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Publications 2020

Bottom-up Construction of Dynamic Density Functional Theories for Inhomogeneous Polymer Systems from Microscopic Simulations
Sriteja Mantha, Shuanhu Qi, Friederike Schmid
Macromolecules 53 (9), 3409-3423 (2020);
doi:10.1021/acs.macromol.0c00130

We propose and compare different strategies to constructdynamic density functional theories (DDFTs) for inhomogeneouspolymer systems close to equilibrium from microscopic simulationtrajectories. We focus on the systematic construction of the mobilitycoefficient,Λ(r,r′), which relates the thermodynamic driving force onmonomers at positionr′to the motion of monomers at positionr.Afirstapproach based on the Green−Kubo formalism turns out to beimpractical because of a severe plateau problem. Instead, we propose toextract the mobility coefficient from an effective characteristic relaxationtime of the single chain dynamic structure factor. To test our approach, we study the kinetics of ordering and disordering in diblockcopolymer melts. The DDFT results are in very good agreement with the data from correspondingfine-grained simulations

Using Copolymers to Design Tunable Stimuli-Reponsive Brushes
Shuanhu Qi, Leonid I. Klushin, Alexander M. Skvortsov, Friederike Schmid
Macromolecules 53 (13), 5326-5336 (2020);
doi:10.1021/acs.macromol.0c00674

Recently, a new design for switch sensors has been proposed that exploits a conformational transition of end-grafted minority adsorption-active homopolymers in a monodisperse polymer brush [Klushin et al. Phys. Rev. Lett.2014, 113, 068303]. The transition is sharp and first-order type if the minority chain is longer than the brush chains. However, the intrinsic nature of the system imposes a constraint on the relation between the sharpness of the transition and the height of the free energy barrier controlling the transition kinetics: The sharper the transition, the slower the transition time. Here we demonstrate that adopting diblock copolymers with the adsorption-active block anchored at the substrate as the minority chains allows a much more flexible control of the three main characteristics of the transition, i.e., the transition point, its sharpness, and the barrier height. In particular, the barrier height can be greatly reduced without compromising the sharpness. We develop an analytical theory that predicts the relevant characteristics of the transition and verify it with SCF calculations and Monte Carlo simulations. We also demonstrate that from a thermodynamic point of view the transition characteristics of a diblock copolymer are equivalent to those of the active block alone in a modified brush with the same grafting density and reduced length.

Supramolecular Packing Drives Morphological Transitions of Charged Surfactant Micelles
Ken Schäfer, Hima Bindu Kolli, Mikkel Killingmoe Christensen, Sigbjørn Løland Bore, Gregor Diezemann, Jürgen Gauss, Giuseppe Milano, Reidar Lund, Michele Cascella
Angewandte Chemie International Edition, (2020);
doi:10.1002/anie.202004522

The shape and size of self‐assembled structures upon local organization of their molecular building blocks are hard to predict in the presence of long‐range interactions. Combining small‐angle X‐ray/neutron scattering data, theoretical modelling, and computer simulations, sodium dodecyl sulfate (SDS), over a broad range of concentrations and ionic strengths, was investigated. Computer simulations indicate that micellar shape changes are associated with different binding of the counterions. By employing a toy model based on point charges on a surface, and comparing it to experiments and simulations, it is demonstrated that the observed morphological changes are caused by symmetry breaking of the irreducible building blocks, with the formation of transient surfactant dimers mediated by the counterions that promote the stabilization of cylindrical instead of spherical micelles. The present model is of general applicability and can be extended to all systems controlled by the presence of mobile charges.

Kernel-Based Machine Learning for Efficient Simulations of Molecular Liquids
Christoph Scherer, René Scheid, Denis Andrienko, Tristan Bereau
Journal of Chemical Theory and Computation 16 (5), 3194-3204 (2020);
doi:10.1021/acs.jctc.9b01256

The Grignard Reaction – Unraveling a Chemical Puzzle
Raphael Mathias Peltzer, Jürgen Gauss, Odile Eisenstein, Michele Cascella
Journal of the American Chemical Society 142 (6), 2984-2994 (2020);
doi:10.1021/jacs.9b11829

More than 100 years since its discovery, the mechanism of the Grignard reaction remains unresolved. Ambiguities arise from the concomitant presence of multiple organomagnesium species and the competing mechanisms involving either nucleophilic addition or the formation of radical intermediates. To shed light on this topic, quantum-chemical calculations and ab initio molecular dynamics simulations are used to study the reaction of CH3MgCl in tetrahydrofuran with acetaldehyde and fluorenone as prototypical reagents. All organomagnesium species coexisting in solution due to the Schlenk equilibrium are found to be competent reagents for the nucleophilic pathway. The range of activation energies displayed by all of these compounds is relatively small. The most reactive species are a dinuclear Mg complex in which the substrate and the nucleophile initially bind to different Mg centers and the mononuclear dimethyl magnesium. The radical reaction, which requires the homolytic cleavage of the Mg–CH3 bond, cannot occur unless a substrate with a low-lying π*(CO) orbital coordinates the Mg center. This rationalizes why a radical mechanism is detected only in the presence of substrates with a low reduction potential. This feature, in turn, does not necessarily favor the nucleophilic addition, as shown for the reaction with fluorenone. The solvent needs to be considered as a reactant for both the nucleophilic and the radical reactions, and its dynamics is essential for representing the energy profile. The similar reactivity of several species in fast equilibrium implies that the reaction does not occur via a single process but by an ensemble of parallel reactions.

A generalized Newton iteration for computing the solution of the inverse Henderson problem
Fabrice Delbary, Martin Hanke, Dmitry Ivanizki
Inverse Problems in Science and Engineering, 1-25 (2020);
doi:10.1080/17415977.2019.1710504

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