Our overarching goal is to guide the organization of synthetic nanoscale building blocks to create 3-D hierarchical materials with novel properties. Specifically, Thrust 1 focuses on two central research areas: (1) Nanoparticle Gels and (2) Polymer Nanocomposites, which are closely integrated through common intellectual links and the highly collaborative nature of our interdisciplinary research team. A mutual goal is to employ self- and/or directed assembly to control the organization of diverse nanoscale building blocks in the solution, melt, gel, rubbery or glassy state. Understanding and exploiting both enthalpic and entropic forces of assembly is a common and pervasive theme in the two core research areas.
The leading questions that guide our research activities include:
- What are the fundamental structural and thermodynamic parameters that control the slow dynamics and rheological properties of concentrated nanoparticle gels, glasses and polymer nanocomposites?
- What are the effects of nanoparticle size, shape, volume fraction and interparticle forces on the structure, rheology and physical aging of nanostructured gels and glasses and polymer nanocomposites?
- How does the polymer-filler interface influence the self-assembly, structure, and thermal, mechanical and transport properties, of nanocomposites?
- How are polymer dynamics affected by the presence of many highly curved interfaces?
- How do nanoparticles diffuse, aggregate and form networks in dense polymer matrices?
During the last eight years, we have synthesized organic and inorganic nanoscale building blocks with controlled size, composition, and surface functionality, studied the viscoelasticity, phase behavior, and structure of model nanoparticle-polymer mixtures, created 3-D hierarchical structures via direct-write assembly of nanoparticle inks, and synthesized, assembled, characterized, and modeled the behavior of polymer nanocomposites. Some recent results:
Synthesis of Functional Nanoparticles and Colloids
We (Shim) have synthesized novel anisotropic nanocrystalline heterostructures (bringing two or more components together with the goal of developing an entirely new material) with controlled surface chemistry to allow programmable assembly of inorganic structures. We have determined which parameters favor anisotropic growth and have been able to quantify how the number of heterojunctions formed varies with concentration and the size of the seed nanocrystals. We have discovered strain-induced limitations in the number of CdS particles that can nucleate per Fe3O4 seed nanocrystal and we have been able to enhance spatial anisotropy in rods-on-dot heterostructures by increasing the growth rate. These results are summarized in the flowing figure.
Synthesis, Structure and Assembly of Brush-Coated Fillers in Homopolymer Matrices
Polymer nanocomposites in the melt, rubbery or glassy states have many applications that combine novel electrical, optical and/or mechanical responses. Our multi-faceted efforts include the synthesis of nanoscale building blocks with controlled surface functionalization, and fundamental experimental and theoretical studies of the structure, dispersion, directed assembly, and dynamical and optical properties of model and practical systems in the bulk. Complementary experimental and simulation studies of polymer dynamics at model flat interfaces and confined between two surfaces, and nanoparticle diffusion in confined polymer melts, are also performed. We implemented RAFT polymerization techniques that allow us to precisely control the key molecular parameters that regulate the particle-matrix interactions, as well as the placement of functionality (Benicewicz, Schadler, Kumar). Recently, we have combined the RAFT technique with new developments in “click” chemistry to create novel grafted polymer brushes on nanoparticle surfaces and we discovered that spherical inorganic nanoparticles isotropically grafted with polymeric homopolymer “brushes” can self-assemble into anisotropic structures.
Novel Applications: Holographic Assembly of Inorganic and Organic Nanostructures
The directed assembly of inorganic and organic nanosystems into higher order functional structures remains a significant challenge in nanoscience and engineering. Previously, we (Braun and co-workers) demonstrated that nanoparticles can be assembled into predefined regions via the holographic exposure of a mixture of nanoparticles, monomer, liquid crystal (optional) and photoinitiator. In the regions of constructive interference the monomer polymerizes first; during this polymerization, the nanoparticles are generally sequestered into the regions of destructive interference. It was hypothesized that nanoparticle segregation occurs only if their Stokes-Einstein diffusion constant is sufficient to enable nanoparticle transport within the polymerizing matrix before polymerization locks the nanoparticles in place.
In order to investigate this idea, 12, 25, and 50 nm silica nanoparticles were synthesized and assembled via holographic exposure under similar conditions. As expected, given their greater diffusion constant, the smaller nanoparticles were significantly sequestered, while the large particles exhibited very little movement. By replacing the nanoparticles with a monomer with an orthogonal reactivity to the photocurable monomer, we have demonstrated that a second monomer can be sequestered into nanodroplets, which can be polymerized upon demand at a later time as required for the desired application (e.g., films with definable elastic properties, self-healing coatings, dynamic optical coatings). Specifically, acrylate and isocyanate monomers have been used. The acrylate polymerizes via a free radical process, while the isocyanate polymerizes via a cationic polymerization, and the cross reactivity of the monomers is zero. The monomer mixture is placed in a laser interference pattern, which drives the polymerization of the acrylate in the regions of constructive interference and sequestration of the isocyanate into the regions of destructive interference. A TEM micrograph of alternating lines of isocyanate and acrylate can be seen in the next figure. The isocyanate has been polymerized through the addition of a catalyst and a heat treatment prior to microtoming the sample.
While the index of refraction of the acrylate and isocyanate polymers are the same, the indices of the isocyanate monomer and acrylate polymer differ. As a result, holographically defined coatings reflect a certain wavelength when the isocyanate remains in monomeric form, but become transparent when isocyanate is polymerized. The color of the coating can be tuned by changing the indices of the monomers or by changing the periodicity of the monomer within the polymer. The transmission spectrum of a typical sample is shown on the right side of the following figure. Notice the transmission notch (reflectance peak) in the sample with unpolymerized isocyanate; after a heat treatment of the same sample, the isocyanate polymerizes and the reflectance peak disappears. This holographic dual-cure nanocomposite is currently under investigation for both self-healing and optically responsive applications.
Novel Applications: Polymer Nanocomposites for Restoring Electrical Functionality
To overcome the cycle life and safety issues that plague lithium-ion battery technology, new approaches are needed that can stabilize the electrode-electrolyte interface and restore electrodes degraded by microcracks formed from charge-discharge recycling. Toward this goal, we (Moore and Braun) are developing a new type of polymer nanocomposite, in which carbon nanotubes (CNTs) are suspended in organic solvents encapsulated within polymer-based microcapsules. Shells that erode under conditions of high electrical potential, temperature spikes, mechanical damage or other appropriate stimuli can release and deliver conductive components, where they are needed, thus restoring current in damaged electrical conductors. The migration of CNTs in an organic solvent driven by an external electrical field has been previously reported, suggesting that triggered release of CNTs from microcapsules suspensions, even at small CNT weight fraction, could indeed provide an autonomous mechanism of self-repair of electronic functionality.
Specifically, we (Moore and Braun) have encapsulated single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs) suspended in chlorobenzene (PhCl) and ethyl phenylacetate (EPA) into microcapsules using an in situ emulsification polymerization of urea-formaldehyde. The capsules were characterized using optical microscopy, TGA, and SEM to determine size and thermal stability. The resulting microcapsules had an average diameter of 300 μm. Microcapsules containing suspensions of SWNTs in EPA at various weight fractions ranging from 0.025-0.1 wt% CNTs were prepared by this method. The release of CNTs from these capsules was monitored when crushed onto a silicon wafer mounted on top of a carbon tape-coated stage, which was observed by SEM. When the electrical conductivity of the released core material from the capsules was evaluated, the electrical resistance decreased with increasing concentration of CNTs in the initially encapsulated suspension.
Our results are being transferred to industry for pre-commercial trials (see Thrust 3).