Session Y44: Focus Session: Directed Assembly of Hybrid Materials - Particle Organization and Arrays

Simulations of nanoparticle ionic and organic hybrid materials

We have used molecular dynamics simulations over microsecond time scales to study the structure and dynamics of coarse-grained models for nanoparticle-based ionic and organic hybrid materials [1]. The systems of interest consist of particles with charged surface groups and linear or three-arm counterions, which also act as the solvent. A comparable uncharged model of nanoparticles with tethered chains is also studied. The pair correlation functions display a rich structure resulting from the packing of cores and chains, as well as electrostatic effects. Even though electrostatic interactions between oppositely charged ions at contact are much greater than the thermal energy, we find that chain dynamics at intermediate time scales are dominated by chain hopping between core particles. We have also used molecular dynamics simulations and density functional theory to investigate the structure of solvent-free oligomer-grafted nanoparticles [2,3]. At low temperatures and moderate to high oligomer lengths, the qualitative features of the core particle pair probability, structure factor, and the oligomer brush configuration obtained from the simulations can be explained by a density-functional theory that incorporates the configurational entropy of the space-filling oligomers. In particular, the structure factor at small wave numbers attains a value much smaller than the corresponding hard-sphere suspension, the first peak of the pair distribution function is enhanced due to entropic attractions among the particles, and the oligomer brush expands with decreasing particle volume fraction to fill the interstitial space. At higher temperatures, the simulations reveal effects that differ from the theory and are likely caused by steric repulsions of the expanded corona chains. \\[4pt] [1] B. Hong, A. Chremos, and A. Z. Panagiotopoulos; Faraday Disc., DOI: 10.1039/c1fd00076d (2012).\\[0pt] [2] A. Chremos, A. Z. Panagiotopoulos, H.-Y. Yu, and D. L. Koch; J. Chem. Phys., 135:114901, 12pp (2011).\\[0pt] [3] A. Chremos and A. Z. Panagiotopoulos; Phys. Rev. Lett., 107:105503, 5pp (2011).   

Reverse-engineering the anisotropic assembly of grafted nano-particels

We use computer simulations to study the self assembly and directed phase separation of nanoparticles grafted with polymeric chains in an implicit solvent. The formation of anisotropic clusters from building blocks that are symmetric (both in shape and interactions) has been the subject of recent studies. This anisotropy has roots in many body physics, which manifests itself as a directional assembly. Here we apply both full and coarse-grained simulations to get a better insight of the physics behind the assembly of such systems.   

Crystalline assembly of hard polyhedra via directional entropic forces

Entropic forces are effective forces that result from a system's statistical tendency to increase its entropy. Hard rods and disks spontaneously align and can assemble into layers and columns if those structures increase the configurational space available to the particles. Hard spheres, cubes and even tetrahedra order for the same reason. Here we extend those findings by showing that hard polyhedra can self-assemble into a variety of complex phases, most of them never before reported in systems of single-component hard particles. The role of shape and directional entropic forces in stabilizing these structures will be discussed. Our results suggest new possibilities for self-assembling complex target structures from colloidal building blocks. \\[4pt] [1] Damasceno, PF; Engel, M; Glotzer, SC. arXiv:1109.1323v1   

Facile Fabrication of 3-D Nanoparticle Arrays in Thin Films toward Photonic Materials

3-D hierarchical assemblies of nanoparticles in thin films enable one to exploit their collective properties to generate functional electronic, magnetic, and photonic materials. Among many templates, block copolymer-based supramolecules is one of the most promising candidates since they combines solution processiblity and high precision in nanoparticle spatial distribution. Herein, we report a facile way of fabricating 3-D nanoparticle arrays in thin films of diblock copolymer-based supramolecules exhibiting bandgap in the visible regime. The entropy-dominant assembly drives the formation of 3-D lattice of nanoparticle arrays with precise inter-array spacings in thin films. The coupling effect between the precisely positioned nanoparticle arrays with adjustable lattice spacings in the hierarchically-structured nanocomposites allows potentially interesting optical properties.   

Creating Opal Templated Continuous Conducting Polymer Films with Ultralow Percolation Thresholds Using Thermally Stable Nanoparticles

We propose a novel and robust strategy for creating continuous conducting polymer films with ultralow percolation thresholds using polymer-coated gold nanoparticles (Au NPs) as surfactant. Continuous poly(triphenylamine) (PTPA) films of high internal phase polymeric emulsions were fabricated using an assembly of crosslinked polystyrene (PS) colloidal particles as template. Polymer-coated Au NPs localize at the PS/PTPA interface and function as surfactant to efficiently produce a continuous conducting PTPA polymer film with very low percolation thresholds. The volume fraction threshold for percolation of the PTPA phase with insulating PS colloids was found to be 0.20. In contrast, with the addition of an extremely low volume fraction of surfactant Au NPs, the volume fraction threshold for percolation of the PTPA phase was dramatically reduced to 0.05. The SEM and TEM measurements clearly demonstrated the formation of a continuous PTPA phase within the polyhedral phase of PS colloids. To elucidate the influence of the nanoparticle surfactant on the blend films, the morphology and conductivity of the blends at different PS colloid/PTPA volume ratios were carefully characterized as a function of the Au NP concentration.   

Magnetic fields in long-range alignment of functional hybrid soft materials

We present a magnetic field-based method to impose long range order in self-assembled soft materials including polymer-nanowire composites, block copolymers and surfactant mesophases. We discuss the broad utility of this approach, indicating its advantages and limitations. Our method yields highly anisotropic materials with quality of alignment in many cases comparable to that of single crystals as assessed by X-ray scattering techniques. We take advantage of the high fidelity of alignment to systematically explore and characterize the anisotropic properties of these materials. We present a perspective for improving electron and hole transport, as well as exciton utilization in magnetically doped ZnO nanowire-polythiophene composites for photovoltaic applications by global alignment of the nanowires. For block copolymers, we focus on enhancing Li-ion transport in membranes with self-assembled cylindrical and lamellar morphology by alignment of the Li-conducting PEO domains.   

$^{2}$H NMR study of the Assembly of Gold Nanoparticles Dispersed in Nematic Liquid Crystal

Gold nanoparticles (AuNP) with mesogenic ligands, tailored to be highly miscible in the liquid phase of 4-n-pentyl-4'-cyanobiphenyl (5CB) liquid crystal (LC), form reversible, micron-scale cellular networks upon cooling to the nematic phase. The network topology and LC director field orientation are controlled by the cooling rate, film thickness, ligand shell composition and AuNP concentration. Isotopically labeled samples allow selective probing of the orientational order of the NP ligands and host LC by $^{2}$H NMR. The $^{2}$H NMR spectra of AuNPs dispersed in nematic 5CB display isotropic and doublet peaks assigned to disordered and field aligned ligands respectively. The intensity of the ligand doublet signal increases relative to the isotropic peak with decreasing temperature and NP concentration. Conversely an isotropic peak along with the expected doublet is observed for the host 5CB in the nematic phase. The intensity of the host 5CB isotropic peak, which persists far into the nematic phase and decreases with NP concentration, shows the coexistence of isotropic and nematic phases. The NP ligand and host LC orientational orders are related the formation of the cellular network and theoretical phase diagrams of NP-LC dispersions.   

Organization of Gold Nanorods in Cylinder-Forming Block Copolymer Films

The addition of gold nanorods (AuNRs) to copolymer films can impart unique optical and electrical properties. To take full advantage of this system, the AuNRs must be dispersed in a self-organizing copolymer that directs the orientation of the anisotropic particle. In the present work, AuNRs with aspect ratio 3.6 (8 nm x 29 nm) are grafted with poly(2-vinyl pyridine) (P2VP) brushes and dispersed in a cylindrical forming diblock copolymer of polystyrene-b-P2VP (180K-b-77K, 29.6 wt{\%} P2VP). Films are spun cast and solvent annealed in chloroform to produce a perpendicular cylindrical morphology at the surface. Using TEM and UV-ozone etching combined with AFM, the AuNRs are well dispersed and co-locate (top down view) with the P2VP cylinders, $\sim $50nm diameter. However, the AuNRs mainly lie parallel to the surface indicating that they likely locate at the junction created at the intersection between P2VP cylinders and P2VP brush layer adjacent to the silicon oxide surface. Self-consistent field calculations of the Au:PS-b-P2VP morphology as well as the effect of adding P2VP homopolymer to the nanocomposite will be discussed.   

Nanopatterned poly(ethylene glycol) brushes: A route for highly tunable assembly of Au nanoparticles

Assembly of metallic nanoparticles (NPs) on surfaces offers many interesting opportunities for scientific studies as macroscopic properties of the assemblies depend on organization of the particles at the nanometer length scale. On the other hand, new/improved functionalities of the assemblies are promising in the potential technological applications such as chemical sensing and metamaterials. Chemical patterning of surfaces for directed assembly of ex-situ synthesized NPs is advantageous due to controllable substrate-particle interaction and applicability to a range of different size and type of particles. Here we present assembly of Au NPs (15-50 nm) on lithographically patterned poly(ethylene glycol) (PEG) brushes. The density of NPs on the patterns can be controlled with the molecular weight of PEG brush and the size of particles. The ability to pattern PEG brushes with high chemical contrast and resolution (sub 50 nm) with the interesting particle brush interaction provides routes for highly controllable assembly of NPs. For example, number of particles per spot can be precisely controlled with high yields (single 95{\%} and dimer 70{\%} for 30 nm Au NP) and NPs of two different sizes (heterostructures) can be assembled on patterned spots.   

Distribution of POSS Nanoparticles in Symmetric Diblock Copolymer Thin Films

The distribution of Polyhedral Oligomeric Silsesquioxanes (POSS) nanoparticles) in thin films of symmetric diblock copolymer polystyrene-b-poly dimethyl siloxane (PS-$b$-PDMS) was studied by neutron reflectometry. Blending of nanoparticles (NPs) with polymers provide a pathway to tune properties of the polymer such as conductivity, mechanical strength, optical activity as well as dewetting, where the properties depend on the distribution of the NPs. Controlling the distribution of NPs in polymers thin films however, remains a challenge where entropic contributions drive segregation of the NPs to the interface. Different approaches were taken including modifying the interfacial interactions of the NP. The current study investigates effects of the relative size of the NP with respect to that of the polymer layers following distribution of POSS cages which consist of a silicon cage in a symmetric PS-PDMS. The temperature was varied from room temperature to 150$^{o}$C, investigating the distribution above and below the glass transition temperature of the styrene blocks. The distribution of the NPs as well as structural changes for two molecular weights of the diblock 10kg/mol and 50kg/mol within the films will be discussed.   

Long-range Ordering of Symmetric Block-copolymers by Chaining of Superparamagnetic Nanoparticles in External Magnetic Fields

Chaining of superparamagnetic nanoparticles is numerically investigated as a new method to align the block copolymer without additional lithographic processing. The method relies on the chaining of superparamagnetic nanoparticles, sequestered preferentially in one of the blocks, in the direction of in-plane external magnetic fields to achieve long-range order in the block copolymer. Effects of nanoparticle size, concentration, and magnetization strength are explored using Hybrid-Particle-Field technique (Sides et al, 2006). The 2D simulations reveal that, for the same nanoparticle loading, the nanoparticle sizes commensurate with the domain sizes yield defect-free alignment. While small sizes lead to jamming and kinetic trapping of defects, larger sizes lead to swelling of domains and break the symmetry of the lamellar phase. A window of optimal nanoparticle concentrations exists over which orientational order is achieved. For low concentrations, only local alignment is observed, while high concentrations lead to order-order phase transition from lamellae to cylindrical phase. Scaling calculations corroborate the effect of high magnetization strengths in lowering the equilibrium defect density for such nematic-isotropic phase transitions observed in monolayers.   

Highly Ordered Superstructures of Single-Walled Carbon Nanotubes in Polymeric Systems

Fabrication of highly ordered arrays of single-walled carbon nanotubes (SWNTs) has been of great interest for a wide range of potential applications. Block copolymers exhibit rich phase behavior and have been extensively used as excellent templates for highly ordered nanostructure materials with various architectures. Therefore, utilization of the rich phase behavior of block copolymers may provide a general and inexpensive way for fabricating a large variety of self-assembled and highly ordered arrays of SWNTs without going through complicated preparative procedures. Here, we investigated the cooperative self-assembling behavior of functionalized isolated SWNTs in Pluronic block copolymer systems using small angle neutron and x-ray scattering techniques, which show highly ordered superstructures of SWNTs with different symmetries.   

Block copolymer modified epoxies: Role of localized network damage

Adding block copolymers to epoxy resins has proven to be an effective approach to toughening these materials while retaining commercially relevant properties such as high modulus and glass transition temperature. When properly designed, block copolymers self-assemble into spherical micelles that disperse in the monomer resin. These structures survive curing into a dense network. At this point, a complete description of the toughening mechanism for block copolymer modified epoxies is still lacking. Here we present new experimental evidence that challenges the current understanding in this area. We compared the toughening effect of spherical micelle forming block copolymers with rubbery and glassy cores. Consistent with previous reports rubbery cores produced significant toughening, but surprisingly the glassy core micelles also imparted some toughness. These results suggest that the block copolymer/epoxy interface plays a significant role during deformation. We propose that the micelles compromise the integrity of the glassy matrix by damaging or plasticizing the crosslinked network in contact with the corona blocks, thereby facilitating shear yielding.