Polymer nanocomposites (PNCs), prepared by incorporating nanoparticles within a polymer host, generally exhibit properties that differ significantly from those of the host, even with small amounts of ...nanoparticles. A significant challenge is how to tailor the properties of these materials for applications (structural and biomedical to optoelectronic), because PNCs derive their properties from a collective and complex range of entropic and enthalpic interactions. Here, we show that PNCs, prepared from athermal mixtures of polymer-chain-grafted gold nanoparticles and unentangled polymer chains, may exhibit increases or decreases in their relaxation dynamics, and viscosity, by over an order of magnitude through control of nanoparticle concentration, nanoparticle size, grafting density and grafting chain degree of polymerization. In addition, we show how the glass transition may also be tailored by up to 10 degrees with the addition of less than 1.0 wt% nanoparticles to the polymer host.
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.
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.
A cross-sectional study.
The aim of this study was to evaluate the quality of online scoliosis information available on the video sharing site YouTube.
The Internet is an increasingly utilized ...resource for accessing information about a variety of heath conditions. YouTube is a video sharing platform used to both seek and distribute information.
A search for "scoliosis" was carried out using YouTube's search engine and data were collected on the first 50 videos returned. A JAMA score to determine currency, authorship, source and disclosure, and scoliosis-specific score that measures the amount of information on the diagnosis and treatment options (as devised by Mathur et al in 2005; scored 0-32) was recorded for each video to measure quality objectively. In addition, the number of views, number of comments, and feedback positivity was documented for each. Data analysis was conducted using R 3.1.4/R Studio 0.98 with control for the age of each video in analysis models.
The average number of views per video was 71,152 with an average length of 7 minutes 32 seconds. Thirty-six percent of the videos fell under the authorship category of personal experience. The average JAMA score was 1.32/4 and average scoliosis specific score was 5.38/32. There was a positive correlation between JAMA score and number of views (P = 0.003). However, in contrast, there was a negative correlation between scoliosis-specific score and number of views (P = 0.01).
Online health information has historically been poor and our study shows that in an environment like YouTube that lacks a peer review process, the quality of scoliosis information is low. Further work is needed to determine whether accessing information on YouTube can play a role in patient care other than simple education pertaining to the disease and its management.
3.
To devise a reliable strategy for achieving specific HOMO and LUMO energy level modulation via alternating donor‐acceptor monomer units, we investigate a series of conjugated polymers (CPs) in which ...the electron withdrawing power of the acceptor group is varied, while maintaining the same donor group and the same conjugated chain conformation. Through experiment and DFT calculations, good correlation is identified between the withdrawing strength of the acceptor group, the HOMO and LUMO levels, and the degree of orbital localization, which allows reliable design principles for CPs. Increasing the acceptor strength results in an enhanced charge transfer upon combination with a donor monomer and a more pronounced decrease of the LUMO level. Moreover, while HOMO states remain delocalized along the polymer chain, LUMO states are strongly localized at specific bonds within the acceptor group. The degree of LUMO localization increases with increasing polymer length, which results in a further drop of the LUMO level and converges to its final value when the number of repeat units reaches the characteristic conjugation length. Based on these insights we designed PBT8PT, which exhibits 6.78% power conversion efficiency after device optimization via the additive assisted annealing, demonstrating the effectiveness of our predictive design approach.
A strong acceptor lowers both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of conjugated polymers (CPs), ultimately producing a narrowed band‐gap. The energy level difference between the CP and the constituent monomers converge to a constant value, providing an energy level prediction tool. Organic photovoltaic performance is correlated with the CP's energy levels, and a 6.78% power conversion efficiency is achieved.
The segmental relaxation times τA and τB of the A and B components, respectively, of an A/B polymer/polymer blend typically exhibit dissimilar temperature dependences and can differ by orders of ...magnitude, thereby manifesting the influence of spatial compositional heterogeneity. We show that for weakly miscible A/B blends the relaxations of the faster A component occur via separate and distinct mechanisms. In the melt state, τA increases in a nonlinear manner as temperature T decreases toward blend glass transition temperature T g (blend) (or toward the local effective glass transition temperature of its component T g (A)); this is the typical α relaxation process. For temperatures below the transition, 1/τA exhibits an Arrhenius temperature dependence; this is identified as the α′ process. A third relaxation process, a so-called α0 process, also occurs in the melt state; it is slower than the α process and exhibits a significantly stronger dependence on temperature. Each relaxation process, characterized by a different dependence on temperature, occurs via a different mechanism and associated local composition. This behavior, the existence of the α0 and α′ relaxations that accompany the α relaxations, would occur in miscible and weakly miscible blends whose component T gs differ significantly.
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.
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}<M_{e} (M_{e} is the entanglement molecular weight), is shown by molecular dynamics simulations to be associated with a preferential localization of these macromolecules at the free surface. This new phenomenon is in notable contrast to that of linear-chain polymer thin film systems, where the surface relaxations are enhanced in relation to the bulk; this enhancement persists only for a limited temperature range above the bulk T_{g}^{bulk}. Evidence of the slow surface dynamics, compared to the bulk, for temperatures well above T_{g} and at length and time scales not associated with the glass transition has not previously been reported for polymers.
Dodecanethiol-stabilized gold nanoparticles (5 nm diameter) are shown to self-organize to form a two-dimensional hexagonal structure in poly(methyl methacrylate) films upon spin-casting from solution ...onto a substrate, using high-angle annular dark-field scanning transmission electron microscopy. Through use of the distribution functions describing particle distributions, we show that the particle coarsening dynamics is self-similar, characterized by two distinct growth stages. During the initial stage, coarsening occurs via simultaneous Ostwald ripening and coalescence mechanisms, whereas during the second stage, the dominant coarsening mechanism is coalescence.
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