Session Q44: Focus Session: Interparticle Interactions in Polymer Nanocomposites - Transport and Dynamics

Multiscale structure, interfacial cohesion, adsorbed layers, miscibility and properties in dense polymer-particle mixtures

A major goal in polymer nanocomposite research is to understand and predict how the chemical and physical nature of individual polymers and nanoparticles, and thermodynamic state (temperature, composition, solvent dilution, filler loading), determine bulk assembly, miscibility and properties. Microscopic PRISM theory provides a route to this goal for equilibrium disordered mixtures. A major prediction is that by manipulating the net polymer-particle interfacial attraction, miscibility is realizable via the formation of thin thermodynamically stable adsorbed layers, which, however, are destroyed by entropic depletion and bridging attraction effects if interface cohesion is too weak or strong, respectively. This and related issues are quantitatively explored for miscible mixtures of hydrocarbon polymers, silica nanospheres, and solvent using x-ray scattering, neutron scattering and rheology. Under melt conditions, quantitative agreement between theory and silica scattering experiments is achieved under both steric stabilization and weak depletion conditions. Using contrast matching neutron scattering to characterize the collective structure factors of polymers, particles and their interface, the existence and size of adsorbed polymer layers, and their consequences on microstructure, is determined. Failure of the incompressible RPA, accuracy of PRISM theory, the nm thickness of adsorbed layers, and qualitative sensitivity of the bulk modulus to interfacial cohesion and particle size are demonstrated for concentrated PEO-silica-ethanol nanocomposites. Temperature-dependent complexity is discovered when water is the solvent, and nonequilibrium effects emerge for adsorbing entangled polymers that strongly impact structure. By varying polymer chemistry, the effect of polymer-particle attraction on the intrinsic viscosity is explored with striking non-classical effects observed. This work was performed in collaboration with S.Y.Kim, L.M.Hall, C.Zukoski and B.Anderson.   

Organically Modified Nanoclay-Reinforced Rigid Polyurethane Films

The nanodispersion of vermiculite in polyurethanes was investigated to produce organoclay-reinforced rigid gas barrier films. Reducing gas transport can improve the insulation performance of closed cell polyurethane foam. In a previous study, the dispersion of vermiculite in polyurethanes without organic modification was not sufficient due to the non-uniform dispersion morphology. When vermiculite was modified by cation exchange with long-chain quaternary ammonium cations, the dispersion in methylene diphenyl diisocyanate (MDI) was significantly improved. Dispersion was improved by combining high intensity dispersive mixing with efficient distributive mixing. Polymerization conditions were also optimized in order to provide a high state of nanodispersion in the polyurethane nanocomposite. The dispersions were characterized using rheological, microscopic and scattering/diffraction techniques. The final nanocomposites showed enhancement of mechanical properties and reduction in permeability to carbon dioxide at low clay concentration (around 2 wt percent).   

Shear viscosity of polymer nanocomposites from NEMD simulations

Polymer nanocomposites (PNC) are preferred for a variety of applications since they offer excellent thermal, electrical, and mechanical properties. The viscosity of PNC is primarily a strong function of filler size. For micron-sized fillers, the shear viscosity increases with filler volume fraction following the well-known Einstein relationship. However as the particle size approaches the nanoscale, the viscosity is found to either increase or decrease depending on the strength of interactions between the particle and polymer. In this study, the shear viscosity of an entangled polymer melt of N=400 beads of diameter $\sigma $ with and without fillers is estimated using NEMD simulations. The diameter of the nanoparticles (1-10$\sigma )$ and volume fraction (0.05-0.3) are varied for shear rate from 10$^{-2}$ to 10$^{-6 }\tau ^{-1}$. The viscosity of PNC decreases when compared to pure melt for nanoparticles of size 1$\sigma $ and recovers to the value of a pure melt when the size approaches 10$\sigma $, provided all the interactions are neutral. It thus appears that the increase in viscosity embodied in the Einstein relationship only manifests itself for large nanoparticle size $>$10$\sigma $.   

Towards the Next Generation of Polymer Nanocomposites: More Rigid, Lighter Weight, Yet Easier to Process

Polymer nanocomposites are attractive for their lightness in weight and excellent mechanical reinforcement. Current nanocomposites incorporate high aspect ratios fillers such as clays, carbon nanotubes (CNT), and graphenes. Exfoliation of clay, for example, is essential to obtain maximum mechanical reinforcement, yet this results in a 2 to 4 order of magnitude melt viscosity increase. This high viscosity leads to a reduction in the production rate and/or increased processing cost, which is undesirable. In order to obtain reduced melt viscosity without sacrificing the advantages of current nanocomposites, we prepared poly(styrene) (PS) nanocomposites by inclusion of various spherical nanoparticles using a technique that ensures good dispersion. Tensile tests demonstrated enhanced tensile modulus in the glassy state while a reduced melt viscosity was observed. The results show that nanoparticle geometry is extremely important and nanoscale effects can lead to the next generation of polymer nanocomposites.   

A Comparative Study of Interfacial Slip in Polymer Blends with Nanoparticles and Diblock Copolymer Compatibilizers

The interfacial region in polymer blends has been identified as a low viscosity region in which considerable slip can occur when the blend is subjected to shear forces. Here we use Molecular Dynamics simulations to establish and compare the roles that added nanoparticle fillers and diblock copolymers play in modifying the interfacial rheology. By choosing conditions under which the fillers and diblocks are localized, either in the two phases or at the interface, we can look at the interplay between their strengthening capabilities and the change in the interfacial slip behavior. We examine particle size, attraction between the particle and the polymer component, and the amount of filler in the material and compared this to systems including diblock copolymers at the same volume fraction. Our studies are performed, for a variety of shear values, both above and below the point at which the filler particles form a transient network in the blend.   

The Effect of Elongational Flow on the Placement and Orientation of Nanorods in Polymer: Modeling and Experiments

Nanorods are often incorporated into a polymer to enhance its functionality. Gaining control over the placement of nanorods in polymer is important to improve the desired nanocomposite material property and for application like solar cell. First, we used coarse-grained molecular dynamics (CGMD) simulation to quantitatively examine the effect of elongational flow on the placement of model nanorods in homopolymer matrix. We have investigated how flow strength, concentration, interaction, and aspect ratio of nanorods affect its placement in homopolymer. As an analogous experiment, we have electrospun polyvinyl alcohol (PVA) in water with Au nanorods. The experimental result showed dispersion and alignment of Au nanorods, as predicted by the simulation. We also demonstrated selective placement of nanorods along the outer layer of fiber by co-axially electrospinning PVA/Au nanorods and pure PVA as shell and core, respectively. The material properties of the PVA/Au nanocomposite fiber are tested to show its potential application such as electromagnetic interference (EMI) shielding. The good agreements between simulation and experiment suggest that CGMD simulation can be used as a predictive tool for controlling nanorod placement in a polymer under extensional flow.   

Directed Self-Assembly of Nanoparticles via Flexible-Blade Flow Coating

We present a facile, non-lithographic, one-step method termed flexible-blade flow coating to direct the assembly of quantum dots. This versatile technique exploits the phenomenon known as the ``coffee ring effect'' coupled with confined convective flow and controlled stick-slip motion to fabricate ribbons and fabrics with a broad range of length scales of nearly any material. We achieve nanostripe dimensions of width below 300 nm, thickness of a single nanoparticle ($\sim$8 nm), and continuous length exceeding 5 cm. This multi-length scale control is facilitated by the use of a flexible blade, which allows capillary forces to self-regulate the uniformity of convective flow processes. We exploit solvent mixture dynamics and nanoparticle chemistry to enhance intra-assembly particle packing, leading to novel assembly properties including conductivity and free-standing mechanical flexibility and strength. This simple technique and the use of novel materials open up a new paradigm for integration of nanoscale patterns over large areas for various applications.   

Properties of Macroscopic Nanoparticle Assemblies Fabricated by Flexible Blade Flow Coating

Nanoparticle assemblies have gained much interest for their potential in electronic, photonic, optical, chemical, and biological applications. Although one of the greatest challenges is the controlled positioning of nanoscale components into the desired multi-length scale structures, understanding the properties of nanoparticle assemblies has also remained elusive. We have developed a technique termed flexible blade flow coating to direct the assembly of nanoparticles into ribbons and fabrics with a broad range of length scales. Ribbons and fabrics constructed with photoreactive quantum dots are crosslinked by UV irradiation affording long, flexible and robust structures that allow for subsequent liftoff from the substrate by dissolution of a sacrificial underlayer. The structural integrity of freely floating fabrics and extremely high aspect ratio ribbons are observed through fluorescence microscopy. Physical properties of these assemblies are explored with varying dimensions and ligand chemistry. We find that nanoparticle ribbons and fabrics possess unique properties in comparison to continuous polymer thin films, such as spherical wrapping and two-dimensional flexibility.   

Reversible Shear-Flow-Induced Polymer and Colloid Aggregates

Using hydrodynamic simulations and a coarse-grained interaction model, we show that self-associating polymer and colloid mixtures can form reversible flow-induced aggregates in shear flow. We find that when increasing shear rates, the mixtures go through four distinct conformations from no aggregation to dense aggregates. The different conformations are verified by analyzing their radial distribution functions, g(r), as well as by visual inspection. Furthermore, we find that the formation of the aggregates is reversible. That is, the shear-induced aggregates disappear when we decrease the shear rates, and reappear when we increase the shear rates again.   

Impact of Carbon Nanoparticle Shape on Polymer Dynamics in Nanocomposites

In forming quality nanocomposites of carbon-based nanoparticles (CNPs) in a polymer matrix, achieving and maintaining a high degree of dispersion is a crucial problem. One method to attain well-dispersed CNP nanocomposites is to incorporate non-covalent interactions between the CNP and matrix, which also impacts the dynamics of the polymer chain. In this work, T2 NMR relaxometry (T2 NMR) examines the effect on polymer chain dynamics of incorporating CNPs into polystyrene-co-acrylonitrile (SAN) random copolymer matrices. In SAN-CNP composites, the segmental-chain dynamics can be influenced by the non-covalent interactions formed with the nanoparticle (interacting), but are also influenced by the CNP steric bulk alone (non-interacting). The use of T2 NMR allows for the examination of the influence of the extent of non-covalent interactions on this segmental chain level. This segmental level view also allows for the distinction between relaxation dynamics of the interacting and non-interacting regimes. Current data indicates that increased acrylonitrile content in the copolymer results in increased non-covalent interactions and overall slowing of chain dynamics.   

Modifying Fragility and Length Scales of Polymer Glass Formation with Nanoparticles

We investigate the effects of nanoparticles on glass formation in a model polymer melt by molecular dynamics simulations. The addition of nanoparticles allows us to change the glass transition temperature $T_g$, the fragility of glass formation, and both static and dynamical length scales in a controlled fashion. We contrast the length scales of static density changes with the length scale over which nanoparticles perturb the dynamics, as well as the length scale of cooperative string-like motion. Using the Adam-Gibbs approach, we show how the changes of fragility can be interpreted as a measure of the scale of cooperative string-like motion. We contrast the behavior along isobaric and isochoric paths to $T_g$, and find that changes along an isobaric path (most relevant experimentally) are much smaller than those along an isochoric path.   

Glass transition temperature of polymer nano-composites with polymer and filler interactions

We systematically studied versatile coarse-grained model (bead spring model) to describe filled polymer nano-composites for coarse-grained (Kremer-Grest model) molecular dynamics simulations. This model consists of long polymers, crosslink, and fillers. We used the hollow structure as the filler to describe rigid spherical fillers with small computing costs. Our filler model consists of surface particles of icosahedra fullerene structure C320 and a repulsive force from the center of the filler is applied to the surface particles in order to make a sphere and rigid. The filler's diameter is 12 times of beads of the polymers. As the first test of our model, we study temperature dependence of volumes of periodic boundary conditions under constant pressures through NPT constant Andersen algorithm. It is found that Glass transition temperature (Tg) decrease with increasing filler's volume fraction for the case of repulsive interaction between polymer and fillers and Tg weakly increase for attractive interaction.   

Payne effect in model nanocomposite the role of polymer confinement

Payne effect is a non-linearity observed in polymer nanocomposites at small strain amplitudes, abiove the bulk glass transition temperature. The origin is the non-linear mechanical response of the polymer located near the solid fillers. However the exact nature of the mechanical response is still the object of debate. We have developed since ten years model nano-composite systems consisting in monodisperse spherical particles dispersed in an elastomer matrix. Thanks to NMR and Neutrons scattering, we have been able to determine precisely the amount of polymer confined between pairs of particles and with a modified dynamics. From that we have deduced that the Payne effect clearly originates in two mechanisms: i) around each particle, a glass transition temperature gradient ii) around this first layer, a modified topology of the polymer --originating itself in the glassiness of the polymer very near the particles. Hence, we are able to build a master curve for the Payne effect amplitude versus the number of particles connected to their neighbors by these two layers, that gathers measurements at various temperatures, volume fractions and frequencies.