Nonmonotonic Composition Dependence of Viscosity upon Adding Single-Chain Nanoparticles to Entangled Polymers.

Well-characterized single-chain nanoparticles (SCNPs), synthesized from a linear polystyrene precursor through an intramolecular [4 + 4] thermal cycloaddition cross-linking reaction in dilute conditions, were added to entangled polystyrene melts at different concentrations. Starting from the pure linear melt, which is much more viscous than the melt of SCNPs, the zero-shear viscosity increased upon the addition of nanoparticles and reached a maximum before eventually dropping to the value of the SCNP melt. Molecular simulations reveal the origin of this unexpected behavior, which is the interplay of the very different compositional dependences of the dynamics of the two components.

Nanostructured Single-ion Polymer Blend Electrolytes Composed of Polyanionic Particles and Low Molecular Weight PEO.

The development of single-ion solid polymer electrolytes with high ion conductivity holds the key to the realization of safe, long-lasting, high-energy batteries. Here we introduce the use of core-shell nanostructured polyanionic particles, composed of polyanion asymmetric miktoarm stars with a large number of glassy polystyrene-based polyanion arms that complement longer poly(ethylene oxide), PEO, arms, as additives to low molecular weight, liquid PEO. Due to the proposed macromolecular design approach, the polyanion particles are well dispersed for wt % ≤ 55 that enables the formation of a nanostructured single-ion electrolyte with highly interconnected channels composed of liquid PEO that promotes fast ion transport.

Ionic Conductivity and Mechanical Reinforcement of Well-Dispersed Polymer Nanocomposite Electrolytes.

Nanoparticles are commonly added to polymer electrolytes to enhance both their mechanical and ion transport properties. Previous work reports significant increases in the ionic conductivity and Li-ion transference in nanocomposite electrolytes with inert, ceramic fillers. The mechanistic understanding of this property enhancement, however, assumes nanoparticle dispersion states─namely, well-dispersed or percolating aggregates─that are seldom quantified using small-angle scattering.

How Does the Number of Arms Affect the Properties of Mikto-Arm Stars in a Selective Oligomeric Matrix? Insights from Atomistic Simulations.

We present a simulation study of amphiphilic mikto-arm star copolymers in a selective polymer host. By means of atomistic molecular dynamics simulations, we examine the structural and dynamical properties of mikto-arm stars with varying number, , of poly(ethylene oxide) (PEO) and polystyrene (PS) arms, (PEO) (PS) in a 33% wt blend with an oligomeric PEO host (o-PEO). As the number of arms increases, the stars resemble more spherical particles with less separated PEO and PS intramolecular domains.

Spatio-temporal heterogeneities in nanosegregated single-molecule polymeric nanoparticles.

The study of the coupling between structural and dynamical heterogeneities in nanostructured systems is essential for the design of hybrid materials with the desired properties. Here, we use atomistic molecular dynamics simulations to closely examine the dynamical heterogeneities in nanostructured single-molecule nanoparticles consisting of mikto-arm star copolymers with poly(ethylene oxide), PEO, and polystyrene, PS, arms. The particles exhibit an internally nanostructured morphology, resembling either "Janus-like" or "patchy-like" morphology when the functionality of the stars varies.

Designing All-Polymer Nanostructured Solid Electrolytes: Advances and Prospects.

Multi-phase nanostructured polymer electrolytes, where the one phase conducts ions while the other imparts the desired mechanical properties, are currently the most promising candidates for solid-state electrolytes in high-density lithium metal batteries. In contrast to homogeneous polymer electrolytes, where ion transport is coupled with polymer segmental dynamics and any attempt to improve conductivity via faster polymer motions results in a decrease in stiffness, nanostructured materials efficiently decouple these two antagonistic parameters. Nevertheless, for reasons discussed herein the synthesis of a polymer electrolyte that simultaneously has a shear modulus of ' ≈ GPa and an ion conductivity of σ > 10 S/cm (in the case dual ion conductor) or of σ > 10 S/cm (in the case of single-ion conductor) remains a challenge.

Nanostructuring Single-Molecule Polymeric Nanoparticles via Macromolecular Architecture.

Heterogeneous polymer-based nanoparticles comprise a very promising family of materials for a broad range of applications. Here, we present a detailed study of structural heterogeneities in nanostructured single-molecule nanoparticles in various environments by means of atomistic molecular dynamics simulations. The nanoparticles consist of mikto-arm star copolymers with two types of chemically incompatible arms, namely poly(ethylene oxide) (PEO) and polystyrene (PS), (PS) ,(PEO) , where n is the number of arms.

Effect of macromolecular architecture on the self-assembly behavior of copolymers in a selective polymer host.

We present a detailed simulation study of the structural and dynamical behavior of star-shaped mikto-arm (polystyrene)8(poly(ethylene oxide))8, (PS)8(PEO)8, copolymers with eight arms of each type, versus that of a linear polystyrene-block-poly(ethylene oxide), PS-b-PEO, diblock, in a selective homopolymer host. Both copolymers are blended at the same weight fraction 33% with an oligomeric PEO host. We use atomistic molecular dynamics simulations to account for the molecular interactions present in the blends and to study quantitatively the dynamical and structural properties of these systems.

Free Surface Relaxations of Star-Shaped Polymer Films.

The surface relaxation dynamics of supported star-shaped polymer thin films are shown to be slower than the bulk, persisting up to temperatures at least 50 K above the bulk glass transition temperature T_{g}^{bulk}. This behavior, exhibited by star-shaped polystyrenes with functionality f=8 arms and molecular weights per arm M_{arm} View Article and Find Full Text PDF

Rheological State Diagrams for Rough Colloids in Shear Flow.

To assess the role of particle roughness in the rheological phenomena of concentrated colloidal suspensions, we develop model colloids with varying surface roughness length scales up to 10% of the particle radius. Increasing surface roughness shifts the onset of both shear thickening and dilatancy towards lower volume fractions and critical stresses. Experimental data are supported by computer simulations of spherical colloids with adjustable friction coefficients, demonstrating that a reduction in the onset stress of thickening and a sign change in the first normal stresses occur when friction competes with lubrication.

Glass transition of polymers in bulk, confined geometries, and near interfaces.

When cooled or pressurized, polymer melts exhibit a tremendous reduction in molecular mobility. If the process is performed at a constant rate, the structural relaxation time of the liquid eventually exceeds the time allowed for equilibration. This brings the system out of equilibrium, and the liquid is operationally defined as a glass-a solid lacking long-range order.

Elastic Mechanical Response of Thin Supported Star-Shaped Polymer Films.

We show evidence of thickness-dependent elastic mechanical moduli that are associated largely with the effects of architecture (topology) and the overall shape of the macromolecule. Atomic force microscopy (AFM) based nanoindentation experiments were performed on linear chain polystyrene (LPS) and star-shaped polystyrene (SPS) macromolecules of varying functionalities (number of arms, ) and molecular weights per arm . The out-of-plane elastic moduli () increased with decreasing film thickness, , for less than a threshold film thickness, .

Zinc oxide nanoparticle suspensions and layer-by-layer coatings inhibit staphylococcal growth.

Despite a decade of engineering and process improvements, bacterial infection remains the primary threat to implanted medical devices. Zinc oxide nanoparticles (ZnO-NPs) have demonstrated antimicrobial properties. Their microbial selectivity, stability, ease of production, and low cost make them attractive alternatives to silver NPs or antimicrobial peptides.

Structure and dynamical intra-molecular heterogeneity of star polymer melts above glass transition temperature.

Structural and dynamical properties of star melts have been investigated with molecular dynamics simulations of a bead-spring model. Star polymers are known to be heterogeneous, but a systematic simulation study of their properties in melt conditions near the glass transition temperature was lacking. To probe their properties, we have expanded from linear to star polymers the applicability of Dobkowski's chain-length dependence correlation function [Z.

The elastic mechanical response of supported thin polymer films.

Nanoindentation studies of the mechanical properties of sufficiently thin polymer films, supported by stiff substrates, indicate that the mechanical moduli are generally higher than those of the bulk. This enhancement of the effective modulus, in the thickness range of few hundred nanometers, is indicated to be associated with the propagation and impingement of the indentation tip induced stress field with the rigid underlying substrate; this is the so-called "substrate effect". This behavior has been rationalized completely in terms of the moduli and Poisson's ratios of the individual components, for the systems investigated thus far.

thin films of poly(isoprene-b-ethylene oxide) diblock copolymers on mica: an atomic force microscopy study.

The structural behavior of three amphiphilic semicrystalline poly(isoprene-b-ethylene oxide) block copolymers (PI-b-PEO) with different PEO volume fraction (f(PEO) = 0.32, 0.49, and 0.

Structural relaxations of thin polymer films.

Time-dependent changes of thermodynamic properties due to structural relaxations and physical aging occur in all glasses. We show that the physical aging of thin supported films of star-shaped macromolecules, with f arms of length N(arm), exhibits average aging dynamics that are sensitive to f and N(arm). Regions of the films in proximity to interfaces age at substantially different rates than the interior of the film; this is also true of linear chain systems.

Effect of thickness-dependent microstructure on the out-of-plane hole mobility in poly(3-hexylthiophene) films.

Regioregular poly(3-hexylthiophene) (RR-P3HT) is a widely used donor material for bulk heterojunction polymer solar cells. While much is known about the structure and properties of RR-P3HT films, important questions regarding hole mobilities in this material remain unresolved. Measurements of the out-of-plane hole mobilities, μ, of RR-P3HT films have been restricted to films in the thickness regime on the order of micrometers, beyond that generally used in solar cells, where the film thicknesses are typically 100 to 200 nm.

Physical Aging of Star-Shaped Macromolecules.

Time-dependent structural relaxations, physical aging, of films with thicknesses in the range 0.4 μm < < 2 μm of star-shaped polystyrene (SPS) macromolecules were investigated. Our studies reveal that the aging rates of star-shaped PS macromolecules are appreciably slower than their linear chain analogs.

Nanomechanical properties of phospholipid microbubbles.

This study uses atomic force microscopy (AFM) force-deformation (F-Δ) curves to investigate for the first time the Young's modulus of a phospholipid microbubble (MB) ultrasound contrast agent. The stiffness of the MBs was calculated from the gradient of the F-Δ curves, and the Young's modulus of the MB shell was calculated by employing two different mechanical models based on the Reissner and elastic membrane theories. We found that the relatively soft phospholipid-based MBs behave inherently differently to stiffer, polymer-based MBs [Glynos, E.

Elastic modulus of a polymer nanodroplet: theory and experiment.

We redevelop a theoretical model that, in conjunction with atomic force microscopy (AFM), can be used as a noninvasive method for determination of the elastic modulus of a polymer nanodroplet residing on a flat, rigid substrate. The model is a continuum theory that combines surface and elasticity theories for prediction of the droplet's elastic modulus, given experimental measurement of its adsorbed height. Utilization of AFM-measured heights for relevant droplets reported in the literature and from our own experiments illustrated the following: the significance of both surface and elasticity effects in determining a polymer droplet's spreading behavior; the extent of a continuum theory's validity as one approaches the nanoscale; and a droplet size effect on the elastic modulus.

Wetting of a multiarm star-shaped molecule.

The equilibrium contact angles and line tensions of macroscopic droplets, composed of star-shaped polystyrene (PS) macromolecules, on silicon oxide substrates, are shown to be smaller than their linear analogs, by up to approximately 1 and 2 orders of magnitude, respectively, depending on the size and functionality of the star-shaped molecule. A precursor layer, of lateral dimensions and of thicknesses on the order of nanometers, surrounds each droplet of low molecular weight linear PS chains. Droplets composed of star-shaped molecules possessing a sufficient number of arms, reside on a layer adsorbed to the substrate.

Role of molecular architecture on the vitrification of polymer thin films.

We show that thin film star-shaped macromolecules exhibit significant differences in their average vitrification behavior, in both magnitude and thickness dependence, from their linear analogs. This behavior is dictated by a combination of their functionality and arm length. Additionally, the glass transition temperature at the free surface of a star-shaped molecule film may be higher than that of the interior, in contrast to their linear analogs where the opposite is true.

Nanomechanics of biocompatible hollow thin-shell polymer microspheres.

The nanomechanical properties of biocompatible thin-shell hollow polymer microspheres with approximately constant ratio of shell thickness to microsphere diameter were measured by nanocompression tests in aqueous conditions. These microspheres encapsulate an inert gas and are used as ultrasound contrast agents by releasing free microbubbles in the presence of an ultrasound field as a result of free gas leakage from the shell. The tests were performed using an atomic force microscope (AFM) employing the force-distance curve technique.