67 nm silica nanoparticles in selective homopolymer matrices. PtBA and poly(cyclohexyl methacrylate) (PCHMA) are two selective homopolymers, which are compatible with PtBA and PS brushes but immiscible with PS and PtBA brushes, respectively. Compared with the uniformly collapsed mixed brushes on silica particles, mixed brushes presented different and responsive assembly structures when blended in various homopolymer matrices. These responsiveness of mixed polymer brushes could be explained by wet- and dry-brush theories. Chapter 4 focuses on the interactions between polymer brush-grafted silica nanoparticles and block copolymers as well as polymer blends. An onion-like structure can form in the low molecular weight diblock copolymer with PtBA homopolymer brush-grafted 67 nm silica nanoparticles or PtBA/PS mixed brush-grafted nanoparticles, where the PtBA brushes are much longer than the PS brushes. In polymer blends, due to either the wet- or the dry-brush effect, hairy particles uniformly dispersed in the preferred domains or segregated in the interfaces. The knowledge gained on hierarchical self-organization of homo and mixed polymer brush-grafted nanoparticles in block copolymer and polymer blend matrices will help us design better polymer nanocomposites with uniform dispersion of nanoparticles.
It is easy to understand the self-assembly of particles with anisotropic shapes or interactions (for example, cobalt nanoparticles or proteins) into highly extended structures. However, there is no experimentally established strategy for creating a range of anisotropic structures from common spherical nanoparticles. We demonstrate that spherical nanoparticles uniformly grafted with macromolecules ('nanoparticle amphiphiles') robustly self-assemble into a variety of anisotropic superstructures when they are dispersed in the corresponding homopolymer matrix. Theory and simulations suggest that this self-assembly reflects a balance between the energy gain when particle cores approach and the entropy of distorting the grafted polymers. The effectively directional nature of the particle interactions is thus a many-body emergent property. Our experiments demonstrate that this approach to nanoparticle self-assembly enables considerable control for the creation of polymer nanocomposites with enhanced mechanical properties. Grafted nanoparticles are thus versatile building blocks for creating tunable and functional particle superstructures with significant practical applications.
Polymer-nanoparticle composites have attracted considerable interest over the past few decades. While many traditional applications of composites require the nanoparticles (NPs) to remain well dispersed within the polymer matrix, some of the newer proposed applications rely on higher-order organization of NPs. Self-assembly provides a powerful bottom-up approach for organizing nanoparticles in a highly parallelized fashion. However, directing nanoparticles to self-assemble into anisotropic architectures more complex than the isotropic, close-packed structures or random aggregates observed under equilibrium or non-equilibrium conditions is highly challenging. In this dissertation, I will demonstrate how we have used molecular dynamics simulations to investigate and propose new polymer-mediated strategies for assembling spherical NPs into anisotropic, and often unique, configurations. We first investigated the underlying basis for anisotropic interactions between spherical NPs uniformly grafted with polymer chains, which were recently shown to assemble into anisotropic phases like strings and sheets. The anisotropy was shown to arise from the expulsion of polymer grafts between two contacting NPs, which led to anisotropic graft-mediated steric repulsion felt by a third approaching NP. Our computed phase diagram for formation of isotropic versus anisotropic 3-particle clusters agreed qualitatively with that obtained experimentally for larger aggregates of NPs. Next, we proposed a new strategy for assembling spherical nanoparticles into unique, anisotropic architectures in a polymer matrix. The approach takes advantage of the interfacial tension between two mutually immiscible polymers forming a bilayer to trap NPs within two-dimensional planes parallel to the interface. We demonstrated both trapping NPs at tunable distances from the interface and assembling them into a variety of unconventional nanostructures. We also developed a theoretical model to predict the preferred positions and free energies of NPs. Lastly, we studied the dynamics of polymer-grafted gold nanoparticles loaded into polymer melts. Under certain annealing conditions, the diffusion is one-dimensional and related to the direction of heat flow during annealing and is associated with an dynamic alignment of the host polymer chains. We used molecular dynamics simulations to investigate a single gold nanoparticle diffusing in a partially aligned polymer network which semi-quantitatively reproduce the experimental results to a remarkable degree.
The controlled organization of nanoparticle (NP) constituents into superstructures of well-defined shape, composition and connectivity represents a continuing challenge in the development of novel hybrid materials for many technological applications. Surface modification of NPs with grafted polymer ligands has emerged as a versatile means to control the interaction and organization of particle constituents in polymer-matrix composite materials. In this study, by incorporating polymer-grafted nanoparticles (PGNPs) into polymeric thin films, we aim to understand and control the spatial organization of PGNPs through the interactions between polymer brush layer and matrix chains. As model systems, we investigate thermodynamic behaviors of polystyrene-tethered gold nanoparticles (denoted as AuPS) dispersed in polymer thin film matrices with identical and different chemical compositions (PS and PMMA, respectively), and evaluate the influence of external perturbation fields on directed organization of nanofillers.With the presence of unfavorable enthalpic interactions between grafted and free polymer chains (i.e. AuPS/ PMMA blend thin films), phase-separated structures are generated upon thermal annealing, characterized with morphologies ranging from discrete droplets to spinodal structures, which is consistent with composition-dependent classic binary polymer blends phase separation. The phase separation kinetics of AuPS/ PMMA blends exhibit distinct features compared to the parent PS/ PMMA homopolymer blends. We further illustrate phase-separated AuPS-rich domains can be directed into unidirectionally aligned anisotropic structures through soft-shear dynamic zone annealing (DZA-SS) process with tunable domain aspect ratios.To exert exquisite control over the shape, size and location of phase-separated PGNP domains, topographically patterned elastomer confinement is introduced to PGNP/ polymer blend thin films during thermal annealing. When the phase-separated lengthscale coincides with confined pattern dimension, long-range ordered submicron-sized AuPS domains are generated in PMMA matrices with dense and well-dispersed nanoparticle distribution. Furthermore, preferential segregation of AuPS nanoparticles at patterned mesa regions can be induced in PS matrices where enthalpic interactions are absent. This selective segregation is achieved due to the local perturbation of grafted chains when confined in a restricted space. The efficiency of this particle segregation process within patterned mesa-trench films can be tuned by changing the relative entropic confinement effects on grafted and matrix chains. This physical pattern directed PGNP organization strategy is applicable to versatile pattern geometries and nanoparticle compositions.
Incorporating nanoparticles and polymers into one composite material have opened new pathways for generating novel material structures and advancing the properties of conventional materials. The developments in the field of nanocomposites have been accelerated by the progress in fabrication of nanoparticles with designed shape and precise size control, surface modification techniques covering a variety of nano-scale materials including clay sheets, carbonaceous materials, metal oxide particles, etc., as well as new syntheses of polymers with targeted architecture and functionality. The control of interfacial interactions is the key to property enhancement of almost all nanocomposite materials. Grafting polymer chains directly onto the surface of nanoparticles is a relatively new approach for obtaining novel nanocomposite structures and it offers better control of grafting density and maximizes the interfacial interactions between nanoparticles and polymeric matrices. The first project in this thesis describes the preparation of nanocomposites via surface initiated polymerization of block copolymer chains directly from the surface of montmorillonite clay. A ‘graft-from’ synthesis protocol was developed for the preparation of the nanocomposites. Comprehensive material characterization was performed to understand the structure and properties of the nanocomposites. Crystallization behavior of the bulk material and optical properties of nanocomposite films were examined. The relationship between material synthesis, structure and properties is also discussed in these chapters. The second project involves grafting polyelectrolytes onto magnetic nanoparticles for the application of electromagnetic imaging in high temperature, high salinity gas and oil reservoir environments. The fabrication of magnetic nanoparticles is described with a focus on both size control and achieving colloidal stability. The synthesized nanoparticles were used as core materials for their outstanding magnetic properties. Subsequent surface functionalization and a ‘grafting-to’ method was developed to coat the nanoparticles with a surface layer of polyelectrolytes, which provides nanoparticles with excellent transport mobility for high temperature, high salinity aqueous flow conditions through porous rock and sediment.
Since their inception, polymers have been used in the formulation of composite materials that capitalize on the ease of processing, low density, and low cost of plastics while incorporating specific filler materials that enhance mechanical properties or add functionality. Synthesizing polymer matrix composites with a high content of particulate additives can maximize the particular functionality imparted by the additive phase and lead to materials with advantageous property combinations. Critically, the distribution of particulate fillers has a profound influence on the properties of the composite material. For many applications, such as optically transparent or high strength composites, maintaining a uniform distribution of non-aggregated filler is vital. Obtaining such a uniform distribution, particularly for high loadings or nanoscale particles, is a significant challenge, and substantial research and engineering effort has been dedicated to establishing methods of compatibilizing and dispersing filler particles within a polymer matrix. Of these methods, polymer grafted nanoparticles (PGNPs) provide a unique and tunable platform for controlling composite composition and mediating interparticle interactions while precluding aggregation of the particle cores. While the utility of PGNPs as filler materials has been demonstrated extensively, their independent use as single-component composites remains a rapidly developing area of investigation. A pivotal challenge in the development of PGNP composites is the trade-off between filler loading and the mechanical robustness and processability of the composite. In this work, multiple strategies for bridging this gulf are presented and investigated in order to create highly-filled, single-component PGNP composites without compromising mechanical performance or processability. Specifically, the introduction of interparticle bonds between PGNPs via traditional chemical crosslinking, thermal self-crosslinking, and embedding inside of a polymer network are explored as routes to functional nanocomposites.
This edited volume brings together the state of the art in polymer nanocomposite theory and modeling, creating a roadmap for scientists and engineers seeking to design new advanced materials. The book opens with a review of molecular and mesoscale models predicting equilibrium and non-equilibrium nanoscale structure of hybrid materials as a function of composition and, especially, filler types. Subsequent chapters cover the methods and analyses used for describing the dynamics of nanocomposites and their mechanical and physical properties. Dedicated chapters present best practices for predicting materials properties of practical interest, including thermal and electrical conductivity, optical properties, barrier properties, and flammability. Each chapter is written by leading academic and industrial scientists working in each respective sub-field. The overview of modeling methodology combined with detailed examples of property predictions for specific systems will make this book useful for academic and industrial practitioners alike.