Session B10: Polymer Nanocomposites - Structure and Driven Assembly

Driven Assembly and Properties of Interfacial Polymer-Nanoparticle Composite Layers.

Specific associations between an oil-soluble polymer and a water-soluble nanoparticle can drive formation of a robust composite polymer-nanoparticle layer at the oil/water interface. Here, for a toluene/water interface, the polymer is oligomeric amine-terminated polystyrene and the nanoparticle is citrate-functionalized gold, with the nanoparticle size varied from 2 to 15 nm. At appropriate pH, the polymer amines and the nanoparticle carboxyls associate strongly by salt-bridging, efficiently segregating both components to the interface and reducing the interfacial tension dramatically. The size of the gold nanoparticles plays a significant role in the process, with smaller nanoparticles better coupling to the polymer, raising the extent to which the surface tension drops. By dilatational interfacial rheology, the composite layers show two relaxations, one by adsorption-desorption and one by polymer configurational rearrangement. For the smaller nanoparticles, the former relaxation disappears, and the interfacial layers are effectively solid-like whereas with the larger nanoparticles, the latter disappears, and the layers are effectively fluid-like. Interesting viscoelasticity is observed in-between.   

Magnetic Alignment of $\gamma $-Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$ Nanoparticles in Polymer Nanocomposites

Recent work in nanocomposites has been heavily focused on controlling the dispersion state of filler particles. The use of internal self-assembly based on matrix properties provides a limited solution to the desire for specified organizations. By introducing a magnetic field during the casting of a polymer solution it has been shown that particles can be oriented to form anisotropic structures -- commonly sought after for improved mechanical properties. Here, magnetic nanoparticles were cast in two different polymer matrices to study the effect of various forces that lead to this highly desired alignment. The addition of the magnetic field as an external trigger was shown to not necessarily force the clustering, but rather orient the agglomerates already available in solution. This demonstrates the importance of other dominant forces introduced into the system by characteristics of the polymers themselves. While this magnetic field provides a direction for the sample, the key forces lie in the interactions between the polymers and nanoparticles (as well as their solvent). The study shows a dependence of anisotropy on the particle loading, matrix, and casting time, from which continued work hopes to quantify the clustering necessary to optimize alignment in the composite.   

Nanoparticle order through soft patterned confinement

As has been proposed in colloidal science, visual order transitions can be achieved with entropy contributions alone. We compare ``athermal'' NP/polymer blends in which the NPs are densely grafted with a polymer brush of the same chemical composition as the polymer matrix with chemically repulsive interactive ones. Visual order of the NPs is induced by geometrically soft confining the thin film blends with topographic patterns. When the residual layer thickness of the patterned blend films approaches the nanoparticle dimension, exclusive segregation of NPs to less confining imprinted mesa region occurs, defined by partition coefficient K, the particle density ratio in confined residual layer to mesa region. The associated free energy change is calculated to explain NP segregation preference. Particle aggregation and anisotropy effects are examined.   

Tunable Multiscale Nanoparticle Ordering by Polymer Crystallization

Achieving the controlled, multiscale assembly of nanoparticles (NPs), which is known to underpin the unusual mechanical properties of some biomaterials, such as Nacre, remains a major challenge in nanoscience. Here we achieve such Nacre-inspired hierarchical NP ordering in a one-pot approach by leveraging the kinetics of polymer crystallization, which yields a multiscale structure spanning lamellae [(10 nm)], fibrils (m) and spherulites (mm). NPs are engulfed by the growing crystals, ordered into layers in the interlamellar zone [spacing of (10-100 nm)], or assembled into fractal objects at the interfibrillar scale, (1-10 m), with the relative NP populations in this hierarchy readily manipulated by crystallization speed. Adding NPs always increases the Young's modulus, but the effects of multiscale ordering are nearly an order of magnitude larger than those for a randomly mixed state. Since fracture toughness remains practically unaffected, this assembly strategy allows us to create high strength materials that retain the attractive high toughness and low density of polymers.   

Predictive Modeling and Design of Corona Driven Self-Assembly

Nanoparticle (NP) self-assembly that result in ordered arrangements can produce materials with interesting emergent properties. However, control of the assemblies is challenging as NPs can easily phase separate out of solution. Efforts to control dispersion have focused on surface modifications to program additional interactions to NPs. While these approaches have resulted in increased miscibility and morphological control, the emphasis has predominantly focused on spherical NPs, which have a limited range of accessible morphologies. One method to search for new morphologies is to utilize differently shaped cores. Here, we propose a theory that predicts corona conformation for polymers grafted to an anisotropic core. Our results indicate a preferential partitioning of chains of differing lengths to positions of maximum curvature, giving rise to NPs with different coronal conformations. By taking advantage of corona complementarity, we can optimize NPs to pack into ``lock-and-key'' configurations as well as design crystal motifs that can produce morphologies with long range ordering. These results strongly suggest that corona-amplified shape complementarity is a powerful handle for precise control of self-assembly.   

Influence of dielectric inhomogeneities in aggregation of charged nanoparticles in polymer solutions

We study the structural characteristics in a system of charged nanoparticles in a neutral polymer solution while accounting for the differences in the dielectric constant of the particle from the polymer and the solvent. We use a hybrid computational methodology which uses a combination of single chain in mean-field simulations and the solution of the Poisson-Boltzmann equation. In the absence of polymers, similarly charged macro-ions experience increased repulsion with increase in dielectric inhomogeneities, leading to less aggregation. In presence of polymers, similar effects manifest, but with the additional complexity of the polymer density profiles being influenced by their dielectric constant. We discuss the resulting radial distribution functions as a function of particle charge and polymer concentrations.   

An insight on hierarchical nanoparticle order by polymer crystallization

We investigate multiscale nanoparticle (NP) order via solidification in polymer nanocomposites tunable by a single parameter: crystallization speed G. NPs are fully dispersed for crystallization faster than a threshold Gc. However, for slower crystallization rates, G < Gc, anisotropic NP order arises. Given that polymer crystallization is inherently anisotropic, it can organize nanoparticles in different length scales (from sub-100nm to cm) determined by the crystalline superstructure morphology. Via molecular dynamics simulations on a geometric-kinetic model, we study how crystallization-driven NP assembly and threshold speed depends on surface tension as well as solvent and NP size. These studies agree with experimental observations and provide an insight on the key factors for achieving tunable multiscale NP order in these polymeric materials.   

Scaling and phase behavior of polymer-linked particles.

The entropy-driven depletion force is known to drive phase separation between polymers and modestly sized nanoparticles. Recent experiments have demonstrated that it is now possible to synthesize nanoparticles that are linked by a single polymer chain through hybridization of single stranded DNA grafts on disparate nanoparticles, and by crosslinking the two ends of a triblock copolymer, among other methods. We have used integral equation theory to study the phase behavior of polymer linked nanoparticles, and have found that a mesoscopically ordered phase emerges even with only hard core interactions, and modest size asymmetries between the polymer segment, and attached nanoparticles. This ordering occurs at melt-like densities, and is a result of depletion forces acting between polymers, and nanoparticles, which are now chemically linked to each other. The liquid-to-ordered phase transition density initially decreases with polymer length (in contrast to what is seen in mixtures of polymer and nanoparticles) and becomes independent of it at very long polymer lengths. The transition density is a nearly linear function of the ratio of the bead diameter to the polymer radius of gyration (Rg).~ Furthermore, it attains a unique value when this ratio is nearly one. The predicted ordering length scale is several times Rg.~ It rises linearly with Rg and then slowly plateaus.~ For all bead sizes, and polymer lengths studied, the ordering length scale follows a universal scaling behavior as a function of the nanoparticle diameter, and polymer density.   

Structure and dynamics of polymer-tethered colloids in dilute solutions

Photon correlation spectroscopy was employed in order to probe the form factor and the translational diffusion of a series of well-characterized polymer-tethered colloidal particles (particle brush) in dilute solution, as function of concentration, grafting density and polymer degree of polymerization (N) for constant core size. Analysis of form factor and diffusion coefficient revealed a pronounced dependence of the brush height on the grafting density of tethered chains. Hence, the thicknesses for seven different particle brush systems as functions of N do not superimpose on a single curve. An unexpected finding is the changing values of the apparent second virial coefficient under good solvency conditions, from negative for densely tethered particles to positive as the grafting density decreases. A tentative explanation calls for a change in the effective monomer-solvent interactions as the conformation of tethered chains is affected by their grafting density. This can be considered in a coarse-grained approach by means of a temperature-dependent potential developed originally for star polymers of varying functionality. The results give insight into the role of `brush architecture' in determining interactions in polymer-tethered particle systems that could ultimately benefit the processing and application of particle-brush based materials.   

Quantifying the brush structure and assembly of mixed brush nanoparticles in solution

The arrangement of nanoparticles in a polymer melt or solution is critical to the resulting material properties. A common strategy to control the distribution of nanoparticles is to graft polymer chains onto the surface of the nanoparticles. An emerging strategy to further control the arrangement of nanoparticles is to graft polymer chains of different types and/or different lengths onto the surface of the nanoparticle, though this considerably increases the parameter space needed to describe the system. Theoretical models that are capable of predicting the assembly of nanoparticles in a melt or solution are thus desirable to guide experiments. In this talk, I will describe a recently developed non-equilibrium method that is appealing in its ability to tractably account for fluctuations and that can directly relate to experiments. To showcase the utility of this method, I apply it to mixed brush grafted nanoparticles in solution where fluctuations are prominent. Specifically, I investigate the role of experimentally relevant parameters on the structure of the brush and the corresponding effects on the assembly of the nanoparticles in solution. These results can be directly linked to experiments to help narrow the relevant parameter space for optimizing these materials.   

Preparation and Structural Studies on Hybrid Core-Shell Nanoparticles Consisting of Silica Core and Conjugated Block Copolymer Shell Prepared by Surface-Initiated Polymerization

Controlled Kumada catalyst-transfer polymerization occurring by chain-growth mechanism was developed for the synthesis of conjugated polymers and block copolymers from the surface of inorganic substrates such as silica nanoparticles. Although synthesis of conjugated polymers via Kumada polymerization became an established method for solution polymerization, carrying out the same reaction in heterogeneous conditions to form monodisperse polymer chains still remains a challenge. We developed and described a simple and efficient approach to the preparation of surface-immobilized layer of catalytic Ni(II) initiator, and demonstrated using it to prepare polymers and block copolymers on silica nanoparticle. The structure of the resulting hybrid nanostructures was thoroughly studied using small-angle neutron and X-ray scattering, thermal analysis, and optical spectroscopy. The photoexcitation energy transfer processes in the conjugated polymer shell were studied via steady-state and time resolved transient absorption spectroscopy. This study uncovered important details of the energy transfer, which will be discussed in this presentation.   

Light Irradiation as Key to Shape and Function of Nano-Assemblies in Solution

Developing strategies to exploit solar energy become more and more important. Inspired by natural systems it is highly promising to self-assemble functional species into effective tailored supramolecular units. Here we report self-assembled polymer structures in solution, taking advantage of optical properties of hybrid structures and light responsiveness. A new type of photocatalytically active self-assembled polymer structure in aqueous solution consists of supramolecular nano-objects obtained from macroions and multivalent inorganic ``counterions'' such as nanoparticles or clusters. These can exhibit expressed selectivity or even allow catalytic reactions in solution that are not possible with the building blocks only. Further, polyelectrolyte-porphyrin nanoscale assemblies exhibit tunable optical properties including strong fluorescence and an up to 20-fold higher photocatalytic activity than without polymeric template. A different approach is to transfer light energy into mechanical energy. Here, light energy is converted into nanoscale shape changes. This route for the conversion of light is highly promising for applications in drug delivery, nanosensors and solar energy conversion.   

From Nano- to Meso-Scale Order in Block Copolymer Self-Assembly-Derived Gyroidal Mesoporous Niobium Nitride

Niobium nitride is of academic and technological interest in fields including electrochemical energy storage and conversion and low-temperature superconductivity. Mesostructured nitrides can be obtained via sol-gel synthesis routes to oxides followed by conversion to nitrides via reactive heat treatment. In many applications of niobium nitride, the high specific surface area and pore accessibility available from block copolymer and oxide nanoparticle self-assembly could significantly improve material performance. Furthermore, mesoscale crystallographic order derived from block copolymer self-assembly could allow exploration of emergent properties in mesostructured superconductors. We report the first synthesis of gyroidal NbN superconductors from gyroidal block copolymer self-assembly-derived Nb$_2$O$_5$. The resulting materials have a mesoscale lattice with the I4$_1$32 (alt. gyroid) structure and d spacings between 27 and 36 nm. The materials are superconducting with a Tc of about 8 K. We expect that block copolymer-inorganic hybrid co-assembly will be a scalable, tunable platform for exploration of the impacts of mesoscale order and porosity on superconducting properties, and will discuss recent efforts to vary the structure type and grain structure of the mesoscale lattice.