Search results for "Chemical physics"

Critical adsorption of a single macromolecule in polymer brushes.

2014

The adsorption of long flexible macromolecules by polymer brush-coated surfaces is studied by molecular dynamics simulations and by calculations using density functional and self-consistent field theories. The case of repulsive interactions between the substrate surface and the monomers of both the brush polymers and the extra chains that can get absorbed into the brush is considered. Under good solvent conditions, critical absorption can occur, if the interaction between the monomers of the brush polymers and the extra chain is (weakly) attractive. It is shown that it is possible to map out the details of the critical absorption transition, if the chain length and/or the grafting density o…

Structure of Polymer-Grafted Nanoparticle Melts.

2020

The structure of neat melts of polymer-grafted nanoparticles (GNPs) is studied via coarse-grained molecular dynamics simulations. We systematically vary the degree of polymerization and grafting density at fixed nanoparticle (NP) radius and study in detail the shape and size of the GNP coronas. For sufficiently high grafting density, chain sections close to the NP core are extended and form a dry layer. Further away from the NP, there is an interpenetration layer, where the polymer coronas of neighboring GNPs overlap and the chain sections have almost unperturbed conformations. To better understand this partitioning, we develop a two-layer model, representing the grafted polymer around an N…

Positron Annihilation in Alkali Halides at Low Temperatures

1986

We report on low-temperature positron studies for pure, single crystals of the alkali halides KCl, NaCl and NaF. Strong temperature dependences are observed in the lifetime and angular correlation measurements in the temperature range 10–300 K. Delocalized para-positronium is observed at low temperatures in the three crystals. The broadening, with rising temperature, of the narrow peak in the angular correlation curve can be accounted for by the self-trapping model of positronium. The results are discussed in the light of this model, in terms of positronium localization at imperfections and in terms of positron interactions with imperfections created in the spur of the positron. The increas…

Structure Formation of Polymeric Building Blocks: Complex Polymer Architectures

2013

This chapter describes macromolecules with a complex structure, their defined aggregation in solution, their adsorption to surfaces, and their possible aggregation on surfaces. The term “complex structure” implies that the macromolecules show different, distinct structural elements or building blocks on a supra-atomic length scale. Key to understanding the complex structure of macromolecules, their aggregation, and adsorption to surfaces are intra- and intermolecular interactions such as van der Waals, electrostatic, π–π interactions, and hydrogen bonds.

Isostructural compartmentalized spin-crossover coordination polymers for gas confinement

2016

[EN] Here we present two FeII coordination polymers that possess discrete compartments suitable for CO2 physisorption despite the lack of permanent channels. The two crystalline materials, of general formula [Fe(btzbp)3](X)2 (X = ClO4 or BF4), present voids of ca. 250 Å3, which each can accommodate up to two CO2 molecules. The abrupt spin transition can be modified upon CO2 sorption, and different magnetic behaviour is observed depending on the number of molecules sorbed.

A Self-Assembled 2D Molecule-Based Magnet: The Honeycomb Layered Material {Co3Cl4(H2O)2[Co(Hbbiz)3]2}

2003

Research in the area of molecule-based magnets is rapidly expanding, owing, in part, to numerous breakthroughs in the past decade.[1] The preparation of solid-state architectures of varying dimensionalities from specifically tailored paramagnetic building blocks has proven to be very successful, and also clearly multidisciplinary. Materials science and supramolecular chemistry efforts have joined forces with classical organic and inorganic chemistry in the design of solid-state materials whose magnetic properties rival, and sometimes even exceed, those of classic inorganic solids.[2] The moleculebased strategy allows for the preparation of unusual materials that cannot otherwise be obtained…

Disentangling the Role of Chain Conformation on the Mechanics of Polymer Tethered Particle Materials

2019

[Image: see text] The linear elastic properties of isotropic materials of polymer tethered nanoparticles (NPs) are evaluated using noncontact Brillouin light spectroscopy. While the mechanical properties of dense brush materials follow predicted trends with NP composition, a surprising increase in elastic moduli is observed in the case of sparsely grafted particle systems at approximately equal NP filling ratio. Complementary molecular dynamics simulations reveal that the stiffening is caused by the coil-like conformations of the grafted chains, which lead to stronger polymer–polymer interactions compared to densely grafted NPs with short chains. Our results point to novel opportunities to …

Pearl-necklace structures of molecular brushes with rigid backbone under poor solvent conditions: A simulation study

2010

Bottle-brush polymers, where flexible side chains containing N=20 to 50 effective monomers are grafted to a rigid backbone, are studied by molecular dynamics simulations, varying the grafting density σ and the solvent quality. Whereas for poor solvents and large enough σ the molecular brush is a cylindrical object, homogeneous in axial direction, for intermediate values of σ an axially inhomogeneous structure of "pearl-necklace" type is formed. The "pearls," however, have a strongly nonspherical ellipsoidal shape, due to the fact that several side chains cluster together in one pearl, qualitatively consistent with predictions of Sheiko et al. [Eur. Phys. J. E 13, 125 (2004)] We analyze the …

Molecular Simulation of Polymer Melts and Blends: Methods, Phase Behavior, Interfaces, and Surfaces

2016

Polymer brushes under flow and in other out-of-equilibrium conditions

2011

Polymer brushes are formed from flexible linear macromolecules tethered at one chain end to a solid substrate, forming a dense polymeric layer of polymer chains which are more or less stretched in the direction perpendicular to the substrate surface. These systems find interest also due to numerous applications (colloid stabilization, improvement of lubrication properties when the surfaces are exposed to shear, protection of the surface against adsorption of nanoparticles or proteins, etc.), for which often the dynamic non-equilibrium response of these brushes to external perturbation is important. The present review summarizes recent computer simulation studies pertinent to these questions…