Session X44: Focus Session: Directed Assembly of Hybrid Materials - Nanoparticles in Micelles

Multigeometry nanoparticles and higher-order assemblies from block copolymer blends via kinetic control and chemical modification

Multigeometry micellar structures, due to segregation of unlike hydrophobic domains trapped within the same micelle core, have been produced via self-assembly of block copolymer mixtures in tetrahydrofuran/water solution. The mixture is composed of two block copolymers with distinctive hydrophobic blocks but the same poly(acrylic acid) (PAA) hydrophilic block. By taking advantage of the complexation in the hydrophilic corona between the acid side chains of the PAA block and added organoamine molecules, unlike hydrophobic blocks can be trapped in the same micelle core. Locally, the unlike hydrophobic blocks can segregate into compartments and even express different interfacial curvatures, or geometries, within the same nanoparticle. Through controlled kinetic pathways, block copolymer design and mixing ratios, both micelle compartment size and shape can be controlled to generate sphere-sphere, sphere-cylinder, cylinder-cylinder, cylinder-bilayer, and bilayer-bilayer blended multigeometry nanoparticles. Furthermore, higher order assembly behavior of the micelles has been investigated by taking advantage of chemical modification on the hydrophilic PAA shells. New mixtures using functionalized-PAA-containing block copolymers produce nanoparticles with a compartmentalized surface. These patchy surfaces can be used as templates for asymmetric hybrid nanoparticles, but also as building blocks for hierarchical assembly of the nanoparticles to produce one-dimensional arrays or three dimensional networks.   

Polymer nano-particle hybrid micelles: Encapsulation of POSS into semi-fluorinated polymer micelles

Self-assembly of block copolymers in selective solvents was used to form a nanoparticle (NP)/polymer hybrid micelles. These micelles can be used as a cargo vehicle for other substances such as drug delivery, and as building blocks for polymer-nanocomposites with controlled NP distribution. Association of NPs into specific blocks of the copolymer depends on the compatibility between the NPs and the block as well as their preference to the solvent that micellization takes place. The current work introduces a small angle neutron scattering study of association of Polyhedral Oligomeric Silsesquioxane (POSS) NPs into micelles of a highly segregating random copolymer, Biphenyl Perfluorocyclobutane (BPh-PFCB), in toluene, which is a good solvent for BPh. Incompatibility between the blocks drives copolymer into micelles with PFCB in the core and BPh in swollen corona. Modification of NPs with polymer chains drives POSS cages into the micelle core and prevents the micelle dissociation at higher temperatures.   

Efficient Encapsulation of Gold Nanorods into Block Copolymer Micelles

Gold nanorods (GNRs) have the potential to be used as an imaging and/or hyperthermia agent for cancer theragnosis. Clinical applications of as-synthesized GNRs (i.e., CTAB-coated CNRs) are currently limited by their cytotoxicity and insufficient colloidal stability. With an aim to address these problems, we developed a self-assembly processing technique for encapsulation of GNRs in block copolymer (BCP) micelles. This technique uses simple steps of solvent exchange processes designed based on the known principles of block copolymer self-assembly. It will be demonstrated that BCP-encapsulated GNRs are stable against aggregation under physiological conditions and non-toxic to cells.   

Kinetic pathways to organized polymer/nanoparticle assemblies

Processes that allow for controlled access to kinetically trapped non-equilibrium states have the potential to significantly expand the range of structures and properties that may be achieved by self-assembly. We will describe several recent examples from our group wherein new types of polymer/nanoparticle assemblies are enabled by designed processing pathways. In the first case, we study the formation of amphiphilic polymer micelles through hydrodynamic instabilities of solvent/water interfaces induced during emulsion processing. We show that this route allows for efficient co-encapsulation of multiple types of hydrophobic nanoparticles within the micelle cores. Second, we consider the influence of nanoparticles on spinodal decomposition of a polymer blend and find that the inclusion of aggregating particles provides a route to kinetically stabilize co-continuous structures through particle gelation in one of the polymer phases. Finally, we show how the structures of hybrid nanoparticle/conjugated polymer nanowires can be tuned using solution-state crystallization.   

A coarse-grained molecular dynamics approach to shear-directed assembly of nanoparticle arrays in amphiphilic block copolymer solutions

Experiments by Pozzo and Walker (2007) demonstrated shear ordering of binary nanoparticle block copolymer micelle crystals and its potential in the development of new nanostructured materials. Specifically, they have shown that nanoparticles occupying interstitial sites within the micelle crystal affect a low-shear, long-range order and that the shear type and rate are important to the crystal structure. However, the connection between macroscopic variables and resulting crystal structure is yet to be understood. We present results which elucidate the underlying mechanisms governing shear-directed assembly of these binary nanocrystals. Our approach employs a coarse-grained molecular dynamics model (CGMD) that includes the hydrodynamics of the solvent and preserves dynamic effects by making no assumptions regarding the micelle structure. In fact, micelle self-assembly of the amphiphilic block copolymers occurs in-situ with the nanoparticles along with the shear ordering of the binary crystal. Tunability of the crystal structure is determined by varying shear rates/types, nanoparticle size/concentration, and block copolymer chemistry as simulation parameters. The simulation predicted results of shear-directed assembly of nanoparticles will be compared to the experimental results.   

Nanoparticle Polymer Suspensions: Structure and Dynamics

We present structure and rheology measurements for model nanoparticle suspensions, which comprise of silica nanoparticles, densely grafted with small polyethylene glycol (PEG) chains and suspended in PEG oligomers. Structure characterization, using electron microscopy and X-ray scattering, reveals well-dispersed nanoparticles leading to stable suspensions across a range of particle volume fractions. At the same time, X-ray photon correlation spectroscopy studies reveal arrested dynamics and extremely small nanoparticle diffusivities in the high volume fraction suspensions, consistent with the expectations for a soft glass. The liquid-soft glassy solid transition is found to occur at strikingly low core volume fractions and the glassy suspensions are found to exhibit a range of unique features including strong shear thinning, presence of a zero shear Newtonian plateau, strain accelerated relaxation and prominent stress overshoots in flow startup. Comparisons of our experimental findings with the SGR model are provided and on this basis, we propose our suspensions as model systems for understanding the jamming transition and other properties of soft glasses. Further, we elucidate the form of particle interactions and compare them with models for spherical polymer brushes.   

Pristine graphene dispersions and solution-cast composites

Graphene holds potential as strong, conductive fillers in polymer nanocomposites; however, difficulties in dispersion quality and interfacial strength between filler and matrix have been a persistent problem for graphene-based nanocomposites, particularly for pristine, unfunctionalized graphene. We utilize a triphenylene based molecule (C10) to stabilize pristine graphene in water with a high graphene/stabilizer ratio. C10 molecules pi-pi stack with the graphene surface and prevent reaggregation. This dispersion can be reversibly destabilized based on pH and is stable against heat and lyophilization. Solution cast poly (vinyl alcohol) (PVA) composites prepared from these dispersions have enhanced mechanical and electrical properties (percolation threshold: 0.26 vol {\%} graphene). Also, for the first time, pristine graphene/PVA dispersions are electrospun to form graphene/polymer composite nanofibers.   

Simulation of viscoelastic suspensions using regularized singularities

Particles interacting through viscoelastic fluids exhibit behavior that differs qualitatively from corresponding systems with Newtonian suspending fluids. Depending on the type of fluid involved, the polymer contribution to the stress can induce clustering or particle chaining, and impede efforts to form a homogeneous suspension. We are using regularized singularities in conjunction with the finite volume method to calculate velocity fields, stress fields, and particle displacements in viscoelastic suspensions. These singularities consist of Stokelets and stresslets, or regions of enhanced body forces and stresses, and provide a simple model of suspensions of weakly deformable, non-neutrally buoyant particles. Simulations of hundreds of two-dimensional particles in multimode viscoelastic fluids show behavior similar to what is seen experimentally, and provide insight into the physical causes of particle aggregation in complex fluids.   

Polymer mediated solution self-assembly of nanorods

Control over the self-assembly of nanorods in polymer composites enables the design of hybrid materials in which the anisotropic properties of the nanocrystals can be harnessed efficiently. Here, we demonstrate that a delicate balance between entropic and enthalpic interactions controls the self-assembly behavior of nanorods in solutions and can lead to the formation of ordered nanorod arrays. Small angle X-ray scattering techniques were used to elucidate the phase behavior of CdSe nanorods in polymer solutions and to identify the concentration space that allows for the formation of nanorod superlattices. Furthermore, this work demonstrates that enthalpic interactions have strong effects on the nanorod self-assembly, and that the presence of reversible thermal transitions can allow for the growth of large nanorod arrays. The solution self-assembly behavior discussed in this study allows for the fabrication of solution processable composite thin films with vertically aligned nanorods over large areas.   

Quantifying the Thermodynamic Interactions in Carborane Nanoparticle Solutions

The dissolution of nanoparticles, particularly those containing boron, is an important area of interest for polymer nanocomposite formation and material development (1-2). In this work, the solubility of four boron cage molecules are quantified in toluene, THF, and methyl ethyl ketone with static light scattering, refractometry, UV-Vis spectroscopy, and physical observations. UV-Vis spectroscopy provides a method to determine the concentration and solubility limits of the solutions tested. Using light scattering, the second virial coefficient, A2, was determined and used to calculate $\chi$, the solute-solvent interaction parameter. The Hildebrand solubility parameter, $\delta$, was then extracted from this data using the Hildebrand-Scatchard solution theory (3-4). A list of potential good solvents based on the extracted $\delta$ value is provided for each nanoparticle. Of the systems tested, 1,3-di-o-carboranylpropane was shown to be a thermodynamically stable in toluene, with a $\chi$ less than 0.5, a solubility limit of 2.47 mg/mL, and all solutions remaining clear with no visible particle settling.