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A number of grand challenges are emerging in the development of new polymeric materials which cannot be solved by traditional academic, single investigator, single institution approaches. As a result it is proposed to effectively address these challenges by the establishment of a multi-disciplinary, University-National Laboratory research team whose members can exploit a wide variety of different skills and industrial connections. The expertise of Hawker and Phillips ranges from polymer synthesis to nanoparticle production with the synergism of the team based on the LANL PI's (Phillips) recent development of novel methods for the production of a wide variety of different nanoparticles, nanorods, etc., and the ability of the laboratory of the UCSB PI (Hawker) to exploit these nanoparticles for creating unique polymer-nanoparticle composites, interfacial agents, etc. A critical aspect of this proposal is the inherent flexibility of this multi-discipline team (synthesis of all starting materials, base polymer and nanoparticle are present) which allows quick and efficient optimization of material properties for a variety of applications.
A broad effort over the last five years to develop novel technology for the synthesis of nanoscale materials has produced dramatic new methods for creating materials such as carbon nanotubes, nano-oxide, nano-metal, and nano-catalysts. Yet, it is still not clear that these nanostructured materials can be employed to make truly unique, and manufacturable devices as their accurate and controllable dispersion in a matrix (typically polymeric due to the ease of processing) is a significant challenge. We propose to overcome this difficulty by employing the synergistic talents/skills/facilities of UCSB and LANL to answer a fundamentally important question: Can nanostructures be assembled to produce materials or devices of unique character and value? The unique feature of this proposal is access to well-defined nanoparticle/nanorods of various compositions, novel polymeric dispersing agents, functionalized polymer matrices and responsive polymeric materials. During the course of these studies we will devise strategies for the homogeneous dispersion of nanostructures in a variety of polymeric matrices while at the same time controlling their position at interfaces, surfaces, bulk phase, etc.
The recent availability of many new types of polymeric surfactants (e.g., block and graft copolymers) and functional nanoparticles/colloids creates exciting new opportunities for novel structures, properties, and functions that substantially builds on the promise of important classes of materials such as nanocomposites, polymeric foams and cellular structures. This proposal seeks to understand the interactions of surface-modified nanoparticles and novel, interfacially active copolymers with polymeric interfaces and thin films, both in and out of equilibrium, and to apply this understanding to the development of innovative polymer-based materials.
The starting point for this work will be the unique availability of an atmospheric pressure plasma systems or plasma torch, from the pioneering work of Phillips at LANL which will be used for the synthesis of novel nano-metal particles, nano-oxide particles, graphite coated nano-metal particles, etc. The ability to create core-shell structures with well-defined surface chemistry and significantly enhanced stability to air-oxidation and high temperatures permits a wide range of functionalization reactions and surface treatments to be performed. Another key feature of this technology is facile preparation of synthetically useful amounts of material (1+ gram) which greatly enables the following physical property and application studies.
Initial work will concentrate on developing general strategies for the production of core-shell nanoparticles containing a metallic core with either a graphite or alumina shell. Phillips has recently shown that thin (5-10 nm) graphite shells can be formed on either Al or Ni nanoparticles (20-50 nm) and it is proposed to elaborate this technology to other metal nanoparticles of interest, specifically Au and Fe. Once synthetic strategies have been developed for the preparation of graphite coated Au and Fe nanoparticles structural optimization will be performed to elucidate the level of control over the shell thickness and core diameter that can be achieved while still maintaining low polydisperisty for the overall nanoparticle. Using the knowledge gained from the graphite functionalized materials, alumina coated nanoparticles will also be prepared based on Au and Fe cores. Both sets of Au core-shell nanoparticles will then be examined for the synthesis of hollow nanostructures by dissolution of the internal Au core through treatment with aqueous cyanide. The relationship between Au core removal and the thickness and chemical nature of the shell layer will be studied in detail.
Two different synthetic strategies will be examined for surface functionalization of the core-shell nanoparticles. For the alumina shell materials, the residual Al-OH groups will use for either the attachment of initiating groups for living free radical chains or the covalent attachment of preformed linear polymers. The second approach will involve the non-covalent surface functionalization of the nanoparticles using procedures developed by Hawker. In this case, the polar, H-bonding Al-OH groups will be used to form self-assembled nanostructures through non-covalent interactions with dendritic precursors, functionalized polymer chains, block copolymers, etc.
For the graphite coated nanoparticles, initial experiments will concentrate on determining the presence of surface chemistry, for example carboxylic acid or phenolic groups by IR and chemical methods. The surface chemistry of these particles are proposed to be analogous to either carbon black or carbon nanotubes and these known forms of nanostructured carbons will be used as reference materials. If the graphite coated nanoparticles are found to have low levels of surface functionality, in further parallel with carbon black/nanotubes, it is proposed to examine a range of techniques for attaching polymer chains to the surface. This includes treatment with oxidizing agents such as nitric acid to introduce carboxylic acid groups which can then be amidated/esterified with the corresponding amino- or hydroxyl functionalized living free radical initiator followed by growth of covalently attached polymer brushes by surface initiated polymerizations. Alternatively, block copolymers containing a pyrene-substituted anchoring block can be prepared and used to self-assemble at the graphitic surface through stacking which has been shown to be highly effective for the solubilization of carbon nanotubes. The overall goal of these experiments is to introduce polymer chains to the surface of the nanoparticles leading to increased solubility in solvents as well as polymer matrices.
Three main strategies will be examined for synthesis of the polymeric dispersing agents; surface initiated polymerization, polymer grafting and non-covalent self assembly using surface active polymers. The method of choice will be based on the nature of the surface chemistry, level of surface functionalization and final use of the nanoparticles. For surface initiated polymerization, it is proposed to examine functionalized dithioester living free radical initiators which will be prepared using a novel approach. Esterification of the surface active group with bromophenyl acetic acid gives the ester which can then be coupled with the anion derived from dithiobenzoic acid to afford the surface active dithioester initiator. Reaction of this with the available surface groups of the core-shell nanoparticles then gives a functionalized derivative which can be used to initiate the polymerization of a wide range of different vinyl monomers.
For polymer grafted systems, it is proposed to prepare a range of chain end functionalized macromolecules using living polymerization techniques and reactively blend these materials with the core-shell nanoparticles followed by separation and purification of the grafted nanoparticles. Finally, the recently discovered Click chemistry concept will be exploited by Hawker in the design of reactive homopolymers and block copolymers from simple, readily available starting materials. The critical features of Click chemistry are essentially quantitative yields, compatibility with functional groups and benign reaction conditions which are perfectly suited for the controlled attachment of 'binding' groups, such as pyrene (for graphite shells) and phosphonic acid (for alumina shells) to copolymeric systems. It is envisaged that these surface active copolymers will adsorb and self-assemble on the surface of the core-shell nanoparticles leading to dispersion.
It should be noted that these proposed targets are not exclusive and only serve to represent the wide spectrum of surface chemistries which will be accessed as well as the wide availability of different functional groups that will be incorporated for covalent and non-covalent bonding with the host, polymeric matrix. This diversity will allow the chemical nature of the shell of such nanostructures to be tuned independently of the core, the nature of which will be dictated by the specific applications envisaged for these materials.
The ability to fabricate core-shell nanoparticles with controlled surface functional allows for the use of these novel materials in a variety of different applications. These applications range from the stabilization of nanostructured foams and polymer composites to the preparation of water soluble materials for diagnosis and treatment of various diseases. To demonstrate the potential of these materials, two main areas will be targeted, dispersion of core-shell nanoparticles in solution, specifically aqueous solution and dispersion in polymer matrices.
The dispersion of nanoparticles based on Au or Fe cores and graphite or alumina shells requires the attachment of water soluble, biocompatible chains to the surface of the nanoparticles. Two strategies will be examined, one based on the direct attachment of monofunctional poly(ethylene glycol) derivatives to the surface of the core-shell nanoparticles via amidation, or Click chemistry. Alternatively, functionalized LFRP initiators will be attached to the surface functional groups of the nanoparticles with both alkoxyamine and RAFT based systems being examined in detail. Subsequently the homo-and random copolymerization of PEG-based macromonomers from the surface will be examined in detail. Once water soluble polymer chains are attached to the surface, the dispersibility of these materials in aqueous solution will be studied by dynamic light scattering, ultra-centrifugation and X-ray scattering in order to determine the relationship between nanoparticles size, surface layer and density/length of attached polymer chains. For the materials that are fully dispersed in aqueous solution, biodistribution studies will be undertaken in collaboration with researchers at Washington University in St. Louis. Following biodistribution studies it is proposed to examine the use of the Fe cored nanoparticles for MRI imaging applications and for magnetic separation of biological macromolecules while the Au cored nanoparticles will be studied as near-IR absorbers for tissue destruction and as SERS scaffolds for analysis of complex biological mixtures.
By varying the chemistry of the nanostructure surface through the introduction of low or high surface energy groups it is proposed to specifically direct the nanostructures to interfaces/surface or away from interfaces/surfaces. The ability to prepare random copolymer functionalized surfaces and patterned, Janus-like surfaces will allow these interactions to be systematically controlled and varied. In addition, the ability to prepare well-defined nanostructures of various sizes (10-50 nm) and shapes (spheres, rods, tubes) will allow a complete understanding to be developed.
Once this understanding is developed the application of these materials and strategies will be examined in two critical areas: highly efficient solar cells and high refractive index plastic lens. As an example, it is proposed that for polymer-based solar cells, metal nanostructures embedded in organic devices will beneficially improve light absorption, charge separation and charge collection processes. In addition, if located at the donor-acceptor interface, electrically isolated metal nanostructures will further boost this effect and the flexibility of the overall approach outlined in this proposal will allow tuning of the spectral properties of the solar cells through the appropriate choice of metal and nanostructure shape, size and location. In fact, Phillips at LANL has already had created metal supported on titania nanotube photocatalysts with unprecedented activity levels and the interfacial location of these structures will be controlled by the surrounding polymer shell, in this case a functionalized poly(thiophene) derivative. Similar strategies, though different materials can be used to make high refractive index, plastic lens which can withstand changes in temperature without changes in optical performance.
In a separate, though related application, the structure of polymer foams and oil-water emulsions are largely determined by the stability of thin films against rupture as the volume fraction of gas or other liquid component increases. It is proposed to take advantage of the synthetic ability to prepare functionalized nanostructures to develop new functionalized syntactic foams with unique or improved properties. Initial focus will be on the stabilization of sub-100 nm pores (initially polystyrene or PMMA) by the direction of nanostructures (graphite coated silica or titania) to the polymer air interface and the role of size, shape and chemical composition of the nanostructures on the stability of the syntactic foam examined in detail. Besides influencing structure, the presence of nanoparticles, e.g. carrying charge or of high polarizability, may also directly impact foam properties relevant for applications such as polymers electrets. Alternatively, the plethora of functional groups in the polymer shell surrounding the nanostructure can be exploited for a variety of applications such as high surface area sensors. It should be noted that while colloidal particles have been known to stabilize micron-sized foams the promise of nanoparticles for stabilizing the cell walls of foams is unknown. However recent work from Hawker has shown that 2-10nm nanoparticles stabilize and inhibit the dewetting of polymer films on substrates.
Finally once the ability to direct nanostructures to surfaces and interfaces has been developed and understood it is proposed to examine the reverse scenario, destabilization of interfaces. If this can be accomplished by depletion of nanostructures at interfaces significant impact may occur in the vitally important field of oil recovery which is today hampered by the formation of aggregates. Present strategies such as the use of polymers and viscoelastic surfactants cannot overcome this issue. We propose to exploit the knowledge gained in this program to better understand aggregation phenomena in oil fields and to devised nanostructure approaches to disrupting/dispersing aggregate formation.
