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. The SCNPs become much slower than the linear chains as their concentration decreases because they are threaded by the linear chains, reaching a maximum viscosity which is higher than that of the linear chains at a fraction of about 20%. This behavior is akin to that of single-loop ring polymers when added to linear matrices. This finding provides insights into the design and use of SCNPs as effective entropic viscosity modifiers of polymers and contributes to the discussion of the physics of loopy structures.

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. Noticeably, while the ion conductivity remains fairly unaffected and close to 10-5 S/cm at room temperature with nanoparticle loading, the shear modulus monotonically increases by several order of magnitudes indicating a very strong decoupling between the antagonistic properties of mechanical modulus and ion conductivity.

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. In this work, we carefully control the inter-silica nanoparticle structure (where each NP has a diameter D = 14 nm) in a model polymer electrolyte system (PEO:LiTFSI). We find that hydrophobically modified silica NPs are stabilized against aggregation in an organic solvent by inter-NP electrostatic repulsion. Favorable NP surface chemistry and a strongly negative zeta potential promote compatibility with PEO and the resulting electrolyte. Upon prolonged thermal annealing, the nanocomposite electrolytes display structure factors with characteristic interparticle spacings determined by particle volume fraction. Thermal annealing and particle structuring yield significant increases in the storage modulus, G', at 90 °C for the PEO/NP mixtures. We measure the dielectric spectra and blocking-electrode (κb) conductivities from -100 to 100 °C, and the Li+ current fraction (ρLi+) in symmetric Li-metal cells at 90 °C. We find that nanoparticles monotonically decrease the bulk ionic conductivity of PEO:LiTFSI at a rate faster than Maxwell's prediction for transport in composite media, while ρLi+ does not significantly change as a function of particle loading. Thus, when nanoparticle dispersion is controlled in polymer electrolytes, Li+ conductivity monotonically, i.e., (κbρLi+), decreases but favorable mechanical properties are realized. These results imply that percolating aggregates of ceramic surfaces, as opposed to physically separated particles, probably are required to achieve increases in bulk, ionic conductivity.

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, n, of poly(ethylene oxide) (PEO) and polystyrene (PS) arms, (PEO) n (PS) n 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. As a result of their internal morphology and associated geometrical constraints, the mikto-arm stars self-assemble either into cylindrical-like objects or a percolated network with increasing n, within the o-PEO matrix. The segmental dynamics is mostly governed by the star architecture and the heterogeneous local environment, formed by the intra- and intermolecular nanosegregation. We discuss the role of each factor and compare the results with previously published studies on mikto-arm stars.

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. The differences in the local environment result in strong intramolecular dynamical heterogeneities. In the proximity of the star core, geometric constraints promote unfavorable PEO:PS contacts that lead to a behavior similar to dynamically asymmetric miscible polymer blends or disordered copolymers. In contrast, further away from the core, the nanosegregation induces segmental dynamics very similar to the one found in the homopolymer star analogues.

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 G' ≈ GPa and an ion conductivity of σ > 10-4 S/cm (in the case dual ion conductor) or of σ > 10-5 S/cm (in the case of single-ion conductor) remains a challenge. This review focuses on recent designing strategies for the synthesis of all-polymer nanostructured electrolytes, and protocols for introducing a single-ion character in such materials.

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) n,(PEO) n, where n is the number of arms. The immiscibility between the two components gives rise to intramolecularly nanostructured particles. The nanostructured objects resemble either "Janus-like" or "patchy-like" particles, depending on the number or the length of the arms (or both) as well as the interaction with the surrounding medium. The degree of intramolecular heterogeneity increases with increasing number of arms and with decreasing affinity of star components to the polymer host. We provide a detailed analysis of the internal structure of the star-shaped particles, focusing on the intramolecular packing and the spatial arrangement of the arms. The results of our study can be used to design heterogeneous, internally nanostructured particles with two phases of distinct static properties for challenging specific applications of next-generation materials.

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. The presence of the selective oligomeric PEO host leads to the formation of complex self-assembled structures. While cylindrical structures are formed in the case of linear diblock copolymers, mikto-arm star copolymers form percolated interconnected assemblies within the PEO host. The cylindrical objects formed by the linear diblock copolymers exhibit a higher degree of compactness and a weaker temperature dependence than the percolated network formed by their star-shaped analogues. The dynamics is governed primarily by the local structural heterogeneity, i.e., the environment around a segment, which is determined by the interaction between the different components, the macromolecular architecture of the copolymer as well as the associated geometrical constrains. Our data further stress the fact that the structural and dynamical properties in these blends may be controlled/tuned by the macromolecular architecture of the copolymer and/or by adjusting the temperature.

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}

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. In the quasi-Newtonian flow regime, roughness increases the effective packing fraction of colloids. As the shear stress increases and suspensions of rough colloids approach jamming, the first normal stresses switch signs and the critical force required to generate contacts is drastically reduced. This is likely a signature of the lubrication films giving way to roughness-induced tangential interactions that bring about load-bearing contacts in the compression axis of flow.

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. Despite almost 100 years of research on the (liquid/)glass transition, it is not yet clear which molecular mechanisms are responsible for the unique slow-down in molecular dynamics. In this review, we first introduce the reader to experimental methodologies, theories, and simulations of glassy polymer dynamics and vitrification. We then analyse the impact of connectivity, structure, and chain environment on molecular motion at the length scale of a few monomers, as well as how macromolecular architecture affects the glass transition of non-linear polymers. We then discuss a revised picture of nanoconfinement, going beyond a simple picture based on interfacial interactions and surface/volume ratio. Analysis of a large body of experimental evidence, results from molecular simulations, and predictions from theory supports, instead, a more complex framework where other parameters are relevant. We focus discussion specifically on local order, free volume, irreversible chain adsorption, the Debye-Waller factor of confined and confining media, chain rigidity, and the absolute value of the vitrification temperature. We end by highlighting the molecular origin of distributions in relaxation times and glass transition temperatures which exceed, by far, the size of a chain. Fast relaxation modes, almost universally present at the free surface between polymer and air, are also remarked upon. These modes relax at rates far larger than those characteristic of glassy dynamics in bulk. We speculate on how these may be a signature of unique relaxation processes occurring in confined or heterogeneous polymeric systems.

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. Here we sought to (1) determine the relative efficacy of ZnO-NPs on planktonic growth of medically relevant pathogens; (2) establish the role of bacterial surface chemistry on ZnO-NP effectiveness; (3) evaluate NP shape as a factor in the dose-response; and (4) evaluate layer-by-layer (LBL) ZnO-NP surface coatings on biofilm growth. ZnO-NPs inhibited bacterial growth in a shape-dependent manner not previously seen or predicted. Pyramid shaped particles were the most effective and contrary to previous work, larger particles were more effective than smaller particles. Differential susceptibility of pathogens may be related to their surface hydrophobicity. LBL ZnO-NO coatings reduced staphylococcal biofilm burden by >95%. From the Clinical Editor: The use of medical implants is widespread. However, bacterial colonization remains a major concern. In this article, the authors investigated the use of zinc oxide nanoparticles (ZnO-NPs) to prevent bacterial infection. They showed in their experiments that ZnO-NPs significantly inhibited bacterial growth. This work may present a new alternative in using ZnO-NPs in medical devices.

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, f) and molecular weights per arm Mwarm. The out-of-plane elastic moduli E(h) increased with decreasing film thickness, h, for h less than a threshold film thickness, hth. For SPS with f ≤ 64 and Mwarm > 9 kg/mol, the dependencies of E(h) on h were virtually identical for the linear chains. Notably, however, for SPS with f = 64 and Mwarm = 9 kg/mol (SPS-9k-64), the hth was over 50% larger than that of the other polymers. These observations are rationalized in terms of the structure of the polymer for high f and sufficiently small Mwarm and not in terms of the influence of interfacial interactions.

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. Dobkowski, Eur. Polym. J. 18, 563 (1982)]. The density and the isokinetic temperature, based on the canonical definition of the laboratory glass-transition, can be described well by the correlation function and a subtle behavior manifests as the architecture becomes more complex. For linear polymer chains and low functionality star polymers, we find that an increase of the arm length would result in an increase of the density and the isokinetic temperature, but high functionality star polymers have the opposite behavior. The effect between low and high functionalities is more pronounced for short arm lengths. Complementary results such as the specific volume and number of neighbors in contact provide further insights on the subtle relation between structure and dynamics. The findings would be valuable to polymer, colloidal, and nanocomposites fields for the design of materials in absence of solution with the desired properties.

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. Here we show that for thin supported polymer films, in general, information regarding the local chain stiffness and local vibrational constants of the polymers provides an appropriate rationalization of the overall mechanical response of polymers of differing chemical structures and polymer-substrate interactions. Our study should provide impetus for atomistic simulations that carefully account for the role of intermolecular interactions on the mechanical response of supported polymer thin films.

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 0.66), spin-coated on freshly cleaved mica surfaces from aqueous solutions, was investigated by atomic force microscopy. We focus on the dependence of the resulting thin film nanostructures on the molecular characteristics (f(PEO) and molecular weight) and the adsorbed amount. The nanostructures obtained immediately after spin-coating were robust and remained unchanged after annealing and/or aging. The PEO affinity for the highly hydrophilic mica and the tendency of the hydrophobic and low surface energy PI to dewet and be at the free interface caused the soft PI-b-PEO micelles to collapse leading to the formation of 2D dendritic networks over mica. We show that, for all three polymers, the dendritic monolayer thickness can be predicted by a model consisting of a PEO crystallized layer (directly on top of mica) of the same thickness in all cases and a PI brush layer on top. In thicker areas, polymer material self-assembled into conelike multilamellar bilayers on top of the monolayer and oriented parallel to the substrate for both symmetric and asymmetric diblock copolymers with the lowest f(PEO). We compare the lateral morphology of the films and discuss the thickness heterogeneity, which results from the coupling and competition of crystallization kinetics, phase separation, and wetting/dewetting phenomena highlighting the role of the two blocks to inhibit or enhance certain morphologies. We show that the deviation of the f(PEO) = 0.32 thin film from its bulk phase structure (cylinders in hexagonal lattice) continues for several lamellar bilayers away from the substrate. For the asymmetric PI-b-PEO polymer with the higher PEO volume fraction (f(PEO) = 0.66) and higher APT, laterally extensive stacks of flat-on lamellar crystallites formed on the surface demonstrating the crucial role of the PEO crystallization.

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. Studies of in-plane carrier mobilities have been conducted in thinner films, in the thickness range 100-200 nm. However, the in-plane and out-of-plane hole mobilities in RR-P3HT can be significantly different. We show here that the out-of-plane hole mobilities in neat RR-P3HT films increase by an order of magnitude, from 10(-4) cm(2)/V·s, for a 80 nm thick film, to a value of 10(-3) cm(2)/V·s for films thicker than 700 nm. Through a combination of morphological characterization and simulations, we show that the thickness dependent mobilities are not only associated with the differences between the average morphologies of thick films and thin films, but specifically associated with changes in the local morphology of films as a function of distance from the interfaces.

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. This behavior may be reconciled in terms of a universal picture that accounts only for changes in the local T(g) of the films.

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.; Koutsos, V.; McDicken, W. N.; Moran, C. M.; Pye, S. D.; Ross, J. A.; Sboros, V. Langmuir2009, 25 (13), 7514-7522] and that elastic membrane theory is the most appropriate of the models tested for evaluating the Young's modulus of the phospholipid shell, agreeing with values available for living cell membranes, supported lipid bilayers, and synthetic phospholipid vesicles. Furthermore, we show that AFM F-Δ curves in combination with a suitable mechanical model can assess the shell properties of phospholipid MBs. The "effective" Young's modulus of the whole bubble was also calculated by analysis using Hertz theory. This analysis yielded values which are in agreement with results from studies which used Hertz theory to analyze similar systems such as cells.

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.