Using Time–Temperature Superposition for Determining Dielectric Loss in Functionalized Polyethylenes

Using Molecular Dynamics simulations, we probe the effect of various pendant polar groups on the dielectric loss of polyethylene copolymers. The dielectric loss was computed using the autocorrelation function of the total dipole moment of a completely relaxed PE–X sample. Since this calculation is computationally expensive (wall time ≥ 4200 h), we explore the use of the time–temperature superposition (tTS) principle to make it more tractable. An important point is that short time MD simulations do not allow the dipole autocorrelation function to decay completely to zero. However, we find that the tTS method performed well in determining dielectric losses in the system as long as these unrelaxed components are not included in the calculation. This methodology, which provides us with a significantly faster and reliable pathway for calculation of dielectric loss, allows us to identify the role of polar side groups on the behavior of nonpolar polymeric dielectrics

Molecular Simulations of Solute Transport in Polymer Melts

Polymer membranes are typically used to separate gas mixtures on the basis of molecular size differences (“sieving”). The gas purity is known to be inversely proportional to the membrane flux, and the slope of this plot in glassy polymers is empirically found to be determined by the sizes of the gas molecules being separated, λ = (<i>d</i>_<i>B</i>/<i>d</i>_<i>A</i>)² – 1. Despite potential mechanistic differences, the separation performance of rubbery polymers is often discussed in the same framework as their glassy counterparts. Here we perform molecular dynamics simulations of spherical solutes in coarse-grained high-density, high temperature polymer melts to gain a molecular understanding of their transport and separation behavior. We find that the diffusion coefficient follows an exponential law <i>D</i> ∼ <i>e</i>^–<i>ad</i>. Since this dependence results in λ = <i>d</i>_<i>B</i>/<i>d</i>_<i>A</i> – 1, these findings do not provide a direct understanding of the experimentally deduced slope of the Robeson plot

Role of Casting Solvent on Nanoparticle Dispersion in Polymer Nanocomposites

We investigate the influence of casting solvent on the final spatial dispersion of nanoparticles (NPs) in polymer nanocomposites (PNCs). We prepared nanocomposites of bare silica NPs and poly­(2-vinylpyridine) (P2VP) by casting from two different solventsmethyl ethyl ketone (MEK) and pyridinewhich are theta/good solvents, respectively, for both the polymer and the NPs. In MEK, we show that P2VP strongly adsorbs onto the silica surface to create a temporally stable bound polymer layer. The resulting “hairy” particles are sterically stabilized against agglomeration, and thus good NP dispersion in PNCs is always achieved, independent of P2VP molecular weight, concentration, or NP loading. On the contrary, in pyridine, P2VP does not adsorb on the silica NPs. The phase behavior in this case is thus governed by a subtle balance among electrostatic repulsion, polymer-induced depletion attraction, and the kinetic slowdown of diffusion-limited NP aggregation. While there is little remnant solvent in the dry PNC, and since these dispersion states are hardly altered on annealing, these results serve to emphasize the crucial role played by the casting solvent in the spatial dispersion state of NPs in a polymer matrix

Controlling the Thermomechanical Behavior of Nanoparticle/Polymer Films

We show that the mesoscale (∼200 nm) thermomechanical properties of polymer nanocomposites formed from silica nanoparticles (NPs) and poly(2-vinyl­pyridine) (P2VP) critically depend on their interfacial structure, which can be controlled by the casting solvent. The composite films are solvent cast from either pyridine (PYR) or methyl­ethyl­ketone (MEK), with uniform NP spatial distribution obtained in both cases. In the films cast from MEK, our previous work has shown that a bound layer of P2VP is formed at the NP surfaces, while no such bound layer is formed when PYR is used as the casting solvent. In PYR as-cast films, Brillouin light scattering reveals a single acoustic phonon with its longitudinal sound velocity increasing with NP loading. This implies a homogeneous mixture of the NP and the polymer on the mesoscopic scales for all compositions examined. However, in the MEK as-cast films, two longitudinal and two transverse acoustic phonons are observed at NP loadings above ∼20 wt % (or ∼11 vol %), reminiscent of two metastable microscopic phases. The dense microphase is attributed to the bridging of NPs by P2VP chains, whereas for the softer medium, we conjecture that there exists an interfacial lower density P2VP layer whose longitudinal sound velocity barely changes with NP loading. These solvent-induced differences in the (elastic) mechanical behavior disappear upon thermal annealing, suggesting that these nanocomposite interfacial structures in the as-cast state (far from equilibrium) locally approach equilibrium (<i>i.e.</i>, near equilibrium after annealing). Consistent with these conclusions, the abrupt decrease of the longitudinal sound velocity with temperature occurs at a single glass transition temperature for the annealed nanocomposites irrespective of the casting solvent used, which assumes only a slightly higher (∼5 K at 45 wt % or ∼29 vol %) value than that in bulk P2VP. The results emphasize the important role of solvent in determining the interfacial structure of nanocomposites, which can be used to tailor their thermomechanical behavior

Reducing Strain and Fracture of Electrophoretically Deposited CdSe Nanocrystal Films. II. Postdeposition Infusion of Monomers

Thick electrophoretically deposited (EPD) films of ligand-capped colloidal nanocrystals (NCs) typically crack when removed from the deposition solvent due to the loss of residual solvent. We report the suppression of fracture in several micrometers thick EPD films of CdSe NCs by treating the wet, as-deposited films with solutions of polymer precursor monomers, followed by UV-initiated polymerization. The monomers diffuse into voids and, for several monomers, dissolve the NCs to form a uniform dispersion in the film

Reducing Strain and Fracture of Electrophoretically Deposited CdSe Nanocrystal Films. I. Postdeposition Infusion of Capping Ligands

Thick electrophoretically deposited (EPD) films of ligand-capped colloidal nanocrystals that adhere to the substrate typically crack after they are removed from the deposition solvent due to the loss of residual solvent. We report the suppression of fracture in several micrometers thick EPD films of CdSe nanocrystals by treating the wet, as-deposited films with solutions containing the NC core-capping ligand, trioctylphosphine oxide (TOPO). The increase in TOPO ligand density increases photoluminescence of the dried film and leads to a decrease in elastic modulus

Directionally Interacting Spheres and Rods Form Ordered Phases

The structures formed by mixtures of dissimilarly shaped nanoscale objects can significantly enhance our ability to produce nanoscale architectures. However, understanding their formation is a complex problem due to the interplay of geometric effects (entropy) and energetic interactions at the nanoscale. Spheres and rods are perhaps the most basic geometrical shapes and serve as convenient models of such dissimilar objects. The ordered phases formed by each of these individual shapes have already been explored, however, when mixed, spheres and rods have demonstrated only limited structural organization to date. Here, we show using experiments and theory that the introduction of directional attractions between rod ends and isotropically interacting spherical nanoparticles (NPs) through DNA base pairing leads to the formation of ordered three-dimensional lattices. The spheres and rods arrange themselves in a complex alternating manner, where the spheres can form either a face-centered cubic (FCC) or hexagonal close-packed (HCP) lattice, or a disordered phase, as observed by <i>in situ</i> X-ray scattering. Increasing NP diameter at fixed rod length yields an initial transition from a disordered phase to the HCP crystal, energetically stabilized by rod-rod attraction across alternating crystal layers, as revealed by theory. In the limit of large NPs, the FCC structure is instead stabilized over the HCP by rod entropy. We, therefore, propose that directionally specific attractions in mixtures of anisotropic and isotropic objects offer insight into unexplored self-assembly behavior of noncomplementary shaped particles

Impact of the Distributions of Core Size and Grafting Density on the Self-Assembly of Polymer Grafted Nanoparticles

It is now well-accepted that hydrophilic nanoparticles (NPs) lightly grafted with polymer chains self-assemble into a variety of superstructures when placed in a hydrophobic homopolymer matrix or in a small molecule solvent. Currently, it is thought that a given NP sample should only assemble into one kind of superstructure depending on the relative balance between favorable NP core–core attractions and steric repulsion between grafted polymer chains. Surprisingly, we find that each sample shows the simultaneous formation of a variety of NP-assemblies, e.g., well-dispersed particles, strings, and aggregates. We show through the generalization of a simple geometric model that accounting for the distributions of the NP core size and the number of grafted chains on each NP (which is especially important at low coverages) allows us to quantitatively model the aggregate shape distribution. We conclude that, in contrast to molecular surfactants with well-defined chemistries, the self-assembly of these NP analogues is dominated by such fluctuation effects

Role of Filler Shape and Connectivity on the Viscoelastic Behavior in Polymer Nanocomposites

We compare the rheological behavior of three classes of polymer nanocomposites (PNCs) to understand the role of particle shape and interactions on mechanical reinforcement. The first two correspond to favorably interacting composites formed by mixing poly­(2-vinylpyridine) with either fumed silica nanoparticles (NPs) or colloidal spherical silica NPs. We show that fumed silica NPs readily form a percolated network at low NP volume fractions. We deduce that the NPs act as network junctions with the effectively irreversibly bound polymer chains serving as the connecting bridges. By comparing with colloidal spherical silica, which has a significantly higher percolation threshold, we conclude that the fractal shape of the fumed silica is responsible for its unusually low percolation threshold. The third system corresponds to polystyrene grafted colloidal silica nanoparticles (PGNPs) in a polystyrene matrix. These PNCs have an even lower percolation threshold probably because the grafted chains increase the effective volume fraction of the NPs. When we take these different thickness of the polymer layers in the two cases into account (i.e., grafted layer vs adsorbed layer thickness), the percolation threshold for the fumed and the grafted system occurs at similar effective loadings, but the NP network with fumed silica has a higher low-frequency plateau modulus than that formed with the PGNPs. These findings can be reconciled by the fact that the fumed silica NPs are composed of fused entities, thus ensuring that they have a higher modulus than the PGNPs where the modulus is largely attributed to interactions between the grafts. Our results systematically stress the important role of the nanofiller shape and connectivity on the mechanical reinforcement of PNCs

Melt State Reinforcement of Polyisoprene by Silica Nanoparticles Grafted with Polyisoprene

We systematically vary the nanoparticle (NP) dispersion state in composites formed by mixing polyisoprene homopolymers with polyisoprene grafted silica particles, and demonstrate how creep measurements allow us to overcome the limitations of small amplitude oscillatory shear (SAOS) experiments. This allows us to access nearly 13 orders in time in the mechanical response of the resulting composites. We find that a specific NP morphology, a percolating particle network achieved at intermediate graft densities, significantly reinforces the system and has a lower NP percolation loading threshold relative to other morphologies. These important effects of morphology only become apparent when we combine creep measurements with SAOS re-emphasizing the role of synergistically combining methods to access the mechanical properties of polymer nanocomposites over broad frequency ranges