51 research outputs found
Multiple glass transitions in star polymer mixtures: Insights from theory and simulations
The glass transition in binary mixtures of star polymers is studied by mode
coupling theory and extensive molecular dynamics computer simulations. In
particular, we have explored vitrification in the parameter space of size
asymmetry and concentration of the small star polymers at
fixed concentration of the large ones. Depending on the choice of parameters,
three different glassy states are identified: a single glass of big polymers at
low and low , a double glass at high and low
, and a novel double glass at high and high which is
characterized by a strong localization of the small particles. At low
and high there is a competition between vitrification and phase
separation. Centered in the -plane, a liquid lake shows up
revealing reentrant glass formation. We compare the behavior of the dynamical
density correlators with the predictions of the theory and find remarkable
agreement between the two.Comment: 15 figures, to be published in Macromolecule
A spatiotemporal Data Envelopment Analysis (S-T DEA) approach:the need to assess evolving units
One of the major challenges in measuring efficiency in terms of resources and outcomes is the assessment of the evolution of units over time. Although Data Envelopment Analysis (DEA) has been applied for time series datasets, DEA models, by construction, form the reference set for inefficient units (lambda values) based on their distance from the efficient frontier, that is, in a spatial manner. However, when dealing with temporal datasets, the proximity in time between units should also be taken into account, since it reflects the structural resemblance among time periods of a unit that evolves. In this paper, we propose a two-stage spatiotemporal DEA approach, which captures both the spatial and temporal dimension through a multi-objective programming model. In the first stage, DEA is solved iteratively extracting for each unit only previous DMUs as peers in its reference set. In the second stage, the lambda values derived from the first stage are fed to a Multiobjective Mixed Integer Linear Programming model, which filters peers in the reference set based on weights assigned to the spatial and temporal dimension. The approach is demonstrated on a real-world example drawn from software development
Slow dynamics, aging, and crystallization of multiarm star glasses
Multiarm star polymers are model systems with tunable intermediate colloid to polymerlike character, exhibiting rich phase behavior, internal relaxations, and flow properties. An important puzzle for several years has been the lack of clear experimental proof of crystalline states despite strong theoretical predictions. We present unambiguous evidence via multispeckle dynamic light scattering (MSDLS) and small-angle neutron scattering (SANS) for such crystallization in a solvent of intermediate quality. An unexpected speed up of the short-time star diffusion observed in MSDLS was attributed by SANS to crystallization, via aging, of the multiam star glass. This delayed glass to crystal transition establishes a pathway for star crystallization that might be generic in colloidal glasses
Polymer-mediated melting in ultrasoft colloidal gels
Star polymers with a high number of arms, f = 263, become kinetically trapped when dispersed in an athermal solvent at concentrations above the overlapping one, forming physical gels. We show that the addition of linear chains at different concentrations and molecular weights reduces the modulus of the gel, eventually melting it. We explain this linear polymer-induced gel-liquid transition in terms of effective interactions and star depletion. In the limit of very high linear-chain molecular weight a "reentrant gelation" is detected and attributed to bridging flocculation, analogous to that observed in colloidal dispersions
Kinetic arrest of crowded soft spheres in solvents of varying quality
Crowded solutions of multiarm star polymers, representing model colloidal spheres with ultrasoft repulsive interactions, undergo a reversible gelation transition upon heating in solvents of intermediate quality (between good and Theta). This unusual phenomenon is due to the kinetic arrest of the swollen interpenetrating spheres at high temperatures, forming clusters, in analogy to the colloidal glass transition. In this work we demonstrate that the choice of the solvent has a dramatic effect on the gelation transition, because of the different degree of star swelling (at the same temperature) associated with the solvent quality. We construct a generic kinetic phase diagram for the gelation of different stars in different solvents (gelation temperature against effective volume fraction, phi) and propose a critical "soft sphere close packing" volume fraction phi(c) distinguishing the temperature-induced (for phiphi(c)) glass-like gelation. We conclude that appropriate selection of the solvent allows for manipulation of the sol-gel transition in such ultrasoft colloids
Suppressed twist in droplets of cholesteric rod-like virus as identified by single particle imaging
The director field configuration of a colloidal chiral nematic liquid crystal confined in droplets is studied in this work. We employ microfluidics to produce monodisperse droplets containing nematic dispersions of fd virus surrounded by a carrier oil phase, while we vary the size of the droplet as well as the concentration of the virus. The resulting director fields within the droplets are studied at the single-molecule level, using confocal microscopy. The 3-D structures are linked to polarisation microscopy observations of bright rings, which can be attributed to a cholesteric twist of the director field. We identify boundaries of concentration and size where one and two rings are observed, suggesting suppression of the cholesteric twist by confinement. Single particle confocal observations confirm that indeed the twist in the director field underlies the ring formation, but they also show that the twist is non-monotonous throughout the droplet and much smaller than observed in bulk
Depletion and cluster formation in soft colloid - polymer mixtures
The dynamics of colloidal star —linear homopolymer mixtures is
investigated by photon correlation spectroscopy. In dilute star solutions,
osmotic forces due to the added polymers lead initially to a shrinkage of
the stars and eventually, at higher polymer concentrations, to stable star
clusters. Furthermore, concentrated glassy star solutions melt upon addition
of small amounts of linear polymers, as manifested by the remarkable
speed-up of the star self-diffusion. Quantitative description of the
experimental findings is provided by calculations of the effective star-star
pair potential
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