Session Q22: Focus Session: Directed Assembly of Hybrid Materials II

Role of Polymer-Graft Architecture on the Cohesive Interactions, Assembly and Thermo-Mechanical Properties of Particle Brush Materials

Recent progress in the area of surface-initiated controlled radical polymerization (SI-CRP) has enabled the synthesis of polymer-grafted particulates with precise control over the architecture of grafted chains. The resulting ``particle brush materials'' are of interest both from a fundamental as well as applied perspective because structural frustrations (that are associated with the tethering of chains to a curved surface) imply a sensitive dependence of the conformation of surface-grafted chains on the architecture of the particle brush. The opportunity to control chain conformation in hierarchically organized hybrid materials with precisely controlled microstructure renders particle brush materials intriguing building blocks for innovative material systems that could have a transformative impact on a range of ``soft material'' technologies. In the first part, this presentation will discuss experimental results that illustrate the role of polymer graft modification on the interaction between brush particles in solution and the solid state as well as the assembly characteristics of particle brushes in the solid state. The opportunities provided by ``merging'' of the physical properties of ordered particle superlattice structures with the mechanical properties and processibility of polymer materials will be demonstrated for the example of ``plastic colloidal crystal'' structures. In the second part, this presentation will showcase results of ongoing experimental studies that aim to harness the distinctively different property characteristics associated with ``stretched'' and ``relaxed'' polymer segments to facilitate novel property combinations in particle brush materials that are absent in binary particle/polymer blend systems.   

Ordering Nanoparticles with Polymer Brushes

Ordering nanoparticles into a desired super-structure is crucial for their technological applications. We use molecular dynamics simulations to study the assembly of nanoparticles with polymer brushes randomly grafted to a plane surface and with varying densities. In the starting state, the nanoparticles are mostly dispersed in the solvent that wets the polymer brush. After the solvent is evaporated, the nanoparticles either enter the brush or straddle the top of the brush, depending on the strength of the nanoparticle/brush interaction. In the case of engulfed nanoparticles, a 2-dimensional array is only formed when the brush density is finely tuned to accommodate just a single layer of nanoparticles. When the brush density is higher or lower than this optimal value, the packing of nanoparticles shows large fluctuations in space and its quality diminishes. In the case of weak nanoparticle/brush interactions, a hexagonal packing with almost no defects is always found as long as the brush density is higher than some critical value. We also report an interesting healing effect of nanoparticles weakly interacting with the brush that can make a low-density brush more uniform.   

Nonisotropic Assembly of Single-Component Hairy Nanoparticles

Solvent-free assemblies of hairy nanoparticles (HNPs) are providing avenues to avoid issues of mixing, agglomeration and limited inorganic content that plague traditional nanocomposites that are based on polymer-nanoparticle blending. We demonstrate that for a range of graft densities, depletion forces acting on high molecular weight poly(styrene) (120kDa) grafted to SiO$_{2}$ (r0 $=$ 8nm) lead to non-isotropic organization of the nanoparticle center of mass. The order within the neat HNP assembly (aHNP) and its elongational characteristics evolve as the architecture of the polymeric corona in solution transitions from concentrated (CBP) to semidilute (SDPB) polymer brush regimes. Specifically, local HNP packing adopts a non-isotropic arrangement at intermediate graft densities ($\sigma \quad =$ 0.01 -- 0.1 chains/nm$^{2})$ where the CPB-to-SDPB transition in solution is approximately r0. In concert, the neat HNP assembly responds to elongational deformation in a manner analogous to semi-crystalline elastomers. The correlation between the corona architecture of the HNP and the physical characteristics of the solvent free aHNP point toward a possible approach to tune mechanical, optical and electrical properties of single component hybrids in a manner analogous to block-copolymer mesoscale morphology.   

Study of Rigid Polymers Grafted on Silica Nanoparticle as a Function of Coverage using Molecular Dynamics Simulations

Nanoparticles (NPs) hybrids consisting of para dialkyl phenyleneethynylenes (PPEs) grafted to a silica NPs were studied in solution using molecular dynamic simulations. PPEs are rigid polymers whose conformation determines their degree of conjugation and assembly which in turn affects the electro-optical response of the NPs-polymer hybrids. Here we report the effect of coverage of PPE chains on their conformation, correlated with their interaction with the solvent. We have previously shown that at low coverage in good solvent, a star like hybrid is formed with the PPEs chained assuming a fully extended configuration in the corona, and associated with each other to form clusters with reduced solvent quality. Here we show that similar to the low concentration regime, increasing the coverage of PPEs in good solvents results in a homogenous corona with starched out PPE molecules. However, at higher coverage, clustering of chains becomes distinctive and their number increases with increase in the coverage of PPEs. We find that the clusters are temperature responsive and dissociate with increasing temperature. This control over clustering of the corona chains offers a mechanism to tune the electro-optical behavior of the hybrid as well as direct their assembly.   

Polymer Conformation and Topological Defects in Systems of Hairy and DNA hybridized Nanoparticles

Systems of hairy and DNA hybridized Nanoparticles are able to self-assemble into an array of superlattices. Understanding the role the polymer plays is critical to predicting the superlattice structure. In this talk, we use Molecular Dynamics to study hairy nanoparticles where the grafted polymer is modeled explicitly. We study self-assembly starting from a liquid and following the nucleation and growth of large nanoparticle superlattices (2000NP). We explore the role of polymer stretching as well as the geometric frustration of the polymer for both spherical and cubic nanoparticles. We also provide a characterization of the dynamics, including topological defects. Further, we will discuss the difficulties and methods for simulating large lattices in molecular dynamics.   

Polymer-Grafted Nanoparticles in Polymer Melts: Modeling using Combined SCFT-DFT Approach

Nanoparticles (silica, carbon black, etc.) are often used as fillers to improve physical (thermal conductivity, coefficient of thermal expansion) and mechanical (modulus, strength) properties of polymer materials. In many cases, however, lack of nanoparticle dispersion in the polymer limits the utility of a resulting nanocomposite material. To improve dispersion, one often grafts organic chains (``ligands'') onto the surface of the particles; if the ligands are chemically miscible with the matrix polymer, it helps the particles to disperse more uniformly. In recent years, many new morphologies (``wires'', ``sheets'', ``networks'', etc.) were observed in such nanocomposites. Here, we adapt our earlier formalism combining Self-Consistent Field Theory (SCFT) for polymers with Density Functional Theory (DFT) for the particles; the modified formalism explicitly incorporates the grafted chains into SCFT. We then perform several simulations to study the dependence of morphology on the length and density of grafted chains, as well as the nanoparticle loading. The results are in qualitative agreement with predictions of earlier theories in the limit of lower particle loadings, and predict new morphologies (``bundles of wires'') for the case of larger particle loadings. The method can be easily extended to more complex cases (for example, where the matrix and/or ligand itself is a blend or block copolymer).   

Self-Assembly of Supramolecular Composites under Cylindrical Confinement

Block copolymer (BCP) or BCP-based supramolecules are useful platforms to direct nanoparticle (NP) assemblies. However, the variety of NP assemblies is rather limited in comparison to those shown by DNA-guided approach. By subjecting supramolecular nanocomposites to 2-D cylindrical confinement afforded by anodic aluminum oxide membranes, a range of new NP assemblies such as stacked rings, and single and double helices can be readily obtained, as confirmed by TEM and TEM tomography. At low NP loadings (3 v{\%}), the nanostructure conforms to the supramolecule morphology. However, at higher NP loadings (6-9 v{\%}), the nanostructure deviates significantly from the morphology of supramolecular nanocomposites in bulk or in thin film, suggesting that frustrated NP packing, in addition to simple supramolecule templating, may play a significant role in the self-assembly process. The present studies demonstrate that 2-D confinement can be an effective means to tailor self-assembled NP structures and may open further opportunities to manipulate the macroscopic properties of NP assemblies.   

Polymer Structure and Dynamics in Polymer / Layered-Silicate Nanocomposites

Polymer/layered silicate nanocomposites are of particular interest among different nanohybrids because of their anticipated superior properties. Mixing polymers with layered inorganic materials can lead to three different types of structure, depending on the interactions between the constituents: phase separated, intercalated and exfoliated. Intercalated hybrids, where the polymer is confined within the inorganic galleries, can serve as model systems for the study of the static and dynamic properties of macromolecules in nano-confinement. We describe our recent efforts to elucidate the effects of severe confinement utilizing hydrophilic nanohybrids of PEO or hyperbranched polymers mixed with Na$^{+}$-MMT. Intercalated hybrids with mono-, bi- and tri-layers of chains are obtained for all compositions covering the complete range from pure polymer to pure clay. Severe confinement influences significantly the structure of the polymer: the PEO chains intercalated within the inorganic galleries as well as those in close proximity to the outside walls are purely amorphous; it is only when there is significant excess polymer outside the completely filled galleries that the bulk polymer crystallinity is abruptly recovered. In contrast, when the inorganic is incorporated as silica nanoparticles, the crystallinity varies smoothly with composition whereas a population with a lower melting temperature near the inorganic surfaces is observed under strong confinement. The dynamics of the polymers confined within the galleries is probed by quasi-elastic neutron scattering and dielectric spectroscopy. The very local dynamics of the confined chains show similarities with those in bulk, whereas the segmental dynamics depend very strongly on the polymer/inorganic interactions varying from much faster to much slower or even frozen dynamics as the strength of the interactions increases.   

Sulfonated Poly(styrene) Chains Grafted on Magnetic Nanoparticles

Iron oxide nanoparticles functionalized with poly(styrene) (PS) chains at various grafting densities and loadings present stable and ordered nanostructures for tuning the mechanical and conductive properties in polymer composites. Strings, spherical and anisotropic clusters and well-dispersed particles are achieved with PS-grafted Fe$_{3}$O$_{4}$ nanoparticles in PS matrices upon varying the system parameters. In this work, we report the effect of sulfonic group locations on the aggregation state of polymer-grafted nanoparticles. Structures formed by the random and diblock copolymers of PS-poly(styrene sulfonate) (PSS) grafted particles will be discussed with small-angle x-ray scattering (SAXS) measurements in solution and melts. The conformational changes in PS-grafted chains and ion-containing grafts will be also presented in small-angle neutron-scattering (SANS) results to understand the role of polymer on the assembly of particles at the low grafting density.   

Assembly of diblock copolymer grafted nanoparticles in a homopolymer blend matrix

Hybrid materials comprised of nanoscale fillers embedded in a polymer matrix, also terms polymer nanocomposites, are used in many applications, such as photovoltaics, photonics, automobile parts, where their macroscopic properties are governed by the nanocomposite morphology. The structure and composite morphology is controlled by the interactions of the nanoscale fillers and the polymer matrix. In this talk we show using molecular simulations that functionalization of the nanoparticle surface with AB diblock copolymer grafts is a way to tune the interactions between the grafted particle and the A and B homopolymer blend matrix. Specifically, our work demonstrates that by tailoring the copolymer composition and the copolymer grafting density one can tune the location of the copolymer grafted particles in the matrix, (e.g. within a domain versus interface of two domains). Additionally, in the case where the grafted particles locate themselves at the interface between the two domains, the interfacial tension is reduced below that possible with bare ungrafted particles at the interface.   

Additive-Driven Self-Assembly of Well Ordered Mesoporous Carbon/Iron Oxide Nanoparticle Composites for Supercapacitors

Supercapacitors have attracted significant attention as energy storage devices for applications to meet the requirements of fast charge and discharge, high power density, and long cycle life. Recent research efforts demonstrate that the metal oxide- mesoporous carbon nanocomposite materials are indeed a class of promising electrode materials for high performance supercapacitors. However several major drawbacks for metal oxide-carbon nanocomposite materials remain, such as relatively low loadings of the metal oxide, aggregation of nanoparticles, and the lack of an ordered mesoporous structure. Here we demonstrate that well ordered mesoporous carbon/iron oxide composites can be prepared through simple carbonization of blends of block copolymers serving as the source of carbon and a porogen, e.g., poly(t-butyl acrylate)-block-polyacrylonitrile (PtBA-b-PAN), and iron oxide nanoparticles (NPs). Strong interactions between phenol-functionalized iron oxide NPs and polyacrylonitrile result in a preferential dispersion of the nanoparticles within the PAN domains and leads to ordered nanostructured mesoporous carbon framework containing upto 30 wt% iron oxide nanoparticles after pyrolysis. The specific capacitance of composites with 30 wt.% Fe2O3 NPs reaches 235 F/g.